Advanced Technologies to Treat Manure Generated on Dairy Farms: Overview and Perspectives for Intensifying Australian Systems

Abstract

Livestock production systems are considered some of the most environmentally degrading due to greenhouse gas (GHG) emissions and their contribution to poor air, soil and water quality, amongst other impacts. Advanced manure treatment technologies are required in response to intensification of dairy production worldwide, and the considerably greater volumes of manure generated that require collection and management. Similarly, in Australian dairy systems cows spend more time off pasture, with increased collection of larger manure volumes from a range of contained housing facilities. Adoption of advanced treatment is required to capture nutrients at risk of loss, and ideally to valorise manure to support uptake of these technologies. This review describes the generation of manure and the manure sources found in commercial Australian systems, including grazing-based and intensive dairy farms, supporting zero grazing. The review draws on manure data from pasture-based industries elsewhere and summarises their properties for comparison with Australian systems. Manure treatments that recover and retain nutrients, water and energy are reviewed. These include additives, mechanical/chemical/membrane separation, thermochemical and biological treatments which produce organic and inorganic soil amendments, clarified or potable water, gases (N₂, H₂), biofuels and energy. The review describes the technical and operational details of the technologies, and where there are opportunities for the Australian dairy industry. Treatment technologies need to be validated for Australian systems based on the collated data of local manure properties, as differences with international manure data have been observed. The relative costs, technological maturity, and the benefits and challenges associated with adoption are discussed. Many advanced technologies are ready for adoption, but others are experimental or at pilot stage and relative costs range from low to very high. However, to accurately assess feasibility of manure treatments, environmental, and production benefits should be balanced against capital and operating expenses and account for costs associated with current management. For large intensive farms, implementing advanced manure technologies may be required to ensure approval to operate/expand and to meet regulatory compliance. Future research for the Australian industry should investigate nutrient retention and further develop separation treatments incorporating chemical and mechanical technologies. Bioconversion of manure through insect composting as well as investigating co-digestion opportunities to enhance biogas production would support famers currently using these systems.

1. Introduction

The projected growth in global population to over 9 billion by 2050 is expected to increase protein demand by 20% and further intensify dairy production worldwide [1]; resulting in increased on-farm excreta deposition, an important contributor to environmental pollution [2]. Similar intensification has occurred in the largely grazing-based Australian and New Zealand dairy systems, where average herd sizes and per cow milk yield have increased due to greater feed and fertiliser inputs [3,4,5]. The considerably greater volumes of manure generated on intensively managed dairy farms require collection and management including adoption of advanced treatment technologies.

1.1. Intensification of Australian Dairy and Manure Generation

The Australian dairy industry is the third largest rural industry with farms located in all states and across climatic regions that vary from sub-tropical to temperate and include Mediterranean and arid zones (Figure 1; [6]). Australia has 4% of global market share and ranks fifth in world dairy trade. Consequently, the recent international focus on greenhouse gas (GHG) emissions reduction has implications for an industry with high dependence on export markets.

Intensification of the Australian industry is demonstrated by an almost four-fold increase in average herd sizes and more than double per cow milk production over the last 40 years (Table S1). Australian dairy systems are largely pasture-based, with animals spending most (mean of 74%; ranging from 0 to 98%) of their time grazing paddocks [7]. Despite this, only 3% of national milk production comes from cows solely grazing pasture compared to milk produced from the variety of forages, grains, concentrates and by-products imported onto farms and the feed systems used (Table S2, Figure S1; [8,9]). For instance, dairy diets fed in the 2023/24 lactation consisted of an average of 1.8 tonnes of supplementary grain and concentrate, although up to an average of 2.5 tonnes per cow per year of supplements were fed in some regions [10]. Data collected over a lactation from 43 commercial Australian dairy farms ranging in herd sizes from 51 to 1263 show supplementary intakes comprised on average 52% of dietary dry matter (ranging from 0 to 100%) and varied between and within farms at different times of the year [11]. Average annual nutrients excreted by these herds were 34, 5, 26, 3 t N, P, K and S/lactation, representing 62, 57, 116, and 41% respectively, of nutrients imported onto these farms [12] with excreta management required to minimise nutrient losses.

The move to partial mixed ration (PMR) and total mixed ration (TMR) diets in Australia, away from complete dependence on pasture, has been driven by factors including, pasture and animal management in hot, drier regions, the desire for larger per cow and more consistent seasonal milk production, as well as access to equipment and technology to minimise farm labour [13]. In parts of the country intensification has seen increased installation of feeding infrastructure and use of housed structures such as Freestalls and Loose Housing Composted Bedding Pack Barns, similar to that found in the USA and Europe. This change has led to ‘Intensive Animal Production’ land-use where dairy cow diets are imported, with the provision of minimal or incidental grazing, and contrasts with ‘Grazing Animal Production’ where grazing is the main source of feed. Consequently, feeding and housing infrastructure installed on Intensive and Grazing Animal Production system farms has contributed to greater volumes and nutrient content of manures accumulating on farms, influencing environmental and planning requirements.

Concreted or earthen feedpads, which may or may not be covered, are usually found on Grazing Animal Production systems, most commonly where farmers use PMR systems. Up to 60% of the animals’ diets may be provided in feedpads, and although these systems can be considered intensive within the context of Australian dairy farms, cows still graze freely and spend most of their time on pasture. On the other hand, ‘Freestall’, ‘Loose Housing-barn’ and ‘dry lot’ are long-term housing options associated with zero grazing, classified under ‘Intensive Animal Production’ and are associated with TMR systems (Figure 2). Barn systems are most common in the Australian dairy industry, comprising 55% of TMR systems and typically holding less than 1000 animals. Freestalls are 26% of TMR farms and hold over 1000 cows, while dry lots comprise the smallest (13%) proportion of farms. Manure sources accumulating in these places can differ greatly.

More recently, emerging corporate consolidated dairy farms, exploring opportunities to house herds greater than 10,000 cows, are experiencing difficulties navigating planning pathways, due to manure generation. Using the relationship developed by Nennich et al. [14] and industry mean per cow milk production, a fully housed 2000 cow herd was estimated to produce about 120 t manure/herd per day. However, this estimate does not consider the volume of water used to wash down concreted surfaces (dairy shed, yards, housing, feed pads). Williams et al. [13] calculated a primary pond storage requirement of 90 ML which would lead to difficulties in siting lagoons on large farms, and in securing planning approval. Irrespective of land-use categories, the industry has seen greater use of imported feed nutrients, larger herd sizes, and infrastructure installation, where deposited and collected manure requires treatment.

1.2. Impact of Intensification on Manure Sources Present on Dairy Farms

Houlbrooke et al. [15] describe the variety of manure sources/types found on a survey of commercial grazing-based dairy farms in New Zealand. Classification of the manure types described are similar to those on Australian dairy farms, and contrast to those found in Europe and the US [15,16]. Dirty water, or dairy soiled water has been reported for Ireland, England and Wales [17,18,19], while in South American countries, dairy washed water is classified as slurry [20]. Diluted/washed manure or effluent is present on all Australian dairy farms resulting from the washing of excreta from the dairy shed (milking shed/parlour), feed alleys or other concreted surfaces. On most farms, this diluted manure is stored in at least one (but up to three) ponds or lagoons although some farmers directly irrigate paddocks with the washdown manure (Table S4; [16]). Other manure sources commonly found on farms include solids removed from effluent ponds (sludge), and manure scraped from hard surfaces which is either stored in ponds or stockpiled depending on total solids (TS) concentration. In barns, about two-thirds of manure is deposited on bedding, which can be periodically removed and stockpiled before land application. On some farms, separation systems used to increase the TS concentration and improve manure handling leads to the presence of other manure sources, such as bedding packs (Loose Housing Compost or Deep Litter).

2. Dairy Manure Characteristics

Data of chemical and physical properties for manure collected from grazing system dairy farms in Australia and elsewhere were collated for this review (Supplementary Table S3). Sources include a review and comparison of farm dairy effluent as well as slurry and manure data for farms in New Zealand [15,21,22], slurry management information for pasture-based dairy farms in South America [20,23], dairy soiled water data from England and Wales [17] and Ireland [18,19], as well as data published in peer-reviewed and grey literature for dairy effluent and sludge samples from farms in south east Queensland and Victoria, Australia [16,24,25,26]. In addition, de-identified effluent, solids and sludge data for samples collected from farms across Australia (2008 to 2009; Table S5) and in the major dairy regions in Victoria in the early to mid-2000s and more recently (2016 to 2023) were also accessed from government research and extension databases for summary statistical analysis. Summary statistics, such as mean, range, and coefficient of variation (CV), were calculated in Excel and tabulated for international manure data (

Table 1. Summary of mean (CV) physicochemical properties for diluted manure (farm dairy effluent, dairy soiled water) collated for countries with pasture-based dairy systems.

	Houlbrooke et al. [27]	Wang et al. [22]	Cumby et al. [17]	Minogue et al. [18]	Salazar et al. [20]			Salazar et al. [23]
	New Zealand (n = 7)	New Zealand (n = 23 to 117)	England & Wales (n = 20)	Ireland (n = 780)	Argentina (n = 63)	Chile (n = 151)	Uruguay (n = 25)	Southern Chile (n = 50)
pH		7.9 (np)	6.88 (np)					7.70 (np)
TS (%)	1.10 (0.83)	0.99 (np)	1.08 (np)	0.50 (1.04)	1.21 (5.51)	2.70 (1.05)	1.05 (3.88)	0.04 (np)
N (g/L)	0.58 (0.75)	0.20 (np)	0.83 (np)	0.59 (0.91)	0.42 (5.94)	1.28 (0.84)	0.27 (1.67)	1.12 (1.15)
P (g/L)	0.10 (0.76)	0.04 (np)	0.42 (np)	0.08 (0.85)	0.20 (4.16)	0.47 (0.99)	0.13 (2.69)	0.09 (0.63)
K (g/L)	0.41 (0.46)	0.22 (np)	1.18 (np)	0.57 (0.90)	0.33 (2.73)	1.06 (0.82)	0.49 (0.92)	0.47 (0.84)

CV—coefficient of variation; n–number of samples; np—data not provided.

) and effluent samples collected on Victorian commercial farms (

Table 2. Mean (minimum–maximum; CV) pH, N, P, K, S (mg/L) for effluent samples collected between 2001 and 2006 by government extension staff from different effluent sources on commercial dairy farms in the three Victorian dairy regions.

Region	Manure Source	No.	pH	N	P	K	S
Gippsland	Single pond	12	-	429 (190–1200; 0.72)	113 (56–300; 1.05)	479 (240–860; 0.53)	112 (28–390; 1.28)
(2006; n = 196)	First pond	64	-	527 (95–1500; 1.03)	119 (24–710; 0.89)	477 (69–1300; 0.54)	140 (9.0–890; 1.24)
	Second pond	79	-	286 (8–1600; 0.94)	107 (2–1400; 1.92)	474 (17–3900; 0.94)	58 (3.6–890; 2.05)
	Third pond	6	-	202 (170–280; 0.65)	63 (42–80; 0.59)	312 (210–590; 0.40)	16 (6.1–27; 0.94)
	Sump	21	-	517 (120–1400; 0.71)	99 (17–260; 0.62)	519 (130–1200; 0.69)	61 (21–140; 0.58)
	Other	14	-	419 (23–1300; 0.21)	112 (8–300; 0.21)	431 (91–800; 0.45)	224 (17–760; 0.53)
	Average			405 (8–1600)	110 (2–1400)	472 (17–3900)	98 (3.6–890)
Northern Victoria	Single pond	20	7.3 (6.5–8.8; 0.07)	311 (82–820; 0.67)	86 (32–350; 0.81)	361 (35–970; 0.71)	113 (19–220; 0.71) ‡
(2006; n = 20)	Average		7.3 (6.5–8.8)	311 (82–820)	86 (32–350)	361 (35–970)	113 (19–220)
South-west Victoria	First pond	191	7.2 (6.2–8.8; 0.05)	678 (7.4–3370; 1.04)	115 (8.2–610; 0.94)	425 (34–1520; 0.57)	56 (5.2–380; 1.13)
(2001, 2004; n = 316)	Second pond	123	7.6 (5.7–8.6; 0.06)	261 (5.1–2900; 1.35)	69 (0.05–520; 1.14)	355 (44–1340; 0.56)	36 (1.1–244; 1.05)
	Third pond	2	8.2 (8.2–8.2; 0)	14 (10.4–18; 0.38)	7 (2.2–11; 0.94)	236 (233–239; 0.02)	10 (3.8–17; 0.90)
	Average		7.4 (5.7–8.8)	512 (5.1–3370)	97 (0.1–610)	397 (34–1520)	48 (1.1–380)
	Overall average		7.4 (5.7–8.8)	465 (5.1–3370)	101 (0.1–1400)	423 (17–3900)	68 (1.1–890)

No pH data available for the Gippsland samples, ‡ Only 6 out of 20 samples analysed for S.

). More detailed summary statistics were performed and visualised using ggplot2 package from R Studio (version 2025.09.0+387, © 2025 Posit Software) and R (version 4.5.1, © The R Foundation) for sample data from commercial dairy farms (Figure 3, Figure 4, Figure 5, Figure 6 and Figure 7). Methodology details are given in the Supplementary Methods.

Macronutrients (N, P, K), pH, and electrical conductivity (EC) were the parameters most frequently analysed in collated manure sample data, reflecting their importance facilitating manure use as fertiliser on grazing system dairy farms (Table S3). Diluted manure (dairy soiled water, farm dairy effluent) data (Table 1) for pasture-based dairy systems were highly variable due to the influence of animal (age, breed, stage of lactation, diet) and environmental (rainfall) factors as well as milking and washdown/scraping procedures [18,21]. Repeated sampling from farms rarely occurred, but where this occurred, no clear seasonal trends were observed in some studies [17,18], unlike others, which indicated greatest N concentrations early in the lactation [22]. Longhurst et al. [21] also reported increases in farm dairy effluent nutrient concentrations over two decades that were associated with larger herd sizes and decreased wash water volumes per cow used in the dairy shed, as well as greater use of N fertiliser.

Manure samples collected for analysis between 2001 and 2006 were only from effluent sources on Australian commercial farms, highlighting the relative importance and prevalence of washed manure for nutrient re-use at that time (Table 2). On average these effluents were alkaline (pH = 7.4) as reported by others [22,23], and ranged widely in N, P, K and S concentrations with coefficients of variation generally greater than 50% (Table 2). Most samples were collected from first and second ponds, except in northern Victoria, where samples collected were only from single ponds.

Samples collected for research in the late 2000s (Table S5) from 39 farms across Australia and in different seasons, came from different on-farm sources, such as first or second effluent ponds, composted or separated solids piles. Similarly, more recent (2016–2023) sample collection in Victoria by government extension staff assisting farmers improving manure management also comprised different manure sources. These were categorised as effluent, sludges and stockpiled manures for data analysis (Figure 3, Figure 4 and Figure 5; Table S4), although within these categories the sources were diverse. The variety now present on commercial farms is likely to be a consequence of intensification of dairy systems leading to different manure sources since sample collection between 2001 and 2006. Analysis of the recent manure data showed pH for all sources was on average close to or greater than pH 7.5, while sludges generally had the lowest mean EC (Figure 3). Mean TS concentration of stockpiled manure (50%) was greater than that of sludges (24%), which had been collected from ponds and sumps. Unlike most other data in the literature (Table 1), TS were not measured in effluent samples from Victorian dairy farms. Consequently, the increased effluent TS associated with reduced wash water volumes per cow observed in New Zealand [21] cannot be verified for Australian dairy farms.

Total N, P and K in sludges and stockpiled manure were more than an order of magnitude greater than for effluent samples, due to dilution with wash water (Figure 4). Mean total N was 0.3 g/L in effluent and about 11 and 12 g/kg in sludge and stockpiled manure respectively. Total P and K in effluent were 0.07 and 0.48 g/L, compared with sludge (2.3, 4.3 g/kg) and stockpiled (3.7, 7.7 g/kg) manure, respectively. Victorian effluent nutrient data were generally lower than that reported for dairy wash waters elsewhere, especially the total P data (Table 1). The N:P ratios for effluent, sludges, and stockpiled manures were 4.2, 4.9 and 3.3:1, respectively. Manure N:P ratios fell within the previously reported range of 3 to 5:1 with lower ratios for stockpiled manure likely due to N volatilisation losses. Nitrate (NO₃) N concentrations were very low, although these were only measured in a third of the samples. Median ammonium (NH₄⁺) N was highest in sludges (Figure 5) but comprised 50%, 13% and 5% of total N in effluent, sludge, and stockpile manures, respectively, likely decreasing with moisture contents. Soluble P was measured in 58% of sludge and 38% of stockpile samples and comprised 10% and 6% of total P in those manure types, respectively (data not presented). No volatile solids data were found for any of the records sourced from the literature (Table 1), and similarly, they were not measured for Victorian farm manure samples.

Effluent samples collected between 2001 and 2006 and between 2016 and 2023 were analysed to compare pH, EC, and total N (Figure 6), and total P, K, and S (Figure 7). Higher median effluent pH was observed for Northern and South West Victoria, and a trend to greater EC in the north and lower in South West Victoria. Median total N was lower in effluent collected in South West Victoria after 2016, and was not different for the other regions, although there was a trend to greater effluent TN in Gippsland and lower in Northern Victoria. Within the 2016–2023 period effluent was most alkaline in the south-west, while no difference was observed for Northern and South West Victoria in the 2001–2006 data. In both time periods, effluent EC was lowest in Northern Victoria. Total N was similar across regions for the earlier data, but between 2016 and 2023 was lower for effluent from the north and south-west than for Gippsland (Figure 6).

Median effluent total P was similar between the two time periods except for South West Victoria which was lower than Gippsland and lower than the 2001–2006 data. In contrast, median total K only appeared to be greater between 2016 and 2023 than the earlier time period. Total K was highest in Gippsland in the later sampling time but only appeared to be greater between 2001 and 2006. The opposite was observed for total S which was greater in the 2001 and 2006 sample collection with the highest median values in the north. No apparent differences were observed between the three regions for the 2016–2023 data (Figure 7).

These data provide chemical analytical data for the manure sources on commercial dairy farms, and like the ManureDB [28] in the USA, the Australian data are highly variable. Manure samples are seldom collected by farmers to determine application rates [29]. Similarly, Australian data in this review were collected either by advisors to support manure management system design or by researchers. Manure analysis was for properties that supported the use of manure as a nutrient amendment [30]. Further development of an Australian database similar to ManureDB could ensure more uniform collection of manure samples and reporting of data, while supporting the availability of up-to-date data and the implementation of treatment technologies on commercial farms.

Labile N concentration in manure is primarily a result of conversion of urinary urea to NH₄⁺ and ammonia (NH₃) through the action of urease enzymes abundantly present in the environment [31]. Scotford et al. [32] describe ammoniacal N, representing both NH₄⁺-N and NH₃-N, as a better indicator of readily plant available N, than NH₄⁺-N. Urine initially is the major contributor to NH₃-N which exponentially decays and above 400 g dietary N cow⁻¹ day⁻¹, exponentially greater excretion of N as urea occurs in urine compared with a linear increase in faecal N [33]. High average lactating cow N intakes of 545 g N/cow/day (range = 268 to 983 g N/cow/day; [11]), show the potential for considerable volatilisation of NH₃ from manure in these systems. Organic bound N needs to be mineralised to NH₄⁺ before plant uptake. Nitrification and denitrification of NH₄⁺ can lead to nitrous oxide (N₂O) emissions or NO₃ leaching losses both during storage and after land application [34].

Dissolved/soluble P formed a variable proportion of manure P in collated data of washed dairy water and effluent, amounting to 60% [23], 19% [19], and 45% [18] as well as 68 to 74% from studies reviewed by the latter researchers. Insignificant amounts of P are found in urine with virtually all P excreted by dairy cows in faeces, which increases with dietary P [35]. This research, albeit for housed dairy systems, showed that soluble P forms over half of total faecal P, explaining the high proportions reported in washed dairy effluents. While no data are available for Australian dairy effluents, it is expected that this soluble P would contribute to point source pollution losses if land application is not appropriately managed, as reported for other grazing systems [36]. Phosphorus readily binds to particulate matter and therefore settles in sludges, where nutrient recovery methods seek to reduce the amount of soluble P through physical or chemical means (see nutrient recovery Section 3.2).

Potassium in manure is susceptible to leaching losses and can contribute to luxury uptake by plants and subsequent animal health issues (hypomagnesaemia, grass tetany) if applied according to pasture N requirements [36]. Minogue et al. [18] observe the disparity in N:K ratio of soiled dairy water (1:1) and plant requirements (2:1) which may lead to over-fertilisation with K and potential animal health problems. Averaged N:K of Australian effluent, sludges, and stockpiled manure from collated data were 0.6, 2.6 and 1.6:1, respectively. Few studies describe dairy manure S forms and their loss pathways from manure [37]. Hydrogen sulphide (H₂S) gas emissions are one form of S loss that contributes to odour impacts of dairy farms and which are influenced by animal diet, environmental factors (temperature, season) and manure management [38,39,40]. Manures from cows fed concentrates or high S content diets emitted more H₂S, while emissions were greatest during summer or high temperatures. Largest emissions of H₂S occurred between 5% and 9% TS manures, which could mean lower emissions for dairy effluents and stockpiled manures.

Little information is available for heavy metal concentrations in dairy manures in Australia. Unpublished data for deposited faecal manure samples collected in spring 2008 for a study of nutrient fluxes on 43 grazing-based dairy farms across Australia [4] gave concentrations; count for cobalt (2.08 mg/kg; 48), copper (47 mg/kg; 15), iron (2.17 g/kg; 51), manganese (366 mg/kg; 51), and zinc (167 mg/kg; 50). Based on information in the literature of heavy metal concentrations in dairy manures, slurry, and composts from the UK [41] and China [42], these Australian values fell within the reported ranges, which were lower than limits for composts worldwide, except for UK specifications for copper and zinc [43]. Heavy metal concentrations in faeces were shown to be related to their contents in animal feed [44], and may need to be monitored for housed systems in Australia, where TMRs and PMRs are fed.

In addition to NH₃ and H₂S losses previously mentioned, carbon dioxide (CO₂), methane (CH₄), and N₂O emissions occur at all stages of the manure management chain [45] including from dairy effluent in grazing systems [46]. On Australian dairy farms, manure GHG emissions are estimated to be about 21% of farm losses with CH₄ comprising half of manure GHG emitted [47]. Chemical and physical properties of manure sources, determined by dairy cow diets and manure management (storage, flushing etc), influence gas emissions [48]. The high nutrient content of dairy manures and the evidence of on-farm GHG emissions from manure sources indicate the requirement for technologies to enable better recovery and use of manure nutrients, as well as the reduction of GHG emissions, especially CH₄.

3. Advanced Manure Treatment

Technologies to improve manure treatment have increased in response to intensification of dairy production, environmental regulations to minimise nutrient losses to water and air, as well as greater emphasis on reductions in GHG emissions, energy recovery and supporting bio-circularity. This interest has resulted in a large number of reviews and meta-analyses, e.g., [31,45,48,49,50,51]. A Scopus search of literature relating to recovery and retention of manure nutrients shows publication of 7950 articles, exponentially increasing from 1995 to present, while dairy manure publications (1114) increased at a slower linear rate over that time (Figure S2). The literature highlights technologies that have been and are being developed to treat manures for nutrient retention and recovery of more sustainable on-farm soil amendments, biogas or biofuel production, environmental impact reduction, and to close nutrient cycling loops through developing circular economies for the livestock and broader agricultural sectors. Manure treatments that can be applied to manure sources typically found in Australian dairy systems (Table 3) are discussed in the following sections. Some of these treatments are more immediately available and appropriate while others need further assessment of their suitability for grazing-based systems.

3.1. Nutrient Retention

Nutrient retention is usually focused on preventing N loss from manures through NH₃ volatilisation [52]. The equilibrium between NH₄⁺ and NH₃ is influenced by urine N concentration, where partial pressure difference between NH₃ gas and NH₃ liquid is affected by pH and temperature, while convective NH₃ transfer from the manure surface is influenced by turbulence and wind speed particularly before crust formation [31,34,52]. Less information is available regarding volatilisation during rain events, and Kupper et al. [49] suggest losses are low. Prior to land application, N volatilisation from manure can be reduced through increased removal frequency, acidification/bioacidification, enzyme inhibition, or the use of NH₃ adsorbents, biological additives or covers depending on manure source (Table 3). The frequency with which manures are collected and land applied reduces interaction of dung and urine with urease enzymes and therefore the extent of NH₃ volatilisation. Acidifying manure pushes the equilibrium of the ion away from gas production particularly if manure is maintained at least at pH 5.5 [53,54].

NH₄⁺ ⇌ NH₃ (l) ⇌ NH₃ (g)

(1)

Inorganic (e.g., sulphuric acid, alum) and biological (e.g., simple sugars) amendments [55] have been recommended, but while some acids are effective at reducing NH₃ volatilisation others are not economic (phosphoric acid), and many are corrosive and hazardous [52,54]. The addition of sulphuric acid to stored manure can lead to evolution of CO₂ and H₂S gases. That said, acidification of manure slurry with sulphuric acid is routinely commercially available in Denmark and some countries in the European Union, which has added advantages of reducing costs of mineral fertiliser application (fertiliser, diesel, machinery) and associated soil compaction [56]. Base precipitating salts such as aluminium and magnesium chlorides are safer, but less effective acidifiers. Bioacidification, either through the addition of lactic acid bacteria or through degradation by innate microbial populations of added labile C sources, also reduces manure pH. Lowering pH changes the physicochemical properties of manure leading to increases in EC, dissolution of Ca-P complexes, concentration of dissolved organic compounds and reductions in surface negative charge and viscosity. Acidification can also influence emissions of other gases, (such as increased H₂S evolution when sulphuric acid is used), the effectiveness of other manure treatment options (e.g., anaerobic digestion) as well as potential for increased availability and loss of P.

Absorbents such as zeolite are porous and have high cation exchange capacity which allow ready capture of NH₃ and NH₄⁺ respectively, although ion exchange selectivity can vary based on chemical properties of zeolites [57]. Similarly carbonaceous materials (biochars, sphagnum peats) also possess large surface areas that readily trap positive ions and gases. Biochar properties such as surface charge and area, porosity, and CEC are variable depending on feedstock and pyrolysis conditions, influencing the choice of biochar to retain manure N [58,59]. Adsorbent use has additional co-benefits when treated manures are land applied including reductions in nitrification, urease activity and improvements in N and P use efficiency and soil properties [58,60]. Early research using saponins to abate NH₃ emissions from pig slurry indicated lower effectiveness than either zeolite or peat [52,53].

Other manure nutrient retention treatments include the use of covers and enzymes. Inhibition of the ubiquitous urease enzyme, while successful, it required repeated application to reduce NH₃ emissions in various laboratory studies [53]. Permeable (straw, peat, zeolite) and impermeable (e.g., polyethylene) covers can trap NH₃ and include surface crusts formed naturally on manures. Both cover types include synthetic and natural materials, but generally impermeable covers are more effective at retaining N in manures [53,61], although increased GHG emissions can occur [62].

Table 3. Categories of treatment technologies with technology types relevant to each of three manure sources, and their potential to mitigate gas emissions, the products produced and comments including relative cost, technology maturity and advantages/disadvantages.

Treatment Category	Manure Sources			Mitigation Potential			Products	Comments	Citations
	Effluent (<5% TS)	Scraped (~10–20% TS)	Solids (>20% TS)	NH3	CH4	N2O
None	Removal and application frequency to minimise loss to the environment			High	High	High	None	Lowest cost with reduced losses for well-designed and managed systems. Storage is generally required when conditions prohibit land application. Land area required for effluent storage for large intensive farms can make this option prohibitive, limiting development.	[63]
Nutrient retention		Acidification /bioacidification	Acidification /bioacidification	High	Expected to be high for acidification; unknown for bioacidification		Enhanced nutrient soil conditioner/green water	Sulphuric acid is not recommended for solid manures. Alternatives to concentrated acids already exist. More research is needed to improve both their technical and economic impacts.	[54,56]
		Enzyme inhibitors							[53]
	NH3 adsorbents	NH3 adsorbents	NH3 adsorbents	High			Enhanced nutrient soil conditioner	Adsorbents can be applied as covers.	[53]
	Biological additives								[53]
	Covers	Covers	Covers	High	Low	Low	Enhanced nutrient soil conditioner	More work required to assess GHG mitigation.	[62]
Nutrient recovery Mechanical	Solid–liquid separation Mechanical/physical - sedimentation basin				Low	Low	Enhanced nutrient soil conditioner	Least costly and may require extensive land area for large herds.	[64]
	Solid–liquid separation Mechanical/physical - screens, screw press, centrifuge				High	Low		Efficiency of separation increases from screens to screw presses to centrifuges. Costs vary: low/medium (screw press) and high (centrifuge). Mitigation potential is influenced by separation efficiency of mechanical separators and any requirement for and configuration of manure storage.	[48,64]
Nutrient recovery Chemical	Solid–liquid separation Chemical; coagulants/flocculants				High	Low	Enhanced nutrient soil conditioner/clarified water	Relatively simple process. Costs (medium) are associated with chemical use. Chemical choice may influence handling, nutrient availability of soil amendments.	[64]
	Solid–liquid separation Chemical; coag/flocc + dissolved air flotation				High	Low		Dissolved air flotation increases separation costs.
	Struvite Recovery–chemical addition, biomineralisation, microbial electrochemical technologies						N/P/K fertiliser	Mostly tested in labs. Some patented. Higher cost and more complex when applied to dairy manure. Can include other more costly N recovery methods. Added salts increase effluent salinity. Bioelectrochemical systems very experimental, can generate energy or H2 gas and have been tested on manure.	[65,66,67,68,69]
	NH3 stripping						N fertiliser	High cost and operationality. pH adjustment required. Applied to digestates in Europe.	[56,70]
	Precipitation as Ca-P, Al-P, Fe-P using acids or metal salts		Liming (CaO.MgO) manure (and associated NH3 stripping)	High			Organo-mineral fertiliser with high pH, Ca, available S and is pathogen-free	Liming needs high temp (>70 °C) and TS (10 to 15%). Needs air-scrubber (to capture NH3) and biofilter (for odorous compounds).	[70,71]
	Electrochemical including electrocoagulation, redox reservoir/ion selective system (RR/IS)						Precipitation of N/P, K/P salts N/K fertiliser + H2 or H2O2	N/P struvite precipitate can foul electrodes but uses less chemicals. Electrocoagulation is adept at suspended solids, turbidity, heavy metal removal and good for manure. RR/IS system at proof of concept. Produce low-cost NH4+ uptake from manure.	[69,72]
Nutrient recovery Membrane	Pressure- and non-pressure-based membrane and filtration technologies, as well as electrodialysis, forward osmosis (see Table 4 for more details)						Potable water and transportable solids	Costs medium to high, operationality: moderate, Membrane fouling is reduced with manure pre-treatment. Electrodialysis is more suited to digestates than higher TS manures. Other membrane technologies are often used to process anaerobic digestates. Bioelectrochemical systems can be combined with membranes.	[70,73,74,75,76,77]
	Reverse osmosis			Potentially high			High-quality water	Also pressure-based. Membranes highly subject to fouling. Very energy demanding and costly. Often used to process digestates.
Thermo-chemical		Wet torrefaction or hydrothermal carbonisation	Thermo-chemical conversion				Biochar, bio-oil, gas	High cost and operationality.	[50,78]
		Pyrolysis/gasification	Thermo-chemical conversion						[71]
Biological	Biological nutrient removal/recovery	Biological nutrient removal/recovery			Potential high GHG mitigation from nitrification–denitrification			Includes: anaerobic NH3 oxidation, biological nitrification–denitrification, microalgae and plant-based recovery; benefits and costs vary depending on technology.	[79]
	Biological P removal (EBPR)	Biological P removal						EBPR has lower cost and reduces environmental impact.	[80,81]
			Compost	Gas emissions affected by C/N ratio			Soil conditioner, recycled solids for bedding	Relatively low cost but requires management of process (equipment for turning piles and labour), takes time, kills most pathogens.	[79,82]
			Vermicompost				Frass–soil conditioner	Unknown costs but would require infrastructure and labour.	[83]
			Insect compost		BSFL GWP similar to untreated manure and half that of windrow compost		Frass soil conditioner, livestock/fish feed, chitosan, biodiesel	Unknown costs but would require infrastructure and labour. Currently largely lab-based. Organic, regenerative farmers would pay more for granular BSF-based fertiliser.	[84,85,86]
		Biological nutrient removal to produce N2 gas							[53]
	Covered anaerobic lagoons (CAL) lowest cost for these systems, but managing digestates and sludges is more difficult than for other built structures.	Digester options that can be applied to scraped manures varying in TS range from continuous stirred tank to USB reactors	Digesters most suited to high TS manures include Plug Flow digesters	Low	High	High	Soil conditioner (digestate), biogas/biofuels, clarified/potable water	Various anaerobic digestion and co-digestion options are available for digesters that can treat diverse feedstocks (e.g., poultry manure, farmyard manure, silage effluent, waste silage, discarded milk, green waste, potatoes, sugar beets or any other organic substrates), contributing to circular economy. Dilute manures may need separation technologies to increase TS and reduce tank sizes. Treatment options above can be applied to digestates to recover nutrients and water.	[70,71,75,87]

Manure sources: effluent-washed/flushed into storage lagoons/ponds; scraped and solids are from concreted/hard surfaces and stockpiled; mitigation potential: possibility of treatment reducing gas losses where known from the literature; comments: include costs, operationality, benefits where known, potential issues, technology details and examples, pathogen impacts; EBPR: enhanced biological P removal; BSFL: black soldier fly larvae; GWP: global warming potential.

Table 3. Categories of treatment technologies with technology types relevant to each of three manure sources, and their potential to mitigate gas emissions, the products produced and comments including relative cost, technology maturity and advantages/disadvantages.

Treatment Category	Manure Sources			Mitigation Potential			Products	Comments	Citations
	Effluent (<5% TS)	Scraped (~10–20% TS)	Solids (>20% TS)	NH3	CH4	N2O
None	Removal and application frequency to minimise loss to the environment			High	High	High	None	Lowest cost with reduced losses for well-designed and managed systems. Storage is generally required when conditions prohibit land application. Land area required for effluent storage for large intensive farms can make this option prohibitive, limiting development.	[63]
Nutrient retention		Acidification /bioacidification	Acidification /bioacidification	High	Expected to be high for acidification; unknown for bioacidification		Enhanced nutrient soil conditioner/green water	Sulphuric acid is not recommended for solid manures. Alternatives to concentrated acids already exist. More research is needed to improve both their technical and economic impacts.	[54,56]
		Enzyme inhibitors							[53]
	NH3 adsorbents	NH3 adsorbents	NH3 adsorbents	High			Enhanced nutrient soil conditioner	Adsorbents can be applied as covers.	[53]
	Biological additives								[53]
	Covers	Covers	Covers	High	Low	Low	Enhanced nutrient soil conditioner	More work required to assess GHG mitigation.	[62]
Nutrient recovery Mechanical	Solid–liquid separation Mechanical/physical - sedimentation basin				Low	Low	Enhanced nutrient soil conditioner	Least costly and may require extensive land area for large herds.	[64]
	Solid–liquid separation Mechanical/physical - screens, screw press, centrifuge				High	Low		Efficiency of separation increases from screens to screw presses to centrifuges. Costs vary: low/medium (screw press) and high (centrifuge). Mitigation potential is influenced by separation efficiency of mechanical separators and any requirement for and configuration of manure storage.	[48,64]
Nutrient recovery Chemical	Solid–liquid separation Chemical; coagulants/flocculants				High	Low	Enhanced nutrient soil conditioner/clarified water	Relatively simple process. Costs (medium) are associated with chemical use. Chemical choice may influence handling, nutrient availability of soil amendments.	[64]
	Solid–liquid separation Chemical; coag/flocc + dissolved air flotation				High	Low		Dissolved air flotation increases separation costs.
	Struvite Recovery–chemical addition, biomineralisation, microbial electrochemical technologies						N/P/K fertiliser	Mostly tested in labs. Some patented. Higher cost and more complex when applied to dairy manure. Can include other more costly N recovery methods. Added salts increase effluent salinity. Bioelectrochemical systems very experimental, can generate energy or H2 gas and have been tested on manure.	[65,66,67,68,69]
	NH3 stripping						N fertiliser	High cost and operationality. pH adjustment required. Applied to digestates in Europe.	[56,70]
	Precipitation as Ca-P, Al-P, Fe-P using acids or metal salts		Liming (CaO.MgO) manure (and associated NH3 stripping)	High			Organo-mineral fertiliser with high pH, Ca, available S and is pathogen-free	Liming needs high temp (>70 °C) and TS (10 to 15%). Needs air-scrubber (to capture NH3) and biofilter (for odorous compounds).	[70,71]
	Electrochemical including electrocoagulation, redox reservoir/ion selective system (RR/IS)						Precipitation of N/P, K/P salts N/K fertiliser + H2 or H2O2	N/P struvite precipitate can foul electrodes but uses less chemicals. Electrocoagulation is adept at suspended solids, turbidity, heavy metal removal and good for manure. RR/IS system at proof of concept. Produce low-cost NH4+ uptake from manure.	[69,72]
Nutrient recovery Membrane	Pressure- and non-pressure-based membrane and filtration technologies, as well as electrodialysis, forward osmosis (see Table 4 for more details)						Potable water and transportable solids	Costs medium to high, operationality: moderate, Membrane fouling is reduced with manure pre-treatment. Electrodialysis is more suited to digestates than higher TS manures. Other membrane technologies are often used to process anaerobic digestates. Bioelectrochemical systems can be combined with membranes.	[70,73,74,75,76,77]
	Reverse osmosis			Potentially high			High-quality water	Also pressure-based. Membranes highly subject to fouling. Very energy demanding and costly. Often used to process digestates.
Thermo-chemical		Wet torrefaction or hydrothermal carbonisation	Thermo-chemical conversion				Biochar, bio-oil, gas	High cost and operationality.	[50,78]
		Pyrolysis/gasification	Thermo-chemical conversion						[71]
Biological	Biological nutrient removal/recovery	Biological nutrient removal/recovery			Potential high GHG mitigation from nitrification–denitrification			Includes: anaerobic NH3 oxidation, biological nitrification–denitrification, microalgae and plant-based recovery; benefits and costs vary depending on technology.	[79]
	Biological P removal (EBPR)	Biological P removal						EBPR has lower cost and reduces environmental impact.	[80,81]
			Compost	Gas emissions affected by C/N ratio			Soil conditioner, recycled solids for bedding	Relatively low cost but requires management of process (equipment for turning piles and labour), takes time, kills most pathogens.	[79,82]
			Vermicompost				Frass–soil conditioner	Unknown costs but would require infrastructure and labour.	[83]
			Insect compost		BSFL GWP similar to untreated manure and half that of windrow compost		Frass soil conditioner, livestock/fish feed, chitosan, biodiesel	Unknown costs but would require infrastructure and labour. Currently largely lab-based. Organic, regenerative farmers would pay more for granular BSF-based fertiliser.	[84,85,86]
		Biological nutrient removal to produce N2 gas							[53]
	Covered anaerobic lagoons (CAL) lowest cost for these systems, but managing digestates and sludges is more difficult than for other built structures.	Digester options that can be applied to scraped manures varying in TS range from continuous stirred tank to USB reactors	Digesters most suited to high TS manures include Plug Flow digesters	Low	High	High	Soil conditioner (digestate), biogas/biofuels, clarified/potable water	Various anaerobic digestion and co-digestion options are available for digesters that can treat diverse feedstocks (e.g., poultry manure, farmyard manure, silage effluent, waste silage, discarded milk, green waste, potatoes, sugar beets or any other organic substrates), contributing to circular economy. Dilute manures may need separation technologies to increase TS and reduce tank sizes. Treatment options above can be applied to digestates to recover nutrients and water.	[70,71,75,87]

Manure sources: effluent-washed/flushed into storage lagoons/ponds; scraped and solids are from concreted/hard surfaces and stockpiled; mitigation potential: possibility of treatment reducing gas losses where known from the literature; comments: include costs, operationality, benefits where known, potential issues, technology details and examples, pathogen impacts; EBPR: enhanced biological P removal; BSFL: black soldier fly larvae; GWP: global warming potential.

3.2. Nutrient Recovery

Nutrient recovery implies technology that retrieves nutrients usually in solid form from manures [51] to be reused to reduce/substitute for synthetic fertiliser nutrient use. These technologies can equally be applied to raw manures as well as digestates produced from anaerobic digestion and co-digestion of manures. Many manure nutrient recovery technologies can be too costly for typical grazing system farms, where most manure is deposited on pasture. However, these technologies can provide management options that help reduce manure storage volumes required for the growing number of proposed TMR systems [13] and will be discussed below.

3.2.1. Mechanical/Physical Solid–Liquid Separation

Lower TS manures and slurries can be separated into solid and liquid fractions using technologies that range from simple physical sedimentation to mechanical separation with or without pre-treatment [64,88]. The aim is to generate a solid fraction with greater nutrient content that can be more cost-effectively exported off farm for crop production but also may be used for energy generation. Co-benefits include reduced odour and availability of low-nutrient clarified liquid for on-farm use.

Identification of the most suitable separation system is influenced by manure properties and desired end products, which determine the separation technology used [64,88]. Settling lagoons or ponds, found on virtually all Australian dairy farms, depend on gravity to sediment solids; operating most effectively for manures with less than 6.5% TS [48]. Almost all NH₄⁺-N and K remain in the liquid, whereas P settles in conjunction with small particles and increases over time, while settling of large particles decreases exponentially with time [64]. Mechanical separation of liquid manure includes centrifuges that increase gravitational settling through the use of centrifugal force, thereby reducing the time for separation. Centrifuges are most efficient for separating TS and P where the liquid fraction can have N:P and N:K ratios more similar to crop requirements. Screens (stationary inclined, vibrating, rotating) and filter belts gravity-drain the liquid from diluted manure influent, where most N, P, K are in the liquid, but up to about 25% can be retained in the solid fraction due to its high moisture content. Zhang and Westerman [88] defined screens, centrifuges and sedimentation as primary separators that generate high-moisture-content (85–95%) solids that can be further dewatered using presses. Screw, belt or roller presses, and press augers apply pressure to force the liquid fraction (including small particles) through a screen, although these lead to the poorest N and P separation, where solids have very little N, P and K, as most small particles are in the liquid fraction. Retention time and cost assist in the selection of the mechanical separator best suited to meet nutrient recovery/manure management objectives. Beyond settling ponds, these technologies require additional investment in equipment, built infrastructure, and energy, as well as maintenance costs and have a limited lifespan. For instance, Fournel et al. [89] compared roller, screw press and decanting centrifuge separators based on the production of solids for use in bedding. Solids produced from the decanting centrifuge were most suitable for dairy cow bedding but were also more costly to produce. Furthermore, the final TS concentration of the solid fraction also varied over time, influenced by the properties of the influent manure, which almost doubled operational costs of the screw press and roller press.

3.2.2. Chemical Solid–Liquid Separation

Chemical separation is used to improve nutrient recovery especially when used in conjunction with physical separation methods above. Coagulation/flocculation has historically been used in industrial and urban wastewater treatment but has also been applied to manure management [88]. Coagulants and flocculants reduce charge repulsion between negatively charged suspended colloidal particles in liquid manures, resulting in aggregation and subsequent sedimentation. Aggregation mechanisms include precipitation of dissolved ions as insoluble Ca-, Al-, or Fe-P complexes, coalescing of manure colloids through charge neutralisation, and flocculation. Electrolytes such as ferric chloride (FeCl₃), magnesium chloride (MgCl₂), aluminium sulphate (alum, Al₂ (SO₄)₃), ferrous sulphate (FeSO₄) and lime (Ca(OH)) are used in coagulation and precipitation of manures. Flocculants are polymer molecules that aid the coagulants to increase particle size and accelerate settling by using their tails and loops to bridge coagulated particles [90]. Flocculation produces larger, stronger, less reversible aggregates than coagulation [91]. Modern polymers have been modified to adsorb particles through charge neutralisation, electrostatic patching, bridging and depletion flocculation mechanisms [92]. Even though polymers are known as flocculants, those with both coagulation and flocculation properties, are better known as coagulant-flocculants [93,94]. Coagulant-flocculants are water soluble, either synthetic or natural, and are categorised based on size, charge (cationic, anionic, neutral, amphoteric) and architecture (branched or linear). Most synthetic polymers are polymerised/co-polymerised acrylamides (polyacrylamide, PAM) but also include polymers of ethylene oxide or diallyl dimethyl ammonium chloride (DADMAC), as well as copolymers with other synthetic monomers and functional groups [91]. Cationic PAM is recommended for dairy manures and were shown to have greater removal efficiencies when combined with metal-based salt coagulants such as aluminium chloride [95] and hydrated lime [96]. Pre-polymerized metal-ion coagulants or inorganic polymeric flocculants are a newer category of coagulants (e.g., polyaluminium chloride, PAC; polyferric sulphate, PFS) that demonstrate higher coagulation efficiencies than traditional salts of Al and Fe [97,98]. Synthetic polymers such as PAM are highly efficient coagulant-flocculants as they are required in small volumes, and do not affect the medium pH, and as such, have been widely investigated for their effectiveness in nutrient recovery from dairy manure [99,100,101,102]. However, PAM degradation products (acrylamides) are potentially toxic to animals as a neurotoxin and carcinogen, although there is evidence of microbial degradation of acrylamide in the environment [103]. Natural bio-flocculants, (such as chitosan and tannin extracts) have been identified as safer alternatives for solid–liquid separation [91,94,104,105]. Removal efficiencies of over 95%, 73% and 54% for TSS, TN, and TP, respectively, were observed when chitosan was used with screened diluted dairy manure [104]. While tannin-based flocculants are used in wastewater treatment [91,94], no reports describe their on-farm efficacy for livestock manures. A challenge with using chemical separation is the potential to overdose with coagulants and flocculants, thereby increasing costs, reversing charge reactions, and leading to re-suspension of aggregated solids. Many studies have used jar tests to identify suitable coagulants, flocculants or their combination and their optimum doses for maximum removal efficiencies from dairy manures (e.g., [95,101,104]). However, more rapid automated approaches are required for instantaneous dosing in the field. Parameters such as pH, turbidity, and suspended solids can be measured using sensors and have been shown to have potential for determining optimal doses for wastewater [106] with a turbidity sensor used to dose dairy effluent [98] treatment. A comparison of removal efficiencies of five coagulants and coagulant-flocculants showed that the optimal dose for effluent clarification could be estimated with only turbidity of the untreated liquid manure [107].

3.2.3. Phosphorus Precipitation

Accumulation of P and K within easy application distance of collection infrastructure around dairy sheds and feed pads of Australian dairy farms and to depth on paddocks routinely receiving farm dairy effluent in New Zealand has been reported [108,109]. Mechanical solid–liquid separation systems most commonly found in Australia produce solids that are more easily transported greater distances, but the liquid fractions are relatively high in P. Recovery of P from these liquid fractions would assist in reducing soil P levels. Crystallisation of struvite to recover P produces a low-solubility fertiliser with slow P release that minimises eutrophication potential while providing plant nutrients as required [81,110,111]. Struvite (MgNH₄PO₄.6H₂O) consisting of equal molar concentrations of PO₄³⁻, Mg₂⁺, and NH₄⁺, has been successfully crystallised from a wide variety of wastes, including dairy manures, although mostly in laboratory trials rather than commercially [112,113,114,115]. Other precipitates can form or struvite crystallisation can be slowed due to interference from the many ionic species present in manure, as well as its high (>0.1%) concentration of suspended solids. Ca concentration, higher in dairy than swine manure, hinders struvite formation in preference for Ca-P compounds such as apatite/hydroxyapatite [116]. Additionally, the soluble to total P ratio is generally low in dairy manures [81,110,112]. Consequently, struvite precipitation was enhanced by pre-treatment of dairy manures to reduce solids (solid–liquid separation), acid treatment to solubilise bound P, use of chelating agents to trap interfering ions such as Ca, Fe, and microwave treatment with heat or chemicals [110,113,114,117]. Anaerobically digesting dairy manure was considered a pre-treatment that increases total and soluble P, as well as NH₄⁺ and Mg⁺ availability in manure [112]. However, further treatment of digestates through pH adjustment favours struvite crystallisation, and struvite recovery is increased by minimising Ca-P formation, increasing percent soluble P and managing ionic strength [111,118]. Enhanced biological P removal, where polyphosphate accumulating microflora hydrolyse organically bound P increasing soluble P in liquid fractions, has also been recommended to complement struvite recovery [81,112].

In addition to chemical precipitation (where the simplest method requires Mg addition in conjunction with stirring and pH elevation), Kataki et al. [112] describe three other P recovery methods, (i) electrochemical deposition of struvite (based on the reduction of water leading to increased pH near a cathode, and which requires energy but no chemical additions), (ii) ion exchange of PO₃⁴⁻ and NH₄⁺ from NaCl regenerated columns and Mg addition to precipitate struvite, and (iii) biomineralisation through mineralisation of organic N compounds to NH₄⁺ by a variety of microorganisms which increases pH of the medium and leads to struvite production. These techniques have largely been tested at laboratory scale, are more costly than chemical precipitation and require further investigation for dairy manures.

K-struvite (MgKPO₄.6H₂O), similarly containing equimolar ionic concentration, can be an alternative product to recover K. However, K-struvite is more soluble than MgNH₄PO₄.6H₂O, which preferentially forms when NH₄⁺ concentration is high [67], and therefore, it is best recovered from denitrified liquid manure fractions [56,115]. Further research is required to investigate factors influencing K-struvite production from dairy manures and assess the potential to recover this mineral at scale [67].

A new approach using ‘redox reservoirs’ to electrochemically take up NH₄⁺ and K⁺ from manure, producing N-K fertiliser salts and either H₂ gas or a peroxide disinfectant has been demonstrated at laboratory scale. Modelling based on laboratory findings suggest that this technology showed economic, environmental, and production benefits for a large 1000-cow dairy farm [72].

Calcium, iron and aluminium are other P compounds that can be recovered from manures. Calcium precipitation has mostly been reported for swine and poultry manures, where patents for technologies such as Quick Wash have been filed [68]. These technologies are based on initial acidification of swine lagoon sludge, raw poultry litter or fresh swine manure solids to between pH 3 and 5, using mineral or organic acids to extract P into the liquid fraction. The low-P solids are subsequently separated leaving a high-P liquid from which P is recovered through the addition of hydrated lime (to raise pH to between 9 and 11) and a polymer flocculant. The recovered solids are purported to have low heavy metal concentration but high P, N and C [78]. Iron and aluminium phosphate precipitation uses metal salts mentioned previously for coagulation of diluted manures (i.e., sulphates and chlorides of iron and aluminium). The precipitates have low P availability and therefore are useful for minimising P loss to the environment in run-off.

Variability in Australian dairy manure nutrients suggests that P recovery technologies require research to investigate their feasibility, including addition of Mg with and without pH elevation, as well as potential pre-treatments. The lower TS of Australian effluents compared with liquid manures reported in the literature may support P recovery due to their lower ionic strengths [111]. High K concentrations of Australian dairy manures indicate potential for K-struvite crystallisation. However, these recovery technologies are likely to be costly to implement on commercial farms. Despite this, struvite crystallisation is considered a more cost-efficient method of recovering P while retaining solids that can be used to return organic matter to soils [65] when compared with technologies that yield liquid fractions with greater purity.

3.2.4. Membrane Separation

Membrane technologies aim to concentrate and separate soluble ions through micro, ultra and nano-filtration, as well as reverse osmosis using pressure. Neither membrane distillation (gas-permeable membranes) nor electrodialysis require pressure for ion separation ([119]; Table 4). More recently forward osmosis using osmotic pressure to separate nutrients across a semi-permeable membrane has been trialled in laboratory scale studies [120,121]. Separation is based on size, charge, affinity, vapour or osmotic pressure and uses porous, dense or ionic membranes of different materials and configurations [73,74,122]. Materials used for membranes can include polyvinyl fluoride (PVDF), polyethersulfone (PES), polypropylene (PP), polyamide (PA), polytetrafluorethylene (PTFE) and polymer composites, ceramic, silica, cation/anion exchange, mixed cellulose ester.

The main limitation to membrane treatment is fouling of membranes and the requirement for cleaning strategies to minimise deterioration in transmembrane flux and the resultant increase in pressure to maintain operation. To that end, pre-treatment of manure (e.g., solid–liquid separation to produce an enriched liquid fraction) is usually required and filtration methods like micro- and ultrafiltration can also be used in sequence with other membrane techniques to reduce fouling. Of the main membrane configurations (plate and frame, spiral wound, tubular, hollow fibre, capillary), the latter two are most easily fouled, difficult to clean are require extra pre-treatment, while flat sheet or tubular membranes are more suitable for manure as they are easier to clean. The economics of use of membrane technologies is usually unfavourable compared with other technologies due both to the installation (tanks, pre-treatment, pumps, and compressors), as well as operational (energy consumption, cleaning and maintenance and labour) costs [74]. For this reason, membrane technology is unlikely to be readily adopted by Australian dairy, although a number of commercial membrane systems have been piloted on livestock farms in Europe and elsewhere.

Table 4. Membrane filtration types used to treat manures including exclusion sizes functions performed, and their disadvantages.

Filtration Type	Size Exclusion (µm) Molecular Weight Cut off (kDa)	Functions	Manures Used	Disadvantages	Citations
Micro	0.1–5 µm	High efficiency S-L separation based on size. Particle removal (SS, colloids, bacteria)	Manure effluents from biological reactors, pretreated slurries and digested manures; 1.2 to 2% TS	Membrane fouling requiring regular/planned cleaning	[73,74,119]
Ultra	0.001–0.2 µm 100 kDa	High efficiency S-L separation based on size. Particle removal (SS, colloids, bacteria)	Manure effluents/liquors from biological reactors, pretreated slurries and digested manures; 2 to 4% TS	Membrane fouling requiring regular/planned cleaning	[74,119,123]
Nano	0.15–0.40 kDa	Retains or separates soluble nutrients (e.g., NH4+, K+) based on size. Recovers concentrated soluble nutrients, water		Requires removal of suspended solids by centrifugation. Use of ultrafiltration suggested to improve performance	[74]
Reverse osmosis	<0.0001 µm 0.1 kDa	Concentrates or separates soluble nutrients based on charge. Reduces manure volume and recovers water. Most efficient concentration of soluble nutrients	Pretreated, raw and digested manures, from 0.1 to 5% TS	High cost of operation at industrial scale but most efficient of membrane methods	[70,73,74,119]
Electrodialysis	-	Charge-based separation to concentrate N and separate P through highly conductive ion exchange membrane	Solid–liquid separated/acidified manure	Potential N volatilisation and fouling although cleaning restores membrane functionality	[69,119,123,124]
Membrane distillation (gas permeable membranes)	-	Separation based on vapour pressure allowing NH4+ to pass through a porous hydrophobic membrane	High pH (>9) liquid raw or digested manure; aeration can substitute for alkali addition	To minimise clogging suspended solids removal by ultrafiltration is advised	[74,78,119]
Forward osmosis	-	Osmotic pressure used to separate NH3-N/recover water across a semi-permeable membrane, requiring less energy and resulting in lower fouling	Digested pretreated (separated; acidified) manure liquid fraction	Relatively new method only tested at lab-scale; relatively resistant to fouling	[119,120,121,123]

Table 4. Membrane filtration types used to treat manures including exclusion sizes functions performed, and their disadvantages.

Filtration Type	Size Exclusion (µm) Molecular Weight Cut off (kDa)	Functions	Manures Used	Disadvantages	Citations
Micro	0.1–5 µm	High efficiency S-L separation based on size. Particle removal (SS, colloids, bacteria)	Manure effluents from biological reactors, pretreated slurries and digested manures; 1.2 to 2% TS	Membrane fouling requiring regular/planned cleaning	[73,74,119]
Ultra	0.001–0.2 µm 100 kDa	High efficiency S-L separation based on size. Particle removal (SS, colloids, bacteria)	Manure effluents/liquors from biological reactors, pretreated slurries and digested manures; 2 to 4% TS	Membrane fouling requiring regular/planned cleaning	[74,119,123]
Nano	0.15–0.40 kDa	Retains or separates soluble nutrients (e.g., NH4+, K+) based on size. Recovers concentrated soluble nutrients, water		Requires removal of suspended solids by centrifugation. Use of ultrafiltration suggested to improve performance	[74]
Reverse osmosis	<0.0001 µm 0.1 kDa	Concentrates or separates soluble nutrients based on charge. Reduces manure volume and recovers water. Most efficient concentration of soluble nutrients	Pretreated, raw and digested manures, from 0.1 to 5% TS	High cost of operation at industrial scale but most efficient of membrane methods	[70,73,74,119]
Electrodialysis	-	Charge-based separation to concentrate N and separate P through highly conductive ion exchange membrane	Solid–liquid separated/acidified manure	Potential N volatilisation and fouling although cleaning restores membrane functionality	[69,119,123,124]
Membrane distillation (gas permeable membranes)	-	Separation based on vapour pressure allowing NH4+ to pass through a porous hydrophobic membrane	High pH (>9) liquid raw or digested manure; aeration can substitute for alkali addition	To minimise clogging suspended solids removal by ultrafiltration is advised	[74,78,119]
Forward osmosis	-	Osmotic pressure used to separate NH3-N/recover water across a semi-permeable membrane, requiring less energy and resulting in lower fouling	Digested pretreated (separated; acidified) manure liquid fraction	Relatively new method only tested at lab-scale; relatively resistant to fouling	[119,120,121,123]

3.3. Thermochemical Treatment Technologies

Thermochemical processes use high temperatures to convert carbonaceous materials like manures to biochar, hydrocarbon fuels and synthesis gases in different proportions depending on temperature, pressure and oxygen availability (

Table 5. Thermal conversion process conditions and percentages of products (solid, liquid, gas) produced [43,50,71,78].

					Percentage
Process	Temperature (°C)	Pressure (mPa)	Oxygen	Duration	Solid	Liquid	Gas	Comments
Torrefaction	200–300	0.10133	0	1 h	75	20	5	Lower heat pyrolysis.
Hydrothermal carbonisation	80–280	0.1–10						No drying pre-treatment required. An aqueous inorganic fertiliser liquid fraction (containing PO43− and most of the K) is produced, as well as a hydrochar fraction.
Hydrothermal liquefaction	200–350	4–22						Treats, sterilises and deactivates antibiotic-resistant genes in manure, but concentrates heavy metals in solids produced.
Pyrolysis	300–850		0					Catalysing pyrolysis with zeolite increased biofuel production from cow manure.
Slow pyrolysis	220	2.2	0	Hours	35	30	35
Flash pyrolysis	350–550		0	Seconds	12	75	13
Gasification	600–1300	0.10133	1/3 O2	Minutes	10	5	85 (H2 rich)	Not yet deployed for animal manures at industrial scale. Syngas produced is used for fuel, to produce chemicals such as methanol, or to produce H2.
Super critical water gasification (SCWG)	374	22.1						SCWG does not require drying pre-treatment. Shows potential for H2 production. The technology is not commercialised. Future research is required to integrate with C capture.
Combustion	700–1400	0.10133	High O2	Hours	1 (ash)		99 (CO2)

). In these processes, nutrients are recovered from manures, soil conditioners are produced, and alternative energy (biofuels, biogas) generated [50]. Chars produced from dairy manure were shown to have the highest heating value (15.3 MJ/kg), energy density and C in comparison with other manures, and Cantrell et al. [125] considered dairy manure a favourable feedstock for thermochemical processing. Bio-oils produced from manures, on the other hand, are not suitable for internal combustion engines due to impurities (ash, alkali and alkaline elements), water content, and low pH. Despite the potential nutrient recovery, soil conditioner and green fuel production as well as circular economy benefits to be gained from thermochemical treatment of dairy manure, these technologies are not as well established and would be costly to implement in Australian dairy systems. Further research is required to optimise thermochemical processes, including manure source and reactor design with more emphasis on quantifying the impact of variation in manure characteristics on properties of biochars produced and their impact on soils and pastures.

3.4. Biological Nutrient Removal and Recovery

An alternative to thermochemical processes is biochemical treatment of manure using organisms ranging from prokaryotes through cyanobacteria and algae to plants, in aerobic or anaerobic conditions. These treatments generally require less energy and take longer than thermochemical processes which occur in hours or less [43]. In contrast to thermochemical treatment, biological nutrient removal can be applied to either diluted or high solids manures.

3.4.1. Low Solids Manures

Nitrifying and denitrifying bacteria grown in sequential aerobic and anaerobic tanks or in one tank with intermittent aeration are effective in converting NH₄⁺ to N₂ gas [78]. Both planctomycetes-like bacteria as well as Candidatus Brocadia caroliniensis are also able to produce N₂ from NH₄⁺. This chemolithoautotrophic bioconversion occurs via an anaerobic NH₃ oxidation pathway of high NH₄⁺–low C manures, which requires solid removal from dairy effluents. In contrast to releasing N to the air, nutrient accumulating organisms such as purple non-sulphur bacteria, cyanobacteria and algal-bacterial systems [78,122], allow recovery of N and are very effective at assimilating nutrients with high N accumulation rates.

Enhanced biological P removal (EBPR) from liquid manures by polyphosphate accumulating organisms (PAOs) can sequester P in an aerobic environment and release P under anaerobic conditions [80]. P uptake is associated with energetic use by PAOs of accumulated polyhydroxylalkanoates (PHAs), to increase biomass. During anaerobic stages, formation and accumulation of PHAs depends on energy released from cleavage of polyphosphates. Separation of the P-enriched biomass yields a liquid fraction with reduced P concentration, and P can be released from the solids using previously described technologies. This process is favoured at pHs between 7 and 8, and a C:P of >5. However, performance of this process can be influenced [78] by environment (e.g., rainfall), operational conditions (pH, mixing speed), and manure characteristics (microbial composition).

Algae and plants can be used to accumulate N and P or to support metabolism of N [78,122]. These processes include N accumulation in algae which can be harvested for alternative uses including animal feed, thereby contributing to circularising nutrient use on farms. Vegetative filters and constructed wetlands are other methods that enable nitrification–denitrification, NH₃ volatilisation, and N accumulation in plants, as well as P assimilation/immobilisation in soils and plants. These methods slow the movement of nutrient rich waters, allowing conversion mechanisms and plant accumulation. Vanotti et al. [78] suggest that natural wetlands are not suitable for treatment of manures, in contrast to constructed wetlands which are a relatively inexpensive treatment method and typically require large areas of land. This requirement for land is potentially less of an issue for most grazing-based dairy farms, although land would be removed from production.

3.4.2. Bioconversion

Irrespective of herd size, on many Australian dairy farms with concreted or earthen feed pads (e.g., holding areas) or housed animals manure deposited in these areas is scraped and stockpiled, which requires considerably lower volumes of water than used in flood-wash systems. Composting is a frequently used bioconversion method to treat scraped manure, concentrating and stabilising (humifying) nutrients, reducing odour, pathogens, and weed seeds [126,127,128,129]. This traditional composting can generate soil amendments that retain nutrients and enhance soil condition. Composted manures have also been used as bedding for cows and were shown to have similar microbial counts compared with untreated recycled manure solids [130]. To compost manure requires more active management than simply stockpiling. Traditional aerobic composting, usually in windrows, proceeds through mesophilic, thermophilic, cooling and maturation phases, and requires raw materials of suitable C:N ratio (30–40:1) to be regularly turned with temperature (at least 55 °C) and moisture (40–65%) monitoring [129]. The resulting compost has stabilised nutrient contents and low pathogen levels [128]. However, composting of manure showed greatest cumulative CO₂-equivalent emissions while emissions from stockpiled scraped manure was a third of Australian emissions factor for ‘waste composting’ [131].

Conversion of organic refuse by saprophage (CORS) systems treat manures through the action of invertebrates. Useable products and benefits include enhanced, non-synthetic soil amendments that improve soil biological and physico-chemical properties and plant growth, biopolymers, alternative energy sources and biofuels, and lower GHG emissions (compared with stockpiled manures) [83,132,133]. Vermicompost produced using earthworms, e.g., Eisenia spp. was shown to have reduced heavy metal concentrations, modified microbial diversity and chemical composition, with the added benefit of earthworm biomass for aquaculture and poultry feed in addition to worm casts. While a usable organic amendment was obtained after 30 days, maximising earthworm biomass required vermicomposts to be managed for 120 days [83,134]. Cammack et al. [85] describe insects that degrade manure such as dung beetles which recycle nutrients deposited on pasture, lesser mealworms (Alphitobius diaperinus) and house flies (Musca domestica L). Black soldier flies (Hermetia illucens; BSF) however, are considered most suitable as they are neither a pest species nor a vector of pathogens unlike mealworms and house flies. Black soldier flies degrade a variety of manures, reduce odour, degrade pharmaceutical residues and antibiotic resistance genes, and may decrease manure pathogens. Products of BSF composting arise from both the larvae and the frass. The latter makes a soil conditioner with reduced mass and lowered N and P content compared with untreated manure thereby reducing nutrient accumulation and loss on farms. The larvae can be used to produce biodiesel (e.g., 1.36 t biodiesel/day from a 2000 head dairy farm), livestock and aquaculture feed (although usually not from manures), and chitin. Chitin can be extracted from BSF prepupae/pupae, insects, cocoons as well as from larvae [135], and while extraction percentage and purity is lower than from crustaceans [136], further processing to chitosan gives almost 90% deacetylation [137]. Chitin and chitosan are valuable products of insect farming required in many industries, such as wastewater treatment and flushed dairy manure solid–liquid separation [104]. There are few examples of black soldier fly composting facilities at commercial scale [86], although a mobile unit was tested using dairy manure from commercial farms [138]. Despite potential circular economy benefits of insect composting, limited data exist at farm scale, where management of these and other bioconversion systems would require farmers to devote time and labour, or depend on commercial contractors. Despite favourable perceptions of BSF-derived fertilisers and a generally willingness to use these amendments, Australian farmers’ choice of and formulation preferences were influenced by their categorisation with ‘alternative’ farmers willing to pay greater costs for BSF frass [139].

3.4.3. Anaerobic Digestion

Bioconversion of manure under anaerobic conditions yields CH_4, which, when captured from a digester, reduces GHG emissions attributed to manure on commercial dairy farms, and is considered the “most effective and recommended technology for reducing CH₄ emissions from livestock manure” [87]. Biogas generated on commercial dairy farms is most commonly converted to electricity to offset energy, a major and growing cost on dairy farms [140]. In addition to reducing energy costs, other benefits of anaerobic digestion include decreased odour, reductions in animal, human, and plant pathogens, and weed seeds, improved air quality (where biogas is used to replace traditional fuels that generate particulate matter), substitution for fossil fuel use, and production of high-value soil amendments, with a resultant decrease in fertiliser use and associated emissions [87,141].

Anaerobic digesters are increasingly installed on commercial dairy farms in countries around the world. In the US, as of February 2026, of the total 473 digesters across the country, 379 were on dairy farms ([142]; filtered for dairy animal type). Forty-five digesters were operational across livestock farms in Canada processing over 1 million tonnes of manure, mainly from dairy farms in Ontario [143]. Most digesters in North America produce electricity and/or heat for on-farm use, with extra exported for off-farm use. Europe is the largest biogas producer although China has an order of magnitude greater number of biogas plants than Germany the European country with the greatest number [144]. Manure, agricultural waste and energy crops (specifically grown for anaerobic digestion) constitute about 74% of biogas production in Europe although this varies for individual countries [141,144,145].

The four-stage anaerobic digestion process has specific requirements at each step, starting with hydrolysis, where complex organic molecules (e.g., carbohydrates, proteins) are broken down. Fermentation of the resulting smaller molecules produces volatile fatty acids (VFA), as well as carbon dioxide, and hydrogen. In the third acetogenesis stage, VFA are converted to acetate. Finally, methanogenesis yields biogas consisting of CH₄ (45 to 75%), CO₂ (20 to 50%), and small amounts of other gases. The type of methanogens present will determine whether hydrogen or acetate are primarily used to produce CH₄. Biogas yield, volume and composition is affected by factors, such as the chemical and physical characteristics of the feedstock (C:N ratio, acidity (VFA), alkalinity (NH₃)), temperature, and frequency of feedstock supply and mixing [145,146,147,148,149]. Additionally, inhibitory substances occurring in dairy manure (antibiotics, sanitisers/cleaners/disinfectants, salts, and sulphate/sulphide) will reduce biogas yield.

Biogas yield of cow manure is generally lower than for other agricultural substrates due to pre-digestion in the animal’s rumen [150]. Gas production also varies depending on farm system, likely influenced by animal dietary intake and milk production [151,152]. Dilution of manures during the washing of milking facilities reduces TS and volatile solids concentrations, further decreasing gas production. Solid–liquid separation of diluted dairy manures increases TS concentration, with Frear et al. [153] reporting that over 70% of methanogens were associated with the higher dry matter solid fraction. A further advantage of physical separation is reduced digester size [154]. An alternative approach to increase biogas yield is to amend diluted manure with more than 10% solid manure [155].

Co-digesting dairy manure with C-rich substrates increases biogas yield, through reducing potential NH₄-N toxicity associated with manure [145,156]. Co-digestion substrates for dairy manure include food waste, residues of other agricultural industries (e.g., tomato, potato, fruit, hops), milk, and cheese processor residue, and other livestock manures although accurate agricultural residue data are required for Australia [157,158]. Manure is also a recommended digestion co-substrate due to its year-round availability and an ability to buffer VFA changes during digestion [159]. The benefits of co-digestion includes greater microbial diversity priming microbial activity, balanced nutrients (macro and micro), trace elements and moisture, and dilution of inhibitors. [156]. In addition to increased biogas, the fertiliser value (nutrient content and bioavailability) of digestates produced from co-digestion is also greater. Minerals, metal oxides, monomer and ions, and oxidation agents were generally shown to increase biogas produced from mono- and co-digestion of dairy manure [160]. Industrial slags and clay residues generally increased biogas yield, but the benefits of nanoparticle additives were less definitive. Added iron enhanced biogas production which could be a benefit of chemical solid–liquid separation using salts such as FeCl₃ or FeSO₄. However, as digestion is inhibited by low pH additives, separation of manure with iron salts, which typically reduce pH of solids, may also reduce biogas production. Trace element ions also effectively counteracted high NH₄⁺ in manure, increasing methanogenesis. Of other manure pre-treatment additives investigated, alkaline agents increased biogas yield, in contrast to sulphuric acid. Within Australian livestock pre- and post-farmgate sectors, co-digestion is an opportunity for treating dairy manure. However, the availability and generation of biomass streams, their properties, biomethane potential and organic loading for effluent are limitations for the development of this technology [156].

Manure and co-digestion substrates may need pre-treatment for anaerobic digestion, including removal of debris and unwanted material, increasing TS content, degrading complex chemical components, reducing particle size and removing inhibitory constituents [150,161]. The aim of pre-treatment is to enhance biogas production, speed anaerobic digestion through creating a more homogenous material that does not stratify thereby reducing the requirement for agitation. Manure pre-treatments have been characterised as physical, chemical, physicochemical, and biological and have been demonstrated to increase biogas yield on average by 40%, primarily through enhancing the hydrolysis of hard to degrade lignin, hemicellulose and cellulose (Table S6; [145,150,160,161,162]).

Temperature is one of the most important factors influencing anaerobic digestion which is particularly relevant for digesters in temperate climates, such as that in the location of the majority of the Australian dairy industry. Lower temperatures slow methanogenic activity and longer hydraulic retention times are required. However, adaptations for psychrophilic anaerobic digestion include use of acclimatised microbes and development of new digester reactors [146].

Anaerobic digesters can be categorised as passive, and low- or high-rate systems with their selection influenced by composition and volume of the material to be digested. Schematics of some common digesters are shown in Supplementary Figure S3 (adapted from [163]). Passive systems include covered anaerobic lagoons or ponds, also called ambient temperature digesters, and require little management of the digestion process, although challenges with removal of sludge and sedimented material that accumulates at the base of ponds can make on-going management of these systems difficult. Also, in temperate regions gas production from covered anaerobic lagoons is likely to be limited during cold weather when temperatures fall below 20 °C.

Complete Mix, Plug Flow and Mixed Plug Flow digesters are low-rate systems. Complete Mix digesters comprise one or more tanks where inflow of material is matched with the outflow to keep a constant volume for between 20 and 30 days. As the name implies the contents are mixed, continuously (continuously stirred tank) or intermittently, and these systems are best suited to manure solids contents of 3 to 6%. Plug Flow digesters use material of at least 15 and up to 20% solids content to allow the material to move as a ‘plug’ through the digester over a 15- to 20-day period, also with equal inflow and outflow volumes. The length of these digesters is usually five times greater than their width. Mixed Plug Flow digesters include a “hairpin” turn in the manure path through the digester and heating tubes to encourage mixing of the manure.

In high-rate systems, active anaerobic digestion organisms in effluent leaving the digester are returned to the inflow which reduces digestion times to as little as five days. This approach can be applied to both Plug Flow and Complete Mix systems with the latter now called Contact Stabilisation (Anaerobic Contact) digesters. Other high-rate systems are Fixed Film, Suspended Media (including Fuidized Bed, Upflow Anaerobic Sludge Blanket, Induced Blanket) and Sequencing Batch Reactors. In Fixed Film digesters microorganisms form a biofilm around material in a digestion column within the digester. Biofilms are formed around larger manure particles by microorganisms suspended in the upflowing liquid in Suspended Media digesters. Sequencing Batch reactors are only required to retain manure for five days or less and can operate with very dilute manure.

Solid State Anaerobic Reactors digest higher solids content organic matter such as food waste and yard waste. Briefly, in these reactors a ‘wet’ methanogenesis stage that converts VFA to CH₄ is separated from the ‘dry’ fermentation stage where organic matter is converted into VFA. The process occurs in single (bin) or two (silo) phase reactors, distinguished by whether the fermentation phase occurs in the same or separate place as methanogenesis. Any of the previously mentioned digesters can be used for the ‘wet’ methanogenesis stage in the digesters, but high efficiency reactors such as UASB and ASBR are most frequently used.

A variety of processing technologies are being developed to enhance the use of digestate produced from anaerobic digestion, through improving nutrient, fibre and water recovery [70,164,165]. The characteristics of the digestate produced will depend on the initial primary (and any co-) substrates digested and the anaerobic digester used. Digestates from manure are expected to have lower TS and organic carbon, higher pH and NH₄ N compared with undigested animal manure. While decreased GHG emissions were reported, NH₃ volatilisation increased, due to greater NH₄-N concentration [166]. Technologies to post-process digestates are similar to those mentioned previously in this paper for dairy manures, such as mechanical and chemical separation, nutrient recovery, biological and thermochemical conversion [159,164,165].

4. Adoption Considerations and Conclusions

Advanced manure treatment technologies are required by the dairy industry to (1) assist farmers to more easily manage larger manure volumes generated, (2) reduce the accumulation of nutrients on farms and in particular parts of farms, (3) recover nutrients and increase the value of manure, (4) reduce the potential for negative impact on the broader environment through loss of nutrients to surface and ground water and the air, and (5) reduce GHG emissions associated with increased manure volumes. Without adoption of these technologies, expansion of the industry is likely to be constrained and the ability of the industry to meet climate and sustainability goals will be limited, potentially restricting market access. Global companies manufacturing dairy products, such as Unilever and Nestlé, have committed to meeting Sustainable Development Goals and emissions reduction targets, in turn, requiring dairy farmers to improve their environmental credentials [167,168]. For instance, Unilever has set targets to reduce its Scope 3 (i.e., supplier) GHG emissions from forest, land and agriculture by 30% by 2030, through regenerative agricultural and low carbon practices. For the dairy sector, manure management, including bioenergy generation [169] has been piloted to meet this goal. Similarly, Nestlé highlights ‘manure management’ as a key area for reducing the significant contribution of dairy to their Scope 3 GHG emissions, and identifies suppliers who access milk from pasture-based systems such as in Oceania, Europe and South America. Nestlé’s approach to meet their targets includes on-farm manure technologies that produce solid biofertilisers and reduce CH₄ emissions [170]. Manure treatments also have the benefit of contributing to circular bio-based economies [171] and meeting Sustainable Development Goals [172] through reducing reliance on unsustainable agricultural inputs and mitigating their environmental impacts, improving resource use efficiency, minimising waste and adding value to biomass.

Selection of treatment technologies for commercial dairy farms will be influenced by the types, amounts and characteristics of manure sources collected on farms for management. Current Australian manure data for determining on-farm use are largely based on mean nutrient concentrations for samples collected by Victorian government extension staff, as farmers seldom sample prior to application of manures. These data are highly variable, do not always include all major plant nutrients, and lack information (e.g., Ca, Mg), which could inform potential suitability, or operational requirement of different advanced technologies. For instance, many effluent properties were lower than for international diluted/flushed manure data. Additionally, limited data are available for TS concentration of effluents and volatile solids for all manure sources, information that is required to inform anaerobic digestion and which currently relies on international data [156]. The manure data collated for this review are more extensive than available elsewhere nationally and for many locations internationally. Furthermore, these data focus on effluents, while the amounts of stockpiled manure on dairy farms can be large and are unknown. Building on this data repository for the range of dairy farms nationally could aid in treatment selection and manure management. Further improvement in manure use would be supported by rapid and accurate nutrient analysis and quantification of the volumes of manure stores on farms, noting that technologies used in Europe and the US to measure slurry manure nutrients in real time need to be validated for manure types in Australia.

Immediate removal and land application of collected manure is expected to be the lowest cost treatment for most dairy farms, although inability to apply to wet soils means that some form of storage is currently recommended for farmers to best manage on-farm manure sources. Manure treatment is therefore required to minimise gas emissions associated with the storage in stockpiles and lagoons that is typically practiced in grazing systems. The treatment options selected can differ depending on manure TS (Table 3). Effluent, present on all Australian dairy farms, can be treated to prevent gas losses, and to recover nutrients mechanically, chemically, and with membrane and nutrient purification technologies. Biological (bacterial, algal and plant-based) nutrient removal is also appropriate for low TS manures. Thermochemical and biological (composting, anaerobic digestion) treatments to retain and recover nutrients are generally more appropriate for higher dry matter materials.

Technology uptake has been mixed within Australian dairy systems. In contrast to elsewhere globally, little nutrient retention research has been undertaken to demonstrate its effectiveness (Table 3 and Table 6), although application of an adsorbent (lignite) has been shown to reduce NH₃ loss from feedlots [173]. On the other hand, mechanical separators (e.g., screens, screw presses, decanting centrifuges) are available commercially but are more commonly adopted on larger Intensive Animal Production dairy systems rather than smaller grazing-based farms. Similarly chemical separation of dairy farm effluents has not been adopted. The effectiveness of chemical separation of solid and clarified liquids has been demonstrated for New Zealand pasture-based dairy, and the treatment was also shown to reduce CH₄ emissions. Research should be undertaken to investigate other chemical recovery treatments for the manure sources typically found on Australian dairy farms. Currently, industry advisers are considering chemical treatments in conjunction with mechanical treatment to enhance separation of solids from effluents for a few farms. Additionally, international companies offering these technologies are seeking to enter the Australian market (Table 6). Although examples of more advanced recovery technologies (precipitation/membrane/NH₃ stripping) exist internationally (e.g., [56,70,71]), these treatments are generally less commonly used for dairy manure, and are not currently applied in Australian systems. Biological treatments are more commonly adopted with composting most frequently implemented on commercial dairy farms in Australia. Off-setting energy costs while treating manures is a benefit, particularly for dairy farmers managing larger herds. However, despite significant interest from farmers, only a small number of anaerobic digesters have been installed in Australia, in contrast to elsewhere globally. It is important to note that the manure treatments described in this review can be used in combination, incorporating up to five technologies. Sequential deployment of technologies were most frequently used to treat liquid manure fractions [174].

The technologies reviewed in this paper are available to varying degrees for dairy farmers internationally as shown in catalogues of equipment suppliers for small and large scale enterprises (e.g., [43,174,175,176]). For instance, commercial treatment options available to US dairy farmers have been catalogued and evaluated based on six indicators related to liquid manure storage, N and P recovery and environmental impact minimisation, with scores assigned if the technology can be verified [175]. In an earlier report Flotats et al. [174] described technical and economic indicators and environmental impacts of treatment equipment in use commercially or at pilot/laboratory scale in Europe and elsewhere. Similar assessment of treatment technologies, taking into consideration the factors unique to local dairy systems, would benefit Australian farmers.

Operational feasibility of many treatments described in this review has only been demonstrated at a lab, or at best, pilot scale (Table 3 and Table 5). Alternatively, they have been incorporated in livestock systems other than dairy, and seldom for grazing-based farms. For instance, NH₃ scrubbers are common in housed systems, particularly poultry while forward osmosis is still to become a more commonplace treatment of high salt and foul-prone liquid residues [79]. Technological maturity, particularly for thermochemical recovery as well as biogas/biofuel production from manures, is still low and requires further development to support the Australian dairy sector. Electrochemical treatment of liquid manures, while promising to both precipitate nutrients and produce either H₂ gas or disinfected liquid, is still in early development and needs to progress from lab to farm scale. Likewise, bioelectrochemical separation of either P or N from manures as well as other electrochemical technologies need further development and have seldom been validated at scale [69,72,177].

Table 6. Treatments used with different manure sources on Australian dairy farms, their maturity and relative costs.

Category	Treatments	Applied to Australian Dairy Manure Sources			Maturity	Relative Cost
		Effluent (<5% TS)	Scraped (~10–20% TS)	Solids (>20% TS)
Nutrient retention	Acidification/adsorbents/covers/enzymes	No	No	No	High	Low to Moderate
	Biological additives	Yes	No	No		Low
Nutrient recovery: - mechanical	Physical sedimentation	Yes	Yes		High	Low
	Screens/screw presses/decanting centrifuges	Yes	No	No	High	Moderate to High
Nutrient recovery: - chemical/membrane	Coagulant/flocculants (±DAF)	Being considered by consultants	No	No	High	Moderate to High (+DAF)
	Struvite recovery/liming and/or NH3 stripping/precipitation	No	No	No	High	Moderate to High
	Membrane filtration technologies/reverse osmosis/electrochemical	No	No	No	Experimental to High	Moderate to High
Thermochemical	Hydrothermal carbonisation/pyrolysis/gasification	No	No	No	Low to High	Very High
Biological	N and P removal	No	No	No	Low to High	Moderate to High
	Compost	No	Yes	Yes	High	Low
	Vermicompost	No	No	No	Moderate	Low
	Insect (e.g., black soldier fly) compost	No	No	No	Experimental	Moderate
	Anaerobic digestion	Yes	No	No	High	Very High

Maturity and relative cost of technology based on the international literature (e.g., [74,79,174,176]).

Table 6. Treatments used with different manure sources on Australian dairy farms, their maturity and relative costs.

Category	Treatments	Applied to Australian Dairy Manure Sources			Maturity	Relative Cost
		Effluent (<5% TS)	Scraped (~10–20% TS)	Solids (>20% TS)
Nutrient retention	Acidification/adsorbents/covers/enzymes	No	No	No	High	Low to Moderate
	Biological additives	Yes	No	No		Low
Nutrient recovery: - mechanical	Physical sedimentation	Yes	Yes		High	Low
	Screens/screw presses/decanting centrifuges	Yes	No	No	High	Moderate to High
Nutrient recovery: - chemical/membrane	Coagulant/flocculants (±DAF)	Being considered by consultants	No	No	High	Moderate to High (+DAF)
	Struvite recovery/liming and/or NH3 stripping/precipitation	No	No	No	High	Moderate to High
	Membrane filtration technologies/reverse osmosis/electrochemical	No	No	No	Experimental to High	Moderate to High
Thermochemical	Hydrothermal carbonisation/pyrolysis/gasification	No	No	No	Low to High	Very High
Biological	N and P removal	No	No	No	Low to High	Moderate to High
	Compost	No	Yes	Yes	High	Low
	Vermicompost	No	No	No	Moderate	Low
	Insect (e.g., black soldier fly) compost	No	No	No	Experimental	Moderate
	Anaerobic digestion	Yes	No	No	High	Very High

Maturity and relative cost of technology based on the international literature (e.g., [74,79,174,176]).

Adoption of advanced manure treatment ideally requires technologies to be simple to operate and have low capital and maintenance costs [74]. However, these can vary significantly depending on the treatment and equipment selected [174]. The equipment, infrastructure and maintenance required will also be influenced by system intensity and farm layout. Costs for nutrient retention treatments are expected to be relatively low [174] with expenses dependent on amendment choice and application equipment. If amendments can be applied using equipment already on farms costs will be reduced. On the other hand, expenses associated with mechanical separation can range. For instance, where costs are provided, they are generally highest for decanting centrifuges and lowest for screens and will be influenced by any additional equipment (e.g., pumps) and concrete infrastructure required. The outlay for chemical separation will depend on the system installed and associated infrastructure. Automated dosing systems and post separation treatment (mechanical separation) will add to costs of chemicals required. Membrane technologies are generally operated in combination with other separation to reduce fouling and minimise cleaning required. The replacement costs of membranes as well as cleaning chemicals contribute to the variation in costs for this category of technologies. Compared to anaerobic digestion, biological technologies, such as turned compost, have lower investment costs than some chemical, membrane and thermochemical technologies, despite the requirement for equipment such as windrow machines and mixers. However, depending on manure amounts generated, the land area required for bioconversion processes will generally be greater, and these treatments could also require infrastructure [79,174].

To more accurately assess costs of implementing new technology, labour and energy expenses also need to be considered, which will vary depending on the treatment(s) selected (Table 3; [56,70,79,175,176]). Bioconversion treatments are potentially more labour intensive, with composting, for example, needing regular turning and management of moisture and temperatures. Reverse osmosis is very energy intensive unlike electrochemical oxidation of manure and electrocoagulation which do not depend on membranes [72,79].

There is interest in anaerobic digestion due to its potential to offset farm energy use, particularly for more intensive systems with greater animal numbers and which generate large dairy manure volumes. Feedstock from about 1000 fully housed dairy cows is thought to be required for traditional biodigesters with an output of 100 kWe combined heat and power [87]. Similarly, a minimum of 1000 cow herds were required for anaerobic digestion to be feasible on Australian dairy farms. Despite this, recent research has investigated the feasibility of ‘Small-Scale Digesters’ for use on smaller livestock farms present in Europe and North America [87,178]. Many dairy farms in these regions have average herd sizes of 100 to 133 cows which can generate less than 20 kWe. In certain instances, small anaerobic digesters were cost effective, easy to operate and an efficient way for farmers to manage manure. However, a study to assess small-scale anaerobic systems for herd sizes of 100 to 300 showed that capital costs decreased as herd sizes increased. Systems that used the biogas directly or accounted for on-farm energy needs had lower net costs per cow than businesses solely reliant on electrical generation [179]. Policy incentives and accounting for avoided costs made small-scale digesters profitable if designed to meet on-farm needs through increasing manure value and reducing GHG emissions [87].

The average Australian grazing-based dairy herd would generate considerably smaller manure volumes than produced on these European dairy farms small-scale digesters (

Table 7. Estimated manure collected from small-scale housed systems (EU, [87]) and Australian grazed and housed systems.

	EU	Australia
	Housed	Grazed-Average	Grazed-Large	Housed
Herd size a	100	342	1500	1000
Milk production (L/cow/year) a	9000	6443	9000	9000
Estimated manure production (kg/cow/day) b	64	59	64	64
Amount collected (t/herd/day) c	6.4	3.0	29.0	57.9
TS of collected manure (%)	13 d	2	2	17 d
Volatile solids collected (t/day) e	0.69	0.05	0.48	8
Potential CH4 production (m3) f	104	8	72	1226

^a Average herd size and average milk production. For larger herds milk production is increased, although housed animals could be expected to be up to 13,000 L/cow [180]. ^b Calculated based on Nennich et al. [14]. ^c Amount collected accounting for time: in dairy (grazed-average; 15% of day), on feedpads and yards (grazed-large; 30%), and away from dairy (housed; 90%). ^d No water used in collection, TS of collected manure based on scraped manure (housed), except for that washed from the dairy shed (grazed herds). ^e. Assuming manure volatile solids concentration is 83% of TS. ^f Based on a specific CH₄ potential of 0.15 m³/kg VS [87].

). When compounded with the flush systems used in dairy sheds and yards, much greater volumes of effluent with TS contents of less than 5% are produced which require pre-treatment to reduce water contents (e.g., solid–liquid separation) and/or larger infrastructure and longer retention times during digestion.

These estimates suggest that even some of the largest grazing herds in Australia still do not generate manure with equivalent potential CH₄ to small housed systems, suggesting that farm-based anaerobic digestion without co-digestion would not be economically feasible. Combining technologies to pre-treat diluted manure to increase solids content could be advantageous for these systems [174].

To fully assess the costs and benefits of technologies, financial outlay is usually balanced against environmental benefits, productivity gains (e.g., substitution for purchased fertilisers), and value of co-products generated. For example, membrane technologies and thermal conversion generate higher value products (potable water/high purity fertilisers, biogas/diesel), although equipment and maintenance outlay can be high. However, for more advanced technologies at scale cost data are not available. Improved air and water quality (lower NH₃, GHG and odour emissions and other nutrient loss) are environmental benefits of technologies such as acidification, chemical separation, electrochemical synthesis, anaerobic digestion. Worms and insects have been shown to reduce heavy metal accumulation in manures while BSFL can degrade antibiotic resistance genes and pharmaceuticals in manure [82]. Although persistent PFAS compounds have only been found in very low concentrations in animal manures [181] pyrolysis can destroy these organic chemicals [182]. On the other hand, increased secondary pollution can be associated with some manure treatments [79,183,184]. Composting can lead to odour and increased GHG emissions [79]. Chemicals and membranes used during operation of some technologies could generate waste that cannot be recycled. Use of plastic covers to retain NH₄⁺ in manures could contribute microplastics that are then applied to pasture and soil [184]. Furthermore, addition of off-farm residues in co-digestion could introduce contaminants to digestates that need to be managed and accounted for [183,185].

Additional benefits of implementing manure treatment technologies should consider avoided costs, including capital, maintenance and labour, as well as compliance/regulatory levies associated with current manure management. More difficult to quantify is the inability to secure permits to expand, or indeed commence, operation of a dairy enterprise if the farm does not meet state or local planning requirements for manure storage [13]. The financial benefits associated with helping markets meet their Scope 3 targets and industries and governments meet their climate goals should also be counted. For instance, Garnett and Eckard [186] suggest that chemical solid–liquid separation or a passive anaerobic digester could contribute between 5 and 10% reduction in whole farm GHG emissions. Thus, policy instruments could significantly influence cost/benefit estimates and the feasibility of adopting manure treatment technologies. Including these considerations in a decision-making framework could identify trade-offs that need to be taken into account when identifying and selecting treatment technologies [48].

In conclusion, advanced manure treatment technologies offer advantages of improved nutrient and clarified/potable water recovery and reuse, and generation of byproducts including organic soil amendments/fertilisers, energy and biofuels. Manure treatment research continues to grow, as evidenced by the literature citations, particularly for chemical, thermochemical and biological nutrient recovery technologies. Many technologies have been developed as evidenced by catalogues available in Europe and the US. These range from relatively simple low-cost solutions (biological additives, composting) to more costly options (anaerobic digestion, membrane filtration, gasification), with higher value products generated by the latter. Research is required to validate these technologies for Australia as a review of the literature indicates differences in manure characteristics for local compared with international farms. Furthermore, there is an absence of some manure physicochemical data required to support implementation of these technologies in Australia. Currently, within Australian dairy systems, advanced treatment adoption is low and focused on medium-cost mechanical separation, or bioconversion technologies through composting (lower cost) or higher cost anaerobic digestion. Recommending technologies for Australian farms is difficult as choice will be determined by farm size, manure characteristics and volume generated, as well as farmer appetite for technology and the costs involved. That said, manure amendments to retain nutrients should be identified and investigated for manure sources on commercial farms. Further exploration combining mechanical and chemical separation both for raw manure and digestates would enhance treatment benefits for the industry. Current farmer interest in compost could be supported by investigating the potential for vermi- and insect-based conversion, with the latter potentially generating a high-value chitin by-product. Larger farms (>1000 cows), and where animals spend significant periods on concreted infrastructure, need more research into anaerobic digestion and co-digestion options for these systems, including co-use of technologies. Pyrolysis treatment may also be an alternative for dairy farms generating large volumes of solid manures. Collation of industry-wide data of manure generation, TS and volatile solids of effluents and manures will support technology selection. Real-time manure nutrient measurement would aid implementation of treatment options. Furthermore, manure treatment adoption would be supported by up-to-date technology catalogues similar to those available elsewhere. These should include assessment against criteria linked to regulatory compliance and local planning requirements, as well as market access. It is likely that going forward, grazing-based dairy farms will need to demonstrate improvements in manure management. Research into treatment technologies that retain nutrients, reduce carbon loss and environmental and human health impact, minimise noxious emissions and contribute to better soil health will benefit the Australian dairy industry.