Conversion of Kitchen Waste into Sustainable Fertilizers: Comparative Effectiveness of Biological, Microbial, and Thermal Treatments in a Ryegrass Growth Trial

Abstract

This study investigated the conversion of kitchen waste into environmentally friendly fertilizers and soil improvers using five various treatments: (1) inoculation with effective microorganisms followed by anaerobic fermentation, (2) 12 days of decay before adding a double dose of effective microbes, (3) sterilization at 70 °C for 1 h after 12 days of decay, and (4) sterilization followed by effective microbes addition. In (5), waste was decayed, sterilized, and digested without effective microbes. For comparison, a commercial NPK fertilizer and pelleted cow manure were also applied. These treatments were tested in a ryegrass growth experiment in a glasshouse under warm conditions in northern Poland, with four successive harvests. Measurements included dry matter yield, N uptake, N utilization per hectare, and soil properties. Results showed that granulated cow manure and (5) were the most effective fertilizers during the first two months, while other kitchen waste treatments performed better in months 3 and 4, functioning as slow-release fertilizers. The relative agronomic effectiveness after four months was 88%, 67.8%, 60.2%, 48.6%, and 48.6% for (5), (4), (2), (3), and (1), respectively, compared to cow manure, and 36.3%, 27.9%, 24.8%, 20%, and 20% compared to NPK fertilizer. The study indicated that using a double dose of effective microbes produced yields comparable to sterilized waste, suggesting that microbial treatments could replace energy-intensive sterilization, reducing costs. Soil properties were largely unaffected, but residual soil N levels increased from 0.5 to 1.1 g/kg.

1. Introduction

1.1. Kitchen Waste Generation

The current economic strategy heavily relies on non-renewable raw materials, resulting in hazardous waste generation and various environmental issues [1]. In response, innovative concepts of minimizing waste production and alternative feedstock materials have emerged, aligning with circular economy and green production principles. These systems, particularly developed in the EU, aim to address environmental concerns, mitigate the depletion of non-renewable resources, and facilitate a transition from linear to circular material flows [2].

In Poland, approximately 12.5 million tons of municipal waste is generated annually, with around 3.27 million tons recycled (26%) and about 1.01 million tons composted or digested. In 2018, the average waste production per person was 325 kg, with significant variations between urban areas (up to 384 kg) and rural areas (under 200 kg). Compared to the EU average of 480 kg, Poland has one of the lowest waste production rates, second only to Romania (280 kg). Notably, the potential for innovative management of biodegradable waste is significant, particularly since it constitutes 28% of domestic waste (approximately 26 kg per inhabitant), which can be effectively utilized for fertilizing green areas. The organic fraction of municipal solid waste (MSW), primarily kitchen waste, is a nitrogen-rich resource often composted by municipal companies, yet it often holds low economic value.

Kitchen waste (KW) should be managed according to a sustainable way of waste management. KW is defined as organic residues from home kitchens and food waste (FW) is defined as food (plate) waste—that food which has not been completely eaten or spoilt food. Both kitchen waste and food waste contain a high level of organic matter and both macro- and micro-nutrients: nitrogen (N) 0.5–2.5%, phosphorus (P) 0.2–1.2%, potassium (K) 0.4–2.0%, calcium (Ca) 0.5–3.5%, magnesium (Mg) 0.1–0.5%, and organic carbon (C) 40–60% with moisture contents 70–80% and the carbon-to-nitrogen (C/N) ratio from 15:1 to 25:1 [3,4,5]. For this reason, it is worth valorizing fertilizers. However, the high water content, heterogeneity, and variability of the composition are problematic. KW should be processed so that, when discharged directly into the soil, it has no negative impact on the environment (emissions, leaching into groundwater), and fertilizer nutrients are fully assailable.

1.2. Methods for Processing Kitchen Waste

Biological methods for the processing of organic waste have a substantial role in waste management. Both composting and fermentation offer unique benefits and drawbacks as waste processing methods, and the decision to use one over the other is influenced by local conditions. Nonetheless, for biowaste treatment, methane fermentation appears to hold increasing potential due to its distinct advantages over composting. Over the past two decades, the technology and design of fermentation plants have seen steady optimization. Early operational challenges have been addressed, and technological adjustments have been made to suit the specific characteristics of solid waste. Consequently, methane fermentation has become a reliable, proven solution for biowaste processing, enabling the production of high-quality digestate.

Anaerobic digestion (AD) is an effective and popular method for treating various types of organic waste, including kitchen waste. This biological process breaks down biodegradable material in the absence of oxygen, producing biogas—a renewable source of energy—and a nutrient-rich residue known as digestate. The process serves a dual purpose: waste management and resource recovery.

Kitchen waste presents a unique challenge for anaerobic digestion due to its high moisture content, rapid biodegradability, and the presence of inhibitory substances such as fats, oils, and cleaning agents. To address these challenges, techniques such as pre-treatment, co-digestion, additive dosing, and process optimization have been developed. These strategies improve the microbial stability of the digestion process and reduce the inhibitory effects of harmful components. The efficiency of kitchen waste processing can also be enhanced by choosing suitable system configurations and adjusting parameters to match the local composition of kitchen waste. Reusing the treated residues, particularly the digestate, further adds value to the process and enhances the economic viability of biogas projects. One of the most promising innovations in this field is the use of anaerobic dynamic membrane bioreactors (DMR). Compared to traditional anaerobic digestion systems, DMR provide better solid–liquid separation, allowing for higher loading rates and improved retention of microbial biomass. These features help overcome many of the limitations traditionally associated with kitchen waste treatment and expand the range of usable feedstocks [6].

The digestate produced in anaerobic digestion consists of undigested organic matter and microbial biomass. It is typically separated into liquid and solid fractions to facilitate storage, handling, and application. The liquid fraction generally contains a large share of nitrogen (N) and potassium (K), while the solid fraction is rich in residual fibers and phosphorus (P). The nutrient content and composition of digestate determine its agronomic value—that is, its effectiveness as a fertilizer or soil amendment [7,8].

One of the key features of digestate is its high concentration of plant-available nutrients, especially nitrogen in the form of ammonium (NH₄⁺). During anaerobic digestion, organic nitrogen compounds are decomposed by microbes into ammoniacal nitrogen, with only a small portion consumed for microbial growth. As a result, the digestate often contains high concentrations of ammonium and ammonia. The literature reports total nitrogen concentrations in digestates ranging from 11.2 to 25.7 g/kg of total solids. This readily available nitrogen enhances the short-term fertilization effect, promoting rapid plant growth following application [9,10,11].

In addition to nitrogen, digestates are also rich in phosphorus, derived primarily from adenylic acid, nucleic acids, and phospholipids present in the raw feedstock. The pH of the digestate, typically between 7.5 and 8.0, affects the solubility and form of phosphorus. Under alkaline conditions, phosphates tend to precipitate as struvite (MgNH₄PO₄·6H₂O) or apatite, making phosphorus more stable and potentially easier to recover [7,11].

Beyond nutrient content, digestates contribute to long-term soil health. The organic matter in the digestate helps improve the soil’s carbon balance, stimulates microbial activity, and enhances enzymatic functions, which support continuous nutrient release and soil regeneration. Digestates can increase germination rates, improve root development, and enhance soil structure and water retention, leading to better overall soil quality [10].

However, digestate management must be aligned with land carrying capacities to avoid nutrient overload, especially in regions with intensive agriculture where large quantities of digestate are produced daily. In such contexts, the concept of nutrient recovery is gaining traction. Technologies that recover nitrogen and phosphorus from digestate can help transform surplus waste into valuable agricultural inputs while reducing environmental risks.

In summary, anaerobic digestion is not only a practical solution for kitchen waste treatment but also a promising approach for producing renewable energy and sustainable fertilizers. The digestate—when properly processed and applied—can significantly enhance soil fertility, reduce dependency on mineral fertilizers, and close the nutrient loop in agriculture. Future improvements in digestion technology and digestate utilization strategies will further strengthen the role of anaerobic digestion in the circular bioeconomy.

1.3. Usage of Effective Microorganisms

Effective microorganisms (EMs) are commercially available microbial inoculants composed of a consortium of beneficial microorganisms. This mix includes five families, ten genera, and over 80 types of both anaerobic and aerobic microbes. EMs contain lactic acid bacteria, photosynthetic bacteria, yeast, actinomycetes, and fungi, but their effectiveness is influenced by the balance among these primary microorganisms [12,13,14]. Their efficiency is influenced by the ratios of these microorganisms [14]. In this study, the product from Greenland Technologia EM Ltd., Janowiec, Poland was used.

EMs have a wide range of applications; they can be used for intensification of animal production [15], maximization of crop production [16,17,18,19], production of health drinks [20], bioremediation [21,22,23], biodegradation [24] but also in concrete industry [25,26,27], improved humus formation and altering composting processes [28,29], and as coatings additives [30].

Composting with microorganisms has been shown to enhance total microbial populations and biodiversity, increasing degradation rates and mineralization [31]. Researchers concluded that the use of effective thermoacidophilic microorganisms significantly enhanced the overall quality of the final product. Zhong et al. [32] evaluated the composting process with and without EMs addition through the chemical and physical analysis of compost to assess the potential as a partial replacement for peat in potting soil, observing the growth parameters of the grown plants. On the basis of the obtained results, they concluded that bamboo residues can be composted successfully regardless of the addition of EMs. The addition of EMs during composting improved the properties of the compost, although no significant differences were found in plant growth.

Jusoh et al. [33] explored the impact of EMs on the co-composting of rice straw, goat manure, and green waste, finding that while composting processes with or without EMs followed similar trajectories, the addition of EMs significantly increased nitrogen (N), potassium (K), and iron (Fe) contents in the compost, with no notable changes in copper (Cu) and zinc (Zn) levels. Fan et al. [13] studied the effects of EM on the co-composting of organic wastes at a household level, noting that microbial activity shifted pH levels from acidic to neutral, leading to the conversion of organic acids into CO₂.

Studies have demonstrated that EMs can enhance composting processes by increasing microbial populations and biodiversity, thereby improving the quality of the final compost product [31]. For instance, Zhong et al. [32] found that the addition of EMs improved compost properties, though it did not significantly affect plant growth outcomes. Jusoh et al. [33] observed higher nitrogen and potassium levels in compost treated with EMs, suggesting an enhanced nutrient profile. EMs do not elevate the absolute amount of nutrients, but could only transform already existing nutrients into a more available form. However, the effectiveness of EMs remains debated, with some labeling it a ‘myth’ [34,35], indicating the necessity for further exploration into its role in waste management.

Hu and Qi, 2013 [36] performed a long-term field experiment which was conducted to examine how compost application affects crop growth and yield. Findings indicated that using EMs in conjunction with compost over an extended period led to improved crop yields. The highest NPK content observed in plant tissues with EM-treated compost suggested that nutrient release was more efficient with this organic and microbial application. Researchers concluded that compost with EM enhanced soil fertility more effectively than compost alone.

Li and Luo et. al. [37] discovered that nutrient-rich kitchen waste, containing carbohydrates, proteins, and minerals, can be used as a fermentation substrate to produce probiotics, which then act as microbial feed for cultivating rotifers. This approach not only repurposes kitchen waste but also supports rotifer growth. In the study, kitchen wastewater and solid waste were fermented with yeast, lactic acid bacteria, compound bacteria (a mix of yeast and lactic acid bacteria), and effective microorganisms to create microbial feed for rotifers. The results showed that using fermented kitchen waste notably increased rotifer population density, egg-holding rate, body length, and egg volume. The effectiveness was ranked as follows: EM bacteria > yeast > compound bacteria > lactic acid bacteria > control. Thus, EM bacteria are the most effective for fermenting kitchen waste for rotifer cultivation, proving the approach feasible.

1.4. Focus of This Study—Decaying, EMs, Sterilization, Anaerobic Digestion, and Their Combinations

In summary, biological methods such as EM composting and anaerobic digestion are crucial for organic waste management, with anaerobic digestion gaining prominence due to its efficiency and ability to produce valuable byproducts. Technological advancements over the past two decades have optimized fermentation processes, making them reliable for producing high-quality digestate. The primary goal of this paper is to evaluate whether anaerobic digestion or treatment with effective microorganisms (EMs) enhances kitchen waste’s potential as a fertilizer. Specifically, this research investigates the effects of various treatments of kitchen waste on ryegrass growth, biomass yield, and nitrogen uptake. Ryegrass (Lolium) is a pollutant-resistant and fast-growing plant, which can be effectively used from early spring to late autumn in central and northern European regions, e.g., for urban green areas, which do not generate any food for consumers. The choice of the ryegrass was due to two main reasons: (1) rapid growth and high biomass yield: (1a) ryegrass grows quickly and produces a substantial amount of biomass, allowing researchers to assess the effectiveness of fertilizers within a short period, (1b) its fast growth makes it an ideal candidate for repeated measurements, offering consistent and reliable data; and (2) high nutrient uptake: ryegrass is efficient in taking up nutrients such as nitrogen (N), phosphorus (P), and potassium (K), which are often present in processed waste fertilizers. This allows researchers to evaluate the nutrient availability and potential environmental impacts, such as leaching or runoff, from waste fertilizers. That is why it was decided to use this crop in the current study. The Monod model kinetics, a well-established framework in biological and ecological studies, could be later employed to describe biomass growth [38,39]. Moreover, this paper proposes a comprehensive quantitative utilization model based on a literature review, establishing a closed-loop resource chain to achieve efficient management and complete nutrient consumption. While previous studies primarily focused on the effects of EM in composting, this research presents an alternative approach to valorizing kitchen waste without a composting step. By utilizing decaying EMs, thermal sterilization, and anaerobic digestion, this study aims to produce solid organic fertilizers that are non-toxic and environmentally friendly. Of course, generating extra energy, if it comes from fossil fuels for sterilization, may not be considered as an environmentally friendly process, but in this case, this process would be considered as an obligatory step in meeting the national and international regulations on organic and organic–mineral fertilizer production. The production of high-quality organic fertilizers and the use of unpolluted natural resources are critical for achieving a sustainable and clean environment. Ultimately, this paper seeks to demonstrate the potential of effective microorganisms and anaerobic digestion in producing solid organic fertilizers, promoting sustainable resource use and agricultural productivity.

Fertilizers’ agronomic properties are evaluated through plant testing, with germination tests being the most common. These tests measure germination strength, germination index (percentage of seeds that germinate), and phytotoxicity, providing insight into fertilizer effects during the early stages of plant growth. Pot tests allow for the assessment of later growth phases (e.g., up to the third leaf stage in cucumbers) under controlled conditions in a phytotron or greenhouse. Germination tests have the benefit of near-real conditions and enable the evaluation of the transfer factor, or nutrient uptake within the soil-plant system. Various model plants, such as ryegrass, are used in pot trials [40,41,42] as in the present study.

The aim of this paper is to examine the effect of using differently treated kitchen waste on ryegrass growth through the greenhouse study, in relation to effects on plant growth, biomass yield, nitrogen uptake, and residual soil properties.

Since previous studies mainly focused on effects of EM addition to the composting process, this paper describes another approach to valorizing kitchen waste without a composting step, using (1) decaying, (2) effective microbes (EMs) addition, (3) sterilization, (4) anaerobic digestion, and combinations of these for the subsequent production of solid organic fertilizers. Production of non-toxic organic fertilizer using waste is a key method for bringing back nutrients to the environment, what is strongly in line with both circular economy and sustainability.

2. Materials and Methods

The effect of kitchen waste on the growth of ryegrass was studied in pot experiments carried out in a glasshouse—Gdynia Wiczlino, Pomerania, Northern Poland—during a four-month period (mid-April to mid-August 2022).

2.1. Fertilizers

In total, five kitchen waste streams were investigated in a glasshouse experiment and compared with the commercially available mineral and organic NP fertilizer.

Table 1. Basic characteristics of soil and fertilizer materials applied.

Material	Symbol	TS	VS	N-Total	P-Olsen	P-Total	K-Olsen	K-Total
Unit		%	%	g/kg
Soil	-	88.77	6.08	1.32	0.0190	0.186	0.0649	0.610
S1 Kitchen waste 1 × dose EM-incubated	KW 1EM	95	91.42	33.82	0.0486	1.511	5.715	8.482
S2 Kitchen waste decayed, 2 × dose EM-incubated	DKW 2EM	95	89.30	30.04	NA	NA	NA	NA
S3 Kitchen waste decayed, 1 × dose, sterilised, EM-incubated	DKW 1EM ST	95	89.30	32.18	NA	NA	NA	NA
S4 Kitchen waste decayed, sterilized	DKW ST	95	92.37	36.11
S5 Kitchen waste sterilized, digested	DKW ST-dig	95	89.53	42.67	NA	NA	NA	NA
Organic cow manure, granulated	CMG	100	NA	30.00
Mineral fertilizer FLOROVIT NP.	MF	100	0	190	NA	NA	NA	NA

LOD: 0.1% for TS and VS, 2 mg/L for P, 0.01 g/kg for N, 1 ppm for K; SD: 22.7% for N-total, 3.4% for P-total, 4.3% for K-Total, 3.6% for P-Olsen, 29.6% for K-Olsen.

shows the basic characteristics of the materials used.

The first step towards preparing the final fertilizer was to prepare the substrate mixture (model waste) for its production. The mixture consisted of 100 g of each of the following ingredients: apple, lemon, roll, butter, sour cream, milk, cottage cheese, yogurt, eggs, meat with bones, sausage, fish meat, potatoes, banana, tomato, lettuce, fruit juice, and bun and 200 g of each flowers and paper. Then, the ingredients were pre-cut with a knife and then ground twice in a meat mincer in order to obtain a homogenous paste. For a given mixture, total solids (TS%, % fresh mass) and volatile solids (VSS%, %TS) were determined which were 54% and 90%, respectively.

Before planting, soil samples were analyzed for pH and electrical conductivity (EC) (1:5 H₂O), along with total soil nitrogen, phosphorus (both Olsen and total), and potassium (both Olsen and total). After the final harvest, samples were again tested for pH, EC, and total nitrogen [43]. Phosphorus concentration in liquid samples (soil extracts) was measured using a portable spectrophotometer DR3900 (Hach GmbH, Düsseldorf, Germany) with Hach Method 8048, including a mineralization step. Prior to analysis, soil–water samples were filtered through a paper filter followed by a 0.45 µm syringe filter. Total K was measured on solid, ground samples using Energy Dispersive X-Ray Fluorescence EDX 7000 spectrometer (Shimadzu Scientific Instruments Inc., Columbia, MD, USA). Available phosphorus using the Olsen method (P-Olsen) was determined by phosphorus extraction using 4.2% NaHCO aqueous solution with the addition of 0.05% polyacrylamide solution adjusted with 50% sodium hydroxide to pH 8.5.

Commercially available fertilizers were used as a reference material, mineral FLOROVIT NP ‘fast growth’ with additional S and Fe contents and an organic one—granulated cow manure (REF CM). Although the intention is to compare organic fertilizers to organic reference fertilizer (cow manure), the mineral fertilizer was used as an additional reference. No treatment (control) without any fertilizer materials was added for comparison as well.

Then, model wastes were placed in 5 jars for which different pre-treatments were implemented like microorganism addition, decaying, sterilization, and fermentation (

Table 2. Pre-treatment scenarios implemented for model wastes before preparing final fertilizers.

Scenario	Sample Weight [g]	Pre-Treatment					Fertilizer Symbol
		EM [ml]	Water [mL]	Decaying Time [Days]	Sterilization	Fermentation
S1	500	1	249	-	-	-	KW 1EM
S2		2	248	12			DKW 2EM
S3		1	249		1 h, 70 °C		DKW1 EM ST
S4		-	-				DKW ST
S5		-	-			21 days, 38 °C	DKW ST-dig

).

In the first scenario, the model wastes were inoculated with EMs after being placed in the jar. Commercially available (mainly anaerobic) effective microorganisms (Greenland Technologia EM Ltd., Janowiec, Poland) were used. Inoculation with a single dose was achieved by dispersing 1 mL of the bacterial product (EM) in 250 mL of deionized water, and mixed with 1 kg of substrate. In the second scenario, however, the waste was left for 12 days; during this time the wastes decayed, then the solution with a double dose of EM was added. In the third and fourth scenarios, the wastes were sterilized for 1 h at 70 °C according to [44] after 12 days; however, only in the third scenario was the EM solution added, but after the sterilization. In the last scenario, the waste was digested. Mesophilic methane digestion was carried out in 2 L reactors for 30 days in accordance with the methodology described in [45,46,47]. The fermentation residue was centrifuged in a laboratory centrifuge (MPW-260RH, MPW Med. Instruments, Warszawa, Poland) for 10 min at 5000 RPM without the use of coagulants. Later, it had been decayed and sterilized; no EM solution was added.

The effective microorganisms are the Greenland EM BIO formula because of its originality [24]. The composition of the applied EMs is as follows: lactic acid bacteria, photosynthetic bacteria, yeast, nitrobacteria, cane molasses; total nitrogen (0.3%), K₂O (0.2%).

The prepared wastes were pre-dried, then pellets were made, also using a meat mincer, and then dried. Finally, the content of organic nitrogen was determined for all fertilizers by Kjeldahl method in order to calculate the right amount of fertilizer. Samples were digested in concentrated sulphuric acid in the presence of a copper-based catalyst with titanium, the next was a steam distillation step into boric acid solution with Tashiro indicator, then titrated with hydrochloric acid to measure the released ammonia.

Assuming a positive response of the plant to nitrogen-rich organic waste, fertilizer doses were applied at rates from 20 up to 270 kg N/ha in the cool season and up to 370 kg N/ha in the warm season to reach the plateau of the N response curve.

Table 3. Amounts of kitchen waste fertilizers based on N content, added to the soil in the glasshouse experiment.

Scenario				S1	S2	S3	S4	S5	REF CM
Dosage Nr	kg N/ha	g N/pot	mg N/kg TS Soil	g Fertilizer/pot
1 (normal)	20	0.033	0.023	1.027	1.157	1.080	0.962	0.814	1.100
2	70	0.116	0.079	3.596	4.048	3.779	3.368	2.850	3.851
3	120	0.198	0.135	6.164	6.940	6.479	5.773	4.886	6.602
4 (max in PL)	170	0.281	0.192	8.733	9.832	9.178	8.179	6.922	9.353
5	220	0.363	0.248	11.301	12.723	11.877	10.585	8.957	12.103
6	270	0.446	0.304	13.870	15.615	14.577	12.990	10.993	14.854
7	370	0.611	0.417	19.007	21.399	19.976	17.801	15.065	20.356

outlines the experimental design, showing the assumed fertilizer dose and the corresponding (calculated) nitrogen and fertilizer amount per pot. Initial doses started at 20 kg N/ha, the standard recommendation for ryegrass based on mineral fertilizer needs, and were increased by 50 kg N/ha increments until reaching 170 kg N/ha, the maximum yearly limit for nitrogen in natural fertilizers on Polish farmland [48]. The dose was increased further; the last dose was 270 kg N/ha (for the cool season) and 370 kg N/ha (for the warm season, assuming higher growths) to reach the plateau of over-fertilization.

2.2. Soil and Plants Preparation

Plants were cultivated in the <2 mm sieved fraction of a sandy soil mixed with peat in a weight ratio of sand/peat = 5/1, equivalent to a volume ratio of 1:1.5. Soil properties are detailed in Table 1. Additionally, the following soil parameters were measured: for the cool-season experiment, pH was 7.48, redox potential was 33.39 mV, and electrical conductivity (EC) was 191.6 µS/cm; pH was 8.29, redox potential was 63.8 mV, and EC was 159.7 µS/cm. Approximately 1.75 kg of this prepared soil was placed in pots with an internal diameter of 14.5 cm (surface area: 0.0165 m²). Each pot received supplementary nutrient solutions (except nitrogen), according to the recipe: 12 mL/pot and 6 mL/pot of K₂SO₄ (42 g/L); CaCl₂·2H₂O (90 g/L); MgSO₄·7H₂O (24 g/L); MnSO₄·H₂O (6 g/L); ZnSO₄·7H₂O (5.4 g/L); CuSO₄·5H₂O (1.2 g/L); H₃BO₃ (0.42 g/L); CoSO₄·7H₂O (0.16 g/L); Na₂Mo₄·2H₂O (0.12 g/L).

The soil in each pot was pre-watered with 120 mL of deionized water (DIW), then thoroughly mixed with nutrients in the top 5 cm layer. Eighty grains of annual ryegrass—0.5 g in total (containing lolium perenne 40%, lolium multiflorum-estanzuela 20%, festuca rubra 25%, and lolium hybridum 15%)—were sown on the soil surface and covered with an additional 80 g of soil. Experiments were conducted in duplicate, with pots re-randomized every 7 days to equalize light exposure, and maintained at a constant weight at field capacity (20 g H₂O/g TS soil), around 26.4% (cm³ H₂O/cm³ soil). Although the duplicated experimental set-up is not often used in agri-environmental studies, this was due to the space limitations available in the greenhouse this time. Harvests took place monthly over a 4-month period, cutting plants about 1 cm above the soil. The harvested material was then placed in paper bags and dried at 105 °C until reaching a constant weight.

2.3. Soil and Plant Analysis

After each of the four harvests, ryegrass tops were dried, ground, and analyzed for Total Kjeldahl Nitrogen (TKN). Samples were digested in concentrated H₂SO₄ with a titanium-based catalyst using a SpeedDigester K-436 (Büchi Labortechnik, Flawil, Switzerland), then steam-distilled (K-355 distillation unit, Büchi Labortechnik, Flawil, Switzerland) into boric acid solution with a Tashiro indicator before titration with HCl to determine ammonia content. After the finalization of the experiment, soil samples were analyzed for pH and electrical conductivity EC (1:5 H₂O), along with total soil nitrogen.

3. Theory and Calculation

The absolute agronomic effectiveness (AAE) and relative agronomic effectiveness (RAE) of the materials were calculated for each of the four harvests and cumulatively at the experiment’s conclusion, using nitrogen (N) uptake data (for each harvest and cumulatively) and cumulative dry matter yield data. AAE is defined by the slope of the best-fit line representing the relationship between plant N uptake and the rate of N application. RAE is calculated as the ratio of the AAE of the tested material to the AAE of the reference fertilizer [49]. This is a standard method to evaluate the performance of various fertilizers.

4. Results

4.1. Response of the Ryegrass Biomass Yield to Kitchen Waste-Based Fertilizers

The dry matter yields versus the fertilizer dosage range constitute the first and fairly visible measure of fertilizer performance. Figure 1 shows the response of the dry matter yield to a given fertilizer application rate. Plant yields were up to 4.5 g TS/pot with the highest ones and best increasing response noticed between 30 and 60 days. Best yields were provided by CMG, followed by DKW ST-dig; however, these two materials were not that efficient during month 4. Other treatments had similar effect on plant growth, especially visible during month 4, where they provided better yields than CMG.

4.2. Nitrogen Uptake by Ryegrass Fertilised with Kitchen Waste-Based Fertilizers

Figure 2 indicates the amount of N taken up by ryegrass in each month. The highest contents of N after month 1 ranging from 30 to 45 g/kg were noticed for DKW 1EM ST treatment. All treatments except DKW ST followed the linear increase in N in plants with more N input to the soil for this month. Subsequent growths provided less N in plant cells (7–21 g/kg after month 2, 8–14 g/kg after month 3 and 7–13 g/kg after month 4). Most of the N was absorbed during the first 2 months of the growth, whereas months 3 and 4 showed more or less stable N content in plants with increasing N application rate. Higher N contents in plants for kitchen waste treatments (10–13 g/kg) were noticed after month 4 in comparison to kitchen waste digestate (DKW ST-dig) and cow manure (CMG) which ranged from 7 to 10 g/kg, which again proves that these materials in undigested form could work as slow-N-release fertilizers even after sterilization or EM addition. However, in order to see the full picture, the N use per ha has to be calculated and compared, as seen in Figure 3.

N use per ha is displayed in Figure 3, where N content from Figure 2 was multiplied by ryegrass dry matter yields from Figure 1. Again, most N was used during the first 2 months with CMG and DKW ST-dig having the best absolute agronomic effectiveness, expressed by the slopes of the linear trend lines (if the regression was possible) visible on the plots for each material. After month 1, both CMG and DKW ST-dig were a few times better than other materials in utilizing N from the soil, as they provided up to 25 (DKW ST-dig) and up to 35 (CMG) kg N/ha, whereas other treatments only provided 4–17 kg N/ha. After month 3, agronomic effectiveness of reference cow manure dropped from 6.7% (month 1) to 2.9%, whereas all other treatments showed similar effects ranging from 3.57% for DKW 1EM ST to 4.38% for DKW ST-dig. Although N use values after month 4 were only up to 8.5 kg N/ha, this month showed the domination of treated kitchen waste in the following order: DKW 2EM, DKW ST, KW 1EM, and DKW 1EM ST with in contrary DKW ST-dig and CMG expressing the weaker effectiveness (1.55 and 1.02%, respectively). The differences between 2EM addition (DKW 2EM), sterilization without EM addition (DKW ST), and EM addition coupled with sterilization (DKW 1EM ST) were small as their effectiveness ranged from 1.84% to 2.20%. However, the slow-N-release fertilizer character of these materials and fast exploitation of N bank from digested kitchen waste and cow manure is again backed up here by Figure 3.

Here in Figure 4, the final N use is taken into account after summing up ryegrass yields with their N contents after months 1, 2, 3, and 4. This final picture evidences that in absolute terms CMG has the highest agronomic effectiveness (20.3%), followed by DKW ST-dig (17.9%), DKW 2EM (13.8%), and DKW 1EM ST (12.2%), and the weakest one for DKW ST and KW 1EM (9.9%). Anaerobic digestion allows the bioenergy in the form of methane to be extracted and provides more NH₃-N into the digestate; however, both 2EM addition without sterilization has still a very high effect on N use. Sterilization which should be applied according to EU and PL regulations to utilize potential salmonella eggs, E. Coli, and other pathogens does not have as much of a substantial effect as EM addition.

The mineral fertilizer data were not displayed in Figure 1, Figure 2 and Figure 3 because cow manure granulated as organic fertilizer serves as a more representative reference for kitchen waste-based organic fertilizers, especially that agronomic effectiveness in terms of N use per ha of MF is ca. 2.5 times higher (49.3) than CMG (20.3). Mineral fertilizer data are presented in a cumulative way in Figure 4 to generally show their higher effectiveness; these data are also used in Figure 7 when RAE is calculated.

Figure 5a shows the ryegrass growth dynamics across treatments for four subsequent harvests. Generally, 40–50% of the biomass was formed during the second month, 20–30% of the biomass during the third month, whereas ca. 10–15% each during first and fourth months. This was probably due to a combination of already well-developed root zones, temperature/lighting (mid-May to mid-June) conditions, and availability of nutrients. Only the control treatment (without any fertilization) expressed more even distribution of biomass in the first three months, probably due to nutrient scarcity, and stress factors resulting in rapid biomass accumulation in the first month. Among all experimental treatments, DKW ST-dig provided the highest total cumulative yields reaching 35 g TS/pot with CMG reaching a slightly higher value of the 38 g TS/pot. No significant differences in growth dynamics over time were noticed across treatments, with reference fertilizers MF and CMG working a bit faster for the first month already (15–20% of the biomass versus 7–15% for kitchen waste treatments).

Figure 5b shows the N utilization dynamics across treatments for four subsequent harvests. Generally, 35–45% of the nitrogen was utilized during the second month, 20–40% during the first month, whereas ca. 20–40% during the third and fourth month together. Only the control treatment expressed the largest share of N utilization in the first month (up to 60%). Among all experimental treatments, DKW ST-dig provided the highest total cumulative N utilization rate reaching 350 kg N/ha with CMG reaching a slightly higher value of 390 kg N/ha. Among the organic materials, CMG provided over 40% of N utilization in the first month, whereas step-wise treatments of kitchen waste also improved this share, namely, 20% for KW 1M, 25% for DKW-ST, 28% for DKW 2EM, 30% for DKW 1EM ST, and 35% for DKW ST-dig. This proves that the suggested treatment brings the kitchen waste agronomic characteristics closer to the behavior of commercial organic fertilizer.

4.3. Residual Soil Properties After the Growth of Ryegrass

Final soil properties (Figure 6) after 4 months of ryegrass growth on treated kitchen waste varied in terms of pH (7.5–8.4), reaching the highest values for CMG and DKW 1EM ST (more alkaline as typical organic commercial products) and the lowest for KW 1EM and MF (more acidic as typical for untreated waste and mineral fertilizer); however, these differences are not significant. EC equaled mainly 240–460 μS/cm with some visible outliers for DKW ST and DKW 2EM, possibly due to unwanted impurities elevating or decreasing the EC and redox potential (160–200 mV)—the highest for KW 1EM and DKW 1EM ST, medium for DKW ST and MF, and the lowest for DKW 2EM, CMG, and DKW ST-dig. Residual soil N content increased with increasing application rate and varied from 0.5 to 1.1 g N/kg soil, with most of it retained after KW 1EM and CMG for certain fertilizer application rates. Standard error variations: 0.06–3.73% for soil pH, 1.11–21.3% for soil EC, 0.29–9.45% for soil redox, and 0.14–20.39% for soil N content.

5. Discussion

5.1. Absolute Agronomic Effectiveness

Absolute agronomic effectiveness (AAE) is represented by the slope of the best-fit linear regression model that illustrates the relationship between plant growth and fertilizer dosage. This can be expressed through either (1) nitrogen (N) use or (2) dry matter yield as a function of the amount of fertilizer applied. In simpler terms, nitrogen-based AAE quantifies the percentage of nitrogen absorbed in relation to the nitrogen input provided by the fertilizer at varying dosages. Calculating AAE is a widely accepted method for evaluating fertilizer performance in the literature and has been reported in other studies as well [50,51].

5.2. Relative Agronomic Effectiveness

Figure 7 summarizes all previously presented data by showing the calculated relative agronomic effectiveness for the fertilizer materials, where it was possible to calculate AAE. RAE is actually the AAE related to the AAE of the reference mineral fertilizer. The RAE based on the utilization of N (RAE(N)) was calculated and presented for each subsequent harvest and also as a total value after adding all the harvest data (total N). N-utilization-based RAE(N) better characterizes the fertilizer material as it also contains the dry matter yield when calculating the total N-utilisation per area. In general, RAE differed for the materials across time of growth; however, in most of the cases, DKW ST-dig reached the highest values: 88.3%, 147.9%, and 88% after months 1, 2, and cumulative. Then, DKW 1EM ST and DKW 2EM showed similar values: 60.1%, 120.6%, 180.4% (DKW 1EM ST), and 58.4%, 127.4%, and 215.7% (DKW 2EM) for three subsequent months, meaning that twice the dose addition of EMs provided similar agronomic effects in terms of N use. The weakest RAE was noticed for either only thermally treated kitchen waste (DKW ST) or only microbially treated kitchen waste (KW 1EM), where the final RAE after 120 days was about 48.6%. However, these materials performed quite well only after month 2 and 3 (RAE equalled 95.6–210.8%) exceeding (after month 3) or remaining similar values to others (after month 2). In absolute terms, taking into account the whole growth period, the dominating processes that decided about the highest agronomic performance of kitchen waste were anaerobic digestion, coupling EM addition with sterilization, twice EM addition, sterilization solely, and EM addition solely. Optionally mixing processed kitchen waste with commercial organic fertilizers like cow manure granulated could be a good idea to alter the agronomic effectiveness.

The results above are partially in line with the previous authors’ works [52] when they concluded that in the warm season, in comparison to effective microorganism-incubated kitchen waste (KW 1EM), its anaerobic digestion (KW-dig) improved the relative agronomic effectiveness twice after 30 days of growth (82% versus 43%). However, the total effectiveness for anaerobically digested kitchen waste (KW-dig) versus pelleted and effective microorganism-incubated kitchen waste (KW 1EM) was 32% versus 27%. This is quite comparable to the results in the current study. When it comes to the external literature, many works addressed the processed kitchen waste and its valorization towards organic fertilizers [53,54,55,56,57,58,59,60]; however, none of them used the same set of technologies applied on ryegrass.

The theory behind better agronomic performance of digestates vs. undigested feedstocks is as follows: application of biogas digestate, containing mainly ammonium nitrate, typically has a pH of about 5–6. This ammonium nitrate will be immediately available to plants, allowing ’just-in-time’ application. The remaining solvated ammonium hydroxide (NH₄OH) will be nitrified by soil bacteria to form nitrate (NO³⁻) and volatile nitrous oxide (N₂O), but with less emissions because the ammonium concentration is reduced. Excess nitrate will still be lost either by denitrification (right) or by leaching to groundwater. Denitrification adds to the undesirable (N₂O) loss, but the acidic conditions may reduce the action of the denitrification bacteria, thus reducing N₂O emissions.

Obtaining clean kitchen waste for applications like composting or fertilizer production requires careful logistical planning to ensure that the waste is free from contaminants, sorted effectively, and collected efficiently. A detailed description of logistics is provided in

Table 4. Logistic key steps when upscaling kitchen waste into the real-scale valorization towards fertilizer production.

Logistic Step	Recommendations
1. Source identification	Residential sources: Individual households can provide large quantities of kitchen waste. Commercial sources: Restaurants, cafeterias, supermarkets, and food processing units. Institutional sources: Schools, hospitals, and offices with cafeterias.
2. Waste segregation at source	Awareness campaigns: Educate contributors on the importance of clean waste segregation. Provision of sorting bins: - Green bin: For organic waste (fruit peels, vegetable scraps, eggshells, etc.). - Red bin: For non-organic waste (plastic, metal, glass). - Special bin: For hazardous or non-compostable items like oils, chemicals, or bones (if these are excluded).
3. Collection system	Dedicated waste collection services: - Arrange daily or weekly collection schedules for organic waste. - Provide sealed containers or biodegradable liners to prevent leakage and odors. Local drop-off points: - Establish community collection points for kitchen waste.
4. Transportation	Use specialized vehicles with compartments to avoid contamination between organic and non-organic waste. Ensure proper sealing to prevent leakage, odors, and pest attraction during transit.
5. Initial processing and screening	Inspection stations: - Screen for contaminants like plastics, metals, and glass that may be missed during sorting. Sorting technologies: - Use manual or automated systems for an additional layer of sorting.
6. Storage and treatment	Short-term storage: - Store organic waste in controlled conditions to prevent decomposition before processing. Pre-treatment: - Shredding or grinding waste to uniform sizes for composting or biogas production. - Anaerobic digestion and/or sterilization. - Dosing and adding effective microorganisms.
7. Partnerships and incentives	Collaborate with local governments, waste management companies, and NGOs to support collection and processing. Provide incentives for proper segregation - Reduce waste management fees. - Rewards like compost or processed fertilizers for contributors.
8. Monitoring and quality control	Regularly inspect collected waste to ensure it remains clean and free from contaminants. Provide feedback to sources (household, businesses) on segregation quality to improve future collections.

.

With respect to the sustainability aspects, the authors calculated that the theoretical kitchen waste-based fertilizer would cost ca. 84.8 PLN/t including drying electricity (39% cost share), drying heat (34% cost share), EM addition (20% cost share), and pelleting (7% cost share). This estimation does not take into account sterilization cost. Economically speaking only about the basic scenario (KW 1EM), a different pricing policy for various package/supply size could be applied with price/ton increasing with decreasing package mass (from >0.5 t supplies costing 200–800 PLN/t; to <20 kg bags costing each 30 PLN/bag and in total 1500–6000 PLN/t). The calculated payback time of the plant, allowing to produce 35–152 t/year of solid fertilizer, corresponding to ca. 140–600 t/year of input fresh feedstock (25% Total Solids), was from 2 to 7 years depending on the pricing policy of various supplies.

The study has some limitations, namely (1) it is carried out in duplicates due to time and space restrictions in the greenhouse, which can result in less representative data than what would have been when running the study in at least triplicates, (2) the study should be followed by the long-term field study to include the varying weather conditions and diversity of the soil system, and (3) the study is based on model kitchen waste processed by various techniques and it would be worth investigating the post-segregated kitchen waste from the municipal waste sorting/management plant; however, this would imply removal of the potential hazards and contamination from the material prior to land application.

Future studies could build on the presented results in the following aspects: (1) Long-term field studies: The greenhouse experiment, though controlled, does not fully capture field variability. Long-term field trials are needed to validate the results under real-world conditions. (2) Microbial community analysis: Further investigation into microbial dynamics during EM treatments could provide deeper insights into their role in enhancing nutrient availability. (3) Energy consumption assessment: A detailed life cycle assessment (LCA) quantifying energy use and emissions could quantify the sustainable aspects of the novel waste treatment processes.

6. Conclusions

Source-separated food waste at the household level (kitchen waste) is a clean and abundant source of nutrients and carbon that could constitute a feedstock for organic fertilizer manufacturing. In this paper, five scenarios of treated kitchen waste were tested towards agronomic performance in relation to N use via glasshouse studies, using anaerobic digestion, thermal sterilization, microbial addition, and combinations of these.

In absolute terms, taking into account the whole growth period, the dominating processes that decided on the highest agronomic performance of kitchen waste were anaerobic digestion, coupling EM addition with sterilization, twice EM addition, sterilization solely, and EM addition solely. In absolute terms, CMG had the highest agronomic effectiveness (20.3%), followed by DKW ST-dig (17.9%), DKW 2EM (13.8%), and DKW 1EM ST (12.2%), and the weakest one for DKW ST and KW 1EM (9.9%). Anaerobic digestion allows the bioenergy in the form of methane to be extracted and provides more NH3-N into the digestate; however, both 2EM addition without sterilization has still a very high effect on N use. Sterilization which should be applied according to EU and PL regulations to utilize potential salmonella eggs, E. Coli, and other pathogens does not have as much of a substantial effect as EM addition. The calculated relative agronomic effectiveness (RAE) after 4 months (cumulative growth data) equalled 88%, 67.8%, 60.2%, 48.6%, and 48.6% for DKW ST-dig, DKW 1EM ST, DKW 2EM, DKW ST, and KW 1EM, respectively, compared to cow manure and 36.3%, 27.9%, 24.8%, 20.0% and 20.0%, respectively, compared to the mineral NPK fertilizer.

The final soil properties after 4 months of ryegrass growth on treated kitchen waste remain rather stable in terms of pH, EC, and redox potential. Residual soil N content increased with increasing application rate and varied from 0.5 to 1.1 g N/kg soil, with most of it retained after KW 1EM and CMG for certain fertilizer application rates.

Organic nitrogen-based fertilizers, including the ones examined in this study, release nitrogen more slowly than synthetic fertilizers due to their reliance on microbial mineralization processes. However, their effect on soluble nitrogen (primarily nitrate, NO₃⁻) leaching and groundwater contamination varies depending on their composition, application rate, and timing. Animal-based organic fertilizers (e.g., poultry litter, manure) tend to have higher proportions of readily mineralizable nitrogen, increasing the risk of nitrate leaching, especially when applied in excess or just before heavy rainfall. Compost and stabilized organic materials release nitrogen more gradually, generally resulting in lower leaching risks. However, under conditions of high soil moisture and poor plant uptake (e.g., fall application or sandy soils), even these can contribute to nitrate losses. Plant-based organic fertilizers (e.g., alfalfa meal, cover crop residues) usually have higher carbon-to-nitrogen (C:N) ratios, promoting microbial immobilization of nitrogen and thus reducing short-term nitrate leaching. Nevertheless, as they decompose over time, they can contribute to delayed nitrate release, posing a risk to groundwater if not managed correctly. In summary, while organic nitrogen fertilizers can enhance soil health and reduce synthetic input reliance, their improper use can lead to significant nitrate leaching and groundwater contamination. Best management practices—such as appropriate timing, application rates, and use of cover crops—are crucial to minimizing these risks.

Kitchen waste generally has a lower leaching risk than raw manure or poultry litter but can pose a greater risk than well-aged plant-based composts with high C:N ratios—especially if improperly processed. Kitchen waste-based fertilizers, when properly composted and applied at agronomically appropriate times and rates, pose a moderate risk of nitrogen leaching. However, immature or over-applied material can contribute significantly to nitrate pollution in groundwater. As with other organic fertilizers, careful management and matching N release with crop demand are essential to minimize environmental impact.

More studies on varying kitchen waste morphology and pre-treatment methods should further address this issue. However, the study shows the high potential of kitchen waste to be the slow-N-release fertilizer with processes like anaerobic digestion, EM addition, sterilization, and combinations of these to better regulate the gradual N release to plants.