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Article

Reviewing Improved Anaerobic Digestion by Combined Pre-Treatment of Waste-Activated Sludge (WAS)

by
Miao Yang
1,
Margot Vander Elst
2,
Ilse Smets
2,
Huili Zhang
3,
Shuo Li
3,
Jan Baeyens
1,* and
Yimin Deng
4,*
1
Beijing Advanced Innovation Centre for Soft Matter Science and Engineering, Beijing University of Chemical Technology, Beijing 100029, China
2
Chemical Engineering Department, KU Leuven, 3001 Leuven, Belgium
3
School of Life Science and Technology, Beijing University of Chemical Technology, Beijing 100029, China
4
School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 100811, China
*
Authors to whom correspondence should be addressed.
Sustainability 2024, 16(15), 6419; https://doi.org/10.3390/su16156419
Submission received: 24 April 2024 / Revised: 20 July 2024 / Accepted: 21 July 2024 / Published: 26 July 2024
(This article belongs to the Section Waste and Recycling)
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Abstract

:
The anaerobic digestion of wastewater treatment sludge (WAS) produces a “green” biogas while reducing the amount of residual sludge. To increase the yield of biogas, several individual or combined pre-treatment methods of WAS can be used. These pre-treatment methods substantially reduce the amount of volatile suspended solids (VSSs) and their associated total chemical oxygen demand (TCOD). Pre-treating the sludge will increase the methane yield by 15 to 30%. Although the individual methods have been dealt with in research and large-scale operations, the combined (hybrid) methods have not previously been reviewed. Here, different hybrid treatment methods are reviewed, including (1) thermochemical hydrolysis pre-treatment, using an alkaline or acid addition to enhance solubilization of the sludge cells and increase biogas production; (2) alkaline and high-pressure homogenizer pre-treatment, combining a chemical and mechanical treatment; (3) alkaline and ultrasound pre-treatment, capable of solubilizing organic sludge compounds by different mechanisms, such as the fast and effective ultrasound disruption of cells and the increasing effect of the alkaline (NaOH) treatment; (4) combined alkaline and microwave pre-treatment, which enhances sludge solubilization by at least 20% in comparison with the performance of each separate process; (5) microwave (MW) and peroxidation pre-treatment of WAS suspended solids (SSs), which are quickly (<5 min) disintegrated by MW irradiation at 80 °C; (6) ultrasound and peroxidation pre-treatment, with ozone and peroxides as powerful oxidizing agents; and (7) pulsed electric field (PEF) pretreatment. All literature findings are assessed, discussing relevant operation conditions and the results achieved.

Graphical Abstract

1. Introduction

1.1. Anaerobic Digestion and Worldwide Application

Anaerobic digestion (AD), as a widely applied biological method, converts organic material under anaerobic conditions into biogas, a biomass residue and nutrient-rich digestate [1]. AD can treat various biodegradable wastes (e.g., sludge, manure, food waste) in an industrial context. The conversion to biogas is carried out using different types of microorganisms, each responsible for a particular digestion reaction in the mechanism. The most basic anaerobic digestion system contains one reactor, where four stages are distinguished throughout the process, namely, hydrolysis, acidogenesis, acetogenesis, and methanogenesis. This system is called a single-stage digestion system [1].
In general, the produced biogas stream consists mainly of methane (CH4) and carbon dioxide (CO2). On average, biogas is composed of 60% CH4 and 40% CO2, but it can be different depending on the organic stream used as a feedstock [2]. Table 1 summarizes the different biogas compositions. The vol % of H2O in the biogas is commonly around 6%.
Figure 1 illustrates a biogas plant operating in Dafeng City (Jiangsu Province) that includes four stirred digesters, each 875 m3 in volume. The anaerobic digestion operates at a temperature of 35 °C with a retention time of 28 days. Annually, it processes 200,000 tonnes of poultry manure and produces 7 million m3 of biogas, of which 4 million m3 is biomethane. This fraction is fed into the China gas grid. In addition to the produced biogas, 150,000 tonnes of digestate is retrieved. Another example of a Chinese biogas plant, on a smaller scale, using livestock manure is the Liangjia biogas plant. Annually, the plant processes 1800 tonnes of livestock manure, which produces 70,000 m3 biogas for an annual power generation of 120 MWh. As a by-product, 100 tonnes of solid residue and 1500 tonnes of digestate are produced [4].
The yields of digestion units in Malaysia were described by Hanum et al. [5]. The wastewater treatment plant (WWTP) “Pantai 2” in Greater Kuala Lumpur produces 9600 m3 biogas per day, resulting in 400–500 kW of electricity. Another WWTP called “Bunus Centralized sewage treatment plant” in Kuala Lumpur produces 2500 m3 of biogas daily and would generate 15,000 kWh if conversion losses were negligible.
Manure from livestock or poultry shows many applications and promising results. Arshad et al. [6] described the possibilities in Pakistan, where around 42.4 million buffaloes and 51.5 million cattle live. This number of livestock produces large volumes of manure, of which 30% can be collected. The daily collected amount of manure could equal 92.53 million tons. Considering that 1 kg of manure can be anaerobically digested into 40–50 L biogas, a total amount of 4.63 billion m3 biogas is produced daily. Hereof, only 70%, 3.24 billion m3, can be recovered, which results in a daily power generation of 19.72 TWh. Hidayati et al. [7] added further that 1 kg of chicken manure yields a volume of biogas of 70 L.
To stress the importance of AD, some additional literature data for various countries are gathered below as examples.
Starting with sewage sludge, Eurostat statistics were consulted. These show that in the EU-27 of Europe, 10.4 million tonnes of sewage sludge, expressed as dry solids (DSs), are produced annually. If all of this sludge was anaerobically digested, this would allow 3500 GWh of energy to be recovered [8]. It is estimated that 1 PE (population equivalent) produces an average of 18 to 28 L of biogas per day when using the common AD principles. The calorific value of biogas is proportional to the heating fraction of methane since CO2 cannot be further combusted. It is calculated that 1 m3 of biogas with a composition of 60% CH4 and 40% CO2 can provide 6 kWh of energy. Depending on the electrical efficiency of the power generation, this can also be converted to an amount of kWh electric [9].
The quantities of biogas plants in various countries are shown in Table 2. The abbreviation “n.a.” stands for “data not available”. Although biogas is mainly used in Combined Heat and Power (CHP) applications, both Sweden and Germany stand out in using upgraded biogas as transport fuel after the removal of H2S and CO2 [4].
Details of the impacts are illustrated for Yorkshire and Humberside, United Kingdom, which have 610 sewage treatment plants (STPs) and a population equivalence of 6,000,000, generating 390 GWh of power per year [2].
Worldwide, Germany is the largest producer of biogas, with a total of 11,500 biogas plants to process agricultural residues, industrial wastewater, bio-waste, leachate, and sewage sludge, generating 54,100 GWh of electricity annually.
The situation in China is even more important. According to Xu et al. [10], and country statistics, there are over 110,000 biogas production plants, producing about 12.5 × 1012 m3 of biogas. These plants deal with 78.9% of wastewater sludge or household waste, 19.2% of agricultural waste, and 1.9% of industrial residue.

1.2. The Circular Economy Concept of Anaerobic Digestion

A lot of research has been carried out on the full utilization of WAS-AD products, even in producing E-fuels and E-chemicals. Within the goals of a circular economy, the digestate could be upgraded to, e.g., fertilizer and biochar, among others.
A possible strategy for WAS is illustrated in Figure 2.
The circular economy potential is even greater when dealing with AD of food waste, as previously examined by Jin et al. [11] and illustrated below (Figure 3). A complete circle is established from food waste to AD, agriculture, and food. Biogas remains the target energy product.
Within the circular economy concept, AD digestate plays an important role since it is rich in organic matter and nutrients, such as nitrogen, potassium, and phosphorus.
The organic matter (OM) contains the poorly biodegradable or stable organic fraction, the living organic matter composed of microorganisms, and the minor biodegradable fraction that is not transformed into biogas.
The same amount of stabilized organic matter is present in the digestate and the raw feedstock. Consequently, the digestate retains its ability to form humus. When the digestate is spread on soils, the stable organic matter can bind with the clay fraction of the soil, thereby boosting the clay–humus complex essential for water and nutrient retention. The porosity of the soil is improved, which is essential for microbial activity and, thus, carbon and nitrogen cycles.
Through its organic fraction, digestate can provide a long-lasting increase in and release of organic carbon and nitrogen pools to sustain microbial growth and plant nutrition. Soil microbial activity is important to break down organic matter in the soil, such as dead plant materials, into nutrients that plants can use for growth.
The raw digestate can either be used without processing or can be mechanically separated into liquid and solid fractions by, e.g., centrifuges or belt presses. The compositions of both fractions are different, as illustrated in Figure 4 [12,13].
Nutrients are preserved during digestion. According to Fertilizers Europe [14], the total nutrient demand in Europe, based on the average value of the three growing seasons between 2018 and 2021, is 11.1 Mt nitrogen, 2.8 Mt phosphorus, and 3.1 Mt potassium per year. Comparing the total nutrient demand with the nutrient volumes provided through digestate usage as illustrated in Table 3 highlights the considerable share that digestate can cover.
The valorization of the digestate is diverse, as illustrated in Figure 5. As previously stated, the agronomic application provides nutrients for plant growth, ensuring crop yields, and stable organic matter that increases the humus content of the soil and its carbon sequestration [16,17].
When applying digestate, both nitrate leaching (from the nitrification of NH4+ to NO3− by soil bacteria) and the possible emission of organic compounds (VOCS) and odors should be carefully considered. Ammonia (NH3) and nitrous oxide (N2O) emissions may occur during digestate application. Effective mitigation strategies are however available, such as direct digestate injection into the soil, real-time nutrient monitoring, and precision agriculture methods. Reducing the digestate pH to ~6 inhibits NH3 volatilization and significantly limits emissions [18].
Novel digestate treatment methods [19,20] have been progressively developed, as depicted in Figure 5, together with the more common application methods. Digestate for the cultivation of insects was demonstrated at the pilot scale [21], whereas other valorization routes are described by Zheng et al. [22] and Ma et al. [23].
Clearly, the application of the AD digestate, as produced, or after transformation, significantly contributes to the circular economy model.

1.3. The Degradation of WAS to Enhance Its AD

The structure and composition of WAS determine its degradation properties to a large extent. Next to water, sludge essentially consists of micro-organisms (mainly bacteria), extracellular organic matter, and inorganic ions.
The cell wall of a microorganism (composed of glycan strands crosslinked by peptide chains, i.e., peptidoglycan) is a semi-rigid structure and protects the cell from osmotic lysis. The crosslinks make the cells difficult to biodegrade under natural circumstances and render them an unfavorable substrate for (anaerobic) microbial degradation.
Extracellular organics have been identified as extracellular polymeric substances (EPSs) and account for 50 to 80% of the sludge biomass; they exist in two different ways: EPSs are (i) formed by the bacterial cells of the activated sludge through cell metabolism and cell lysis or (ii) originate from various compounds present in the wastewater influent. It forms a 3D gel-like network, surrounding and protecting the micro-organisms embedded in the matrix, and consists of a very heterogeneous mixture of proteins, humic substances, carbohydrates, uronic acids, DNA, and lipids, as illustrated in Table 4. EPSs are responsible for the structural integrity, floc stability, flocculation properties, and dewaterability of the sludge.
Since EPSs comprise the major organic fraction in activated sludge, it is reasonable to assume that they have a significant impact on the digestibility of the sludge, and the total protein and carbohydrate fractions were identified by Appels et al. [24] as having a large impact of the biochemical methane potential (BMP) of activated sludge.
Since the cells and EPSs are not easily biodegradable, pre-treatment methods are frequently applied to enhance anaerobic digestion: they result in a cleavage of sludge flocs and a partial degradation of the biopolymers. This observation leads to the conclusion that, depending on the nature of the sludge, a specific method would be optimal for pre-treating the sludge.
When using a treatment with a low energy input, only floc-destruction will occur, with little influence on the digestion process. A noticeable increase in gas production is, however, obtained when effective cell disruption and degradation of the EPS is achieved. The specific working mechanisms and effects of each type of combined pre-treatment will be discussed in the following paragraphs of this review.

1.4. Objectives of This Review

In all of the different biogas valorization and circular economy schemes, the amount of produced biogas per tonne of DS sludge and the conversion of organic matter are important parameters, and an extra volume of biogas produced by AD will increase its application potential. A research review by Sharma et al. [25] illustrated this conversion potential. A further treatment was presented by Deng et al. [26].
This review will hence assess the different combined techniques that were previously investigated to decrease the TCOD and VS contents and increase the biogas yield.

2. Reviewing the Literature Data on Combined WAS Pre-Treatment

2.1. Thermochemical Acid/Alkaline Hydrolysis Pre-Treatment

Through the action on EPS, the addition of a base or acid together with an increasing temperature will increase the sludge cell solubilization and biogas production [27,28,29,30,31,32,33]. As an alkali addition, NaOH leads to greater VSS solubilization than Ca(OH)2, with >20% for NaOH and <7% for Ca(OH)2, respectively.
The pre-treatment is advantageous since operating at low to moderate temperatures, a 10 h treatment with NaOH addition to pH 11 solubilizes 42.3% of sludge cells at 90 °C [34]. Kim et al. [28] demonstrated sludge disintegration of 77.8% and excess methane production of nearly 73.9% when operating with 0.10 to 0.16 M NaOH at ~74 to 90 °C. These results were confirmed by Demir [35] when operating at 90 °C with 0.2 mol/L NaOH and 25 min retention time. The methane production increased by up to around 54% compared to raw sludge.
Guo et al. [36] studied the influence of alkali on the thermal treatment of activated sludge during high-solids anaerobic digestion at 135 °C and adding 23.77 mg NaOH/g total solid. The SCOD, soluble proteins, and VFA concentrations were considerably higher with alkali thermal combined pretreatment. The daily methane yield of anaerobic digestion was increased from 35.25% with single thermal treatment to 52.95% with combined treatment, where synergistic effects were clearly demonstrated. It was also demonstrated that the NaOH addition was more effective on the structure of the methanogen community than just increasing the temperature. When applying the pre-treatment at 60 °C and pH 12, Rani et al. [30] found that COD solubilization, SSs reduction, and biogas production increased by 23%, 22%, and 51% in comparison with the control, respectively.
Thermochemical hydrolysis also occurs when adding an acid such as HCl or H2SO4. Due to metal leaching and 100% pathogen removal, acidic thermal treatment at 100 °C for 60 min makes land applications of residual sludge results possible [37,38].
Obtained experimental results differ significantly. In conclusion, however, it can be expected that thermochemical pre-treatment will have a significant impact on sludge digestion. Both combined thermal and chemical effects lead to greater volatile solids redaction, up to ~50%; increased dewaterability due to the lower viscosity and EPS disruption; and increased methane production between 30% and 54%.

2.2. Alkaline and High-Pressure Homogenizer Pre-Treatment

The positive impact of alkaline pre-treatment is further enhanced by its combination with mechanical sludge disintegration.
Mechanical sludge disintegration methods transfer mechanical energy (provided as pressure, translational, or rotational energy) to the sludge flocs. Compared to other methods, literature sources are positive. Their effects are based on the disruption of microbial cell membranes by externally applied shear stress or pressure; when the tension resulting from mechanical stress becomes higher than the strength of the cell membrane, the cell disrupts. Stirred ball mills, high-pressure homogenizers (HPHs), and mechanical jet smash techniques have been reported for mechanical treatment applications. In some studies, ultrasonic disintegration was also treated as a mechanical pre-treatment method, but since it was shown previously that the disintegration effects are in fact a combination of mechanical shear, heat effects, and chemical effects, ultrasonic disintegration is discussed in a separate section in this review paper.
The application of mechanical disintegration on sewage sludge is driven by three main incentives: (i) making the organic material of the disintegrated sewage sludge available for digestion, (ii) reducing the bulking and foaming of wastewater, and (iii) enhancing the anaerobic digestion process.
A number of technologies use high-pressure gradients to rupture cell walls. Most methods disrupt the sludge by jetting it to collide with a collision plate or impact ring. Sludge is pumped under high pressure (typically between 300 and 700 bar) into a high-pressure homogenizer (HPH). An HPH is a rather simple device consisting of a multi-stage high-pressure pump and a homogenizer valve. The pump forces the sludge through the valve at a speed of 300 m/s and the static pressure in the valve reaches that of the vapor pressure of the sludge. The resulting cavitation bubbles induce the disintegrating forces.
Wet milling or stirred ball milling uses small beads (mostly made of steel) for the disruption of cell walls. The soluble COD in the liquid fraction of municipal sludge increased from 1–5% to 47% after 9 min of milling, but no increase was noticed for industrial sludge. The overall COD degradation and gas production from mesophilic anaerobic digestion was improved by a factor of 1.2–1.5. The energy requirement of this pre-treatment was estimated to be between 1 and 1.25 kW/m3 of sludge treated per day. Better results in terms of energy consumption were obtained for sludge with a higher DS content. In comparison with ultrasonic homogenizers, the high-pressure homogenizer and a stirred ball mill are favorable in terms of energy consumption. Relevant literature results are summarized in Table 5. It should be noted that high-pressure pre-treatment has virtually been abandoned since 2014.
The results show that the treatment and an increased sludge solubilization caused a simultaneous release of organic matter.
A combined chemical and mechanical treatment (MicroSludgeTM) using a secondary high-pressure homogenizer [40] is also very efficient. An increase or decrease in pH facilitates the disruption of the cell walls [41]. Further solubilization is achieved when a high-pressure homogenizer is subsequently applied [39], with up to 80% solubilization achieved. A significant VS reduction was observed from 18 to 78% in the mesophilic digester, reducing the hydraulic residence time (HRT) from 18 to 13 days [40].
Fang et al. [42] varied the homogenization pressure and cycle duration and investigated the effects on anaerobic sludge digestion. The pressure applied in the homogenizer was of major importance. Compared with a sole alkaline pre-treatment, the optimal pre-treatment was achieved at a single homogenization cycle of 60 MPa, combined with an alkaline dosage of 0.04 mol/L. The sludge TCOD, VS removal, and cumulative biogas production were increased by 24.68%, 18.95%, and 95.81%, respectively.
Sun et al. [43] introduced the hydrocyclone into sludge pretreatment. It was found that the hydrocyclone could effectively disrupt the flocs of sludge, and with a specific energy input of 0.18 W/g TS, SCOD release of 2142 mg/L was obtained. When the process was combined with alkali treatment, a maximal SCOD release of 4409 mg/L was observed. Combined pretreated sludge also exhibited VFA accumulation of 1017 mg/L and methane production of 134.29 mL CH4/g TS.
Early large-scale applications were reported by [40] and the increase in sludge degradation exceeded 50%.
Again, mostly laboratory-obtained results are not quantitatively convincing. The sludge TCOD, VS removal, and biogas production are significantly increased. The hybrid treatment could moreover reduce the hydraulic residence time of the digester by several days, thus increasing the WAS feeding rate in a given AD.

2.3. Alkaline and Ultrasound Pretreatment

Energy-intensive ultrasound is, however, mostly regarded as the most powerful technology. The best combined energy input and disintegration effectiveness can, however, be reached when combining alkaline and ultrasound pre-treatment [44]. The fast cell disruption by the ultrasound treatment enhances the NaOH treatment.
A combined treatment of US at 120 W and 0.04 mol/L of NaOH was investigated by [45]. This resulted in solubilization of the COD of 89% and an increased hydrolysis rate during subsequent digestion. A combined alkaline/ultrasound treatment was investigated by [46]. A NaOH dosage of 100 g/kg DS and an SE of 7500 kJ/kg DS were used and resulted in an improvement in degradation efficiency (from 38 to 50.7%).
Relevant literature data are combined in Table 6.
Experiments with alkali integrated with ultrasound require just 30 to 60 min to complete [51,52,53,54,55]. VSS solubilization of 60% was achieved after 1 h at 28 kHz and pH 12 [56]. VSS solubilization of 70% was also demonstrated [57].
Bao et al. [58] compared the results of alkali, ultrasound combined alkali (UA), and high-temperature micro-aeration (TM) pre-treatments on the enhancement of the productivity of methane from WAS. The results indicated that the combined treatment has the best effect and the concentrations of VFAs and SCOD exceeded the results of the alkali pre-treatment by a factor between 2.6 and 2.8, whereas TM pretreatment was lower by a factor of 2.1. Cho et al. [48] observed a 70% sludge disintegration when combining a 4 g KOH/L dosage and ultrasound treatment at a specific energy of 12 kJ/g TS.
Lu et al. [59] found that the combined alkali/ultrasound pre-treatment increased the sludge solubilization and biogas generation rate but also released humic substances (HSs) and complex proteins. The residuals’ dissolved organic matter, however, had adverse effects on the digestate treatment. Further polishing steps are necessary for removing the remaining soluble recalcitrant compounds.
Although the single ultrasound pre-treatment has been industrially applied, the combination with alkali seems to offer the additional merits of short treatment times, greater VSS solubilization, and reduced specific energy (SE) requirement. The cost of adding alkali is certainly outweighed by the extra merits and the reduced VS operation costs.

2.4. Alkaline and Microwave (MW) Pre-Treatment

This combined pre-treatment has similar benefits as the above techniques and the solubilization is 20% higher than when applying each process alone [60]. Solubilization of 45.5% for COD was observed with NaOH at pH 12 and 600 W for 2 min, and the VSS reduction was 20% higher than that of WAS without pretreatment [61]. A semi-batch thermophilic reactor fed with pre-treated, thickened, waste-activated sludge showed 28% and 18% reductions in VSS and TCOD, respectively, and a 17% increased methane yield at 170 °C for 1 min with 0.05 g NaOH/g SSs addition [62]. The combined MW/alkali pretreatment of sludge killed pathogens effectively after pre-treatment at 70 °C for 5 min, enabling safe residual sludge land application [63]. The complete destruction of E. coli was demonstrated [64].
Table 7 illustrates some literature data.
It was recommended to apply the alkali pre-treatment before the microwave to increase the combined efficiency [60], preferably using NaOH. It was also found that the combined pretreatment achieved a disintegration degree of 65.87% at energy inputs of 38,400 kJ/kg TS and pH 11.0. The pH showed a more significant effect on the disintegration degree than the energy input. Wang et al. [66] found that CaO2 also exhibited an excellent combinative effect with the MW on the treatment of sludge. Under the conditions of 0.1 g/gVSS CaO2 and microwave of 480 W with 2 min of irradiation, the methane accumulation yield of anaerobic digestion was 80.2% higher than that of the control. Enzyme and Illumina MiSeq sequencing analyses indicated that microwave irradiation increased hydroxyl radical generation from CaO2 and alleviated the inhibitory effect of CaO2 on methanogens.
A novel alkali rhamnolipid combined microwave disintegration (ARMD) was investigated by Banu et al. [67] and demonstrated that, as a bio-surfactant, rhamnolipid under alkali conditions significantly enhanced the liquefaction of sludge. With an alkali pH of 10, the energy consumption (1620 kJ/kg TS) was remarkably reduced from 6480 kJ/kg TS for the MW treatment process. Methane production of 379 mL/g COD was obtained for ARMD, which was 58% higher than that of the MD treatment (239 mL/g COD).
Despite the reported lower operation costs related to the microwave application in comparison with ultrasound, the overall increase in solubilization and biogas production is lower than for previous pre-treatment techniques. Additional research is recommended to improve the efficiency of the method.

2.5. Microwave and Oxidation Pre-Treatment

Microwave and oxidation pre-treatment is clean, efficient, and “green”. The disintegration of sludge SS was achieved within 5 min of MW irradiation at 80 °C [68,69,70]. Minor additions of peroxidants are required (e.g., 1 g H2O2/g TS). Moderate MW temperatures (80 °C) transformed H2O2 into reactive OH• radicals, and this increased both the oxidation of COD and the solubilization of particulate COD. Liu et al. [56] investigated the effects of microwave/H2O2 pretreatment on sludge anaerobic digestion. In their experiment, pretreatment resulted in a change in organic matter solubilization and decreased sludge viscosity. The AD methane production after pretreatment was increased by about 20%. Yu et al. [71] observed a SCOD increase from 52.8 mg/L (control) to 812 mg/L after combined pre-treatment at 70 °C with 0.04% to 0.08% of H2O2 dosage. The concentration of fecal coliforms was below the detection limit (1000 CFU/L). Ambrose et al. [72] demonstrated that oxidative pre-treatment resulted in the accumulation of superoxide radicals in the thermophilic phase and thus increased sludge activity and the biodegradability of WAS. After anaerobic digestion, the sludge achieved more than 90% fecal coliform reduction, which caused the sludge to achieve a “class-A” biosolid level.
Some important results are illustrated in Table 8.
In conclusion, this hybrid method has the merits of increased solubilization and peroxide-driven disinfection. Although the microwave enhanced the peroxidation effect of H2O2, it did not lead to increased methane production. The extra solubles, which were released after H2O2 and subsequent microwave irradiation, were probably slower to biodegrade or more refractory than those generated during MW treatment alone. Liu et al. [74] studied the effects of residual H2O2 on the methanogenesis of AD after the H2O2/microwave combined pretreatment of WAS. The result showed that refractory compounds were generated with residual H2O2. The residual H2O2 was harmful to the methanogens and inhibited the following biodegradation stage through a long lag phase and low methane production.

2.6. Ultrasound and Peroxidation Pre-Treatment

Combining peroxidation and ultrasound cavitation treatment offers synergistic advantages similar to the microwave/peroxidation effects, with the disruption of activated sludge flocs and enhanced effectiveness of the peroxide activity as major results. Ozone decomposition occurs in the cavitation bubbles, forming free oxidation radicals [51,52], with a significant increase in SCOD concentration after 60 min of 0.6 g/h O3 treatment and a subsequent application of ultrasound during 60 min with an energy input of 0.26 W/mL.
The oxidation effect was also provided by Fenton oxidation [75,76], and sulfate radicals formed from ultrasound treatment of the persulfate/sulfate. These radicals participated in the advanced sludge oxidation and were highly effective for EPS disintegration. After 96 h bio-digestion, a hydrogen yield of around 50 mmol/L was observed with persulfate/ultrasound combined sludge treatment, which was two times higher than that of ultrasound or persulfate treatment alone. Additional data are given in Table 9.
From the analysis, and considering the few publications with efficiency results available, it appears too early to include this hybrid pre-treatment in the priority techniques.

2.7. Pulsed Electric Field (PEF) Pre-Treatment

The pulsed power technique is another option suggested to enhance solubilization and biogas production. Compared to other reported pretreatment methods, PEF is an effective technique with lower energy consumption, which could enhance methane production and COD removal during the anaerobic digestion process of sludge [78,79]. WAS was introduced into the reactor at 600–800 mL/min, with an HRT of 1.5 s, prior to anaerobic digestion. It was shown that the soluble COD concentration and EPS content were increased by 4.5 and 6.5 times, respectively. The biogas production was improved too, by 150%.
Results from Ki et al. [80] indicated that the impact of PEF pretreatment was weaker for primary sludge (PS) compared to the effect of pretreatment on WAS. The PEF treatment caused the microbial inactivation of PS and, at the same time, due to the disruption of the microstructure of microbial cells, soluble components were increased in PEF-treated PS by 78–86%, but the PEF only slightly enhanced the conversion of methane by around 8%. Thus, the researchers considered that the PEF method was more suitable for the pretreatment of WAS or WAS + PS. In another work by the same author, it was found that PEF pretreatment increased the content of acetate in total VFAs by 35% compared to untreated PS. Since acetate could be utilized as the electron donor for microbial electrolysis cells (MECs), the PEF pretreatment was effective in achieving a higher maximum current density in the batch MEC experiments with PS as a substrate [81].
Westerholm et al. [82] investigated the anaerobic degradation of PEF-pretreated WAS in continuous mode at the pilot scale with a 50 L digester. The whole degradation process was continued for 132 days and divided into two phases. WAS with PEF an energy input of 0.066 kJ/kg sludge was used in the first phase (days 0–72) and WAS pretreated with an energy input of 0.091 kJ/kg sludge was used in the second phase (days 73–132). The results indicated that PEF significantly improved SCOD (180–450%) as well as carbohydrate solubility (110–150%). The biogas production was increased by 10–11% compared with untreated sludge.
Many studies have demonstrated that PEF is an effective pretreatment method for sludge anaerobic digestion [10,83,84]. It was found that the effect of PEF could be further enhanced with a DC corona assistant [85]. In an experiment, WAS was treated with a DC corona process, with DC voltage in the range of 1–5 kV before PEF treatment. The results indicated that relative variation in SCOD (∆S%) was improved by this combined work and, under the condition of a positive DC voltage of 4 kV, the ∆S% was promoted twice compared to the individual treatment of PEF. The influence of DC corona on the PEF performance depended upon the magnitude and polarity of DC voltage.
Through an investigation of the effects of frequency and temperature [86] on the efficiency of the treatment, the results indicated that the efficiency increased along with the magnitude as well as the temperature, but tended to decrease with the frequency. This phenomenon may have been because the higher frequency caused the time between two consecutive pulses to be too short for the organic matter to diffuse into the sludge. On the contrary, it accumulated around the cell surface, thus resulting in low efficiency.
Recent results by Ning et al. [87] confirm the merits of local electric field applications regarding overall AD efficiency and in situ CO2 bioconversion.
Despite research mostly since 2015, the PEF application is still at the lab-scale technology readiness level. The results of biogas production increase from 8 to 11% are very low in comparison with the competing combined treatments.

3. Discussion

Although the present review demonstrates that the combined pre-treatment of WAS prior to anaerobic digestion offers significant benefits in both VSS and TCOD reduction, the enhancement of the biogas production varies considerably among the research results and the applied methods.
In general, however, it can be expected that thermochemical pre-treatment will have a significant impact on sludge digestion. Combining both thermal and chemical effects leads to greater volatile solids reduction, up to ~50%, higher biogas production, up to 30%; increased dewaterability due to the lower viscosity and EPS disruption; and increased methane production, between 30% and 54%.
The alkaline and high-pressure homogenizer pre-treatment methods have been examined at lab and pilot scales. The obtained results are not quantitatively conclusive but are qualitatively convincing. Sludge TCOD, VS removal, and biogas production were significantly increased. The hybrid treatment could moreover reduce the hydraulic residence time of the digester by several days, thus increasing the WAS feeding rate in a given AD.
Although the single ultrasound pre-treatment has been industrially applied, the combination with alkali seems to offer the additional merits of short treatment times, greater VSS solubilization, and reduced specific energy (SE) requirements. The cost of adding alkali is certainly outweighed by the extra merits and reduced VS costs.
Despite the reported lower operation costs related to the microwave application in comparison with ultrasound, the overall increase in solubilization and biogas production is remarkably lower than the reported results of the other pre-treatment techniques. Additional research is recommended to improve the efficiency of the method.
Although microwaves enhanced the peroxidation effect of H2O2, they did not lead to increased methane production. The extra solubles, which were released after H2O2 and subsequent microwave irradiation, were probably slower to biodegrade or more refractory than those generated during MW treatment alone. The results indeed showed that refractory compounds were generated with residual H2O2 that was harmful to the methanogens and inhibited the following biodegradation stage through a long lag phase and low methane production.
The ultrasound and peroxidation method has scarcely been investigated. It appears to be too early and too costly to include this hybrid pre-treatment as a priority technique.
Finally, and despite research mostly since 2015, the PEF application is still at the lab scale. The results of biogas production increase from 8 to 11% are very low in comparison with the results of competing combined treatments.
Despite the improved performance merit of applying pre-treatment of WAS prior to AD, some problems and demerits need to be considered.
Thermal and hybrid thermal pre-treatments can suffer from reduced methanogenesis from compounds formed during the Maillard’s reaction. The compounds can be formed above 100 °C by the reaction of amino acids and carbohydrates and inhibit microbial activity.
Some pre-treatment technologies incur high operation and maintenance (O&M) costs such as ultrasonication, high-temperature thermal methods, microwaves, high-pressure homogenization, and peroxidation. The evaluation of capital and operational costs is difficult because of the low TRL of these pre-treatment techniques. The energy demand of thermal pre-treatments makes their application costly, although the enhanced biogas yield and composition can be effectively exploited in on-site electricity generation or by producing value-added chemicals.
It should finally be noted that some pre-treatments can lead to secondary effects such as the generation of recalcitrant compounds and odor, both problems mainly associated with thermal and ozone or peroxide pre-treatments.
Prior to deciding on implementing one of the combined pre-treatment techniques, these potential demerits need to be accounted for, and both the capital and operation costs and the economic efficiency must be critically examined.
In general, most of the investigated methods are still at technology readiness levels 1 to 2. Only the thermal alkaline, mechanical WAS disintegration, and alkali ultrasound methods surpass this TRL. Only pilot-plant or large-scale investigations will be able to prove their maturity.

4. Conclusions

The present review demonstrated that the combined pre-treatment of WAS prior to AD offers significant benefits for both VSS and TCOD reduction, whereas the enhancement of the biogas production varies considerably among the research results and the applied methods.
Most of the combined enhancement techniques were assessed on a laboratory scale. Only a few techniques were applied on a pilot scale. All applied techniques increased VS removal and biogas production, but the obtained results are not quantitatively convincing, with findings of different levels of % increase.
Within the investigated techniques, high prospects were presented for the thermo-chemical and ultrasound pre-treatments. All other methods were not fully investigated, and results that differed significantly were reported. The overall increase in solubilization and biogas production was remarkably lower than in other pre-treatment techniques. Although microwaves enhanced the peroxidation effect of H2O2, they did not lead to increased methane production, while refractory compounds generated by H2O2 were harmful to the methanogens and inhibited the following biodegradation stage through a long lag phase and low methane production. The ultrasound and peroxidation method was scarcely investigated. PEF application is still at the lab scale, with low biogas production increases reported.
Except for the thermal alkaline, mechanical WAS disintegration, and alkali ultrasound methods, the additional investigated methods still lack maturity. Only pilot-plant or large-scale experiments will be able to increase their technology readiness levels.

Author Contributions

Conceptualization, I.S. and J.B.; Methodology, J.B. and Y.D.; Formal analysis, M.Y. and M.V.E.; Investigation, M.Y.; Resources, I.S.; Writing—original draft, M.Y. and J.B.; Writing—review & editing, H.Z. and J.B.; Visualization, M.V.E., S.L. and Y.D.; Supervision, J.B.; Project administration, H.Z. All authors have read and agreed to the published version of the manuscript.

Funding

The authors acknowledge the financial support of the Beijing Advanced Innovation Centre for Soft Matter Science and Engineering, Beijing University of Chemical Technology.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available on request from the corresponding author.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. Biogas plant in Dafeng City, Jiangsu Province. Adapted from [4].
Figure 1. Biogas plant in Dafeng City, Jiangsu Province. Adapted from [4].
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Figure 2. Tentative products from WAS-AD biogas: heat, power, and E-chemicals (dimethyl ether, formic acid, ethanol, methanol, ammonia, and, ultimately, Fischer–Tropsch synthesis of CxHy-fuels).
Figure 2. Tentative products from WAS-AD biogas: heat, power, and E-chemicals (dimethyl ether, formic acid, ethanol, methanol, ammonia, and, ultimately, Fischer–Tropsch synthesis of CxHy-fuels).
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Figure 3. The circulate economy model with AD of food waste [11] (reproduced courtesy of Elsevier).
Figure 3. The circulate economy model with AD of food waste [11] (reproduced courtesy of Elsevier).
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Figure 4. The liquid and solid fractions of AD digestate.
Figure 4. The liquid and solid fractions of AD digestate.
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Figure 5. Selected digestate valorization.
Figure 5. Selected digestate valorization.
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Table 1. Biogas composition of organic waste streams. Adapted from [3].
Table 1. Biogas composition of organic waste streams. Adapted from [3].
Components (vol%)Domestic WasteSewage SludgeManure
CH4 55 ± 568 ± 868 ± 8
CO2 36 ± 226 ± 726 ± 7
N2 2.5 ± 20.50.5
O2 0.5<0.25<0.25
H2S mg/m3100–9002500 ± 15006500 ± 3000
Table 2. Biogas production in several countries. Data adapted from [4,5].
Table 2. Biogas production in several countries. Data adapted from [4,5].
CountryYearTotal Biogas ProductionBiogas Production in WWPTs Only
Number of Plants[GWh/y]Number of Plants[GWh/y]
Australia2021247158752381
Austria 201729134893818
Belgium 2015284955n.a.n.a.
Brazil 2021638117,000n.a.n.a.
Finland 202010987716221
France2017687352788442
Germany202010,55154,10012714000
Ireland202059752.415n.a.
The Netherlands2015268301180541
Norway201916278227305
Sweden20202822161134721
Switzerland20194341519271638
UK 201698726,457162950
USA2017210010301240n.a.
Table 3. Expected total digestate production volumes [Mt dry matter (DM)] and nutrient content (Mt) in 2022, 2030, and 2050 [15].
Table 3. Expected total digestate production volumes [Mt dry matter (DM)] and nutrient content (Mt) in 2022, 2030, and 2050 [15].
Parameter202220302050
Total digestate production (Mt DM/year)3175177
Nutrient content (Mt)
Nitrogen (TKN): total Kjeldahl nitrogen1.74.19.7
Phosphorous (P)0.30.71.7
Potassium (K)0.20.40.8
Table 4. Organic EPS composition in waste-activated sludge and extracted EPS.
Table 4. Organic EPS composition in waste-activated sludge and extracted EPS.
Organic Compound Abundance in Activated Sludge (%)Abundance in Extracted EPS (%)
Proteins>4345–55
Humic substances 15–4230–33
Carbohydrates 10–18±10
Uronic acids1–2±1
DNA1–63–14
Table 5. Alkaline and HPH pre-treatment research.
Table 5. Alkaline and HPH pre-treatment research.
ReferenceCombined PretreatmentMain Result
[39]900 mg/L NaOH and an 83 MPa homogenizerSolubilization increased by 37%
[40]pH 10, 83 MPa, and 1 h reaction time80% solubilization of SSs, VS reduction improved from 18 to 78%, and HRT reduced from 18 to 13 days
[40]Alkaline and high-pressure homogenizer pretreatmentSludge degradation from 50% to 57%
Table 6. Alkaline and ultrasound pre-treatment research.
Table 6. Alkaline and ultrasound pre-treatment research.
ReferenceCombined PretreatmentMain Result
[45]US (120 W) and NaOH (0.04 mol/L, 24 h)Solubilization of the COD of 89% and an increased hydrolysis rate
[46]NaOH dosage of 100 g/kg DS and an SE of 7500 kJ/kg DSDegradation efficiency improved from 38 to 50.7%
[47]US of 28 kHz and pH 12, 1 h60% of VSSs were solubilized
[48]4 g/L of KOH and US at 12 kJ/g TS70% sludge disintegration
[49]Ultrasound of 220 MHz for 9 min + 0.05 M NaOH
Stepwise NaOH addition/ultrasound combined pretreatment with 0.02 M + ULS for 5 min and then 0.02 M + ULS for 4 min		31% increase in methane production for NaOH ultrasound combined treatment
Stepwise NaOH addition/ultrasound pre-treatment resulted in a 40% increase in methane production and the chemical dosage for the process could be reduced by 20%
[50]0.04 g CaO/g ultrasound at a low frequency of 20 kHz and average power of 150 W for 30 min SCOD and VFAs increased by 270.30% and 159.52% in comparison with the untreated control, respectively; the cumulative methane production was between ~163 and 167 mL/g·VS
Table 7. Alkaline and microwave pre-treatment.
Table 7. Alkaline and microwave pre-treatment.
ReferenceCombined PretreatmentMain Result
[61]MW/NaOH pre-treatment (pH 12, 600 W, and 2 min)93% SCOD reduction; VSS reduction was improved by 20%
[62]MW at 170 °C, 1 min, and 0.05 g NaOH/g SS28% and 18% reductions in VSS and TCOD and 17% increase in methane yield
[63]MW output 400 W, 102 °C, and 2.3% TSSludge solubilization degree of 17.9%
[60]MW at 38,400 kJ/kg TS and pH 11.0Sludge disintegration degree of 65.87% and VSS removal in AD improved by up to 40%
[65]MW for 1250 W, 2450 MHz, temperature range 25–260 °C, with pH of 10.0MW pretreatment at 175 °C increased solubilization by 68.2%; the maximum CH4 yield was 37% higher than that of the control
Table 8. Microwave and oxidation pre-treatment.
Table 8. Microwave and oxidation pre-treatment.
ReferenceCombined PretreatmentMain Result
[73]5 min of MW irradiation at 80 °CSolubilization of SCOD (1954 mg/L) and most of the SSs were disintegrated
[71]MW (70 °C) with 0.04% of H2O2 dosageIncreased SCOD from 52.8 (control) to 812 mg/L; negligible residual fecal coliform concentrations (<1000 CFU/L)
Table 9. Ultrasound and peroxidation pre-treatment.
Table 9. Ultrasound and peroxidation pre-treatment.
ReferenceCombined PretreatmentMain Result
[34]60 min O3 at 0.6 g/h and US at 0.26 W/mL)Increased SCOD from 83 to 3040 mg/L
[77]pH of 3.0, H2O2/Fe2+ weight ratio of 10:1, ultrasonic power of 100 W, and treatment time of 10 minPetroleum hydrocarbon removal rate of up to 84.25%
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Yang, M.; Vander Elst, M.; Smets, I.; Zhang, H.; Li, S.; Baeyens, J.; Deng, Y. Reviewing Improved Anaerobic Digestion by Combined Pre-Treatment of Waste-Activated Sludge (WAS). Sustainability 2024, 16, 6419. https://doi.org/10.3390/su16156419

AMA Style

Yang M, Vander Elst M, Smets I, Zhang H, Li S, Baeyens J, Deng Y. Reviewing Improved Anaerobic Digestion by Combined Pre-Treatment of Waste-Activated Sludge (WAS). Sustainability. 2024; 16(15):6419. https://doi.org/10.3390/su16156419

Chicago/Turabian Style

Yang, Miao, Margot Vander Elst, Ilse Smets, Huili Zhang, Shuo Li, Jan Baeyens, and Yimin Deng. 2024. "Reviewing Improved Anaerobic Digestion by Combined Pre-Treatment of Waste-Activated Sludge (WAS)" Sustainability 16, no. 15: 6419. https://doi.org/10.3390/su16156419

APA Style

Yang, M., Vander Elst, M., Smets, I., Zhang, H., Li, S., Baeyens, J., & Deng, Y. (2024). Reviewing Improved Anaerobic Digestion by Combined Pre-Treatment of Waste-Activated Sludge (WAS). Sustainability, 16(15), 6419. https://doi.org/10.3390/su16156419

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