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Recent studies have shown that anaerobic co-digestion (AnCoD) is superior to conventional anaerobic digestion (AD). The benefits of enhanced bioenergy production and solids reduction using co-substrates have attracted researchers to study the co-digestion technology and to better understand the effect of multi substrates on digester performance. This review will discuss the results of such studies with the main focus on: (1) generally the advantages of co-digestion over mono-digestion in terms of system stability, bioenergy, and solids reduction; (2) microbial consortia diversity and their synergistic impact on biogas improvement; (3) the effect of digester mode, i.e., multi-stage versus single stage digestion on AnCoD. It is essential to note that the studies reported improvement in the synergy and diverse microbial consortia when using co-digestion technologies, in addition to higher biomethane yield when using two-stage mode. A good example would be the co-digestion of biodiesel waste and glycerin with municipal waste sludge in a two-stage reactor resulting in 100% increase of biogas and 120% increase in the methane content of the produced biogas with microbial population dominated by Methanosaeta and Methanomicrobium.

This study aimed to investigate the effect of combined Free Nitrous Acid (FNA)-Heat (i.e., FNH) pretreatment on single- and two-stage anaerobic digestion (AD) of thickened waste-activated sludge (TWAS). Single-stage AD was conducted in batches, while two-stage AD involved acidogenic fermentation under semi-continuous flow followed by batch methanogenesis. FNH pretreatment was applied before the acidogenic stage, using 1.4 mg HNO2-N/L FNA concentration at 25 °C, 37 °C, and 60 °C for 24 h. Among the scenarios, the most promising results were observed with two-stage AD fed with FNH-pretreated TWAS at 60 °C, showing higher COD solubilization and a reduction in volatile solids. Combined FNA-Heat pretreatment in two-stage AD yielded elevated methane production (363–415 mL CH4/g VS added) compared to single-stage digestion. Methane yields from FNA-Heat pretreated single-stage ranged from 332 to 347 mL CH4/g VS added, contrasting with 212 mL CH4/g VS added for untreated TWAS. Methane generation commenced early in both untreated and pretreated samples, attributed to soluble substrate abundance.

Anaerobic digestion and fermentation processes in wastewater sludge treatment are limited by several factors, including the slow breakdown of complex organic matter and solubilization of solids. In this study, thermochemical pretreatment of thickened waste activated sludge using high temperature (>170 °C) was investigated to understand the impact of the pretreatment on the volatile fatty acids (VFA) production and its fractions during the fermentation process. Furthermore, the influence the thermochemical pretreatment on sludge disintegration and methane recovery was investigated. A range of acidic and alkaline conditions over the pH range of 4.5–10 was examined. Sludge (pH adjusted) was exposed to hydrothermal pretreatment (HTP) at a temperature of 170 °C for 30 min. Pretreated samples were then subjected to batch fermentation and methane potential tests which revealed that acidic and alkaline conditions resulted in increased sludge solubilization during HTP. Acidic conditions were associated with a higher VFA production yield of up to 185 mg chemical oxygen demand/g total chemical oxygen demand. Alkaline conditions led to a higher methane production yield where the maximum yield (276 mL CH4/g total chemical oxygen demandadded) was found to occur at pH 10. Therefore, alkaline sludge used for fermentation has shown technical and economic feasibility for sludge carbon recovery.

This study evaluated the impact of hydrothermal pretreatment on thickened waste activated sludge (TWAS) for solubilization enhancement and biomethane production improvement through the mesophilic anaerobic digestion process. In order to assess the effect of temperature, retention time and severity index (SI) of the hydrothermal pretreatment, TWAS was exposed to fifteen different pretreatment conditions within a combination of 10 different pretreatment temperature range (150–240 °C), five different retention times (5–30 min) and five different severity indexes (SI = 3, 3.5, 4, 4.5 and 5). The solubilization enhancement was observed in all hydrothermally pretreated samples with the highest solubilization efficiency of 49% in pretreatment conditions of 200 °C and 10 min retention time within the corresponding SI = 4. Biomethane production was not improved in all fifteen pretreatment conditions, pretreatment with SI beyond 4 decreased the biodegradability of TWAS. The highest biomethane production was observed in the pretreatment condition of 170 °C and 10 min with a 40% increase compared to non-pretreated TWAS.

Municipal solid waste treatment and disposal have become one of the major concerns in waste management due to the excessive production of waste and higher levels of pollution. To address these challenges and protect the environment in sustainable ways, the hydrothermal pretreatment (HTP) technique coupled with anaerobic digestion (AD) becomes a preferred alternative technology that can be used for municipal solid waste stabilization and the production of renewable energy. However, the impact of HTP parameters such as temperature, retention time, pH, and solid content on the fermentation of TWAS is yet to be well studied and analyzed. Hence this study was conducted to review the effect of hydrothermal pretreatment of thickened waste-activated sludge (TWAS) on fermentation and anaerobic digestion processes. Many studies reported that fermentation of TWAS at pretreatment temperature ranges from 160 °C to 180 °C resulted in a 50% increase in volatile fatty acid (VFA) yields compared to no pretreatment. However, for the AD process, HTP in the range of 175 °C to 200 °C with a 30–60 min retention time was considered the optimal condition for higher biogas production, with 30% increase in biodegradability and greater than 55% increase in biogas production. Even though there is a direct relationship between increased HTP temperature and the hydrolysis of TWAS, a pretreatment temperature range beyond 200 °C alters the biogas production. The solid content (SC) of sludge plays a crucial role in HTP, where in practice up to 16% SC has been utilized for HTP. Further, a combined alkaline-HTP enhances the process performance.

Cellulose from toilet paper is a significant fraction of particulate organics, which is recoverable. For the first time, comprehensive mapping and tracking the fate of cellulose across various unit processes at full-scale in two water resource recovery facilities located in North America and Europe was undertaken. The influent cellulose content accounted for approximately one-third of the total suspended solids (TSS). Although about 80% of the raw wastewater cellulose was removed in primary treatment, the type of primary treatment process (rotating belt filter [RBF] vs. primary clarification [PC]) had a significant impact on cellulose capture and diversion. The high cellulose content of the RBF sludge accounting for 35% of the TSS facilitates cellulose recovery. For the North American plant, with a conventional activated sludge process (SRT of 6–7 days, preceded by PC), cellulose biodegradation efficiencies of 70%–90% of the PC effluent were observed in summer and winter. For the European plant, with a modified University of Cape Town process (SRT of 14 days, without primary treatment in train 2, or preceded by RBF in train 1), comparable cellulose biodegradation efficiencies were also observed. Results from laboratory SBRs indicated that cellulose biodegradation efficiency at room temperature was 86% of the influent cellulose.

Pre-treatment of feedstock prior to anaerobic digestion has been explored and adopted in full-scale anaerobic digestion facilities [8], while further research is required for process optimization. [...]in recent years, amending conductive materials in digesters [7], integrating anaerobic digestion with microbial electrochemical systems has also been demonstrated to be a potential approach to overcome process kinetics and stability issues faced in conventional digesters [9]. [...]anaerobic fermentation of sludge has been adopted by many wastewater treatment plants to produce organic acids to supplement exogenous electron donors to the biological nutrient removal process [10]. Notably, the higher moisture content, lower energy density, non-uniformity of waste biomass can significantly influence process performance [12]. [...]efficient and economically viable thermochemical conversion processes for various waste streams are yet to be developed. Based on their results, the temperature used in thermal hydrolysis was the most important factor influencing the yield of volatile organic acids from anaerobic fermentation. [...]the authors commented that pretreatment conditions could be adjusted to manipulate product spectrums for target applications, including biological nutrient removal, biopolymer synthesis, and so on.

Challenges associated with mesophilic digestate (MD) involve volume, odor, and pathogens, which effective post-digestion treatments can address. The efficiency of MD post-treatment can be enhanced by conditioning with ferric chloride (FeCl3), hydrogen peroxide (H2O2), and polymer. This study aimed to observe the effect of combined chemical conditioning on volume reduction, phosphorus (P) release, odor, and pathogen reduction potential for MD. MD was conditioned with polymer only, polymer and FeCl3 at pH adjusted to 8.0 with lime (Ca(OH)2), and a blend of polymer, FeCl3, and hydrogen peroxide (H2O2) at pH 8.0. The results show that adding all three chemicals improved post-treatment efficiency at 2.1 kg/t DS FeCl3, 2.1 kg/t DS polymer, and 600 mg/L H2O2 at pH 8.0, compared with polymer or dual conditioning. At the combined dose, cake solid content, centrate P removal, and odor reduction capability improved compared with raw MD by 20%, 99%, and 66%, respectively. Combined chemical treatment reduced fecal coliform by 98% but does not fulfil class A requirements and showed 50% regrowth potential. The synergic effect of polymer, FeCl3, H2O2, and alkaline pH breakdown EPS, reduced water holding capacity and formed compacted flocs for better water removal and settling. This combination also precipitated P through FeCl3 while H2O2 oxidation curbs odor, enhancing further P removal from centrate.

Anaerobic co-digestion (AnCoD) presents several advantages over conventional mono-digestion. Various factors can impact the efficiency of the co-digestion process, including the mixing ratio of the feedstocks. This study primarily investigates the effects of different mixing ratios on methane production during the co-digestion of source-separated municipal organic waste (SSO) with thickened waste activated sludge (TWAS). While the C/N or COD/N ratio has generally been used for optimizing the mixing ratios of co-digested feedstocks, a new approach is introduced in this study to evaluate the effects of the lipid, protein, and carbohydrate (L:P:C) ratios on the efficiency of AnCoD with respect to methane production, kinetics, and synergism at mixing ratios of TWAS:SSO of 10:90, 30:70, 50:50, 70:30, and 10:90. AnCoD improved methane production and kinetics relative to TWAS at all mixing ratios, the highest of which was at the 10:90 ratio, corresponding to a methane yield, maximum methane production rate, and an L:P:C ratio of 353 mL CH4/g COD, 25 mL CH4/g COD/d, and 8:1:18, respectively. Improvements in methane yields and kinetics due to synergy were evident at all mixing ratios, with improvements in methane yields ranging from 11 to 23% and improvements in kinetics ranging from 18 to 58%. Improvements in methane yields and kinetics were insensitive to the feedstock composition beyond the 50:50 mixing ratio.

Investigating the impact of hydrothermal pretreatment (HTP) on organic solubilization and anaerobic acidification of thickened waste activated sludge (TWAS) was the objective of this study. For this purpose, 15 different hydrothermal pretreatment scenarios were investigated. The HTP temperature, retention time, and pressure for this study extended from 150 to 240 °C, 5 to 30 min, and 69 to 488 psi respectively. Mesophilic batch acidification tests were conducted for all pretreated and raw samples. The highest solubilization percentage after HTP and acidification was observed at HTP condition of “200 °C–10 min–225 psi” and “180 °C–15 min–145 psi” accounting for 48% and 40% respectively. Moreover, The HTP condition of “190 °C–10 min–181 psi” indicated the highest volatile fatty acid (VFA) production yield of 5.38 g/L or 1076.6 mgVFAs/gVSSadded. For all hydrothermally pretreated samples, acetic acid was determined to be the most abundant VFA. There was no significant hydrogen produced during the acidification process.