Mesophilic and thermophilic anaerobic digestion of municipal sludge and fat, oil, and grease

Effective grease separator management is the key to enhancing bioenergy recovery of fat, oil, and grease (FOG) and contributing to a circular bio-economy

Management of fat, oil and grease (FOG) is crucial for the recovery of renewable resources and the protection of sewer systems. This study aims to identify the potential quantities and qualities of FOG that can be acquired through optimised grease separator (GS) management approaches in hotels and restaurants during seasonal tourism. A technical survey of 20 GS from hotels and restaurants in the federal state of Tyrol, Austria was conducted. The findings revealed that 55 % of the GS were in poor condition, often due to infrequent maintenance and limited operator's knowledge. The FOG layer quality and quantity was monitored over three years and physicochemical parameters including total residue, volatile solids, total organic carbon, lipid content, and biomethane yield, were analysed. An optimised management approach, which involved up to 4 GS emptying per season, revealed a significant increase in FOG quantity for the majority of the inspected establishments, with an overall doubling of the acquired FOG volume. Based on these results, the energy potential of GS is presented in three potential management scenarios. The energy recovered from GS increased by 246 %. This highlights the importance of proper GS management in the hospitality sector, which can play a critical role in promoting environmental sustainability and renewable energy production.

Environmental impact of sewage sludge co-digestion with food waste and fat-oil-grease: Integrating plant-wide modeling with life cycle assessment approach

Anaerobic co-digestion of fat-oil-grease (FOG) and food waste (FW) with sewage sludge (SS) in wastewater treatment plants is a method used to increase biogas production. In this study, digestion scenarios were compared using plant-wide modeling and life cycle assessment: Scenario-0 (mono-digestion of waste-activated sludge (WAS)), Scenario-1 (co-digestion of WAS with FOG), and Scenario-2 (co-digestion of WAS with FW). Scenario-0, with the highest energy use and landfilling of FOG/FW, has the worst environmental impact. Scenario-1 and Scenario-2 minimize the environmental load by energy recovery and avoiding landfilling of organic waste. Scenario-wise, the change in greenhouse gas (GHG) emissions from treatment was negligible. However, due to the impact of landfilling, GHG emissions in Scenario-0 were 21% and 30% higher than in Scenario-1 and 2, respectively. The environmental benefit of anaerobic co-digestion of FOG/FW with SS is not only in the contribution to energy production but also in the recycling of organic waste.

Digesters as heat storage: Effects of the digester temperature on the process stability, sludge liquor quality, and the dewaterability

Variation of the digester temperature during the year enables the operation of digesters as seasonal heat storage contributing to a holistic heat management at water resource recovery facilities. Full- and lab-scale process data were conducted to examine the effect of the digester temperature on process stability, sludge liquor quality, and dewaterability. Both full- and lab-scale digesters show a stable anaerobic degradation process with a hydraulic retention time of more than 20 days and organic load rates up to 2.2-kg COD/(m3 ·day) at temperatures between 33 and 53°C. The concentrations of soluble COD and ammonium-nitrogen in the sludge liquor digested at 53°C are 2.6 to 5.8 times and 1.3 times higher, respectively, than in the sludge liquor digested at 37°C. Dewatering tests show an enhancement of the dewaterability but a clear increase in the polymer demand at increased digester temperature. PRACTITIONER POINTS: Digesters can operate as seasonal heat storage within mesophilic and thermophilic temperatures Stable anaerobic degradation process for HRT above 20 days Maintenance of process stability as well as quantity and quality of biogas Increase of soluble COD in sludge liquor at higher temperatures Better dewaterability but higher demand for polymers with increasing temperature.

Enhancement of biomethane potential of brown sludge by pre-treatment using vortex based hydrodynamic cavitation

Novel, non-thermal and economically benign pre-treatment process was developed for enhancing valorisation potential of brown sludge generated by dairy industry wastewater treatment plant (WWTP). Vortex-based hydrodynamic cavitation (HC) device was used to quantify influence of pretreatment by measuring biomethane potential (BMP) of untreated and treated brown sludge. Pre-treatment parameters, primarily, pressure drop and number of passes through the cavitation device were varied to quantify influence on BMP. BMP tests were performed at 39 °C containing 5% of total solids in each reactors using an automatic BMP measurement system containing 15 reactors with each volume of 500 mL fitted with overhead stirrer. HC treatment increased the soluble chemical oxygen demand (sCOD) by more than 25% which increased the BMP. HC treatment was able to push the BMP of treated sludge to more than 80% of the theoretical BMP. Volatile solids (VS) removal was more than 65%. Highest methane yield was 376 mL/g-VS of sludge. The methodology and results presented here show significant potential to valorise brown dairy sludge via vortex based hydrodynamic cavitation.

Anaerobic co-digestion of municipal sludge with fat-oil-grease (FOG) enhances the destruction of sludge solids

The objective of this study was to investigate the benefits of co-digestion of a sludge-mix of primary sludge (PS)/thickened waste activated sludge (TWAS) with concentrated fat-oil-grease (FOG) over a wide range of FOG/sludge-mix volumetric feed ratios. The biodegradability (i.e., COD to methane conversion) of PS, TWAS, sludge-mix, and FOG was 43.0, 38.6, 41.8, and 97.7%, respectively, with a pseudo first-order rate of 0.13, 0.12, 0.13, and 0.18 d^-1, respectively. Batch co-digestion of sludge-mix and FOG at COD ratios ranging from 93.2:6.8 to 27.3:72.7% resulted in methane production linearly correlated to both the total waste blend and FOG COD feed concentration. An enhanced extent of degradation of the sludge-mix COD to as much as 10.9% (increased from 42.2 to 53.1%) and an increased degradation rate by 17% (increased from 0.12 to 0.14 d^-1) was observed when the feed FOG COD was 18.5% of the total waste COD feed. Overall, co-digestion of mixed municipal sludge with FOG is feasible and recommended to meet target biogas/methane levels at municipal wastewater treatment facilities taking into account the trade-off between energy production and solids destruction to fit their particular needs.

Current advances and future outlook on pretreatment techniques to enhance biosolids disintegration and anaerobic digestion: A critical review

Waste activated sludge (biosolids) treatment is intensely a major problem around the globe. Anaerobic treatment is indeed a fundamental and most popular approach to convert organic wastes into bioenergy, which could be used as a carbon-neutral renewable and clean energy thus eradicating pathogens and eliminating odor. Due to the sheer intricate biosolid matrix (such as exopolymeric substances) and rigid cell structure, hydrolysis becomes a rate-limiting phase. Numerous different pretreatment strategies were proposed to hasten this rate-limiting hydrolysis and enhance the productivity of anaerobic digestion. This study discusses an overview of previous scientific advances in pretreatment options for enhancing biogas production. In addition, the limitations addressed along with the effects of inhibitors in biosolids towards biogas production and strategies to overcome discussed. This review elaborated the cost analysis of various pretreatment methods towards the scale-up process. This review abridges the existing research on augmenting AD efficacy by recognizing the associated knowledge gaps and suggesting future research.

Comparison between thermophilic and mesophilic anaerobic digestion of waste activated sludge by combined NaOH-microwave pretreatment

The purpose of this study was to investigate the performance of the thermophilic and mesophilic anaerobic digestion process (TADP, MADP) fed with NaOH-microwave pretreated waste activated sludge. The experiment was conducted in anaerobic CSTR reactors. During this experiment, the reactors were stable in operation and were not inhibited by ammonia. The methane production and reduction of organic matters from MADP were less than those from TADP. The dewatering performance of mesophilic sludge was better than that of the thermophilic sludge. The experimental results showed that the continuous TADP and MADP were effective, when the reactors were fed with the waste activated sludge pretreated by NaOH-microwave. MADP was more suitable to combine the NaOH-MW pretreatment process than TADP.

Determination of long-chain fatty acids in anaerobic digester supernatant and olive mill wastewater exploiting an in-syringe dispersive liquid-liquid microextraction and derivatization-free GC-MS method

Long-chain fatty acids (LCFA) are commonly found in lipid-rich wastewaters and are a key factor to monitor the anaerobic digesters. A new simple, fast, precise, and suitable method for routine analysis of LCFA is proposed. The system involves in-syringe-magnetic stirring-assisted dispersive liquid-liquid microextraction (DLLME) prior to gas chromatography-mass spectrometry (GC-MS) without a derivatization process. Calibration curves were prepared in an ethanol solution (R² ≥ 0.996), which was also useful as disperser solvent. Hexane was chosen as the extraction solvent. Several parameters (pH, ionic strength, extraction solvent volume, stirring time) were optimized in multivariate and univariate studies. Limits of detection (LODs) were found in the range 0.01-0.05 mg L^-1 and good precision inter-day (RSDs≤7.9%) and intra-day (RSDs≤4.4%) were obtained. The method was applied to quantify LCFA in supernatants of anaerobic digesters and olive mill wastewaters (OMW). Palmitic, stearic, and oleic acids were the most abundant fatty acid in the analyzed samples and the relative recoveries for all of them were between 81 and 113%.

In-situ alkaline enhanced two-stage anaerobic digestion system for waste cooking oil and sewage sludge co-digestion

Anaerobic digestion is a promising way for resource recovery from waste cooking oil (WCO) due to its high bio-methanation potential. In-situ mild alkaline (pH 8) enhanced two-stage continuous stirred tank reactors (ALK-2-CSTRs) were implemented to explore its efficiency in co-digesting WCO and sewage sludge with stepwise increase of WCO in the co-substrates. Results demonstrate that the ALK-2-CSTRs effectively promoted methane yield from the co-substrates via promoting hydrolysis, long chain fatty acids (LCFAs) degradation and protecting methanogens from exposure to high concentration of LCFAs directly. The maximum methane yield of the ALK-2-CSTRs is 39.2% higher than that of a single stage CSTR system at the optimal feed mixture of 45:55 (WCO:SS [VS]). The thermophilic operation applied to the stage-1 of the ALK-2-CSTRs failed to improve the methane yield when the methanogenic performance was stable; while upon WCO overloaded, the elevated temperature mitigated the deterioration of methanogenesis by stimulating the bioconversion of the toxic LCFAs, especially the unsaturated oleic acid. Microbial community analysis reveals the ALK-2-CSTRs stimulated the growth of lipolytic bacteria and hydrogenotrophic methanogens, which suggests the hydrogenotrophic methanogenic pathway was promoted. Cost evaluation demonstrates the economical superiority of the ALK-2-CSTR over the prevailing strategies developed for enhancing methane yield from the co-substrates.

Anaerobic Digestion for Producing Renewable Energy-The Evolution of This Technology in a New Uncertain Scenario

Anaerobic digestion is a well-known technology with wide application in the treatment of high-strength organic wastes. The economic feasibility of this type of installation is usually attained thanks to the availability of fiscal incentives. In this review, an analysis of the different factors associated with this biological treatment and a description of alternatives available in literature for increasing performance of the process were provided. The possible integration of this process into a biorefinery as a way for producing energy and chemical products from the conversion of wastes and biomass also analyzed. The future outlook of anaerobic digestion will be closely linked to circular economy principles. Therefore, this technology should be properly integrated into any production system where energy can be recovered from organics. Digestion can play a major role in any transformation process where by-products need further stabilization or it can be the central core of any waste treatment process, modifying the current scheme by a concatenation of several activities with the aim of increasing the efficiency of the conversion. Thus, current plants dedicated to the treatment of wastewaters, animal manures, or food wastes can become specialized centers for producing bio-energy and green chemicals. However, high installation costs, feedstock dispersion and market distortions were recognized as the main parameters negatively affecting these alternatives.

First approaches to valorizate fat, oil and grease (FOG) as anaerobic co-substrate with slaughterhouse wastewater: Biomethane potential, settling capacity and microbial dynamics

Anaerobic digestion (AD) is the biological preferred treatment applied to Slaughterhouse wastewaters (SWW) due to its effectiveness. The aim of the study is to investigate the effect of different percentages of fats, oil and grease (FOG) on biomethane production in anaerobic co-digestion with slaughterhouse wastewater using BMP tests under mesophilic conditions (35 °C). For this purpose, three percentages of FOG from 1% to 10% were tested. Biodegradability, biomethane production and the microbial population were studied. In addition, settling capacity has been evaluated at different conditions: i) before and after anaerobic co-digestion; ii) at different temperature 25 °C and 35 °C. The settling rates as well as the characterization of the digestate were recorded. Experimental results showed that all the co-digestion mixtures (FOG percentages = 1-10%) enhanced biomethane production and biodegradability compared to AD of sole SWW. The best conditions were achieved at 5-10% of FOG, showing biodegradability of 66-70% CODt_removal and specific biomethane productions of 562 and 777 mL_CH4·g^-1_CODsremoved, respectively. Regarding microbial dynamics, Eubacteria was reduced with the increase in %FOG but Acetate utilizing methanogens was increased. Regarding settling capacity, mesophilic temperatures (35 °C) increased the settling rate of digestate in 1.76 times and reduced the lag-phase to 0.92 min; obtaining a more concentrated sludge and leaving a clarified whose TSS represent only 8% of TS.

Comparison of mesophilic and thermophilic methane production potential of acids rich and high-strength landfill leachate at different initial organic loadings and food to inoculum ratios

Landfill leachate (LL), which can contaminate both ground and surface water is a major global environmental issue. The aim of the present study was to investigate the biomethane potential (BMP) of a high-strength LL with low pH (5.0), high solids concentration (16%), and high organic matter (170 g/L of chemical oxygen demand (COD); 55 g/L of volatile fatty acids (VFA)) with ammonia nitrogen (NH₃-N) (17 g/L). We investigated the BMP of LL at four different initial organic loadings (IOL) of 170 g/L, 85 g/L, 42.5 g/L and 21 g/L of COD and Food to inoculum (F/I) ratios of 0.5; 1; 2 and 3 at mesophilic (35 ± 2 °C) and thermophilic temperatures (55 ± 2 °C). We found that the highest cumulative CH₄ could be obtained at an IOL of 42.5 g/L of COD regardless of the F/I ratio and temperature. The highest methane content results in biogas at an IOL of 42.5 g/L were 72% and 74% at mesophilic and thermophilic temperatures respectively. About 80-100% of cumulative methane was produced within 15 days in thermophilic reactors, and 40-72% in mesophilic reactors. The kinetic study revealed a fourfold reduction of lag phase in thermophilic compared to mesophilic reactors. The methane yield and organic matter removal rate increased as the concentration of IOL in LL decreased from 170 g/L to 21 g/L regardless of temperature. There exists an inverse correlation between IOL and organic matter removal efficiency. About 80% COD reduction was obtained at mesophilic temperature, and 90% at thermophilic temperature, at an IOL of 42.5 g/L and 21 g/L of COD. The modified Gompertz model showed a good fit to the experimental data, with R² > 0.98 in all cases. Overall, the findings of this study conclude that treatment of acids rich and high-strength LL both at mesophilic and thermophilic temperature is feasible at an optimum IOL of 42.5 g/L of COD. However, treatment of LL at thermophilic temperature outperformed compared to mesophilic over the digestion time.

Increased loading stress leads to convergence of microbial communities and high methane yields in adapted anaerobic co-digesters

Enhancing biogas production, while avoiding inhibition of methanogenesis during co-digestion of grease interceptor waste (GIW), can help water resource recovery facilities reduce their carbon footprint. Here we used pre-adapted and non-adapted digesters to link microbial community structure to digester function. Before disturbance, the pre-adapted and non-adapted digesters showed similar methane production and microbial community diversity but dissimilar community composition. When exposed to an identical disturbance, the pre-adapted digester achieved better performance, while the non-adapted digester was inhibited. When re-exposed to disturbance after recovery, communities and performance of both digesters converged, regardless of the temporal variations. Co-digestion of up to 75% GIW added on a volatile solids (VS) basis was achieved, increasing methane yield by 336% from 0.180 to 0.785 l-methane/g-VS-added, the highest methane yield reported to date for lipid-rich waste. Progressive perturbation substantially enriched fatty acid-degrading Syntrophomonas from less than 1% to 24.6% of total 16S rRNA gene sequences, acetoclastic Methanosaeta from 2.3% to 11.9%, and hydrogenotrophic Methanospirillum from less than 1% to 6.6% in the pre-adapted digester. Specific hydrolytic and fermentative populations also increased. These ecological insights demonstrated how progressive perturbation can be strategically used to influence methanogenic microbiomes and improve co-digestion of GIW.

Anaerobic Co-Digestion of Wastewater Sludge: A Review of Potential Co-Substrates and Operating Factors for Improved Methane Yield

Anaerobic digestion has been widely employed in waste treatment for its ability to capture methane gas released as a product during the digestion. Certain wastes, however, cannot be easily digested due to their low nutrient level insufficient for anaerobic digestion, thus co-digestion is a viable option. Numerous studies have shown that using co-substrates in anaerobic digestion systems improve methane yields as positive synergisms are established in the digestion medium, and the supply of missing nutrients are introduced by the co-substrates. Nevertheless, large-scale implementation of co-digestion technology is limited by inherent process limitations and operational concerns. This review summarizes the results from numerous laboratory, pilot, and full-scale anaerobic co-digestion (ACD) studies of wastewater sludge with the co-substrates of organic fraction of municipal solid waste, food waste, crude glycerol, agricultural waste, and fat, oil and grease. The critical factors that influence the ACD operation are also discussed. The ultimate aim of this review is to identify the best potential co-substrate for wastewater sludge anaerobic co-digestion and provide a recommendation for future reference. By adding co-substrates, a gain ranging from 13 to 176% in the methane yield was accomplished compared to the mono-digestions.

Perspectives on variabilities in biomethane potential test parameters and outcomes: A review of studies published between 2007 and 2018

Biomethane Potential (BMP) test continues to be a useful and inexpensive assay to estimate the digestibility and maximum methane production of various organic substrates in anaerobic digestion or co-digestion processes. Despite its usefulness and several published efforts toward standardizing it, the BMP test still do not follow a universally accepted standard protocol. This makes the comparison of results among studies quite challenging. In this context, this paper analyzes 78 peer-reviewed BMP studies published between 2007 and 2018 that used the BMP test primarily to assess methane potential of commonly digested substrates, such as food waste, wastewater sludge and manure. We focused on the similarities and differences in the methodologies used and, where possible, the results obtained from these studies were compared and discussed. It was observed that many studies do not provide adequate information on salient aspects of the BMP methodology, and results are sometimes reported in different units of measurements. The inoculum to substrate ratio (ISR), substrate concentration and/or load should be clearly indicated in future studies, and positive controls should be included to validate BMP results. It is recommended that more studies assess the impact of nutrient addition, potential effects of continuous and intermittent mixing and mixing intensities and the influence of reactor size and headspace volume on BMP results.

Improving biogas production from anaerobic co-digestion of Thickened Waste Activated Sludge (TWAS) and fat, oil and grease (FOG) using a dual-stage hyper-thermophilic/thermophilic semi-continuous reactor

This paper investigates the feasibility and advantages of using a dual-stage hyper-thermophilic/thermophilic semi-continuous reactor system for the co-digestion of Thickened Waste Activated Sludge (TWAS) and Fat, Oil and Grease (FOG) to produce biogas in high quantity and quality. The performance of the dual-stage hyper-thermophilic (70°C)/thermophilic (55°C) anaerobic co-digestion system is evaluated and compared to the performance of a single-stage thermophilic (55°C) reactor that was used to co-digest the same FOG-TWAS mixtures. Both co-digestion reactors were compared to a control reactor (the control reactor was a single-stage thermophilic reactor that only digested TWAS). The effect of FOG% in the co-digestion mixture (based on total volatile solids) and the reactor hydraulic retention time (HRT) on the biogas/methane production and the reactors' performance were thoroughly investigated. The FOG% that led to the maximum methane yield with a stable reactor performance was determined for both reactors. The maximum FOG% obtained for the single-stage thermophilic reactor at 15 days HRT was found to be 65%. This 65% FOG resulted in 88.3% higher methane yield compared to the control reactor. However, the dual-stage hyper-thermophilic/thermophilic co-digestion reactor proved to be more efficient than the single-stage thermophilic co-digestion reactor, as it was able to digest up to 70% FOG with a stable reactor performance. The 70% FOG in the co-digestion mixture resulted in 148.2% higher methane yield compared to the control at 15 days HRT. 70% FOG (based on total volatile solids) is so far the highest FOG% that has been proved to be useful and safe for semi-continuous reactor application in the open literature. Finally, the dual-stage hyper-thermophilic/thermophilic co-digestion reactor also proved to be efficient and stable in co-digesting 40% FOG mixtures at lower HRTs (i.e., 9 and 12 days) and still produce high methane yields and Class A effluents.

Evaluating analytical methods for the characterization of lipids, proteins and carbohydrates in organic substrates for anaerobic co-digestion

This study provides insights into the characterization of lipids, proteins and carbohydrate content in substrates for codigestion, and evaluates their effects on biogas yield. Among the analytical methods evaluated, the Bligh and Dyer, Hach Total Nitrogen and the Anthrone method were found to be most suitable for lipids, proteins and carbohydrates analysis, respectively. The co-digestibility of ten co-substrate mixes prepared using various volume-to-volume ratios of foodwaste (FW), fats, oils and grease (FOG), and waste activated sludge (WAS) were tested using biomethane potential assays. The three main substrates were mono-digested as well. WAS mono-digestion yielded the lowest methane yield of 118mL CH₄/g VS, while a 50:50 mix of WAS and FOG, containing 85% lipid and 15% protein produced the highest methane yield of 1040mL CH₄/g VS. In general, lipid-rich samples yielded more biogas than samples rich in proteins and carbohydrates. However, samples rich in proteins and carbohydrates had faster biogas production rates.

Co-digestion performance of organic fraction of municipal solid waste with leachate: Preliminary studies

The main aim of the study was to evaluate the co-digestion performance of OFMSW with different wastes. Leachate, reverse osmosis (RO) concentrate collected from a leachate treatment facility and dewatered sewage sludge taken from a wastewater treatment plant (WWTP) were used for co-digestion in this paper. An extra effort was made to observe the effect of leachate inclusion in the co-digestion. In the study, the mono-digestion of OFMSW, leachate, RO concentrate and sewage sludge as well as digestion of 7 different waste mixtures were carried out for this objective. The experiments were carried out for approximately 50days under mesophilic conditions. The highest methane yield was 785L CH₄/kg VS_added in the reactor, which had only OFMSW. While the methane yield derived from OFMSW was found higher than previous studies, methane yield of leachate was found to be 110L CH₄/kg VS_added, which was lower than findings in the literature. The mono-substrate of OFMSW was followed by the reactor of having waste mixture of leachate+sewage sludge+OFMSW+water (C7) with 391L CH₄/kg VS_added, which was the only combination included water. In order to understand the effect of leachate and water inclusions on co-digestion, two separate waste combinations; leachate+sewage sludge+OFMSW+water (C7) and leachate+sewage sludge+OFMSW (C1) were prepared that had different amounts of leachate but same amounts of other wastes. The methane yield of leachate+sewage sludge+OFMSW+water (C7) indicated that addition of some water instead of leachate could stimulate biogas production. Methane yield of this reactor was found to be 71% higher than the waste combination of leachate+sewage sludge+OFMSW (C1). It could be thought that the high amount of non-biodegradable matters in leachate could be responsible for lower methane yield in leachate+sewage sludge+OFMSW (C1) reactor. Methane yields of the reactors showed that co-digestion of OFMSW and leachate could be a solution not only for treatment of leachate and but also increasing the biogas potential of leachate. Leachate addition could also adjust optimum total solids (TS) content in anaerobic digestion. It was also understood that RO concentrate did not affect the methane yield in a negative way. The similar characterization of leachate and RO concentrate in this study could offer the utilization of RO concentrate instead of leachate. The findings showed that volatile solids (VS) removals were changed from 32% to 61% in the reactors. While the reactor of leachate+RO concentrate+OFMSW (C6) had the highest VS removal, the reactor of the sole substrate leachate had the lowest VS removal.

Elucidating microbial community adaptation to anaerobic co-digestion of fats, oils, and grease and food waste

Despite growing interest in co-digestion and demonstrated process improvements (e.g., enhanced stability and biogas production), few studies have evaluated how co-digestion impacts the anaerobic digestion (AD) microbiome. Three sequential bench-scale respirometry experiments were conducted at thermophilic temperature (50 °C) with various combinations of primary sludge (PS); thickened waste activated sludge (TWAS); fats, oils, and grease (FOG); and food waste (FW). Two additional runs were then performed to evaluate microbial inhibition at higher organic fractions of FOG (30-60% volatile solids loading (VSL; v/v)). Co-digestion of PS, TWAS, FOG, and FW resulted in a 26% increase in methane production relative to digestion of PS and TWAS. A substantial lag time was observed in biogas production for vessels with FOG addition that decreased by more than half in later runs, likely due to adaptation of the microbial community. 30% FOG with 10% FW showed the highest increase in methane production, increasing 53% compared to digestion of PS and TWAS. FOG addition above 50% VSL was found to be inhibitory with and without FW addition and resulted in volatile fatty acid (VFA) accumulation. Methane production was linked with high relative activity and abundance of syntrophic fatty-acid oxidizers alongside hydrogenotrophic methanogens, signaling the importance of interspecies interactions in AD. Specifically, relative activity of Syntrophomonas was significantly correlated with methane production. Further, methane production increased over subsequent runs along with methyl coenzyme M reductase (mcrA) gene expression, a functional gene in methanogens, suggesting temporal adaptation of the microbial community to co-digestion substrate mixtures. The study demonstrated the benefits of co-digestion in terms of performance enhancement and enrichment of key active microbial populations.

Co-digestion of municipal sludge and external organic wastes for enhanced biogas production under realistic plant constraints

A bench-scale investigation was conducted to select external organic wastes and mixing ratios for co-digestion with municipal sludge at the F. Wayne Hill Water Resources Center (FWHWRC), Gwinnett County, GA, USA to support a combined heat and power (CHP) project. External wastes were chosen and used subject to two constraints: a) digester retention time no lower than 15 d; and b) total biogas (methane) production not to exceed a specific target level based on air permit constraints on CO2 emissions. Primary sludge (PS), thickened waste activated sludge (TWAS) and digested sludge collected at the FWHWRC, industrial liquid waste obtained from a chewing gum manufacturing plant (GW) and dewatered fat-oil-grease (FOG) were used. All sludge and waste samples were characterized and their ultimate digestibility was assessed at 35 °C. The ultimate COD to methane conversion of PS, TWAS, municipal sludge (PS + TWAS; 40:60 w/w TS basis), GW and FOG was 49.2, 35.2, 40.3, 72.7, and 81.1%, respectively. Co-digestion of municipal sludge with GW, FOG or both, was evaluated using four bench-scale, mesophilic (35 °C) digesters. Biogas production increased significantly and additional degradation of the municipal sludge between 1.1 and 30.7% was observed. Biogas and methane production was very close to the target levels necessary to close the energy deficit at the FWHWRC. Co-digestion resulted in an effluent quality similar to that of the control digester fed only with the municipal sludge, indicating that co-digestion had no adverse effects. Study results prove that high methane production is achievable with the addition of concentrated external organic wastes to municipal digesters, at acceptable higher digester organic loadings and lower retention times, allowing the effective implementation of CHP programs at municipal wastewater treatment plants, with significant cost savings.

Enhanced biogas production by anaerobic co-digestion from a trinary mix substrate over a binary mix substrate

The synergetic enhancement of mesophilic anaerobic co-digestion of trinary and binary mix of organic fraction of municipal solid waste (OFMSW) + primary sludge (PS) + thickened waste activated sludge (TWAS) as substrates was investigated through batch biological methane potential (BMP) and semi-continuous flow reactor tests. Cumulative biogas yield (CBY) yield for the binary mix of OFMSW:TWAS was 555, 580, and 660 mL/g volatile solids (VS)added for an OFMSW:TWAS ratio of 25:75, 50:50, and 75:25, respectively, which was 48, 78.5, and 140% higher than the calculated expected biogas (CEB) yield from the corresponding individual substrates. The trinary mixture of OFMSW:TWAS:PS at ratios of 25:37.5:375.5, 50:25:25 and 75:12.5:12.5 was able to produce 680, 710 and 780 mL/g VSadded, respectively, which was 25.5, 62.0 and 135.6% more biogas than the calculated expected biogas yield from the corresponding individual substrates. Cumulative methane yield (CMY) of trinary mixtures was also higher than the corresponding binary mixtures (20, 27, and 12 % increase for OFMSW:TWAS:PS at a ratio of 25:37.5:37.5, 50:25:25, and 75:12.5:12.5 compared to the binary mix of OFMSW:TWAS at a ratio of 25:75, 50:50, and 75:25, respectively). Methane content of the biogas varied from 54 to 57%. The results from semi-continuous flow anaerobic reactors under hydraulic retention times (HRT) of 15, 10 and 7 days supported the results of batch biological methane potential tests. The results were conclusive that enhancement in biogas production was noticeably higher from the co-digestion of trinary mix of organic fraction of municipal solid waste+ thickened waste activated sludge + primary sludge than the binary mix organic fraction of municipal solid waste+thickened waste activated sludge or thickened waste activated sludge+primary sludge with concomitant improvements in VS removal and biodegradability for tri-digestion of organic fraction of municipal solid waste, thickened waste activated sludge and primary sludge.

Pilot-scale anaerobic co-digestion of municipal wastewater sludge with restaurant grease trap waste

The maximum feasible loading rate of grease trap waste (GTW) to the municipal wastewater sludge (MWS) was investigated using two 1300 L pilot-scale (1200 L active volume) digesters under mesophilic conditions at a 20 day solids retention time. During the co-digestion, the test reactor received a mixture of GTW and MWS while the control reactor received only MWS. The test digester loading was increased incrementally to a maximum of 280% of the control digester COD loading. The highest feasible GTW loading was determined to be 23% and 58% in terms of its total 1.58 kg VS/(m(3) d) and 3.99 kg COD/(m(3) d) loadings, respectively. This test digester COD loading represented 240% of the control digester COD loading. At this loading, test digester biogas production was 67% greater than that of the control. During the test digester quasi steady state loading period when VS from GTW represented 19% of its total VS loading, the test digester COD and VS removal rates were 2.5 and 1.5 fold those of the control digester, respectively. The test digester biogas production declined markedly when the percentage of VS from GTW in its feed was increased to 30% of its total VS loading. Causes of the reduced biogas production were investigated and attributed to inhibition due to long chain fatty acid accumulation.

Determining the limits of anaerobic co-digestion of thickened waste activated sludge with grease interceptor waste

Anaerobic co-digestion of thickened waste activated sludge (TWAS) with grease interceptor waste (GIW) from a food service establishment was conducted in lab scale semi-continuous digesters. GIW included the entire contents of the grease interceptor (GI) including fat, oil, and grease (FOG), food residuals, and associated wastewater. GIW was added in step increases to identify the maximum methane production and the corresponding threshold input of GIW that led to inhibition of methanogenesis. The experiment was performed at mesophilic conditions (37 °C) with a solids retention time (SRT) of 20 days. The highest GIW addition rate achieved without digester failure was 20% (v/v), or 65.5% (w/w) of volatile solids (VS) added, enhancing the methane yield from 0.180 to 0.752 m3(CH4)/kg(VS added), biogas production from 2.2 × 10(-3) to 1.4 × 10(-2) m(3)/d, and methane content from 60.2% to 70.1%. The methane yield of 0.752 m3(CH4)/kg(VS added) is the highest value reported to date for co-digestion of GIW. Stepwise increases in co-substrate addition led to better microbial acclimation and reduced the GIW inhibitory effect. The limit for GIW addition leading to an inhibited digestion process was identified to be between 20 and 40% (v/v) or 65.5 and 83.5% (w/w) of VS added. The results show the significant benefits of anaerobic co-digestion of GIW and the positive impacts of gradual addition of GIW.

Anaerobic co-digestion of grease sludge and sewage sludge: the effect of organic loading and grease sludge content

The objective of this study was to assess the feasibility of co-digesting grease sludge (GS) originating from domestic wastewater along with sewage sludge (SS) and to assess the effect of organic loading rate (OLR) and GS content on process performance. Three lab-scale semi-continuous fed mesophilic anaerobic digesters were operated under various OLRs and SS-GS mixtures. According to the results, addition of GS up to 60% of the total VS load of feed resulted in a 55% increase of biogas yield (700 vs. 452m(3)/tVSadded) for an OLR of 3.5kg VS/m(3)/d. A stable and satisfactory operation of anaerobic co-digestion units can be achieved for a GS-OLR up to 2.4kg VSGS/m(3)/d. For such values biogas yield is linearly proportional to the applied GS-OLR, whereas biogas yield is minimal for GS-OLR higher than this limit and acidification of the anaerobic digestion units is taking place.

Effects of organic loading rate on reactor performance and archaeal community structure in mesophilic anaerobic digesters treating municipal sewage sludge

In this study, the organic loading rate (OLR) of a high-solids anaerobic digestion (HSAD) system was increased from 3.4 to 5.0 gVS L(-1) day(-1) and reactor stability, performance and microbial community structure were determined. Laboratory simulations (3.5 L) of the full-scale process (500 dry ton year(-1)) were conducted using continuously stirred-tank mesophilic reactors. OLRs of 3.4 gVS L(-1)day(-1) (equal to the full-scale HSAD), 4.0, 4.5 and 5.0 gVS L(-1)day(-1) were evaluated. Biochemical parameters and archaeal community dynamics were measured over 42 days of steady state operation. Results showed that increasing OLR increased the amount of organic matter conversion and resulted in higher organic matter removal and volumetric methane (CH₄) production (VMP) rates. The highest volatile solids (VS) removal and VMP results of 54 ± 2% and 1.4 ± 0.1 L CH₄ L(-1)day(-1) were observed for 5.0 gVS L(-1) day(-1). The efficiency of reactor conversion of organic matter to CH(4) was found to be similar in all the treatments with an average value of 0.57 ± 0.07 LCH(4) gVS(-1) (removed). 16S rRNA gene terminal restriction fragment polymorphism (T-RFLP) analyses revealed that archaeal TRFs remained stable during the experiment accounting for an average relative abundance (RA) of 81 ± 1%. Archaea consistent with multiple terminal restriction fragments (TRFs) included members of the Euryarchaeota and Crenarchaeota phyla, including acetoclastic and hydrogenotrophic groups. In conclusion, this laboratory-scale study suggests that performance and stability as well as the archaeal community structure in this HSAD system was unaffected by increasing the OLR by nearly 50% and that this increase resulted in a similar increase in the amount of CH(4) gas generated.

Methane recovery from the anaerobic codigestion of municipal sludge and FOG

The anaerobic biodegradability of a mix of municipal primary sludge (PS), thickened waste activated sludge (TWAS) and fat, oil, and grease (FOG) was assessed using semi-continuous feed, laboratory-scale anaerobic digesters operated at mesophilic (35 degrees C) and thermophilic (52 degrees C) temperature. Addition of a large FOG fraction (48% of the total VS load) to a PS+TWAS mix, resulted in 2.95 times larger methane yield, 152 vs. 449 mL methane @ STP/g VS added at 35 degrees C and 2.6 times larger methane yield, 197 vs. 512 mL methane @ STP/g VS added at 52 degrees C. The high FOG organic load fraction was not inhibitory to the process. The results of this study demonstrate the benefit of sludge and FOG codigestion.