Anaerobic Digestion Of Organic Fraction Of Municipal Solid Waste Research Articles

Exploring resource recovery from diverted organics: Life cycle assessment comparison of options for managing the organic fraction of municipal solid waste

Diversion of the organic fraction of municipal solid waste (OFMSW) from landfills is increasing. Previous life cycle assessment studies have evaluated subsets of OFMSW management options, but conclusions are inconsistent, and none have evaluated diverse applications of material by-products. The primary objective of this work was to identify sustainability-based improvements to the selection, design, implementation, and operation of organics waste diversion management technologies. Process modeling and life cycle assessment were used to compare OFMSW composting, anaerobic digestion, and pyrolysis, with biochar used as a landfill cover, leachate treatment sorbent, and land applicant. Material and energy flows, calculated by newly developed models for the defined functional unit (1 kg MSW over a 20-year timeframe), were translated to environmental performance using ecoinvent and USLCI databases and TRACI method. Additionally, uncertainty, sensitivity, and scenario analyses were conducted to evaluate the implications of model uncertainties, design decisions, and resource recovery tradeoffs. OFMSW pyrolysis usually (65 % of uncertainty assessment simulations) had the best global warming performance mostly due to energy recovery and biochar's carbon sequestration benefit, which was independent of fate. Pyrolyzing the biosolids from OFMSW anaerobic digestion recovered the most energy and had the best performance in 34 % of uncertainty simulations. Material recovery amounts were large (e.g., more biochar was produced than required for novel uses) and warrant feasibility considerations. Global warming performance was more sensitive to uncertainty in carbon sequestration and primary energy production than in fertilizer offset, energy conversion, or heat offset approach. Practical implications include the potential for biochar supply to outweigh demand, and inconsistent revenue from the sale of recovered energy and carbon credits; future research could focus on evaluating the relative social and economic sustainability of the OFMSW management technologies.

Electronic regulation to achieve efficient anaerobic digestion of organic fraction of municipal solid waste (OFMSW): strategies, challenges and potential solutions

Anaerobic digestion (AD) of organic fraction of municipal solid waste (OFMSW) is prone to system breakdown under high organic loading rates (OLRs) condition, which subsequently reduces the efficiency of digestion process and results in substantial economic losses. In this perspective paper, the substances metabolisms, electrons flow, as well as microbial interaction mechanisms within AD process are comprehensively discussed, and the underlying bottleneck that causes inefficient methane production is identified, which is “electrons surplus”. Systems encountering severe electron surplus are at risk of process failure, making it crucial to proactively prevent this phenomenon through appropriate approaches. On this basis, the present perspective proposes three potential electronic regulation strategies to prevent electrons surplus, namely, electron shunt, accelerating electron transfer and regulating methanogenic metabolism pathway, and presents specific methodologies for each strategy. Furthermore, the potential solutions to challenges that may occur during the electronic regulation process are also presented in this paper. This perspective aims to provide innovative approaches to achieve the efficient and stable operation of OFMSW anaerobic digestion, especially under high OLRs condition.

Effect of direct enzyme addition on the anaerobic digestion of OFMSW

The organic fraction of municipal solid waste (OFMSW) has high biodegradability and is a good feedstock for anaerobic digestion (AD). However, the solubilization of OFMSW is a limiting factor for AD. In this study, enzyme treatment was investigated to increase the conversion of complex materials of OFMSW into soluble fractions. Six different concentrations (0.15, 0.3, 0.5, 1.0, 1.5 and 3.0 mg/VSadded) of the alpha amylase enzyme (obtained from Aspergillus oryzae) were directly added as a powder and automated biochemical methane potential (BMP) tests were performed under thermophilic conditions. According to BMP tests, the methane (CH4) yield increased up to 17.5 % and the lag phase decreased to 19.3 % after enzyme addition. The highest CH4 yield and shortest lag phase were obtained when 0.5 mg of enzyme/VSadded was added to the reactor. These findings suggest that direct alpha amylase enzyme application obtained from Aspergillus oryzae can be used to increase biogas production by enhancing solubilization of OFMSW for AD.

Characteristics of the digestates from OFMSW methane production at psychrophilic, mesophilic, and thermophilic conditions under different organic loading rates

ABSTRACT The anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) has shown to be a viable alternative since it allows energy recovery in the form of methane and generates a residue (digestate) that can be applied effectively as a soil improver or fertiliser. The potential for methane production and the digestates’ characteristics depend on the substrate characteristics and the process variables such as temperature, solids retention time, and organic load. This study dealt with OFMSW anaerobic digestion under different organic loading rates and temperatures and the characteristics of the resulting digestates. Three semi-continuous reactors were operated at 20, 35, and 55°C and fed daily with ground, fresh OFMSW from Mexico City. The inoculum was temperature-adapted UASB granular sludge. The main results indicate that the anaerobic digestion was adequate, as the pH values were slightly alkaline, which is sufficient for methanization, and the alkalinity was not a limiting factor. Potassium and PO4-P increased with the organic load, and Kjeldahl nitrogen decreased. At 20°C, total organic carbon (TOC) increased substantially with the organic load; at 35°C, it remained without significant changes; and at 55°C, TOC slightly decreased with the organic load. The C/N ratio changed accordingly to TOC variations. At 20°C, the residual biogas potential increased with the organic load; at 35 and 55°C, it decreased with increasing organic load; the residual biogas potential increased with residual fatty acids concentrations. To comply with international standards for agricultural use, the digestates need only dewatering and supplementing with PO4-P.

Improvement of Biogas Production Using Biochar from Digestate at Different Pyrolysis Temperatures during OFMSW Anaerobic Digestion

Anaerobic digestion (AD) was utilized to treat the ever-growing amount of organic fraction of municipal solid waste (OFMSW) generated due to population growth and the expansion of the global economy. The widespread application of AD has led to a continuous increase in residual solid digestate that necessarily requires further disposal. Improving AD efficiency and reducing the large amount of digestate is necessary. This study investigated the chemical and physical characteristics of biochar derived from digestate at different pyrolysis temperatures (300 °C, 500 °C, and 700 °C), as well as corn stover biochar at 500 °C, and their effects on AD performance. The pH value of the biochar increased with an increase in pyrolysis temperature while the electrical conductivity decreased. Macropores dominated the biochar’s pore size, and decreased with an increased pyrolysis temperature. The biochar preparation temperature significantly influenced the AD efficiency. Biochar prepared at 700 °C outperformed the other groups, improving the biogas production yields by 10.0%, effectively shortening the lag time, and increasing the average chemical oxygen demand (COD) degradation rate by 14.0%. The addition of biochar (700 °C) and corn stover biochar increased the relative abundance of the volatile fatty acid (VFAs)-oxidizing bacteria Syntrophomonadaceae, which expedited the acid conversion in AD systems. Biochar facilitated direct interspecies electron transfer between DMER64 and Trichococcus with Methanosaeta, enhancing the biogas production performance. These findings confirmed that the biochar derived from digestate promoted biogas production and acid conversion in the AD system of OFMSW. Furthermore, biochar has an improved AD stability, which represents a promising approach to recycling digestate.

Characterization of the separately collected organic fraction of municipal solid waste (OFMSW) from rural and urban districts for a one-year period in Germany.

Knowledge on material properties is beneficial to fully exploit inherent utilization potentials of the organic fraction of municipal solid waste (OFMSW). The objective of this study was to analyze and compare the physico-chemical characteristics of separately collected OFMSW (biowaste bin) originating in southwestern Germany. Therefore, 22 rural and 20 urban OFMSW samples, each from the same location were analyzed in the course of one year. Next to the basic characteristics such as the impurity, dry matter (DM) and organic dry matter (oDM) contents, this study focused on the analysis of 37 major, minor and trace elements. In addition, stoichiometric CH4 potentials for the anaerobic digestion were calculated. The fresh mass (FM) based DM contents were significantly (p=0.001) higher in rural OFMSW (32.86±2.35% vs. 30.50±1.75%) while the DM based oDM content was higher (p=0.07) in urban OFMSW (84.59±3.90% vs. 82.22±4.16%). The impurities in rural OFMSW were significantly lower (2.83±1.67% DM vs. 5.07±2.71% DM with p=0.004) while oDM based CH4 potentials were higher for urban OFMSW (533±22 L/kg vs. 519±26L/kg). For both OFMSW types, contents >1000mg/kgDM were detected for Ca, K, Si, Na, Al, Fe, Mg, P and S while Ti, Mn, Ba, Zn, Sr, Cr, Cu, V, Ni, Li, Pb and B were measured between 1 and 1000mg/kgDM. The determined element concentrations are useful for an improved classification of OFMSW as a biorefinery resource.

Treatment of the Supernatant of Anaerobically Digested Organic Fraction of Municipal Solid Waste in a Demo-Scale Mesophilic External Anaerobic Membrane Bioreactor.

Conventional aerobic biological treatments of digested organic fraction of municipal solid waste (OFMSW) slurries–usually conventional activated sludge or aerobic membrane bioreactor (AeMBR)–are inefficient in terms of energy and economically costly because of the high aeration requirements and the high amount of produced sludge. In this study, the supernatant obtained after the anaerobic digestion of OFMSW was treated in a mesophilic demo-scale anaerobic membrane bioreactor (AnMBR) at cross flow velocities (CFVs) between 1 and 3.5 m⋅s–1. The aim was to determine the process performance of the system with an external ultrafiltration unit, in terms of organic matter removal and sludge filterability. In previous anaerobic continuous stirred tank reactor (CSTR) tests, without ultrafiltration, specific gas production between 40 and 83 NL CH4⋅kg–1 chemical oxygen demand (COD) fed and removals in the range of 10–20% total COD (tCOD) or 59–77% soluble COD (sCOD) were obtained, for organic loading rates (OLR) between 1.7 and 4.4 kg COD⋅m–3reactor d–1. Data helped to identify a simplified model with the aim of understanding and expressing the process performance. Methane content in biogas was in the range of 74–77% v:v. In the AnMBR configuration, the COD removal has been in the ranges of 15.6–38.5 and 61.3–70.4% for total and sCOD, respectively, with a positive correlation between solids retention time (SRT, ranging from 7.3 to 24.3 days) and tCOD removal. The constant used in the model expressing inhibition, attributable to the high nitrogen content (3.6 ± 1.0 g N-NH4+⋅L–1), indicated that this inhibition decreased when SRT increased, explaining values measured for volatile fatty acids concentration, which decreased when SRT increased and OLR, measured per unit of volatile suspended solids in the reactor, decreased. The alkalinity was high enough to allow a stable process throughout the experiments. Constant CFV operation resulted in excessive fouling and sudden trans-membrane pressure (TMP) increases. Nevertheless, an ultrafiltration regime based on alternation of CFV (20 min with a certain CFVi and then 5 min at CFVi + 1 m⋅s–1) allowed the membranes to filter at a flux (standardized at 20°C temperature) ranging from 2.8 to 7.3 L⋅m–2⋅h–1, over 331 days of operation, even at very high suspended solids concentrations (>30 g total suspended solids⋅L–1) in the reactor sludge. This flux range confirms that fouling is the main issue that can limit the spread of AnMBR potential for the studied stream. No clear correlation was found between CFV or SRT vs. fouling rate, in terms of either TMP⋅time–1 or permeability⋅time–1. As part of the demo-scale study, other operational limitations were observed: irreversible fouling, scaling (in the form of struvite deposition), ragging, and sludging. Because ragging and sludging were also observed in the existing AeMBR, it can be stated that both are attributable to the stream and to the difficulty of removing existing fibers. All the mentioned phenomena could have contributed to the high data dispersion of experimental results.

Methane Production from the Organic Fraction of Municipal Solid Waste Under Psychrophilic, Mesophilic, and Thermophilic Temperatures at Different Organic Loading Rates

Methane production from the organic fraction of municipal solid waste (OFMSW) can be achieved under different operational conditions, but authors cannot conclude which conditions yield the best methane production. This research deals with how different organic loading rates (OLR) at different temperatures affect methane production during the anaerobic digestion of OFMSW. For 48 days, anaerobic granular sludge was adapted to OFMSW at 35 °C, and then, for another 85 days, using a semicontinuous digester, the mixture was further acclimated using OFMSW as substrate. This digestate was then used to inoculate three digesters at 55, 35, and 55 °C. The digesters were fed with OFMSW under different OLR. The digester at 55 °C reached stable methane production after 90 days at a low OLR. At 35 °C, methane production decreased with increasing OLR. At 20 °C, with increasing OLR, methane production declined as VFA concentration increased. The digester operating at 35 °C reached stable operation faster than the other two under the higher OLR. Slowly increasing the OLR allowed the adequate adaptation of the microorganisms to the substrate and temperature.

Biochemical methane potential of water hyacinth and the organic fraction of municipal solid waste using leachate from Mexico City’s Bordo Poniente composting plant as inoculum

In Mexico City, both the organic fraction of municipal solid waste (OFMSW) and the spread of water hyacinth among water bodies have been reported to increase greatly each year. These wastes have been associated with health problems and with the contamination of water bodies, soil, and air. Previous studies have demonstrated that water hyacinth and OFMSW can be sustainably managed using anaerobic digestion (AD) and anaerobic co-digestion (ACD) processes. In the present study, the biochemical methane potential of water hyacinth and OFMSW was determined. Leachate from Mexico City’s Bordo Poniente composting plant was used as inoculum, and the effects of two substrate-to-inoculum (S:I) ratios on volatile solids (VS) were investigated at lab-scale and under thermophilic conditions with a hydraulic retention time of 50 d. Based on S:I ratio, the AD of OFMSW with an S:I of 1:1 yielded more methane (387 ± 25 NLCH4/kgVSadded) than the ACD process (337 ± 15 NLCH4/kgVSadded), but not significantly (p > 0.05). Inoculum showed great potential to digest wastes used in this research. The ACD S:I ratio of 1:1 demonstrated that the AD process is both to produce renewable energy and to resolve problems associated with water hyacinth, OFMSW, and leachate.

Production and characterization of β-glucosidase from Aspergillus niger fermentation: Application for organic fraction of municipal solid waste hydrolysis and methane enhancement.

The anaerobic digestion of the organic fraction of municipal solid waste (OFMSW) is currently an attractive treatment process with energy production in the form of biogas. Hydrolysis is the rate-limiting step for the anaerobic digestion of solid wastes. Thus, in the present study fungal enzymatic pretreatment of OFMSW was applied to enhance biogas production. Two enzyme cocktails rich on β-glucosidase were produced from submerged fermentation of Aspergillus niger on basal medium using OFMSW as carbon source and urea (Urea cocktail) and Ulva rigida as nitrogen source (Ulva cocktail). Ulva cocktail displayed an important effect on OFMSW solubilization. Therefore, an increase of reducing sugar concentration about 60% was obtained which was in correlation with chemical oxygen demand (COD) increase. The performance of enzymatic pretreatment on anaerobic digestion of OFMSW was studied by conducting biochemical methane potential tests. Results showed that the enzymatic pretreatment improved methane yield of OFMSW even at high solid concentration. High methane yield about 500 ml/g total volatile solid was obtained, which corresponds up to 68% enhancement over the control.

Enhancement of biogas production from organic fraction of municipal solid waste using hydrothermal pretreatment

Anaerobic digestion is a pragmatic approach to recover energy and treat organic fraction of municipal solid waste (OFMSW). This study employs hydrothermal pretreatment to enhance biogas generation during anaerobic digestion of OFMSW in India. OFMSW was pretreated at 80, 100, 120, 140 and 160 °C over heating durations of 0, 15, 30, 60 and 120 min each. Thereafter, batch anaerobic digestion assays of untreated and pretreated OFMSW were carried out using cow excrement as inoculum. The results show that hydrothermal pretreatment is beneficial in improving biodegradability; thereby increasing methane yield and reducing digestion time. Increase in cumulative methane generation ranged between 3% and 32%. Pretreatment at 140 °C for 30 min resulted in highest cumulative methane generation of 200 ml/gVS, with methane content of 68.6% after 18 days; whereas the same for untreated OFMSW was 152 ml/gVS with a methane content of 50.3%, after a digestion period of 28 days. Effect of hydrothermal pretreatment on characteristics of OFMSW and energy required for each pretreatment step were also analysed.

Fe0 inhibits bio-foam generating in anaerobic digestion reactor under conditions of organic shock loading and re-startup

Bio-foaming during the anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) is a major cause of unstable reactor operations, especially under high organic loading rates (OLRs). Here, we used two 40-liter anaerobic reactors to deal with the actual organic fraction of municipal solid waste at OLRs of 5 and 7.5 kgVS/m3/d and tried to inhibit bio-foam generation by dosing with Fe0. One of reactors had 400 g of Fe0 added before operation and additional 200 g of Fe0 was added when the operating conditions changed; the other one was the control reactor with no Fe0 dosing. The results show that bio-foam was generated in the control reactor at an organic shock loading of 10 kgVS/m3/d and re-startup at 5 kgVS/m3/d, while the reactor containing Fe0 operated normally. Due to the surfactant attributes of volatile fatty acids (VFAs), the probability of bio-foam generation was positively correlated with the effluent’s degree of surface activity (SA) induced by VFAs, which is a parameter representing the foaming potential. Because VFA biodegradation was enhanced in the reactor with added Fe0, at the position of Fe0 accumulation, the SA was decreased by 49.7–59.2%. Furthermore, the effect of Fe0 dosing on inhibiting bio-foam was evaluated by comparison with traditional commercial anti-foam agents. The results suggest possible reasons for bio-foam generation in OFMSW AD and suggest the reasonable usage of Fe0 to inhibit bio-foam formation.

Digestion Performance and Microbial Metabolic Mechanism in Thermophilic and Mesophilic Anaerobic Digesters Exposed to Elevated Loadings of Organic Fraction of Municipal Solid Waste

Mesophilic and thermophilic anaerobic digestion reactors (MR and TR) for the organic fraction of municipal solid waste (OFMSW) were tested to reveal the differential microbial responses to increasing organic loading rate (OLR). MR exhibited faster adaptation and better performance at an OLR range of 1.0–2.5 g VS·L−1·d−1, with average profiles of a biogas yield of 0.38 L/gVSadded*d at 0.5 g/L*d OLR and 0.69 L/gVSadded*d at 2.5 g/L*d OLR, whereas TR had a biogas yield of 0.07 L/gVSadded*d at 0.5 g/L*d OLR and 0.44 L/gVSadded*d at 2.5 g/L*d OLR. The pyrosequencing results of amplicons revealed the microbial mechanisms of OFMSW anaerobic digestion. Larger shifts in the bacteria composition were observed in the TR with OLR elevation. For methanogens in both reactors, Methanothrix dominated in the MR while Methanosarcina was favored in the TR. Moreover, analysis of the mode and efficiency of metabolism between the MR and TR demonstrated different performances with more efficiency related to the limiting hydrolytic acid step.

Comprehensive exergy analysis of a gas engine-equipped anaerobic digestion plant producing electricity and biofertilizer from organic fraction of municipal solid waste

This study was devoted to comprehensively investigating the exergetic performance of a gas-engine equipped anaerobic digestion plant producing electric power as well as biofertilizer from organic fraction of municipal solid waste (OFMSW). The main aim of the current survey was to reveal the reasons and sources of thermodynamic losses occurring in the plant based on real operational data. The required data for the analysis were collected from a local OFMSW anaerobic digestion plant located in Tehran, Iran. After writing energy and exergy balances for all components of the plant, their exergetic performance parameters were measured individually. An attempt was also undertaken to quantify the contributions of the products to the overall exergetic efficiency of the plant. The exergetic value of the net electric power was determined at 1596.0kW, while the chemical exergetic content of the biofertilizer was found to be 8758.3kW. The overall exergetic efficiency of the plant was determined at 72.8%. The contributions of the electric power and the biofertilizer to the overall exergetic efficiency of the plant were found to be 15.4% and 84.6%, respectively. Generally, the exergetic analysis presented herein could provide important guidelines and methodological blueprints for futureinvestigations in order to develop thermodynamically-efficient and environmentally-benign waste-to-energy plants.

Organic fraction of municipal solid waste – a valuable source of green energy in India

Purpose The purpose of this paper is to evaluate the green energy generation potential of the organic fraction of municipal solid waste (OFMSW) through anaerobic digestion (AD) route in India and its benefits. Design/methodology/approach In this study, performances of some AD plants presently operating successfully in India have been studied in the field (Section 3.1). Primary data collected from this study has been used to evaluate the biogas generation potential of OFMSW in Indian condition (Section 4). To ensure the validity, this gas generation potential has been compared with the gas yield data observed by the other researchers and with the gas yields of AD plants of some technology providers at some parts of the world (Section 4.1). From the future population projection (year 2030) and the future per capita waste generation rate obtained from the literature survey, estimation has been made for future quantity of municipal solid waste (MSW) (year 2030) (Section 2.3). Based on these data, the green energy generation potential from the bio-degradable portion of MSW through AD route, in India, has been evaluated (Section 4.2), and its economic and environmental benefits have been analyzed (Section 5) . Findings This secondary research work reveals that from the bio-degradable portion of MSW, India can generate about 583 MW of green energy daily and produce about 5.1 mil MT of bio fertilizer per annum presently, and these may go up to 2,273 MW and 19.5 mil MT, respectively, in the year 2030. Generation of green energy from OFMSW, in India, may save coal consumption of about 3.04 mil MT and reduce 35.42 mil MT of CO2 emissions per annum presently. Moreover, utilization of the bio-degradable portion of the MSW stream may save about 550 acres (2.23 sq. km) of landfill area per year presently and in total may save about 8,182 hectare (82.5 sq. km) of landfill area during the period of 15 years time. Research limitations/implications The population growth and future per capita waste generation rate are based on census report of Govt of India and survey report of World Bank, respectively. Separate collection of bio-degradable portion of MSW has not yet been developed properly in India. Practical implications This study reveals that in India, the high-moisture-content, low-calorific-value bio-degradable waste in India can be used for the generation of substantial amount of green energy in India, which in addition to financial gains would reduce the waste quantity at landfill site, conserve natural resources, save land, reduce green house gas emission, generate employment and help to protect environment. Considering these benefits and advantages, evaluated in this study, policy makers and city managers may review their approaches toward solid waste management system of their cities to meet the challenges of huge increase of MSW in the years to come in India. More research works may be initiated to improve the AD system of organic waste, and more capital may be employed in waste management business in India. Originality/value Numbers of research works have been carried out by other researchers for estimation of energy generation potential through AD of OFMSW for different countries; but no such work could be found to identify such potential and its benefits in India. This research work demonstrates how MSW can be used as a wealth for green energy production in India. The originality of this paper is the analysis of green energy generation potential from the low calorific value MSW in India.

Comparing the anaerobic digestion of Ascophyllum nodosum, the organic fraction of municipal solid waste and white rice in batch reactors

The biogas potential of marine macroalga Ascophyllum nodosum was compared with the organic fraction of municipal solid waste (OFMSW) and white rice to determine the applicability of the feedstock for anaerobic digestion. For OFMSW three dry matter contents were compared, 3%, 10% and 25%, to determine the effect of dry matter on methane yield. Biogas was evolved in each system, but the rate of evolution of biogas changed with moisture content. The highest total methane yield was obtained from 3% OFMSW but A. nodosum yielded more methane at 176±37.62 L/kg VS than white rice and the drier anaerobic digestion of OFMSW. The substrates were digested using wastewater treatment plant inoculum to determine gas yield and gas quality under batch mesophilic digestion conditions.

Startup and Stability of Thermophilic Anaerobic Digestion of OFMSW

Anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW) is promoted as an energy source and more recently as a greenhouse gas (GHG) mitigation measure. In this context, AD systems operating at thermophilic temperatures (55–60°C)—compared to mesophilic temperatures (35–40°C)—have the unique feature of producing hygienic soil conditioners with greater process efficiency, higher energy yield, and more GHG savings. Startup of AD systems is often constrained by the lack of acclimated seeds, leading to process instability and failure. The authors focus on strategies to startup thermophilic digesters treating OFMSW in the absence of acclimated seeds and examines constraints associated with process stability and ways to overcome them. Relevant gaps in the literature and future research needs are delineated.

Effect of Sewage Sludge Addition on the Start-Up of the Anaerobic Digestion of OFMSW

The performances and stabilities of three laboratory-scale semi-batch anaerobic digesters, operated at 35°C, treating organic fraction of municipal solid waste (OFMSW) and primary sewage sludge (PSS) mixed at various ratios were compared during the start-up period. The OFMSW:PSS ratios in the feeds to the three digesters were 100:0, 80:20, and 60:40 on a total solids (TS) basis and their organic loading rates (OLR) were 5.2 kg VS m−3 d−1, 3.9 kg VS m−3 d−1, and 3.2 kg VS m−3 d−1, respectively. There were no indications of failure in any of the digesters. However, addition of primary sewage sludge significantly decreased the imbalances observed during the start-up and improved the process performance. The best performance in terms of specific gas production rate (SGPR) was obtained with a feedstock having an OFMSW:PSS ratio of 80:20. The use of inoculum from an operating anaerobic digester enabled a quick start-up and the reactors were capable of handling high OLRs from the beginning of the experiment.