Anaerobic digestion of organic fraction of municipal solid waste combining two pretreatment modalities, high temperature microwave and hydrogen peroxide

In order to enhance anaerobic digestion (AD) of the organic fraction of municipal solid waste (OFMSW), pretreatment combining two modalities, microwave (MW) heating in presence or absence of hydrogen peroxide (H₂O₂) were investigated. The main pretreatment variables affecting the characteristics of the OFMSW were temperature (T) via MW irradiation and supplemental water additions of 20% and 30% (SWA20 and SW30). Subsequently, the focus of this study was to evaluate mesophilic batch AD performance in terms of biogas production, as well as changes in the characteristics of the OFMSW post digestion. A high MW induced temperature range (115-175 °C) was applied, using sealed vessels and a bench scale MW unit equipped with temperature and pressure controls. Biochemical methane potential (BMP) tests were conducted on the whole OFMSW as well as the liquid fractions. The whole OFMSW pretreated at 115 °C and 145 °C showed 4-7% improvement in biogas production over untreated OFMSW (control). When pretreated at 175 °C, biogas production decreased due to formation of refractory compounds, inhibiting the digestion. For the liquid fraction of OFMSW, the effect of pretreatment on the cumulative biogas production (CBP) was more pronounced for SWA20 at 145 °C, with a 26% increase in biogas production after 8 days of digestion, compared to the control. When considering the increased substrate availability in the liquid fraction after MW pretreatment, a 78% improvement in biogas production vs. the control was achieved. Combining MW and H₂O₂ modalities did not have a positive impact on OFMSW stabilization and enhanced biogas production. In general, all samples pretreated with H₂O₂ displayed a long lag phase and the CBP was usually lower than MW irradiated only samples. First order rate constant was calculated.     

Study of the VOC emissions from a municipal solid waste storage pilot-scale cell: comparison with biogases from municipal waste landfill site

The emission of volatile organic compounds (VOCs) from municipal solid waste stored in a pilot-scale cell containing 6.4 tonnes of waste (storage facility which is left open during the first period (40 days) and then closed with recirculation of leachates during a second period (100 days)) was followed by dynamic sampling on activated carbon and analysed by GC–MS after solvent extraction. This was done in order to know the VOC emissions before the installation of a methanogenesis process for the entire waste mass. The results, expressed in reference to toluene, were exploited during the whole study on all the analyzable VOCs: alcohols, ketones and esters, alkanes, benzenic and cyclic compounds, chlorinated compounds, terpene, and organic sulphides.The results of this study on the pilot-scale cell are then compared with those concerning three biogases from a municipal waste landfill: biogas (1) coming from waste cells being filled or recently closed, biogas (2) from all the waste storage cells on site, and biogas (3) which is a residual gas from old storage cells without aspiration of the gas. The analysis of the results obtained revealed: (i) a high emission of VOCs, principally alcohols, ketones and esters during the acidogenesis; (ii) a decrease in the alkane content and an increase in the terpene content were observed in the VOCs emitted during the production of methane; (iii) the production of heavier alkanes and an increase in the average number of carbon atoms per molecule of alkane with the progression of the stabilisation/maturation process were also observed.Previous studies have concentrated almost on the analysis of biogases from landfills. Our research aimed at gaining a more complete understanding of the decomposition/degradation of municipal solid waste by measuring the VOCs emitted from the very start of the landfill process i.e. during the acidogenesis and acetogenesis phases.     

Benefits of supplementing an industrial waste anaerobic digester with energy crops for increased biogas production

Currently, there is increasing competition for waste as feedstock for the growing number of biogas plants. This has led to fluctuation in feedstock supply and biogas plants being operated below maximum capacity. The feasibility of supplementing a protein/lipid-rich industrial waste (pig manure, slaughterhouse waste, food processing and poultry waste) mesophilic anaerobic digester with carbohydrate-rich energy crops (hemp, maize and triticale) was therefore studied in laboratory scale batch and continuous stirred tank reactors (CSTR) with a view to scale-up to a commercial biogas process. Co-digesting industrial waste and crops led to significant improvement in methane yield per ton of feedstock and carbon-to-nitrogen ratio as compared to digestion of the industrial waste alone. Biogas production from crops in combination with industrial waste also avoids the need for micronutrients normally required in crop digestion. The batch co-digestion methane yields were used to predict co-digestion methane yield in full scale operation. This was done based on the ratio of methane yields observed for laboratory batch and CSTR experiments compared to full scale CSTR digestion of industrial waste. The economy of crop-based biogas production is limited under Swedish conditions; therefore, adding crops to existing industrial waste digestion could be a viable alternative to ensure a constant/reliable supply of feedstock to the anaerobic digester.     

Enhancement of biogas production potential from <Emphasis Type="Italic">Acacia</Emphasis> leaf waste using alkaline pre-treatment and co-digestion

The objective of this research was to evaluate possibility of utilizing Acacia leaves (A. mangium and A. auriculiformis), which is an agro-industrial waste from the pulp and paper industry. The effects of alkaline pre-treatment and co-digestion with Napier grass for the enhancement of biogas production from Acacia leaf waste (ALW) were investigated. Six continuous stirred tank reactors with a working volume of 5 L were carried out at the laboratory scale. The results showed that pre-treatment of Acacia leaf waste (pretreated ALW) by soaking in 3 % NaOH for 48 h increased the biogas and methane productivity to 0.200 and 0.117 m³/kgVS_added compared to 0.098 and 0.048 m³/kgVS_added of raw ALW digestion, respectively. Meanwhile, the co-digestion of Acacia leaves with different proportions of Napier grass at ratios of 1:1–1:3 in volatile solid basis also increased the production of biogas and its productivity. The maximum gas production yields of 0.424 and 0.268 m³/kgVS_added for biogas and methane were obtained at 1:3 ratio. This finding affirms the potential of ALW and its possibility to use as biogas feedstock in both single and co-substrate with Napier grass.     

Anaerobic co-digestion of the organic fraction of municipal solid waste with FOG waste from a sewage treatment plant: Recovering a wasted methane potential and enhancing the biogas yield

Anaerobic digestion is applied widely to treat the source collected organic fraction of municipal solid wastes (SC-OFMSW). Lipid-rich wastes are a valuable substrate for anaerobic digestion due to their high theoretical methane potential. Nevertheless, although fat, oil and grease waste from sewage treatment plants (STP-FOGW) are commonly disposed of in landfill, European legislation is aimed at encouraging more effective forms of treatment. Co-digestion of the above wastes may enhance valorisation of STP-FOGW and lead to a higher biogas yield throughout the anaerobic digestion process. In the present study, STP-FOGW was evaluated as a co-substrate in wet anaerobic digestion of SC-OFMSW under mesophilic conditions (37 °C). Batch experiments carried out at different co-digestion ratios showed an improvement in methane production related to STP-FOGW addition. A 1:7 (VS/VS) STP-FOGW:SC-OFMSW feed ratio was selected for use in performing further lab-scale studies in a 5 L continuous reactor. Biogas yield increased from 0.38 ± 0.02 L g VS_feed^?1 to 0.55 ± 0.05 L g VS_feed^?1 as a result of adding STP-FOGW to reactor feed. Both VS reduction values and biogas methane content were maintained and inhibition produced by long chain fatty acid (LCFA) accumulation was not observed. Recovery of a currently wasted methane potential from STP-FOGW was achieved in a co-digestion process with SC-OFMSW.     

Tank-connected food waste disposer systems – Current status and potential improvements

An unconventional system for separate collection of food waste was investigated through evaluation of three full-scale systems in the city of Malmö, Sweden. Ground food waste is led to a separate settling tank where food waste sludge is collected regularly with a tank-vehicle. These tank-connected systems can be seen as a promising method for separate collection of food waste from both households and restaurants. Ground food waste collected from these systems is rich in fat and has a high methane potential when compared to food waste collected in conventional bag systems. The content of heavy metals is low. The concentrations of N-tot and P-tot in sludge collected from sedimentation tanks were on average 46.2 and 3.9 g/kg TS, equalling an estimated 0.48 and 0.05 kg N-tot and P-tot respectively per year and household connected to the food waste disposer system. Detergents in low concentrations can result in increased degradation rates and biogas production, while higher concentrations can result in temporary inhibition of methane production. Concentrations of COD and fat in effluent from full-scale tanks reached an average of 1068 mg/l and 149 mg/l respectively over the five month long evaluation period. Hydrolysis of the ground material is initiated between sludge collection occasions (30 days). Older food waste sludge increases the degradation rate and the risks of fugitive emissions of methane from tanks between collection occasions. Increased particle size decreases hydrolysis rate and could thus decrease losses of carbon and nutrients in the sewerage system, but further studies in full-scale systems are needed to confirm this.     

Enhanced hydrolysis and methane yield by applying microaeration pretreatment to the anaerobic co-digestion of brown water and food waste

Microaeration has been used conventionally for the desulphurization of biogas, and recently it was shown to be an alternative pretreatment to enhance hydrolysis of the anaerobic digestion (AD) process. Previous studies on microaeration pretreatment were limited to the study of substrates with complex organic matter, while little has been reported on its effect on substrates with higher biodegradability such as brown water and food waste. Due to the lack of consistent microaeration intensities, previous studies were not comparable and thus inconclusive in proving the effectiveness of microaeration to the overall AD process. In this study, the role of microaeration pretreatment in the anaerobic co-digestion of brown water and food waste was evaluated in batch-tests. After a 4-day pretreatment with 37.5 mL-O₂/L_R-d added to the liquid phase of the reactor, the methane production of substrates were monitored in anaerobic conditions over the next 40 days. The added oxygen was consumed fully by facultative microorganisms and a reducing environment for organic matter degradation was maintained. Other than higher COD solubilization, microaeration pretreatment led to greater VFA accumulation and the conversion of other short chain fatty acids to acetate. This could be due to enhanced activities of hydrolytic and acidogenic bacteria and the degradation of slowly biodegradable compounds under microaerobic conditions. This study also found that the nature of inoculum influenced the effects of microaeration as a 21% and 10% increase in methane yield was observed when pretreatment was applied to inoculated substrates, and substrates without inoculum, respectively.     

Optimization of micro-aeration intensity in acidogenic reactor of a two-phase anaerobic digester treating food waste

Micro-aeration is known to promote the activities of hydrolytic exo-enzymes and used as a strategy to improve the hydrolysis of particulate substrate. The effect of different micro-aeration rates, 0, 129, 258, and 387 L-air/kg TS/d (denoted as LBR-AN, LBR-6h, LBR-3h and LBR-2h, respectively) on the solubilization of food waste was evaluated at 35 °C in four leach bed reactors (LBR) coupled with methanogenic upflow anaerobic sludge blanket (UASB) reactor. Results indicate that the intensity of micro-aeration influenced the hydrolysis and methane yield. Adequate micro-aeration intensity in LBR-3h and LBR-2h significantly enhanced the carbohydrate and protein hydrolysis by 21–27% and 38–64% respectively. Due to the accelerated acidogenesis, more than 3-fold of acetic acid and butyric acid were produced in LBR-3h as compared to the anaerobic treatment LBR-AN resulting in the maximum methane yield of 0.27 L CH₄/g VS_added in the UASB. The performance of LBR-6h with inadequate aeration was similar to that of LBR-AN with a comparable hydrolysis degree. Nevertheless, higher aeration intensity in LBR-2h was also unfavorable for methane yield due to significant biomass generation and CO₂ respiration of up to 18.5% and 32.8% of the total soluble hydrolysate, respectively. To conclude, appropriate micro-aeration rate can promote the hydrolysis of solid organic waste and methane yield without undesirable carbon loss and an aeration intensity of 258 L-air/kg TS/d is recommended for acidogenic LBR treating food waste.     

Anaerobic co-digestion of municipal solid organic waste with melon residues to enhance biodegradability and biogas production

Management of solid organic waste has become a major challenge in developing countries. Raw solid organic waste can be converted into biogas through anaerobic digestion; however, the efficiency of the process is influenced by various factors including the composition of the substrate. The present study was designed with the objective of enhancing the biodegradability of the organic fraction of municipal solid waste (OFMSW) and biogas production through co-digestion of the substrate with melon residues. The study was conducted in batch mode in four phases. The results revealed that an addition of melon waste at the rate of 300?g?kg^?1 OFMSW substantially increased the biodegradation rate and biogas production compared to OFMSW alone. The removal of up to 57.2?% volatile solids and a carbon to nitrogen (C/N) ratio of 15.9 was achieved at a 60?% water level when the digestion mixture was treated with inocula collected from partially-degraded food waste. The findings of this study reveal that melon residues could be used as a potential co-substrate to enhance the biodegradability of OFMSW and biogas production.     

Evaluating the biogas potential of the dry fraction from pretreatment of food waste from households

At the waste handling company NSR, Helsingborg, Sweden, the food waste fraction of source separated municipal solid waste is pretreated to obtain a liquid fraction, which is used for biogas production, and a dry fraction, which is at present incinerated. This pretreatment and separation is performed to remove impurities, however also some of the organic material is removed. The possibility of realising the methane potential of the dry fraction through batch-wise dry anaerobic digestion was investigated. The anaerobic digestion technique used was a two-stage process consisting of a static leach bed reactor and a methane reactor. Treatment of the dry fraction alone and in a mixture with structural material was tested to investigate the effect on the porosity of the leach bed. A tracer experiment was carried out to investigate the liquid flow through the leach beds, and this method proved useful in demonstrating a more homogenous flow through the leach bed when structural material was added. Addition of structural material to the dry fraction was needed to achieve a functional digestion process. A methane yield of 98 m³/ton was obtained from the dry fraction mixed with structural material after 76 days of digestion. This was in the same range as obtained in the laboratory scale biochemical methane potential test, showing that it was possible to extract the organic content in the dry fraction in this type of dry digestion system for the production of methane.     

Need for improvements in physical pretreatment of source-separated household food waste

The aim of the present study was to investigate the efficiency in physical pretreatment processes of source-separated solid organic household waste. The investigation of seventeen Swedish full-scale pretreatment facilities, currently receiving separately collected food waste from household for subsequent anaerobic digestion, shows that problems with the quality of produced biomass and high maintenance costs are common. Four full-scale physical pretreatment plants, three using screwpress technology and one using dispergation technology, were compared in relation to resource efficiency, losses of nitrogen and potential methane production from biodegradable matter as well as the ratio of unwanted materials in produced biomass intended for wet anaerobic digestion. Refuse generated in the processes represent 13–39% of TS in incoming wet waste. The methane yield from these fractions corresponds to 14–36 Nm³/ton separately collected solid organic household waste. Also, 13–32% of N-tot in incoming food waste is found in refuse. Losses of both biodegradable material and nutrients were larger in the three facilities using screwpress technology compared to the facility using dispersion technology.¹ Thus, there are large potentials for increase of both the methane yield and nutrient recovery from separately collected solid organic household waste through increased efficiency in facilities for physical pretreatment. Improved pretreatment processes could thereby increase the overall environmental benefits from anaerobic digestion as a treatment alternative for solid organic household waste.     

Semi-continuous anaerobic co-digestion of thickened waste activated sludge and fat, oil and grease

Co-digestion of thickened waste activated sludge (TWAS) and fat, oil and grease (FOG) was conducted semi-continuously under mesophilic conditions. The results showed that daily methane yield at the steady state was 598 L/kg VS_added when TWAS and FOG (64% of total VS) were co-digested, which was 137% higher than that obtained from digestion of TWAS alone. The biogas composition was stabilized at a CH₄ and CO₂ content of 66.8% and 29.5%, respectively. Micronutrients added to co-digestion did not improve the biogas production and digestion stabilization. With a higher addition of FOG (74% of total VS), the digester initially failed but was slowly self-recovered; however, the methane yield was only about 50% of a healthy reactor with the same organic loading rate.     

Molten carbonate fuel cells fed with biogas: Combating H2S

The use of biomass and waste to produce alternative fuels, due to environmental and energy security reasons, is a high-quality solution especially when integrated with high efficiency fuel cell applications. In this article we look into the coupling of an anaerobic digestion process of organic residues to electrochemical conversion to electricity and heat through a molten carbonate fuel cell (MCFC). In particular the pathway of the exceedingly harmful compound hydrogen sulphide (H₂S) in these phases is analysed. Hydrogen sulphide production in the biogas is strongly interrelated with methane and/or hydrogen yield, as well as with operating conditions like temperature and pH. When present in the produced biogas, this compound has multiple negative effects on the performance and durability of an MCFC. Therefore, there are important issues of integration to be solved.Three general approaches to solve the sulphur problem in the MCFC are possible. The first is to prevent the formation of hydrogen sulphide at the source: favouring conditions that inhibit its production during fermentation. Secondly, to identify the sulphur tolerance levels of the fuel cell components currently in use and develop sulphur-tolerant components that show long-term electrochemical performance and corrosion stability. The third approach is to remove the generated sulphur species to very low levels before the gas enters the fuel cell.     

A life cycle approach to the management of household food waste - A Swedish full-scale case study

Environmental impacts from incineration, decentralised composting and centralised anaerobic digestion of solid organic household waste are compared using the EASEWASTE LCA-tool. The comparison is based on a full scale case study in southern Sweden and used input-data related to aspects such as source-separation behaviour, transport distances, etc. are site-specific. Results show that biological treatment methods - both anaerobic and aerobic, result in net avoidance of GHG-emissions, but give a larger contribution both to nutrient enrichment and acidification when compared to incineration. Results are to a high degree dependent on energy substitution and emissions during biological processes. It was seen that if it is assumed that produced biogas substitute electricity based on Danish coal power, this is preferable before use of biogas as car fuel. Use of biogas for Danish electricity substitution was also determined to be more beneficial compared to incineration of organic household waste. This is a result mainly of the use of plastic bags in the incineration alternative (compared to paper bags in the anaerobic) and the use of biofertiliser (digestate) from anaerobic treatment as substitution of chemical fertilisers used in an incineration alternative. Net impact related to GWP from the management chain varies from a contribution of 2.6 kg CO₂-eq/household and year if incineration is utilised, to an avoidance of 5.6 kg CO₂-eq/household and year if choosing anaerobic digestion and using produced biogas as car fuel. Impacts are often dependent on processes allocated far from the control of local decision-makers, indicating the importance of a holistic approach and extended collaboration between agents in the waste management chain.     

Anaerobic digestion of food waste: comparing leachate exchange rates in sequential batch systems digesting food waste and biosolids

Over 35,000 tonnes of food waste are generated from high concentration point sources (i.e., restaurants, hospitals and markets) in metropolitan Adelaide (Australia) each year. Anaerobic digestion is a preferred method of treatment to degrade highly putrescible waste streams such as food waste due its high methane potential. To maximise methane yield, a sequential batch anaerobic system was chosen as the most appropriate system. Two sets of sequential batch systems consisting of mature and start-up reactors in triplicate exchanged leachate. One set exchanged twice as much leachate by volume as the other set to determine the effects of different leachate exchange volumes. Results show that by increasing the leachate volume between mature and start-up reactors, the time to degrade feedstock decreases, but total methane generation yields did not markedly differ, being 229LCH4 kg(-1) VSadded and 214LCH4 kg(-1) VSadded. Process parameters used to determine when to cease leachate exchange in start-up reactors were a pH of 6.5 and methane generation rate of >0.5LCH4 kg(-1) VSadded day(-1). Changes in carbon dioxide and methane biogas composition in the mature reactors reflected process stress caused by the addition of leachate with high VFA concentrations from the start-up reactors and indicate there may be limits to leachate exchange rates and subsequent loading of mature reactors.     

Effect of inoculum to substrate ratio (I/S) on municipal solid waste anaerobic degradation kinetics and potential

The goal of this study is to evaluate the impact of the inoculum to substrate ratio (I/S) on the anaerobic degradation potential of municipal solid waste (MSW). Reconstituted MSW samples were thus incubated under batch anaerobic conditions and inoculated with an increasing amount of inoculum originating from a mesophilic sludge digester. I/S tested values were 0 (no inoculum added), 0.015, 0.03, 0.06, 0.12, 0.25, 1, 2 and 4 (gVM_inoculum/gVM_waste). The results indicate that the apparent maximal rate of dissolved organic carbon accumulation is reached at I/S = 0.12. Under this level, the hydrolysis process is limited by the concentration of biomass and can thus be described as first order kinetics phenomena with respect to biomass for I/S ratios below 0.12. The maximum methane production rate and the minimal latency are reached at a ratio of 2. In addition to that, both methane signature and ARISA show a shift in the methanogenic populations from hydrogenotrophic to acetoclastic.     

Feasibility assessment of thermophilic anaerobic digestion process of food waste

In this study, a lab-scale thermophilic anaerobic digestion of food waste collected from G-district in Seoul was performed to assess its feasibility and applicability in field-scale biogas plants. Monitoring parameters included biogas production, methane composition, pH, alkalinity, and volatile fatty acid (VFA) concentrations. Accumulation of VFA caused successive depression in pH, which inhibited microbial activity of methane-forming microorganisms. Signals of biological instability and inhibition of methanogenesis suggest possible process failure, as indicated by reduction in methane production. Results revealed that modifications in certain conditions, such as decreased organic loading rate (OLR) or additional insertion of alkalinity, must be made for its application in industrial-scale biogas plants, and that thermophilic anaerobic digestion of food waste may not be feasible without any modification.     

Leachate recirculation effects on waste degradation: study on columns

The purpose of this study is to determine the impact of leachate recirculation on the degradation of municipal solid wastes (bioreactor concept). The study was carried out using columns containing approximately 50 kg of waste, in order to follow waste degradation over a limited time. Three types of waste were studied: fresh waste of standard composition, fresh waste of fermentable composition and some 8-yr-old waste extracted from a site in France. Measurement of the global parameters, such as chemical oxygen demand (COD), volatile acidity, alkalinity, leachate conductivity, methane potential of the wastes and biogas production monitoring (volume of CO2 and CH4 produced), were carried out. The quantity of oxydizable matter and biogas production was increased by the leachate recirculation, and the duration of the first degradation phases was reduced in all cases. Chloride, ammonium and organic pollution accumulation was observed according to the duration of recirculation. After 400 days of degradation, waste stabilization seemed to be reached for all of the recirculated columns (COD<300 mg/L O2, and methane potential reached).     

Determination of biogas generation potential as a renewable energy source from supermarket wastes

Fruit, vegetable, flower waste (FVFW), dairy products waste (DPW), meat waste (MW) and sugar waste (SW) obtained from a supermarket chain were anaerobically digested, in order to recover methane as a source of renewable energy. Batch mesophilic anaerobic reactors were run at total solids (TS) ratios of 5%, 8% and 10%. The highest methane yield of 0.44 L CH₄/g VS_added was obtained from anaerobic digestion of wastes (FVFW + DPW + MW + SW) at 10% TS, with 66.4% of methane (CH₄) composition in biogas. Anaerobic digestion of mixed wastes at 5% and 8% TS provided slightly lower methane yields of 0.41 and 0.40 L CH₄/g VS_added, respectively. When the wastes were digested alone without co-substrate addition, the highest methane yield of 0.40 L CH₄/g VS_added was obtained from FVFW at 5% TS. Generally, although the volatile solids (VS) conversion percentages seemed low during the experiments, higher methane yields could be obtained from anaerobic digestion of supermarket wastes. A suitable carbon/nitrogen (C/N) ratio, proper adjustment of the buffering capacity and the addition of essential trace nutrients (such as Ni) could improve VS conversion and biogas production yields significantly.     

Zeolites relieves inhibitory stress from high concentrations of long chain fatty acids

Protein and fat rich slaughterhouse waste is a very attractive waste stream for the production of biogas because of the high biochemical methane potential of the substrate. The material has however some drawbacks as the sole material for biogas production due to the production of several process disturbing metabolites such as ammonia, sulfides and long chain fatty acids. We can in this work present results that show that zeolites have the potential to relieve inhibitory stress from the presence of long chain fatty acids. Moreover, the results strongly indicate that it is mainly acetic acid consumers that are most negatively affected by long chain fatty acids and that the mechanism of stress relief is an adsorption of long chain fatty acids to the zeolites. In addition to this, it is shown that the effect is immediate and that only a small amount of zeolites is necessary to cancel the inhibitory effect of long chain fatty acids.