Influence of composition on the biomethanation potential of restaurant waste at mesophilic temperatures

A synthetic waste was used to study the effect of waste composition on anaerobic degradation of restaurant waste. It was made by blending melted pork lard, white cabbage, chicken breast, and potato flakes, to simulate lipids, cellulose, protein, and carbohydrates, respectively. Four blends of the four constituents with an excess of each component were assayed and compared with a fifth blend containing an equal amount of chemical oxygen demand (COD) of each of the four components. The methane production and the time course of soluble COD and volatile fatty acids were assessed in batch assays. A high reduction of volatile solids (between 94% and 99.6%) was obtained in all the assays. The methane yield was between 0.40 m(3) CH(4)/kg VS(initial) (excess of carbohydrates) and 0.49 m(3) CH(4)/kg VS(initial) (excess of lipids). The degradation of the lipid-rich assays differed from the others. Fifty percent of the biochemical methane potential was obtained after 3-6 days for all of the assays, except for the one with excess of lipids which achieved 50% methanation only after 14.7 days of incubation. In the assay with excess of lipids, a considerable fraction of COD remained in the liquid phase, suggesting an inhibition of the methanogenic process that was likely due to the accumulation of long chain fatty acids. The hydrolysis rate constants, assuming first order kinetics, over the first 6 days were between 0.12d(-1) (excess of lipids) and 0.32 d(-1) (excess of carbohydrates). The results indicate that anaerobic digestion facilities with large variations in lipid input could have significant changes in process performance that merit further examination.     

Optimization of anaerobic digestion of municipal solid waste in combined process and sequential staging.

The optimization of anaerobic digestion aims to maximize organic waste stabilization after a short digestion period. This paper presents the optimization performance of the combined anaerobic digestion and sequential staging concept in a thermophilic, solid-state batch system as a treatment technology prior to landfill. The former involves enhanced pre-stage flushing with the addition of microaeration and inoculum in the methane phase. The latter involves leachate cross-recirculation between the mature and fresh waste reactors without conducting a pre-stage operation. The optimized process for combined anaerobic digestion showed that reducing the pre-stage operation with the maximum removal of organics from the waste bed is beneficial. Moreover, the sequential staging concept offers an improved process over the combined anaerobic digestion wherein the specific methane yield of 11.9 and 7.2 L CH4 kg(-1) volatile solids (VS) per day was achieved, respectively. After 28 days of operation, the sequential staging process showed an improved waste stabilization with 86 and 79% mass and volume reduction, respectively. A higher methane yield of 334 L CH4 kg(-1) VS with 86% VS reduction, which is equivalent to 84% process efficiency was obtained.     

Long-term anaerobic digestion of food waste stabilized by trace elements

The purpose of this study was to examine if long-term anaerobic digestion of food waste in a semi-continuous single-stage reactor could be stabilized by supplementing trace elements. Contrary to the failure of anaerobic digestion of food waste alone, stable anaerobic digestion of food waste was achieved for 368 days by supplementing trace elements. Under the conditions of OLR (organic loading rates) of 2.19-6.64 g VS (volatile solid)/L day and 20-30 days of HRT (hydraulic retention time), a high methane yield (352-450 mL CH(4)/g VS(added)) was obtained, and no significant accumulation of volatile fatty acids was observed. The subsequent investigation on effects of individual trace elements (Co, Fe, Mo and Ni) showed that iron was essential for maintaining stable methane production. These results proved that the food waste used in this study was deficient in trace elements.     

Methane production potential of leachate generated from Korean food waste recycling facilities: a lab-scale study

This paper examines the applicability of food waste leachate (FWL) in bioreactor landfills or anaerobic digesters to produce methane as a sustainable solution to the persisting leachate management problem in Korea. Taking into account the climatic conditions in Korea and FWL characteristics, the effect of key parameters, viz., temperature, alkalinity and salinity on methane yield was investigated. The monthly average moisture content and the ratio of volatile solids to total solids of the FWL were found to be 84% and 91%, respectively. The biochemical methane potential experiment under standard digestion conditions showed the methane yield of FWL to be 358 and 478 ml/g VS after 10 and 28 days of digestion, respectively, with an average methane content of 70%. Elemental analysis showed the chemical composition of FWL to be C(13.02)H(23.01)O(5.93)N(1). The highest methane yield of 403 ml/g VS was obtained at 35 degrees C due to the adaptation of seed microorganisms to mesophilic atmosphere, while methane yields at 25, 45 and 55 degrees C were 370, 351 and 275 ml/g VS, respectively, at the end of 20 days. Addition of alkalinity had a favorable effect on the methane yield. Dilution of FWL with salinity of 2g/l NaCl resulted in 561 ml CH(4)/g VS at the end of 30 days. Considering its high biodegradability (82.6%) and methane production potential, anaerobic digestion of FWL in bioreactor landfills or anaerobic digesters with a preferred control of alkalinity and salinity can be considered as a sustainable solution to the present emergent problem.     

Enhanced methane production from ultrasound pre-treated and hygienized dairy cattle slurry

The effect of hygienization (70 °C, 60 min) and ultrasound (6000 ± 500 kJ/kg total solids (TS)) pre-treatments on hydrolysis and biological methane (CH(4)) potential (BMP) of dairy cattle slurry was studied. The BMP of the untreated slurry (control) was 210 ± 10 Nm(3) CH(4)/ton volatile solids (VS) added; after ultrasound pre-treatment it was 250 ± 10 Nm(3) CH(4)/ton VS(added) and after hygienization 280 ± 20 Nm(3) CH(4)/ton VS(added). The specific methanogenic activity (SMA) of the inoculum increased from 22 (untreated) to 26 (ultrasound treated) and up to 28 N ml CH(4)/g VS d, after hygienization. However, only hygienization achieved a positive net energy balance. Both pre-treatments increased the VS-based hydrolysis of slurry (10-96%), soluble nitrogen (N(sol)) content in digestates (20 ± 5%) and biodegradability of the slurry (8 ± 3%) as estimated via elevated VS removal.     

Methane yield in source-sorted organic fraction of municipal solid waste

Treating the source-separated organic fraction of municipal solid waste (SS-OFMSW) by anaerobic digestion is considered by many municipalities in Europe as an environmentally friendly means of treating organic waste and simultaneously producing methane gas. Methane yield can be used as a parameter for evaluation of the many different systems that exist for sorting and pre-treating waste. Methane yield from the thermophilic pilot scale digestion of 17 types of domestically SS-OFMSW originating from seven full-scale sorting systems was found. The samples were collected during 1 year using worked-out procedures tested statistically to ensure representative samples. Each waste type was identified by its origin and by pre-sorting, collection and pre-treatment methods. In addition to the pilot scale digestion, all samples were examined by chemical analyses and methane potential measurements. A VS-degradation rate of around 80% and a methane yield of 300-400Nm(3) CH(4)/ton VS(in) were achieved with a retention time of 15 days, corresponding to approximately 70% of the methane potential. The different waste samples gave minor variation in chemical composition and thus also in methane yield and methane potential. This indicates that sorting and collection systems in the present study do not significantly affect the amount of methane produced per VS treated.     

Anaerobic digestion of cattail by rumen cultures

The anaerobic digestion of aquatic plants could serve the dual roles for producing renewable energy and reducing waste. In this study, the anaerobic digestion of cattail (Typha latifolia linn), a lignocellulosic aquatic plant, by rumen microorganisms in batch cultures was investigated. At a substrate level of 12.4 g/l volatile solids (VS) and pH 6.7, maximum VS conversion of 66% was achieved within an incubation time of 125 h. However, a decrease in pH from 6.7 to 5.8 resulted in a marked reduction in VS conversion. The total volatile fatty acids (VFAs) yield was about 0.56 g/g VS digested. Acetate and propionate were the major aqueous fermentation products, while butyrate, i-butyrate and valerate were also formed in smaller quantities. Biogas that was produced was composed of carbon dioxide, methane and hydrogen. A modified Gompertz equation was developed to describe substrate consumption and product formation. The hydrolysis of insoluble components was the rate-limiting step in the anaerobic digestion of cattail.     

Continuous high-solids anaerobic co-digestion of organic solid wastes under mesophilic conditions

With increasing concerns over the limited capacity of landfills, conservation of resources, and reduction of CO(2) emissions, high-solids (dry) anaerobic digestion of organic solid waste (OSW) is attracting a great deal of attention these days. In the present work, two dry anaerobic co-digestion systems fed with different mixtures of OSW were continuously operated under mesophilic conditions. Dewatered sludge cake was used as a main seeding source. In reactor (I), which was fed with food waste (FW) and paper waste (PW), hydraulic retention time (HRT) and solid content were controlled to find the maximum treatability. At a fixed solid content of 30% total solids (TS), stable performance was maintained up to an HRT decrease to 40 d. However, the stable performance was not sustained at 30 d HRT, and hence, HRT was increased to 40 d again. In further operation, instead of decreasing HRT, solid content was increased to 40% TS, which was found to be a better option to increase the treatability. The biogas production rate (BPR), CH(4) production yield (MPY) and VS reduction achieved in this condition were 5.0m(3)/m(3)/d, 0.25 m(3) CH(4)/g COD(added), and 80%, respectively. Reactor (II) was fed with FW and livestock waste (LW), and LW content was increased during the operation. Until a 40% LW content increase, reactor (II) exhibited a stable performance. A BPR of 1.7 m(3)/m(3)/d, MPY of 0.26 m(3) CH(4)/g COD(added), and VS reduction of 72% was achieved at 40% LW content. However, when the LW content was increased to 60%, there was a significant performance drop, which was attributed to free ammonia inhibition. The performances in these two reactors were comparable to the ones achieved in the conventional wet digestion and thermophilic dry digestion processes.     

Rapid screening procedure to optimise the anaerobic codigestion of industrial biowastes and agricultural livestock wastes in Cyprus

Small-scale experimental investigations were undertaken on the anaerobic digestion (AD) and codigestion of livestock waste and industrial biowastes. A simple procedure was developed to rapidly determine the suitability of wastes for digestion. The experiment was split into two phases; initially, the seed (digested brewery waste) was replaced by the test waste over a period of 5 days. During the second phase, the test waste was incubated and monitored for methanogenesis. Dairy cattle slurry was the most efficient co-substrate which, when codigested with pig slurry in an equal ratio achieved volatile solids destruction of 32%, CH(4) production rate of 97.4 ml d(-1), maximum CH(4) content of 61.6% and total gas yield of 2229 ml after 529 h. High fat content wastes were unsuitable for AD due to low pH value and because the dominant microbial reaction was fermentation. Codigestion was investigated to overcome any inhibitions; however, dairy cattle slurry, abattoir wastewater and NaOH additions did not lead to methanogenesis. Treating these wastes by AD is feasible but without CH(4) production.     

Method for determination of methane potentials of solid organic waste

A laboratory procedure is described for measuring methane potentials of organic solid waste. Triplicate reactors with 10 grams of volatile solids were incubated at 55 degrees C with 400 ml of inoculum from a thermophilic biogas plant and the methane production was followed over a 50-day period by regular measurements of methane on a gas chromatograph. The procedure involves blanks as well as cellulose controls. Methane potentials have been measured for source-separated organic household waste and for individual waste materials. The procedure has been evaluated regarding practicality, workload, detection limit, repeatability and reproducibility as well as quality control procedures. For the source-separated organic household waste a methane potential of 495 ml CH4/g VS was found. For fat and oil a lag-phase of several days was seen. The protein sample was clearly inhibited and the maximal methane potential was therefore not achieved. For paper bags, starch and glucose 63, 84 and 94% of the theoretical methane potential was achieved respectively. A detection limit of 72.5 ml CH4/g VS was calculated from the results. This is acceptable, since the methane potential of the tested waste materials was in the range of 200-500 ml CH4/g VS. The determination of methane potentials is a biological method subject to relatively large variation due to the use of non-standardized inoculum and waste heterogeneity. Therefore, procedures for addressing repeatability and reproducibility are suggested.     

Optimization of mixing ratio of ammoniated rice straw and food waste co-digestion and impact of trace element supplementation on biogas production

The anaerobic co-digestion of biomass waste, a promising process of reusing resources, is capable of improving methane production. However, the characteristics and composition of fermenting raw material negatively influence the efficiency of methane production. Optimization experiments were systematically performed in this study through anaerobic co-digestion with urea-ammoniated rice straw (UARS) and food waste (FW) as co-substrates. Anaerobic co-digestion of UARS and FW in biogas production under mesophilic conditions (35 °C) was investigated in a 1 L enclosed triangular flask with a total organic load of 6 g volatile solids (VS)/L. The optimal mixing ratio of UARS to FW was close to 1:3, and the methane yield increasing by 8.83% compared with the sole substrate. Furthermore, based on the optimization ratio, supplementation of cobalt (Co) and nickel (Ni) on co-digestion were significantly superior to that of a single element. Additionally, kinetic analysis indicated that trace element remarkably facilitated the reaction rate of co-digestion. Noteworthy, the addition of Co, Ni, and the combination of Co and Ni achieved very significant (p < 0.01) improvement of 6.45, 8.36, and 13.65%. Meanwhile, Ni was substantially promoted the removal rate of VS, enhanced the operational stability of co-digestion and increased the methane content significantly.     

Ensilage of pineapple processing waste for methane generation

Pineapple peel wastes, which are seasonal, comprise of peels and rags. Their disposal poses a serious environmental pollution problem. Since pineapple peel is rich in cellulose, hemicellulose and other carbohydrates it was found to be a potential substrate for methane generation by anaerobic digestion. Ensilaging of pineapple peel resulted in the conversion of 55% carbohydrates into volatile fatty acids. The ensilage of pineapple processing wastes reduced the biological oxygen demand by 91%. Biogas digester fed with ensilaged pineapple peel resulted in the biogas yield of 0.67 m3/kg volatile solids (VS) added with methane content of 65% whereas fresh and dried pineapple peels gave biogas yields of 0.55% and 0.41 m3/kg VS added and methane content of 51% and 41% respectively.     

Effect of alkaline pretreatment on anaerobic digestion of solid wastes

The introduction of the anaerobic digestion for the treatment of the organic fraction of municipal solid waste (OFMSW) is currently of special interest. The main difficulty in the treatment of this waste fraction is its biotransformation, due to the complexity of organic material. Therefore, the first step must be its physical, chemical and biological pretreatment for breaking complex molecules into simple monomers, to increase solubilization of organic material and improve the efficiency of the anaerobic treatment in the second step. This paper describes chemical pretreatment based on lime addition (Ca(OH)2), in order to enhance chemical oxygen demand (COD) solubilization, followed by anaerobic digestion of the OFMSW. Laboratory-scale experiments were carried out in completely mixed reactors, 1 L capacity. Optimal conditions for COD solubilization in the first step of pretreatment were 62.0 mEq Ca(OH)2/L for 6.0 h. Under these conditions, 11.5% of the COD was solubilized. The anaerobic digestion efficiency of the OFMSW, with and without pretreatment, was evaluated. The highest methane yield under anaerobic digestion of the pretreated waste was 0.15 m3CH4/kg volatile solids (VS), 172.0% of the control. Under that condition the soluble COD and VS removal were 93.0% and 94.0%, respectively. The results have shown that chemical pretreatment with lime, followed by anaerobic digestion, provides the best results for stabilizing the OFMSW.     

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.     

A side-by-side comparison of two systems of sequencing coupled reactors for anaerobic digestion of the organic fraction of municipal solid waste.

The objective of this work was to compare the performance of two laboratory-scale, mesophilic systems aiming at the anaerobic digestion of the organic fraction of municipal solid wastes (OFMSW). The first system consisted of two coupled reactors packed with OFMSW (PBR1.1-PBR1.2) and the second system consisted of an upflow anaerobic sludge bed reactor (UASB) coupled to a packed reactor (UASB2.1-PBR2.2). For the start-up phase, both reactors PBR 1.1 and the UASB 2.1 (also called leading reactors) were inoculated with a mixture of non-anaerobic inocula and worked with leachate and effluent full recirculation, respectively. Once a full methanogenic regime was achieved in the leading reactors, their effluents were fed to the fresh-packed reactors PBR1.2 and PBR2.2, respectively. The leading PBR 1.1 reached its full methanogenic regime after 118 days (Tm, time to achieve methanogenesis) whereas the other leading UASB 2.1 reactor reached its full methanogenesis regime after only 34 days. After coupling the leading reactors to the corresponding packed reactors, it was found that both coupled anaerobic systems showed similar performances regarding the degradation of the OFMSW. Removal efficiencies of volatile solids and cellulose and the methane pseudo-yield were 85.95%, 80.88% and 0.109 NL CH4 g(-1) VS(fed) in the PBR-PBR system; and 88.75%, 82.61% and 0.115 NL CH4 g(-1) VS(fed0 in the UASB-PBR system [NL, normalized litre (273 degrees K, 1 ata basis)]. Yet, the second system UASB-PBR system showed a faster overall start-up.     

Effects of lipid concentration on anaerobic co-digestion of municipal biomass wastes

The influence of the lipid concentration on the anaerobic co-digestion of municipal biomass waste and waste-activated sludge was assessed by biochemical methane potential (BMP) tests and by bench-scale tests in a mesophilic semi-continuous stirred tank reactor. The effect of increasing the volatile solid (VS) concentration of lipid from 0% to 75% was investigated. BMP tests showed that lipids in municipal biomass waste could enhance the methane production. The results of bench-scale tests showed that a lipids concentration of 65% of total VS was the inhibition concentration. Methane yields increased with increasing lipid concentration when lipid concentrations were below 60%, but when lipid concentration was set as 65% or higher, methane yields decreased sharply. When lipid concentrations were below 60%, the pH values were in the optimum range for the growth of methanogenic bacteria and the ratios of volatile fatty acid (VFA)/alkalinity were in the range of 0.2–0.6. When lipid concentrations exceeded 65%, the pH values were below 5.2, the reactor was acidized and the values of VFA/alkalinity rose to 2.0. The amount of Brevibacterium decreased with increasing lipid content. Long chain fatty acids stacked on the methanogenic bacteria and blocked the mass transfer process, thereby inhibiting anaerobic digestion.     

Thermal and thermo-chemical pre-treatment of four waste residues and the effect on acetic acid production and methane synthesis

In this study four diverse solid waste substrates (coal, Kraft pulp solids, chicken feathers and chicken processing waste) were thermally pre-treated (70, 140 and 200 °C), under an inert (nitrogen) or oxidative (oxygen) atmosphere, and then anaerobically digested. Membrane inlet mass spectrometry during the thermal and thermo-chemical reactions was successfully used to establish oxygen and carbon dioxide gas fluxes and product formation (acetic acid). There was significant solids hydrolysis pre-treatment at 200 °C under an oxidative atmosphere, as indicated by a decrease in the volatile suspended solids and an increase in dissolved organic carbon. Greater concentrations of volatile fatty acids were produced under oxidative conditions at higher temperatures. The methane yield more than tripled for feathers after pre-treatment at 140 °C (under both atmospheres), but decreased after oxidative pre-treatment at 200 °C, due to the destruction of available carbon by the thermo-chemical reaction. Methane yield more than doubled for the Kraft pulp solids with the 200 °C pre-treatment under oxidative conditions. This study illustrated the power of wet oxidation for solids destruction and its potential to improve methane yields generated during anaerobic digestion.     

Energy recovery from grass using two-phase anaerobic digestion

Municipal solid wastes are major sources of air, water and soil contamination. There is a need for alternative waste management techniques to better utilize the waste and minimize its adverse environmental impact. A two-phase pilot-scale bio-fermentation system was used to evaluate the feasibility of producing methane from grass waste, a major constituent of solid wastes. The bi-phasic system consists of a solid phase and a methane phase. Leachate is re-circulated through the solid phase until a desired level of volatile fatty acid (VFA) is accumulated in the leachate. The leachate is then transferred to the methane reactor where the VFA is converted to methane. The results showed that 67% of the volatile solids in the waste can be converted into soluble chemical oxygen demand in a period of six months. The system produced an average of 0.15 m3 of methane per kg of grass. The average methane concentration in the produced gas was 71%. A mathematical model was developed to estimate the methane and carbon dioxide concentrations in the gas phase as a function of reactor properties.     

Solubilization and methanogenesis of a particulate industrial waste: Impact of solids loading and temperature

Effective anaerobic treatment of particulate wastes requires solubilization and acid formation prior to methanogenesis. In this case study of a particulate waste from a corn-processing industry, the influence of solids loading in solubilization, acid formation and methanogenesis was studied under mesophilic (35°C) and thermophilic (60°C) conditions. The waste was concentrated by centrifugation to initial suspended solids concentrations (TSS_i) of 150 to 350 g/L (15% to 35%). Anaerobic batch tests were conducted for 20 days, and significant solubilization of the particulate organic matter occurred in all cases. The thermophilic systems were more effective than the mesophilic systems with respect to solubilization of particulates, volatile solids destruction, acetic acid uptake, and methane generation. Methanogenesis appreared to be a rate-limiting step at higher TSS_i values, indicated by accumulation of volatile organic acids in the batch systems. Slower rates of methane production led to identification of the limiting solids loading for both temperature regimes. The results of this study can be used to evaluate the limitations of a single stage system for anaerobic treatment of organic particulate industrial wastes.     

Improved biogas production from rice straw by co-digestion with kitchen waste and pig manure

In order to investigate the effect of feedstock ratios in biogas production, anaerobic co-digestions of rice straw with kitchen waste and pig manure were carried out. A series of single-stage batch mesophilic (37 ± 1 °C) anaerobic digestions were performed at a substrate concentration of 54 g/L based on volatile solids (VS). The results showed that the optimal ratio of kitchen waste, pig manure, and rice straw was 0.4:1.6:1, for which the C/N ratio was 21.7. The methane content was 45.9–70.0% and rate of VS reduction was 55.8%. The biogas yield of 674.4 L/kg VS was higher than that of the digestion of rice straw or pig manure alone by 71.67% and 10.41%, respectively. Inhibition of biogas production by volatile fatty acids (VFA) occurred when the addition of kitchen waste was greater than 26%. The VFA analysis showed that, in the reactors that successfully produced biogas, the dominant intermediate metabolites were propionate and acetate, while they were lactic acid, acetate, and propionate in the others.