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.     

Digestion of frozen/thawed food waste in the hybrid anaerobic solid-liquid system

The hybrid anaerobic solid-liquid (HASL) system, which is a modified two-phase anaerobic digester, is to be used in an industrial scale operation to minimize disposal of food waste at incineration plants in Singapore. The aim of the present research was to evaluate freezing/thawing of food waste as a pre-treatment for its anaerobic digestion in the HASL system. The hydrolytic and fermentation processes in the acidogenic reactor were enhanced when food waste was frozen for 24h at -20 degrees C and then thawed for 12h at 25 degrees C (experiment) in comparison with fresh food waste (control). The highest dissolved COD concentrations in the leachate from the acidogenic reactors were 16.9g/l on day 3 in the control and 18.9g/l on day 1 in the experiment. The highest VFA concentrations in the leachate from the acidogenic reactors were 11.7g/l on day 3 in the control and 17.0g/l on day 1 in the experiment. The same volume of methane was produced during 12 days in the control and 7 days in the experiment. It gave the opportunity to diminish operational time of batch process by 42%. The effect of freezing/thawing of food waste as pre-treatment for its anaerobic digestion in the HASL system was comparable with that of thermal pre-treatment of food waste at 150 degrees C for 1h. However, estimation of energy required either to heat the suspended food waste to 150 degrees C or to freeze the same quantity of food waste to -20 degrees C showed that freezing pre-treatment consumes about 3 times less energy than thermal pre-treatment.     

Methane production from food waste leachate in laboratory-scale simulated landfill

Due to the prohibition of food waste landfilling in Korea from 2005 and the subsequent ban on the marine disposal of organic sludge, including leachate generated from food waste recycling facilities from 2012, it is urgent to develop an innovative and sustainable disposal strategy that is eco-friendly, yet economically beneficial. In this study, methane production from food waste leachate (FWL) in landfill sites with landfill gas recovery facilities was evaluated in simulated landfill reactors (lysimeters) for a period of 90 d with four different inoculum–substrate ratios (ISRs) on volatile solid (VS) basis. Simultaneous biochemical methane potential batch experiments were also conducted at the same ISRs for 30 d to compare CH₄ yield obtained from lysimeter studies. Under the experimental conditions, a maximum CH₄ yield of 0.272 and 0.294 L/g VS was obtained in the batch and lysimeter studies, respectively, at ISR of 1:1. The biodegradability of FWL in batch and lysimeter experiments at ISR of 1:1 was 64% and 69%, respectively. The calculated data using the modified Gompertz equation for the cumulative CH₄ production showed good agreement with the experimental result obtained from lysimeter study. Based on the results obtained from this study, field-scale pilot test is required to re-evaluate the existing sanitary landfills with efficient leachate collection and gas recovery facilities as engineered bioreactors to treat non-hazardous liquid organic wastes for energy recovery with optimum utilization of facilities.     

Digestion of waste bananas to generate energy in Australia

This paper presents results from laboratory studies to measure the methane yield and rate of digestion of reject bananas. These parameters were determined in experiments that took into account the likely configuration of a full-scale plant in the banana growing region of north Queensland. The digestion was conducted in a 200-l reactor using fed-batch operation, relying entirely on the natural microbial consortia on the reject bananas to avoid reliance on external inocula such as sludge, an undesirable material around food packaging facilities. An enrichment culture was first established in a highly buffered 200-l batch digestion unit. The fed-batch digester was then started by exchanging leachate with the mature batch reactor. Under loading conditions of 0.6 kg VS m(-3)d(-1) over 70 days where the average working volume was 160 l, the digester produced 398+/-20 l CH4 kg VS(-1). Increasing the loading rate to 1.6 kg VS m(-3)d(-1) resulted in a reduced methane yield of 210 l CH4 kg VS(-1) over 23 days of operation, with a concomitant accumulation of banana waste in the digester. The leachate at the end of digestion contained over 4000 mg l(-1)K, 200 mg l(-1) N and 75 mg l(-1), levels that exceed acceptable limits for general agricultural irrigation.     

Anaerobic co-digestion of food waste and landfill leachate in single-phase batch reactors

In order to investigate the effect of raw leachate on anaerobic digestion of food waste, co-digestions of food waste with raw leachate were carried out. A series of single-phase batch mesophilic (35 ± 1 °C) anaerobic digestions were performed at a food waste concentration of 41.8 g VS/L. The results showed that inhibition of biogas production by volatile fatty acids (VFA) occurred without raw leachate addition. A certain amount of raw leachate in the reactors effectively relieved acidic inhibition caused by VFA accumulation, and the system maintained stable with methane yield of 369–466 mL/g VS. Total ammonia nitrogen introduced into the digestion systems with initial 2000–3000 mgNH₄–N/L not only replenished nitrogen for bacterial growth, but also formed a buffer system with VFA to maintain a delicate biochemical balance between the acidogenic and methanogenic microorganisms. UV spectroscopy and fluorescence excitation–emission matrix spectroscopy data showed that food waste was completely degraded.We concluded that using raw leachate for supplement water addition and pH modifier on anaerobic digestion of food waste was effective. An appropriate fraction of leachate could stimulate methanogenic activity and enhance biogas production.     

Co-digestion of cattle manure with food waste and sludge to increase biogas production

Anaerobic co-digestion strategies are needed to enhance biogas production, especially when treating certain residues such as cattle/pig manure. This paper presents a study of co-digestion of cattle manure with food waste and sewage sludge. With the aim of maximising biogas yields, a series of experiments were carried out under mesophilic and thermophilic conditions using continuously stirred-tank reactors, operating at different hydraulic residence times. Pretreatment with ultrasound was also applied to compare the results with those obtained with non-pretreated waste. Specific methane production decreases when increasing the OLR and decreasing HRT. The maximum value obtained was 603 LCH(4)/kg VS(feed) for the co-digestion of a mixture of 70% manure, 20% food waste and 10% sewage sludge (total solid concentration around 4%) at 36°C, for an OLR of 1.2g VS/Lday. Increasing the OLR to 1.5g VS/Lday led to a decrease of around 20-28% in SMP. Lower methane yields were obtained when operating at 55°C. The increase in methane production when applying ultrasound to the feed mixtures does not compensate for the energy spent in this pretreatment.     

Effect of leachate recirculation on mesophilic anaerobic digestion of food waste

The effects of using untreated leachate for supplemental water addition and liquid recirculation on anaerobic digestion of food waste was evaluated by combining cyclic water recycle operations with batch mesophilic biochemical methane potential (BMP) assays. Cyclic BMP assays indicated that using an appropriate fraction of recycled leachate and fresh make up water can stimulate methanogenic activity and enhance biogas production. Conversely increasing the percentage of recycled leachate in the make up water eventually causes methanogenic inhibition and decrease in the rate of food waste stabilization. The decrease in activity is exacerbated as the number cycles increases. Inhibition is possibly attributed to accumulation and elevated concentrations of ammonia as well as other waste by products in the recycled leachate that inhibit methanogenesis.     

Potential methane yield of food waste/food waste leachate from the biogasification facilities in South Korea

This study was to find out potential of methane yield on food waste and food waste leachate as biomass in Korea. The seven biogasification facilities were selected for comparison of theoretical methane yield and actual methane yield. The theoretical methane yield was calculated based on organic constituents (carbohydrate, protein, fat) and based on element analysis. The actual methane yield was investigated based on volatile solids and CODcr. Theoretical methane yields by organic constituents were 0.52 Sm³CH₄/kg VS and 0.35 Sm³CH₄/kg CODcr and these by element analysis were 0.53 Sm³CH₄/kg VS and 0.36 Sm³CH₄/kg CODcr. Actual methane yields were 0.36 Sm³CH₄/kg VS_in and 0.26 Sm³CH₄/kg CODcr_in. Considering the average removal efficiency of organic materials of seven FWL biogasification facilities, actual methane yields were 0.48 Sm³CH₄/kg VS_rem and 0.33 Sm³CH₄/kg CODcr_rem. Methane yield by organic constituents is very similar to that by element analysis and actual methane yields of volatile solids and CODcr were similar to theoretical value. The actual methane yield in this study showed approximated boundary values with previous other references which conducted in lab-scale or biochemical methane potential (BMP) tests. In conclusion, Korean food waste and food waste leachate have sufficient potential of methane yield in the ongoing biogasification facilities.     

Microbial diversity and dynamics during methane production from municipal solid waste

The objectives of this study were to characterize development of bacterial and archaeal populations during biodegradation of municipal solid waste (MSW) and to link specific methanogens to methane generation. Experiments were conducted in three 0.61-m-diameter by 0.90-m-tall laboratory reactors to simulate MSW bioreactor landfills. Pyrosequencing of 16S rRNA genes was used to characterize microbial communities in both leachate and solid waste. Microbial assemblages in effluent leachate were similar between reactors during peak methane generation. Specific groups within the Bacteroidetes and Thermatogae phyla were present in all samples and were particularly abundant during peak methane generation. Microbial communities were not similar in leachate and solid fractions assayed at the end of reactor operation; solid waste contained a more abundant bacterial community of cellulose-degrading organisms (e.g., Firmicutes). Specific methanogen populations were assessed using quantitative polymerase chain reaction. Methanomicrobiales, Methanosarcinaceae, and Methanobacteriales were the predominant methanogens in all reactors, with Methanomicrobiales consistently the most abundant. Methanogen growth phases coincided with accelerated methane production, and cumulative methane yield increased with increasing total methanogen abundance. The difference in methanogen populations and corresponding methane yield is attributed to different initial cellulose and hemicellulose contents of the MSW. Higher initial cellulose and hemicellulose contents supported growth of larger methanogen populations that resulted in higher methane yield.     

Nitrogen removal from recycled landfill leachate by ex situ nitrification and in situ denitrification

A three-compartment system, comprising a landfill column with fresh municipal solid waste, a column with a well-decomposed refuse layer as methane producer, and a sequential batch reactor as ex situ nitrifying reactor, was employed to remove nitrogen from municipal solid waste leachate. Since food waste comprised a major portion of refuse collected in Shanghai, an intense hydrolysis reaction occurred and caused the rapid accumulation of ammonia nitrogen (NH(3)-N) and total organic carbon in the leachate. This paper discusses the role of the three mentioned units and the design and operation of the proposed system. With most NH(3)-N being converted to nitrite nitrogen (NO(2)(-)-N) or nitrate nitrogen (NO(3)(-)-N) by the nitrifying reactor, and with the well-decomposed refuse layer transforming most dissolved organic compounds to CO(2), carbonates and methane, it was found that the fresh refuse column could efficiently denitrify the hydrolyzed nitrogen to N(2) gas. The role of the three mentioned units and comments on the design and operation of the proposed system are also discussed.     

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.     

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.     

METHANE POTENTIAL OF FOOD WASTE AND ANAEROBIC TOXICITY OF LEACHATE PRODUCED DURING FOOD WASTE DECOMPOSITION

The objective of this study was to characterize the anaerobic biodegradation of food waste, including its methane potential and the anaerobic toxicity of leachate associated with food waste decomposition. Biodegradation experiments were conducted in 2.2-litre reactors and were seeded with well-decomposed refuse. Despite pH neutralization, reactors seeded with 30% old refuse failed to undergo methanogenesis. Food waste in a second set of reactors, containing 70% seed, produced 300.7 ml CH₄dry g⁻¹. Leachate toxicity was evaluated by a modified anaerobic toxicity assay (ATA). The results of ATAs were typically consistent with the methane production behavior of the reactors. However, the toxicity observed in the ATA test could not be simulated with synthetic leachate containing high concentrations of carboxylic acids and sodium. Tests with 20, 5, 15 and 12 g l⁻¹of acetate, propionate, butyrate and sodium, respectively, suggested that high concentrations of butyric acid and sodium inhibited the onset of methane production but that refuse micro-organisms could acclimatize to these concentrations within 5–10 days. The refuse ecosystem was shown to tolerate higher concentrations of undissociated carboxylic acids than previously reported for anaerobic digesters.     

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.     

Improving hydrolysis of food waste in a leach bed reactor

This paper examines the rate of degradation of food waste in a leach bed reactor (LBR) under four different operating conditions. The effects of leachate recirculation at a low and high flow rate are examined with and without connection to an upflow anaerobic sludge blanket (UASB). Two dilution rates of the effective volume of the leach bed reactors were investigated: 1 and 6 dilutions per LBR per day. The increase in dilution rate from 1 to 6 improved the destruction of volatile solids without connection to the UASB. However connection to the UASB greatly improved the destruction of volatile solids (by almost 60%) at the low recirculation rate of 1 dilution per day. The increase in volatile solids destruction with connection to the UASB was attributed to an increase in leachate pH and buffering capacity provided by recirculated effluent from the UASB to the leach beds. The destruction of volatile solids for both the low and high dilution rates was similar with connection to the UASB, giving 82% and 88% volatile solids destruction respectively. This suggests that the most efficient leaching condition is 1 dilution per day with connection to the UASB.     

Effect of long chain fatty acids removal as a pretreatment on the anaerobic digestion of food waste

This study investigated the effect of long chain fatty acids (LCFAs) removal as a pretreatment prior to anaerobic digestion on the production of methane from food waste. The results showed that the anaerobic digestion of food waste containing 1.6 g COD/L of LCFAs was not inhibited (4 days lag-time, 78.3 % methane recovery in 35 days) compared to that of lipid free food waste (3 days lag time, 72.5 % methane recovery in 35 days); however, some unsaturated LCFAs, which are toxic to microorganism, were accumulated in the batch anaerobic digestion reactor. Meanwhile, in a methanogenic activity study, the activity of methanogens was observed to be linearly inhibited by the presence of more than 1 g COD/L of LCFAs. The possibility of the accumulation of unsaturated LCFAs in the reactor should be considered when operating a large-scale continuous system.     

Enhanced methane recovery by food waste leachate injection into a landfill in Korea

The current food waste leachate (FWL) disposal practice in Korea warrants urgent attention and necessary action to develop an innovative and sustainable disposal strategy, which is both environmentally friendly and economically beneficial. In this study, methane production by FWL injection into a municipal solid waste landfill with landfill gas (LFG) recovery facility was evaluated for a period of more than 4 months. With the target of recovering LFG with methane content ~50%, optimum LFG extraction rate was decided by a trial and error approach during the field investigation in five different phases. The results showed that, upon FWL injection, LFG extraction rate of ~20 m(3)/h was reasonable to recover LFG with methane content ~58%. Considering the estimated methane production potential of 31.7 m(3) CH(4) per ton of FWL, methane recovery from the landfill was enhanced by 14%. The scientific findings of this short-term investigation indicates that FWL can be injected into the existing sanitary landfills to tackle the present issue and such landfills with efficient liner and gas collection facility can be utilized as absolute and sustainable environmental infrastructures.     

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.     

Anaerobic co-digestion of coffee waste and sewage sludge

The feasibility of the anaerobic co-digestion of coffee solid waste and sewage sludge was assessed. Five different solid wastes with different chemical properties were studied in mesophilic batch assays, providing basic data on the methane production, reduction of total and volatile solids and hydrolysis rate constant. Most of the wastes had a methane yield of 0.24-0.28 m3 CH4(STP)/kg VS(initial) and 76-89% of the theoretical methane yield was achieved. Reduction of 50-73% in total solids and 75-80% in volatile solids were obtained and the hydrolysis rate constants were in the range of 0.035-0.063 d(-1). One of the solid wastes, composed of 100% barley, achieved a methane yield of 0.02 m3 CH4(STP)/kg VS(initial), reductions of 31% in total solids, 40% in volatile solids and achieved only 11% of the theoretical methane yield. However, this waste presented the highest hydrolysis rate constant. Considering all the wastes, an inverse linear correlation was obtained between methane yield and the hydrolysis rate constant, suggesting that hydrolysis was not the limiting factor in the anaerobic biodegradability of this type of waste.