Anaerobic Digesters Treating Food Waste Research Articles

Pretreatment of cottage cheese to enhance biogas production.

This study evaluated the possibility of pretreating selected solid fraction of an anaerobic digester treating food waste to lower the hydraulic retention time and increase the methane production. The study investigated the effect of different pretreatments (thermal, chemical, thermochemical and enzymatic) for enhanced methane production from cottage cheese. The most effective pretreatments were thermal and enzymatic. Highest solubilisation of COD was observed in thermal pretreatment, followed by thermochemical. In single enzyme systems, lipase at low concentration gave significantly higher methane yield than for the experiments without enzyme additions. The highest lipase dosages decreased methane yield from cottage cheese. However, in case of protease enzyme an increase in concentration of the enzyme showed higher methane yield. In the case of mixed enzyme systems, pretreatment at 1 : 2 ratio of lipase : protease showed higher methane production in comparison with 1 : 1 and 2 : 1 ratios. Methane production potentials for different pretreatments were as follows: thermal 357 mL/g VS, chemical 293 mL/g VS, and thermochemical 441 mL/g VS. The average methane yield from single enzyme systems was 335 mL/g VS for lipase and 328 mL/g VS for protease. Methane potentials for mixed enzyme ratios were 330, 360, and 339 mL/g VS for 1 : 1, 1 : 2, and 2 : 1 lipase : protease, respectively.

Biological nitrate removal using waste-derived extracts as sole carbon source

Waste-derived multicomponent extracts were investigated as carbon source for biological nitrate removal. Aerobic and anaerobic sludge from a municipal wastewater treatment plant and leachate from an anaerobic digester treating food waste were studied. Alkaline-heat treatment was used for sludge solubilisation. More than 90% carbon was recovered at pH 10–11. The extracts contain very high volatile fatty acids (VFAs), protein and carbohydrate. The protein/carbohydrate ratio was 2/1 for both aerobic and anaerobic sludge extracts and 9/1 for leachate. Of the total chemical oxygen demand (COD), 60% was solubilised in both the aerobic and anaerobic sludge extracts, while 80% was solubilised in the leachate. The extracts were successfully tested in batch experiments for different time intervals as a sole carbon source for biological nitrate removal in synthetic wastewater. More than 95% nitrate removal was achieved in 24 h batch studies at carbon-to-nitrate (C/N) ratio of 8 and at 35oC. The nitrate removal performance was comparable to that obtained with glucose as the external carbon source. Further, the nitrate removal efficiency of all the three extracts is similar. Thus, the result of this study shows the potential of these waste derived materials for the low-cost treatment of nitrate rich wastewaters characterised by low C/N ratio that are generated from many industries.

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 387L-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 0.27L CH4/g VSadded 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 258L-air/kg TS/d is recommended for acidogenic LBR treating food waste.

A pilot scale two-stage anaerobic digester treating food waste leachate (FWL): Performance and microbial structure analysis using pyrosequencing

Food waste leachate (FWL) from the food waste recycling facilities in Korea is a serious environmental problem. Much research was done on anaerobic digestion of FWL in a lab-scale; however, there is little information on a large scale anaerobic digestion system (ADS). In this study, a two-phase ADS in a pilot scale was operated using FWL and the ADS performance and microbial structure dynamics using pyrosequencing were investigated. The ADS was operated for 136 days using FWL containing a high concentration of volatile fatty acid (12,435±2203mg/L), exhibiting volatile acid (VS) removal efficiency of 74–89% and CH4 yield of 0.39–0.85Nm3/kg of reduced VS. The microbial structure at 76, 101, and 132 days indicated the methanogen population shift from acetoclastic methanogens (Methanosarcina and Methanosaeta) to hydrogenotrophic methanogens (Methanobacterium and Methanoculleus). The bacterial community also shifted to the taxa syntrophically related with hydrogenotrophic methanogens (Clostridia). The statistical analysis revealed the positive correlation of VS removal efficiency with Methanosarcina, but the negative correlation with Methanobacterium. The results presented here suggest that acetoclastic methanogens and their associated bacteria were more efficient for VS removal in the pilot scale ADS system, providing useful information for FWL treatment in a large scale ADS.

Field performance of small scale anaerobic digesters treating food waste

Internationally, it has been reported to have over 30 million small scale anaerobic digesters. Despite the large number of existing small scale anaerobic digesters and a growing interest in food waste digestion, there has been a paucity of research regarding low cost digester's performance. This paper aims to provide a snap shot of the performances of small scale digesters treating food waste exclusively. The performance of six small scale digesters, ranging from 1m3 to 25m3 were recorded and analysed. Results showed consistently high substrate degradation ranging from 92.5% to 97.5% COD with daily methane production ranging between 0.25m3/kg VS and 0.46m3/kg VS. Composition of biogas analyzed yielded between 63.1%–66.8% CH4 and between 27.4%–33.3% CO2. Results also revealed a negative linear correlation between gas production and % total solids and a positive linear correlation between gas production and the organic loading rate. While small scale digesters can effectively degrade food waste; the total dependency on digester for their gas requirements is unlikely and alternative fuel source is required.

Biochemical methane potential (BMP) of food waste and primary sludge: Influence of inoculum pre-incubation and inoculum source

Biochemical methane potential tests were conducted to evaluate the effect of using a blank versus a pre-incubated inoculum in digestion of primary sludge at different waste to inoculum ratios (S/X). In addition, this study explored the influence of using two different anaerobic inoculum sources on the digestion of food waste: digested sludge from a municipal wastewater treatment plant and from a digester treating the organic fraction of municipal solid wastes. The results revealed that although there was no significant difference in methane yield (on average 114mLCH4/g TCODsub) or biodegradability (on average 28.3%) of primary sludge using pre-incubated or non-incubated inocula, the maximum methane production rates using non-incubated inoculum were higher than those using pre-incubated inoculum at all S/X ratios. Moreover, interestingly the inoculum from an anaerobic digester treating municipal wastewater sludge was superior over the inoculum from anaerobic digester treating food waste in digesting food waste.

Trace element requirements for stable food waste digestion at elevated ammonia concentrations

The work investigated why anaerobic digesters treating food waste and operating at high ammonia concentrations suffer from propionic acid accumulation which may result in process failure. The results showed deficiency of selenium, essential for both propionate oxidation and syntrophic hydrogenotrophic methanogenesis, leads to this while supplementation allows operation at substantially higher organic loading rates (OLR). At high loadings cobalt also becomes limiting, due to its role either in acetate oxidation in a reverse Wood-Ljungdahl or in hydrogenotrophic methanogenesis. Population structure analysis using fluorescent in situ hybridization showed only hydrogenotrophic methanogens. Critical Se and Co concentrations were established as 0.16 and 0.22mgkg−1 fresh matter feed at moderate loading. At this dosage the OLR could be raised to 5gVSl−1day−1 giving specific and volumetric biogas productions of 0.75m3kg−1VSadded and 3.75STPm3m−3day−1, representing a significant increase in process performance and operational stability.

Performance and Microbial Analysis of Anaerobic Digesters Treating Food Waste and Dairy Manure

The stability and microbial population of completely mixed anaerobic digesters were evaluated during the treatment of manure, food waste, and their mixtures at a temperature of 35C 2C and a hydraulic retention time of 20 days. Two mixtures were tested; the first mixture composed of 32% and 68% and the second of 48% and 52% food waste and dairy manure, respectively, based on volatile solids (VS). The digesters treating manure and the two mixtures showed stable performance at an organic loading rate (OLR) of 4 g VS L-1 day-1. However, the digester treating food waste was not stable at this OLR nor at the reduced OLR of 2 g VS L-1 day-1, as indicated by high volatile fatty acid concentrations and low pH in the digester and low biogas producion rate. The results of this study show that the co-digestion of food waste and dairy manure significantly increases the biogas production rate as compared to digestion of dairy manure alone. The measured biogas production rates were 1.26, 1.91, and 2.02 L L-1 day-1 for manure, first mixture, and second mixture, respectively, at OLR of 4 g VS L-1 day-1. Thus, the addition of food waste into dairy digesters is a feasible method for improving the economics of dairy farm digesters. Microbial analysis of the digester contents revealed that the food waste digester contained statistically greater numbers of clones related to the phyla Thermotogae and Actinobacteria, and the manure digester contained greater amounts of the phyla Firmicutes, Bacteriodetes, and Spirochetes. The archaeal population structure had little difference in the digesters fed with different substrates and was composed of hydrogenotrophic, acetotrophyic, and methylotrophic methanogens.