Effect of operational pH on biohydrogen production from food waste using anaerobic batch reactors.

This study was performed to investigate the influence of operational pH on dark H(2) fermentation of food waste by employing anaerobic batch reactors. The highest maximum H(2) yield was 1.63 mol H(2)/mol hexoseadded at operational pH 5.3, whereas the lowest maximum H(2) yield was 0.88 mol H(2)/mol hexoseadded at operational pH 7.0. With decreasing operational pH values, the n-butyrate concentration tended to increase and the acetate concentration tended to decrease. The highest hydrogen conversion efficiency of 11.3% was obtained at operational pH 5.3, which was higher than that (8.3%) reported by a previous study (Kim et al. (2011) 'Effect of initial pH independent of operational pH on hydrogen fermentation of food waste', Bioresource Technology 102 (18), 8646-8652). The new result indicates that the dark fermentation of food waste was stable and efficient in this study. Fluorescence in situ hybridization (FISH) analysis showed that Clostridium species Cluster I accounted for 84.7 and 13.3% of total bacteria at operational pH 5.3 and pH 7.0, respectively, after 48 h operation.

Effect of granular porous media on the composting of swine manure.

This study investigated the feasibility of a bulking agent of granular porous media (GPM) for the composting of swine manure. Two lab-scale composting reactors were operated to evaluate the general performances and maturity parameters using GPM made of wastes from the Portland cement manufacturing processes as an alternative bulking agent. The overall volatile solid (VS) removal was 38.5% (dry basis). During the experiments, moisture content ranged between 41% and 53%, ensuring feasibility of microbial activity in composting. Cured compost showed proper maturity and low phytotoxicity, despite the slight decreases of CO2 production and VS removal at the second batch operation. Various physico-chemical parameters of the cured compost met the regulatory standards reported elsewhere. The pH, carbon-to-nitrogen ratio, ammonia nitrogen and soluble organic carbon (SOC) of the cured compost were significantly correlated to the germination index (GI) using the seeds of Chinese cabbage and lettuce, indicating the progressive biodegradation of phytotoxins as well as organic matter. Consequently, the results obtained in this study demonstrate that GPM could contribute to the environmentally friendly and economical composting of problematic swine manure as a recyclable bulking agent.

Enhancement of methane production in anaerobic digestion of sewage sludge by thermal hydrolysis pretreatment.

This study was performed to optimize thermal hydrolysis pretreatment (THP) of sewage sludge for enhanced anaerobic digestion (AD). Using the response surface methodology (RSM), the optimal conditions were found 180 °C of reaction temperature and 76 min of reaction time. Through THP under optimal conditions, high molecular substances in sewage sludge such as soluble microbial by-products (SMPs) and extracellular polymeric substances (EPSs) were hydrolyzed into low molecular ones without the generation of refractory compounds. The microbial community analysis revealed that relative abundances of Methanomicrobia such as Methanosarcina, Methanosaeta (acetoclastic methanogens), and Methanoculleus (hydrogenotrophic methanogens) in AD with THP were higher than those in conventional AD.

Sludge characteristics in anaerobic SBR system producing hydrogen gas.

In this study, sludge characteristics of anaerobic sequencing batch reactor (ASBR) were investigated to improve and optimize the efficiency of the system converting starch into biohydrogen. The effect of stratification in settling phase on H2-producing ASBR, which results in settleable and non-settleable sludge, was observed using a batch experiment. It was concluded that specific H2 activity of decanting non-settleable sludge was higher than that of settleable sludge, which may be the reason of low yield in H2-producing ASBR. In addition, effect of settling time on settleable sludge, which is another key operational parameter, was also analysed using another set of batch experiment. Settling time of the sludge was found to be an important parameter in H2-producing ASBR. Specific H2 activity varied inversely with the duration for which settleable microorganisms were contained in settling phase. Microbial species, responsible for H2 activity in each condition, were identified using polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) analysis.

Response surface optimization of substrates for thermophilic anaerobic codigestion of sewage sludge and food waste.

This study investigated the effects of food waste constituents on thermophilic (55 degrees C) anaerobic codigestion of sewage sludge and food waste by using statistical techniques based on biochemical methane potential tests. Various combinations of grain, vegetable, and meat as cosubstrate were tested, and then the data of methane potential (MP), methane production rate (MPR), and first-order kinetic constant of hydrolysis (kH) were collected for further analyses. Response surface methodology by the Box-Behnken design can verify the effects and their interactions of three variables on responses efficiently. MP was mainly affected by grain, whereas MPR and kH were affected by both vegetable and meat. Estimated polynomial regression models can properly explain the variability of experimental data with a high-adjusted R2 of 0.727, 0.836, and 0.915, respectively. By applying a series of optimization techniques, it was possible to find the proper criteria of cosubstrate. The optimal cosubstrate region was suggested based on overlay contours of overall mean responses. With the desirability contour plots, it was found that optimal conditions of cosubstrate for the maximum MPR (56.6 mL of CH4/g of chemical oxygen demand [COD]/day) were 0.71 g of COD/L of grain, 0.18 g of COD/L of vegetable, and 0.38 g of COD/L of meat by the simultaneous consideration of MP, MPR, and kH. Within the range of each factor examined, the corresponding optimal ratio of sewage sludge to cosubstrate was 71:29 as the COD basis. Elaborate discussions could yield practical operational strategies for the enhanced thermophilic anaerobic codigestion of sewage sludge and food waste.

Performance of an innovative two-stage process converting food waste to hydrogen and methane.

This study was conducted to evaluate the performance of an innovative two-stage process, BIOCELL, that was developed to produce hydrogen (H2) and methane (CH4) from food waste on the basis of phase separation, reactor rotation mode, and sequential batch technique. The BIOCELL process consisted of four leaching-bed reactors for H2 recovery and post-treatment and a UASB reactor for CH4 recovery. The leaching-bed reactors were operated in a rotation mode with a 2-day interval between degradation stages. The sequential batch technique was useful to optimize environmental conditions during H2 fermentation. The BIOCELL process demonstrated that, at the high volatile solids (VS) loading rate of 11.9 kg/m3 x day, it could remove 72.5% of VS and convert VS(removed) to H2 (28.2%) and CH4 (69.9%) on a chemical oxygen demand (COD) basis in 8 days. H2 gas production rate was 3.63 m3/m3 x day, while CH4 gas production rate was 1.75 m3/m3 x day. The yield values of H2 and CH4 were 0.31 and 0.21 m3/kg VS(added), respectively. Moreover, the output from the post-treatment could be used as a soil amendment. The BIOCELL process proved to be stable, reliable, and effective in resource recovery as well as waste stabilization.

Enhanced acidogenic fermentation of food waste in a continuous-flow reactor.

This study was performed to improve acidogenic fermentation of food waste in a continuous-flow reactor. The fermentation of food waste is affected by the fermentation constraints such as the biodegradability of substrate, the degrading capability of microorganisms and the environmental conditions. The key factors were, therefore, examined to control the fermentation constraints, such as the effect of seed inoculation and the effect of adjusting dilution rate. Acidogenic fermentation of food waste employing rumen microorganisms resulted in the enhanced efficiency (71.2%) as compared with that (59.8%) employing mesophilic acidogens. In addition, the fermentation efficiency increased from 71.2 to 82.0% by adjusting dilution rate from 3.0 to 1.0 d(-1) depending on the state of the fermentation. The main component of the acidified product was shifted from butyric to acetic acid. This meant that the increase of the fermentation efficiency was mainly caused by the enhanced degradation of vegetables and meats. The control of the fermentation constraints was, therefore, very effective in improving the fermentation efficiency of food waste.

Kinetics of LCFA inhibition on acetoclastic methanogenesis, propionate degradation and beta-oxidation.

Kinetics of long-chain fatty acids (LCFAs) inhibition on acetoclastic methanogenesis, propionate degradation and beta-oxidation were studied with granular sludge under mesophilic batch conditions. Mathematical expressions used for reaction rates were as shown below: [formula: see text]. The simulated results revealed that the methane production rates from acetate decreased with an increase in both concentration and the number of double bonds of LCFAs. The concentrations of oleate (C18:1), linoleate (C18:2), palmitate (C16:0), and stearate (C18:0) were 0.54 mM, 0.11 mM 1.62 mM, and 2.58 mM, respectively, at which the methane production rates from acetate dropped 10%, and 3.10 mM, 0.72 mM, 5.71 mM, and 5.37 mM, respectively, at which the rates dropped 50%. The inhibitory effects of LCFAs on propionate degradation showed a similar tendency with acetoclastic methanogenesis; however, were less severe. The concentrations of oleate, linoleate, palmitate, and stearate were 1.02 mM, 0.18 mM, 2.34 mM, and 1.92 mM, respectively, at which the propionate degradation rates dropped 10%, and 4.38 mM, 1.17 mM, 5.88 mM, and 5.18 mM, respectively, at which the rates dropped 50%. The observed maximum beta-oxidation rates of oleate, linoleate, palmitate, and stearate were 0.21 mmol (gVSS)(-1) d(-1), 0.09 mmol (gVSS)(-1) d(-1), 0.12 mmol (gVSS)(-1) d(-1), and 0.08 mmol (gVSS)(-1) d(-1), respectively. The lag-phase times in beta-oxidation were also dependent on LCFA concentrations. The concentrations of oleate, linoleate, palmitate, and stearate, at which the lag-phase times became 5 days, were 5.93 mM, 2.24 mM, 4.02 mM, and 2.81 mM, respectively.

The optimisation of food waste addition as a co-substrate in anaerobic digestion of sewage sludge.

Food waste has been regarded as the main source of various environmental pollution in Korea due to the high volatile solids (VS) and moisture content caused by the features of dietary habits. The feasibility of food waste as a co-substrate in anaerobic digestion of sewage sludge was investigated in mesophilic and thermophilic conditions using batch tests. Cumulative methane production, dissolved organic carbon (DOC) and volatile fatty acids (VFA) were monitored to find the optimal mixing ratios of food waste and sewage sludge for the enhanced performance of co-digestion. It was observed that adequately mixed food waste led to the enhanced methane production both at mesophilic and thermophilic conditions. However, a conventional linear regression conducted for the optimisation of co-substrate mixing ratios was not accurate in describing exact methane production trends of co-digestion because of the different biodegradability of substrates. Therefore, a remodified Gompertz equation showing nonlinear relationship between variables was developed to find exact information with the same experimental data obtained at 2g VS/l generally used in biochemical methane potential (BMP) tests. Based on an influential parameter, methane production rate (MPR), the optimal mixing ratios of food waste were 39.3% and 50.1% in mesophilic and thermophilic conditions, respectively. To confirm the application of the remodified Gompertz equation, secondary batch tests were conducted with the substrate concentrations of 1-4g VS/l. In overall range tested, the confident mixing ratios of food waste was adjusted to 30-40% and 40% in mesophilic and thermophilic conditions, respectively. The most significant factor for enhanced performance was the improved organic carbon content provided by additional food waste.