Environmental Management
P.V. Dinh; T. Fujiwara; A.N. Peni; C.K. Tran
Abstract
BACKGROUND AND OBJECTIVES: Advantages such as high stability and high biogas production when recirculating the effluent stream in two-stage anaerobic digestion systems have been demonstrated on a variety of substrates, but there is limited information regarding the use of this practice on organic municipal ...
Read More
BACKGROUND AND OBJECTIVES: Advantages such as high stability and high biogas production when recirculating the effluent stream in two-stage anaerobic digestion systems have been demonstrated on a variety of substrates, but there is limited information regarding the use of this practice on organic municipal waste. Therefore, this study aimed to investigate how effluent recirculation affects the two-stage anaerobic digestion of biodegradable municipal solid waste.METHODS: Firstly, biodegradable municipal solid waste substrate was fermented under conditions of 12 percent initial total solids and a temperature of 36 degrees Celsius for 5 days. After that, the substrate continued to be diluted using tap water or the effluent stream with a rate of 2:1. In the case of using the effluent stream, the experiment was further performed with dilution rates of 3:1, 1:1, and 1:2. Then, the liquid part was collected and pumped into the methane reactor at an organic loading rate of 7.64 grams of total solids per liter per day at 36 degrees Celsius. The methane reactor was an up-flow reactor that contained both granular sludge and suspended sludge. The effectiveness of the experimental stages was evaluated through biogas production and chemical oxygen demand removal.FINDINGS: In the fermentative reactor, using the effluent stream to dilute solid-state feedstock helped keep the reactor stable at pH 5.5 without alkali addition. In the case of using tap water for dilution, it required a dose of 115.8 grams and 75.3 grams of sodium hydroxide per kilogram of volatile solids to attain pH conditions at 6.5 and 5.5, respectively. Maintaining the reactor at pH 6.5 increased the concentration of fermentation products compared to pH 5.5, including 5.9 percent total chemical oxygen demand, 5.5 percent soluble chemical oxygen demand, and 10.6 percent total volatile fatty acids. In the case of recirculating the effluent stream in the methane reactor, increasing the dilution rate from 0.5 to 3.0 resulted in a methane yield of 227.5-278.9 milliliter per gram of volatile solids and 85-93 percent chemical oxygen demand removal. The methane reactor’s best digestion performance was attained at recirculation rate 2. Methane formation mainly occurred in granular sludge via the hydrogenotrophic pathway. Methane formation in suspended sludge occurred in a secondary manner, mainly via both the hydrogenotrophic and acetotrophic pathways. Among methanogen families, Methanobacteriaceae was found to have the highest relative abundance (7.5 percent in granular sludge and 0.8 percent in suspended sludge).CONCLUSION: Recirculating the effluent provided significant benefits, including the ability to stabilize the hydrolysis process and increase the methane yield. A recirculation rate of 2 to obtain a total chemical oxygen demand of 35.2 grams per liter was the best condition for methanogenesis. Acetotrophic methanogens were better adapted to difficult conditions than hydrogenotrophic methanogens. The formation of methane mainly occurred in granular sludge via a dominant hydrogenotrophic pathway. Methane formation in suspended sludge occurred in a secondary manner, mainly via both the hydrogenotrophic and acetotrophic pathways. Among methanogen families, Methanobacteriaceae was found to have the highest relative abundance.
D. Pham Van; M.G. Hoang; S.T. Pham Phu; T. Fujiwara
Abstract
Kinetic models which can express the behaviors of hydrolysis and biogas generation more precisely than the conventional models were developed. The developed models were evaluated based on the experimental data of six batch reactors. Anaerobic digestion test was co-digestion of food and vegetable waste ...
Read More
Kinetic models which can express the behaviors of hydrolysis and biogas generation more precisely than the conventional models were developed. The developed models were evaluated based on the experimental data of six batch reactors. Anaerobic digestion test was co-digestion of food and vegetable waste with inoculating horse dung by 15% of the total wet weight, at the temperature of 37oC. For hydrolysis, the modified model was developed from an original first-order kinetic model. The modified first-order kinetic model was proved to be better than the original one with the hydrolysis rate constant in the range of 0.22-0.34/day and hydrolyzable rate of 0.80 to 0.84. Kinetics of carbon dioxide and methane were developed from a current potential model. The comparison between experimental data and modeling values had the high correlation of determination (0.9918-0.9998) and low root mean square errors (0.08-4.51) indicating the feasibility of the developed model. In which, the evolution of methane showed the rate constant in the range of 0.031-0.039/day. The carbon dioxide from fermentation accounted for 12-44% of the total observed carbon dioxide. Thus, separation of fermentation and methanogenesis by various reactors may reduce the price of methane enrichment significantly. There was a lag time between methanogenesis and fermentation in reactors (λ = 7-11 days). Also, the biogas yield was in the range of 431.6-596.9 Nml/g-VS with the CH4 concentration of 56.2-67.5%. The best methane yield (393.7 Nml/g-VS) was in a reactor with food waste to the vegetable waste ratio by 1.8:1 (wet basis) and C/N ratio by 25.4.
D. Pham Van; M.G. Hoang; S.T. Pham Phu; T. Fujiwara
Abstract
Coming out from the growth kinetics, the Gompertz model has been developed and considered as the best one for simulating the biogas production from anaerobic digestion. However, the model has failed to describe the starting point of the process, and no-sense of lag phase constant has been pointed out. ...
Read More
Coming out from the growth kinetics, the Gompertz model has been developed and considered as the best one for simulating the biogas production from anaerobic digestion. However, the model has failed to describe the starting point of the process, and no-sense of lag phase constant has been pointed out. Thus, the goal of this study is to develop a new kinetic model of biogas production with meaningful constants that can alternate the Gompertz model. The kinetic constants of the model were determined by applying the least squares fitting method for experimental data. The experimental data were taken from running seven batch reactors of co-digestion of vegetable, sludge and horse manure under 37oC, pH of 6.7, and total solids of 2.5%. The result of the high coefficient of determination (0.9611-0.9906) demonstrated that the new biogas production kinetic model was feasible to simulate the biogas generation process. This finding has opened a new choice that can deal with simulation of the biogas production. Moreover, co-digestion of vegetable, horse manure, and sludge was also evaluated under strong attention. The biogas potential was in the range of 183-648 Nml/g-VS with the best carbon-to-nitrogen ratio of 16. Vegetable waste played a major role in producing the biogas yield while horse manure and sludge contributed to balancing nutrient of the digestion process. Also, the strong correlation between carbon-to-nitrogen ratio and kinetic constants confirmed that the carbon-to-nitrogen ratio was the key factor that influenced biogas generation.
