Enhancement Of Methane Production In Anaerobic Digestion Of Swine Manure By Microbial Electrolysis Cell

A large amount of swine manure(SM)is produced as a result of pig breeding in China,but almost half of the SM is not properly disposed of,which leads to serious environmental pollution.Anaerobic digestion is an effective method for recycling SM.The fermented sludge and slurry produced can be used as fertilizer,and biogas can also be generated.However,the traditional anaerobic digestion has some disadvantages,such as low methane yield and poor system stability.In order to realize the resource utilization of the SM,improvements to this system are warranted.Microbial electrolysis cell(MEC),a new technology based on microbial fuel cells,can improve anaerobic digestion performance,thereby increasing the production of the methane(biogas).Organic wastes differ greatly in terms of material composition and physical and chemical characteristics;thus,the effects of MEC on the anaerobic digestion of different organic substrates also varies,making it necessary to study the influence of MEC coupled with anaerobic digestion(MEC-AD)of SM.In this study,a MEC-AD model based on anaerobic digestion model NO.1(ADM1)was used to study the effects of MEC-AD from swine manure.The factors affecting the MEC-AD system were investigated,with the aim of improving the practical application of MEC-AD systems.Because the MEC-AD system requires an input of electrical energy,when optimizing the parameters of the MEC-AD system,both methane production and total energy efficiency should be considered.Accordingly,both methane production and total energy efficiency are considered key objectives in multi-objective optimization of MEC-AD system.The main results are as follows:(1)Influence and analysis of applied voltage on MEC-AD systemBy studying the effects of different applied voltages on MEC-AD from SM,it was found that the amount of methane produced from MEC-AD was as high as264.52 m L/g TS when the applied voltage was 0.90 V,and the total energy efficiency was 53.12%,the added methane has more energy than the incoming electricity,electrochemical contribution efficiency was closer to the theoretical electrochemical contribution efficiency.The optimum range of applied voltage was determined to be0.45 V-1.46 V.In order to understand the effects of MEC on anaerobic digestion of the SM,an MEC-AD model was built based on ADM1 and was fitted with the cumulative methane production under different applied voltages.The results of the MEC-AD model showed that,overall,the microbial biomass of the MEC-AD system improved,and there was a clear increase in electroactive microorganisms,indicating that they play an important role in the MEC-AD system.The proportion of hydrogenophilic methanogens in the archaea increased.Direct interspecies electron transfer(DIET)played an important role in MEC-AD;the methane production through DIET reached 18.41%,and the electrochemical contribution rate reached21.94%.The results show that the microbial electrolysis cell can improve anaerobic digestion performance by increasing microbial biomass,enhancing the methanogenesis pathway and increasing the methanogensis pathway.(2)Influence of electrode position,substrate concentration,initial pH,and electrode side length on MEC-AD systemWhen the electrodes were located at the bottom of the reactor(as opposed to elsewhere in the reactor)the MEC-AD system was superior in terms of methane production,system stability,lignocellulosic degradation rate,and energy efficiency.At the same time,the energy efficiency relative to the electrical input was 137.12%.When the substrate concentration was 10%,the methane production of MEC-AD increased by 27.93%,compared to the ordinary AD system,reaching 284.89 m L/g TS,and the total energy efficiency was 53.12%,the added energy was greater than the electricity consumed.When the initial pH was 7.0,the methane production of MEC-AD increased by 36.79%,compared to the AD system,reaching 301.20 m L/g TS,and the total energy efficiency was 55.58%,the increased methane energy made up for the incoming electricity.Based on Gauss Amp analysis,we determined that the optimal initial pH range in the MEC-AD system is 6.21-9.53.Microbial electrolysis cell can expand the initial pH range of anaerobic digestion of SM.When the electrode side length was 6.0 cm,the methane production of the MEC-AD system was as high as 316.90 m L/g TS and the total energy efficiency was 55.53%,the energy output was positive.Microbial electrolysis cell can accelerate the decomposition of volatile fatty acids,soluble chemical oxygen demand,degradation of lignocellulose,maintain pH stability,and improve the performance of anaerobic digestion by increasing the protein mass of extracellular polymeric substances(EPS)on the electrode.(3)Multi-objective optimization of MEC-AD systemThrough multi-objective optimization of methane production and total energy efficiency,the results showed that the multi-objective optimization model constructed using a Back-Propagation Neural Network(BP-NN)was superior to the model constructed by response surface methodology(RSM).The non-dominated sorting genetic algorithm Ⅱ(NSGAⅡ)was used to optimize the conditions of the multi-objective model,and the optimization results of the BP-NN model were verified.The error between the predicted values and verification results of the BP-NN model was smaller to that obtained from the model constructed using RSM model.The results showed that when the applied voltage was 1.13 V,the electrode side length was 4.4 cm,the substrate addition amount was 191 g,and the initial pH was 7.50,the methane production was 327.84 m L/g TS,and the total energy efficiency was 61.38%.When the applied voltage was 1.05 V,the electrode side length was 5.4 cm,the substrate addition amount was 192 g,and the initial pH was 7.39,the methane production was331.36 m L/g TS,and the total energy efficiency was 61.24%.When the applied voltage was 1.00 V,the electrode side length was 6.0 cm,the substrate addition amount was 197 g,and the initial pH was 7.21,the methane production was 333.97 m L/g TS,and the total energy efficiency was 60.98%.However,the methane production and total energy efficiency cannot simultaneously reach the maximum.The results of our study show that it is feasible to solve multi-objective optimization of SM treatment by using Box-Behnken design(BBD)for experimental design,BP-NN for multi-objective modeling,and NSGAⅡ was used for multi-objective optimization in the MEC-AD system.