Research On Methane Production Of Swine Wastewater Treated By Microbial Electrolysis Cell Coupled Anaerobic Digestion

Intensive pig farms discharge a large amount of wastewater during the pig breeding process,which often contains high concentrations of suspended solids,organic matter,ammonia nitrogen and phosphorus.If the swine wastewater is not properly treated,it will cause harm to the surrounding environment and human health.Therefore,the swine wastewater needs to be treated and then discharged.Anaerobic digestion（AD）is a common method for treating swine wastewater,which can effectively remove organic matter and recover energy biogas.This treatment method is conducive to reducing pollution and energy consumption in the process of pig farming.However,AD has problems such as low processing efficiency and unstable operation.Recent studies have found that microbial electrolytic cell assisted anaerobic digestion（MEC-AD）can induce the enrichment of hydrogenotrophic methanogens by hydrogen evolution at the cathode to improve the methane production performance and operational stability,and enrich the electroactive microorganisms at the anode to accelerate the degradation of organic matter.However,the performance of electrodes has a significant impact on the effectiveness of MEC,and the energy consumption issue of long-term operation of MEC is also worth paying attention to.In addition,the structure and performance of the MEC coupled continuous wastewater treatment reactor still need further design optimization.1.Firstly,the cathode material of MEC was designed.By applying metal-organic frameworks（MOFs）with catalytic properties to the cathode of MEC-AD,a Fe Ni-MOFs electrospun fiber membrane composite cathode wrapped by stainless steel mesh was designed.Experiments on the treatment of swine wastewater found that the carbonized MOFs composite cathode（Fe Ni₂-PAN2）reactor achieved the highest SCOD removal rate of 82.92%and the maximum cumulative methane production of 213.47 m L CH₄/g COD.33.14%and 57.56%higher than the traditional AD control reactor,respectively,and also higher than the carbon paper（CP）cathode reactor.The faster current response and smaller charge transfer resistance of the Fe Ni₂-PAN2 cathode promoted the enrichment and activity of functional microorganisms;the Fe Ni₂-PAN2 cathode enriched the most abundant hydrogenotrophic methanogens（Methanobacterium and Methanoculleus）,which promotes the transformation of the methanogenic pathway,improves the system stability and catalytic methanogenic effect.2.Secondly,from the perspective of reducing cost and improving electrode biocompatibility,Ni Co-BDC composite hydrothermal carbon materials were designed and used to modify the cathode.In addition,the wind energy and solar energy complementary power generation device was also used for the energy supply of MEC,and the energy consumption was further reduced by setting different power supply times.The comparative experimental results of different cathodes and power supply methods showed that the MOFs composite hydrothermal carbon cathode group（HCM-18）achieved the highest cumulative methane production（305.11 m L/g COD）after 18 hours of power-on and 6-hour power-off,which was 36.89%higher than the AD group.This was also slightly higher than the HCM-24 group with 24 h continuous power supply.The intermittent power supply improves the activity of the cathode biofilm,and also achieves higher energy recovery efficiency,which effectively reduces the energy consumption of the MEC-AD system.The comparison between different cathodes found that the hydrothermal carbon cathode group HC-18formed a thicker biofilm,but due to the lower catalytic activity,its cumulative methane production was 11.6%lower than that of the stainless-steel mesh SSM-18 group,which illustrates the importance of the cathode having catalytic activity.Microbial community analysis found that the surface of HCM-18 group was enriched with higher hydrogenotrophic methanogens（Methanoculleus）,which may be the reason why the HCM cathode still maintained high methane production performance under the highest ammonia nitrogen concentration.3.Finally,in order to realize the continuous treatment of wastewater,the MEC was coupled to the Anaerobic Baffled Reactor（ABR）,and an ABR-MEC reactor with electrodes capable of intercepting sludge was designed.By designing the reactor and the electrode group separately,and simplifying the electrode fixing method,the manufacturing difficulty and cost of the reactor are reduced.The multiphase flow simulation of the reactor showed that the reactor has a certain effect of intercepting sludge.Finally,the start-up experiment of the trial-manufactured ABR-MEC reactor showed that MEC promoted the rapid start-up of the reactor,the current in the reactor was positively correlated with the methanogenic activity,and the improvement of the methanogenic activity promoted the rapid consumption of VFAs,and finally realized the The highest COD removal is 83.41%and the highest daily methane production is 344.17 m L/d.The above results show that the designed reactor has a good treatment effect of low-concentration wastewater.