Dewatered Swine Manure-Derived Biochar As An Additive To Facilitate Anaerobic Digestion Of Swine Wastewater

China is the world’s largest pork consumer.In recent years,with the continuous improvement of national living standards,the level of pork consumption has been increasing year by year.Meanwhile,the production of pig farm waste is increasing rapidly,and the development of pig farm waste resource treatment technology becomes of great significance for the green development of the pig industry.Waste from large-scale pig farms usually includes dewatered swine manure（DSM）and swine wastewater.The application of landfill and composting and other traditional technologies to DSM has a low level of resource utilization.The goal of resource utilization of swine wastewater can be realized by anaerobic digestion（AD）technology.However,the composition of swine wastewater is complex,and it contains antibiotics and resistance genes,characteristic metals and other pollutants,which may inhibit the productivity of digestion system.Based on this,in this study,pyrolysis technology is proposed to treat DSM to prepare DSM-derived biochar（DSMB）,and DSMB can be used as the adding material to enhance the AD efficiency of swine wastewater.The regulation effect of DSMB on AD efficiency of swine wastewater is systematically studied.The alleviating effect of DSMB on the inhibition of AD of swine wastewater under the condition of metal inhibition is investigated.The antibiotic resistance genes（ARG）attenuation and the migration and transformation mechanisms of characteristic metals copper and zinc in the process of DSMB preparation and enhanced AD of swine wastewater are analyzed.And finally,a new resource utilization model of pyrolysis coupled with AD of pig farm waste is proposed.The research results are as follows:（1）In this study,the physicochemical properties of DSM and the six pretreatment techniques to enhance its capacity of AD are explored.It is found that DSM exists in an incomplete dry state with low moisture and ash content.Conventional AD shows a low resource utilization effect of DSM under different substrate-inoculum ratios,and it can only achieve no more than 15.0%theoretical methane production.For the original DSM rich in cellulose that is difficult to biodegrade,six common pretreatment methods,such as preheating,co-fermentation,bioaugmentation,micro-aeration,acidic and alkaline preheating,cannot solve the problem of low hydrolysis rate of AD.（2）The physicochemical properties of DSMB prepared by slow pyrolysis to realize resource utilization of DSM are analyzed,and the regulatory effects of DSMB as an additive agent mediating the AD efficiency of swine wastewater and alleviating the inhibition of characteristic metals are proposed.The results shows that DSMB with excellent physicochemical properties can be prepared by using pyrolysis when treating DSM,so as to realize resource disposal of DSM.DSMB has 3D cellular pore structure and presents more abundant surface organic functional groups and redox-based electron exchange capacity（EEC）at lower pyrolysis temperatures,while its electrical conductivity（EC）shows an opposite trend.With 20 g/L DSMB assistance,the methanogenic lag time of swine wastewater was shortened by 17.4-21.1%,and the maximum CH₄production rate increased from 40.8 m L/d to 48.3-50.5 m L/d,among which DSMB prepared under 300°C exhibited a better performance than that prepared under 500°C and 700°C.Integrated analysis of DSMB electrochemical properties and microbial community,DSMB,as redox-active electron transfer mediators,may be found to establish the potential direct interspecies electron transfer（DIET）between syntrophic microbes Defluviitoga/Syntrophomonas and Methanosaeta/Methanosarcina,so as to achieve the efficient syntrophic methanogenesis.In addition,the characteristic metals Cu and Zn inhibits the AD process,and the lag time increases from 4.8 days to6.9 days.The CH₄production potential decreases by 2.7%,and the peak of daily CH₄production is delayed by 2-4 days.Adding DSMB accelerates the efficiency of AD and attenuates the inhibition of characteristic metals mainly by promoting the methanogenesis process.The CH₄production lag time is shortened by 24.5-30.2%,and the peak of daily CH₄production is recovered.DSMB5-M shows relatively excellent production capacity and rate mitigation performance at different digestion stages.（3）The effects of DSMB on characteristic pollutants in swine wastewater are explored,including ARG attenuation,occurrence forms and transformation of characteristic metals Cu and Zn,and the potential action mechanism of microbial toxicity inhibition.The results shows that DSMB preparation by slow pyrolysis dramatically reduced ARG abundance by almost 4 logs.Adding DSMB into AD not only strengthens the ARG attenuation in the original swine wastewater,but also effectively controls the potential risk of horizontal gene transfer by mitigating 74.8%of the mobile gene elements abundance.The characteristic metals Cu and Zn in swine manure have direct and potential environmental risks.During the AD of swine wastewater,Cu exists mainly in organic bonded and residual states.Zinc is mainly in carbonates,iron manganese oxidation binding and residual states.With the AD process and the ecological toxicity of DSMB to Cu,the potential risk can be reduced and gradually transformed into a stable residual state,but the regulation effect on Zn morphology is not obvious.According to the microbial metabolic analysis,characteristic metals inhibite the growth of AD hydrolytic and acidate dominant families and methanogenic archaea,and increase the growth of Volatile Fatty Acids producing bacteria families and hydrophilic methanogens.The addition of DSMB increases the total proportion of dominant families and the methanogens relying on extracellular electron transport.Especially the common Syntrophomonadaceae and Methanosaeta,which promote the syntrophic methanogenesis by participating in the DIET approach.Based on the above research results and the consideration of the application of resources utilization to agriculture,a resource disposal strategy of pig farm waste with pyrolysis and AD as the core is proposed.Compared with the existing traditional methods,this strategy has advantages in efficient energy recovery and reduction of characteristic pollutants,and can realize the recycling of residual energy of DSMB,the recovery of nutrients or reclaimed water from effluent,and the fertilization of AD products under the stable operating conditions of the system,etc.