Study On Abatement And Agricultural Risk Of Heavy Metals And Antibiotics In Manure From A Large Pig Farm

Pig breeding plays an important role in China’s breeding industry.According to the data form the National Bureau of statistics,China’s pork output in 2020 was about 41.13 million tons,accounting for 54%of the total output of livestock and poultry meat.The annual slaughter volume of live pigs was 527.04 million and the year-end stock volume was 406.5million.The rapid development of pig breeding industry has brought voluminous of feces and wastewater,which contains trace elements and veterinary antibiotics that cannot be absorbed by animals.Direct discharge will not only cause environmental pollution,but also do harm to human health and food safety,which is detrimental to the sustainable development of breeding industry.Taking a large-scale pig farm in Shandong Province as an example,breeding wastewater,pig feces,and compost products were collected,the residual law and removal efficiencies of typical heavy metals and antibiotics were investigated,and the ecological risks from wastewater discharge and application of compost products were evaluated.At the same time,aerobic composting experiments were carried out using fresh pig feces.The effects of exogenous bacteria and passivators on compost maturity,antibiotic degradation,and bioavailability of heavy metals were studied,as well as the appropriate addition proportion of exogenous bacteria and passivators was determined,which will provide reference for manure treatment and resource utilization in pig breeding.The main results of this paper are as follows:(1)The removal laws of six heavy metals(Cu,Zn,As,Cd,Cr,and Pb)and six antibiotics(oxytetracycline,chlortetracycline,doxycycline,sulfadiazine,sulfamethazine,and sulfamethoxazole)from different treatment units(temporary storage tank,primary settling tank,secondary settling tank,final settling tank,and oxidation pond)and an aerobic composting reactor were determined.The ecological hazards of compost products and effluents from oxidation ponds were assessed using the Nemerow index method and risk quotient(RQ_s)method,respectively.All six heavy metals and all six antibiotics were detected in all wastewater and compost samples.The main heavy metals in wastewater were Cu and Zn,respectively.Oxytetracycline,chlortetracycline and doxycycline were the main antibiotics in wastewater.The treatment process"solid-liquid separation+UASB+multistage A/O+oxidation pond"effectively removed heavy metals and antibiotics,with removal rates of 74.8%～99.7%(except Pb)and 39.4%～99.8%,respectively.The effluent quality met Standards for Farmland Irrigation Water Quality(GB 5084-2005).The degradation rate of antibiotics in pig feces ranged from 49.8%to 90.9%during the composting process in the aerobic reactor.Pb and Zn concentrations in aerobic composting products of pig feces exceeded the limits specified in the Organic Fertilizer(NY 525-2012),and the Control Standards of Pollutants in Sludge for Agricultural Use(GB 4284-2018).In conclusion,doxycycline and oxytetracycline are the main pollutants associated with high risk in the discharge water of oxidation ponds,posing certain ecological risk to the environment.Further reductions or other mitigation measures should be considered.Pb and Zn are the main pollutants in compost products,which could pose potential risks if applied directly to farmlands.(2)Exogenous bacteria were added to study their effect on compost maturity,antibiotic degradation,and heavy metal bioavailability in the process of aerobic composting of pig manure.The results showed that the exogenous bacteria could effectively promote compost maturity,increase compost temperature and prolong the duration of high temperature period,increase heap p H and reduce ammonia volatilization,as well as reduce the toxicity of compost product to plant growth.The concentration of heavy metals in the treatments with bacterial agent addition were higher than that of the control treatment.The passivation rates of Cu,Zn,Cr and Pb were the best with 1%bacterial agent addition,which were 46.1%,35.9%,39.1%and 26.1%respectively.The results of redundancy analysis showed that the total amount of heavy metals,DTPA-Pb,DTPA-As and DTPA-Cr were positively correlated with p H and total nitrogen(TN),and negatively correlated with temperature and organic matter(OM).DTPA-Cu was positively correlated with OM and negatively correlated with p H.DTPA-Zn and DTPA-Cd were positively correlated with conductivity(EC).The total degradation rate of the five antibiotics was the highest in the treatment with 2%bacterial agent addition,which was91.6%,higher than 86.9%in the control treatment.At the same time,the half-life of antibiotics in the composting system was reduced.The half-life of oxytetracycline and aureomycin in the T3 treatment with 1%bacterial agent addition was the shortest,which was2.04 and 4.92 days shorter than CK respectively.The degradation rate of sulfadiazine in the treatment with 2%bacterial agent addition was the fastest,with half-life reduced from 11.9days to 5.87 days.(3)Passivators(woody peat and biochar)were added to study their effects on compost maturity,antibiotic degradation and heavy metal bioavailability in pig manure aerobic composting.The results showed that the temperature and physicochemical indexes of the seven treatments after composting met the requirements of harmless composting,and the contents of Cu,Zn and Cr increased by 29.0%～55.4%,21.7%～33.4%and 17.6%～48.3%respectively.15%woody peat addition showed the best passivation effect on heavy metals,with passivation rates of 52.2%,42.7%,and 27.2%for Cu,Zn and Cr respectively.For biochar treatments,15%biochar addition showed the best passivation effect on Cu,with passivation rate of 41.4%.However,for Zn and Cr,10%biochar addition showed the best passivation effect,with passivation rates of 36.8%and 29.0%respectively.The addition of woody peat and biochar can improve the degradation of antibiotics.Addition of 15%woody peat showed the highest degradation of antibiotics,with degradation rates of 96.4%,100.0%,93.1%,98.2%and 97.2%for OTC,CTC,DOX,SDZ and SMZ respectively,and the total degradation rate is 97.2%.T5 treatment with 10%addition of biochar presented the best degradation effect on antibiotics.The degradation rates of OTC,CTC,DOX,SDZ and SMZ were 100.0%,95.4%,90.8%,97.6%and 97.8%respectively,and the total degradation rate was 95.5%.The residual amount of antibiotics showed a very significant negative correlation with p H(P<0.01),and a significant positive correlation or very significant positive correlation with EC,C/N,DTPA-Cu and DTPA-Zn(P<0.05 or P<0.01),indicating that p H,EC,C/N,DTPA-Cu and DTPA-Zn are important factors affecting the degradation of antibiotics.