Efficiency And Mechanism Of A Hybrid A/O Baffled Reactor In Treating Manure-free Piggery Wastewater

The rapid development of large-scale pig farms has caused the centralized discharge of large piggery wastewater,which is considered as a great threat to environment and human health.As a traditional management model of piggeries,urine-free manure collection before flushing has been extensively practiced with the discharge of manure-free piggery wastewater.Though most of pollutants have been removed by the collection of manure,ammonium（NH₄⁺-N）in the manure-free piggery wastewater remains very high,resulting in a low ratio of chemical oxygen demand（COD）to total nitrogen（TN）in the wastewater.It is a great challenge to the traditional anaerobic-aerobic（A/O）processes for nitrogen removal from the wastewater via complete nitrification-denitrification and/or shortcut nitrification-denitrification.It is a common urgency for the sustainable development of pig farms to develop a biological process for the cost-effective treatment of manure-free piggery wastewater characterized by high NH₄⁺-N and low COD/TN ratio.Based on the previous studies,a hybrid A/O baffled reactor（HAOBR）with four compartments was further constructed in the present research work for the synchronous removal of COD,TN and total phosphate（TP）in manure-free piggery wastewater.In addition to the startup process,the effects of HRT and reflux ratio on the HAOBR performance were evaluated stage by stage.Investigations into the microbial community,functional microorganisms and stoichiometry were further conducted to get a better understand of the nitrogen removal mechanism in the HAOBR.The novel findings will provide theoretical guidance and technical support for the cost-effective treatment of piggery wastewater and the sustainable development of pig farms,making contribution to the ecological civilization construction of our motherland.With the introduction of the novel anammox to the traditional A/O process,a HAOBR was constructed firstly for the synchronous removal of COD,TN and TP from manure-free piggery wastewater.The HAOBR included two anaerobic compartments（A1and A2）and two aerobic compartments（O1 and O2）in sequence.The four compartments were same in size and volume,as well as the packed beds constructed by VC carriers.Inoculated with the seed sludge taken from the secondary sedimentation tank of a local domestic wastewater treatment plant,the HAOBR could be successfully started up and got steady in performance within 25 days under the condition of HRT 36 h,reflux ratio200%and 32℃.Within the steady performance,the removal of COD,NH₄⁺-N,TN and TP in the HAOBR averaged 86.8%,100%,91.3%and 40.1%,respectively,with the effluent well meeting the requirement of Discharge Standard of Pollutants for Livestock and Poultry Breeding of China（GB18596-2001）.The effects of HRT and reflux ratio on the HAOBR performance were further evaluated stage by stage.The results showed that HRT 20 h and reflux ratio 50%were more cost-effective in the synchronous removal of COD,TN and TP from manure-free piggery wastewater.Under the most favorable HRT and reflux ratio,the removal of COD,NH₄⁺-N,TN and TP in the HAOBR was as high as 75.8%,99.5%,86.6%and 68.6%,respectively,with the residues all well meeting the requirement of GB18596-2001.COD in the wastewater was mainly removed in the first 3 compartments with a load removal averaged 0.20,0.43 and 0.50 kg/（m³·d）,respectively.It was interesting to find that the removal of NH₄⁺-N and TN were mainly occurred in the first anaerobic compartments of the HAOBR.The load removal of NH₄⁺-N and TN in the first two anaerobic compartments and the last two aerobic compartments averaged 0.43 and 0.37 kg/（m³·d）,and 0.60 and 0.48 kg/（m³·d）,respectively.The analysis of stoichiometry revealed that the TN in the anaerobic compartments was mainly removed by heterotrophic denitrification,while the TN in the aerobic compartments was dominated by partial nitrification-anammox（PN/A）along with some heterotrophic denitrification.The microbial community and functional microorganisms revealed by high-throughput sequencing showed a large number of methanogens such as Methanothrix and Methanobacterium had been enriched in the first anaerobic compartments（A1 and A2）with the relative abundance of 12.29%and 3.36%,respectively.Heterotrophic denitrification bacteria were also detected in compartment A1 and A2 with a relative abundance of 5.42%and 2.73%,respectively.On the other hand,the last two aerobic compartments（O1 and O2）were dominated by ammonia oxidation bacteria（AOB）such as Nitrosomonas and anaerobic ammonium oxidation bacteria（An AOB）such as Candidatus Kuenenia,Candidatus Brocadia and Candidatus Anammoxoglobus,all of which were involved in the nitrogen removal in the last two compartments.The relative abundances of AOB and An AOB were detected to be 3.32%and 3.05%,and 3.11%and8.37%in the O1 and O2,respectively.The results suggested that the large amounts of chemoheterotrophic bacteria and methanogenic bacteria both enriched in the first two anaerobic compartments had played a key role in COD removal,while the abundant of AOB and An AOB in the last aerobic compartments were mainly responsible for the synchronous removal of NH₄⁺-N and TN.It was the cooperation between the anaerobic and aerobic compartments had ensured the HAOBR to perform efficiently and steady.Further kinetics analysis revealed that COD degradation rate in A1 and A2 of the HAOBR was 0.0125 and 0.0291 L/（g·h）,respectively,under the favorable HRT 20 h and reflux ratio 50%.The stoichiometric analysis showed that the contribution of PN/A to the removal of NH₄⁺-N and TN in the aerobic compartment O1 was 9.6%and 19.78%,respectively.While,the denitrification via nitrite and nitrate reduction had made a contribution of 50.3%and 29.92%to the TN removal,respectively,in the same aerobic compartment.However,the contribution of PN/A to the TN removal in the last aerobic compartment O2 was as high as 67.2%,with 23.6%TN removed by the reduction of nitrite.The results confirmed again that PN/A was the dominant approach to the TN removal in the HAOBR treating manure-free piggery wastewater.In conclusion,the developed HAOBR was cost-effective in treating manure-free piggery wastewater,with a satisfactory synchronous removal of COD,TN and TP.Though the manure-free piggery wastewater was characterized by high NH₄⁺-N and low COD/TN ratio,the TN in the wastewater could be efficiently removed without the supplement of additional organic carbon source,resulted in the well performance of PN/A in the HAOBR.The novel HAOBR was not only efficient in pollutants removal,but also simple in management with low energy consumption,presenting a great potential in treating organic wastewater with high NH₄⁺-N and low COD/TN ratio.