Research On Operational Performance And Mechanism Of Biological Oxytetracycline Wastewater Treatment Processes

Oxytetracycline（OTC）pharmaceutical wastewater contains high concentration of refractory organic matter,oxytetracycline residue and ammonia nitrogen which could seriously inhibit the biological treatment unit.It is important to develop a stable,efficient and low energy consumption biological treatment process to remove pollutants from OTC wastewater.The expanded granular sludge bed（EGSB）-partial nitritation-anammox process was used to treat OTC wastewater for exploring the changes of operational performance and the mechanism of microbial community succession under the influence of OTC,COD/N and volume loading rates.Meanwhile,OTC wastewater also contains the risk pollutants,such antibiotics resistance genes（ARGs）,which caused a huge threat to the ecological environment.So that the distribution of ARGs in sewage and sludge and the propagation mechanism of ARGs in different reactors were investigated.The purpose of this project was to provide theoretical basis and technical support for the actual OTC wastewater treatment process,which has the important scientific research value.Considering the high concentration of OTC could inhibit microbial activity and even cause reactor collapse,the performance of EGSB reactor under different concentrations of OTC was investigated by gradient increase of OTC concentration.With the concentration of OTC increase to 200 mg L^-1,the COD removal efficiency was over 90%.The removal efficiency of OTC in EGSB was limited（36%）,and the effluent concentration was about 149.4 mg L^-1.The EGSB reactor experienced particles disintegration,which resulted in VFAs accumulation and decrease of COD removal efficiency.Then the anerobic sludge secreted more EPS to protect microbes from OTC with the increase of sludge particle size and recovery of EGSB performance.OTC reduced the diversity of archaeal community,but the addition of OTC did not affect the preponderant methanogen type.The dominated genus was Methanosaeta.Additionally,the fungi genus Apiotrichum might have the ability of degradation of complex organic contaminants.Considering that the effluent of EGSB still contained high concentration of OTC residues,the influence of OTC on PN process was further explored.Even influent OTC concentration was around 150 mg L^-1,the PN process still had good performance of partial nitritation.The nitrate/ammonia ratio in effluent was around 1.44,which was suitable for subsequent anammox reactor.At the same time,the removal efficiency of OTC was about 94%,and the concentration of OTC in the effluent was below 9 mg L^-1.According to situ activity experiment and quantitative analysis of functional genes,it was found that NOB was significantly inhibited by OTC,suggesting that stability and high efficiency of partial nitritation were enhanced.As an important factor affecting anammox process,organic carbon could significantly affect the nitrogen removal performance and microbial structure of anammox process.Consequently,it was necessary to determine the COD/N threshold of anammox process.At COD/N ratio of 0.63,the highest SAA was also achieved with the value of 0.52±0.03 g N g^-1 VSS d^-1,indicating the appropriate COD was beneficial to anammox process.However,the activity of An AOB was significantly inhibited and the nitrogen removal efficiency decreased to 49.2%when COD/N increased to 1.31.It was found that An AOB secreted more extracellular protein,especially a specific humic acid-like substance to form fine and coarse granules.Metagenomics sequencing results further revealed that responses of Ca.Kuenenia＿stuttgartiensis to acetate and propionate was distinct.The acetate increased the relative abundances of genes related to anammox pathway（hzs and hdh）,DNRA pathway and reductive acetyl-Co A pathway,which might be the mechanism for An AOB maintaining the predominant position.Ca.Kuenenia＿stuttgartiensis could participate in acetate metabolism,indicating that An AOB has multiple carbon metabolic potential.On the contrary,propionate was utilized by denitrifying bacteria.The influence of volume load rates on EGSB-PN-anammox process was also studied.When organic loading rate and nitrogen loading rate were 11.7 kg·m^-3·d^-1 and kg·m^-3·d^-1,respectively,the combined process maintained the removal efficiency of COD and OTC at 98.8%and 87.7%,respectively.While the TN removal efficiency decreased to 67.3%,and the TN concentration in the effluent reached to 220.5 mg L^-1.When organic loading rate and nitrogen loading rate were reduced to 5.2 kg·m^-3·d^-1 and 0.6 kg·m^-3·d^-1,respectively,the nitrogen removal performance of the combined process was restored.Then the TN and COD concentrations in the effluent were about 66.4 mg L^-1 and 44.8 mg L^-1,respectively.The performance of combined process met the requirements of water pollutant discharge standard of fermentation pharmaceutical industry（GB 21903-2008）.As an emerging pollutant,ARGs have attracted increasing attention in recent years.In this study,q PCR assay was used to quantify the ARGs and MGEs in influent,effluent and sludge of each reactor,as well as the transmission mechanism of ARGs.The most abundant ARG in EGSB system was tet Q,and the main mechanism of this gene is ribosomal protein protection.The main TRG in PN process was tet X,and the resistance mechanism was enzyme inactivation.The main TRG in Anammox reactor was tet A,and the resistance mechanism is tetracycline efflux pump.PN-Anammox process could effectively remove ARGs,mainly due to the membrane’s closure effect in PN process.The hosts of ARGs in different biological systems were obviously different.The horizontal gene transfer（HGT）of ARGs in the EGSB reactor is mainly through the three kinds of MGEs:int I 1,Tnp A/Tn21 and IS26,and Trichococcus,vadin HA17＿norank and Bacteroides were the representative hosts.In PN system,sul1 and tet A mainly spreaded among Petrimonas,Tissierella,unclassified＿Bacteria,unclassified＿Anaerolineaceae,unclassified＿Bacteroidetes and Anaerovorax through int I1.In Anammox reactor,tet A,the dominant TRG,was enriched through vertical gene transfer,and the potential hosts were norank＿f＿＿A4b,Mycobacterium and norank＿f＿＿AKYG1722.