Study On The Effect Of Disposing Residual Sludge And The Change Of Tetracyclines Resistance Genes By Thermal-alkaline Combined With Hydrolysis Process

The residual sludge of municipal wastewater treatment plants?WWTPs?contains a large amount of organic matter,such as protein and carbohydrate,which had potential utilization value.The recovery of carbon source from residual sludge for biological nitrogen removal of sewage could not only reduce the amount of residual sludge,but also compensate the operation cost of WWTPs.As a new type of environmental pollutant,most of antibiotic resistance genes?ARGs?accumulated in residual sludge,which had potential biological risk.This paper puts forward the treatment process of the residual sludge by thermal-alkaline combined with hydrolysis.The effect of thermal-alkaline pretreatment process solution on the sludge was investigated,and the effect of pH,reaction time and temperature on the sludge was studied to determine the pretreatment condition of the most suitable thermal-alkaline pretreatment process.After thermal-alkaline process treatment and further to hydrolysis of sludge to determine the volatile fatty acid to obtain the optimal hydrolysis hydraulic retention time,proved the hydrolysis process of material transformation,discusses material use and feasibility in the thermal-alkaline combined with hydrolysis process.And the change of ARGs and microbial community concentration and abundance in thermal-alkaline pretreatment process and hydrolysis process were further explores.A preliminary assessment of the risk of residual sludge treatment by thermal-alkaline combined with hydrolysis process.The results of the study were as follows:?1?According to the results of single factor thermal-alkaline treatment process,the optimum condition for thermal-alkaline sludge disintegration as follows:initial pH was 11,reaction time was 1 h,and temperature at 70°C.Under these conditions,the concentrations of SCOD,TP,TN,ammonia nitrogen,protein and sugar were 11159.4 mg/L,491.1 mg/L,878.8mg/L,169.1 mg/L,752.6 mg/L and 621.2 mg/L in the supernatant of thermal-alkaline pretreatment process,respectively.Lysis of rate was 40.3%.?2?Thermal-alkaline pretreatment could effectively promote sludge hydrolysis.The optimum hydraulic retention time?HRT?was 24h.At this time,the VFAs and SCOD concentrations in the supernatant were higher than 2400 mg/L and 5800 mg/L,respectively.The thermal-alkaline combined with hydrolysis process reached equilibrium in about 120 h.At this time,the protein and sugar in the supernatant were stable at 130 mg/L and 190 mg/L,respectively.Among them,nitrogen and phosphorus were mainly in the forms of ammonia nitrogen and PO₄^3-,NH₄⁺-N/TN was 89%,PO₄^3-/TP was 94%,respectively.?3?All selected tetracyclines resistance genes?tetA,tetG,tetL,tetO,tetQ,tetW,tetX?and mobile gene elements?intl 1?were detected in the raw sludge,reached a higher concentration of 10^6-10⁸ copies/g dry sludge.Thermal-alkaline process had a significant reduction effect on all selected ARGs,with a reduction of 1.2-2.3 log units.However,after the thermal-alkaline process,a relatively high concentration of ARGs was still detected in the excess sludge,with a concentration of 10⁶ copies/g dry sludge.It showed that the sludge after thermal-alkaline process still had a very high biological risk.?4?ARGs rebounded at the end of thermal-alkaline combined with hydrolysis process,with tetL increasing by approximately 2.11 log units and tetO,tetQ,and tet W increasing by1.24,1.06,and 0.88 log units,respectively.The concentration of all ARGs were decreased?tetA,tetG,tetL,tetM,tetO,tetW,tetX,and intl 1?during the direct hydrolysis,and the decrease range was 0.16-1.1 log units.There was a significant positive correlation between intl 1 and tetL and tetG in the thermal-alkaline combined with hydrolysis process?P<0.05?,and tetL,tetQ,and intl1 showed significant correlation in the direct hydrolysis process?P<0.05?.It was shown that the probability of the combination of the int1 gene and the ARGs in the sludge was large,which easily led to the proliferation and spread of the ARGs.?5?The main dominant bacteria in the process of direct sludge hydrolysis were Proteobacteria and Chloroflexi,accounting for?37.61%?and?19.69%?,respectively.At the genus level,the predominant species of the thermal-alkaline combined with hydrolysis process process evolved into Acinetobacter,accounting for 24.81%,while the Litorilinea abundance increased during direct hydrolysis,indicating that Acinetobacter and Litorilinea were more likely to be bacteria that carry and spread ARGs.?6?Through the analysis of the KEEG metabolic pathway and COG function genes in the hydrolysis process,it was shown that Carbohydrate Metabolism,Energy Metabolism,Amino Acid Metabolism,Membrane Transport,Genetic Information Processing,Replication and Repair were the main metabolic pathways;Cellular Processes and Signaling,Information Storage and Processing,and Metabolism were main functional genes in the hydrolysis process...