Study On The Degradation Law Of Sulfamethoxazole By Nitrifying Bacteria Under Different Carbon Source Conditions

Many studies have shown that nitrifying bacteria have a significant biotransformation effect on refractory organic matter and that this biodegradation is associated with ammonia oxidizing bacteria(AOB).In many incubation conditions where inorganic matter is the only carbon source,nitrification-enriched sludge demonstrates better biodegradation of antibiotics.However,the substrate carbon source is often complex under actual environmental conditions.It is worthwhile to investigate further whether activated sludge cultivated under different carbon source conditions still has advantages for removing refractory organic matter.Sulfonamide antibiotics(SAs)are synthetic antimicrobial agents widely used to treat humans and livestock.Therefore,this study used the most widely produced and used sulfamethoxazole(SMX)as the target pollutant.Three typical activated sludge were domesticated and cultured: nitrated activated sludge(NAS)with sodium bicarbonate as substrate,mixed activated sludge(MAS)with glucose and sodium bicarbonate as substrate,and conventional activated sludge(CAS).The degradation of SMX by different activated sludge was investigated by changing the initial reaction conditions.The microbial communities of the three types of activated sludge were analyzed using high-throughput sequencing.A kinetic model of pollutant degradation rate was fitted further to investigate the advantages of AOB on SMX degradation.The main findings of this study are as follows.(1)In the SMX domestication stage,the three activated sludge species were adapted to SMX as CAS>MAS>NAS,but NAS degraded SMX at the fastest rate after the domestication was completed.The delay in the degradation of SMX by MAS and NAS,which preferentially oxidized ammonia nitrogen,increased with the increase of ammonia nitrogen concentration.Therefore,the high concentration of ammonia nitrogen was not conducive to the rapid degradation of SMX.(2)The removal rate of SMX at 5 h increased from 76% to 97.7% after the injection of 50 mg/L glucose into the NAS without providing glucose domestication.Furthermore,the genus OLB8 in the microbial community increased from 7.6% to 67%after 7 days of NAS incubation with 50 mg/L glucose,while the higher content of nitrifying bacteria decreased significantly but did not affect the composition of the AOB functional flora.It is hypothesized that the glucose injection increased OLB8 competitiveness,promoting SMX degradation.(3)The degradation of SMX by MAS and CAS was inhibited by the addition of the nitrification inhibitor ATU,and the degree of inhibition in CAS was about 70%higher than that in MAS when ATU was added at 3 mg/L.As the concentration of ATU increased,the degree of inhibition increased for both,and the inhibition rates were essentially the same.Since CAS provided only the ammonia concentration(C: N= 20:1)required to maintain the essential growth of heterotrophic bacteria,the percentage of nitrifying bacteria in CAS was small.Presumably,while nitrifying bacteria were inhibited,heterotrophic bacteria degrading SMX were also affected by ATU.The inhibition of glucose mineralization by ATU in MAS also demonstrated that ATU might inhibit some specific heterotrophic bacteria.Therefore,ATU dosing could not wholly distinguish the role of nitrifying and heterotrophic bacterial flora.(4)Genus Ahniella detected in CAS(30.8%)There is no previous study on the ability of this genus to degrade antibiotics,and this study is the first time that this genus was found to be associated with the degradation of SMX.The domestication of complex substrates resulted in a very species-rich microbial community in MAS,containing a large number of nitrifying bacteria(16.40%),denitrifying bacteria(18.96%),and other heterotrophic bacteria,which may be potential hosts for antibiotic resistance genes and play an essential role in the degradation of SMX.(5)The degradation of SMX by NAS and CAS under the conditions without the provision of glucose and ammonia nitrogen was following the zero-level kinetics.The degradation rate of SMX by NAS was 1.51 times higher than that by CAS.The degradation of ammonia nitrogen by both activated sludge under the condition of no glucose injection was also consistent with zero-level kinetics.(6)The degradation of SMX by CAS and NAS under different SMX concentrations follows the Aiba model.With the initial SMX concentration(0-10mg/L)increasing,the degradation rate of SMX increased and then decreased.The degradation rates for different initial concentrations of ammonia nitrogen(0-200 mg/L)were also consistent with the Aiba inhibition model,and the Aiba model data showed that the maximum degradation rates of NAS for SMX and ammonia nitrogen were 3and 1.04 times higher than those of CAS,respectively.Therefore,the enriched nitrifying sludge has a significant long-term advantage for the degradation of SMX.