| Purified terephthalic acid,or PTA,is an important raw material widely used in petrochemical industry to make various synthetic products including polyethylene terephthalate bottles,polyester textile fiber,polyester films,insulating varnish,etc.During the PTA manufaction process,bulk of wastewater containg aromatic compounds are produced.If these pollutants are not effectively removed before final discharge,it will induce serious environmental pollution.Among these polluatants,terephthalic acid(TA)and p-Toluic acid(p T acid)are generally recognized as two of the most refractory organics,and p-Toluic acid was proved to have an inferior biodegradability than TA.Therefore,in this study,Thermophilic-mesophilic staged anaerobic process and UAFB-SBR process were employed in the treatment of PTA wastewater for better degradation of TA and p-Toluic acid.The effects of temperature,organic loading rate and quality of inlet water on these two processes were evaluated as well.The major microbial communities structure and their succession were also detected in different stage of these two processes.The study of operating parameters and major microbial community shifts would be helpful in elucidating the complex digestion pathway as well as providing references to enhance reactor performance.Two thermophilic reactor startup strategies were investigated in this theis.The results showed that two-step increase startup strategy with mesophilic cultivation was more suitable for the enrichment of thermophilic TA degradable microorganism and efficient degradation of TA than one-step temperature increase strategy without mesophilic cultivation.Meanwhile,the relatively short run time was unfavorable for the thermophilic reactor startup and operation.During the two-step startup process,55 days were needed for the recovery of operation performance after the sharp temperature elevation,which was longer than stepwise strategy(40 days).Therefore,a reasonable startup strategy and operation time were critical for the efficient cultivation of thermophilic microorganisms,as well as the startup of thermophlic reactor.Considering a signgle thermophilic anaerobic reactor is insufficient for the complet removal of organic pollutants within PTA wastewater,for better enhancement of PTA wastewater treatment efficiency,a thermophilic-mesophilic staged anaerobic system sequentially connected with thermophilic and mesophilic anaerobic treatment system was developped.The treatment efficiencies under different organic loading rate and water quality were investigated by changing the influent compositions and HRT.Most of the organics could be effectively removed when the organic loading rate was lower than 4.45 kg·COD/(m3·d),while the COD concentration in effluent was well below 80 mg/L.However,after the organic loading rate was elevated to 6.68 kg·COD/(m3·d),the organics concentration rapidly increased to around 140 mg/L,which was dominated by p-Toluic acid,as well as slight TA,benzoic acid and acetic acid.Moreover,according to the study on different PTA wastewater compositions,thermophilic-mesophilic staged anaerobic process showed prevalent tolerance of concentration variation in TA,benzoic acid and acetic acid,but poor tolerance on variation of p-Toluic acid.An apparent COD increase to 400 mg/L in effluent was observed after the increase of p-Toluic acid concentration in influent.The microbial diversity analysis demonstrated that bacterial diversity in the thermophilic stage was significantly lower than that in mesophilic stage.Meanwhile,the bacterial diversities in both stages decreased directly with the increase of organic loading rate.Bacterial community structure in mesophilic was significantly affected by the concentration of benzoic acid,acetic acid,TA and p-Toluic acid,while TA and p-Toluic acid had slight influence on bacterial diversity in thermophilic stage.The microbial community structure analysis revealed that TA and benzoic acid were mainly degraded by TA06 and TTA-B1 in thermophilic stage of anaerobic digestion.Aromatic compounds were sufficiently utilized by some syntrophs,including Syntrophorhabdus,Syntrophus,Syntrophobacteraceae,Syntrophaceae.In addition,one of the aceto-trophic methanogens Methanosaeta was frequently detected in both mesophilic and thermophilic stage,while the hydrogenic Methanolinea was mainly observed in thermophilic stage,which showed strong temperature sensibility.For the circumastance of PTA wastwater composition fluctuations,especially for PTA wastewater with high p-Toluate concentration,the organics cannot be completely removed by through the thermophilic-mesophilic staged anaerobic process.Therfore,the connection of UAFB reactor and SBR process was recommended for complete degradation of PTA wastewater.According to the experimental operation,UAFB-SBR process showed strong resistance to temperature shock,and the UAFB-SBR process operated at ambient temperatures(15-25?)was proved to be stable and highly efficient for the organics removal.The total organic removal efficiencies were well over 98%,while the COD effluent fluctuated between 40-100 mg/L.Among these organics,99% p-Toluic acid was removed,and the other pollutants in effluent were simutaneously well below the detection limit.The microbial analysis demonstrated that,within the UAFB system at amobient temperature,Syntrophorhabdus was one of the most critical bacteria genus promoting the anaerobic degradation of aromatic compounds.Meanwhile,the increase of p-Toluic acid concentrations in influent also accelerate the growth of both Aminicenantes and Acetothermia;Within the SBR system,Rhodobacteraceae and Thauera could prompt the efficient degradation of p-Toluic acid,and Azoarcus was maily efficient for TA degradation. |
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