| The high rate activated sludge process(HRAS)is one of the mainstream processes for recovering organic carbon from wastewater,which is composed of an adsorption tank followed by a sedimentation tank.The optimal recovery of different forms of organic carbon in the wastewater cannot be achieved simultaneously through the optimization of adsorption tank process parameters,especially it is difficult to improve the recovery of colloidal and dissolved organic carbon,which makes the organic carbon recovery rate of HRAS process only about 30%,which is not conducive to its transformation to the direction of energy neutralization and carbon neutralization.In this study,an inverted high-load activated sludge process(R-HRAS)was constructed by front-loading the HRAS process sedimentation tank,using the pre settling tank to precipitate and recover the particulate organic carbon first,while the remaining colloidal and dissolved organic carbon was targeted by the post adsorption tank for sludge adsorption and organic carbon storage.This sorted recovery of organic carbon from wastewater may be the key to break the bottleneck of organic carbon recovery from wastewater and provide a solid foundation for energy neutral and carbon neutral operation of the process.The operational performance of the R-HRAS process was examined in terms of organic carbon recovery effect,energy neutralization and carbon neutralization potential,the organic carbon recovery mechanism of the R-HRAS process was examined from the perspective of sludge physicochemical properties and microbiology,and the effects of dissolved oxygen(DO)and hydraulic retention time(HRT)on the R-HRAS process were investigated,and the details and conclusions of the study are as follows.(1)The operational performance and organic carbon recovery mechanism of R-HRAS was analyzed using HRAS process as a control.The study showed that the COD removal effect of R-HRAS process(60.4%)was comparable to that of HRAS process(61.7%),and the removal of particulate organic carbon was mainly concentrated in the front sedimentation tank,while the removal of colloidal and dissolved organic carbon was mainly concentrated in the back adsorption tank.recovery.Compared with HRAS,the organic carbon recovery rate(49.2%)was improved by 13.4%,the energy neutralization rate(250%)by 146%,and the carbon neutralization rate(91.1%)by 55.0%in R-HRAS.From the perspective of sludge physicochemical properties,compared with HRAS,the average particle size of the post-adsorption tank sludge was reduced by 25.5%and the specific surface area was increased by 81.6%,which enhanced the sludge adsorption performance;the zeta potential was reduced by 35.5%and the contact angle was reduced by 18.5%,which may lead to enhanced carbon storage activity of the sludge;the TB-EPS,LB-EPS and S-EPS concentrations were increased by 130.9%,112.5%and 73.6%,respectively,which may be an important reason for the enhanced sludge adsorption performance and carbon storage activity,and the enhanced sludge adsorption performance and carbon storage activity are the key to the enhanced organic carbon recovery effect of the R-HRAS process,and the key to the process achieving energy neutrality as well as carbon neutrality.From the microbiological analysis,the relative abundance of Chloroflexi,a phylum of green curvilinear bacteria in the post-adsorption cell was only 11.%of that in HRAS,which may lead to a lower effect of organic carbon mineralization in the post-adsorption cell,while the carbon storage of Flavobacterium spp.Flavobacterium,Zoogloea spp.and Plasticicumulans spp.in the post-adsorption cell The relative abundance of the genera Flavobacterium,Zoogloea and Plasticicumulans in the post-sorption cell increased by 27.08%compared to HRAS,which may be the underlying reason for the higher carbon storage activity and increased carbon storage capacity of the sludge in the post-sorption tank.(2)The DO of the R-HRAS process was optimized and its effect on the organic carbon recovery mechanism was investigated.It was shown that the organic carbon recovery rate,energy neutralization rate and carbon neutralization rate of the R-HRAS process showed an increasing and then decreasing trend with the increase of DO at 0.5,1.0 and 1.5 mg/L,and reached the highest at 1.0 mg/L DO(56.6%,255%and 100.8%).The DO of 1.0 mg/L was determined to be a better control parameter for the R-HRAS process from the perspective of energy neutralization and carbon neutralization.The analysis of sludge properties showed that with the increase of DO,the average particle size of the post-sorption tank sludge decreased,the specific surface area increased,the zeta potential decreased,the contact angle decreased,the concentration of TB-EPS,LB-EPS and S-EPS increased and then decreased,and the relative abundance of carbon storing bacteria genera such as Flavobacterium spp.increased by more than 13%.This indicates that the sludge adsorption performance and carbon storage activity were enhanced with the increase of DO,and the adsorption and carbon storage capacity of sludge was subsequently increased,which drove the improvement of organic carbon recovery effect,but the mineralization effect of sludge on organic carbon might be more intense when DO increased to 1.5 mg/L,and the organic carbon adsorbed and stored by sludge was mineralized in large amounts to reduce the organic carbon recovery effect,which was not conducive to energy neutralization and carbon neutralization.(3)The HRT of the R-HRAS process was optimized and its effect on the organic carbon recovery mechanism was investigated.The study showed that under the conditions of HRT of 180,210 and 240 min,the organic carbon recovery rate of the R-HRAS process increased and then decreased with the extension of HRT,and the highest rate(59.5%)was achieved at the HRT of 210 min,and the energy neutralization rate and carbon neutralization rate decreased continuously from 255%and 100.8%at the HRT of 180 min to 145%and 55.1%at the HRT of 240 min.The energy and carbon neutral rates decreased continuously from 255%and 100.8%at HRT of 180 min to 145%and 55.1%at HRT of 240 min.The HRT of 180 min was determined to be a better control parameter for the R-HRAS process in terms of energy and carbon neutrality.The analysis of sludge properties showed that with the extension of HRT,the average particle size of the post-adsorption tank sludge decreased,the specific surface area increased,the zeta potential decreased,the contact angle decreased,the concentrations of TB-EPS,LB-EPS and S-EPS increased and then decreased,and the relative abundance of carbon storing bacteria spp.increased by more than 7%.This indicates that the adsorption performance and carbon storage activity of sludge increased with the extension of HRT,which in turn led to the improvement of organic carbon recovery,but the extension of HRT may make the mineralization effect of sludge on adsorbed and stored organic carbon more intense,which increases the energy consumption and carbon emission of the process while reducing the organic carbon recovery effect,and hinders the energy neutralization and carbon neutralization of the process. |
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