Inveatigating The Effect Of Packing Carriers On In Situ Side-stream Sludge Reduction System

Massive waste activated sludge generated from the activated sludge process is still one of the most pressing challenges in the operation and management of wastewater treatment plants,although the process has been serving society for more than 100 years.Sludge in situ reduction strategy has been proposed as a cost-efficient way to minimize waste activated sludge production within the biological wastewater treatment process rather than struggling with posterior sludge treatment and disposal,which was a burden for wastewater treatment plants from economic,environmental and managerial perspective.Biological sludge in situ reduction process with an anaerobic side-stream reactor?ASSR?placed in the sludge return loop and its derivation,oxic-settling-anaerobic process,are promising ways that can be employed in full-scale applications.Nevertheless,deterioration of effluent quality owing to the increase of suspended solids,and high hydraulic retention time requirements for ASSR were two major obstacles for ASSR application.Therefore,the combination of membrane bioreactors?MBR?with ASSR?A-MBR?provides a promising alternative for sludge reduction by preventing suspended solids overflow,decreasing footprint and enhancing biological maintenance.However,membrane fouling could result in a sharp flux decrease,hydraulic retention time changed or swift jump in trans-membrane pressure,high energy consumption,and frequent membrane cleaning or replacement,which typically bring out increasing costs in maintenance and operation.The regulation mechanisms and crucial parameters of sludge reduction and membrane fouling in A-MBR will be advanced to provide theoretical support for its applications.Anoxic/oxic membrane bioreactor?AO-MBR?and A-MBRs were constructed to investigate the effects of ultrasonication and packing carriers to sludge reduction,biological nutrient removal and membrane fouling.On this basis,the effect of ultrasonication coupled carriers and filling fractions on system operation was further analyzed to determine the optimal enhanced strategy filling fraction conditions and analyze the mechanisms of sludge reduction and membrane fouling in the systems.Through the analysis of microbial population structure and sludge characteristics,the sludge reduction and membrane fouling mechanism of the SSR process was clarified.The leading mechanism of sludge reduction and the mechanism of membrane fouling will be studied to provide theoretical support for its applications.An AO-MBR,an A-MBR,an A-MBR with carriers packed in ASSR?AP-MBR?and an AP-MBR with sludge ultrasonicated before ASSR?AUP-MBR?were operated for 261 d to investigate effects of ASSR,packing carriers and ultrasonication on sludge reduction and microbial population.Sludge reduction efficiencies?SRE?of A-MBR,AP-MBR and AUP-MBR were 36.2%,46.4% and 51.4%,respectively.Compared to AO-MBR,A-MBR showed more severe membrane fouling,AUP-MBR showed the most severe membrane fouling and obvious cake layer fouling;on the contrary,packing carriers in ASSR alleviated membrane fouling of ASSR-MBR.Packing carriers and ultrasonication strengthened the sludge cell lysis and humification of the system,and improved the release of the secondary materials.Anaerobic hydrolysis of particulate organic matters mainly occurred in ASSR,while cell lysis mainly occurred in aerobic conditions.In addition,packing carriers in ASSR further reduced sludge mass by 2.46 and 3.63 g/d in AP-MBR and AUP-MBR,respectively,while ultrasonication further enhanced sludge reduction by 2.34 g/d in AUP-MBR,indicated that there is a non-significant coupling effect between ultrasonication and packing carriers,which slightly increased the SRE.Packing carriers and ultrasonication both enhanced hydrolysis by stimulating activities of ?-glucosidase and protease,while uncoupling metabolism was enhanced greatly by packing carriers but slightly by ultrasonication.Linear discriminant analysis of effect size results showed that packing carriers promoted the growth of hydrolytic and fermentative bacteria in bulk sludge,and enriched anaerobes and fermentative bacteria on the surface of carriers.Ultrasonication screened ultrasonication-resistant bacteria,and created an anaerobic environment beneficial to hydrolytic and fermentative bacteria.Ultrasonication is often not so economic to reach higher enhancement of sludge reduction because it requires very high energy levels,therefore,packing carriers is probably more cost-effective and advantageous to enhance sludge reduction.An AO-MBR,an A-MBR and an AP-MBR were operated parallelly to investigate mass balance,kinetics and functional bacteria of biological nutrient removal in sludge reduction systems.Compared to AO-MBR,AP-MBR and A-MBR were both efficient in chemical oxygen demand?COD?and ammonium nitrogen?NH₄⁺-N?removal,had significantly higher nitrogen removal,but deteriorated biological phosphorus removal.Nitrosomonadaceae and Nitrospira were major bacteria responsible for ammonium and nitrite oxidation in the three systems.Inserting ASSR and packing carriers both favored denitrifying bacteria enrichment and organic substances release,and thus resulted in higher nitrate uptake rate in the anoxic tank.Higher endogenous nitrate uptake rate in ASSR than in anoxic tank also indicated that ASSR and carriers both accelerated sludge decay.Denitrification and sludge reduction occurred in ASSR played important roles in biological nutrient removal.In order to further shorten the hydraulic retention time of A-MBR and reduce the footprint requirements,the effect of filling fraction and contributions of different mechanisms on the sludge reduction of A-MBR was studied.An AO-MBR and three pilot-scale A-MBRs,which packed with 0%?A-MBR?,25%?APL-MBR?and 50%?APH-MBR?carriers in ASSRs,were continuously operated to study pollutants removal and mechanisms for sludge reduction.Four systems showed efficient COD and NH₄⁺-N removal,while packing carriers significantly enhanced nitrogen removal.APL-MBR achieved the highest sludge reduction efficiency of 50.5% compared to A-MBR?21.7%?and APH-MBR?39.7%?.Compared to conventional ASSR-MBR,packing carriers enhanced the release of dissolved organic matters,accelerated the secretion of enzyme for cell lysis and hydrolysis.Cell lysis was dominated route for sludge reduction in aerobic MBR,while inserting ASSR and packing carriers greatly enhanced anaerobic hydrolysis.For the roles of carriers in APL-MBR,the enhancement by carriers themselves and to the bulk sludge accounted for 25% and 75%,respectively,suggesting that the presence of carriers prompted the formation of environment propitious to sludge reduction in bulk sludge.APL-MBR tended to enrich hydrolytic,fermentative and denitrifying bacteria to accelerate hydrolysis process,while excessive carriers had negative effect on biomass stability and movement of carriers.Archaea taxonomic analysis showed that packing carriers provided a place for the life of microorganisms.After packing carriers,microorganism micro-habitat and community structure were varied with the growing status of biomass,and thus reported as an essential strategy bringing out discrepancies in membrane fouling.Accordingly,further operated AO-MBR,A-MBR,APL-MBR and APH-MBR to investigate the appropriate filling fraction of carriers packed in ASSR,influences on membrane fouling and microbial community of A-MBR.Carbohydrates and protein were the major organic substances in membrane foulants,and protein were the most important substances causing membrane fouling.Compared to AO-MBR,inserting ASSR without carriers induced the release of viscous components in extracellular polymeric substances and the formation of calcium carbonate-related bacteria on membrane surface,and thus deteriorated membrane fouling.Packing carriers with 25% filling fraction promoted the hydrolysis of soluble microbial products and extracellular polymeric substances,whilst reduced the viscoelasticity of sludge flocs.Higher filling fraction of 50% increased the shear forces to the biofilm and biomarkers related to membrane fouling,and thus showed little improvement to alleviate membrane fouling.MiSeq sequencing revealed that although it enriched in the bulk sludge of conventional A-MBR and the coupled reactor with filling fraction of 50%,the floc-forming,hydrolytic and fermentative bacteria were more inclined to attach on the membrane surface and alleviate fouling process.