Effects Of PAC And Sepiolite On The Formation Of Aerobic Granular Sludge Under EBPR Conditions

Aerobic granular sludge is the self-immobilization of microorganisms under aerobic conditions.Compared to the conventional activated sludge,aerobic granular sludge has a regular,dense,and strong structure and good settling properties,and thus enable a high biomass retention and withstand high strength wastewater and shock loadings.Therefore,the aerobic granule system has become a promising technology for wastewater treatment.Most of the conducted research on aerobic granular sludge has focused on the cultivation under aerobic conditions,and less research on Enhanced Biological Phosphorus Removal(EBPR).The EBPR is a wastewater treatment process,which operates under alternating anaerobic and aerobic conditions,this suggests that a simultaneous nitrification,denitrification,and phosphorus removal of aerobic granule is possible under EBPR conditions,which may provide a promising technology.Up to date,the aerobic granular sludge has been successfully cultivated under EBPR conditions and the simultaneous removal of nitrogen and phosphorus was observed.However,a long start-up period and instability are usually required for the development of aerobic granules from flocculent sludge,these shortcomings impede the application of this technology for real wastewater treatment.Previous study showed that adding Polyaluminium Chloride(PAC)can accelerate the start-up of aerobic granules,whether PAC can accelerate granulation under EBPR conditions is unknown.Moreover,high levels of PAC have an inhibiting effect on microbe growth,and studies have shown that adding Sepiolitemay may reduce this effect.In this study,laboratory experiments were conducted with three sequencing batch reactors(SBRs)to investigate the effects of PAC and PAC/Sepiolite on the formation of aerobic granules under EBPR conditions.Then we analyzed the bacterial communities of activated sludge and granular sludge,and the effect of PAC?PAC/Sepiolite on it.The main results are as follows:(1)The granular sludge was formed at 52 days?56 days and 44 days in RC(Control)?RP(PAC)and RPS(PAC and Sepiolite)reactors.The diameter of mature aerobic granular sludge in RC?RP and RPS reactors was 0.426mm?0.409mm and 0.374mm,respectively,and the wet density was 1.037 g/cm~3,1.033 g/cm~3,1.057 g/cm~3.It showed that PAC/Sepiolite could improve the formation of granular sludge,moreover the sludge have a denser and stronger structure.(2)During and after the formation of aerobic granular sludge,the removal rate of TP is higher than 95%and the effluent TP concentration meeted class A standard.The three reactors possess strong phosphorus removal capacity under alternating anaerobic and aerobic conditions.RPS was more stable than RC and RP,and adding PAC and Sepiolite could enable aerobic granular sludge to withstand shock loadings.The COD removal rate can meet the second class standard and the TN removal rate can meet the class A standard in the three reactors.(3)Both the Chao1 and the Ace indices suggested that the species richness of RP is the largest,followed by RC and RPS,and RS is the lowest.And the Shannon and Simpson indices suggested that the species diversity of RS was the highest,and the diversity of RC and RPS was slightly lower and the diversity of RP was the lowest.The above results indicate that the species diversity of the granulation process is reduced,but the species richness increased during granulation.The results of OTU and NMDS analysis showed that there was a great difference in the bacterial community structure between flocculent sludge and granular sludge,and the addition of PAC had a great influence on it.(4)During the granulation of sludge,the dominant class in sludge changed significantly from Bacteroidetes(RS,30.80%)and Sphingobacteriia(RS,13.0%)to Betaproteobacteria(RC,30.7%;RP,47.1%;RPS,32.8%)and Gammaproteobacteria(RC,12.6%;RP,8.8%;RPS,13.1%)The abundance of Betaproteobacteria class in RP group was significantly higher than that in other groups,while Gammaproteobacteria was the lowest.It suggested that PAC can affect bacterial community.(5)After granulation,the abundance of Azoarcus genera with denitrification capability greatly from 1.2%in RS to 15.8-36.1%after granulation and Rhodocyclus(0.3%)increased to 4.0%(RC)?3.3%(RP)and 9.2%(RPS).These results indicated that the addition of PAC may help the growth of denitrifying bacteria or inhibit the growth of other bacteria,thus,the denitrifying bacteria were enriched and the abundance increased.However,the addition of PAC and Sepiolite increased the abundance of phosphorus accumulating organism,so as to improve the phosphorus removal capacity of EBPR system.