Cultivation And Characteristics Of Aerobic Granular Sludge In A Double-sedimentation Tank Continuous Flow Reactor

Aerobic granular sludge has a compact structure,good settling ability,biodiversity of microbial community,ability to degrade toxic pollutants and capacity to resist the loading fluctuation.This study aims to investigate the cultivation and characteristics of aerobic granular sludge in a double-sedimentation tank continuous flow reactor?DST-CFR?.In this study,the aerobic granular sludge was fed with a relatively low concentration wastewater from a wastewater treatment plant in an industrial park.And the gas stripping method was used in the inverse sludge system of DST-CFR.Through the clean water test and formal test of DST-CFR,the feasibility of gas stripping for return granular sludge and doublesedimentation tank selector was successfully validated.The small granules in the reactor were first observed on day 60.After 100 days,the percentage of granules significantly increased and the system gradually kept stable.During the stability period,the mixed liquor suspended solids?MLSS?in the aeration tank varied from 5000 mg/L to 5500 mg/L,and the MLVSS/MLSS ratio was about 45%.The sludge volume index?SVI?of aerobic granular sludge was in the range of 22-30mL/g after 100 days.The removal rates of biological oxygen demand?BOD5?and ammonia nitrogen?NH₄⁺-N?were respectively about 95% and 94%,and NH₄⁺-N was less than 2.3 mg/L.After 150 days of operation,the average particle size of granules fed with relatively low concentration wastewater was 105±3.2 ?m and the maximum particle size was 255±4.1 ?m.This thesis carried out studies aiming at morphology,specific gravity,settleability,extracellular polymeric substance?EPS?,elementary composition and microbial species of different sludge?seed sludge,aerobic granules from sedimentation tank 1 and floc sludge from sedimentation tank 2 on day 150?.The results showed that the double-sedimentation tank had an excellent selection of sludge characteristics.Obviously,almost all aerobic granules settled in the sedimentation tank 1 and floc sludge was selected into the sedimentation tank 2.The specific gravity and SVI30 of aerobic granules were 1.008±0.002 and 23.8±1.9 mL/g,which were significantly better than seed sludge and floc sludge from tank 2.Besides,the EPS content,PN/PS ratio and fluorescence intensity of aerobic granules were evidently higher than other two kinds of sludge.Meanwhile,Fe,Ca,Al,Mg,Mn and P were enriched to varying degrees in the aerobic granular sludge.In addition,the microbial diversity of aerobic granules after 150 days of operation was significantly higher,and the relative abundance of microbial operational taxonomic units?OTU?had significant changes at the levels of phylum,order and genus.And the dominant bacteria of aerobic granular sludge was obviously different from seed sludge.In this study,the unique design and construction of reactor,the influence on settleability and surface hydrophobicity of EPS,the elements enrichment,the influent SS and sludge micropowder,and the change of microbial diversity were the main promotion factors of granulation in DST-CFR.The operation of the reactor was stopped,and the aerobic granules were saved with clean water about 60 days.After the reactor restarted,the morphology,performance,settleability,bioactivity and pollutant removal efficiency of aerobic granules were analyzed.Results indicated that the system recovered well and had high stability in a long-term running.The particle size increased obviously and the selective system of the double-sedimentation tank could operate steadily with the increase of particle size.Besides,the SVI30 was maintained stably at 15mL/g.MLVSS and heterotrophic bacteria activity decreased during the suspension,attributing to the anaerobic hunger.However,in the phase of stable operation,MLVSS,MLSS and heterotrophic bacteria activity increased,the activity of ammonia oxidizing bacteria?AOB?and nitrite oxidizing bacteria?NOB?had not significant change.After 25 days of operation,the removal efficiencies of contaminant were 95% and 97% for BOD5,NH₄⁺-N,respectively.