Cultivation Of Aerobic Granular Sludge At Low Temperature And Its Salt-tolerant Characteristics

The use of sea water for flushing can save fresh water resources in the northern coastal areas of China,with the long winter and low temperature.Due to the high osmotic pressure brought by the salinity of wastewater at low temperatures,it is difficult to treat it with traditional biological methods.Aerobic granular sludge(AGS)has many advantages such as the ability to resist external impacts,good sedimentation performance,large biological concentration and biological community diversity.AGS has been increasingly applied to the treatment of saline wastewater.Although AGS technology has broad application prospects in the treatment of salinity wastewater,its rapid formation mechanism and salinity tolerance threshold at low temperature are not clear,and further research is needed.In the paper,two different ways of adding salinity and the operation of SBR cultured AGS under different temperature conditions are introduced:gradually increasing salinity at low temperature and maintaining the same salinity at normal temperature for a long time,systematically examining the morphological structure,extracellular polymerization,physical and chemical characteristics,microbial community structure,and reactor performance evolved to explore the rapid formation mechanism at low temperatures and salinity tolerance threshold of AGS.The main results are as follows:(1)This study investigated whether and how salinity acclimation strategies can enhance aerobic granulation at low temperature(10-15℃)in three sequencing batch reactors(SBRs).Stepwise increased concentrations of NaCl(2-10 and 4-20 g/L)were added to the influent of R1 and R2 with steps of 1 and 2 g/L per week respectively,while R0 was set as a control(salt-free).The granulation processes in R1 and R2 were rapidly started up within 9 days,and were completed within 21 and 18 days,respectively.By contrast,R0 took 25 days and 49 days to start and complete granulation.The salinity acclimation strategies improved sludge hydrophobicity,reduced repulsion barrier and hydrophilic groups between cells and stimulated EPS production during granulation processes,which simultaneously promoted the formation of AGS.When the influent salinity reached 14 g/L on day 35,granule hydrophobicity,density and size in R2 sharply decreased and granules began to disintegrate afterwards.When operated under salt-free condition,sludge bulking occurred in RO since day 60.The treatment performance was thus impaired in these two reactors,especially in R2 with significant biomass loss.Conversely,the AGS developed in R1 maintained stable structure with high biomass concentration(8.0 gSS/L)and excellent treatment performance for COD(90%),ammonium(95%)and total nitrogen(70%).Genera Thauera,Azoarcus,and Nitrosomonas were more enriched,while Flavobacterium and Meganema were more suppressed in R1,which would have contributed to granule stability and treatment performance.In conclusion,great care has to be taken for cultivating and operating AGS at low temperatures for treating saline wastewater.Increasing salinity with a lower salt gradient provides a possibility for rapid granulation of AGS with excellent treatment performance under such conditions.(2)The salinity tolerance characteristics of AGS at normal temperature(20-25℃)were explored.After the above AGS of R1 was completed and placed at-20℃ for two months,the AGS was cultured in SBR at room temperature for 75 days at 5 and 15 g/L NaCl,respectively.Under the condition of 5 g/L NaCl,the increase of Aazoarcus abundance(40%)and biodiversity made AGS have excellent removal performance on chemical oxygen demand(95%),ammonia(90%),total nitrogen(39%)and total phosphorus(94%).At 15 g/LNaCl,due to increased temperature and excessive salinity,the AGS cultured at low temperature was disintegrated.The polysaccharide concentrations in EPS(230 mg/gVSS),Pseudomonas abundance(40%),Amino Acid Metabolism(11%),Membrane Transport(12%)and other functional genes increased significantly,resulting in the AGS disintegration.During the first 25 days of operation,the performance of AGS basically remained stable;after that,the performance began to deteriorate,and finally its particle size,(SOUR)AOB,(SOUR)NOB,(SOUR)HB,MLSS were reduced to 102 μm,5.3 mgO2/gSS-h,3.2 mgO2/gSS·h,15.6 mgO2/gSS·h,and 0.8 g SS/L;At this time,the removal rate of various nutrients such as chemical oxygen demand(51%),ammonia(8%),total nitrogen(2%)and total phosphorus(21%)dropped significantly.AGS cultured by increasing salinity acclimation at low temperature disintegrates when the temperature and salinity become higher,which means that it may be necessary to further acclimate the temperature and salinity to complete the process,instead of directly increasing temperature without acclimation to treat higher salinity wastewater.In the future,AGS needs to solve such problems urgently in large-scale industrial applications.