| Aerobic activated sludge technology is widely used in urban domestic sewage treatment,but it has the disadvantages of large area,poor pollutant treatment stability and low impact loading resistance,which restricts the energy saving and emission reduction of sewage treatment plants.Aerobic granular sludge(AGS)technology provides a new method for wastewater treatment due to its excellent settling performance,efficient and stable pollutant removal capacity,and small footprint.However,the problems of high energy consumption caused by high superficial gas velocity(SGV)culture condition,and the granule disintegration and nutrient removal deterioration caused by microbial endogenous respiration or filamentous bacteria bulking,restrict the application of this technology.The purpose of this study is to cultivate AGS with low energy consumption and improve its structural strength and operational stability.The experimental study of sludge granulation with skeleton strengthening as the core was carried out under low SGV.The temporal and spatial succession characteristics of microbial community structure in the granulation process was explored from molecular biological level to study the strengthening mechanism on the AGS structure.The aerobic granulation and stable operation of AGS was studied in SBR at low SGV(1.0 cm/s).After 120 d of operation,AGS was successfully cultured,with an average diameter of 3.5 mm and an average settling speed of 56 m/h.?-polysaccharide maintained the integrity of AGS together with filamentous bacteria.In the stable stage,AGS showed high pollutant removal ability.The removal efficiencies of COD,NH4+-N,TN and TP were 93.5%,99.1%,75.2% and 98.5%,respectively.Compared with the optimum SGV,the aeration intensity was reduced by about 60%.However,under low SGV,rapid growth of granule size in the early stage and overgrowth of filamentous bacteria led to a low structural stability and frequent disintegration of AGS.As a result,AGS and flocculent sludge coexisted in the reactor.In order to improve the structural stability of AGS,carbon fiber(CF)and micro sponge(MS)were applied to strengthen the linear and spatial network skeleton of AGS.The results revealed that both of them could effectively promote the maturation and increase the structural strength of AGS.The sedimentation speed of AGS was increased to 71 and 120 m/h,respectively,which was 1.4% and 71.4% higher than AGS cultivated under high SGV.The strengthening efficiency of the spatial network skeleton was more obvious,while the reactor could achieve stable operation within 60 d,which was 60 and 35 d earlier than the conventional granulation process and linear skeleton strengthening process,50% and 36.8% reduction in AGS maturation time.The exogenous skeleton worked by replacing part of the filamentous bacteria and ?-polysaccharide skeletons in AGS.With the strengthening of exogenous skeleton,the total amount of protein and polysaccharides in AGS decreased,but the content of tightly bound polysaccharides increased by more than 38%.The denser the exogenous skeleton,the higher the content of tightly bound polysaccharide,and the higher the degree of substitution for AGS's own skeleton.The structural strength and DO penetration depth were the main factors affecting the microbial community structure.Flavobacterium,Candidatus?Competibacter,Nitrospira,Pseudomonas,norank?f??Saprospiraceae and norank?p??Saccharibacteria showed a high correlation with both.The influencing factors of the skeleton-strengthened granulation process were analyzed.The results showed that the effective compensation of exogenous skeleton for SGV could further reduce the required SGV by 40%.SGV at 0.6 cm/s not only accelerated the granulation process,but also promoted the enrichment of denitrifying phosphorus-accumulating bacteria in the granules,making the TN removal efficiency increase to 99%.The organic matter concentration of influent had a significant effect on the formation of AGS.When COD concentration was 200 mg/L,excessively low organic loading rate(0.8 kg COD/m3/d)led to insufficient microbial proliferation rate and extracellular polymer s(EPS)secretion.TN and TP removal efficiencies were only 64.8% and 50.9%.When the influent COD concentration was 800 mg/L,the removal rate of TN and TP increased to more than 92.6% and 98.5%.However,the excessively high sludge loading rate(1.5 g COD/g SS/d)at the initial stage made the interior space quickly filled with EPS.As the sludge loading rate droped below 0.6 g COD/g SS/d,granulation process was completed quickly.Hydraulic retention time affected the formation of AGS by acting on the feast-famine effect.With the extension of hydraulic retention time,the conversion efficiency of influent organic matter to EPS increased.However,when the hydraulic retention time was 12 h,lower organic loading rate(0.8 kg COD/m3/d)and longer famine period(5 h)during typical cycles made the microbial proliferation rate slower and the endogenous consumption of EPS increase,which was not conducive to sludge granulation.Compared with the characteristics of AGS at room temperature,the formation and stable operation of conventional AGS and skeleton-strengthened AGS at low temperature(10 ?)were also investigated,and the effect mechanism of low temperature on granulation process at low SGV was explored.The results showed that under low temperature conditions,due to the excessive growth of filamentous bacteria and unclassified?f??Comamonadaceae in the conventional granulation process,complete granulation could not be achieved autonomously.The reactor did not achieve stable operation until day 160.Low temperature significantly inhibited the denitrification process in conventional granules,and the removal efficiency of TN in the stable stage was only 60%.In the process of skeleton-strengthened granulation,the time for the reactor to enter a stable state was shortened by about 90 d,and the average removal efficiencies of TN and TP were 70.5% and 99.7%,respectively.The richness and diversity of the microbial community were 1.6 and 1.1 times higher than that of conventional system,and the relative abundance of unclassified?f??Comamonadaceae was kept below 5%,indicating that the skeleton-strengthened granulation had higher resistance to low temperature,stronger control of microbial community structure,and higher removal efficiency for pollutants such as nitrogen and phosphorus. |
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