| Sludge bulking is a globally common problem to wastewater treatment plants(WWTPs),which has always attracted considerable attention because of the high frequency,universality and harmfulness.Bulking sludge has been studied a lot all over the world,however,the characteristics,mechanisms and control of bulking sludge and variations of bulking bacteria in bulking sludge control process are still not clear.In particular,different results have been reported about the effects of nutrients imbalances(especially nitrogen)on the performance of bulking sludge under the single carbon source in laboratory experiments.Furthermore,research on the community migration regularity in bulking sludge(especially bulking bacteria and some functional bacteria)under the single carbon source has been rarely reported.At present,sludge volume index(SVI)is the main indicator to judge the occurrence of sludge bulking(usually SVI>150 mL·g-1 is considered bulking occurs).Other physiological and biochemical indicators that could reflect the performance of bulking sludge,such as Zeta potential(Zeta),extracellular polymeric substances(EPSs),poly-?-hydroxybutyrate(PHB),filamentous index(FI),specific oxygen uptake rate(SOUR)and lipopolysaccharide(LPS),have become research hotspots gradually.Therefore,more research on the performance and microbial community of bulking sludge under the single carbon source are needed,in order to fully understand the characteristics of bulking sludge.In the current work,bulking sludge was the seeding sludge of the system,and sodium acetate was selected as the single carbon source in laboratory experiments.To control the performance and microbial community of bulking sludge,different influence conditions were studied,including normal nutrient condition,nutrient limitation condition,nitrogen limitation with carriers added initially condition,and different nitrogen limitation ratio with carriers added in later stage condition.This article focused on the control and recovery process of bulking sludge settleability,and the improvement of bioreactor performance,bulking sludge performance and the abundance of functional bacteria by adding carriers.Furthermore,the ratio of nitrogen limitation condition that had the relative best control effects was optimized,and then a set of optimized process for improving the settlement performance of bulking sludge was proposed,in order to enrich and improve the theoretical system of bulking sludge,providing a theoretical basis and reference for the control of sludge bulking in practical engineering.The research results are as follows:(1)The effects of normal nutrient and nutrient limitation conditions on the performance of the bulking sludge were preliminarily discussed,results found that the nitrogen limitation(C:N:P=100:1:1)condition could be used to control the settleability of bulking sludge.In the bulking sludge system that using sodium acetate as the single carbon source,results showed that the sludge SVI of R0 would continue to increase if the influent kept the normal nutrient condition continuously;while the sludge settleability of nitrogen limitation alone(RN)reactor could recover to normal(SVI<150 mL·g-1).Under normal nutritional conditions,the Pearson correlation analysis showed a significant negative correlation between the LPS relative content(LPS/MLVSS)and settleability of bulking sludge,while under nutrient limitation conditions,LPS showed high accuracy to reflect biomass of activated sludge.PCoA analysis showed that the migration of the community of the reactors experienced nitrogen limitation(RN,RNP)changes greatly during the stage II and III.Thiothrix is the dominant bulking bacteria in all reactors,and RDA analysis showed that the positive correlation of Thiothrix with settling performance and oxygen consumption rate was significant.(2)The effects of nitrogen limitation with carriers added initially condition on the performance of the bulking sludge were studied,results showed that the emergence of numberous microfauna under the nitrogen limitation(C:N:P=100:1:1)condition could recover the settleability of bulking sludge,which could recover to normal sludge settleability again after the influent return to normal nutrient condition.In the bulking sludge system that using sodium acetate as single carbon source,results showed that the reactor adding carriers(RC)could keep stable settleability(SVI=254-351 mL·g-1)and appear better performance(including higher removal efficiency of COD and TN,and lower effluent SS concentration)than the reactor without carriers(RA)under the nitrogen limitation(C:N:P=100:1:1).While significant effects on bulking control could be observed in RA under the nitrogen limitation.The sludge SVI of RA could keep a stable recovery level(SVI<120 mL·g-1)from 26 d to 60 d until the influent return to normal nutrient condition(C:N:P=100:5:1)since stage ?.And then,RA could recover a great settlement performance(SVI<90 mL·g-1)again from 80 d to 100 d.In addition,it was found that RA could still keep low sludge SVI while with a high FI,presumably because of the growth of numberous microfauna,mainly including vorticella and rotifers.They could ingest filamentous bacteria,accelerate the flocculation of sludge flocs by secreting sugar and mucus,improve the C/N of the substrate,optimize the bacterial community and bound to filamentous bacteria that beneficial to settling performance.Thiothrix and Sphaerotilus are dominant filamentous bacteria in the system,adding carriers could control the growth of them and improve the enrichment of bacteria possessing the function of nitrifying,denitrifying and phosphorus accumulation in the sludge system,and provide a good buffer in the activated sludge system.RDA analysis showed that the sludge settling performance had a significant negative correlation with Thiothrix and Sphaerotilus,while a significant positive correlation with Caldilinea and Haliscomenobacter.Meanwhile,a significant positive correlation of the sludge settling performance with MLVSS and LPS were both observed,as well as the significant positive correlation between MLVSS and LPS,which could both indicate the system biomass.(3)The effects of different nitrogen limitation ratio with carriers added in later stage condition on the performance of the bulking sludge were studied,results found that the ratio of nitrogen limitation condition that had the relative best control effects for bulking sludge was C:N:P=100:2:1(RN-2).In the bulking sludge system that using sodium acetate as single carbon source,results showed that all of the four reactors could control the bulking sludge under the nitrogen limitation of different gradient,but different ratios of nitrogen sources had effects on the rate and time of the sludge settleability recovery.The abundance of Caldilinea,which belongs to filamentous bacteria,had a higher abundance of 2.1%in seeding sludge.It showed decreasing trends in all four reactors,which were consistent with the tendency of sludge settleability.Also,the variations of Thiothrix were basically consistent with the sludge settleability,and adding carriers was beneficial to control the growth of Thiothrix,so that to improve the sludge settleability.Under the nitrogen limitation,the relative abundance of Pseudomonas that belongs to nitrogen-fixing bacteria in all four reactors increased continuously,with the increasing range from high to low as RN-0>RN-1>RN-2>RN-4,respectively.It inferred that the abundance of nitrogen-fixing bacteria,which could fix the nitrogen in the wastewater as inorganic nitrogen so that to provide nitrogen source for the activated sludge system and relieve the nitrogen limitation,may increase with the nitrogen limitation level increased.In addition,adding carriers could promote the growth of nitrogen-fixing bacteria.PCoA analysis showed that the nitrogen limitation more serious,the migration of the community of the reactors more significant.While the community of four reactors kept stable after adding carriers,suggesting that adding carriers could relieve the nitrogen limitation in the system.RDA analysis showed a significant negative correlation between Zeta and PHB,suggesting that the accumulation of PHB in the sludge may increase the electronegativity of the sludge flocs,resulting in the increased Zeta potential and decreased flocculation performance of the sludge flocs.However,adding carriers in later stage(?)could reduce the Zeta potential and improve the flocculation performance of sludge flocs.Based on the results,we suggest that adding carriers could improve the sludge performance and the optimized nitrogen limitation ratio is of RN-2(C:N:P=100:2:1)in order to control bulking.(4)The effects of different influence conditions on the performance of the bulking sludge that used sodium acetate as the single carbon source were analyzed and compared,and the optimized control strategy for bulking sludge was proposed.Results showed that nitrogen limitation could always recover the bulking sludge settleability,and adding carriers could improve the bulking sludge performance,especially promoting the enrichment of functional bacteria such as nitrification,denitrification,phosphorus accumulation,and perchlorate degradation bacteria in activated sludge.Based on the bioreactor performance,sludge settleability,physiological,biochemical properties and microbial community structure(distribution of filamentous bacteria and some functional bacteria),we proposed a set of optimized process for improving the settleability of bulking sludge:?(nitrogen limitation(C:N:P=100:2:1)alone to improve the settleability of bulking sludge)+?(nitrogen limitation+carriers,to improve the performance of bioreactor and sludge)... |
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