The Formation And Enhancement Mechanisms Of Algal-bacterial Symbiotic Granular Sludge Based On Quorum Sensing

Aerobic granular sludge?AGS?is a type of special biofilm structure formed by the microorganisms of activated sludge via self-aggregation.AGS is considered to be very promising due to its relatively dense micro-structure,high biomass retention,excellent settling ability and low footprint.However,the high aeration energy consumption,long start-up time and unstable particle structure during the long process are major problems impeding its large-scale applications.To address these scientific questions,this study suggested that the use of the close ecological relationship between bacteria and algae in the microalgal-bacterial consortia,such as gas exchange,material circulation and nutrient dependence,might provide a new insight for overcoming the above technical bottlenecks.At the same time,it is expected to solve the difficult problems of algal biomass harvesting,large footprint and long treatment process in the microalgal-bacterial consortia.Based on this,this study proposed to couple the AGS technology with the algal-bacterial symbiotic system to construct a new algal-bacterial granular sludge?ABGS?system,in order to explore whether the ABGS system can maintain the stability under the low aeration conditions to reduce the energy consumption,owing to in situ oxygen production by algae.In addition,this study explored the regulatory effect of N-acyl homoserine lactone?AHLs?in the ABGS development and maintainance of stability,with the purpose of revealing the formation mechanism of ABGS.On this basis,a new strategy for rapid formation of ABGS based on AHLs-producing bacteria was proposed,and the stability of ABGS formed by short-term reinforcement in actual sewage treatment was confirmed,which provided a theoretical basis for further expansion of engineering applications.The formation process and characteristics of ABGS were studied in this paper.Based on the sequential batch reactor for developing AGS,the spontaneous growth of algae in the system was promoted by controlling the external light conditions,and thus the ABGS system was constructed.Based on the optimization of the operating parameters of the system,the formation process and characteristics of ABGS were studied.The results showed that the optimal conditions for developing ABGS were light intensity of 142 ± 10 ?mol/m2·s,light/dark cycle of 12 h light/12 h dark,and the aeration volume of 24 L/m2·s.The ABGS formed under the above optimal conditions were intact and compact,and the settling ability was excellent?SVI30 28?34 m L/g?.Moreover,the granulation was obviously enhanced.When the sludge was completely granulated,the aeration rate is gradually reduced to 10 L/m2·s.The microbial activity in the ABGS system still remained at a high level,and the treatment performance of pollutants was excellent.The average removal rates of COD,NH₄⁺-N and PO₄^3--P were 96.7%,99% and 89.9%,respectively.After the formation of ABGS,the species diversity of bacteria and algae was reduced.From the perspective of microbial community composition,Alpharoteobacteria,Betaproteobacteria and Gammaproteobacteria were the most dominant bacterial species,while Chlorophyceae,Trebouxiophyceae and Bacillariophyceae were dominant algae species.The roles of quorum sensing signal molecules?AHLs?in the granulation process of ABGS development,production of extracellular polymer substances and microbial community were further explored,and the formation mechanisms of ABGS were elucidated.The results showed that the production of AHLs in the ABGS system was depended on the biomass density.When the sludge density was lower than 1.0143 g/m L,the genes related to AHLs were not expressed.When the biomass density was higher than 1.0288 g/m L,AHLs concentration decreased slowly.The reasons for this phenomenon was possibly due to i)the enrichment and growth of algicidal bacteria?Flavobacterium?,which led to the lysis of algal cells and thus the original "bacterial-algal" symbiotic relationship was broken,and ii)in order to resist bacterial damage to their activities,AHLs quenching agents or analogues were produced by algae to block or interfere with the bacterial QS system,and the excessive growth of bacterial density was suppressed.As a result,the contents of AHLs reduced.The relative content of AHLs was highly related to sludge density,protein concentrations and microbial attachment.0.2 g/L vanillin was used as quorum quencher for signal molecule AHLs contained in the mature ABGS,and an obvious reduce was observed in microbial attachment,EPS content,relative hydrophobicity and strength of granular sludge after 24 h,indicating that AHLs played an important role in maintaining the stability of ABGS.By HPLC-MS/MS analyses,HSLs with acyl side chain length ? C10 were common signal molecules in ABGS system,of which C6-HSL and 3OC8-HSL were two main signal molecules in the ABGS system.These two signal molecules were involved in mediating the granulation process of ABGS,the synthesis of EPS,and played an important role in regulating the compositions of microbial community.This paper proposed an enhancement technology of granulation based on quorum sensing.The AHLs-producing bacteria were successfully isolated from the granular sludge system.By comparing the effects of the dosage of CEBs and AHLs supernatant on the granulation process,a sustainable and feasible method for rapid granulation of activated sludge was proposed.The treatment performance of real municipal wastewater by ABGS and the stability of the system under short-term high load were investigated.The particle size of the ABGS decreased and the settling performance deteriorated after addition of CEBs at the initial stage of sludge granulation,and the particle size showed a slow upward trend but slowly recovered when CEBs were dissolved,indicating that this method was not practically feasible.On the contrary,the addition of AHLs supernatant accelerated the granulation rate,improved the settling performance of the sludge,and induced the sustained release of AHLs and the secretion of tryptophan and aromatic proteins in TB-EPS.The ABGS suppressed the excessive proliferation of filamentous bacteria due to its dense structure.The above results showed that the addition of AHLs supernatant during the initial stage of granules formation could accelerate the sludge granulation.The short-term addition of AHLs supernatant increased the biomass concentrations and biological activity,and achieved the rapid start-up of ABGS system.The average particle size reached 0.345 mm,and the zeta potential and the contact angle with water were-9.41 ± 0.73 m V and 93.11°,respectively,after 25 days.The total interface free energy G_adh was-106.04 m J/m2.The surface hydrophobicity of the sludge was enhanced after bioaugmentation.The ABGS had a lower repulsion barrier and the aggregation between sludges was enhanced according to the XDLVO theoretical analysis.The ABGS after bio-augmentation treatment had excellent removal efficiencies of COD,TN and TP in real domestic sewage.The effluent concentration was low,which met the Class A emission standard of Pollutant Discharge Standards for Urban Sewage Treatment Plants?GB 18918-2002?.In addition,the ABGS showed a good resistance to short-term influent high carbon,nitrogen and phosphorus loads,and the stability of granular structure was remained for a long time.The successful construction of ABGS system will lay a good theoretical foundation for realizing the high efficiency and low consumption for biological treatment of urban sewage,and provide a new insight for development of sewage treatment field in the future.