Research On Performance And Mechanism Of Biofilm Process For Wastewater Treatment Using Nano-scale Aeration

As a widely used technology of wastewater treatment plants(WWTPs),aerobic biofilm demanded high energy consumption,whereas the traditional air blowing aeration mode has not been concerned and innovated.The loss and pollution of the aerators(aeration devices)aggravate the problem of energy waste,considering their low oxygen transfer efficiency.The in-situ generated nanobubble(NB)was applied to be an aeration method for its high oxygen transfer efficiency and stability in mature wastewater treatment processes.However,there is barely had researched on the oxygen supply capacity of NB in biological wastewater treatment units and its interactions with biological aggregates.Therefore,this dissertation was aimed to have a comprehensive investigation on issues above,to evaluate the suitability of solely NB aeration via assessing the aerating capacity of NB,the effects of stratagem controlling factors of treatment process,and integrated effects of NB aeration,environmental factors on biochemical units,and to propose a simple,stable,and effective aeration approach.Firstly,the structural growth and stability of fixed biofilm,moving bed biofilm and activated sludge were investigated to evaluate the adaptability to nanobubble aeration mode.Results confirmed that the nanobubble has high oxygen supply capacity and oxygen transfer efficiency,which was approximately 56.53 and 1.5 –folds than that in traditional blast aeration mode,respectively.Nanobubble aeration significantly enhanced the secretion of protein and polysaccharides in extracellular polymeric substance(EPS)with a more even distribution,promoting the floc size and thickness of biofilm.It showed benefits on structure development and biological activity.Wherein biofilm showed stable aerobic-anaerobic structure with abundant β-polysaccharides instead of sludge floating,representing to be a more suitable form of nanobubble aeration.Based on this,the performance and the response mechanism of biofilm in the BAF solely NB aeration with internal reflux(NB-BAF)and the BAF using a combination of weak blast aeration/NB were operated and investigated respectively,according to the high stability with weak penetrability of NB.Results indicated that two strategies were able to enhance the penetration of nanobubble via the filtration with optimized oxygen supply.The best removal result was achieved as the removal rates of chemical oxygen demand(COD)were 91.80% and 91.34% and the removal rates of total nitrogen(TN)were 54.98% and 70.69% when the weak blast air/water ratio of 0.5 and reflux ratio of 150% respectively.Two strategies led to differences in biofilm characteristics and microbial community.Proteobacteria was the dominant phyla in BAF,as well as the denitrifying bacteria and denitrifying phosphorus accumulating bacteria that availed for the removal of total nitrogen.To assess the suitabilty of nanobubble aeration under low COD/N and temperature conditions,the performance and the response mechanism of biofilm were investigated in NB-BAF that operated with designed temperature and COD/N conditions.Although the temperature reduction slightly weakened the oxygen diffusion of nanobubble,it kept 9 –folds higher than the ones in blast aeration.Temperature and COD/N showed collaborative interactions with nanobubble aeration to biofilm.Under a low air/water radio,NB-BAF showed well adaption to low temperature and COD/N conditions.Removal rates of COD and total nitrogen reached 91.78% and 76.42%,respectively.Due to the gradual release of nanobubble and the consumption of nitrifying and heterotrophic bacteria,oxygen became a limit of nitrogen removal under high COD/N condition.Below a medium COD/N,DO distribution via the filtration is more uniform,and gained a best removal rate of ammonia at 94.63% and 97.46%,as well as best TN removal rate at 86.64% and93.10%.Biofilms thickened where the aerobic and anaerobic bacteria were enriched.Proteobacteria was still the dominant phyla in BAF while Bacteroidetes and Actinobacteria were significantly affected by the conditions.Enrichment of functional bacteria,including Zoogloea and cold-resistant Nitrosospira,guaranteed the pollutant removal of NB-BAF.In summary,this research revealed the pattern of NB aeration and the response of biofilm,and built high-efficiency operating strategies for NB aerated biofilm processes,to realize the stable performance of them under low temperature and low COD/N conditions.