Study On The Characteristics Of Biofilm During The Start-up Period Of Moving Bed Biofilm Reactor

Moving bed biofilm reactor is a new type of high effective technology for wastewater treatment and become populer.The formation and development of biofilm is very crucial to the start-up of MBBR and the property of biofilm is close to the performance of MBBR.Therefore,it is necessary to study the mechanism of biofilm formation and development during MBBR start-up period.This thesis used synthetic wastewater as the the influent water and studied the biofilm characteristics during the start-up period to reveal the formation and development of biofilm.Biofilm characteristics included biofilm biomass,extracellular polymeric substance(EPS),autotrophic and heterotrophic respiration rate,biofilm morphology and microbial community structure.Moreover,we compared and analysed different conditions of MBBR opertation,including feeding modes,filling ratio of biocarriers and wastewater temperature.The results were as follows:(1)The moving bed biofilm reactor with intermittent influent was much better than the continuous condition for the start-up period.Reasons including:firstly,the ability to degrade NH4+-N was better in intermittent influent MBBR,the removal rate was 28%higher than continuous one.Secondly,during the first four days,the adhered biofilm would detach from the surface of carrier.The detached biofilm was mainly composed by filamentous bacteria and maintained in the continuous reactor,and they would grow quickly by using nutrition.However,the over growth of suspended filamentous bacteria would restrict the development of attached biofilm.The VS content of continuous MBBR was half of the intermittent.Whereas,EPS was promoted by suspended filamentous bacteria.Lastly,the microorganism of biofilm was dominant by filamentous bacteria in a continuous reactor and the diversity of microbial community was degraded.Inversely,the structure of biofilm was clusered or filar.(2)For reactors with different filling ratio.When the filling ratio was increased,the removal rate of N4+-N,accumulation rate of biomass,activity of autotrophic microorganism were improved accordingly.However,the activity of heterotrophic microorganism and the rate of biomass detachment was reduced at the same time.In addition,the diversity of microorganism and the content of filamentous bacteria were more higher with lager filling ratio.In the reactors with 20%,35%and 50%filling ratio,filamentous bacteria accounted for 37.31%,41.41%and 45.83%respectively,including Actinobacteria,Sphaerotilus and Haliscomenobacter.However,the difference was much smaller during the initial attachment.The initially attached microorganism was Gram-negtive bacterium,most belonged to two phylum of Proteobacteria(86.38-87.43%)and Bacteriodetes(4.17-6.05%).In genus level,the dominanted bacteria were Acinetobacter,Sphaerotilus and Zoogloea.Moreover,the content of(PN+PS)/VS was kept to 243.8 ± 12.85 mg/g in mature bifilm with different filling ratio.(3)For reactors with different culture temperatures,the temperature was very important for the start-up of reactors.Compared to room temperature,the reactor with low temperature also could achieve a good efficiency of COD removal.However,the removal rate of NH4+-N was reduced by 6.15%.Low temperature would restrain the growth of biofilm that the biomass was halt of reactor worked in room temperature,reduce the activities of heterotrophic and autotrophic microorganism.Whereas,the low temperture promoted the productivity of EPS and the growth of some cold-adaptive microbes,including Flavobacterium,Lactococcus,Trichococcus and Aeromonas.The dominated microorganism of biofilm cultured in room temperture were Proteobacteria(30.5%),Bacteriodetes(22.2%)and Acinetobacter(25.6%),and filamentous bacteria accouted for 37.8%among these microorganisms,including Actinobacteria,TM7,Haliscomenobacter and Sphaerotilus.However,high temperature increased the removal rate of NH4+-Nand the activities of heterotrophic and autotrophic microorganism.High temperature had no effects on biomass and EPS accumulation.The content of(PN+PS)/VS was similar to the biofilm in room temperature about 262.2 mg/g.The microorganism in high temperature was also dominanted by filamentous bacteria(36.65%),including Actinobacteria and TM7.In addition,the shape of initial attched microbes was bacilliform.And the speed of biofilm formation increased along with the temperature.(4)During the inoculation process,microbial adhered at random,but gram-negative bacteria obtained a preponderant position.Moreover,the initial adhered microorganisms were dominanted by Proteobacteria.However,with the growth of biofilm,the content of Proteobacteria decreased,and filamentous bacteria(the content was 24.32-37.80%)became to the main compisition of mature bifiom(except low temperature).In addition,For different reactors with same oraganic matter and concertration,the relative content of EPS and VS in maure biofilms repesented by(PN+PS)/VS kept 252.50 ±13.43 mg/g(low temperature was exclusive).We speculated that(PN+PS)/VS could reflect the bioflilm station.(5)Under the condition of the same temperature,the growth of biofilm and speed of bioreactor start-up can be promoted through by increasing the concerntration of influent COD and ammonia nitrogen,and the economic benefits were more obvious than the method of increasing filling ratio.