Research On The Causes And Control Measures Of Foam In The Biochemical Tank And Water Replenishment Outlet Of Urban Sewage Treatment Plants

With the acceleration of domestic urbanization in China,the utilization of reclaimed water has gradually become an important part of ecological water replenishment in areas with water shortages.In recent years,when some sewage treatment plants used reclaimed water to replenish water to the river,phenomena such as accumulation of yellow-brown foam and foam floating with the river appeared at the water replenishment outlet.At present,there are many researches on biological foam in biochemical tanks,but there are few reports or studies on the phenomenon of foaming in reclaimed water after disinfection treatment.This paper takes an urban sewage treatment plant in Qingdao as the research object,using Fourier transform infrared spectroscopy(FT-IR),three-dimensional fluorescence spectroscopy(3DEEMFS),gas chromatography-mass spectrometry(GC-MS),and high-throughput sequencing analyzing method.Then analyze the causes of foam generation were preliminarily studied from the perspectives of water quality characteristics,physical properties,organic components,and bacterial community structure of foam,and potential countermeasures for foam elimination in recycled water were proposed.The main conclusions are as follows:(1)The water quality of the foam mixture in the biochemical tank has higher organic matter concentration indicators and viscosity than the sludge mixture,and the surface tension is lower than that of the sludge mixture.The strong negative correlation between polysaccharide concentration and total iron concentration(R=-0.794)was one of the reasons for the low polysaccharide concentration in the foam mixture.The results of Fourier transform infrared spectroscopy showed that the group vibrations of the foam and the sludge mixture were similar,and the database matching results showed that the matching degree of the foam with the protein nisin(Nisin)reached63.5%.The results of three-dimensional fluorescence spectrum analysis showed that the fluorescence intensity of protein substances in foam was significantly greater than that in sludge.Gas chromatography-mass spectrometry analysis showed that long-chain fatty acids(LCFAs),alkanes,and the antioxidant BFK(2,2’-methylenebis-(4-methyl-6-tert-butylphenol))were abundant in the foam higher than sludge.Among them,the correlation coefficients of LCFAs with surface tension and viscosity reached-0.858 and 0.622,but their solubility in water was low.The amphiphilicity of proteinaceous substances exists as foaming substances in the biochemical tank.Based on the above studies,the aeration of the biochemical tank provides the bubble basis for the biological foam;the amphiphilicity of protein substances acts as a surface active substance and is a foaming agent;LCFAs,BFK,alkanes and iron oxide particles serve as stabilizers.By analyzing the microbial community structure of the foam mixture and the sludge mixture in the biochemical tank,it was found that the main microorganisms causing the foam were Actinomycetes(the abundance accounted for 58.46%).At the genus level,the proportion of Nocardia-type microorganisms Skermania,Mycobacterium and Candidatus Microthrix in the foam was significantly higher than that in the sludge mixture,and they were the main bacterial genera that caused the biochemical tank foam.(2)Ozone disinfection has no significant effect on surface tension and viscosity,while chlorine disinfection reduced surface tension.The abundance of alkanes in the water replenishment foam was significantly higher than that in the biochemical tank foam,and LCFAs and BFK still existed.Through FT-IR,3DEEMFS and other characterization methods,it was shown that protein substances were one of the main components of hydrating mouth foam,and the two substances with high matching degree were mucin(76.9%)and nisin(67.2%).The abundance of LCFAs,BFK and alkanes in the effluent of each stage of the process basically did not change.Therefore,these substances would accumulate at the water replenishment outlet with the flow of the process.The high concentration of protein substances at the water replenishment outlet might be related to the outflow of protein substances that have not been completely degraded in each stage of the process;it was also related to the sterilization of microorganisms to break cells,thereby releasing protein substances.Similar to the biological foam in the biochemical tank,the higher DO concentration at the water replenishment outlet provided bubbles for the formation of foam;protein substances were used as foaming agents at the replenishment outlet;LCFAs,BFK,alkanes and iron oxide particles were used as foam stabilizers agent exists.In short,in terms of organic components,the water replenishment foam was basically the same as the biochemical tank foam.Unlike the biochemical tank foam,the microbial community structure in the disinfection effluent and the water replenishment foam changed significantly.The proportions of Acidovorax,Undibacterium and Pseudomonas in the hydrating mouth foam reached 47.05%,30.46%and 4.14%,respectively.Among them,Acidovorax and Pseudomonas are not only chlorine-resistant bacteria,but also genera that can secrete a large amount of EPS.Undibacterium can anabolically utilize lactic acid substances,which may be the reason for the large amount of accumulation in the hydrating mouth foam.Therefore,the formation of hydrating mouth foam is related to three types of bacteria,Acidovorax,Undibacterium,and Pseudomonas.(3)Chlorine disinfection can promote the production of EPS,increased the amount of chlorine,lysed microbial cells,and further increased the protein and polysaccharide content in EPS.The research on the effect of different available chlorine concentrations on the turbidity of activated sludge microorganisms showed that chlorine disinfection would cause microorganisms to release foam-forming substances,and with the increase of chlorine disinfection dose,the foaming height would be higher.Chlorine disinfection is an indirect factor that causes foam at the refill outlet.The small-scale experimental study of available chlorine,pressure,water drop height and foam height shows that the available chlorine concentration can still increase the foam height by 33%at the lowest concentration in the experiment,significantly enhancing the foaming ability,and the effective chlorine concentration and foaming height showed very strong positive correlation(R~2=0.934);both the water drop height and pressure were positively correlated with the foaming height,and R~2reached 0.741 and 0.947,respectively.Chlorine,pressure,and drop height are the three indirect factors that contribute to the formation of fill-in foam.Finally,the laboratory experimental results showed that using granular activated carbon(ACG)as the adsorbent,3 g/L ACG can remove 97%of Nisin within 24 hours,which has a good removal effect on protein wastewater.In the actual operation of the sewage treatment plant,it is recommended to reduce the water drop height,reduce the pipeline pressure,and set up slopes,intermediate tanks or water supply fences to minimize aeration and oxygenation,so as to avoid foam pollution of river environment.