| In the process of social and natural recycling of water,wastewater treatment is a vital part.The degree of wastewater treatment and the level of pollutant discharge directly determine the quality of water in the natural circulation process.Due to its unique technical and economic advantages,the activated sludge process is widely used in municipal wastewater treatment.Activated sludge mainly exists in the form of activated sludge microbial micelles,which are composed of activated microorganisms and extracellular polymeric substances?EPS?.The activated sludge process for the treatment of municipal wastewater produces a large amount of waste activated sludge?WAS?that needs to be treated and disposed.In the WAS,the organic matter in the form of extracellular polymeric substances?EPS?could reach 60%-80%of the quality of WAS.The research on EPS dissociation technology is of great significance for the resource utilization of WAS.From the point of view of WAS resource utilization and taking WAS of MBR system of actual domestic sewage treatment as the research object,the dissociation effects of free nitrous acid?FNA?under low NO2--N concentration on EPS of waste activated sludge from municipal wastewater treatment plants were studied by setting different pH values?6.0,5.0?,NO2--N concentrations?0,100,200mg/L?and reaction times?3h,6h,12h,24h,48h,68h,120h?.At the same time,the dissociation effects of low temperature short-time heat treatment on EPS of waste activated sludge from municipal wastewater treatment plants were studied by setting different reaction temperatures?50,60,70,80??and reaction times?15,30,45,60 minutes?.Finally,the feasibility of FNA and heat composite treatment on WAS was studied,and the dissociation effects of FNA and heat treatment on EPS of waste activated sludge from municipal wastewater treatment plant were compared.The main conclusions were as follows:?1?Considering the economic and EPS dissociation efficiency,under the conditions of FNA concentration of 2.56 mg/L?pH of 5.0,NO2--N concentration of 100 mg/L,reaction temperature of 20?,reaction time of 48h?,the effect of FNA dissociation of waste activated sludge EPS was the best.At this time,the concentration of SCOD,protein?PN?,humic acid?HA?,polysaccharide?PS?were 58.37mg/gVSS,5.48mg/gVSS,12.63 mg/gVSS and 7.79mg/gVSS,respectively.FNA could dissociate the EPS of waste activated sludge and destroy the cells,but reducing the concentration of NO2--N could reduce the damage to WAS cells while ensuring the ability of FNA to dissociate EPS.The ability of FNA to dissociate EPS from waste activated sludge mainly depended on the chemical interaction of FNA and/or its derivatives with EPS components.Under anaerobic conditions,there are denitrification and/or dissimilatory nitrate reduction to ammonium?DNRA?in the process of FNA dissociation of EPS from waste activated sludge,which made the concentration of NO2--N decrease,the pH increase,and the release of NH4+-N and PO43--P with time.FNA had a good dissociation effect on waste activated sludge EPS.FNA could open aromatic ring structure of EPS,release the organic matter in WAS,and convert the macromolecular substances into small molecular substances.?2?The reaction temperature and reaction time studied were 5080?and 1560min,respectively.When the reaction temperature of heat treatment was 80?and the reaction time was 45min,the dissociation effect of EPS from WAS was the best.At this time,the concentrations of SCOD,PN,HA,and PS were 175.21mg/gVSS,29.83mg/gVSS,24.48mg/gVSS,and 32.95mg/gVSS,respectively.Under the same reaction time,the concentration of NH4+-N decreased in varying degrees when the reaction temperature rised from 50°C or 60°C to 70°C or 80°C.The concentration of NO3--N,PO43--P,UV254and E4/E6 increased to a certain extent with the increase of reaction temperature and the extension of reaction time after reaching a certain reaction temperature?60°C?.The increase of E4/E6 value indicated that heat treatment could convert macromolecular substances in waste activated sludge EPS into small molecular substances,but Maillard reaction might occur at high temperature,which is not conducive to the dissociation of waste activated sludge EPS.?3?Compared with only FNA treatment or FNA and thermal combined treatment of WAS,heat treatment alone was the best way to dissolve waste activated sludge EPS,and SCOD concentration was 5.56 times and 1.12 times of FNA treatment or FNA and thermal combined treatment,respectively.Compared with heat treatment alone,the effect of FNA and thermal combined treatment on EPS dissociation of waste activated sludge had not been improved,but the increase of E4/E6 indicated that FNA and thermal combined treatment could further convert macromolecular substances in waste activated sludge EPS into small molecular substances. |
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