The Mechanism Of Residual Flocculants Effects On The Activity Of Activated Sludge In SBR System

Activated sudge process is used in the majority of domestic sewage treatment plants.The discharge of domestic sewage is increasing, the pollution is becoming more and more complex with the improvement of the quality of life and the rapid development of industry. It has lead to the increase useage and dosage of the flocculants in domestic sewage treatment plant. After pretreatment of domestic sewage, the residual of the flocculants is likely to enter the subsequent biochemical treatment, and then affects the activated sludge microorganism and sewage treatment system. There are many studies on the use of flocculants in wastewater treatment, most of them has a bias towards disposal efficiency. Only few studies the effect of flocculants on the activity of activated sludge and the microorganisms, which are relatively simple, and lack of systematic analysis. Therefore, it would be useful and meaningful to have comparative experiment to explore the effect of flocculants on the activity of activated sludge and microorganisms. In this work, there were 3 SBR reaction systems mixing polyaluminum chloride (PAC),polymeric ferric sulfate (PFS), cationic polyacrylamide(CPAM), respectively, after influent in each cycle to simulate the residual flocculants in the biochemical systems. The determination included the characteristics of activated sludge,activated sludge microbial biomass, the activity of the redox enzyme, extracellular polymer substances (EPS), the content of protein and polysaccharide in EPS, oxygen transfer coefficient (kLa) and the removal efficiency of COD, NH4+-N, TP. The main contents were as follows:(1)3 groups of contrast experiments of dosing flocculants in sewage alone and dosing flocculants in sewage with activated sludge were done. Based on the pollutants removal efficiency, reduce the range of dosing concentration by exclusive method. The results show that the suitble dosage of PAC, PFS, CPAM to simulate the residual flocculants in SBR system was 10 mg/L, 10 mg/L, 0.1 mg/L, respectively, in the comparative experiment.(2) In the comparative experiment, the sludge characteristics and the COD, NH4+-N, TP removal efficiency of the reaction system are investigated with adding 10 mg / L PAC to simulate the residual PAC in SBR system (PAC system), adding 10 mg / L PFS to simulate the residual PFS in SBR system (PFS system), reaction system with dosage of 0.1 mg / L CPAM to simulate the residual CPAM in SBR system (CPAM system ) and blank control reaction system (blank). The results show that PAC lead to activated sludge settling performance variation, and reduce the ability of microorganism to decompose the pollutants.PFS and CPAM could improve the structure of activated sludge bacteria micelles and flocs,thus the adsorption efficiency and settlement of activated sludge had been increased. Besides,the pollutants removal efficiency of PFS system is slightly higher than that of CPAM system.(3) In the comparative experiment, it dedicated the adenosine triphosphate (ATP), the activity of dehydrogenase and catalase, the amount of EPS and its components ratio, the change tendency of kLa of activated sludge in the 4 systems, respectively, to analyse the effect of residual flocculants on the growth kinetics of microbial biomass at 10 day, 20 day,30 day, 40 day, 50 day, 60 day after inlet, the activity of enzyme of unit active sludge microorganism, the EPS and its components ratio, and dissolved oxygen mass transfer. The results indicated that PAC inhibited the microbial biomass of avtivated sludge significantly.When the reaction reached 10th day and the concentration of Al3+ in PAC system was about 47.58 mg/L, the residual PAC could inactivate the microbial activity of activated sludge and reduce the amount of microorganisms. It would inhibit the activity of dehydrogenase and catalase to hinder the biochemicall reaction, and slso inhibit the formation of EPS and the mass transfer rate of dissolved oxygen in the reaction system to hinder the mass transfer-reaction process, which result in the deterioration of the sewage treatment. After dosing flocculants, the growth kinetics of microbial biomass in the first 5 hours of PFS and CPAM systems were first-order reactions, and the growth rate of CPAM system was greater than that of PFS system. To the 20th day, the growth rates of microbial biomass in CPAM,PFS and blank systems were 0.0031, 0.0052, 0.0021, respectively. In the PFS and CPAM system, when the cumulative Fe3+ concentration of residual PFS was lower than 115.43 mg/L, the cumulative residual CPAM concentration was lower than 5.08 mg/L, the residual flocculants would significantly promote the increase of the microbial biomass, the activity of dehydrogenase and catalase, and the EPS secretion of the activated sludge. Besides, the effect of PFS on the activity of catalase was slightly greatly than that of CPAM, and the effect of CPAM on the EPS secretion was greater than that of PFS, thus CPAM was more conducive to improve the hydrophobicity of EPS. However, when the cumulative Fe3+concentration of residual PFS and the cumulative residual CPAM concentration were greater than 115.43 mg/L and 5.08 mg/L, respectively, it would not only hinder the biochemical reaction with the activated sludge in the system, but also lead the corresponding inhibition to the activated sludge system.(4) Through the analysis of kLa in PFS and CPAM systems, it is found that PFS would increase the distance and reduce the area of oxygen transfer, because of its flocculation features. Meanwhile, Fe3+ was an essential trace element for organism growth, and it could promote the formation of aerobic granular sludge. Thus with the accumulation of PFS in the PFS system, kLa inhibited at the begining, reached to the level closed to that of the blank system and became stable afterwards. CPAM could improve the flocculation structure of the activated sludge to make the activated sludge have a good flocculation. It was conducive to the mass transfer of oxygen and nutrients, and improved the oxygen transfer rate in CPAM system. The kLa and the protein content of EPS in the PAC, PFS, CPAM and blank systems all show positive correlation. The correlation confficient of kLa and the protein content of EPS in the blank, PFS system, CPAM system and PAC system within 30 days were 0.4837,0.9138, 0.9688 and 0.9462 respectively. It is illustrated that PAC, PFS, CPAM could influence the dissolved oxygen transfer characteristics of the activated sludge system, and CPAM had the largest positive correlation between kLa and the protein content of EPS, that is the greatest impact on the dissolved oxygen transfer in the system.In conclusion, different kinds of residual flocculants with different concentrations would affect the activated sludge and its microbial activity, that led to inhibit or promote the sewage treatment effect. When the accumulation of residual PAC in activated system is greater than 47.58 mg/L,it would inactivate the microbial activity of activated sludge and hinder the growth of the microorganisms, inhibit the activity of dehydrogenase and catalase,hinder the formation of EPS and hinder the mass transfer-reaction process, lead to reduce the sewage treatment effect. Therefore, in the wastewater treatment with activated sludge, PAC is not suitable for the pretreatment which need a large number of flocculants for a long time.When the cumulative Fe3+ concentration in PFS system and the residual CPAM concentration in CPAM system were lower than 115.43 mg/L and 5.08 mg/L, respectively,they would promote the activity of the activated sludge and its microorganisms, which could improve the sewage treatment effect. To the actual activated sludge sewage treatment, PFS and CPAM could be used for the long-term pretreatment, their residual flocculants in the valid concentration range could cooperate with microorganisms and enhance the mass transfer-reaction process, which promote the effect of wastewater treatment. In addition, the PFS hydrolysate contained Fe3+, which would affect the color of effluent and the dosage amount of CPAM was far less than PFS. Therefore, among the three flocculants: PAC, PFS and CPAM, CPAM is the best flocculant used for the pretreatment in the sewage treatment plants.