Mechanism Study On The Effect Of Extracellular Polymer Substance To The Settleability Of Activated Sludge

Urban wastewater treatment plant as an important urban infrastructure, of which plays an irreplaceable role in the governance of water pollution and the protection of natural water environment. Activated sludge process is the mainstream technology of urban wastewater treatment. The existing urban wastewater treatment plants generally have sludge bulking problems in winter, which not only can lead to a significant reduction of settleability and the lose of activated sludge, but also directly lead to the lower of treatment capacity and efficiency, and ultimately affect the normal operation of treatment plant.The issue with the actual production of activated sludge treatment system and sequencing batch reactor activated sludge system as the object, that explored the influence of extracellular polymers substance on sludge floc structure and settleability, analysized the reason of the reduction of settleability and sludge density when the extracellular polymers substance’s accumulate at a low temperature. The main research results are as follows:(1)With the research of the process activated sludge treatment system in one year shows that : with the decrease of the reaction temperature, the settleability of activated sludge becomes worse, and the extracellular polymers substance is accumulated continually, which focus on large LB-EPS and Total-C. With the reaction temperature decreasing from 25.7℃ to 11.1℃, the SVI value increased from 232mL/g to 326mL/g, with an increase of 94 m L/g, which increased by 41%; LB-EPS increased from 88mgCOD/g MLVSS to 207mgCOD/g MLVSS, with an increase of 119mgCOD/g MLVSS, which increased by 135%. The increase of LB-EPS accounted for 73% of the increase of Total-EPS. The Total-C increased from34.51mg/g MLVSS to 56.85mg/g MLVSS, with an increase of 22.34mg/g MLVSS, which increased by 39%. Under different temperature conditions, the SVI value of activated sludge was highly correlated with LB-EPS and Total-C. Therefore, in the process of activated sludge system, the continuous accumulation of Total-C and LB-EPS is the direct cause of the reduction of the activated sludge settling performance.(2) Changed the structure of the sludge flocs by external force and analysized the effect of extracellular polymeric substances on activated sludge transfer matrix. The results shows that: the sequencing batch reactor system used the action of ultrasonic as external force, intensity of ultrasonic(20W/g MLSS, 20kHz) respectively for 0 min,1min, 2min, 5min.The settlement performance and sludge metabolism were improved by varying degrees of t external force. INT-ETS, SVI values were 107, 110, 150, 161 mg/(gVSS*h), 213, 207, 147, 109mL/g.The content of extracellular polymers decreased obviously, the LB-EPS were276, 242,192, 175 mgCOD/gMLVSS; TB-EPS were 421, 423, 368, 339 mgCOD/gMLVSS. The SVI, INT-ETS, TB-EPS and LB-EPS values all had a good correlation with the external force. It showed that the mass transfer capacity of activated sludge can be enhanced by the external forces which acted on the structure of activated sludge, and the metabolic capacity of activated sludge was enhanced, which lead to the decrease of the content of extracellular polymer and improve of settling performance.(3) Through the research on sequencing batch reactor experiment system at different temperatures, results show that:① When the reaction temperature decreased from 24℃ to 8℃, with the reduction of settling performance and the metabolic capacity of activated sludge, the sludge density decreased gradually,and the extracellular polymer of Total-C and LB-EPS were accumulated. The value of SVI increased from 85 m L/g to 328 m L/g, which increased by 243 mL/g, and the sludge specific gravity decreased from 1.0129g/m L to 1.0318g/mL; while LB-EPS increased from 167 mgCOD/gMLVSS to 287 mgCOD/gMLVSS, with an increase of 120 mgCOD/gMLVSS, which increased by 72%; Total-C increased from 47.4 mg/gMLVSS to 72.4 mg/gMLVSS, with an increase of 25 mg/gMLVSS, which increased by 53%. There was a high correlation between SVI and LB-EPS、Total-C; in addition, the proportion of activated sludge and LB-EPS, Total-C also had a high correlation. Therefore, for the sequencing batch reactor system under the different temperatures, the decrease of reaction temperature, the accumulation of LB-EPS and Total-C both are the direct cause of the reduction of the activated sludge settling performance and sludge density.② When the reaction temperature decreased from 24℃ to 8℃,the metabolic capacity of activated sludge decreased gradually; the extracellular polymers substance especially total-C and LB-EPS accumulated. Dehydrogenase activity(TTC-DHA) was decreased from 4.39 mg/(gVSS*h) to 16.74 mg/(gVSS*h), and its value was decreased by 12.35 mg/(gVSS*h); the electron transport system activity(INT-ETS) decreased from 170.86 mg/(gVSS*h) to 41.37 mg/(gVSS*h), and its value was decreased by 129.49 mg/(gVSS*h). There was a high correlation between the reaction temperature of the system and the TTC-DHA and INT-ETS. In addition, there was also a high correlation between TTC-DHA, INT-ETS and LB-EPS, Total-C. Therefore, for the sequencing batch reactor system under different temperatures, the reduction of metabolic activity caused by the decrease of reaction temperature, resulting in an accumulation of LB-EPS and Total-C, which was the fundamental reason for the reduction of settling performance and sludge density of activated sludge system.