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Bioflocculation in EBPR Process Operated at Short Sludge Retention Times

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Title: Bioflocculation in EBPR Process Operated at Short Sludge Retention Times
Author: Wang, Bei
Department: School of Engineering
Program: Engineering
Advisor: Zhou, Hongde
Abstract: Activated sludge processes operated at short sludge retention time (SRT) offer potential to reduce the energy requirement for biodegradation while maximizing the conversion of biodegradable organic matter into the biomass for energy recovery. However, concern has been raised with poor sludge settling due to inadequate bioflocculation at short SRTs. It is hypothesized the problem might be alleviated by incorporating with enhanced biological phosphorous removal (EBPR), thereby, producing heavier phosphorus-rich sludge. The experiments were conducted using three parallel pilot EBPR reactors operated at two short SRTs (2d and 3d) and a regular SRT (6d) for one year, during which the removal efficiency was monitored and the sludge settling performance was related to physicochemical and microbial characteristics. Some short-term complementary operations were also conducted to examine the resistance of short SRT process to shock loadings and to test the hypothesis that incorporation with EBPR could improve the settling performance. Results showed that the effluent total COD increased with decreasing SRT, mainly due to the different performance of bioflocculation. Sludge bulking occasionally happened in short SRT systems, especially at 2d SRT. The underlying reasons for the bulking could be attributed to both the shift in physicochemical characteristics of the floc matrix (surface properties and EPS composition) and the proliferation of some free-living, small-microcolony forming or weakly bounded bacterial groups. The phosphorus content in biomass seasonally varied in the range of 2.8-7.2%. The 2d-SRT system had lower phosphorus content in biomass and higher phosphorus concentration in effluent, although no significant difference was found in the abundance of phosphorus accumulating organisms between different SRTs. Sludge flocs in EBPR system were proved to have better settling performance than those in conventional aeration system operated at the same SRT, associated with more optimal morphology and richer contents of polyphosphate. 3d-SRT EBPR system was found to have good floc stability and high phosphorus contents in biomass with high sludge productivity. Balancing the system removal efficiency and the faction of organic matter and phosphorus converted into biomass, SRT of around 3 days might be an acceptable option suggested for energy and resource recovery.
Date: 2018-04
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