Synthesis of polyhydroxyalkanoates (PHA) from excess activated sludge

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Synthesis of polyhydroxyalkanoates (PHA) from excess activated sludge


Author: Hu, Wenfeng
Title: Synthesis of polyhydroxyalkanoates (PHA) from excess activated sludge
Degree: Ph.D.
Year: 2004
Subject: Hong Kong Polytechnic University -- Dissertations
Polymers -- Biodegradation
Sewage -- Purification -- Activated sludge process
Polyesters -- Synthesis
Sludge bulking
Department: Dept. of Civil and Structural Engineering
Pages: xxiii, 307 leaves : ill. (some col.) ; 30 cm
Language: English
Abstract: This study focused on the polyhydroxyalkanoates (PHA) synthesis from excess activated sludge (EAS), collected from a laboratory-scale sequencing batch reactor or a full-scale municipal wastewater treatment plant (MWWTP), via monitoring and precisely controlling the dissolved oxygen (DO) of the cultural media through an oxidation-reduction potential (ORP) setup instead of conventional DO system from different carbon sources. No matter what types of EAS was submitted to PHA synthesis, by using fatty acids of shorter chain length including acetate and propionate as carbon sources, the total organic carbon (TOC) removal efficiency was higher than those of longer chain length. When C-even numbered fatty acids such as acetate and butyrate was used as sole carbon source respectively, PHB homopolymer or PHA with dominant 3HB unit was synthesized. Otherwise, PHBV copolymer with 3HV major units was synthesized instead of the PHB homopolymer when C-odd numbered fatty acids, propionate and valerate, were used as sole carbon sources. Increase the concentration of C-odd numbered fatty acids in the cultural medium, may result in the decrease of PHA content in EAS. The melting temperatures of the PHBV copolymer produced by EAS decreased with the increasing of 3HV molar fraction in the PHBV. When acetate was used as carbon substrate, the minimum PHA content at ORP -30mV was about 12% (w/w) of cell dry weight (CDW). The maximum PHA content of 35% (w/w) was achieved when ORP was +30mV. By using propionate as carbon substrate and ORP was +30mV, the PHBV copolymer content, polymer production yield and 3HV molar fraction were 25.8%, 0.38 (g/g) and 78.0 mole%, respectively. Decreasing of ORP from +l00mV to -30 mV by adjusting the air flow rate pumped into the cultural broth, equivalent to the DO concentration, resulted in the variation of 3HV mole fraction in the PHBV from 0 mole % to 21 mol% while acetate as carbon source. In other words, these variations were resulted from the changes of DO concentration. When EAS from full-scale MWWTP was conducted for the PHA production from glucose, the polymer content in percentage of CDW increased with the increasing of ORP levels. The 3HV unit content in the PHBV increased from 11.1 mol% to 78.4 mol% with the decreasing of ORP from +l0mV to -20mV by lowering the air flow rate. These results were in accordance with that of acetate and propionate as carbon sources. It was believed that monitoring and controlling the fermentation process by ORP instead of DO provided more sensitive, reliable and precise controll. Slight variation of DO concentration in cultural broth could be observed and realized by ORP system. A hypothesized biochemical metabolic model by using glucose as sole carbon source was estabolished. It is postulated that TCA cycle and glyoxylate pathway are involved in the PHA accmulation even under anoxic and anaerobic condtions. The most important view in this hypothetic model is that the oxygen is the key exogenous factor to regulate 3HV mole fraction in PHBV copolymer. These observations and the hypothesized model in this study may provide experimental evidence to that TCA cycle and glyoxylate pathway might be involved in the PHA production and phosphorus removal in the Enhanced Biological Phosphorus Removal process.

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