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Title: Selected activated sludge for production of environment-friendly bio-plastics
Authors: Zhong, Dan
Degree: Ph.D.
Issue Date: 2013
Abstract: The environmental problems associated with plastics have become increasingly severe in recent years. Technologically- and economically-feasible solutions are in urgent need in order to ensure sustainable development in the plastic industry. While technical and economic constraints remain with plastic recycling and reuse, attempts to development biodegradable plastics have emerged as a potential solution. Polyhydroxyalkanoates (PHAs) are a family of specialized polyesters of hydroxyalkanoates (HAs), artificially synthesized or naturally existing as an intracellular carbon reserve by using different bacteria strains, which are potential biodegradable substitutes to conventional petroleum-based plastics. The significant advantages of PHAs include its biocompatibility with human tissues, biodegradability under natural ambient and thermoplastic properties. Additional beneficial properties, such as zero toxicity and complete recyclability, render PHAs environment-friendly materials to replace conventional plastics. However, it is essential to lower the production costs if PHAs are to be used in large scales. Production of the various types of PHAs in activated sludge processes specifically designed for treating wastewater has emerged as an economically and environmentally promising and attractive alternative to conventional pure culture fermentations. This is because large amount of sewage sludge generated from municipal wastewater treatment works has been continuously increasing with rapid urbanization and industrialization. Conventionally, sewage sludge treatment methods include incineration, composting, land application, landfill, and ocean dumping. However, all these methods have their specific drawbacks, such as the risks of pathogenic proliferation, endotoxins and heavy-metal contaminants spreading from atmospheric, hydrospheric and lithospheric application sites. Consequently, the treatment and disposal of sewage sludge has become a key environmental problem. The method of converting sewage sludge to environment-friendly PHAs is therefore an attractive means for sludge disposal. Till date, little research has been conducted and published pertaining to the application of optimal parameters in laboratory-scale activated-sludge simulator system and pilot-scale bioprocesses for PHA production. In this study, several factors affecting the process efficiency and overall economics of PHA production were investigated. In order to examine how these factors contribute to the process efficiency, main focus of the research works was the design of a laboratory-scale activated-sludge simulator system and pilot-scale sequential batch reactor (SBR) system to determine the optimum operating conditions of activated sludge process for increasing the PHA production yield. Activated sludge from a conventional municipal sewage treatment works was obtained and conducted for the PHA accumulation by using glucose as the sole carbon sources in a laboratory-scale activated-sludge simulator system. Firstly, the optimal aeration time of 2.5 h and the optimal settling time of 1.0 h were determined, based on the quality of the sludge and treated effluent. Secondly, with the fixed aeration and settling time for the activated-sludge simulator system, the optimal carbon-phosphorus (C:P) ratio was fixed at 300, as determined by observing the PHA accumulation rates. In the third stage, investigation was directed at evaluation of the optimal values of carbon-nitrogen (C:N) ratio on PHA production yields under the other pre-determined optimal conditions. Results showed that as the C:N ratio increased from 30 to 120, specific polymeric yield increased to a maximum of 0.291 g polymer/g dry cell weight, while specific growth yield decreased with increasing C:N ratio. The highest overall polymer production yield of 0.099 g polymer/g COD consumed was achieved under the C:N ratio of 90. Therefore, a severe nitrogen deficiency triggered the intracellular accumulation of PHAs as a food reserve in the activated-sludge microbial community. Carefully designed sporadic adjustments and feeding patterns of the C:N ratio did not significantly affect the process performance in terms of COD removal efficiency, which maintained at around 80%. After these three stages of works, the activated sludge was considered to have been selected with a microbial consortium that is best suited for efficient PHA accumulation under the predetermined operating conditions. The aeration and settling times, as physical selection pressures, and the C:P and C:N ratios, as nutrient selection pressures, formed a novel selection procedure for effective establishment of a stable and rigorous sludge suited for PHA accumulation. A relatively high specific polymeric yield of 0.291 g polymer/g dry cell weight indicated that the activated sludge is possibly dominated by hyper accumulators of PHAs such as Nocardia spp., Alkaligenes spp. and Psudomonas spp..
In the fourth stage of the study, the pilot-scale SBR system was built for PHA accumulation based on optimal condition determined from the previous laboratory-scale system. The selected activated sludge was used to seed the system. The PHA production yields and COD removal efficiency were studied on a long-term basis. The results showed that the specific growth yield (Yx/s) from the pilot-scale SBR system was higher than that in the laboratory-scale simulator. The specific polymer yield (Yp/x) was 0.237 g polymer/g dry cell weight in pilot-scale, which was less than the 0.259 g polymer/g cell weight observed in the laboratory-scale simulator system. The overall polymer production yield (Yp/s) was 0.092 g polymer/g COD consumed in pilot-scale, which was less than the 0.099 g polymer/g COD consumed in laboratory-scale simulator under the C:N ratio of 90. The average COD removal efficiency of 83.10% in the laboratory-scale simulator was found to be 2.81% higher than that in the pilot-scale system. In the final stage, the composition of co-polymeric materials, namely poly-hydroxybutyrate-valerate (PHBV), produced by activated sludge bacteria was controlled by regulating the feed composition, namely the concentration of butyric acid (C4) and valeric acid (C5) ratio in the medium, under the established operating conditions of pilot-scale system. The organic feed to PHBV conversion mechanism were studied and mathematically modeled. When butanoic acid, or otherwise known as butyric acid, was used as sole carbon and energy source for fermentation, there was only PHB homo-polymer produced instead of PHBV co-polymers in the culture. On the other hand, the highest 3HV mole fraction 48% in the co-polymer accumulated was when valeric acid was used as sole carbon source. The melting temperature of the PHAs produced by activated sludge decreased from 178.9°C to 98.5°C, with an increase in the 3HV fraction, indicated that 3HV unit act as defects in the PHBV crystal lattice. Therefore, the composition of the co-polymers, the physic-chemical, thermal and mechanic properties, could be fairly accurately controlled by carefully manipulating the influent organic compositions in the culture medium. The carbon source to PHBV copolymers conversion mechanisms and relations were elucidated. In conclusion, PHAs including copolymers of PHBVs with industrially applicable properties can be produced at substantially reduced costs using a specifically selected and enriched activated sludge, through a novel procedure, from sewage treatment works. The PHA accumulation process was scaled up in a pilot scale system and the conversion process mechanism was elucidated. The novel use of dimensionless process design parameters, namely Yp/s, Yp/x and Yx/s, served as the process predictive model for the process design, optimization, operation and control. These model parameters were validated in pilot-scale opeartion, and were valuable tools for full-scale industrial opearations. This is a major step ahead towards environment-friendly plastics production and efficient sewage sludge disposal.
Subjects: Poly-beta-hydroxyalkanoates.
Sewage -- Purification -- Activated sludge process.
Hong Kong Polytechnic University -- Dissertations
Pages: xxix, 227 leaves : ill. ; 30 cm.
Appears in Collections:Thesis

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