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Commercial Production of Polyhydroxyalkanoates in Tomato Cannery Wastewater Treatment

Hsin-Ying Liu and Frank J. Loge. Civil and Environmental Engineering, University of California at Davis, One Shields Avenue, 2001 EUIII, Davis, CA 95616

Bioplastics (polyhydroxyalkanoates (PHAs)) are biodegradable thermoplastics generated from renewable carbon sources. The ability to generate bioplastics is not new, but the production is hindered by high costs ($16/kg versus $1/kg for petroleum derived thermoplastics [1]). This study aims to investigate means to decrease these high costs of bioplastics production that primarily stem from feedstock and aseptic conditions originally deemed necessary for production. Reducing these key costs components was achieved using industrial waste coupled with a mixed microbial culture from activated sludge under non-aseptic conditions.

PHAs are polyesters synthesized as intracellular reserve materials for carbon and energy by over 300 different microorganisms [2, 3]. Currently, commercial PHA production incorporates a pure culture of Ralstonia eutropha with glucose as substrate under aseptic conditions. As previously mentioned, the costs far outweigh those of petroleum derived thermoplastics with 40% of PHA production costs associated with substrate synthesis from agricultural feedstocks. To circumvent this cost, crude carbon substrates from industrial food, agricultural wastes, or industrial by-products have been considered [4, 5] because their chemical composition meets the demands for PHA producing strains [6]. Aseptic conditions require an additional 30~40% of total production costs [7]. Reducing this cost obstacle is feasible via a mixed microbial culture, such as activated sludge.

Previous work incorporating these cost saving approaches have been conducted in either pure culture with defined/undefined media or mixed culture with defined medium with both approaches under aseptic conditions. This study considers undefined media and a mixed culture under non-aseptic conditions.

The selection criteria for industrial wastewaters comprise a wastewater that is rich in organics and biodegradable. A local tomato cannery from a neighboring town (Woodland, CA) was selected and its wastewater was characterized during the summer of 2005. The tomato cannery effluent had an average COD of 3,000 mg/L, average orthophosphate and total phosphate concentrations of 6.3 and 15.5 mg P/L, respectively, and average total kjeldahl nitrogen (TKN) concentration of 120 mg N/L. Overall, the carbon loading in the tomato cannery effluent was found to be at least three times higher than a typical municipal wastewater and the nutrient levels do not impose any macronutrient limitations, which should meet the feedstock criteria.

A 4 liter bench-scale sequencing batch reactor (SBR) was designed, constructed, and implemented to monitor PHA production coupled with biological carbon/nutrient removal. A SBR is an attractive option for treating tomato cannery waste due to its high versatility for operational conditions. The SBR was operated on a 24-hour continuously aerated cycle with a 0.25 d-1 dilution rate. 1 liter completely mixed reaction liquid was discarded daily to maintain both a 4-day hydraulic retention time (HRT) and sludge retention time (SRT). Following 12 days (>3 times SRT) of SBR operation was performed to achieve stabilization. Upon stabilization, the soluble organic carbon was reduced by 84%; nitrification was at 100%, and 76% orthophosphate removal was achieved.

Once SBR reactor results reached steady state, batch studies were implemented that operated under similar conditions as in the parent SBR, but with varying loading levels. The loading levels varied from 0.4 to 2.0 initial food-to-microorganism (F/M) ratios. The batch studies conclusively found that PHA production was directly related to loading rate and that an F/M of at least 2 is recommended based on the experimental results. A maximum 20% PHA content on a wet cell weight basis was obtained during the process. Based on the experimental results, a kinetic model was developed to successfully predict PHA biosynthesis. This model can be utilized for further pilot-scale wastewater treatment plant design.

The proposed process found that industrial wastewater can serve as a feedstock for PHA production and aseptic conditions are not essential. This process converted a waste stream to a value-added product, thus providing an environmentally benign manufacturing process. Furthermore, this study established the foundation for further PHA production studies from various other food processing industrial wastewaters.

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