Impact of dissolved oxygen on PHA production and integration with biological nutrient removal processes

Bio-based and biodegradable polymers are regarded as potential replacements of traditional fossil based plastics. Polyhydroxyalkanoates (PHAs) are widely studied biopolymers due to their broad range of thermal and mechanical properties. The impacts of operational conditions, such as pH, temperature, organic loading rate and sludge retention time, in PHA production process by mixed microbial cultures (MMCs) have been evaluated by previous studies, but the influence of dissolved oxygen (DO), which is directly related to process energetic demand, was rarely discussed. This thesis is focused on the effects of DO on the microbial culture selection and PHA accumulation stages, as well as the impact of DO on a combined PHA production and nutrient removal process for wastewater treatment. Efficient Plasticicumulans dominating microbial cultures were enriched under both high DO (3.47 ± 1.12 mg/L) and low DO (0.86 ± 0.50 mg/L) conditions in the feast phase containing mostly the same populations but with different relative abundance. Butyrate and valerate were found to be the preferred substrates as compared to acetate and propionate regardless of DO concentrations. Compared to acetate and propionate, the butyrate and valerate uptakes were not significantly impacted by low DO levels in the PHA accumulation stage. A metabolic model was developed for the first time to describe the substrate preference among multiple volatile fatty acids (VFAs), providing a successful approach for PHA composition prediction and process efficiency optimization when four competing VFAs are supplied. Further, DO level control through both the feast and famine phases of culture selection was applied, at 3.79±0.65 mg/L and 0.48±0.29 mg/L respectively. The low DO level in both the feast and famine phases proved to be insufficient for successful MMC enrichment. By characterizing the microbial evolution under the unlimited DO conditions, it was found that Paracoccus was the dominating population (>50%) in the selected cultures, and substrate competition was correlated with the abundance of Plasticicumulans during culture selection. In order to optimize the integrated PHA production and nutrient removal process fed with ammonia-rich feedstocks in full scale implementation, the impact of DO on both processes was investigated. Much higher DO affinity for VFA consumption was observed as compared to nitrification. A DO control strategy was proposed based on the observation that the PHA production was not influenced while nitrogen was removed by simultaneous nitrification and denitrification processes when controlling DO at low levels (e.g. 0.4-0.8 mg/L).