|Abstract:||Wastewater treatment is essential for protecting human health and the environment. However, current conventional wastewater treatment, which focuses primarily on aerobic conversion of organic pollutants to CO2, requires significant energy input making it costly and less environmentally sustainable. With increasing economic development, population growth, aging infrastructure, and stricter regulations, the energy and material inputs of wastewater treatment are only expected to increase (EPA, 2006; Mo & Zhang, 2013). Meanwhile, the carbon content of wastewater has potential to be a significant renewable resource for energy and materials production that could be leveraged to offset the cost and resource demands of wastewater treatment. Thus, shifting the current paradigm from pollutant removal to resource recovery is as a promising strategy for improving the economic and environmental impacts of wastewater treatment. To that aim, this work investigated two emerging technologies for resource recovery from wastewater, namely enhanced methane recovery in a novel two-phase anaerobic membrane bioreactor (AnMBR) process incorporating bioaugmentation and ion-exchange resins, as well as bio-polymer recovery via mixed microbial culture (MMC) polyhydroxyalkanoate (PHA) production.
The first study presented in this dissertation investigated the application of bioaugmentation in the acid-phase of a two-phase AnMBR treating primary sludge to improve solids removal and overall process efficiency. Bioaugmentation was carried out using a proprietary bioculture blend containing a mixture of hydrolytic, acidogenic, and acetogenic microorganisms. This mixture was added both on its own and in combination with recycled anaerobic sludge from the methane-phase of the AnMBR. These bioaugmentation strategies increased average percent hydrolysis by 25-38%, and increased average acid-phase acetic acid generation by 31-52% compared to operation without bioaugmentation. These benefits led to subsequent increases in average methane production (10-13%) and greater average overall solids reduction by 25-55%. Finally, microbial community analysis using 16S Illumina MiSeq generated sequences confirmed increased relative abundance of bioaugmented microorganism including Acetobacter, and Syntrophomonas species. Overall, bioaugmentation was found to improve conversion of primary sludge to methane by shifting the microbial community towards one better suited for hydrolysis and acetogenesis.
In the second study, application of ion-exchange resins in the methane-phase of the same two-phase AnMBR system was investigated as a means for improving reactor recovery after organic shock-loading. Four commercially available anion-exchange resins were evaluated for their ability to sorb soluble organics, specifically volatile fatty acids (VFA), from AnMBR effluent. The strong-base resin, Purolite TANEX was determined the best resin for deployment in the AnMBR system having achieved the greatest removal of soluble chemical oxygen demand (COD) (up to 36%) and acetic acid (up to 48%) in batch testing. Addition of 100 and 300 g/L of reactor volume of TANEX resin in a continuous flow AnMBR system improved effluent quality by reducing effluent COD concentrations by 48 and 75%, respectively. After shock-loading with 16,000 mg COD/L acetic acid, reactor recovery in terms of methane production was 9-58% faster with the addition of TANEX than without it. After shock-loading the system twice without the addition of TANEX, it was found that methane production recovery improved from 68 to 55 days, suggesting that acclimation of the microbial community also played a role in reactor recovery. Microbial community analysis using 16S Illumina MiSeq sequencing confirmed changes in the microbial community did occur as a result of shock-loading and the addition of TANEX resin. A higher average relative abundance of Methanoscarcina (up to 51 and 58%) was seen during operating periods with TANEX resin, leading to the conclusion that addition of the TANEX resin benefited reactor recovery by reducing stress on the microbial community via sorption of excess acetic acid, allowing the community time to adjust and become better able to process higher and more variable loadings of acetic acid.
In the third study, production of the biopolymer, polyhydroxyalkanoate (PHA), from hydrolyzed municipal organic waste was investigated as another approach to resource recovery from organic waste streams. The PHA production process was carried out in three phases beginning with (1) fermentation of the waste to produce a VFA-rich liquid effluent, (2) application of that VFA-rich fermentation liquid to select for PHA accumulating biomass, and (3) accumulation of PHA in the selected biomass using varying concentrations of the fermentation liquid to assess the effects of ammonium-nitrogen concentration on PHA accumulation. Preliminary batch testing to determine optimal operating parameters for the fermentation phase revealed that 5.4% solids content, 37°C, and 3.4 day retention time resulted in the greatest VFA production. Up to 14 g/L VFA production was achieved in lab-scale continuous fermentation of municipal organic waste. The liquid fraction of the fermented material was applied using a feast/famine feed strategy to successfully select PHA accumulating biomass. Finally, the PHA accumulation phase achieved an average maximum yield of 38% PHA/g VSS using a low ammonium-nitrogen feed mixture. Application of clinoptilolite was determined to be an effective means for reducing ammonium-nitrogen concentration in the fermentation liquid and improved PHA accumulation by up to 29%. Overall, this study demonstrated the feasibility of using a complex organic waste stream, namely municipal organic waste, for mixed microbial culture PHA production, with the potential for nutrient recovery as well.
Finally, life cycle assessment methodology was applied to evaluate and compare the environmental impacts associated with the two resource recovery options, i.e. methane recovery via AnMBR treatment, and bio-polymer recovery via MMC PHA production, considering primary sewage sludge as the substrate. Overall, the AnMBR process was determined to be the more environmentally sustainable option achieving a reduced environmental impact in 6 out of the 10 impact categories considered. Energy consumption was determined to be the largest contributor to overall environmental impact for both processes. However, in the case of AnMBR treatment, it was estimated that more than enough energy could be recovered as methane to offset energy requirements and achieve a positive energy balance. In the case of PHA production, the high energy requirements for aeration negatively impacted the global warming potential (GWP) of the PHA process, although it performed better in the impact categories of fossil fuel depletion and ecotoxicity compared to the AnMBR process. Uncertainty and sensitivity analysis suggested that, under optimized conditions, it may be possible to achieve a net negative GWP for PHA production from primary sludge. In addition, an initial economic assessment that included only operating input costs and potential revenue from recovered methane and PHA products suggested that the relatively high selling price of PHA could more than offset the operating input costs for its production, potentially leading to greater economic benefits compared to the AnMBR process. In the end, a combination of the two technologies may be an advantageous option for improving the environmental and economic sustainability of wastewater treatment. However, a more detailed techno-economic analysis, including consideration of capital costs and PHA extraction is needed. In addition, LCA predictions should be validated with large-scale, long-term demonstration of the two technologies.