Techno-environmental assessment of integrating polyhydroxyalkanoate (PHA) production with services of municipal wastewater treatment

Abstract

In this paper, the potential impacts in the techno-environmental performance of a municipal wastewater treatment plant with integrated mixed-microbial-culture polyhydroxyalkanoate (PHA) production are presented for the first time. A life cycle assessment was conducted based on mass and energy balances. The techno-environmental performance was evaluated for five wastewater treatment configurations: a reference case and four alternative processes producing PHA-rich biomass using influent municipal wastewater as the only organic carbon source. The integration of PHA-rich biomass production into a municipal wastewater treatment plant with sludge digestion sustains the overall conversion yield for total products of biogas and PHA-rich biomass (around 0.26 gCOD products per influent gCOD treated). PHA production integration has the potential to improve the overall environmental performance with respect to the reference case. Even when no benefits were accounted for substitutions related to the biogas and PHA-rich biomass, similar or improved environmental performances were estimated for all four alternatives for global warming potential, acidification potential, terrestrial eutrophication potential, and photo-oxidant formation potential. When benefits were accounted from substitutions of electricity and heat co-generated from biogas and of PHA-rich biomass by pure-culture PHA-rich biomass from sugar fermentation, gains were even higher due to the diversion of carbon from biogas to PHA-rich biomass. Freshwater and marine eutrophication potentials were dependent on effluent specifications. Case-by-case process configurations influence the mass and energy balance and trade-offs of process integration. The production and export of PHA-rich biomass decreased the aeration requirements for COD and nitrogen removal; however, increased demands for heat, power and chemicals were incurred for the generation of volatile fatty acids from primary solids fermentation. The choice of nitrogen-removal approach (nitrification-denitrification vs. anammox) also impacted energy consumption. Using influent wastewater as the sole carbon supply, the energy balance and PHA production were sensitive to the efficiency of primary treatment and available flux of volatile fatty acids into PHA production. Other regional inputs of organic residuals may improve carbon recovery in the treatment facility. The improved environmental performance of the treatment configurations motivates the idea that individual municipal wastewater treatment plants may become suppliers of renewable raw materials of higher value than that of biogas and/or energy and heat today.