Life Cycle Assessment and Cost Analysis of Water and Wastewater Treatment Options for Sustainability: Influence of Scale on Membrane Bioreactor Systems
Cashman, S., J. Mosley, Cissy Ma, J. Garland, J. Cashdollar, AND D. Bless. Life Cycle Assessment and Cost Analysis of Water and Wastewater Treatment Options for Sustainability: Influence of Scale on Membrane Bioreactor Systems. U.S. Environmental Protection Agency, Washington, DC, EPA/600/R-16/243, 2016.
The main goal of this study is to determine the influence of scale on the energy and cost performance of different transitional membrane bioreactors (MBR) in decentralized wastewater treatment (WWT) systems by performing a life cycle assessment (LCA) and cost analysis. The study explores MBRs as an emerging technology to provide decentralized WWT services while maximizing resource recovery. The efforts will facilitate the applications of decentralization and fit for purpose concepts in water systems.
changes in drinking and wastewater infrastructure need to incorporate a holistic view of the water service sustainability tradeoffs and potential benefits when considering shifts towards new treatment technology, decentralized systems, energy recovery and reuse of treated wastewater. The main goal of this study is to determine the influence of scale on the energy and cost performance of different transitional membrane bioreactors (MBR) in decentralized wastewater treatment (WWT) systems by performing a life cycle assessment (LCA) and cost analysis. LCA is a tool used to quantify sustainability-related metrics from a systems perspective. The study calculates the environmental and cost profiles of both aerobic MBRs (AeMBR) and anaerobic MBRs (AnMBR), which not only recover energy from waste, but also produce recycled water that can displace potable water for uses such as irrigation and toilet flushing. MBRs represent an intriguing technology to provide decentralized WWT services while maximizing resource recovery. A number of scenarios for these WWT technologies are investigated for different scale systems serving various population density and land area combinations to explore the ideal application potentials. MBR systems are examined from 0.05 million gallons per day (MGD) to 10 MGD and serve land use types from high density urban (100,000 people per square mile) to semi-rural single family (2,000 people per square mile). The LCA and cost model was built with existing literature data sources, data from actual commercial units, and wastewater treatment plant design costing software simulations. The results focus on the energy demand and associated greenhouse gases (GHG) for the scenarios examined. However, a full suite of life cycle impact assessment results, including water savings, was calculated.Net energy benefits, considering the drinking water displaced by the delivered recycled water, start at the 1 MGD scale for the AeMBR and at the 5 MGD scale for the AnMBR operated at 35˚C (mesophilic). For all scales investigated, the psychrophilic AnMBR reactor operated at 20˚C results in net energy benefits. This study supports the findings from other literature that AnMBRs operated at lower reactor temperatures are a potential technology for decreasing the environmental impacts of wastewater treatment systems. When examining the energy demand results normalized to a cubic meter of water treated, all energy demand impacts decrease as the scale increases due to economies of scales. While the AnMBR operating at ambient temperature results in notable energy and GHG benefits compared to the AeMBR, the AnMBR costs remain higher than the AeMBR under all scenarios. The main driver for this is the increase in operation and maintenance labor needed to operate the anaerobic reactor and, to a lesser extent, anaerobic reactor capital costs. The study found that all impacts decrease comparatively as the population density increases due to decreased pumping distances and piping requirements, with the highest burdens realized for the semi-rural single family land use and the greatest potential seen for the high-density urban land use. Ambient temperature played a key role, with the most benefits and least energy demand and GHG impacts from psychrophilic AnMBR operated in warm climate conditions with combined heat and power generation from methane recovered from both the headspace and the permeate.While this study focused primarily on net energy demand and GHG impacts of the decentralized MBR systems, there is a potential significant water savings from using recycled wastewater. This study found that use of recycled water from decentralized MBR scenarios avoids 0.94 to 0.96 cubic meters of drinking water per cubic meter of wastewater treated by MBR.