LCA is comprised of four phases including goal and scope definition, inventory analysis, impact assessment, and interpretation. Depending on how the inventory analysis is performed, Similar to LCC, LCA leverages the system inventory and can be tailored to the life cycle stages of interest (e.g., cradle-to-grave, cradle-to-gate, cradle-to-cradle). The system inventory is compiled from the foreground system -the group of unit processes that can be controlled by designers or operators. However, LCA also requires the connection of system inventory to the sources of environmental impact in the background system that consists of processes over which designers and operators may not have direct influence (Table ). In the inventory analysis phase of LCA, the system inventory is converted to the life cycle inventory (LCI) by linking the system inventory to unit impacts incurred in the background system. For example, in operating a membrane reactor, the membrane modules may be replaced during maintenance, which contributes to the system inventory in the foreground. But in LCA, the emissions and raw materials required for the acquisition of these membrane modules (including raw material extraction, manufacturing, transportation, etc.), which are in the background, should also be accounted for to convert the system inventory into the system LCI. While the system inventory could be a specified number of membrane modules replaced at a specified frequency, the system LCI would include all mass and energy flows associated with these membrane modules across their life cycle. With regard to the life cycle impact assessment (LCIA) phase, many methods have been developed in recent years (e.g., ReCiPe, 139,140 TRACI 141-143 ). These LCIA methods provide characterization factors, which translate every individual emission or raw material requirement in the system LCI into normalized, quantitative environmental impacts. Depending on the LCIA method, these characterization factors can be developed at the midpoint or endpoint of the environmental impact cause-effect chain. Midpoints are located along the impact pathway for a particular impact indicator (e.g., kg CO2 equivalents for 100-year global warming potential). In the indicator prioritization step, midpoint characterization factors for different indicators can be aggregated into three main categories (i.e., ecosystem quality, resource scarcity, and human health). Additionally, varying groups of assumptions (e.g., time horizon) may be adopted to reflect the uncertainties and choices associated with these characterization factors. For example, based on the hierarchist perspective in ReCiPe, methane has a midpoint characterization factor of 34 kg CO2 eq•kg -1 for global warming potential, which can be further translated to the human health endpoint using a factor of 9.28×10 -7 disability-adjusted life years (DALYs)•kg CO2 eq -1 . 140 By normalizing LCIA impacts to the defined functional unit (e.g., 1 m 3 of drinking water treated to potable standards, 1 m 3 of wastewater treated to meet effluent permits ), these results can be used to compare environmental sustainability across systems. 117,118