Life cycle comparison of urine source separation and conventional wastewater treatment: Focus on nutrient management

Wastewater systems employing urine source separation demand attention as an alternative to conventional wastewater treatment. The potential benefits of urine source separation, in which urine is collected and treated as a separate waste stream, stem from the largely disproportionate impacts of urine on the treatment requirements of combined wastewater. Typically, less than 1% of the influent volumetric flow at a wastewater treatment plant is attributable to urine, yet urine contributes greater than 50% of the phosphorus (P) and 80% of the nitrogen (N) mass load to municipal wastewater. Urine source separation presents the opportunity to recover N and P from a low flow, nutrient rich solution, as well as to simplify centralized treatment of the remaining nutrient depleted wastewater. In order to pursue such radical changes, system-level evaluations of urine source separation in community contexts are required. This comparative life cycle assessment (LCA) focuses on the environmental and economic impacts of managing nutrients from urine produced in a residential setting with three different urine management scenarios (Figure 1). Scenario A is combined wastewater collection and conventional centralized treatment. Scenario B is urine source separation and subsequent struvite precipitation with high P recovery, which requires magnesium inputs to urine. Scenario C is urine source separation and subsequent struvite precipitation with high P and N recovery, which requires magnesium and phosphorus inputs to urine. The life cycle impacts evaluated in this study pertain to the hypothetical construction of urine source separating systems in residence halls at the University of Florida, production of potable water used as toilet flush water, operation of decentralized urine treatment, and operation of centralized wastewater treatment. System boundaries also include fertilizer offsets resulting from the production of urine based struvite fertilizer. As calculated by the Tool for the Reduction and Assessment of Chemical and Other Environmental Impacts (TRACI), urine source separation with struvite precipitation to achieve high P recovery (Scenario B) has the smallest environmental cost relative to existing centralized wastewater treatment (Scenario A) and urine source separation with struvite precipitation to achieve high P and N recovery (Scenario C) (Figure 2). Preliminary economic evaluations show that the three urine management scenarios are relatively equal on a monetary basis (<13% difference) (Table 1). The environmental impact of Scenario A mostly suffered from high electricity use at the drinking water treatment plant to produce toilet flush water and high electricity usage at the centralized wastewater treatment plant. Although struvite precipitation methods in Scenario C enable high recoveries of both P and N as urine based struvite fertilizer, the upstream and downstream impacts of the chemicals required for these precipitation methods are substantial. This presentation will focus on the major inventory items that contribute to the overall environmental and economic cost of each urine management scenario, the sensitivity of LCA results to model assumptions, and how LCA results highlight the significant roles of flush water production, urine storage, and urine treatment inputs to the overall impacts of urine source separation. The need for alternative disinfection and N recovery methods for source separated urine will also be discussed.