Climate change threatens fresh water supplies around the world, especially in regions already facing water insecurity. Desalination is a critical tool for ensuring adequate water supplies in an uncertain future, and many water-stressed areas already employ desalination to provide drinking and irrigation water. Improving desalination resilience in the face of climate change requires the use of technologies that increase water recovery while reducing brine volumes. Current brine disposal methods largely fail to meet this objective. Typically, brines from desalination plants are disposed of via surface water discharge, sewer discharge, deep-well injection, and/or evaporation ponds. Significant environmental concerns accompany these methods. Regulatory bodies increasingly require the use of Minimal or Zero Liquid Discharge (MLD/ZLD) techniques that maximize water recovery and, ideally, minimize environmental impacts. MLD/ZLD techniques in general involve a set of pre-concentration, concentration, and crystallization steps. These steps can consist of a variety of membrane- or thermal-based technologies, yet despite the importance of utilizing MLD/ZLD technologies, few have been incorporated into desalination life cycle analysis (LCA) studies. The vast majority of desalination-focused life cycle analysis efforts center on the operation stage of the desalination plant; namely the primary separation step involved (typically reverse osmosis). The membrane production process and brine treatment/disposal steps are by comparison understudied. Some have studied the impacts of membrane production, membrane disposal, and brine disposal, but most desalination LCA studies focus on greenhouse gas (GHG) emissions and deem these areas negligible since the energy requirements of desalination operation generally dominate the GHG impacts.
However, the large impacts of energy consumption on GHG emissions do not negate the additional impacts of the membranes and brine, such as impacts relating to ecotoxicity and biodiversity. Working with WaterTAP, a Python-based modeling tool in development at the National Energy Technology Laboratory, we model both conventional brine disposal techniques and MLD/ZLD brine treatment technologies. Surface water discharge and deep well injection were selected for study as conventional brine disposal technologies, and our analysis of MLD/ZLD technologies focuses on mechanical vapor compression and low salt rejection reverse osmosis. For all brine disposal and treatment methods studied, LCA incorporates the GHG emissions that arise from their respective energy intensities, the impacts of materials needed for each technology, and any biodiversity or ecotoxicity impacts that arise. Along with the life cycle analyses, technoeconomic analyses provide an economic comparison of the treatment and disposal options. Ultimately, the work presented here provides a much needed understanding of new MLD/ZLD technologies and how they relate to conventional brine disposal techniques. Such an understanding is critical for developing technologies to effectively and efficiently increase water security around the world.
