Produced water (PW) from oil and gas production contains variable constituents that are difficult to remove with conventional treatment processes. The focus of this study was to explore the long-term performance of a membrane bioreactor (MBR) for the removal of organic constituents from PW and how performance and microbial community composition are affected by progressively increasing salinity and introduction of PW from different shale basins around the United States. Dissolved organic carbon removal from the PW remained consistent throughout the study, averaging 86% from the Denver-Julesburg basin PW and 66% removal from the Permian basin PW. Surfactant removal was less consistent, showing 87% removal of poly(ethylene glycols) (PEGs) at a total dissolved solid (TDS) concentration of 40 g/L but only 58% removal at a TDS concentration of 100 g/L. Diversity in the microbial community decreased during reactor establishment but increased at TDS concentrations above 80 g/L. The results of this study suggest that MBRs can be effective PW pretreatment processes even at high salinities.

Innovation in urban water systems is required to address drivers of change across natural, built, and social systems, including climate change, economic development, and aged infrastructure. Water systems are complex socio-technical systems that interact with biophysical systems to supply and reclaim water. We present a vision for enhancing urban water system resilience through a net zero urban water (NZUW) approach, which meets the needs of a given community with a locally available and sustainable water supply, without detriment to interconnected systems or long-term water supply. NZUW is an integrative approach with progressive targets assessed using a quantitative framework to expand adaptive and responsive solutions for urban water self-sufficiency. Decision makers can use NZUW to understand trade-offs between future interventions to urban water systems across spatial and temporal scales. We present the overall NZUW approach, drivers of change, applications, and research gaps.

Membrane distillation (MD) is a membrane-based thermal desalination process capable of treating hypersaline brines. Standard MD systems rely on preheating the feed to drive the desalination process. However, relying on the feed to carry thermal energy is limited by a decline of the thermal driving force as the water moves across the membrane, and temperature polarization. In contrast, supplying heat directly into the feed channel, either through the membrane or other channel surfaces, has the potential of minimizing temperature polarization, increasing single-pass water recoveries, and decreasing the number of heat exchangers in the system. When solar thermal energy can be utilized, particularly if the solar heat is optimally delivered to enhance water evaporation and process performance, MD processes can potentially be improved in terms of energy efficiency, environmental sustainability, or operating costs. Here we describe an MD process using layered composite membranes that include a high-thermal-conductivity layer for supplying heat directly to the membrane-water interface and the flow channel. The MD system showed stable performance with water flux up to 9 L/m2/hr, and salt rejection >99.9% over hours of desalinating hypersaline feed (100 g/L NaCl). In addition to bench-scale system, we developed a computational fluid dynamics model that successfully described the transport phenomena in the system.

Membrane distillation (MD) is an emerging thermal desalination technology capable of desalinating waters of any salinity. During typical MD processes, the saline feedwater is heated and acts as the thermal energy carrier; however, temperature polarization (as well as thermal energy loss) contributes to low distillate fluxes, low single-pass water recovery and poor thermal efficiency. An alternative approach is to integrate an extra thermal energy carrier as part of the membrane and/or module assembly, which can channel externally provided heat directly to the membrane-feedwater interface and/or along the feed channel length. This direct-heat delivery has been demonstrated to increase single-pass water recovery and enhance the overall thermal efficiency. We developed a bench-scale direct-heated vacuum MD (DHVMD) process to desalinate pre-treated oil and gas "produced water" with an initial total dissolved solids of 115,500 ppm at a feed temperature ranging between 24 and 32 °C. We evaluated both water flux and specific energy consumption (SEC) as a function of water recovery. The system achieved a 50% water recovery without significant scaling, with an average flux >6 kg m^-2 hr^-1 and a SEC as low as 2,530 kJ kg^-1. The major species of mineral scales (i.e., NaCl, CaSO₄, and SrSO₄) that limited the water recovery to 68% were modeled in terms of thermodynamics and identified by scanning electron microscopy and energy-dispersive X-ray spectroscopy. In addition, we further developed and employed a physics-based process model to estimate temperature, salinity, water transport and energy flows for full-scale vacuum MD and DHVMD modules. Model results show that a direct-heat input rate of 3,600 W can increase single-pass water recovery from 2.1% to 3.1% while lowering the thermal SEC from 7,800 kJ kg^-1 to 6,517 kJ kg^-1 in an unoptimized module. Finally, the scaling up potential of DHVMD process is briefly discussed.

The Colorado River supplies >40 million people in the United States Southwest with their daily water supply and is unable to meet the current demands. New approaches are needed to enhance sustainability and resilience. A net zero urban water (NZUW) approach meets the needs of a given community with a locally available and sustainable water supply, without detriment to interconnected systems and the long-term water supply. Transitioning to a NZUW future will require considerable modifications to governance and policy across the Southwest and its cities specifically. We identify five areas of governance and policy challenges: diversified water sources and sinks; planning, design, and operation; monitoring and enforcement; coordination; and addressing equity and justice. Four case study cities are investigated: Albuquerque, Denver, Los Angeles, and Tucson. Across these cities, the policy priorities include supporting potable water reuse, coordinating policies across jurisdictions for alternative water sources, addressing equity and justice, developing and incentivizing water conservation plans, and making aquifer storage and recovery projects easier and more economical to pursue. We conclude that a NZUW transition in the Southwest faces considerable governance and policy challenges, but moving cities toward this goal is crucial.