| The availability of surface and ground water sources for agricultural, industrial, and personal use is becoming increasingly constrained. In response, reverse osmosis (RO) water desalination has been touted as a potential technology for increasing the available water resources in many parts of the world. Different research ideas have been proposed to find the "ultimate" solution to decrease the cost of RO desalination, such as creating more permeable RO membranes, using two-pass nanofiltration (NF) membranes to replace single-pass RO membranes, closed-circuit discharge technology, forward osmosis and etc. Motivated by this, my PhD research focused on creating a framework, from first-principles, to allow for evaluating the cost effectiveness of various "new" ideas and identify the most promising ones and, based on which, minimize the overall cost of RO water desalination with current generation of highly permeable membranes, which enables practical RO processes to be operated up to the thermodynamic limit.;The framework developed in my PhD research led to a conclusion that there is little economic incentive for developing higher permeability membranes if the objective is to lower the cost of water desalination, balancing the energy consumption, membrane expenditure, and concentrate management costs. Future reduction in RO water production cost can arise from a variety of other process improvements including, but not limited to improved fouling-resistant membranes, lower cost of feed pretreatment and brine management, advanced control schemes (e.g. to account for feed salinity fluctuation), process configuration optimization (e.g., multi-stage or multi-pass, mixing and recycling operation), as well as low cost renewable energy sources.;The designed framework is utilized to predict the optimum operating conditions of a single-stage cross-flow RO process, with/without energy recovery devices, under different feed and permeate flow requirement and feed water salinity fluctuation. The algorithms were implemented as the automated energy-optimal based control software in a first-generation pilot mini-mobile-modular (M3) system, equipped with online standard process monitors (i.e., pressure, flow rate, pH and conductivity, and tested both in the lab and in the field desalting the agricultural drainage water at the Panoche Drainage District of the San Joaquin Valley. The framework is also utilized for multi-stage (where the concentrate stream from the previous stage is desalted to increase the overall water recovery) and multi-pass (where the permeate stream from the previous pass is further desalted to meet the product water quality requirement) RO network structures to evaluate their energy efficiency. The analysis revealed that a multi-stage RO process is more energy efficient than a single-stage RO process, but at the expense of more membrane area requirement. The present work also showed that the two-pass NF/RO process is less energy efficient than a single-pass RO process. Notwithstanding, such a process could be necessary if a single-pass RO process cannot achieve the salt rejection requirement. Different recycling and stream mixing options were also evaluated for their energy effectiveness under the framework and the close-circuit discharge operation, even less energy efficient than a single-stage process with full energy recovery, but is more energy efficient at water recoveries lower than a critical value than single-stage without energy recovery and is able to achieve the effect of energy recovery from the brine stream without incurring the capital cost of acquiring an energy recovery device. The close-circuit discharge technology can be even more cost-effective than multi-stage in low recovery (<35%, for example seawater desalination) where ERD and pump costs are high.;The energy optimal policy is also utilized in operating the smart compact modular second-generation RO (CoM2RO) with the integration of ultrafiltration (UF) pretreatment, which is gaining more market share in seawater pretreatment due to its compact size, relatively easy operation and less maintenance required. This system with its adaptive backwash has been tested with the cooling tower water at the UCLA cogeneration plant and is currently being tested with seawater at a navy base for its future deployment as a shipboard desalination unit in the open ocean and coastal areas. |
