Sandeep Sethi, Steven Walker, Pei Xu, and Jörg E. Drewes

OBJECTIVES:

The overall objective was to assess the state-of-science and advance desalination technologies for enhancement of system recovery and minimization of concentrate volume. Phase I focused on a state-of-science assessment of several desalination configurations and technologies, as well as conceptualization of an innovative configuration for increasing recovery and minimizing concentrate. Phase II focused on advancement of desalination technologies via further development and testing of the innovative approach through bench-scale experiments, modeling, and economic assessment.

BACKGROUND:

Key drivers for desalination include increased demand for freshwater due to increases in population, standard of living, industrialization, and agricultural and environmental needs, as well as freshwater supply challenges due to salinity buildup. Furthermore, reduced costs and increased energy efficiency have made desalination technologies more viable. However, a key challenge in the efficient implementation of desalination technologies for treating impaired waters is the limiting process recovery and consequent concentrate management and disposal.

HIGHLIGHTS:

* Various technologies/configurations were assessed and an innovative approach was developed for concentrate minimization that included RO, intermediate precipitation, and ED/EDR.

* Evaluations indicated that recovery of conventional RO can be enhanced by about 10 to 20 percent depending on the source water quality.

* Fluidized bed crystallization process using granular media (sand) as seed provided high removals of cations while the precipitative slurry method provided better silica removals (via magnesium salt co-precipitation).

* Appropriate life-cycle cost comparisons of the innovative approach with conventional RO at an equivalent recovery or zero-liquid-discharge (ZLD) indicated that this approach can be more cost effective.

APPROACH:

Several desalination technologies/configurations were assessed via literature reviews. A screening was performed and coupled with implicit data/technology gaps to aid in development of an innovative configuration for recovery enhancement/concentrate minimization. The project took the approach that practical advancements, while innovative, should be based on established unit processes to provide a near term solution. The innovative configuration includes a conventional RO desalination step and a concentrate treatment scheme including chemical precipitation, possible filtration, and electrodialysis (ED) and/or electrodialysis reversal (EDR). Bench-scale test units and experiments were designed and the configuration systematically tested using four brackish water matrices. Modeling was performed to validate the overall recovery. An economic analysis was performed to compare the innovative configuration with conventional means of concentrate disposal or ZLD.

RESULTS/FINDINGS:

Assessment performed in this work indicated that the recovery of conventional brackish water RO configuration can be enhanced by about 10 to 20 percent depending on the source water quality. Compared to the precipitative slurry method, the fluidized bed crystallization process using granular media (sand) as seed provided higher removals of cations such as calcium, barium, and strontium from RO concentrate (provided adequate alkalinity was present or supplied via addition of chemicals such as soda ash). This process enhanced the crystallization of sparingly soluble salts, as well as minimized the adverse impacts of antiscalant on the efficiency of precipitation. However, for waters with high silica content limiting recovery, the precipitative slurry method provided better silica removals (via magnesium hydroxide or magnesium chloride co-precipitation).

ED treatment recovered over 80 percent RO concentrate by recycling the ED concentrate. Costs comparisons of the innovative approach with the conventional approach at an equivalent recovery (i.e., conventional RO coupled with brine concentrator) or ZLD indicated that the innovative approach can be more cost effective than the conventional approach on a unit cost basis. It is noted that regardless of cost, other non-economic drivers (e.g., regulatory requirements, inland location without access to easy concentrate disposal options, public perception, etc.) can still dictate that an approach including concentrate minimization be considered.

IMPACT:

Limiting process recovery and consequent concentrate management and disposal are implementation challenges for desalination that are becoming more apparent with stricter regulations. Innovative approaches for concentrate minimization, such as the one developed in this work, will provide solutions for enhancing product water recovery and minimizing concentrate volume to increase the viability of desalination and accelerate its effective utilization. The assessment of emerging technologies provides a practical and comprehensive body of knowledge of other key promising technologies/configurations. A guidance chart was also developed to provide broad, general guidance on the options and the associated state-of-science for concentrate minimization technologies.

RESEARCH PARTNERS:

* East Bay Municipal Utility District

* Contra Costa Water District

* San Francisco Public Utilities Commission

* Santa Clara Valley Water District

PARTICIPANTS:

Utilities and agencies from throughout the United States participated in this project.