Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources

Transcription

1 Summary of Issues Strategies Benefits & Costs Key Uncertainties Additional Resources KEY POINT: Stand-alone outfall is the most common method of concentrate disposal at coastal desal facilities. SUMMARY OF KEY POINTS Over 90% of large seawater desal plants in operation dispose of concentrate through a new ocean outfall designed specifically for that purpose (WHO 2007). Ocean outfalls typically consist of a pipe or series of pipes built away from the coast and towards open water. Outfalls can be located in tidal zones in order to utilize the high mixing capacity of the wave and current action. Outfalls are often located beyond the tidal zone, with the addition of diffusers at the ends of outfall pipes to improve mixing. The salinity threshold mixing/transport capacity of the tidal zone and/or necessary diffuser configuration can be estimated with hydrodynamic modeling (Rhodes 2006). Waste streams generated during the desal process (e.g., chemical cleaning and membrane backwash solutions) are an important consideration for concentrate management, as they can affect the quality of the concentrate stream. Well designed ocean outfalls and careful monitoring have been effective in eliminating the impacts of concentrate disposal on marine life. However, the environmental impacts of seawater desal concentrate disposal can be a concern. For more information, see related PIM cell discussion on the environmental impacts of direct concentrate discharge to the ocean. STRATEGIES Outfall location and design Although the tidal zone typically provide much better mixing than that of diffuser outfall systems, tidal zones have limited capacity in transporting saline discharge load to the open ocean (WHO 2007). The mixing and transport capacity of tidal zones should be determined using hydrodynamic modeling to ensure that no excess salinity will accumulate. Such accumulation may result in salinity increments beyond the level of tolerance for aquatic 1

2 life. If discharge load is lower in total dissolved solids (TDS) than the tidal zone s threshold mixing/transport capacity, then concentrate disposal to this zone is preferable and more cost effective than the use of a long open outfall equipped with a diffuser system (WHO 2007). Two models used for salinity plume analysis are CORMIX and Visual Plumes (Voutchkov 2005). Both models depict concentrate plume dissipation associated with different outfall and diffuser designs and operational conditions. Other modeling techniques and criteria enhancing concentrate diffusion have also been described (see Roberts et al. 1997, Purnama and Al-Barwani 2004 and 2006). Large seawater desal plants typically construct outfalls with diffusers beyond the tidal zone, such as in the Perth Desalination Plant in Australia. A well-designed outfall can prevent heavy saline plumes from accumulating at the ocean bottom in the immediate vicinity of the discharge. The length, size, and configuration of the outfall and diffuser structure for a large desal plant should be determined based on hydrodynamic modeling of the site-specific conditions of the discharge location (WHO 2007). Small plants usually use shore discharge to take advantages of the turbulent mixing created by waves. The turbulence in these zones dissipate the concentrate, quickly bringing water quality in the receiving zone back to ambient conditions. Desal plants on the islands of Malta, and Santa Catalina, CA, are examples for such methods. The siting of desal facility or outfall is critical to minimize the ecological impact of concentrate disposal. All possible locations for brine discharge should be evaluated to identify potential ecological impacts, giving preference to the location(s) associated with the least ecological impacts (CDWR 2003). If a disposal outfall has to be built in an environmentally sensitive area, concentrate disposal may require special measures to protect aquatic life and endangered species. Handling of waste streams generated during the desal process Appropriate handling and treatment of other waste streams generated during the operation of a desal facility is an important consideration for concentrate management. Process waste streams, including chemical cleaning solutions and membrane backwash, can affect the quality of the concentrate stream. Chemical cleaning solutions. To ensure product water quantity, most desal plants perform chemical cleaning about two to four times a year. Chemical cleaning solutions usually contain acids, alkaline, and complexing agent (such as ethylenediamine tetraacetic acid (EDTA), dispersants, or surfactants), and disinfectants (Jordahl 2006). Typically, cleaning solutions are either blended with concentrate (using the same 2

3 discharge method) or discharged separately to the sewer (Mickley 2006). Chemical cleaning solutions must be neutralized (e.g., ph adjustment and reducing disinfectants/oxidants) before blending with desal concentrate for ocean disposal. Wastes such as backwash solutions from media filters or MF/UF also need to be treated prior to blending with membrane concentrate and discharging. Significant suspended solids are typically present in filter backwash water. In addition, coagulants such as ferric chloride, ferric sulfate or aluminum sulfate are sometimes used in the pre-treatment process to reduce silt derived from organics, small colloids and other suspended material. These flocculants form flocs of ferric oxyhydroxide (or aluminum hydroxide), which are washed from media filters, cartridge filters, MF, or UF membrane units in the filter backwash. If the filter backwash water is discharged without treatment, ferric oxyhydroxide floc may settle on the seabed or, more likely be dispersed (Khan et al. 2006). In most cases, the filter backwash water is settled prior to removal and the sludge that contains the vast majority of the coagulant is either disposed of to the sewer or dewatered and disposed of to a landfill as solid waste. The settled backwash water can be either recycled to the inlet of the desal plant, upstream of the pretreatment filters, or will be discharged to the ocean via the concentrate disposal pipeline (Poseidon Resources 2005). Careful monitoring and design Well designed ocean outfalls and careful monitoring have been effective in reducing the impacts of concentrate disposal on marine life. For example, the Perth Seawater Desalination Plant undertakes a real-time monitoring in Cockburn Sound to ensure that model predictions are correct and that the marine habitat and fauna are protected (Rhodes 2006). This includes monitoring of dissolved oxygen levels via sensors on the bed of the sound. Visual confirmation of the plume dispersion was achieved by the use of Rhodamine dye added to the plant discharge. The experiment showed that the discharge rapidly mixed with the surrounding waters (Crisp and Rhodes 2007). Recently, an independent report on the environmental impact of the Perth plant concluded that oxygen levels in Cockburn Sound have not been affected by the discharge from the plant (Water Corporation 2007). A documentary film on the adjoining ecosystem near the feed water intake and outfall diffuser has further shed light on this concern. The video shows prolific habitat growth in the area, suggesting a healthy ecosystem. The underwater footage from the Perth Seawater Desalination Plant can be viewed through the website at: 3

4 BENEFITS & COSTS Benefits For coastal facilities, ocean disposal is the most common and least expensive option. Costs The costs for ocean discharge are mainly determined by (Mickley 2006): Concentrate conveyance costs from the desal membrane plant to the ocean discharge outfall. The costs are typically closely related to the concentrate volume and the distance between the desal membrane plant and the discharge outfall. Costs for outfall construction and operation. The costs depend on the outfall size, diffuser system configuration, outfall length and material, and concentrate treatment prior to discharge. Costs associated with monitoring environmental effects of concentrate discharge to surface waters. The costs associated with environmental monitoring of surface water discharge may be substantial, especially if the discharge is in an environmentally sensitive area, or in areas with limited natural flushing. KEY UNCERTAINTIES Although there are a number of hydrodynamic models to estimate the mixing and transport of concentrate disposal, it should be noted that the science of predicting near field dilution achieved by dense fields has not been greatly studied (Khan et al. 2006). There is also lack of knowledge on optimum substrate to help guide the outfall design or to determine the optimum location of a project based on the best available substrate for discharge (CWRD 2003). These information gaps may lead to uncertainty in design and construct an outfall that fully protects marine ecological system. 4

5 ADDITIONAL RESOURCES CDWR Water Desalination Task Force, Issue Paper. Revised Draft August 26, Office of Water Use Efficiency and Transfers Crisp, G., and M. Rhodes Perth Seawater Desalination Plant. AMTA, Summer Khan, S., D. Waite, G. Leslie, and R. Cox Management of Concentrate Waste Streams from Desalination and Water Recycling Membrane Treatment Systems. Report Prepared for Queensland Environmental Protection Agency and ARUP. Draft No. 3. Report Number: CWWT2006/3. 28 February Jordahl, J Beneficial and Nontraditional Uses of Concentrate. Alexandria, VA.: WateReuse Foundation. Mickley, M. 200). Review of Options. Available at: < Mickley, M Membrane Concentrate Disposal: Practices and Regulation. USBR Desalination and Water Purification Research and Development Program Report No. 123, 2 nd Ed. April Available at: < MMWD (Marin Municipal Water District) Report MMWD Seawater Desalination Pilot Program. Prepared by Kennedy/Jenks Consultants in association with CH2M Hill. Available at: < = menuclick&id = 413>. NRC (National Research Council) Desalination: A National Perspective. Washington, D.C.: National Academy Press. Available: < Poseidon Resources Poseidon Resources Carlsbad Desalination Project Environmental Impact Report. Carlsbad, CA. Available: < [Cited April 1, 2007]. Purnama, A. and H.H. Al-Barwani Some criteria to minimize the impact of brine discharge into the sea. Desalination, 171 (2): Purnama, A. and H.H. Al-Barwani Spreading of brine waste discharges into the Gulf of Oman Desalination, 195 (1-3): Rhodes, M Marine management is high priority. The International Desalination and Water Reuse Quarterly, 16:30. 5

6 Roberts, P.J.W., A. Ferrier, and G. Daviero Mixing in inclined dense jets. Journal of Hydraulic, 123(8): Voutchkov, N Alternatives for ocean discharge of seawater desalination plant concentrate. In Proceedings of 20th Annual WateReuse Symposium Water Reuse & Desalination: Mile High Opportunities. Denver, Colorado, Sep 18-21, 2005: WateReuse Association. Water Corporation Perth Seawater Desalination Project. Available: < nt.pdf>. [Cited November 12, 2007]. WHO (World Health Organization) Desalination for Safe Water Supply: Guidance for the Health and Environmental Aspects Applicable to Desalination. WHO/SDE/WSH/07/0?, Geneva. Available: Xu, P., Cath, T, Wang, G., Drewes, J.E. and Dolnicar, S Critical assessment of implementing desalination technology. AwwaRF Project Published by American Water Works Association Research Foundation, Denver, CO. 6