By Joseph A. Cotruvo & Houssain Abouzaid
Table 2 provides some generalized performance expectations for four major categories of membrane systems. The larger pore membranes (MF and UF) are often used as pretreatments to remove larger particulate contaminants and to reduce the loadings on the more restrictive membranes (like RO) and extend their performance and run times.
Reverse osmosis (RO)
In RO, external pressure is applied to the high solute (concentrated) water to cause solvent (water) to migrate through the membrane, leaving the solute (salts and other non-permeates) in a more concentrated brine. Some RO membranes will reject up to 99 percent of all ionic solids and commonly have molecular weight cut off in the range of 100 to 300 daltons for organic chemicals. Increased pressure increases the rate of permeation; however, fouling would also increase. The basic RO process includes pretreatment, membrane transport and posttreatment prior to distribution. RO processes can produce water in the range of 10 to 500 mg/liter TDS.
Feedwater is treated to protect the membranes by removing some contaminants and controlling microbial growth on the membrane. Suspended solids are removed by filtration or larger pore membranes like UF; pH adjustments (lowering) are made to protect the membrane and control precipitation of salts; antiscaling inhibitors control calcium carbonates and sulfates. Manganese iron and some organics cause fouling of membranes. Disinfection to control biofouling can involve chlorine species, ozone or ultraviolet (UV) light etc. Marine organisms, algae and bacteria must be eliminated and if chlorine is used it should be neutralized prior to contact with the membrane.
Product water from distillation or membrane desalination systems must be treated to stabilize it and make it compatible with the distribution system. Adjustment of pH to approximately eight is required. Carbonation or other chemicals such as lime may be applied and blending with some source water may be done to increase alkalinity and TDS and stabilize the water. Addition of corrosion inhibitors like polyphosphates may be necessary. Postdisinfection is also necessary to control microorganisms during distribution, as well as to eliminate pathogens from the blending process. Degasification may also be necessary. Many systems blend back a portion of the source water with the desalinated water for mineralization. With seawater, this is usually limited to about one percent, due to taste contributed by sodium salts. Both blending with source water or treatment with lime or limestone also coincidently reconstitute some of the beneficial minerals. Compatibility between desalinated and local fresh water may be an issue during distribution.
Potentially beneficial chemicals
Water components can supplement dietary intake of trace micronutrients and macronutrients or contribute undesirable contaminants. Some of the chemicals of beneficial interest in drinking water include calcium, magnesium, sodium, chloride, selenium, potassium, boron, iodide, fluoride, chromium and manganese. Seawater is rich in ions like calcium, magnesium, sodium, chloride and iodine, but low in other essential ions like zinc, copper, chromium and manganese. Desalination processes significantly reduce all of the ions in drinking water so that people who traditionally consume desalinated water may be consistently receiving smaller amounts of some nutrients relative to those who consume water from some traditional sources.
Sodium is present in desalinated water. Typical total daily dietary intake of sodium can be in the range of 2,000 to 10,000 mg. Water is usually not a significant contributor to total daily sodium intake except for persons under a physician’s care who are required to be on highly restricted diets of less than 400 mg sodium per day. However, in hot climates the presence of some sodium in the drinking water may actually be beneficial to reduce the risk of hyponatremia among those engaged in strenuous physical activity.
Source selection and source protection are the best ways to avoid contamination of finished water. When contamination occurs, additional pretreatments may be necessary and can involve enhanced disinfection or an adsorption process using activated carbon. Contaminants in blending waters will be transported to the finished water, thus appropriate pretreatment of blending water may also be required.
Disinfection and microbial control in drinking water
Similar to fresh waters, sea and brackish waters can contain pathogenic microorganisms including bacteria, protozoa and viruses. During pretreatment, a disinfectant, often chlorine, will be added to reduce biofouling and protect the membrane from degradation. Membranes can remove microorganisms by preventing their passage to the finished water. If the membranes and seals are intact, virtually complete removals of microorganisms can occur; however, some bacteria can grow through membranes or seals.
Microbial regrowth during storage and distribution is of particular concern when water is stored and distributed, especially in very warm climates. Most regrowth microorganisms are not frank pathogens, but microorganisms such as Legionella that can regrow in any plumbing systems at warm temperatures are a particular health concern and have caused numerous disease outbreaks in hospitals and other buildings.
Significant amounts of disinfection byproducts (DBPs) can be formed in the pretreatment processes that are applied in both membrane and distillation processes and these are mostly removed by the desalination process. Brominated organic products would be expected to predominate due to the bromide in seawater when it is converted to hypobromous acid and other species. Bromate and other DBPs can also be formed in significant amounts under some conditions in finished waters, especially if posttreatment with seawater is practiced.
Waste and concentrates management
Concentrates disposal is one of the most challenging issues with respect to desalination processes and environmental impact. Wastes include concentrated brines, backwash liquids containing scale and corrosion salts and antifouling chemicals and pretreatment chemicals in filter waste sludges. Wastes could be discharged directly to the sea, mixed with other waste streams before discharge, discharged to sewers or treated at a sewage treatment plant, placed in lined lagoons and dried and disposed of in landfills.
Desalination plants require significant amounts of electricity and heat depending upon the process, temperature and source water quality. There is an obvious synergy between desalination and energy plants. Cogeneration facilities provide significant opportunities for efficiencies and are common project components. Energy production plants require large water intakes for cooling purposes. They produce substantial amounts of waste heat that is usable in the desalination facility and the spent water disposal system may also be shared.
Installation and operation of a desalination facility will have the potential for adverse impacts on air quality, water/sea environment and groundwater and possibly other aspects that must be considered in Environmental Impact Assessments (EIAs). Their acceptability and mitigation requirements would be matters of national and local regulation and policies. Examinations of these effects would be conducted at each site and post-installation monitoring programs should be instituted. A partial listing of issues includes:
- Construction: Coastal zone and sea floor ecology, birds and mammals habitat, erosion, non-point source pollution.
- Energy: Fuel source and fuel transportation, cooling water discharges, air emissions from electrical power generation and fuel combustion.
- Air quality: Energy production related.
- Marine environment: Constituents in waste discharges, thermal effects, feed water intake process, biocides in discharge water and toxic metals, oxygen levels, turbidity, salinity, mixing zones, commercial fishing impacts, recreation and many others.
- Groundwater: Seepage from unlined drying lagoons causing increased salinity and possibly toxic metals deposition.
Draft Guidance and recommendations
The Draft Guidance contains a comprehensive overview of technical, health and environmental concepts and practices relating to assuring the optimization of desalination benefits and minimizing concerns. Following are partial summaries of topics and conclusions from the work groups that provide a sense of the direction of the Guidance.
- There are several types of water intakes including wells and open intakes. Well intakes are often favorable because the water has naturally undergone a level of pretreatment. Open intakes are often necessary but the water usually requires pretreatment and they must deal with entrainment of aquatic organisms.
- Physical pretreatment for thermal systems is simpler than that required for membrane processes. Chemical pretreatment is more robust and necessary for minimizing scaling and corrosion and minimizing the effects of oil and grease from source water.
- Pretreatment of source water involves filtration to reduce particulates and also disinfection to control biogrowth on the membranes. The water must be dechlorinated to protect the structural integrity of the polymer membranes.
- Posttreatment of product water requires stabilization by addition of carbonate alkalinity, disinfection, corrosion inhibition, remineralization often by blending with pretreated source water, disinfection and enhanced removal of specific compounds like borate.
- Blending water should be pretreated to assure chemical and microbial safety. Chlorine contact with saline source water will produce disinfection byproducts, especially if electrolytic generation of chlorine from seawater is practiced.
- Mineral composition of concentrates is usually two-to-10-times the source water and disposal should be in an environmentally safe manner. The most common approach for seawater plants is surface water discharge. Proper co-discharge of desalination concentrates with power plant cooling water is advantageous because it accelerates dissipation of the high salinity and thermal plume.
- Recommended work for unresolved questions includes: better analytical methods for saline water applications; documentation of rejection of algal toxins and manmade chemicals; techniques for online determinations of RO membrane integrity; expanded studies of potential applications of disinfectants; management of DBPs; and the need for reliable disinfection and corrosion inhibition methods for water conveyed over long distances in warm climates.
- Most chemicals in desalinated waters are also frequently present in typical drinking water derived from freshwater.
- Candidates either for inclusion or revisions in WHO Guidance for Drinking Water Quality (GDWQ) include brominated organic DBPs , bromate, bromide and borate. These are already under consideration in the GDWQ.
- Governments are encouraged to adopt existing systems or establish systems for specifying the appropriateness and quality of additives and contact surfaces encountered in desalination or to adopt existing credible, recognized standards for those products that would be tailored to desalination conditions.
- Mineral balance in drinking water should be considered, particularly calcium and magnesium, with regard to risks of osteoporosis and ischemic heart disease, respectively; metabolic syndrome associated with risk of diabetes, as well as fluoride in relation to loss of fluoride from the skeleton and tooth decay risk.
- Recommendations for needed work for unresolved questions include: developing a greater understanding of the brominated byproducts and the conditions of their formation and prevention or reduction, particularly in blended systems; quantification of daily drinking water consumption in warm climates for use in improved risk and benefit assessments; safe management and disposal of desalination concentrates should be studied further.
- Monitoring source water for specific pathogens is not recommended, but typical physical, chemical and microbial baseline information is useful for ensuring treatment efficiency. Faecal indicator organisms provide suggestive information of the presence of pathogens.
- Heterotrophic plate counts (HPC) are not an independent indicator of faecal pollution but are useful for indicating microbial community changes or treatment process performance.
- Post-desalination disinfection should be provided and maintained for storage and distribution to ensure that product water quality is delivered to the consumer.
- Water used for blending should be treated to reach microbial quality goals set on the basis of raw water contamination and risk reduction.
- Desalinated water is low in nutrients and possesses a low regrowth potential. However, high temperatures that are frequent in countries using desalination may enhance regrowth and also nitrification when chloramines are used. Growth of Legionella in distribution or domestic plumbing systems is a serious public health threat.
- Analytical techniques specifically applicable to saline water are needed for application in source waters.
Group IV-Monitoring, surveillance, regulation
- Monitoring methods and frequencies and surveillance methods are suggested for small and large plants for source waters, process management, blending waters, storage, distribution and at the tap to assure microbial and chemical safety.
- Water Safety Plan management approaches are recommended for application to both desalination and freshwater systems.
Group V-Environmental impact assessments
- An EIA is a procedure that identifies, evaluates and develops means of mitigating potential impacts of proposed activities on the environment. Its main objective is to promote environmentally sound and sustainable development through the identification of appropriate mitigation measures and alternatives. Based on the EIA results, a decision must be reached which balances the positive and negative effects of a project in terms of the societal and environmental costs and benefits.
- An EIA can also be adopted for plans, policies or programs (e.g., water resources or coastal zone management plans) in the form of a strategic environmental assessment (SEA). The present document is primarily considered as a guide for impact assessment on project level. Depending on the proposed project, it is incumbent upon the national authorities to define the scope and requirements for each EIA.
- An EIA predicts the impacts related directly or indirectly to the implementation of the project. This should comprise implications including ecosystem, socio-economic, cultural and public health effects, as well as cumulative and trans-boundary implications. The EIA should identify the positive effects and offer measures for mitigation of negative impacts.
- Some of the aspects to be considered in an EIA of a desalination project include: siting, impacts of intakes including construction and entrainment and impingement of organisms, concentrates and residuals disposal management, discharges of cooling waters and other waste streams, environmental fate and toxic effects of residuals and DBPs.
- A model EIA procedure and format and 10 basic steps of an EIA are described.
- Future data collection and published case studies are recommended to improve the methodology and utility of EIAs and any future guidance.
WHO encourages all interested parties to examine the Draft Guidance and provide comments. The Guidance is available for free download on the WHO website at http://www.who.int/water_ sanitation_health/gdwqrevision/desalination/en/index.html_
Documentation of comments is encouraged. The closing date for comments is October 31, 2007.
Some questions to consider are:
- Is the Guidance technically accurate?
- Does it contain useful information to assist practitioners and regulators?
- Is there additional information available that would strengthen the Guidance?
- Are the recommendations appropriate and consistent with the text and helpful to users?
This commentary is based upon the draft of the “WHO Desalination Guidance” and does not necessarily represent the policy of WHO. It was also drawn from a paper presented at the Environment 2007 Conference in Abu Dhabi, UAE. Both documents contain the appropriate citations. WHO especially wishes to acknowledge the organizations that generously sponsored the desalination guidance development process. These included: the AGFUND, The Kuwait Foundation for the Advancement of Science (KFAS), the US EPA’s National Risk Management Research Laboratory (Cincinnati, Ohio), the American Water Works Association Research Foundation (AwwaRF) Denver, Colo., USA, The Water Authority of the Cayman Islands, The Bureau of Reclamation (Denver , Colo., USA) and the National Water Research Institute (NWRI, Fountain Valley, Calif., USA). The project was also substantially funded by WHO EMRO. We gratefully acknowledge the expertise and in-kind services provided by all of the expert participants without which it would not have been possible.
About the author
Dr. Joseph Cotruvo (corresponding author) is President of Joseph Cotruvo & Associates, Environmental and Public Health Consultants, Washington, D.C. USA. His Ph.D. is in physical organic chemistry. He was Director of the Drinking Water Standards Division and the Toxic Substances Risk Assessment Division during his almost 25 years of service with US EPA. He currently works extensively on drinking water quality, desalination, water reuse and water delivery systems and health science issues. Cotruvo also serves on several World Health Organization panels and on research panels with the NWRI and the WaterReuse Foundation. He is a member of the Agua Latinoamérica Comité Consultivo de Asesores Téchnicos and President of AIDIS USA. You may contact him by phone/fax: (202) 362-3076 or email: [email protected].