The State of Desalination

• By Joseph A. Cotruvo, Ph.D., Houssain Abouzaid, Ph.D.
• Mar 01, 2007

The more than 12,000 desalination facilities in operation producing 10 billion gallons of water per day worldwide were developed under various governmental and public health authorities and environmental quality specifications. The World Health Organization (WHO) of Geneva, Switzerland, has been working on an authoritative guidance to help facilitate new plants or existing plant upgrades by providing universally acceptable quality goals and environmental, siting and operating considerations.

The guidance, which will be released for public comment in 2007, contains a comprehensive overview of technical, health, and environmental concepts and practices relating to desalination. WHO’s Eastern Mediterranean Regional Office (EMRO) initiated work in 2004 on the desalination guidance: “Desalination for Safe Water Supply: Guidance for the Health and Environmental Aspects Applicable to Desalination.” When released for external review, the Guidance will be accessible at www.WHO.int/water_sanitation_health.

The timing of this document’s release is good. The desalination market is predicted to grow by 12 percent per year to 2010. Over the next 10 years, an estimated $100 billion will be needed for desalination in the Arab states alone -- just to keep up with economic growth and water demand. Desalination plants currently produce 40 million cubic meters of water per day (m³/d). By 2015, production is expected to reach 94 million m³/d.

Many drinking water quality issues are presented in the WHO “Guidelines for Drinking-water Quality” (GDWQ). The new desalination guidance augments the earlier one, focusing on aesthetics and water stability, blending water quality, nutritionally desirable components, direct and indirect additives, process performance and water quality monitoring, and environmental protection.

Quality and Health
Aesthetic factors such as taste, odor, turbidity, and total dissolved solids (TDS) affect water palatability, and thus, consumer acceptance. A lack of acceptance will indirectly affect health as well. Corrosion, hardness, and pH have economic and health consequences and help determine the chemical makeup of the water through the distribution system.

Stabilizers and blending can be used to adjust distribution system effects. Desalinated water is stabilized by adding lime and other chemicals. Blending increases TDS and improves the stability of finished desalinated water. Components of the blending water also can affect quality and safety, because it may not receive any further treatment beyond residual disinfection. Some contaminants are best controlled by pretreating the blending water.

Although drinking water cannot be relied on for supplemental minerals in the daily diet, there is a legitimate question as to the optimal mineral balance of drinking water to ensure quality and health benefits.

Processes
Drinking water production processes can be divided into three broad categories, each of which will affect the quality of the finished water:
• source water,
• treatment technology, and
• distribution system.

Desalination’s source water, especially seawater, requires design and operating engineers to manage TDS in the range of about 5,000 mg/L to 40,000 mg/L; high levels of particular ions, including sodium, calcium, magnesium, bromide, iodide, sulfate, and chloride; the chemical composition of total organic carbon in seawater; petroleum and wastewater contamination potential; and microbial contaminants and larger organisms.

The technologies primarily used in desalination are thermal distillation and reverse osmosis (RO) membranes. In these processes, operators must be aware of potential complications caused by leachate from system components, pretreatment and anti-fouling additives, disinfection byproducts, and blending with source waters.

Operators also must manage high temperatures and large distribution networks in many areas, corrosion control, corrosion products, and bacterial regrowth, including both nonpathogenic heterotrophic plate counts and pathogens, such as Legionella.

Ultimately, desalinated water providers must consider the risks of water with low TDS or corrosivity (caused by degradation of plumbing and distribution system components), the environmental impact of desalination operations and concentrates disposal, and the need to provide consistent quality at the consumer tap. To manage all these issues, water producers or distributors must monitor source water, process performance, finished water, and distributed water.

Source Water Composition
Seawater and brackish waters contain substantial quantities of minerals, and possibly microbial contaminants, and can be affected by waste discharges. Table 1 provides information on the typical mineral composition of seawaters. Brackish water contains less salt. Special technologies are required to convert these waters into acceptable drinking water.

Desalination processes remove the dissolved salts and other materials. Related membrane processes soften freshwaters and reclaim wastewater. Distillation and membrane technologies used in desalination are energy-intensive. Research in this area seeks to improve efficiency and reduce energy consumption. Cogeneration facilities are now the norm for desalination projects.

Distillation Technologies
Distillation plants can produce water in the range of 1 mg/L to 50 mg/L TDS. Alkaline cleaners remove organic fouling, and acid cleaners remove scale and salts. In the distillation process, water is heated and vaporized (see Figure 1); the condensed vapor is drinking water with very low TDS, and concentrated brine is the residual. Inorganic salts and high molecular-weight natural organics are not volatile nor easily separated. Volatile chemicals, however, can be present due to prechlorination, spills, and other contamination.

The heat of vaporization of water amounts to 2,256 kilojoules per kilogram at 100°C (970 btu per pound at 212°F). The same amount of heat must be removed from the vapor to condense it into liquid at the boiling point. Heat generated from vapor condensation is transferred to the feedwater to improve thermal efficiency and reduce fuel consumption.

Multistage Flash Distillation – Plants using this type of distillation are major contributors to the world’s desalting capacity. In these plants, heated water boils rapidly (flash) when the pressure of the vapor is reduced rapidly below the vapor pressure of the liquid at that temperature. The vapor that is generated is condensed to surfaces that are in contact with feedwater, thus heating the feedwater prior to its introduction into the flash chamber. This action recovers most of the vaporization heat. Approximately 25 percent to 50 percent of the flow is recovered as freshwater in multistage plants, which typically have high feedwater volume and flow, corrosion and scaling, and high usage of treatment chemicals.

Multiple-Effect Distillation -- Configurations of these plants include vertical or horizontal tubes. Steam is condensed on one side of a tube with heat transfer causing evaporation of saline water on the other side. Pressure is reduced sequentially in each effect (stage) as the temperature declines, and additional heat is provided in each stage to improve performance.

Vapor Compression Distillation -- These systems compress water vapor, causing condensation on a heat transfer surface (tube) that allows the heat to be transported to brine on the other side of the surface, resulting in vaporization. The compressor, which requires a substantial amount of energy, increases pressure on the vapor side and lowers pressure on the brine side to lower its boiling temperature.

Membrane Technologies
Common membranes are polymeric materials, such as cellulose triacetate, or more likely polyamides and polysulfones. Membranes are layered or thin-film composites. The surface contact layer (rejection layer) adheres to a porous support. Membrane thickness is on the order of 0.05 mm.

Selection factors for membranes include pH stability, working life, mechanical strength, pressurization capacity and selectivity and efficiency for solute removal. Hollow fiber and spiral wound membrane configurations commonly are used. Operating pressures are in the range of 250 pounds per square inch (psi) to 1,000 psi (17 to 68 bar).

Table 2 provides some generalized performance e

This article originally appeared in the 03/01/2007 issue of Environmental Protection.