Of the more than 7,500 desalination plants in operation worldwide, 60% are located in the Middle East. The world's largest plant in Saudi Arabia produces 128 MGD of desalted water. In contrast, 12% of the world's capacity is produced in the Americas, with most of the plants located in the Caribbean and Florida. To date, only a limited number of desalination plants have been built along the California coast, primarily because the cost of desalination is generally higher than the costs of other water supply alternatives available in California (e.g., water transfers and groundwater pumping). However, as drought conditions occur and concern over water availability increases, desalination projects are being proposed at numerous locations in the state.
Desalination is a process that removes dissolved minerals (including but not limited to salt) from seawater, brackish water, or treated wastewater. A number of technologies have been developed for desalination, including reverse osmosis (RO), distillation, electrodialysis, and vacuum freezing. Two of these technologies, RO and distillation, are being considered by municipalities, water districts, and private companies for development of seawater desalination in California. These methods are described below.
In RO, feedwater is pumped at high pressure through permeable
membranes, separating salts from the water (Figure 1). The feedwater
is pretreated to remove particles that would clog the membranes.
The quality of the water produced depends on the pressure, the
concentration of salts in the feedwater, and the salt permeation
constant of the membranes. Product water quality can be improved
by adding a second pass of membranes, whereby product water from
the first pass is fed to the second pass.
Figure 1. Flow Diagram of a reverse osmosis system (courtesy of USAID). (Kahn, 1986.)
In the distillation process, feedwater is heated and then evaporated to separate out dissolved minerals. The most common methods of distillation include multistage flash (MSF), multiple effect distillation (MED), and vapor compression (VC) (Figure 2).
Figure 2. Common methods of distillation.
In MSF, the feedwater is heated and the pressure is lowered, so the water "flashes" into steam. This process constitutes one stage of a number of stages in series, each of which is at a lower pressure. In MED, the feedwater passes through a number of evaporators in series. Vapor from one series is subsequently used to evaporate water in the next series. The VC process involves evaporating the feedwater, compressing the vapor, then using the heated compressed vapor as a heat source to evaporate additional feedwater. Some distillation plants are a hybrid of more than one desalination technologies. The waste product from these processes is a solution with high salt concentration.
Desalination plants may use seawater (directly from the ocean through offshore intakes and pipelines, or from wells located on the beach or seafloor), brackish groundwater, or reclaimed water as feedwater. Since brackish water has a lower salt concentration, the cost of desalting brackish water is generally less than the cost of desalting seawater. Intake pipes for desalination plants should be located away from sewage treatment plant outfalls to prevent intake of discharged effluent. If sewage treatment discharges or other types of pollutants are included in the intake, however, the pre- and post-treatment processes should remove the pollutants.
Distillation plants produce a high-quality product water that ranges from 1.0 to 50 ppm tds, while RO plants produce a product water that ranges from 10 to 500 ppm tds. (The recommended California drinking water standard for maximum tds is 500 mg/L, which is equivalent to 500 ppm.) In desalination plants that produce water for domestic use, post-treatment processes are often employed to ensure that product water meets the health standards for drinking water as well as recommended aesthetic and anti-corrosive standards.
Desalination product water may be used in its pure form (e.g., for make-up water in power plant boilers) or it may be mixed with less pure water and used for drinking water, irrigation, or other uses. The desalinated product water is usually more pure than drinking water standards, so when product water is intended for municipal use, it may be mixed with water that contains higher levels of total dissolved solids. Pure desalination water is highly acidic and is thus corrosive to pipes, so it has to be mixed with other sources of water that are piped onsite or else adjusted for pH, hardness, and alkalinity before being piped offsite.
The product water recovery relative to input water flow is 15 to 50% for most seawater desalination plants. For every 100 gallons of seawater, 15 to 50 gallons of pure water would be produced along with brine water containing dissolved solids. A desalination plant's recovery varies, in part because the particulars of plant operations depend on site-specific conditions. In several locations in California, pilot projects are being proposed to test plant operations before full-scale projects are built.
Pretreatment processes are needed to remove substances that would interfere with the desalting process. Algae and bacteria can grow in both RO and distillation plants, so a biocide (usually less than 1 mg/L chlorine) is required to clean the system. Some RO membranes cannot tolerate chlorine, so dechlorination techniques are required. Ozone or ultraviolet light may also be used to remove marine organisms. If ozone is used, it must be removed with chemicals before reaching the membranes. An RO technology has been developed recently that does not require chemical pretreatment.
In RO plants, suspended solids and other particles in the feedwater must be removed to reduce fouling of the membranes. Suspended solids are removed with coagulation and filtration. Metals in the feedwater are rejected along with the salts by the membranes and are discharged in the brine. With normal concentrations for metals in seawater, the metals present in the brine discharge, though concentrated by the RO process, would not exceed discharge limits. Some distillation plants may also need to remove metals due to potential corrosion problems.
The filters for pretreatment of feedwater at RO plants must be cleaned every few days (backwashed) to clear accumulated sand and solids. The RO membranes must be cleaned approximately four times a year and must be replaced every three to five years. Alkaline cleaners are used to remove organic fouling, and acid cleaners are used to remove scale and other inorganic precipitates. All or a portion of RO plants must be shut down when the membranes are replaced. When RO plants are not used continuously, the RO membranes must be stored in a chemical disinfection/preservation solution that must be disposed of after use. Distillation plants can also be shut down for tube bundle replacement, which is analogous to membrane replacement.
Desalination plant components must be cleaned to reduce scaling-a condition where salts deposit on plant surfaces, such as pipes, tubing or membranes. Scaling is caused by the high salt concentration of seawater and can result in reduced plant efficiency and corrosion of the pipes. In general, scaling increases as temperature increases; thus scaling is of greater concern for distillation plants, since RO plants require lower temperatures to operate. Scaling can be reduced by introducing additives to inhibit crystal growth, reducing temperature and/or salt concentrations, removing scale-forming constituents, or seeding to form particles. Once scales have formed, they can be removed with chemical or mechanical means.
In addition to scaling, both RO and distillation plant intake and outfall structures and pipelines can become fouled with naturally occurring organisms or corroded. Structures and pipelines may be cleaned by mechanical means or by applying chemicals or heat. Feedwater may also be deaerated to reduce corrosion.
Desalination plants produce liquid wastes that may contain all or some of the following constituents: high salt concentrations, chemicals used during defouling of plant equipment and pretreatment, and toxic metals (which are most likely to be present if the discharge water was in contact with metallic materials used in construction of the plant facilities). Liquid wastes may be discharged directly into the ocean, combined with other discharges (e.g., power plant cooling water or sewage treatment plant effluent) before ocean discharge, discharged into a sewer for treatment in a sewage treatment plant, or dried out and disposed of in a landfill. Desalination plants also produce a small amount of solid waste (e.g., spent pretreatment filters and solid particles that are filtered out in the pretreatment process).
For example, the capacity of the City of Santa Barbara's desalination plant is 7,500 AF/yr (about 7.16 MGD). In May 1992, the plant produced 6.7 MGD of product water and generated 8.2 MGD of waste brine with a salinity approximately 1.8 times that of seawater. An additional 1.7 MGD of brine was generated from filter backwash. Assuming that concentrations of suspended solids in the seawater feed range from 10 to 50 ppm, approximately 1.7 to 5.1 cubic yards per day of solids were generated, which is equivalent to one to two truck-loads per week. (Source: Woodward-Clyde Consultants, EIR for the City of Santa Barbara and Ionics, Inc.'s Temporary Emergency Desalination Project, March 1991.)
The energy used in the desalination process is primarily electricity and heat. Energy requirements for desalination plants depend on the salinity and temperature of the feedwater, the quality of the water produced, and the desalting technology used. Estimates for electricity use requirements for various technologies for seawater desalination are:
In addition to electricity requirements, MSF, MED, and some VC plants also use thermal energy to heat feedwater. (Because of the inefficiency of converting thermal energy to electricity, there is a high energy "penalty" if electricity is used to heat feedwater.) For example, in addition to the 3,500 to 7,000 kWh/AF of energy required for electricity, the thermal energy needs for a MSF distillation plant is estimated at 270 million Btu/AF (about 26,000 kWh/AF); for MED plants, the estimated additional thermal energy requirements are 230 million Btu/AF (about 22,000 kWh/AF).[1] Consequently, the total energy needs for distillation technologies are higher than for RO technologies.
Energy use requirements for desalination plants are high. For example, an estimated 50 million kWh/yr would be required for full-time operation of the City of Santa Barbara's desalination plant to produce 7,500 AF/yr of water. In contrast, the energy needed to pump 7,500 AF/yr of water from the Colorado River Aqueduct or the State Water Project to the Metropolitan Water District (MWD) of Southern California is 15 to 26 million kWh/yr. These energy requirements may be compared to the energy use of a small- to medium-sized industrial facility (such as a large refinery, small steel mill, or large computer center) which uses 75,000 to 100,000 kWh/yr.
Both RO and distillation plants can benefit from cogeneration plants to reduce energy use. Since increased energy use may cause adverse environmental impacts, the individual and cumulative impacts of energy use and production at a proposed desalination plant will require case-by-case analysis.
One advantage of distillation plants is that there is a greater potential for economies of scale. Distillation plants also do not shut down a portion of their operations for cleaning or replacement of equipment as often as RO plants, although distillation plants can and have shut down for tube bundle replacement and cleaning. Pretreatment requirements are greater for RO plants, because coagulants are needed to settle out particles before water passes through the membranes. Unlike RO plants, distillation plants do not generate waste from backwash of pretreatment filters.
Advantages of RO plants over distillation include: RO plant feedwater generally does not require heating, so the thermal impacts of discharges are lower; RO plants have fewer problems with corrosion; RO plants usually have lower energy requirements; RO plants tend to have higher recovery rates-about 45% for seawater; the RO process can remove unwanted contaminants, such as trihalomethane-precursors, pesticides, and bacteria; and RO plants take up less surface area than distillation plants for the same amount of water production.
The cost to produce water from desalination depends on the technology used and the plant capacity, among other factors. For example, the cost of desalted water in Santa Barbara ($1,900/AF) results from the following: a write-off of the capital cost over a short five-year period, high financing costs, and high energy costs. The overall costs of water production are about the same for RO and some forms of distillation plants.
Price estimates of water produced by desalination plants in California range from $1,000 to $4,000/AF. Table 2 lists the estimated costs of producing water for existing and proposed plants, where the information is available. (Specific cost estimates for most existing and proposed California desalination plants are also included in Chapter 2 of this report.) The costs include capital and operating and maintenance costs. For long-term projects, capital costs would most likely be amortized over an assumed plant life of 20 to 30 years. Capital costs for RO plants tend to be lower than for distillation plants. Some of the proposals are for plants that would operate for only a few years. Operating a plant on a part-time, rather than full-time, basis may be more expensive in the long run because maintenance and capital costs must be paid while the plant is shut down.
A number of California coastal communities are facing water shortages. Although the communities may have relatively inexpensive existing supplies of water, the supplies are perceived as being insufficient to meet community needs. New water supplies are more expensive than existing supplies, and in some cases the prices are comparable to desalinated water. Table 2 summarizes the costs of various water supplies.
In 1991, the Metropolitan Water District (MWD) of Southern California paid $27/AF for water from the Colorado River and $195/AF for water from the California Water Project. New sources of water would have cost $128/AF from the Imperial Irrigation District and $93/AF from Arvin Edison Water Storage in Kern County (if water was available during the drought). (Source: pers. comm. with Bob Muir, MWD, 1991.)
Noninterruptible untreated water for domestic uses in San Diego is purchased from the MWD for $269/AF; treated water costs an additional $53/AF. The least expensive new supplies, other than desalination, would cost $600-$700/AF. (Sources: pers. comm. with Gordon Hess, SDCWA, 1991 and Robert Yamada, SDCWA, 1992.)
In Santa Barbara, untreated water from the Cachuma reservoir costs $35/AF. Development of new wells to use further the City's groundwater basins would cost $200/AF, while new groundwater wells in the mountains would cost approximately $600-700/AF. Enlarging Cachuma Reservoir, if feasible, is estimated to cost $950/AF. During the recent drought, the City purchased water from the State Water Project on a temporary, emergency basis at a cost of $2,300/AF. This water was made available through a series of exchange arrangements with water agencies between Santa Barbara and the MWD. Permanently tying into the State Water Project is estimated to cost $1,300/AF. (Source: pers. comm. with Dale Brown, City of Santa Barbara, 1992.)
ENDNOTES
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