A number of technologies have been developed for desalination which include distillation, reverse osmosis (RO), electrodialysis, and vacuum freezing. Two of these technologies, distillation and reverse osmosis, are being considered by municipalities, water districts and private companies for development of sea water desalination. A description of these technologies are described below:

Distillation

In the distillation process, sea water is heated and then evaporated to separate out the dissolved salts. The most common methods of distillation include vapor compression (VC), multi-stage flash (MSF) and multi-effect distillation (MED). The vapor compression process involves evaporating the input water and then compressing the vapor. The vapor is then condensed, and the heat released is used to evaporate the input water. In the multi-stage flash process, saline feedwater is heated and the pressure is lowered; so that the water "flashes" into steam. This process constitutes one stage, and there are a number of stages in series, each of which is at a lower pressure. In multi-effect distillation, there are a number of evaporators in series, and vapor from one series is used to evaporate water in the next effect.  Some distillation plants are a hybrid of more than one of these technologies. The waste product from these processes is a high salt concentration brine solution.

Reverse Osmosis

In reverse osmosis, the sea water is pre-treated to remove particles that would clog the membranes. The sea water is pumped at high pressure through membranes, separating the salt from the water. The quality of the water produced is dependent upon the pressure, the concentration of salts in the feedwater and the salt permeation constant of the membrane. The quality of the product water from reverse osmosis plants can be improved by adding a second stage of membranes.

Input Water

Desalination plants may use sea water from the ocean or brackish water from groundwater wells as input water. Brackish water has a lower salt concentration and may produce water at a lower cost than sea water plants. Brackish water plants typically use reverse osmosis technology. Therefore, most plants that are built or proposed in the coastal zone are distillation plants. Desalination intake pipes should be located away from sewage treatment plant discharges; but if some of these discharges or other types of pollutants are included in the intake, the pre-treatment and post-treatment processes should remove them.

Product Water

Distillation plants produce a high-quality product water that ranges from 2 to 50 parts per million (ppm) total dissolved solids (tds). (Note: The recommended California drinking water standard for maximum total dissolved solids level is 500 ppm). Reverse osmosis produces potable product water that ranges from 10 to 500 ppm total dissolved solids. In desalination plants that are producing water for domestic use, post treatment processes are often employed to ensure that the product water meets the health standards for drinking water. The desalination product water may be used in its pure form, (i.e., in power plant turbines, or it may be mixed with less pure water and used for irrigation, drinking water, or other uses. The water that is produced with desalination is usually more pure than the drinking water standards, so when the product water is intended for municipal use, it may be mixed with water that contains higher levels of total dissolved solids. The pure desalination water has a high acidity level, which is corrosive to pipes. Therefore, it needs to be adjusted for the correct ph level and hardness before it is piped offsite. If not, other sources of water that it is mixed with will have to be piped on site.

Pretreatment Processes

Pretreatment processes are needed to remove substances that would interfere with the reverse osmosis membranes. Algae and bacteria can grow in both reverse osmosis and distillation plants, so both require some type of biocide to clean to system (usually chlorine is added - about 1 ppm). Some reverse osmosis 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 prior to reaching the membranes. In reverse osmosis plants, particles in the inflow water must be removed to reduce fouling of the membranes. Metals in the inflow water are removed with acids or ion exchange. Suspended solids are removed with coagulation and tiltration.

Filter Backwashing

Membrane Cleaning and Storage and Pipe Cleaning The filters for pretreatment of plant water at a reverse osmosis plant must be cleaned every few days (backwashed) to clear the accumulation of sand and solids. The reverse osmosis 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 scales and other inorganic precipitates. All or a portion of the plant must be shut down when the membranes are replaced.  When reverse osmosis plants are not used continuously, the membranes must be stored in a chemical disinfection/preservation solution, which must he disposed of after use. For both reverse osmosis and distillation systems, the intake and outfall structures will become fouled with organisms and will have to be cleaned by applying chemicals or high temperatures. The plant components must be cleaned to reduce scaling. Scaling is a condition where salts deposit on pipe surfaces or other parts of the desalination plants. It is caused by the high salt concentration of the sea water and can result in reduced efficiency of the plants and corrosion of the pipes. Scaling can be reduced by introducing additives to inhibit crystal growth, reducing temperature and/or salt concentrations, removing the scale forming constituents, or seeding to form particles. Once scales have formed, they can be removed with chemical or mechanical means. Scaling is a concern for reverse osmosis plants and distillation plants unless lower temperatures are used, which reduces the potential for scaling. The input water may also be deaerated to reduce corrosion.  

 

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