How to obtain fresh water Section - E6.3 Chemistry 12 – SL Isabelle R.

Two Processes Multi-stage Flash Distillation Reverse Osmosis (Membrane Process) Reverse osmosis is the most common method of desalination, although 85 percent of desalinated water is produced in multistage flash plants.

Multi-stage Flash Distillation 1.An evaporator consists of several consecutive stages (evaporating chambers) maintained at decreasing pressures from the first stage (hot) to the last stage (cold). 2. Sea-water flows through the tubes of the heat exchangers where it is warmed by condensation of the vapor produced in each stage. 3. Its temperature increases from sea temperature to inlet temperature of the brine heater. 4.The sea water then flows through the brine heater where it receives the heat necessary for the process (generally by condensing steam). 5.At the outlet of the brine heater, when entering the first cell, sea water is overheated compared to the temperature and pressure of stage 1. Thus it will immediately "flash" ie release heat, and thus vapor, to reach equilibrium with stage conditions. 6. The produced vapor is condensed into fresh water on the tubular exchanger at the top of the stage. 7.The process takes place again when the water is introduced into the following stage, and so on until the last and coldest stage. 8. The cumulated fresh water builds up the distillate production which is extracted from the coldest stage. 9.Sea water slightly concentrates from stage to stage and builds up the brine flow which is extracted from the last stage.

Reverse Osmosis Reverse osmosis is a liquid filtration method which removes many types of large atomic molecules from smaller molecules, by forcing the liquid at high pressure through a membrane with pores (holes) just big enough to allow the small molecules to pass through. Reverse Osmosis has been commercially used since the early 1970s. The energy requirements are low in reverse osmosis compared to other processes of desalination. Largest desalination plant is in Israel.

Pre-Treatment Pre-treatment is important, it consists of removing the solids within the water – reduces the risk of damage. string-wound polypropylene filters that remove between micrometer sized particles Dosing of oxidizing biocides such as chlorine to kill bacteria followed by bisulfite dosing to deactivate the chlorine which can destroy a thin- film composite membrane. Acid is dosed to maintain carbonates in their soluble carbonic acid form -

High Pressure Pump The pump supplies the pressure needed to push water through the membrane, even as the membrane rejects the passage of salt through it.

Membrane Assembly The membrane assembly consists of a pressure vessel with a membrane that allows feed water to be pressed against it. The membrane must be strong enough to withstand whatever pressure is applied against it. Reverse Osmosis membranes are made in a variety of configurations, with the two most common configurations being spiral-wound and a hollow-fiber.

Remineralization and pH adjustment The desalinated water is very corrosive and is "stabilized" to protect downstream pipelines and storages usually by adding lime or caustic to prevent corrosion of concrete or cement lined surfaces. Liming material is used in order to adjust pH at 6.8 to 8.1 to meet the potable water specifications, primarily for effective disinfection and for corrosion control.

Disinfection Post-treatment consists of stabilizing the water and preparing for distribution. disinfection is used to ensure a "safe" water supply. Disinfection (sometimes called germicidal or bactericidal) is employed to sterilize any bacteria protozoa and virus that have bypassed the desalination process into the product water. Disinfection may be by means of ultraviolet radiation, using UV lamps directly on the product, or by chlorination or chloramination (chlorine and ammonia).