Desalination involves removing the salt from water to make it drinkable. There are several ways to do it, and it is not a new idea at all. Sailors have been using solar evaporation to separate salt from sea water for at least several thousand years. Most of the world’s 1,500 or so desalination plants use distillation as the process, and there are also flash evaporation and electrodialysis methods.
All these methods are very expensive, so historically desalination has only been used where other alternatives are also very expensive, such as desert cities. However, an exploding world demand for potable water has led to a lot of research and development in this field and a new, cheaper process has been developed that involves heating sea water and forcing it through membranes to remove the salt from the water. The process is even cheaper if the desalination plant can be located next to an electrical power plant that is already heating sea water to use for cooling the electrical generating units. Even so, it is still more expensive than other alternatives, but it is indeed becoming more competitive and could become a viable alternative.
Seawater desalination plant design
- The raw water intake is beside the neighboring power plant's four discharge tunnels, two of which were tapped to divert around 166,000m³/day of the cooling outflow into the intake structure. From the intake, the water is pumped to the pre-treatment facility.
- Chemical filtration agents and ferric sulfate are added to the inflow, which passes through a two-stage sand filter. The medium is continuously back washed, which further helps to lower the silt density index of the exiting water. There is also provision for dosing the water to adjust pH if required.
- The reverse osmosis (RO) system has seven independent trains, each comprising a transfer pump, cartridge filters, reverse osmosis membranes, associated high-pressure pump and an energy recovery turbine (ERT).
- An 800hp vertical turbine transfer pump in each train draws raw water from the pre-treatment wet well to the 5µm cartridge filter assembly. The water then enters the RO process itself.
- Each battery of reverse osmosis membranes is fed with pressurized water by a 2,250hp, horizontal split case high-pressure pump. These were fitted to the pumps to accommodate the variation in salinity of the water, which naturally ranges between 18 and 32 parts per thousand (ppt) in Tampa Bay, compared with the narrower 28-35ppt of typical seawater. Being able to vary the input pressure allows the plant to match its operating power requirements to salinity changes.
- Each of the plant's seven RO batteries has a minimum rated production of 16,000m³/day and consists of 168 pressure vessels, containing eight SWRO membranes apiece.
- The permeate produced flows into a 1m diameter header pipe, situated below. The high-pressure concentrate returns to the ERT for energy recovery and is then mixed with the power station cooling water in a ratio of 70:1 to dilute its high salinity, before being finally discharged.
- The permeate requires further treatment before distribution and use, leading to the building of a number of chemical storage vessels.
- A 22.5m³ bulk tank and a 4.5m³ day tank have been constructed to store the sodium hypochlorite added to chlorinate the water. The calcium hydroxide, used to introduce hardness, is housed in a 50t silo. It is added to the product water as a slurry, in two up-flow lime contact chambers, some 12m high.
- To balance the pH, a solution of carbonic acid is simultaneously diffused into the chamber. The carbon dioxide used to make the solution is stored in another on-site tank. Chemical diffusers fitted to the lime chamber discharge weirs allow for further pH adjustment, if necessary.
- The finished product water flows into a 20,000m³ storage tank and is eventually pumped 15 miles along a 1m pipeline to the Brandon regional distribution facility, crossing two navigable rivers. Directional drilling was used in one case to position 550m of fiberglass pipe 18m below the river bed.