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Massachusetts Considering Desalination Plants
Iphtashu Fitz writes "Despite a reservoir system containing some 412 billion gallons of water for Boston and surrounding communities, some eastern Massachusetts towns are facing water shortages and are now considering water desalination plants as a new source of fresh drinking water. The city of Brockton, 20 miles south of Boston, has plans in the works to build a $40 million plant and could begin construction as soon as this September. Currently there are fewer than 100 desalination plants in the US and most of them are in smaller communities, but that seems to be changing. The largest desalination plant in the country is located in Tampa, FL, which expects it to provide 10% of the citys drinking water by 2008. California also has at least 10 large scale plants on the drawing board. Some environmental organizations like the Conservation Law Foundation dispute the need for desalination plants however. They argue that many water shortages could simply be solved by better conservation of existing supplies."
At the moment the biggest problem with desalination plants is not just their high build cost, but their high operational cost.
When using technologies such as reverse osmosis the energy costs for pushing high volumes of water at high pressures through the membranes is prohibitive, not to mention the wear on the equipment it's self. In a traditional water treatment plant most of the filtering is done with gravity.
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Probably the environmental impact of the plant itself - it will have to be sited near the coastline, away from already developed areas like harbors or bays, meaning that it will likely displace marshland or other undeveloped coastline. There will be waste discharge as a by-product of the desalianation process, which will increase local salinity. Desalination requires a pressure differential to overcome osmotic forces - the power for this will probably come from electricity. Electricity is in short supply in some places, which means that the water plant will require a coal, nuclear, gas-fired, or hydro plant to contribute part of its output to desalienate the water.
From a tax perspective, these plants will need to be built by somebody, probably with bond issues, and will require taxes to pay off. I'd be more pissed about that than the environmental impact.
The main objection to desalination plants is that they are highly energy intensive. Purifying water from mountain spring water requires seven stages, most of which are chemical/physical:
Filtering of large solids (fish, leaves,twigs)
Removal of unpleasant odors and tastes using carbon filters
Chemical dosing with lime, ferrous sulfate and polymer to remove suspended particles.
Application of chlorine to kill off bacteria.
Application of fluoride to prevent tooth decay.
Filtering through anthracite coal and and sand to remove the last remaining suspended particles.
Desalination plants have the additional task of removing the salt from the water. There are two ways of achieving this. The first method is to boil the water until every last drop has been converted into steam and then recondensed again. Alternatively, membrane filtering can be used, which requires that the water is pumped at high pressure through a water but not salt permeable membrane. Both of these methods require large amounts of energy (Power stations are a good location for this).
More importantly, the areas that require desalination plants, are the same areas which are pouring/or have poured unprocessed sewage and toxic waste into ground water supplies. It would be more energy efficient and environmentally friendly to implement waste water purification, than to run a desalination plant in the first place.
You seem to be completely unfamiliar with all the techniques of water desalination. Saltwater Desalination: Chapter 1 will educate you. Of particular interest is http://www.coastal.ca.gov/desalrpt/dc1tbl1.gif this chart which shows that distillation consumes much less power than reverse osmosis
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Having operated desalination plants for 6 years while in the US Navy (we could produce 200,000 gallons of fresh water daily, so small scale), the idea that you boil of every drop of water is a little misleading.
Actually we would remove only about 10% of the water from the saltwater we pumped through the system. Any higher extraction than that increased scaling problems creating a maintenance nightmare. One poster asked what the communities planned to do with all the "extra" salt. It is pumped back into the ocean with the rest of the brine.
Also, to reduce energy costs and heat loss, all the production is done at partial vacuums to reduce the boiling point. If memory serves, the we reduced the boiling point to 165F, but it was 14 years ago, so my memory is a little fuzzy.