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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.
Life is Short and Hard like a body building Elf
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.
They're even thinking of building a Nuclear Desalinization Plant in the Mideast. At an estimated cost of $200-300 million it will be able to provide enough water for 3 to 4 million people.
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
Even those who arrange and design shrubberies are under considerable economic stress at this period in history.
From Geology 101 - Seawater chemistry
The amount of salt in sea-water is measured in terms of salinity (the number of grams of salt in a kilogram of sea-water). Normal sea-water has a salinity of 35%, or around 35 grams. Thus, one metric ton (1000kg) of sea-water would give you 35,000 grams or 35 Kilograms of salt (35 x 1 Kilogram bags of salt).
Of this, the distribution is as follows:
Chloride: 55.04%
Sodium: 30.61%
Sulphate: 7.64%
Magnesium: 3.69%
Calcium: 1.16%
Potassium: 1.10%
Now, the average adult human need 2 litres of fresh water to drink just to survive each day (2 litres = 2 kilograms at 4.0 C). Although some of this can come from food such as meat, vegetables and fruit.
If a desalination plant is used, that's 70 grams of salt being produced per person/day.
At most an individual is only going to require 1 gram of each mineral (Eg. sodium).
So around 65 grams/day of salt is going to have to be placed somewhere.
Multiply this by 1,000 people for a small town (65kg salt produced per day) and
1 million for a large city (65 tonnes salt produced per day).
And that's not including the requirements for washing machines, dish-washers, garden sprinklers, and toilets.
Boiling a pound of water at atmospheric pressure takes roughly 1000 BTU's, and there are 140,000 BTU's in a gallon of fuel oil. So a gallon of oil can boil 140 pounds of water or about 18 gallons. That is a lot of oil.
But if you boil a pound of water to remove the salt, condense it, you are throwing away all of that heat released when it condenses, almost as much as required to boil it. How can you recover that heat since you are going to boil at a slightly higher temp and condense at a lower temp and heat cannot move uphill?
One technique is multi-effect distillation. You boil and then condense at atmospheric pressure. The condensing at atmospheric pressure is hot enough to boil at some pressure below atmospheric. You condense and then use that heat to boil at an even lower pressure. You keep going until you are what ever vacuum pressure boils water at room temperature. The same 1000 BTU's to boil a pound of water is used several times to boil several pounds of water in several "effects" (stages of the still).
The other method is mechanical vapor compression. If you take the vapor from boiling and compress it in an centrifugal compressor, it can condense at a somewhat higher temperature, and you use that heat to boil the water feeding the compressor. While it seems like pulling yourself up from your bootstraps and violating a thermodynamic law, it is not that much different than a heat pump.
There is some minimum energy required to desalinate water, it is much less than 1000 BTU per pound, and if you know the osmotic pressure for that salt concentration, you take that pressure and the volume of water you want and use work = pressure times volume. That energy is not without consequence, and that is why you probably want to desalinate brackish (slightly salty -- often available from wells when pure water is not available) than going for sea water.
Also, there is some effort in approaching the thermodynamic "reversible" minimum energy of desalination. The multi-effect stills and the vapor compression still have to move large amounts of heat through heat exchangers at small temperature differentials. With reverse osmosis, you probably are pumping harder than the bare minimum to oppose the osmotic pressure so you get enough fluid through the membrane to make it worthwhile.
Multi-effect distillation is probably the way to go for big plants, vapor compression for mid-sized, and reverse osmosis is really probably only effective for small-scale stuff because the membranes are expensive and need replacement. Even with what I said, the energy needs are not trivial -- perhaps you want some kind of cogeneration where you run a multi-effect still from the waste heat stream of a gas turbine.
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.
One of the nice features is that you can buy things like water and electricity from your neighboring cities for a price. This price tends to be higher per unit of supply than you could provide with a structure like a power plant or water pump, but requires far less up front cost. The not so nice thing is that your neighbors will occassionally renegotiate the price with you, meaning you'll pay more each month if you want to continue getting these supplies.
The joke in the previous post is based on the fact that you could import water (based on the bottled water comment) or that you could build a costly desalination plant (as the article suggests is happening). In sim-city you'll get shafted in time if you don't provide your own facilities, thus the neighbors raising the cost of bottled water is funny.
Now I feel like one of those people that analyzes a joke until it isn't funny. However, I went to the trouble of explaining for the poor non-sim-city player so I'm just going to post it... blah. The interesting thing is that bottled water seems to be pretty expensive anyway, and building one of these big plants is probably well worth the trouble in the long run.
If not now, when?