Desalination is a technology that has been around for a long time (a classmate of mine at MIT became one of the founders of Ionics Corporation shortly after he graduated) but it has had little impact on life in the United States. There have been a couple of fairly large plants – one notably in Santa Barbara, California – but the capital and operating expense associated with desalination plants and the general availability of drinking water in this country has kept this country from installing such plants.
A different situation has led to the construction of a large number of desalination units in the Middle East, Israel and in other regions where water supply is limited.
Now comes news of a mammoth desalination plant under construction in Carlsbad, San Diego County using an Israeli firm’s technology. The decision to build this plant should come as no surprise, given the terrible drought now plaguing California(see graphic). The plant will desalinate 100 million gallons of Pacific water daily, providing 10 percent of the county’s water supply. It will use reverse osmosis technology. which uses considerably less energy than distillation or vapor compression, though it is still a very large energy user. It will consume 35 megawatts of electricity, enough to power 30,000 homes This accounts for the fact that the plant’s product will sell for around $2000 per acre-foot, which is 80 percent more than what Carlsbad on the average pays for water from current sources. The plant will add $5 to $7 to the average residential monthly water bill.
With California suffering $ 1.5 billion in agricultural losses in 2014, there is no question that much more desalination will be installed, regardless of its high cost. Since treating brackish or waste water with reverse osmosis costs much less, increasing amounts of currently untreated water will also be recovered in this manner.
Reverse osmosis involves using high pressure to pump a solution of brine against a semipermeable membrane that allows the water to pass through while retaining salt and other inorganic impurities behind. The Carlsbad plant will be built with 2000 fiberglass tubes that hold the membranes. A typical membrane consists of cross-linked aromatic polyamide on a polysulfone support layer. Cellulosic polymers are also used. These membrane are fairly thick, accounting for the high pressures needed to push the water through.
With desalination becoming increasingly important (16,000 plants- many small- built world wide) research to lower the cost is being stepped up. Some nanoengineering approaches in membrane design have shown promise. Graphene membranes could cut energy use substantially, but this is looking far into the future.