Thread: Desalination

How does a desalination plant cause reduced oxygen levels? I've read this statement in a report and assume that it is referring to the oxygen levels of the seawater where the extraction occurs. Would this decrease in oxygen levels then be to do with the continual removal of water from the sea?

I’m not all that familiar with desalination plants, but some desalination plants use multiple stage evaporation of seawater – i.e. the seawater is boiled to produce steam which is condensed as pure water. It seems unavoidable that during boiling dissolved oxygen would also be removed from solution in the seawater and carried off with the steam, leaving a greatly reduced oxygen concentration in the concentrated brine. The waste brine is cooled and dumped back into the sea. If the brine is carefully dispersed and mixed with the natural seawater the effect on local oxygen levels could be minimized, but if it is simply dumped out of the end of a pipe there could be local areas of depleted oxygen.

These days, most new desalination plants are running on reverse osmosis. Distillation is terribly wasteful of energy. Modern reverse osmosis membranes are getting better and better, and cheaper. This makes reverse osmosis the technology of choice.

Loss of oxygen is a minor problem. It applies to the discharge. Since fresh water is the product, the residue is highly salinated water that is sent back into the sea. This discharge water may also contain other pollutants, and tends to be lower in oxygen. This is of concern to marine ecosystems, if the discharged water acts to reduce oxygen levels there. However, there are more potent concerns, if the discharge is poorly mixed with fresh seawater.

A stated goal of the Iranian nuclear programme is water desalination. Waste brine probably into the Caspian Sea or Persian Gulf. Would nuclear-fired plants operating at serious (agricultural) capacity impact those water bodies?