Hi Grace:I agree with ...

Hi Grace: I agree with Valentina. With appropriately designed outfall for diffusing the discharge from desalination and wastewater plants such discharges are equally safe for the environment. Usually well designed discharge diffusers are sized to achieve 1:20 to 1:50 dilution within 100 meters from the discharge and at that point the water quality is indiscernible from ambient waters. Site specific issues however may make one type of discharge more palatable for the environment from another. In Australia for example, the reasoning for preferring desalination over indirect or direct potable reuse was the concern that as we remove fresh water from the wastewater to produce drinking water, we are concentrating man-made compounds contained in the wastewater treatment plant discharge form where the source water for production of frehs water came fro - such as endocrine desruptors (female hormones contained in contraceptives, antibiotics, etc.). Since most of such compounds (i.e., female hormones) act when they reach certain concentration threshold, the more concentrated the discharge the more likely it is that we will reach the threshold levels which may impact the environment - change the sex of male fish into female fish for example. In drinking water produced from purified wastewater such compounds are destroyed by advanced oxidation - combination of peroxide and ultraviolet irradiation - so the water is safe for drinking. We do not apply this advanced oxidation treatment however to the discharge - at least not for now. If we are to apply the same treatment to the concentrate as we do to the drinking water, the energy used for destruction of endocrine disruptors would in many cases be higher than that needed for desalinating seawater. Until we learn how to cheaply destroy endocrine disruptors in the discharge in some water reuse applications this issue may pose a challenge. Desalination plant concentrate does not contain endocrine disruptors and ultimately gets recombined with the water from which it was separated as it turns in wastewater. In over 99 % of the coastal communities worldwide the drinking water produced by desalination is returned as treated wastewater into the ocean within several kilometers distance from the point of intake. That being said, discharge of concentrate from desalination plants into shallow water bodies with limited mixing or into seagrass beds, corals, or other "ocean nurseries" could cause environmental impacts if the volume of discharge is large. So the impacts of both types of plants will be very site specific and it is practically impossible to generalize a conclusion on which of the two water production methods is better for the environment. Most marine species develop natural mechanisms to adopt to increased osmotic pressure originating from concentrate discharge or natural evaporation. However, as we are not born with the ability to walk right way after birth, many marine species in their early embryonic phases of development do not have such mechanisms to deal with higher osmotic pressure yet - so discharging concentrate into such area will have negative impact on embryos - by osmosis - it will suck out the water of the embryos bodies by osmosis - which hinders their development or may cause death. So discharging concentrate in locations where marine organisms go to spawn could be environmentally damaging. If we stay away from such areas (i.e., seagrass beds, corals, etc,.) difusers do adequate job to protect aquatic environment. Practice shows that when done right both methods are complimentary and a part of a diversified water supply portfolio of coastal municipalities worldwide.