Environmental impacts of seawater distillation and reverse osmosis processes

The paper presents and compares the marine and atmospheric discharges of typical large MSF and RO plants. Distillation plants require an inlet seawater flow around 8 to 10 times the fresh water production rate, for, cooling and feed make-up. The discharge is raised in salinity and in temperature, due to heat rejection from the process. A seawater RO plant generally requires a lower feedwater flow rate of around 2.5 to 3 times the production. Salinity of the discharge is correspondingly greater than in distillation, but temperature rise is small. Chemical discharges are compared for the different processes. A comparison is made of atmospheric emissions from generating the power to drive RO process pumps and the emissions resulting from generation of steam and auxiliary power for distillation (in dual purpose desalination/power plants). The factors affecting site selection and requirements for seawater intakes and outfalls are briefly discussed, with examples. The land area needed for a desalination plant is compared with the catchment and reservoir areas needed for a typical surface water collection scheme to illustrate the much lower impact of desalination on land resources. Factors such as noise and aesthetics are also briefly discussed.