Environmental impact and economic costs of brine disposal methods from RO desalination plants in arid coastal regions

Reverse Osmosis (RO) desalination results in production of brine which needs proper disposal to avoid potential negative environmental impact. Three commonly used disposal methods are compared in this paper (sea disposal, well injection and evaporation ponds) with regard to their environmental impact and economic costs. The chemical characteristics of reject brine from five seawater RO plants in Egypt are presented with their possible impact on marine environment and groundwater aquifers. An overview of existing regulations governing brine disposal is given. Assessment criteria to evaluate the impact of the three different brine disposal methods, chemicals concentration limits, and best practices for brine management are suggested. The direct cost of disposing reject brine through discharge into the sea or into a well is approximately 0.05-0.06 US$/m 3 of product water. The direct cost of disposing reject brine by means of evaporation ponds is much larger and is estimated at 0.56 US$/m 3 . However, the latter technology is the only method that allows for resource recovery and combined use with fish farming which may create significant benefits, making this technology competitive with well injection and direct discharge into the sea. The indirect (environmental) costs of these three methods are uncertain. If brine is discharged into wells, the issue is whether aquifer contamination can be excluded. Since the latter is often difficult to rule out, the possible indirect cost may be high. In case of direct disposal of brine into the sea, the damage this may cause on sensitive marine ecosystems is potentially high and thus the associated indirect costs. The indirect cost of evaporation ponds has been estimated to be low. Whereas the direct economic costs of brine disposal vary according to method and location (e.g. whether the plant is inland or coastal), the potential environmental costs are likely to be much higher. The selection of the most appropriate method thus depends on a correct estimate of the associated environmental costs. Consistent with the above environmental considerations is the philosophy of (near) zero discharge and resource recovery. The only disposal method that would allow for resource reuse is brine disposal through evaporation ponds or brine concentrators/crystallizers, combined with economic synergies such as fish production or chemical recovery where applicable.