Treatment and Disposal Methods of Concentrate Stream of Seawater Reverse Osmosis- A Review

The exponentially multiplying population of the world demands increasing freshwater resources. Thelimited resources comprising less than 3% of the earth’s water resources are getting polluted at an alarming rate. To deal with this situation, seawater reverse osmosis is being carried out at large scales across the globe. The concentrate generated in return is two times more concentrated in terms of total dissolved solids when compared to the feed. The adverse effects of the concentrate stream on the marine ecosystem and further pollution of water cause an immediate need to treat the concentrate. In this review, the harm caused by the direct discharge of concentrate stream has been discussed and therefore volume minimization using treatment methods has been addressed. The treatment methods are mainly classified into four types; membrane-based, thermal-based, electricity-based, and chemical-based methods. Integrated methods, which have been mainly tested on a pilot scale for zero liquid discharge, have also been discussed. The treatment methods that are probable for seawater concentrate treatment falling under the above categories for other concentrate sources have also been attended to. Finally, the disposal methods employed for the discharge of the leftover concentrate have been addressed. Thermal methods are well established but require a lot of energy compared to other methods whereas chemical methods can be economic due to the profit obtained from recovered chemicals, but they are mostly employed for pretreatment. Electricity-based and membrane-based methods are emerging technologies. It was also found that seawater reverse osmosis concentrate is usually discharged directly and therefore integrated methods based on zero liquid discharge are to be implemented. To compensate for the intensive research required for zero liquid discharge to become a reality, innovative and environmentally-friendly disposal methods are available to cut the resultant footprint.