The high TDS RO reject typically contains very high levels of dissolved ions, silica, organic compounds and hardness-causing calcium and magnesium salts. It is important to take the bulk of these out of solution before the reject stream is membrane filtered.
In a 2-stage RO system the membrane filtration step normally adds softener and flocculants, inducing solids to settle out of solution in a concentration vessel. In the case of a Zero Liquid Discharge (ZLD) design, the excess solids are sent to a filter press for de-watering and disposal. Further solids removed by the membrane filters are also returned to the concentration tank, while the MF filtrate goes to the second stage RO.
Drawbacks – Recovering water from RO reject / RO concentrate using this approach is very feasible but it has a high maintenance component. The system is continuously combating silt fouling, biofouling and scaling of membrane surfaces and necessitates adding chemicals as part of the process. A far simpler alternative involves Electro Dialysis: