Treatment of severely contaminated waste water by a combination of RO, high-pressure RO and NF — potential and limits of the process

High water recovery rates are essential for waste water treatment. In cases where not scaling but the osmotic pressure limits the recovery rate, the operation of a high pressure reverse osmosis (RO) is feasible. But this study shows that either the compaction of the presently available RO membranes or costs limit the operating pressure to about 150 bar maximum. The integration of a nanofiltration into a staged RO process can solve the problem in cases where chlorides are the reason for high osmotic pressures and scaling is due to calcium sulfates. For the treatment of dumpsite leachate, several large scale plants are operated since 1994 in Germany with an average water recovery rate of 95%!