Direct capillary nanofiltration — a new high-grade purification concept

Capillary nanofiltration (NF) enables raw water to be treated in a single step to produce high quality permeate. The permeate can be used as process water for the industry. Trial studies show that a stable operation is achievable using capillary NF without pretreatment (in other words direct NF), that the technique saves energy, that there is a low use of chemicals and that the permeate is of a high quality. The capillary NF membrane module combines the favorable cleaning properties of capillary ultrafiltration membranes with the favorable separation properties of NF membranes in terms of the removal of bacteria, viruses, color, hardness and pesticides.

The trial studies have demonstrated clearly that direct capillary NF with the capillaries results in a long-term stable flux behavior with a minimal of chemical cleanings. AirFlush® has proved to be a very efficient, powerful way of operation to stabilize the flux and to reduce the amount of chemical cleanings. These conclusions are based on using direct NF with sewage treatment plant effluent and surface water in comparison with the conventional technique: UF as pretreatment for spiral-wound NF. Capillary NF enables raw water to be treated in a single step to produce high quality permeate. The permeate can be used as process water for the industry. Examples are water for the production of ultrapure water and boiler water, and water for the process, chemical, food and dairy industry. A cost comparison shows that direct NF is attractive for use with raw water that is difficult to treat using other treatment steps. Direct NF is in any case a competitive technique for UF-NF. The membrane price of capillary NF membranes The authors wish to thank Program EET – Economy, Ecology and Technology of the Ministry of Economic Affair and NOVEM – The Netherlands Agency for Energy, for their financial support.