Wastewater reclamation by the PAC-NF process

A process combination of powdered activated carbon (PAC) and nanofiltration (NF) called PAC-NF process is currently tested as a process for reclamation of sewage plant effluent on pilot scale. Simple NF is limited by organic membrane fouling which can only partly be removed by chemical cleaning procedures. An analysis of the eluate of a membrane sample shows that small neutral and amphiphilic substances <350 g/mol are accumulated on or in the membrane. When operated with PAC, a layer of adsorbent particles is formed on the membrane which is clogged by organic substances and causes a steep decrease in the membrane permeability. When the organic substances are completely removed by PAC, this effect is not observed and the permeability stays on a comparatively high level. The particle layer also causes a decline in membrane rejection for ionic substances due to concentration polarisation in the particle layer. However the particle layer can at least partly be removed by a flushing procedure consisting of a break in operation and a circulation of retentate without pressure at elevated cross-flow velocities.