Fouling by NOM During Ultra- and Nanofiltration
(Willy-Hager I)

During membrane filtration of natural waters, natural organic material (NOM) is known as the main reason for fouling. It can be distinguished between fouling on the surface of the membrane (surface fouling) and fouling inside the inner structure of the membrane (pore fouling). The effects of each kind of fouling on the permeate flux and on the rejection of the ultra- and nanofiltration membranes are investigated within the three-year research project “Fouling by NOM during ultra- and nanofiltration” sponsored by the Willy-Hager-Stiftung, Stuttgart.

The experiments are carried out in a flat channel test unit which consist of 8 flat channel modules with similar flow conditions as they exist in a spiral wound element.

So far it could be shown that the mechanical removal of the fouling layer by a slide is a suitable instrument to determine the extent of pore fouling and surface fouling of flat sheet membranes: after different filtration periods the percentage of pore fouling on the total fouling of the ultrafiltration membrane was between 44 and 72%, whereas the nanofiltration membrane did not suffer from any significant pore fouling. The flux reduction of the nanofiltration membrane per permeate throughput was 50% less compared to that of the ultrafiltration membrane. Apart from the different membrane materials, this lower fouling tendency of the nanofiltration membrane was explained by its high rejection for NOM, which impeded the penetration of NOM and the blocking of the inner membrane structure.

Further research will focus on the analysis of the surface fouling layer composition by fluorescence in-situ hybridisation (FISH) (microbiological biocenosis) and nuclear magnetic resonance spectroscopy (NMR) (Composition and structure of organic material).