In-line coagulation to reduce high-pressure membrane fouling in an integrated membrane system: a case study

Abstract

Membrane fouling is a critical problem for many nanofiltration (NF) membrane plants. This study demonstrated that the poor spaces of the NF membrane were decreased for the deposition of natural organic matter (NOM) on the membrane surface. In-line coagulation was added prior to the low-pressure filtration step to remove more organic material by microfiltration (MF), thereby improving the quality of the feed-water entering the NF membranes. Three different types of coagulants, i.e. aluminum sulfate, ferric chloride, and polyaluminum chloride, with their different levels at different pH values were used for this study. The optimal dose of each coagulant was determined, and the impact on the NF membrane was assessed by batching and running 20 L of post-coagulation MF permeate through a bench-scale NF membrane for 200 h. Ferric chloride was found to perform the best of these three tested coagulants to reduce NF pressure fouling by reduction of NOM in the NF feed-water. High performance size exclusion chromatography study revealed that higher molecular weight fractions of NOM removed preferentially due to in-line coagulation processes. The scanning electron microscopy analysis of the fouled NF membrane trial revealed that the foulant layer consisted of a very large quantity of inorganic and organic matter, bacteria, which resulted in a very rough surface topography.