In-line coagulation to reduce high-pressure membrane fouling in an integrated membrane system

Abstract

Membrane fouling is a chronic problem for many nanofiltration (NF) membrane plants. Foulant material can range from colloidal, particulate, inorganic minerals and natural organic matter (NOM) (Schäfer et al., 2006). This research project worked with a small community integrated membrane facility (low-pressure membrane followed by high-pressure) in Nova Scotia with membrane fouling concerns associated with dissolved NOM as the primary foulant. Membrane autopsies conducted in our laboratory have demonstrated that NOM deposits on the NF membrane decreased pore space on the membrane (Lamsal et al., 2012). The membrane fouling resulted in a requirement for increased pressure to produce a constant permeate flow.

By adding in-line coagulation prior to low-pressure filtration in an integrated membrane system, the goal was to remove more organic material by MF thereby improving the quality of the feed-water entering the NF membranes. Previous work has shown that for some IMS installations there is a need to reduce the amount of dissolved organic matter prior to NF (Cho et al., 2000; Lamsal et al., 2012; Nilson and DiGiano, 1996; Schäfer et al., 2001). An improved membrane feed-water quality reduces fouling on the membrane and membrane operating cost, and increases productivity and lifespan of the membrane (Choi, 2008). A negative aspect to adding in-line coagulation is it adds another step to the treatment process and sludge removal is required.

This study examined the use of in-line coagulation using coagulants aluminum sulphate, ferric chloride and polyaluminum chloride to improve membrane feed-water quality. The addition of in-line coagulation prior to microfiltration will remove NOM with the MF producing improved feed water quality for NF. After determining the optimal dose of each coagulant, 20 L of post-coagulation MF permeate was batched and run through the bench-scale NF membrane for 200 hours. The water quality of the feed tank, concentrate and permeate were monitored constantly as well as the operational properties of pressure and flow. To simulate a full-scale plant the operating conditions of Collins Park water treatment plant on Fletchers Lake were used in the bench-scale set-up. After the 200h NF run time the membranes were analyzed to assess the fouling on the membrane and the performance of each coagulant. Coagulation was found to reduce NF pressure fouling by reduction of NOM in the NF feed-water. Ferric chloride was found to perform best of the three coagulants at a low dose of 0.5mg/L of Fe at a pH of 5.0.