Evaluating Source Water Quality, Pretreatment, and Particulate Mass Transport in a Nanofiltration Membrane Process

This document details the source and occurrence of particulates in a nanofiltration (NF) process, considers the implementation of additional pretreatment to improve feedwater quality, and investigates the impact of submicron particles on membrane performance. Much of this research was performed in cooperation with the City of Boynton Beach (City) at the West Water Treatment Plant (WTP) and stemmed from particulate fouling concerns in the pretreatment and NF membrane processes. The water quality of the individual wells that supply the West WTP was evaluated to identify the source of particulates which overload the pre-filters (5-μm cartridge filters) and may contribute to membrane fouling. The results indicated that particulates are largely provided by three of the ten wells; however, it was shown that the wells produced poor quality supplies upon well start-up containing increased turbidity relative to the membrane feedwater quality. The effectiveness of sand filtration (SF) as an additional pretreatment step, prior to CF, was investigated in a pilot-scale study. SF consistently reduced feedwater turbidity by at least 70 percent. Furthermore, the distribution of nanoparticles (NPs) and microparticles (MPs) in the NF feed-concentrate channel ranged between 50-nm and 70-μm as identified using particle sizing technologies. Over 65 million submicron particles per milliliter (mL) were detected in the feedwater and diminished to approximately 3.5 million particles/mL in the concentrate stream. Most of the particles had diameters of less than 1-μm, while larger MPs were also identified primarily consisting of silts/clays, calcium carbonate, elemental sulfur, and, in the concentrate streams, organic based matter. The impact of NPs on membrane productivity was probed in laboratory-scale experiments. Small NPs (10-20 nm) caused the greatest flux decline; however, when combined with larger particles (1-μm), the flux decline was not as severe likely due to the formation of a lower density foulant layer on the membrane surface