Electropositive Filtration As a Means for Mitigating Fouling of Ultrafiltration and Reverse Osmosis Membranes By Dissolved Organic Matter and Manufactured Nanoparticles

Organics, colloids and nanoparticles, naturally occurring and manufactured ones, are significant sources of fouling in membrane filtration and desalination applications. These materials persist to some level in microfiltration (MF) and ultrafiltration (UF) filtrate streams as their hydrodynamic diameter approaches and/or is less than the nominal pore size of the MF/UF membrane. Therefore, dissolved organics, small colloids and nanoparticles are particularly troublesome for both the MF/UF membranes and downstream membrane processes like nanofiltration (NF) and reverse osmosis (RO). The former concern arises from the fact that filtrate streams are often used to backwash the MF and UF membranes, subsequently resulting in a compounded fouling of the impacted membranes. The latter concern arises from the fact that nano-sized and dissolved organic materials characteristically form dense cake structures on NF/RO membranes resulting in substantial increases in the hydraulic resistance of the membrane barrier (i.e., flux loss). In light of these concerns, in addition to the growing interest in manufactured nanoparticles and their impacts on water treatment processes, a new type of separation technology is needed. This new technology must be capable of removing nano-sized and dissolved debris without the typically associated requirement of high hydraulic pressures. Electropositive filtration is a relatively new filtration technology in which particulate removal occurs through attractive charge interactions (positively charged filter media and negatively charged particulates). The filter media is characterized by a large porosity and pore size, thus requiring low operating hydraulic pressures. This presentation will cover our work to date with using electropositive filtration to remove dissolved organic matter and manufactured nanoparticles from membrane process streams in an effort to improve the performance of both membrane filtration and desalination processes through reduced membrane fouling.

The benefits and practical implications associated with using an electropositive filter as a pretreatment process for reverse osmosis (RO) has been studied. The specific intended application for the treatment scheme is water reuse/recycling. The Disruptor filter was used to polish water that had been filtered through a UF membrane and would therefore target substances such as, natural organic matter (NOM) – dissolved and particulate forms (< nominal pore size of the UF membrane), colloids and nanoparticles, and biological debris and associated macromolecules.

Two reverse osmosis (RO) process trains were operated in parallel. One train received ultrafiltration (UF) filtrate (control) while the second received UF filtrate that had also been treated with an electropositive filter (Ahlstrom/test). The RO system (DOW FILMTEC^TM XLE-2540) was operated as an open-loop system (i.e., the permeate was wasted). The composition of the source water for the UF membranes was controlled using a chemical dosing pump that injected humic acid into the raw feed water. The raw feed water for the treatment system was filtered tap water. Prefiltration was required to remove trace amounts of sediment and/or iron. The conductivity of the raw source water, which was used as the feed source for the UF membranes, was characterized by a relatively low conductivity (~300 mS cm^-1), and a total organic carbon (TOC) concentration (~2 mg L^-1) prior to the addition of the humic acid. The relatively low conductivity value is attributed to the fact that the tap water during the summer months for the City of Laramie, WY is principally derived from the Laramie Rover, which is fed by snow melt (low conductivity). There was no detectable color in the water and its turbidity was less than or equal to 0.3 NTU.

This presentation will cover our work to date on the development and pilot scale testing of using an electropositive filter as a pretreatment step for both UF and downstream RO processes. We will discuss our findings regarding nanoparticle and organic matter removal using electropositive filters. Specific results pertaining to the removal of nanoparticles like fluorescent nanospheres, Nano silver, titanium dioxide (TIO₂) and fullerene (C60) will be presented. Additionally, we will cover our findings on the fouling mechanisms associated with nanoparticles for both UF and RO membranes. The performance impacts on both the UF and the RO membranes owing to the use of an electropositive filter for polishing the UF filtrate will be discussed in detail.