Microgranular Adsorptive Membrane Filtration

University Of Washington, Seattle WA

Investigators

Abstract

0931739 Benjamin The proposed research will both extend the PI's work on novel, adsorption-based methods to reduce fouling of microfiltration (MF) and ultrafiltration (UF) membranes, and explore a more general water treatment approach - dubbed microgranular adsorptive filtration (ìGAF) - that has emerged from the prior research. Microgranular adsorptive filtration is, essentially, the miniaturization of packed bed processes such as granular media filtration or GAC adsorption to a scale where the whole treatment process takes place in a thin (<500 µm) layer of powder-sized particles that have been pre-deposited on a membrane surface. In such processes, the membrane serves primarily as the support for the powder, rather than in its conventional role as the primary agent for pollutant removal. The research will investigate three implementations of ìGAF: filtration to remove colloidal and particulate matter (i.e., miniaturization of a rapid sand filter); adsorption to remove trace inorganic and organic contaminants (miniaturization of an ion exchange bed or GAC contactor); and adsorption to remove NOM (replacement of an enhanced coagulation process with a miniaturized packed bed process). The experimental procedures will consist of depositing a layer of the microgranular media on a membrane, passing water through the layer and membrane at a constant flow rate, and determining both the contaminant removal efficiency and the buildup of hydraulic resistance. At the end of a treatment step, the membrane will be backwashed to flush the media out of the system, and a new cycle will be initiated. Parameters to be varied include the thickness of the deposited layer, the hydraulic application rate, the length of the treatment step, and the backwashing time and intensity. Process performance will be assessed based on the contaminant removal efficiency, the pressure buildup during individual treatment steps, and the long-term stability of that pressure profile (i.e., the reversibility of any fouling that occurs). The potential applications of ìGAF are enormous, but have not been recognized in the past for two reasons. First, the tendency to focus on using a single technology to address a single water treatment goal has led to the use of membranes only as tools for contaminant removal. And second, until recently, the cost of membranes has made their use simply as supports for other media economically prohibitive. In prior research on the use of adsorbents to control membrane fouling, the PI found that the adsorbents were efficiently removing the contaminants that the membrane was intended to treat. That led to the realizations that (1) the most efficient treatment approach was to pack as much adsorbent on the membrane as possible, and to use the membrane strictly as a support for the adsorbent, and (2) ìGAF has many potential applications beyond fouling control. These applications, which include particle removal, trace contaminant removal, and pre-treatment of water prior to desalination, can potentially reduce the size, energy demands, and cost of numerous water treatment processes, while simultaneously making them more efficient. Preliminary tests of a few applications have been encouraging. The intellectual merit of the proposed work derives from the understanding that it will promote of the behavior of packed layers on membranes in general, and adsorbent layers in particular. In the past, cake layers have been investigated almost exclusively in the context of how severely they foul membranes. This project will introduce the idea that cake layers can sometimes be enormously beneficial to membrane processes, and in the process will demonstrate how they can solve specific current problems associated with membrane fouling, desalination, and control of trace contaminants. The most significant broader impact of the work will be the dissemination of the whole idea of microgranular adsorptive filtration. Several applications of ìGAF will be investigated in the research, but many others will undoubtedly be recognized and pursued by other researchers who have never previously thought about ìGAF as a realistic process. Beyond this, the research will advance the arsenal of possible treatment technologies for improving the quality of impaired water, will train new professionals in the field, and engage K-12 students in thinking about water supply and water quality.