Caustic recovery using membrane filtration

Quick links to navigate to a specific section of this paper:

	Abstract (below)
	Introduction (below)
	Caustic recovery using membrane technology
	Caustic recovery in the citrus industry
	Summary

Abstract

Food and beverage processors that use dilute caustic solutions for cleaning process equipment have shown increased interest in recovering the used caustic. The primary reason is that the price of caustic has increased significantly in the past year or so. Membrane filtration technology can be used to remove suspended solids (clarify with microfiltration) and/or dissolved solids (purify with nanofiltration) from these used caustic solutions. These treated caustic solutions are suitable for reuse within the processing plant as cleaning solutions. While an end-user's specific process, performance, capital and operating cost parameters will require pilot testing and process design evaluations to determine, the existing body of information from commercial and process development work allows for an evaluation of a "typical" caustic recovery application.

Introduction

Membrane technology is widely used in the food, beverage and dairy industries for process applications directly generating commercial products. Membrane technology is also used and increasingly under consideration for by-product or waste stream applications in these same industries.

Membrane technology has four general classifications, microfiltration (MF), ultrafiltration (UF), nanofiltration (NF) and reverse osmosis (RO). The membrane pore sizes (or alternately described by molecular weight cut-offs, MWCO) range up to a couple microns for the most open MF membrane to less than one thousandth of a micron (or ~60 MWCO) for reverse osmosis membranes. Membrane applications can classified as: clarifications (separating suspended and colloidal components from dissolved components), component separations based on molecular size or other parameter, and concentration (removal of water or other solvent from all other components).

A membrane's composition is usually described as being either polymeric or inorganic. While more than a hundred polymers have been used for membranes, there are five to ten polymer types that make up most of the polymeric membranes widely used and available today. The different polymeric membranes have different membrane capabilities (pore sizes, flux rates, etc.) and also process stream capabilities and compatibilities (pH and temperature ranges, solvent stability, oxidizer stability, etc.). Polymeric membranes cover the complete range of pore sizes from MF to RO. Polymeric membranes can be configured as spiral, tubular, hollow fiber, and flat sheets, with spiral being the most widely used configuration. Inorganic membranes predominantly have supporting layers that are either ceramic or stainless steel in composition. The actual membrane layer is usually a ceramic material. The inorganic membranes are limited to MF and UF pore sizes. The inorganic membranes can be described as robust, capable of tolerating pH extremes, temperature extremes, organic solvents, and oxidizers well. Inorganic membranes are almost always configured as a tubular membrane.

Membrane technology is well suited to treating point-source effluent streams. Point-source effluent streams are usually composed of a few components and have a relatively consistent composition and stream characteristics (volumetric flows, temperature, pH, etc.), especially when compared to combined or total plant effluent streams. Because of these factors application of technologies to purify these point-sources for reuse is much more viable than treating combined or total plant effluent.

Point-source streams that are treated for reuse with membrane technology include: contaminated (or dirty) chemical streams like CIP or cleaning caustics and acids, regeneration chemicals, brine solutions, washing/peeling solutions, condensates and permeates, and treated wastewaters.