Membranes

Can A New Polyamide Chemistry Improve RO Rejection of Boron, Nitrates, and Silica?

By Alan D. Sharpe

BORON ION EXCHANGE NITRATE POLYAMIDE REVERSE OSMOSIS REJECTION SILICA

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Abstract

Introduction

The reverse osmosis (RO) membrane process has been applied for the preparation of potable and industrial water applications. More recently, the World Health Organization (WHO) has published guidelines for critical ions like borate and nitrate in drinking water supplies. Likewise, the high-purity water industry has demanded increased control of troublesome ions, like silica, to extremely low parts per trillion (ppt) levels. 

These ions are well rejected by RO membranes based on classic polyamide (PA) chemistry. However, further improvements in rejection performance have been demonstrated by the application of a high cross-linkage in the thin-film PA barrier layer. The improvement in rejection allows operation at lower feed pH levels, and lower ionic burdens on downstream mixed-bed operations, resulting in some cost savings to the final product water cost.

This article will review present data demonstrating the improved rejection performance for specific ions like boron, nitrate, and silicate. In addition, the article will provide model simulations that demonstrate the savings in chemical addition (borate application), and the savings in regeneration costs in a downstream mixed bed operation, post-RO process.

RO Membrane Chemistry

The classic RO membrane chemistry is based on a 0.1-micron (µm) polyamide barrier, and is common in nearly all commercial RO membrane products. The traditional PA barrier layer chemistry is based on the interfacial polymerization (or in situ condensation) of the following monomers: trimesoyl chloride (TMC in decane) and meta-phenylene diamine (m-PDA in water). The polymerization process is essentially a “dip coating” process wherein the polymerization occurs across a moving web (flat sheet) of polysulfone substrate, which itself is separately prepared from a non-woven sheet.

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