Innovative system designs to optimize performance of ultra-low pressure reverse osmosis membranes

Abstract

The reverse osmosis (RO) water treatment industry has been revolutionized in the past several years by the introduction of a new generation of ultra-low pressure RO membrane elements. These new membranes have helped the technology become much more affordable and cost effective by reducing the energy consumption required to operate the RO system. The energy required to pressurize the RO feedwater has always been the largest component of the operating cost of a RO plant. The operating and capital cost savings provided by the new membranes combine to bring the expense for RO water treatment plants in line with the costs for conventional water treatment plants. Although several technical papers have been written over the past few years evaluating the performance of the specific ultra-low pressure membranes, very little information has been shared throughout the industry on observations of the operation of these membrane in conventional RO system design. Although the ultra-low pressure membranes have salinity rejection characteristics comparable to conventional RO membranes, their hydraulic characteristics can be significantly different. Information indicates that the ultra-low pressure membranes have an almost 30% higher design permeate productivity than conventional membranes. This significantly affects the hydraulic behavior of the full-scale membrane water treatment system. Additionally, in multi-stage systems, the lower required feed pressure translates into even lower second pass feed pressure (and third pass, if used). These two factors combine to cause potential hydraulic balance problems in conventionally designed membrane water treatment systems. Modifications to conventional membrane system design practices must be considered to optimize the use of ultra-low pressure membranes. This paper will focus on the behavior of the ultra-low pressure RO membranes in the full-scale system. Several design options will be evaluated for their effectiveness in improving system designs incorporating ultra-low pressure membranes. The design modifications will be evaluated for function, effectiveness and cost impacts. Finally, recommendations will be presented on how to assess which design might be the most applicable for a particular application.