INNOVATIVE SYSTEM DESIGNS TO OPTIMIZE PERFORMANCE OF ULTRA-LOW PRESSURE REVERSE-OSMOSIS MEMBRANES

The reverse osmosis (RO) water treatment industry has been revolutioni zed in the past several years by the introduction of a new generation of ultra-low pressure RO membrane elements. These new membranes have h elped the technology become much more affordable and cost effective by reducing the energy consumption required to operate the RO system. Th e energy required to pressurize the RO feedwater has always been the l argest component of the operating cost of a RO plant. The operating an d capital cost savings provided by the new membranes combine to bring the expense for RO water treatment plants in line with the costs for c onventional water treatment plants. Although several technical papers have been written over the past few years evaluating the performance o f the specific ultra-low pressure membranes, very little information h as been shared throughout the industry on observations of the operatio n of these membrane in conventional RO system design. Although the ult ra-low pressure membranes have salinity rejection characteristics comp arable to conventional RO membranes, their hydraulic characteristics c an be significantly different. Information indicates that the ultra-lo w pressure membranes have an almost 30% higher design permeate product ivity than conventional membranes. This significantly affects the hydr aulic behavior of the full-scale membrane water treatment system. Addi tionally, in multi-stage systems, the lower required feed pressure tra nslates 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 membrane s in the full-scale system. Several design options will be evaluated f or their effectiveness in improving system designs incorporating ultra -low pressure membranes. The design modifications will be evaluated fo r function, effectiveness and cost impacts. Finally, recommendations w ill be presented on how to assess which design might be the most appli cable for a particular application.