Reverse Osmosis

Reverse osmosis is being considered as the second stage of the Water2WATER treatment process, following the microfiltration or ultrafiltration step. Reverse osmosis (RO) relies on the use of a semi-permeable membranes that remove microorganisms (bacteria, viruses and protozoa), dissolved organic compounds, nutrients (nitrogen and phophorus), heavy metals and salts from the water flow. It is an energy intensive process and would operate at a pressure of approximately 10 bar for Water2WATER, compared with 60 to 70 bar for seawater.

The rejection of the chemicals by the membranes is achieved not only by molecular size exclusion but also by factors such as solubility and ionic charge and rejections of close to 99% are generally achieved for both salt (as sodium chloride) and total organic carbon (TOC). The process is also often used in seawater desalination applications.

RO produces a sidestream – known as ‘brine’, ‘concentrate’ or ‘reject’ - that contains all the compounds and elements rejected by the membranes. The volume of this sidestream can be 10-25% of the flow to the RO units and it must be handled appropriately to ensure minimal environmental impact.

The integrity of the reverse osmosis membranes and seals between membrane elements is monitored by on-line instrumentation for conductivity (a measurement of salt concentration) and Total Organic Carbon (TOC), as well as monitoring the pressure drops across individual pressure vessels that house the membrane elements.

For practical application, flat sheet reverse osmosis membranes are typically rolled with support materials and spacers into a spiral wound module – typically 200 mm (8”) or 400 mm (16”) in diameter and approximately 1 m in length (refer Figure 1). A number of cartridges – typically 6 or 7 - are fitted within a pressure vessel, with multiple pressure vessels being mounted in racks (refer to Figure 2).

Figure 1: Spiral wound reverse osmosis membrane cartridge

Figure 2: Reverse osmosis pressure vessels at Seletar NEWater Plant, Singapore