On Tuesday, Sydney and Wollongong’s dams were 60.3 per cent full, marginally above the trigger point for switching on the desalination plant. On current projections, the sub-60 per cent mark is likely to be crossed this weekend.
Reverse Osmosis (RO) is an osmotic process that uses a semi-permeable membrane to separate water from dissolved matter. Applying a pressure greater than the osmotic pressure (determined by the TDS), feed water is forced through RO-membranes which hold back diluted ions and only allow water molecules to pass through. This divides the feed water into a brine stream with a high TDS concentration and permeate stream with pure water (Fichtner, 2011), (Trieb, 2007). Reverse Osmosis plants are used as well for seawater as for brackish water desalination (Fichtner, 2011). As the driving force of the process is the pressure, which rises with increasing feed water salinity, a suitable high-pressure pump is needed. And due to the fact that RO membranes typically are very sensitive to organic matter, several pre-treatment steps like chlorination, cartridge filtration, dissolved air floatation and ultra-/ microfiltration are necessary (Fichtner, 2011). The main advantages of reverse osmosis desalination are the wide range of feed water quality, the flexibility of the location because of independence of adjacent power plants and the scalability (The World Bank, 2012). On the other hand, the membranes high susceptibility for fouling and the subsequent comprehensive need for pre-treatment as well as the complex setup and the required skills for the staff are the main disadvantages (The World Bank, 2012). On the one hand, current research and development focus on the improvement of the membrane technologies (e.g. incorporation of nanocomposites, large diameter spiral wound elements, low bio-fouling feed spacers). On the other hand, efforts are made to improve the overall process performance (better energy recovery, application of renewable energies, new chemical products for anti-scaling and membrane cleaning, reducing maintenance efforts) (Peñate & García-Rodríguez, 2012).