Challenge

Perth’s second largest seawater desalination plant, the 50Gl/year (36 mgd) Southern seawater desalination project (SSDP) is being built in Binningup,150km south of Perth. The coastline is influenced by strong winds and variable currents from the Indian Ocean. The wind, current and bathmetry provide strong mixing, ideal for the marine environment when discharging from the desalination plant. These conditions, however, have introduced challenges for the pre-treatment system design upstream of the reverse osmosis (RO) portion of the plant. Sampling indicates adverse seasonal condition that could result in daily average suspended solids concentration of 10 mg/L, with periodic incident peaks of 50 mg/L.
 
Solution

Options for pre-treatment included two-stage media filtration and single-stage membrane filtration. Although capital and operating costs were similar, a single-stage pressurized membrane filtration system offered sustainability because the process does not use coagulation chemicals and enables backwash from the system to be returned to the ocean without need for a wastewater treatment plant for sludge disposal. The membrane chosen was based on the Memcor® CP system.

Additionally, since the intake site is about 400m offshore in 10m water, a micro-tunnelling solution has been adopted via a 1km-long intake pipeline with a diameter of 2,400mm under the extensive dune system between the intake pump station and the offshore intake. Micro-tunnelling is also being used for the brine disposal pipes that terminate 800m offshore.
 
Results

The Binningup plant is configured into two independent banks, each with a drinking water production capacity of 80 mld. The membrane plant is configured as five separate units in each bank with an overall filtration area of 320,000 m². As well as membrane filtration, the plant incorporates a membrane loading arrangement in the first pass RO System, using a split hybrid approach to reduce energy consumption and minimize the size of the second pass, which is required to reduce bromide concentrations in the drinking water to less than 0.1 mg/L. Testing over a six-month period has demonstrated stable performance at 99% feed water recovery and a design flux of 65 L/h/m² without coagulation addition to the feed. Potable water from the plant will be pumped into Perth Integrated Water Supply Scheme (IWSS).