Wastewater Treatment
Wastewater is 99.97% water and some dissolved and suspended matter such as solids, oil and greases, detergents, nutrients, heavy metals and bacteria, which need to be removed before discharge to the environment or recycling for other uses.
Wastewater treatment plants treat wastewater to a quality suitable for recycling or discharge.
Primary treatment settles out or clarifies about 50% to 60% of the suspended matter in wastewater. The treated wastewater is suitable for discharge through a long ocean outfall. This is used at the Point Peron wastewater treatment plant and is the practice in the large Sydney treatment plants.
Secondary treatment is typically a biological treatment process that is designed to remove suspended solids and 85% of the organic matter. The most commonly used secondary treatment processes are activated-sludge processes and lagoons. Additional sand filtration and disinfection may also be used.
Tertiary (or advanced treatment) is principally designed to remove nutrients, such as phosphorus and/or nitrogen. Most suspended solids are also removed.
Tertiary treatment may additionally target other contaminants of concern, such as toxins and salt. Typical tertiary treatment processes include biological nutrient removal, enhanced pond treatment systems, reverse osmosis and advanced filtration systems.
Most of Perth’s treatment plants are tertiary plants, which treat water to a high standard and are designed to reduce nitrogen to less than 15 milligrams per litre. Where recycling for irrigation occurs there is also sand filtration and disinfection. For industrial recycling in the Kwinana Industrial Area, micro filtration and reverse osmosis are used.
Some Perth wastewater treatment plants have been covered to reduce odours to the atmosphere. The Water Corporation has spent over $60 million over the past 10 years on this program. In addition, there are buffer areas around each treatment plant to reduce the exposure of homes and businesses to odour.