Our treatment plant and the new outfall

Water Services - Wastewater

Image of man and jars. Wastewater treatment and the new outfall

The new outfall will discharge high quality treated effluent into the sea – the end product of a very thorough process carried out at our Rosedale Wastewater Treatment Plant.

Wastewater – the used water and waste from our homes and workplaces, also known as sewage – is carried to the treatment plant by our sewer network, and goes through a comprehensive range of treatments to make it safe to return to our environment.

When the wastewater, which is 99.9 per cent water, arrives at the plant we screen it to remove large inorganic solids. After screening, the remaining liquid (called effluent) is treated and then discharged to the sea. The remaining solids (called sludge or biosolids) are treated then taken to landfill. The way we treat and dispose of effluent and sludge from our wastewater treatment plant is an important issue and we are continually seeking more environmentally friendly ways of doing this.

The treatment process

Primary treatment allows organic solids (called raw sludge) to settle. The plant uses four large primary sedimentation tanks, in which gravity and time play a major role. The heavier solids sink to the bottom while floating solids, such as fat and grease, accumulate on the surface and are removed.

The partially purified liquid, now known as primary effluent, is discharged over weirs at the ends of the tanks. In many cities around the world this is the extent of the treatment process.

The effluent from the primary tanks flows to three large activated sludge reactors where microbes consume the nutrients and organic compounds. These tanks are continually aerated to keep the microbes alive and to make sure the water from these tanks has no oxygen demand on receiving waters. The water and activated sludge is separated in four large clarifiers. The water from the clarifiers is very clear.

The flow then passes to the Rosedale Lakes. These were formerly treatment ponds but now all the treatment is completed in the tank processes, with the exception of disinfection by ultraviolet light from the sun. The lakes now contain clear water that sunlight can penetrate and kill most of the microbes left in the water. Any remaining microbes are killed with the ultraviolet light process that mimics sunlight.

Our UV treatment plant, opened in 2003, is the final stage of treatment before the effluent is discharged, and allows further natural disinfection of the effluent.

After treatment in the ponds and the UV treatment plant, the effluent is discharged to the Hauraki Gulf through a gravity pipeline. The current outfall site is located 600 metres offshore from Kennedy Park between Campbells and Castor Bays.