The Rosedale Wastewater Treatment Plant, located in Rosedale, opened in 1962. At that time, it served a population of around 60,000 people. Since then, the plant has been progressively upgraded using modern processes to effectively treat the wastewater.
The plant now serves almost 220,000 people and sits among houses, industry and recreational facilities that surround it on all sides. The open areas of buffer land around the main plant and the large ponds, which form an integral part of the treatment process, provide relieving, open space vistas within an otherwise urban landscape.
Wastewater is pumped to the site and is treated in a series of different processes to ensure it is safe to be returned into the environment. Solids and liquids in the wastewater are separated at different times through the overall process. Biological processes, similar to those used in our own stomachs, are used to help purify the waste. The main treatment process for liquids is called “activated sludge”, where bacteria in the activated sludge solids use the wastewater as a source of food.
After solids are removed through settlement, the liquids take around two weeks to travel through the pond system, where sunlight provides natural disinfection of remaining bacteria. Further disinfection using ultraviolet light is provided before the treated wastewater is discharged by gravity almost three kilometres out to sea through a completely new discharge system. This was completed in 2010. The treated wastewater easily meets New Zealand Recreational Water Quality guidelines before it reaches the surface. Some of the treated wastewater is used on site for process wash water and irrigation.
The solids removed in the treatment processes are biologically treated in a separate process to produce biogas. This is used to produce around 60 percent of the energy required to run the whole plant. Following this treatment, the biosolids produced are dewatered, with an average of less than two truck and trailer loads discharged to landfill every day.
Treated wastewater quality
The Rosedale plant produces a very high quality of treated wastewater which meets the conditions of resource consents to discharge set following consultation. The plant achieves the following removals of contaminants:
Suspended: Greater than 92%
Oxygen demanding substances: Greater than 92%
Indicator bacteria: Greater than 99.99%