The local government initiated a public consultation in 1991 due to superannuation of the site. Following this, the NSCC decided to upgrade the Rosedale plant and dispose of waste offshore in the Hauraki Gulf. The first stage was to take the immediate load off the oxidation ponds, thereby reducing the potential for nuisance odours to be emitted during the plant's normal operation. The reduction of biochemical oxygen demand and ammonial nitrogen load on the ponds was necessary to minimise odour production. In an ASR, turbine blower units pump air through a series of pipes and diffusers.
Organic matter is removed biologically.
Sludge is a by-product of the process and is extracted and treated in the digesters. Residual sludge settles to the floor of the clarifier and is swept to a central hopper. Sludge wasted from the ASR is very watery, and is thickened in the DAF tank prior to being pumped across to the digesters. This involves injecting air into the liquid under pressure causing the air to be dissolved in the liquid. When the pressure acting upon the liquid is reduced, the aerated sludge immediately effervesces as its capacity to support dissolved gas is reduced.