Secondary treatment of Storm Flows
Abstract
Secondary Treatment of Storm Flows - an economical solution
Secondary treatment of all wastewater, including wet weather flows, is becoming increasingly common in New Zealand as environmental protection standards become more stringent. North Shore City's Rosedale wastewater treatment plant is no exception. It is currently being upgraded to provide secondary treatment under peak storm flow conditions of up to 3.2m3/s or five times average dry weather flow (ADWF).
The provision of storm flow treatment at the Rosedale plant is being implemented at minimum cost through the application of innovative engineering solutions. The hydraulic capacity through the activated sludge process is being increased by separating the influent pipework from the return activated sludge (RAS) pipework and making other hydraulic improvements. Influent flows in excess of 2.4m3/s are being diverted to the outlet from the activated sludge reactor in order to receive treatment by contact stabilisation. In addition, existing secondary sedimentation tanks are being converted to treat part of the influent flow by chemically assisted precipitation (CAS) Use of the secondary sedimentation tanks has been made possible by the decommissioning of trickling filters, which have been a source of odour.
The techniques utilised at the Rosedale plant provide secondary treatment of storm flows are likely to find application elsewhere in New Zealand and may prove particularly economical at treatment plants where it is possible to utilise existing or redundant plant.
Author
J S Powell