Tahuna WWTP - An Innovative Conversion of Primary Sedimentation Tanks to the High rate Activated Sludge Process

CH2M Beca Ltd was engaged by Dunedin City Council (DCC) to undertake the concept and detailed design of Stage 2 of the Tahuna Wastewater Treatment Plant (WWTP) Upgrade. The population served is 120,000 and the peak wet weather flow is 4,000L/s. Prior to the Stage 2 upgrade, the WWTP consisted of primary sedimentation and chlorine disinfection. The Stage 2 upgrade to secondary treatment was required to improve the effluent quality and allow ultraviolet light (UV) disinfection to replace chlorination.

The liquid stream upgrading included the new processes of Grit Removal, and High Rate Activated Sludge (HRAS). The length of the existing primary sedimentation tanks (PSTs) allowed establishment of three zones (grit removal, aeration, sedimentation) using new partition walls and a modified inlet. Conversion of the PSTs included a number of novel approaches such as; converting former sludge hoppers to grit hoppers, rubberlined grit pumps, suction removal of sludge from rectangular tanks, modification of existing travelling bridge scrapers, coarse screw collection of scum, submerged tube launders coupled with a weir at the sedimentation tank outlet, and very high speed aeration blowers with direct-coupled motors and integral variable speed drives (no gearboxes).

This paper describes the methodology and reasons for selecting the HRAS process, the innovations and other unique features required for the conversion of the PSTs to this process, and how the process has performed.