Diffusers in the base of the aeration tank.
Click here to enlarge imageACWa is implementing recommendations from the treatability study to optimise the operation of the DAF system. Raw effluent will be diverted when pH, flow or load is beyond the existing work's design parameters. New run down screens will be installed to remove gross latex solids present in the raw effluent. A new balance tank will provide 12 hours hydraulic retention time for smoothing out loading variations. A new mixer and spray system will reduce foam build up in the tank.
DAF feed pumps will have inverter drives to allow flow control into the DAF system. Ferric chloride, caustic and polyelectrolyte dosing up-stream of the DAF plant will be automated. DAF modifications will provide an averaged flow to secondary treatment.
DAF supernatant enters an existing transfer tank from where new pumps will transfer it to the completely mixed activated sludge aeration tank, with air supplied through fine bubble diffusers. The effluent was deficient in phosphorous in the treatability study. Therefore, phosphoric acid is dosed into the transfer flow to provide micro-organisms with the essential phosphorous nutrient.
A dissolved oxygen (DO) monitor will be used via the control system to control the DO in the aeration tank at two mg/l. The resulting "mixed liquor" suspended solids (MLSS) is expected to be 3,500 to 4,500 mg/l. An MLSS monitor is included to trend suspended solids via the software control and data acquisition (SCADA) system to allow optimisation.
Mixed liquor from the aerobic stage gravitates via a degassing column into a settlement tank. Degassing is included to enhance settlement characteristics, solids being removed from the base of the settlement tank with clarified treated effluent gravitating over a peripheral launder. Solids collected at the base of this tank are normally returned to aeration as return activated sludge (RAS). Periodically, surplus activated sludge (SAS) will be diverted to the sludge holding tank. The amount of SAS wasted will be determined by calculating the sludge yield from the system. The treatability study determined this yield to be 0.15 to 0.25 kg dry solids sludge produced per kg COD treated.