Seqwater Building Flood Resilience into Water Treatment and Supply

 Mt Crosby Water Treatment Plant Since the 2011 flood, Seqwater has been quietly building flood resilience into its major water treatment plants. An example of this is the Mt Crosby Water Treatment Plants at East Bank and West Bank. There were a number of issues that affected the performance of these plants during the 2011 flood that have been or are being addressed by Seqwater to provide greater flood resilience.

During the 2011 flood the East Bank Raw Water Pumping Station which has been operating since 1893, suffered from partial flooding of the pump dry wells through seals and conduits. Fortunately, Seqwater was able to manage this problem without causing major disruption to supply. The problem with leaking seals and conduits has since been rectified. New watertight doors have been installed to enable isolation of individual wells should a similar flood occur in the future. In addition, stabilisation of the river embankment adjoining the pump station has been carried out to afford greater bank stability under flood conditions.

At West Bank, the raw water pumps operate on a fixed speed arrangement but could only be safely operated at a minimum flow of 125 ML/d. (ie one pump operating). As operation of the plant at lower flows was required during the flood because of the high turbidity, this could only be achieved by throttling the pump discharge valves. These valves were not suitable for extended use under throttled conditions. As such Seqwater has installed new Variable Speed Drives to enable safe operation down to 50 ML/d (see photo).

During the 2011 flood, raw water turbidity peaked at over 2000 NTU with the turbidity persisting at high levels for many days after the peak. The high turbidity was accompanied by lower pH levels in the raw water requiring caustic soda dosing to increase the pH to a level sufficient to achieve good coagulation.

Increased alum dosing was also required. The high turbidity (and sediment load) required a reduction in the flow rate to each sedimentation tank to avoid overloading the tanks.

Since the flood substantial improvements to the chemical dosing system have been carried out or are now underway to improve coagulation mixing as well as chemical dosage capability and redundancy. At East Bank additional chemical storage is being provided for alum and caustic soda (refer 3D schematic drawing). During the 2011 flood, tanker delivery access to the site was restricted by the high floodwaters and supplies of chemicals to the Port of Brisbane could not be delivered as it had been closed during the flood peak.

The East Bank the filters are also being upgraded to improve robustness and reliability by the provision of new filter media to provide better control of treated water turbidity and to enable longer run times and by the provision of air scour to improve backwashing effectiveness.

Sludge handling constraints are also being addressed by the installation of additional dewatering capacity at both plants. At East Bank dewatering capacity has increased from 40 t(DS)/day to 60 t(DS)/d. At West Bank, the sizing is proposed to increase from 10t(DS)/day to 28t(DS)/day.

 Mt Crosby Water Treatment Plant