2018-2021: Ross River Dam (Queensland)

DWI-506-21-08862 – Townsville DWS – Cyanobacteria. Event resulting in discoloured water and reduced supply Cyanobacteria have been detected in RRD since 2018. Cell numbers (and biovolume) have been increasing each summer since. Numbers increased significantly  (>120,000 cells/ml) in January and February and diatoms numbers also increased significantly (>80,000 cells/ml). This caused filter binding at Douglas WTP and water production decreased from the 30 January 2021. It also caused filter breakthrough which resulted in a yellow
hue to the treated water. In RRD the cyanobacteria experienced a die off phase which dropped dissolved oxygen (DO) levels in the dam at all levels in the profile. As low DO was at all offtake layers this low DO water was taken into the WTP. This low DO water was high in soluble manganese (low DO enables soluble manganese to be released from the sediments in the bottom of the dam). This soluble manganese passed through the filters and resolubilised to total manganese at the chlorination step in the clear water storages.
Pre-chlorination was added to the filters in module 1 and module 2. This effectively removed the manganese, improved cyanobacterial cell removal, reduced net sodium hypochlorite load on the WTP and improved DO across the WTP. The initial high sodium hypochlorite dose required increased Disinfection By-Product (DBP) formation in the short term but has since reduced due to the reduced sodium hypochlorite dose required for pre-chlorination and the overall net chlorine use across the plant.
Pre-chlorination remains in place and is now prior to filtration on all modules. Powdered Activated Carbon (PAC) dosing is in place to mitigate toxin risk if required. Construction of clarifiers pre–direct filtration modules has begun with a construction end date of
December 2022. A Catchment Management Plan is being developed.