Study of Floc Strength and Stability during Direct Filtration of Surface Water

A project to address conditions at Sydney Water’s Nepean WTP

The current floc strength test is based on turbulence induced shear techDue to the historic low turbidity and colour of many raw surface water supplies, Sydney Water has employed direct/contact filtration systems for drinking water treatment. One challenge for water filtration plants is the rapid deterioration of raw water colour and NOMs during heavy rain events and changes in the dam levels. Removing additional colour introduced under these conditions requires an increase in dose of ferric chloride and polyDADMAC, which has resulted in the reduction of floc strength if the charge of the floc drifts outside the optimum range. niques employing impellers (Jar test). This may not be an accurate representation of the shear conditions in a direct filter operation and cannot be used to define a threshold velocity gradient to prevent the breakthrough of suspended solids.


We have established a mathematic model that can characterise the variation of filter porosity/resistance needs to be developed and tailored from operating data for Sydney Water’s plants. A novel floc strength testing device that can be used to mimic the shear events in the direct filters, and help plant operators diagnose floc strength has been manufactured and tested at UCMST.  

CFD simulations of filter grains Model validation
Computational Fluid Dynamics (CFD) simulations of filter grains Comparison of CFD simulated vs. site reported head loss in direct filters

A novel capillary flow device to tests floc stability has been designed and manufactured, and tested at CMST. This is a substantial improvement that will help operators of direct filtration plants to better manage high colour-low turbidity events by being able to determine floc strength under different coagulation conditions.

Floc device


Research team: Greg Leslie
                            Yuan Wang
                            Boyue Lian

Funding body: Sydney Water

For more information, please contact Dr. Yuan Wang at