A practical method to assess the ‘flocs condition’ and ‘floc strength’ in real-time in water treatment using surrogate parameters of media filter performance

Abstract

Increased frequency and intensity of weather events have greatly affected the surface raw water quality in Australia. It resulted in a 3- to 5-fold increase in True colour and Dissolved Organic Carbon (DOC), representing an increase of Natural Organic Matter (NOM), in Nepean Dam, south of Sydney, New South Wales, Australia. The increased NOM caused the formation of ‘weak flocs’ in the coagulation process that broke up in dual media gravity filters, resulting in a premature backwash and short filter run time. There is no practical method to measure ‘floc strength’ in an operating water treatment facility. This research identified a novel method to assess the ‘flocs condition’ during filtration, either ‘weak’ or ‘strong’, by calculating the rate of turbidity change and reporting it as a ‘turbidity breakthrough slope’ in NTU/day (NTU/d). The end-of-run head loss of the filter represents the energy at which the flocs break. Consequently, this research used the end-of-run head loss of a filter to measure floc strength. The turbidity breakthrough slope and filter end-of-run head loss are linearly correlated, and by using the correlation, a novel Floc Strength Model (FSM) has been developed. Applying the FSM at Nepean Water Filtration Plant (WFP), for ‘very strong flocs’ and ‘very weak flocs’ conditions, the calculated ‘floc strengths’ were 2.2 m and 0.57 m (measured in ‘meter (m)’ of water column), respectively. The FSM is a practical and in-situ method to monitor ‘floc strength’ in real time during a filter operation. It enables dynamic optimisation of water treatment. As soon as the filtered water turbidity starts to increase (start of turbidity breakthrough) in a filter, the FSM calculates and predicts the low ‘floc strength’ (end-of-run head loss) of the ‘very weak flocs’ and accordingly predicts the run time of the filter. Plant operators can then proactively optimise the chemical doses to change the flocs condition to ‘very strong flocs’ and improve ‘floc strength’ and filter run time before the end of the filter operation cycle.