A NOVEL METHOD FOR FILTER BACKWASH PERFORMANCE ASSESSMENT
A method developed by Sydney Water and applied at Water Treatment Plants at Sydney Water
A Smith, H Botham, A Mohiuddin
Publication Date (Web): 3 May 2018
Water Filtration Plants (WFP’s) at Sydney Water use dual media filters to remove coagulated particles and pathogens from raw water. These filters require regular cleaning through backwashing to maintain filtration performance and achieve target drinking water quality and quantity. Historically operators would assess filter backwash performance by visual inspection of the filter during air scour, low and high rate water backwash, and media bed surface post backwash. The operator would then make any process adjustments as required, and record observations.
The novel method developed at Sydney Water combines existing visual inspections with empirical measurement of certain parameters during a backwash to gain an understanding of filter backwash performance. The parameters include:
Water drain down level before air scour
- Media bed expansion and fluidisation
- Verification of backwash flow rate/meter
- Media bed depth and uniformity of bed surface
- Media bed expansion profile over a range of backwash water flow rates
- Inspection of media condition and presence of mudballs
- Backwash effluent water turbidity profile every minute over the period of the backwash
The method also includes assessment of all filters at a treatment plant across a number of conditions, including a baseline condition assessment, and after any process adjustments to the backwash. Stepwise testing assists with root cause analysis and gives a measure of improvement after process changes. Implementation of this method also required developemnt of a tool to perform measurements and collect samples of media. This tool is a modified secchi disk on a telescopic pole of robust construction.
The novel method was first implemented at Cascade Water Filtration Plant where significant mudballs and poor filter runtimes had been observed by operators. The Baseline condition assessment confirmed these observations finding very large mudballs and thick layers of sludge on top of the filter media. The filter bed expansion was found to be 5-10% of the bed depth, well below the target of 20%. The water drain down level before air scouring was also found to be high and variable between all filters, which can lead to ineffective air scouring. To improve the backwash, the drain down level was reduced and the backwash flow rate increased (to increase expansion). A few weeks after each process change the complete condition assessment was conducted to identify any improvements with each change. Increasing the backwash water flow rate, and hence the media bed expansion, gave the greatest improvement in filter bed condition. Minimal improvements were observed after reducing the drain down level, however, it is expected that this may take longer to see an effect. A follow up assessment conducted a few months after all changes were implemented by the plant team identified a significant reduction in the amount and size of mudballs present on the media bed after a backwash. More effective washing at the higher flow rates also allowed the backwash duration to be decreased resulting in a net reduction in water loss due to backwashing. Planned replacement of filter media was also deferred as a result.
Application of this novel method allows physical/empirical measurement of backwash performance following process changes. This improves understanding of the factors influencing filter backwash and identifies opportunity for improvement. At Cascade WFP, implementation of the method lead to significant improvements in the backwash performance and filter condition with a reduction in water loss due to backwashing and also led to deferring filter media replacement.
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