Screening optimisation for indirect potable reuse

An automatic backflush pre-filter used for pre-treatment for secondary wastewater re-use was evaluated and optimised at two different mesh sizes over an 18 month period. The filter was initially run with a 500 μm rating mesh size, as recommended by the supplier of the downstream membrane filtration process, and then at 100 μm to investigate any change in water quality produced and associated improved membrane performance. With the 500 μm mesh in place, the filter fouling rate was low and a backflush was initiated every 3.5 h. For the 100 μm mesh the fouling rate was extremely rapid. Fouling was found to be caused by reverse side blockage of the pre-filter due to biofilm growth, and not by improved solids capture; there was no improvement in water quality with the smaller mesh size, since particle unloading from the biofilm took place. The pre-filter fouling rate was found to be related to turbidity. At a turbidity of 5 NTU the filter backflushed around 200 times per day, while at 10 NTU this increased to over 300 times. Further analysis enabled the backflush water volume to be decreased by reducing the backflush duration and increasing the backflush cycle time (i.e. the time between backflushes).