Filter media washing prior to installation in stormwater treatment facilities

As emphasized in various guidelines for the design of low impact development facilities, cleaning the filter media is a vital preparation step prior to installation to avoid sediment washout. In this study, a rudimentary media washing experiment was done to determine the cleaning intensity in terms of the volume of water needed per volume media. Eleven types of filter media, namely gravel, crushed rock, volcanic stone, coarse sand, vermiculite, zeolite, anthracite, bottom ash, woodchip, synthetic fiber, and pall ring, were studied. Also, basic cleaning models for each media were established for estimating wash water turbidity as a function of the cumulative wash water volume. The results revealed that vigorous manual washing by hand can clean gravel, crushed rock, volcanic stone, zeolite, and anthracite by 39% to up to 200% more as compared with simply passing water through an undisturbed column of the media. This is mainly due to the collision and abrasion causing the detachment of particles less than 20 μm during hand washing. Woodchip, synthetic fiber, and pall ring were proven to be relatively clean and do not need prior washing. For the rest of the media, it was recommended to use a volume of water that is 2–8 times the volume of the media to achieve a 90% decrease in the initial wash water turbidity or 5–11 times to achieve a turbidity less than 50 NTU. Moreover, empirical regression analysis shows that the wash water quality can be estimated by the volume of water used in cleaning the media and that the two parameters follow a power function whose coefficients increase with increasing difficulty of cleaning the media and the initial amount of attached solids on their surfaces.