Backwash System Design

The granular media filter system shall be designed to backwash each filter at a rate of at least 15 gallons per minute per square foot (37 m/h) of filter bed surface area, unless explicitly prohibited by the filter manufacturer and approved at an alternate rate as specified in their NSF/ANSI 50 listing.

For a granular media filter system to be able to backwash at a rate of at least 15 gallons per minute per square foot (37 m/h) of filter bed surface area, the pump(s), pipes, and filters must be designed accordingly. As many professionals have sought to improve water quality by decreasing the filtration rate to values lower than 15 gpm/ft2(37 m/h), they have sometimes failed to recognize that while lowering the filtration rate may generally produce a positive change in performance, a similarly lower backwash rate could lead to a total filtration system failure. In cases where a backwash rate of 15 gpm/ft2 (37 m/h) is explicitly prohibited by the filter manufacturer, the filter may still be used, provided that specially graded filter media is installed that will expand to a minimum of 20% bed expansion at the specified backwash flow rate. Viewing windows are highly recommended in all filters since they will allow direct observation of the bed expansion during backwashing, cleanliness of the media and backwash water, and the depth of the sand in the filter. Croll and coworkers¹⁷⁹ used a backwashing rate of 25 gpm/ft2 (61 m/h) to achieve 25% bed expansion of their filter. WHO Recommends The WHO recommends a backwash rate of 15-17 gpm/ft2 (37-42 m/h for sand filters, but the media specifications are not given nor is it clear whether or not air-scour is expected prior to backwashing.¹⁸⁰ Backwashing swimming POOL sand filters with air scour is common in the UK and elsewhere.¹⁸¹.¹⁸² It has also been reported that air-scour washed AQUATIC VENUE filters are more efficient than filters washed by water only.¹⁸³ It is reasonable that lower backwashing rates would be used for water backwash when following air-scour since the air-scour dislodges most of the particles attached to the media grains (as opposed to relying on the sheer force of the water passing over the surface of the particles). It is not feasible to operate sand filters in drinking water treatment plants without an auxiliary backwash system such as air scour.¹⁸⁴ The practice of operating AQUATIC VENUES and filters (that were not using coagulation) without air scour has been STANDARD practice in the U.S. for many years, which has seen mixed results ranging from no problems to total system failures requiring replacement of all filter media. PWTAG recommends air-scouring filters at 32 m/h (13 gpm/ft2) (at 0.35 bar).¹⁸⁵ Polyphosphate Products Polyphosphate products are sometimes used to sequester metals in POOLS, but this practice is not recommended when granular media filters are used because polyphosphate is an effective particle dispersant that can reduce the removal efficiency. Filter Bed Expansion Sufficient freeboard (or space between the top of the media and the backwash overflow) to allow for a minimum of 35% filter bed expansion during backwashing adds a factor of SAFETY when the target bed expansion is 20% to prevent the washout of filter media during backwashing. The regions underneath the lateral underdrains in granular media filters can become stagnant when filled with sand or gravel, which can lead to low disinfectant residuals and ultimately biofilm growth. Filling this area with concrete at the time of installation may prevent this potential problem.¹⁸⁶ It is fundamentally difficult to suspend (i.e., fluidize) and hence clean filter media or gravel that is below the level where the backwash water enters the filter.