Operate

High-rate granular media filters shall be designed to operate at no more than 15 gallons per minute per square foot (37 m/h) when a minimum bed depth of 15 inches (38.1 cm) is provided per manufacturer.

High-rate granular media filters shall be designed to operate at no more than 15 gpm/ft2 (37 m/h) of filter surface. The minimum depth of filter media above the under-drains (or laterals) shall be set by the filter manufacturer. Filters with bed depths less than 15 inches (38.1 cm) shall operate at no greater than 12 gpm/ft2 (29 m/h) of filter surface area. A minimum bed depth of 15 inches (38.1 cm) is required for flow rates greater than 12 gpm/ft2 (29 m/h) to a maximum of 15 gpm/ft2 (37 m/h). Note: Allowable filter rate is directly related to bed depth. The granular media filter system should 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. Specially graded filter media should be recommended in filter systems backwashing at less than 20 gpm/ft2 (48.9 m/h) to be able to expand the bed at least 20% above the fixed bed height at the design backwash flow rate, which is subject to approval by the local authority. Filtration and backwashing at the same flow rate is likely to lead to poor performance of both processes. Backwashing at double the filtration rate is not all that complicated with a 3-filter system, where the flow of two filters is used to backwash the third. Further, backwashing with unfiltered water is possible in a 2-filter system by backwashing with the entire recirculation flow through each filter individually. Variable drive pumping systems and accurate flow meters also contribute to the likelihood of successful backwashing as well as effective filtration. Effective Filtration Filtration at 10 gpm/ft2 (24 m/h) is really pushing the envelope for attaining effective filtration and would not be recommended for a municipal drinking water system using sand filters due to doubts about the ability of such a filter to remove particulate CONTAMINANTS reliably. There are instances where multi-media deep bed filters or mono-medium filters with large diameter anthracite and 6 foot (1.8 m) deep or greater beds of media are used, such as those owned and operated by the Los Angeles Department of Water and Power. Effective filtration of drinking water at high filtration rates recommends careful and exact management of coagulation. Whereas filtration rates are not explicitly addressed in much of the research on water filtration, the experience of researchers, regulators, and consultants is that high rate filtration recommends extra attention and talent. For example, over three decades ago, the State of California allowed the Contra Costa Water District to operate filters at 10 gpm/ft2 (24 m/h) but other water utilities were not allowed to do this. The exception was permitted because of the design and the high level of operating capability at the plant where the high rate was used. Operation at very high rates either causes very rapid increases of head loss in sand filters (water utility experience resulted in the conclusion that operating sand filters at rates above 3 or 4 gpm/ft2 (7-10 m/h) was impractical) or very little particle removal occurs as water passes through the sand bed, thus enabling filters to operate for a long time at high rates. For this reason following World War II, the use of anthracite and sand filters became the norm for filters designed to operate at 4 or 5 gpm/ft2 (10-12 m/h) or higher. Finally, in the 1980s, workers in Los Angeles showed that a deep (6 foot (1.8 m)) filter with 1.5 mm effective size anthracite media could effectively filter water at rates of close to 15 gpm/ft2 (37 m/h). However, for very high rates of filtration to be effective, pretreatment has to be excellent, with proper pH and coagulant dosage, probably use of polymer, and in some cases, use of a pre-oxidant to improve filter performance. This is well understood by filter designers and professors who specialize in water filtration. Articles published on the Los Angeles work done by James Montgomery Engineers showed the importance of proper pretreatment. Papers written by experts on filtration have noted the importance of effective pretreatment (including proper coagulation) for dependable filter performance, and those writers were focused on rates employed in municipal filtration plants (e.g. 3 to 10 gpm/ft2 (7-24 m/h)). As filtration rate increases, water velocity through the pores in the sand bed increases, making it more difficult for particles to attach to sand grains and remain in the bed instead of being pushed on through the bed and into filter effluent. When filters do not work effectively for pathogen removal, the burden is put on DISINFECTION to control the pathogens. For Cryptosporidium, the DISINFECTION approach that is typically most cost-effective is UV, so a very high rate filter may need to be followed by UV for pathogen inactivation, and the very high rate filters would just have to clarify the water sufficiently that there is no interference from particulate CONTAMINANTS with the UV inactivation process.