Sand Media Filter Efficiency A Case For Reduced Backwashing Using Pre-Filtration

By Randy Delenikos, LAKOS Separators and Filtration Systems

Sand media filtration using media tank filters (with sand as the media) is a respected and proven methodology for removing algae, fine silt and organic matter from pumped water for drip irrigation and micro-spray irrigation systems. By their nature, sand filters not only effectively remove these fine solids, but also (not-so-effectively) remove larger particle matter, too. Solids such as 200 mesh sand and stringy organic matter is even more easily removed from the water stream via a sand filter. Unfortunately, such particle matter/debris is not-so-easily removed from a sand filter. Instead, consider the logic of applying two or more filter techniques in a system to solve the overall contaminant problem, rather than depending on one filter to do all the work – since using a single technique often results in overwhelming a filter and causing it to fail.

Stringy organic material can accumulate quickly on a sand media bed surface, causing pressure loss in a short period of time. This material can also foul the dispersion plates at the inlet to a sand filter, when such debris interweaves or gets impacted onto the plates, vanes and/or coarse screening of a sand filter's dispersion devices. Pre-filtering at the water source, before the debris gets to the pump, is an intelligent alternative. This typically relies on alternative technology designed for the removal of such debris. Self-cleaning pump intake screens (as shown at left), for example, can keep such larger debris from fouling a sand filter and also from damaging a pump. Other filtration devices can be used as well.

More subtle, but more prevalent, is the fouling of a sand media filter due to excessive sand accumulation in the tank. This occurs when the source water contains large amounts of sand and similar size particle matter. This problem almost always begins with the common indicator of a rapid-to-continuous pressure loss. This causes a significant drop in sand filter efficiency.