Sand Media Filters
Water flowing through the filter is collected at the bottom of the tank by an underdrain system. Types of underdrains include an epoxy-aggregate composite or slotted or perforated screens covered with gravel.The size of the sand media affects the filtering capacity. Sand size is designated by number: No. 8 sand is the largest and No. 30 is the smallest (table 5). The type of media also influences filtering ability. Crushed silica filters better than sand because the angular particles of the crushed material fit closer together. For a given sand number, crushed silica retains smaller particles than does sand.
The recommended design filter flow rate depends on the amount of suspended material in the water. For dirty water (100 ppm or more of suspended material), an acceptable filter flow rate is about 15 gallons per minute per square foot of filter area. The recommended rate for “average water” is 20 to 25 gallons per minute; for clean water (less than 10 ppm of suspended solids), the rate is 25 to 30 gallons per minute. An excessive filter flow rate can reduce the filtering ability of a sand media filter by forcing larger particles to pass through.
The design pressure drop in a sand filter is 3 to 5 psi. The filter should be backflushed when the pressure drop becomes about 10 psi. Backflushing reverses the flow through one filter at a time and uses the clean water from the other filters to remove the contaminants. The backflushing flow rate should be sufficient to fluidize or suspend the sand media, allowing the contaminant to be carried out by the backflushing water. The recommended backflushing flow rate is 10 to 15 gallons per minute per square foot of filter area for Nos. 30 and 20 sands, and 20 to 25 gallons per minute for Nos. 16 and 11 sands. The actual backflushing flow rate should be determined by visual observation; it should be sufficient to clean the filter but only a very small amount of sand media should be carried out during backflushing.
Removing fine sand from the water may be difficult, particularly for a filtration system consisting of two sand media tanks. Because the specific gravity of the fine sand may be similar to that of the sand media, the fine sand may not be removed unless the backflushing flow rate is increased to a higher level than is normally recommended. Compacted layers of fine sand in the other tanks also may prevent a sufficient flow rate for backflushing, particularly in a two-tank system. Periodic partial closing of the valve to the field to increase the backflush underdrain pressure to 40 psi during backflushing may solve this problem. Otherwise, replacement of the sand media may be necessary in some cases in order to restore the filtering capacity of sand media filters.
Periodic chlorination of a sand media filter also may be required. Biological matting can occur in the filter, severely degrading its performance. These mats also may prevent an adequate backflushing flow rate. In addition, organic material may grow in the filter during the off season. Using a chlorine concentration of 10 to 20 ppm for several hours has successfully restored filter performance. The chlorine can be added by injecting it upstream of the filter during an irrigation or it can be added by hand to the filter through the filter access port while the irrigation system is shut down. Always be sure that all water pressure has dissipated prior to opening an access port.