(fig. 9-14). In the diatomite filter, water is passed through a layer of diatomaceous silica (also called diatomaceous earth). It consists of skeletal remains of minute algae (diatoms) found in marine deposits that have lifted above sea level.

The diatomite filter accomplishes highly efficient filtration. Properly operated diatomite filters are capable of removing from coagulated and settled water, amoebic cysts, the cercariae of schistosomes, and approximately 90 percent of the bacteria. They also produce water with less than one unit of turbidity.

Before filtering, water is normally pretreated by passing it through sedimentation basins or holding tanks. This process removes heavier suspended solids that may cause rapid clogging of the filter. This water is brought onto the filters as the next step in the purification process. This water contains very finely divided suspended matter such as minute particles of floc, clay, and mud that have not settled, and bacteria and microscopic organisms that have not been removed by sedimentation. The purpose of the filter is to remove this suspended matter and give the water a clear, sparkling, and attractive appearance.

There are basically three types of filters. These are slow sand filters, rapid sand filters, and pressure filters.

Slow sand filters contain fine-grain sand and have low filtration rates. They are usually used when coagulation is not included in the treatment process. Their capacity is about 2 to 10 million gallons per day (mgd) per acre of filter surface. Use of slow sand filters has been practically discontinued because of their high cost per unit of capacity and the labor required to clean them. Rapid sand filters are now universally used in modern water treatment plants. There are two types, gravity and pressure. Gravity filters (fig. 9-1 5) are essentially open-top rectangular concrete boxes about 10 feet deep. An underdrain system at the bottom is covered by gravel, which, in turn, supports a 24 to 30-inch layer of fine filter sand (fig. 9-16). Gravity filters are usually designed to filter about 2 gpm per square foot of filter-bed area. However, in an emergency, up to 4 gpm per square foot can be obtained if prior treatment by flocculation and sedimentation produces very low turbidity and precholorination and postchlorination or both are effectively disinfecting the water. Approval must be obtained from the major command to operate filters at rates in excess of 2 gpm

Figure 9-14. - Diatomite filter, showing one filter element.