SAND FILTERS

Description

Filtering water and other liquids through a bed of sand is an efficient way of separating out solids. Sand filters can be used to polish drinking water or to remove suspended solids and associated BOD from wastewater. Sand filters can be driven by gravity or pumps.

Conventional sand filters accumulate solid at the surface of the bed. The efficiency of filtration can be improved by using layers of graded sand or anthracite in deep bed filters. The unfiltered water or effluent meets the coarsest part of the filter first and is already well cleaned by the time it reaches the finest layers. The materials regain their layered structure after backflushing because of their different sizes and densities.

In continuous sand filters the fouled sand is continuously removed from the filter bed, washed and recycled without interruption of the filtration process. The sand can be removed in an upward or downward flow and the process can be driven mechanically or by air or water pressure. Some sand filters automatically increase the rate of backwashing when the rate of solids loading increases. The discharge of separated solids in suspension must be returned to an earlier process stage or dewatered before disposal.

Application

Sand filters are used in process water treatment, drinking water treatment, recycling, cooling water treatment and industrial and municipal wastewater treatment.

Sizing

Sand filters can reduce the concentration of suspended solids in treated sewage from 100 mg/l to less than 20 mg/l, and the associated BOD from 50 mg/l to less than 20 mg/l.

A conventional sand filter sized to filter about 50 m3/hour of wastewater would require a 2 kW compressor and a 75 kW feed pump. The energy requirement would be about 025 kWh per cubic metre of wastewater filtered.

A typical continuous sand filter treating 50 m3/hour of liquid would require a 15 kW pump. There would also be a requirement for a 15 kW blower and trace heating elements which would operate occasionally.