Under-drainage system:

Under-drainage system supports the sand bed and provides unobstructed passage for filtered water to leave the underside of the filter. This consists of a central drain connected with lateral drains as shown in Figure 1. The laterals are open jointed pipe drains placed 3 to 5 m apart at the floor stopping towards the main central drain. These laterals collect the filtered water and discharge it into the main drain which leads the water to the filtered water reservoir.

Figure 1: Plan View of Under-drainage System

An inlet chamber is constructed for admitting the effluent from the sedimentation tank without disturbing the sand layers of the filter, and disturbing over the filter bed. At the outlet, the filtered water collecting unit is provided. Section view of a typical slow sand filter is shown in Figure 2.

Figure 2: Section of Slow Sand Filter