1.6.6 Rapid Sand Gravity Filters Beds

Filters shall be located in the Filtration Plant Building in two rows separated by a Pipe Gallery. Filters Feeding Channel shall feed each Filter through its respective inlet weir. Each filter shall comprise a sand bed placed on a false slab fitted with filter nozzles. Balancing Tank shall be provided below the false slab to collect and pass on the filtered water to the Backwash & Service Water Sump through pipes and underground channels provided in Pipe Gallery. Backwash & Service Water Sump shall be located below the Machine Hall. Backwash Pumps, Aerators, Service Water Pumps and Chlorine Booster Pumps shall be installed in the machine Hall. An overflow weir shall be provided in the Backwash & Service Water Sump to retain the required quantity of filtered water for backwashing. The excess filtered water shall overflow the weir and pass on to the Clear water Tank through a pipe.

Filter Beds clean water by retaining the turbidity causing particles in the sand bed. Continuous flow of water through the filters eventually completely clogs the filters after which no flow through the filters takes place. Under this situation filtration has to be stopped to restore the filter to the original position so that filtration can be resumed. The process of clearing the filters of the clogged particles is called Backwashing and involves the reverse flow of water and air through the filters which removes the clogged particles and restores the original position of the filters.

To safeguard against any overflowing of the filter beds, an Overflow Weir shall be provided adjacent to the Inlet Weir. Overflow Weirs shall be fitted with level switches and alarm to inform the operators about the overflowing of weirs.

Drains shall be provided to collect and discharge backwash water to the Water Recovery System.

Filter Control Gallery shall be located on the first floor of the Filtration Plant Building. Consoles shall be installed in the Control Gallery to electronically control the backwashing process. The equipped installed for backwashing such as Backwash Pumps, Aerators, valves and actuators shall be electronically linked to the Console. Head loss in the filter beds shall be indicated on the Console. Rate Controller comprising Ultrasonic Level Probe, actuated valves and transmitter shall be installed to control the flow proportionate to head loss caused in the filter beds due to clogging.