Rapid sand filter construction and working

A rapid sand filter (RSF) is a gravity or pressure filter in which water, normally after coagulation and sedimentation, passes down through a sand bed at a high rate (3 to 6 m/h). The bed is cleaned by reversing the flow (backwashing).

Construction

   Inlet (coagulated water)   Wash water trough
        |                      |   |
  ______v______________________v___v____ top
 |  water depth 1.0-1.5 m                |
 |~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~|
 | sand 0.6-0.75 m (ES 0.45-0.7 mm)      |
 |---------------------------------------|
 | gravel 0.45 m (graded, 4 layers)      |
 |----+---+---+---+---+---+---+---+------|
 | laterals with orifices / manifold     |-> filtered
 |_______________________________________|   water out
   wash water in <-    wash waste out ->

• Tank: an open RCC or masonry tank, rectangular in plan with length : width = 1.25 to 1.33, depth about 2.5 to 3.5 m.
• Filter media (sand): 0.6 to 0.75 m depth, effective size 0.45 to 0.7 mm, uniformity coefficient 1.3 to 1.7.
• Base material (gravel): 0.45 m in 4 layers, graded from 2 mm at the top to 50 mm at the bottom, supporting the sand and spreading the flow.
• Under-drainage system: a central manifold with lateral perforated pipes (orifices 6 to 12 mm, area about 0.2 to 0.5 % of bed area). It collects filtered water uniformly and distributes the backwash water. Other types: Wheeler bottom, porous plates, strainers.
• Wash-water troughs above the sand, inlet and outlet pipes with valves, and wash-water and wash-waste pipes.

Working

Coagulated and settled water enters at the top; it flows down through the sand, where particles are removed by straining, sedimentation, and adsorption within the pores (not on a skin like the slow filter). The filtered water leaves via the lateral pipes and manifold to the clear water tank. As impurities collect, the head loss rises from about 0.3 m to the limit of 2.5 to 3.0 m (or the effluent turbidity rises), and the filter must be washed.

Design considerations

• Filtration rate 3000 to 6000 l/h/m² (about 5 m/h) and at least two units (N = 1.22√Q, Q in MLD).
• Area = Q / rate, plus 2 to 4 % of the water for backwashing.
• Depth of water over the sand 1.0 to 1.5 m (prevents negative head), plus 0.5 m free board.
• Terminal head loss 2.5 to 3.0 m.

Operation and maintenance

• Filter runs 24 to 72 h between washes. Record loss of head, turbidity and the rate of filtration; adjust the rate with the rate controller.
• Avoid negative head (which causes air binding) by keeping enough water above the sand.
• Check for mud balls, cracks and sand loss; top up and replace sand every few years. Disinfect the filter after repairs.

Cleaning by backwashing

1. Close the inlet and let water drop to about 0.2 m above the sand.
2. Optionally scour the bed with compressed air (about 0.6 m³/m²·min for 2 to 3 min).
3. Pass clean water upward at 0.6 to 1.0 m/min (600 to 1000 l/min/m²) for 10 to 15 minutes. The bed expands by about 50 %, the sand grains rub together and the dirt is carried out by the wash-water troughs to waste.
4. Stop the wash, close the wash valve, and then reopen the inlet. Run the first filtrate to waste. Backwash uses 2 to 4 % of the filtered water.