Lab 3: Build a Rapid Sand Filter



As a third step in building a water filtration plant you will build a rapid sand filter that can be backwashed. 

Rapid sand filters are a polishing step in water purification. Most of the suspended solids are removed by the flocculation and sedimentation steps that normally precede rapid sand filtration. Rapid sand filters are cleaned by reversing the flow of water and suspending the bed of sand. The agitation during the backwash is sufficient to remove most of the accumulated solids. This makes it possible for a rapid sand filter to treat large quantities of water for many years while only requiring regular backwashing.

Experimental Setup

Filter Specifications



Filter diameter 2.5 cm
Sand density 2.65 g/cm3
Sand depth 30 cm
porosity* 0.4
filtration velocity 5 m/hr
backwash velocity 50 m/hr
* (void volume/total volume of sand plus voids)

The rapid sand filtration apparatus must have the following capabilities

Apparatus schematic


The raw water for this lab contains 100 mg/L kaolin and 50 mg/L alum to make it easier to filter.