Slow Sand Filter:

Slow sand filters are the original form of filtration used for potable water treatment and are becoming obsolete because of their large size and inability to deal with highly turbid water. However, for raw water with less than 30 NTU turbidity they can provide a very effective form of treatment that is particularly good at removing harmful micro-organisms. Much of the purification, which takes place in a slow filter, is achieved within the surface layers of the bed and the biological activity, which generates a surface layer known as the schemultzdeke, contributes to the removal of fine particulate matter and also causes some removal of taste and odour forming organic compounds. Slow filters usually operate for several months before surface clogging grows to the point, while the flow rate can no longer be managed. At this point surface scrapping, manually or through machine restores the flow and the bed can be kept back within service. To avoid disturbance of the bed surface, a depth about 1 m of water above the bed is used and this provides the head required for flow through the unit.

It consists of an open rectangular tank made of masonry or concrete. The bed slopes at around 1 in 100 towards the central drain provided for outlet of filtered water. Depth of tank varies around 2.5 to 3.5 m. The plan area of the tank varies from 100 to 2000 sq.m or more which depends on the quantity of water to be treated. The base material is gravel and the sand layer is supported on this gravel layer, which is 30 to 75 cm thick of different sizes placed in layers. The coarsest gravel is placed in the bottom most layers and the finest one on the top. The bottom most layer of the gravel is having size 40 to 65 mm, the intermediate layers varying from 20 to 40 mm to 6 to 20 mm (when two intermediate layers used) and the top most layer having size range of 3 to 6 mm. Size of sand varies from 0.2 mm to 0.4 mm averaging around 0.3 mm. The sand should be of the same grade in all parts of the same filter so that frictional resistance and hence, the rate of filtration shall be uniform. Chemical composition of sand is also an important factor. A sand containing considerable amount of lime will increase the hardness of filtered water. Presence of aluminous and calcareous materials increases the resistance to the flow of water. In designing a filter, it should be noted that the sand in fact forms the filtering medium; the gravel serves simply to collect the filtered water with little resistance to flow. The original depth of sand must be sufficient to form an effective filteraction and to allow several scrapings without renewal of the sand. The original thickness of sand bed is kept around 90 cm.