Slow Sand Filter

Slow sand filter- Slow sand filter consist of a water tank basin containing sand layer (0.75-0.9 m thick), supported on a layer of gravel (0.2-0.3 m thick). The gravel is underlain by a system of open joint under drains which lead the water towards the outlet where a device is generally located to control the rate of flow through the filter. The effective size of sand to be maintained is 0.2 – 0.3mm and the uniformity coefficient as 2 – 3. The gravel is usually placed in four layers for a total depth of 0.3m graded from 2-45mm. In this type of filter, coagulation as pretreatment is not required, however if the turbidity is more than 50 ppm as in case of rainy season coagulation would be essential.

Filter operation- It is filled with water to a depth of 1-1.5m above the sand bed. The water percolates through the layers at a rate of about 100-150 LPH/Sqm., until the head loss reaches 60cm. When the head loss increases, it is uneconomical to run the filters. The filter is then put out of service and about 20-30mm sand at the top is scrapped off. After this, fresh sand is added to maintain the required depth of sand layer.