Question

One proposal for treating water in developing country is for individual families to use a small, home made slow sand filter followed by storage of filtered water in a clear plastic bottle exposed to the sun for a day. Explain the purpose of the slow sand filter and the mechanism that disinfects the water.

( Please type the answers not hand writing )

Answer 1

Slow sand filters work through the formation of a layer called the hypogeal layer or Schmutzdecke in the top few millimetres of the fine sand layer.Slow sand filters slowly lose their performance as the biofilm thickens and thereby reduces the rate of flow through the filter

They utilise effluents from the plain sedimentation tanks, and are used for relatively clearer waters.

CONSTRUCTION OF SLOW SAND FILTERS •

The various parts of the slow sand filter are:

• Enclosure tank

• Filter media

• Base material

• Under drainage system

• Inlet and outlet arrangements

CONSTRUCTION OF SLOW SAND FILTER Inlet and outlet arrangements: • An inlet chamber is constructed for admitting the clarifier’s effluent without disturbing the sand layers of the filter and to distribute it uniformly over the filter bed. • A ‘filtered water well’ is also constructed on the outlet side in order to collect the filtered water coming out from the main-under drain. • Inlets and outlets are generally governed by automatic valves

OPERATION OF SLOW SAND FILTER • The effluent from the sedimentation tank is allowed to enter the inlet chamber of the filter unit and get distributed uniformly over the filter bed. • The water percolates through the filter media and gets purified. • Through gravels, the water comes out as filtered water. It gets collected in the layers through the open joints, which into the main drain. • The main drain finally discharges into the filtered water well

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