Automatic flushing device

Purpose

An automatic flushing device (flushing tank) is a storage tank that releases a large quantity of water at intervals, to flush the sewer. It is provided at the upper dead ends of sewers and in flat sewers, where the sewage flow is small and the velocity is below the self-cleansing value (0.6 m/s), so solids tend to settle. Flushing avoids blockages and removes deposits.

Features

• Works automatically without manual operation or attention.
• Discharges a fixed quantity of water at set intervals (e.g. once every 24 hours, or every 6 h) with a velocity high enough to scour the sewer for a distance of up to 150-200 m.
• Water is taken from the public water supply (or a tank, a river, a stream) by a small supply pipe with a control valve.
• It consists of a siphon, so there is no moving part.

Construction

        supply pipe (valve)
              |
        ______v______        manhole cover
       |   Water     |
       |   level  ___|___ bell mouth
       |         | cap  |
       |   U-siphon     |
       |_____|__________|---> sewer

1. A flush tank (brick or concrete) of about 1-2 m³ (1,000-2,000 litres) capacity, built in a special manhole at the head of the sewer.
2. A siphon pipe (inverted U) with the long leg connected to the sewer and a bell (cap) fitted over the short leg.
3. A supply pipe with a stop valve admits water slowly; the supply rate sets the interval of flushing.

Working

Water enters slowly and the level rises in the tank and in the siphon. When it reaches the crown of the siphon, the siphon starts, the air is expelled and the full tank flows out rapidly into the sewer. The siphonic action stops when the water level has dropped to the bell mouth and air enters. The cycle then repeats.

Modern practice avoids flushing tanks by giving proper slope, because the cost of water and the risk of cross connection with water supply are high.