Sewage Treatment Plant - Pre Screening (Bar Screen Chamber and Oil & Grease/Grit Trap: Functions, Operation, and Maintenance)

BAR SCREEN CHAMBER

The function of the bar screen is to prevent entry of solid particles above a certain size; such as plastic cups, paper dishes, polythene bags into the STP.

The screening is achieved by placing a screen made out of vertical bars, placed across the sewage flow.

The gaps between the bars may vary between 10 and 25 mm.

Larger STPs may have two screens: A coarse bar screen with larger gaps between bars, followed by a fine bar screen with smaller gaps between bars.

In smaller STPs, a single fine bar screen may be adequate.

1 Inlet pipe for the STP.

2 Debris

3 Muck accumulates

4 Grill. Must be cleaned regularly to avoid

a build-up of debris (2) and muck (3).

5 Screened sewage.

6 Outlet pipe (goes to the Equalization Tank)

7 Platform with weep holes. The STP operator stands here to rake the debris (2).

Operation And Maintenance Considerations

Check and clean the bar screen at frequent intervals
Do not allow solids to overflow/ escape from the screen
Ensure no large gaps are formed due to corrosion of the screen
Replace corroded/ unserviceable bar screen immediately

Oil And Grease/Grit Trap

The grease and grit trap is placed at the discharge point of the canteen/ kitchen area itself to arrest solid and fatty matter at source. The wastewater output from this unit is taken to the equalization tank.

The solids and fats that are separated in this unit are disposed off along with other biodegradable waste.

1 The incoming liquid is released below surface through a T-joint so that the falling water does not disturb (break up) the floating film of fat and scum (3).

2 The tank is always filled till this level.

3 The fat and scum rise to the top and float on the liquid. This needs to be removed periodically, otherwise it will leach into the wastewater.

4 The heavier grit and solids sink to the bottom of the tank. This mass also needs to be removed from the tank periodically.

5 The baffle plate prevents the floating fat and scum (3) from drifting towards the outlet (7).

6 Wastewater reaching the outlet side is free of fat, scum, grit and solids

7 The outlet is through a T-joint pipe, similar to the inlet (1).

Operation And Maintenance Considerations

Check and clean trap at frequent intervals
Remove both settled solids (at bottom) and the floating grease

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