Short note: Catch basin

A catch basin is a small chamber built in the line between a street inlet and a storm or combined sewer, to collect and hold silt, sand, grit and floating refuse so that they do not enter and block the sewer.

Construction

         Cast iron cover
       __________________
      |                  |
      |                  |  hooded outlet
      |            ______|--->  to sewer
      |   water   |
      |   level   |  Outlet 0.3-0.6 m
      |___________|  above floor
      |  silt pit |
      |___________|

• Circular or rectangular, 0.6-1.0 m in diameter, depth 0.9-1.5 m, built of brickwork or concrete, plastered inside.
• The floor of concrete is below the outlet invert to give a sump (silt pit).
• A cast iron frame and cover or the street inlet grating at the top.
• The outlet is connected to the sewer through a trapped or hooded elbow dipping below the water level, which retains floating matter and gives a water seal against foul gas.

Working

Storm water from the inlet falls into the basin, velocity drops, and heavy particles settle in the silt pit while clear water overflows through the outlet. Floating matter stays behind the hood.

Maintenance

Periodic cleaning is needed (silt removed by buckets or a suction machine, especially before the monsoon), otherwise the basin fills up, loses its function and breeds mosquitoes.

Merits: keeps silt out of the sewer, prevents blockages, reduces sewer size by avoiding silting. Demerits: requires frequent cleaning, can give foul smell and breed mosquitoes; not needed if the sewer is self-cleansing.