Sand, grease and oil trap

Why grease and oil must be excluded

1. They are lighter than water, float, and stick to the walls of the sewer, reducing its capacity and causing blockages.
2. They clog pumps, screens and treatment units and interfere with the oxygen transfer, so the biological treatment fails.
3. They form scum layers in the treatment tank and cause foul smell as they decompose.
4. They pollute rivers, forming a film which stops oxygen entering the water.
5. They can ignite and cause explosions if petroleum oils enter the sewer.

Sand, grease and oil trap

It is a small tank built in the line of the sewer from hotels, restaurants, garages, workshops and petrol stations to remove sand (grit), grease and oil from wastewater before it enters the sewer.

Principle of working

It works on gravity separation due to difference in density and a low velocity.

• The tank reduces the velocity of flow (to about 0.1-0.3 m/s), giving a detention time of a few minutes.
• Heavy particles like sand and grit (specific gravity about 2.65) settle to the bottom.
• Grease and oil, which are lighter than water (specific gravity about 0.9), float to the surface and are trapped by a baffle (a T pipe or a submerged outlet).
• The clear water from the middle layer flows out.

   Inlet               Baffle         Outlet
     |                   |              |
  -->|___________________|___           |-->
     |  grease/oil  ~~~~~|~~~~~          |
     |                   |  clear water  |
     |   . . . sand . . .|_______________|
     |___sand pocket_____|
        removable cover

Construction and operation

• A brick or RCC tank, with a cover and an inlet pipe well below the water level, an outlet with a dip pipe (baffle) that leaves the floating layer behind, and a sump at the bottom for sand.
• The trap is cleaned regularly: floating grease is skimmed off and the sand removed manually or by pump, otherwise it will be re-suspended and carried away.