Skip to main content

Sand, grease, oil traps: need, O&M

Short answer

Sand, grease, and oil traps prevent blockages, pump wear, and treatment interference. Sand traps reduce flow velocity to 0.2-0.3 m/s for grit settling; grease/oil traps use baffles to separate floating substances. Clean grease traps weekly or at 25% capacity, sand traps monthly and pre/post-monsoon. Locate at hotels, garages, petrol stations, road inlets, and pumping station entries.

Why are sand, grease and oil traps necessary in sewer lines? Describe them with neat sketches. Briefly discuss their operation and maintenance frequency and locations adopted in road network.

Why sand, grease and oil traps are necessary

  • Sand and grit settle in sewers, reducing the capacity and causing blockages, wear pumps and fill treatment tanks.
  • Grease and oil float and stick on the sewer wall and on screens, blocking sewers, choking pumps, forming scum in tanks, interfering with biological treatment and causing odour. Petroleum oils may cause fire or explosion in the sewer. So the traps are fitted on the wastewater from kitchens, hotels, garages, petrol pumps, workshops and slaughter houses.

Description (with sketch)

Sand trap (grit chamber): a chamber which reduces the velocity of flow to about 0.2-0.3 m/s so that sand and grit settle in a sump, and the lighter organic matter remains in the flow.

Grease and oil trap: a rectangular tank with an inlet deep below the water level, a baffle and an outlet. Flow velocity is reduced, grease and oil rise to the top and are held back by the baffle; the clean water goes out from lower level.

  inlet            baffle        outlet
   -->|_____________|___          |-->
      | oil/grease ~~|~~~~~~~     |
      |              |  water ----|
      |  sand . . . .|__________  |
      |__sand pocket_|

A combined sand, grease and oil trap is used where all are present.

Operation and maintenance

  • Frequency: grease traps of restaurants and hotels should be cleaned weekly or when 25% of the volume is filled (daily in heavy kitchens); sand traps of garages and road traps at least monthly, and before and after the monsoon. Larger traps may be cleaned every 3-6 months.
  • Skimmed grease is removed with a ladle and disposed to a landfill or used for soap making. Sand is shovelled out or removed by suction.
  • Never use hot water or detergents to wash out the grease, as this only moves the problem downstream.

Locations in a road network

  • At petrol stations, workshops, garages and car washes (oil traps).
  • At hotels, restaurants and canteens (grease traps, inside the premises).
  • Along roads with heavy silt (sand traps) at the inlets and at the entry to a pumping station or the treatment plant (grit chamber).
  • At the junctions where storm drains from unpaved areas join the sewer.

✓ Answer checked by Claude Opus 5.5 (high effort)

Related questions

All Sanitary Engineering chapter 4 questions →