A grease trap is a sealed 40-80 L container with an inlet and an outlet near the top and a baffle between them that forces the water to flow underneath, so that fat and oil float to the surface and are held back. At the parent page’s 100-200 L per day, water stays in a trap of that size for 4.8 to 19.2 hours, which is long enough for fat to rise but close enough to a day that a bigger trap would let the water turn septic.
Part of Greywater Recycling Systems
A grease trap is the first treatment stage of a greywater system. It lets floating fat and oil separate from wash water, protecting the sand filter and soil downstream from clogging.
Fat Floats, So A Baffle Holds It Back
Cooking fat and oil are lighter than water. If the water is kept still for a short time, fat rises and floats. The trap uses this: water enters near the top on one side, must pass under a vertical baffle, and leaves from the top on the other side, so the fat floating on the surface stays behind.
What You Need
- A sealed container of 40-80 L: salvaged drum, masonry box or large bucket (parent page)
- Inlet and outlet pipes near the top, on opposite sides
- A baffle, a vertical divider between them that leaves a gap at the bottom
- A lid with a handle, tight against flies and rain
- A skimmer, a ladle or flat scoop, and a container for the fat
Method: Build And Install
Step 1: Set the container level on firm ground, with its rim above the highest water level so it never floods back.
Step 2: Fit the baffle halfway between inlet and outlet, with its bottom edge clear of the floor and its top above the water level.
Step 3: Fit the inlet on one side near the top and the outlet on the other side, also near the top, with the outlet 5 cm or so lower than the inlet so water flows through.
Step 4: Seal the lid. The trap is sealed so flies and smells are kept in; skimming happens through the lid.
Step 5: Check the flow with a bucket of water: it should enter, pass under the baffle and leave without backing up.
Worked Example: How Long Does The Water Stay?
Retention time = volume ÷ daily flow.
- Smallest trap, largest flow: 40 L ÷ 200 L/day = 0.20 day = 4.8 hours.
- Largest trap, smallest flow: 80 L ÷ 100 L/day = 0.80 day = 19.2 hours.
Greywater turns septic within about a day. At the high end the water has spent most of that day in a closed, airless container. This is why bigger is not better: a larger trap raises the hold time and, with it, the chance that the water arrives at the filter already foul. If the trap smells of rotten eggs at the outlet, it is too big for the flow or has not been skimmed.
Skim And Empty On A Schedule
- Weekly: skim the fat from the surface (parent page).
- Annually: empty and clean the trap completely.
- What to do with the fat: the parent page suggests saving it for soap-making (Soap Making) or burning it as fuel. Burn it only in a proper lamp or stove, away from the thatch and the trap, and never pour it into any drain or onto the ground.
Common Mistakes
- Pouring hot fat in and expecting it to separate. Hot fat stays liquid and slides through; scrape pans first.
- No baffle, so the water streams straight across.
- A trap that is never skimmed. The fat layer thickens and blocks the baffle gap.
- A trap too large for the flow.
Safety
A sealed trap holds oxygen-poor air and hydrogen sulphide
The headspace of a sealed trap can contain hydrogen sulphide, which deadens the sense of smell at dangerous concentrations, so smell is no guide to safety, and also flammable gas from rotting waste. Open the lid at arm’s length, do not put your head over the opening, and never climb into a trap or tank. Keep flames away from the open lid; do not test the air with a flame. Rendered fat is a fire hazard when heated.
What’s Next
- Learn next: Sand & Gravel Filters — the next stage
- Combine with: Odor & Clogging Solutions — when a trap goes wrong