Greywater is treated in three gravity-fed stages before it reaches the garden: a grease trap that holds back fat, a sand and gravel filter that catches solids and grows a living layer that eats organic matter, and a reed bed that polishes what is left. Each stage lowers the load but none makes the water drinkable, and in the example worked here the three stages need about 1.22 m of fall between the house drain and the garden outlet.
Part of Greywater Recycling Systems
Greywater filtration is a sequence of simple stages, each removing a different contaminant, that reduces the grease, solids, organic matter and some of the organisms in wash water before it is spread on soil.
Each Stage Removes What The Last One Missed
| Stage | Removes | Does not remove |
|---|---|---|
| Grease trap | Floating fat and oil | Dissolved soap, fine solids, organisms |
| Sand and gravel filter | Solids; organic matter eaten by a living layer on the sand (2-3 weeks to establish) | Dissolved salts; not all organisms |
| Reed bed | Remaining dissolved organic matter, soap and some organisms in the root zone | Salts; guaranteed pathogen removal |
The parent page’s sizes for the three stages: a 40-80 L trap, a filter about 1 m × 1 m × 0.8 m, and a reed bed 1-2 m wide, 3-5 m long and 40-60 cm deep at about 1 m² per person.
Order Matters
Fat fed into a sand filter coats the grains and kills the living layer. Solids fed into a reed bed clog the gravel. So the order is always trap, then filter, then bed. Kitchen water especially must pass the trap first; a household with little cooking fat can run bath and basin water straight to the sand filter, which is a judgement, not a rule from the parent page.
Water Needs Fall To Move By Gravity
Each stage uses height. Worked example, with every distance an assumed design choice, not a standard, and the 2% minimum pipe grade from Gravity-Fed Water & Plumbing Systems:
| Segment | Basis | Drop |
|---|---|---|
| House drain to trap, 5 m of pipe | 5 m × 0.02 | 0.10 m |
| Across the grease trap | Assumed inlet-to-outlet step | 0.05 m |
| Trap to filter, 3 m | 3 m × 0.02 | 0.06 m |
| Through the filter | 0.8 m deep, outlet at the bottom | 0.80 m |
| Filter to bed, 3 m | 3 m × 0.02 | 0.06 m |
| Across the reed bed | Assumed level change inlet to outlet | 0.05 m |
| Bed to garden outlet, 5 m | 5 m × 0.02 | 0.10 m |
| Total | 0.10 + 0.05 + 0.06 + 0.80 + 0.06 + 0.05 + 0.10 | 1.22 m |
A site that cannot give about 1.2 m of fall between the house and the garden needs a slope, a shorter run, or a shallower filter. Do not pump greywater unless there is no choice; a pump stores power and makes a failure point.
No Number Of Filtration Stages Makes Greywater Safe For Raw Produce
Filtration and a reed bed reduce organic load, solids and soap. Neither is a disinfection step, and nothing in this wiki establishes that either makes greywater safe to wet a leaf that will be eaten uncooked. Keep greywater off leaves and off edible parts however many stages it has passed, and put it on crops that will be cooked or on non-food plants (Contamination Risks & Limits).
The parent page carried a “unless fully treated through a reed bed” exception when this article was written; it has since been corrected to allow no exception, so the two now agree.
What You Need
- Containers or pits for each stage: a salvaged drum or masonry box, a lined pit or tank, a lined shallow basin
- Gravel, sand and plants from nearby, washed
- Pipes with a steady fall, and a diverter valve before the trap to send flow to a dry well when needed
- Access lids on trap and filter for cleaning
Common Mistakes
- Skipping the grease trap because the water “looks fine”.
- Building the stages level. Water then backs up into the house drain.
- Disturbing the sand’s living layer during cleaning more than necessary.
- Believing the output is clean. It is treated greywater, not drinking water.
Safety
Greywater at every stage is unsafe to drink, touch carelessly or store
Keep children and animals from the traps, filters and beds. A sealed trap or tank can hold hydrogen sulphide, which deadens the sense of smell, and lack of oxygen: do not lean into it, and never enter one. Keep every stage’s pipes physically separate from drinking-water pipes (Plumbing Separation).
What’s Next
- Learn next: Grease Trap Design — the first stage
- Combine with: Garden Irrigation from Greywater — where the treated water goes