A greywater sand and gravel filter is a container about 1 m × 1 m × 0.8 m deep holding 20 cm of coarse gravel, 20 cm of fine gravel and 30 cm of sand, fed over the top and drained freely from the bottom, taking 100-200 L of greywater a day, which is a loading of 4.2 to 8.3 mm per hour. A living layer forms on the sand in 2-3 weeks, and it works only while the bed stays free-draining and aerobic, because greywater held in saturated sand turns septic in about a day.
Part of Greywater Recycling Systems
A sand and gravel filter is the second stage of greywater treatment. Water trickles down through graded layers; solids are caught on the sand and a thin biological film on the grains consumes dissolved organic matter.
The Layers, Finest On Top
The parent page’s construction, bottom to top:
| Layer | Thickness | Material | Volume in a 1 m² bed |
|---|---|---|---|
| Coarse gravel | 20 cm | 2-5 cm stones | 0.20 m³ |
| Fine gravel | 20 cm | 0.5-2 cm stones | 0.20 m³ |
| Coarse sand | 30 cm | sand | 0.30 m³ |
| Total | 70 cm | 0.70 m³ |
The container is 0.8 m deep, so 10 cm remains above the sand for water to spread. Water enters across the sand surface through a perforated pipe or splash plate and leaves through a pipe at the bottom. Coarse gravel at the bottom keeps the outlet from clogging; the sand does the filtering.
Loading Rate: How Much Water Per Square Metre
Inputs: 100-200 L per day (parent page), a surface of 1 m × 1 m = 1 m².
- Per day = 100 L ÷ 1 m² = 100 L/m² = 0.1 m per day, up to 200 L ÷ 1 m² = 0.2 m per day.
- Per hour = 0.1 m ÷ 24 = 4.2 mm per hour, up to 0.2 m ÷ 24 = 8.3 mm per hour.
The wiki’s Sand Filtration page separates a rapid filter with 0.5-1 mm sand from a slow filter with finer sand (0.15-0.35 mm) that grows a bacteria-removing film. The parent page calls for “coarse sand”, closer to the rapid type. Coarse sand clogs less when fed greywater but treats less. Do not carry the removal percentages given for slow drinking-water filters on Sand Filters to this bed: that is a different bed, ripened and run slowly with clean water.
Why It Must Drain Freely
Sand and gravel hold water in their pores. Taking a porosity of about 0.35 for sand, a typical value I recall from filter design, the sand layer alone holds 1 m² × 0.30 m × 0.35 = 0.105 m³ = 105 L.
- If the outlet were blocked or raised so the sand stayed saturated, the water would sit there for 105 ÷ 200 = 0.53 day = 12.6 hours at 200 L/day, up to 105 ÷ 100 = 1.05 days = 25 hours at 100 L/day.
- Greywater turns septic in about a day, so the saturated sand would put foul, oxygen-free water into the next stage and kill the aerobic film.
- A free-draining bed holds only a thin film of water on the grains, with air in the pores, and the film’s bacteria use that oxygen.
So keep the outlet open to the air, and keep an air vent from the bottom pipe. Feeding the bed in batches, a bucket at a time, rather than as a continuous trickle, lets air refill the pores between doses, as I recall from filter design practice.
The Living Layer
A biological film develops on the sand grains within 2-3 weeks (parent page). It eats organic matter and reduces the load of organisms. During those first weeks the filter is mostly catching solids; do not trust its output.
Method: Build, Ripen, Clean
Step 1: Build the container water-tight or lined, 1 m × 1 m × 0.8 m.
Step 2: Place the layers, washing gravel and sand first so fines do not clog the outlet.
Step 3: Fit the distributor across the sand surface and the outlet at the bottom, with an air vent.
Step 4: Run it for 2-3 weeks before counting on it, sending the water to a mulch basin or dry well away from edible plants.
Step 5: Clean when flow drops. When water stands on the sand, scrape off the top 2-3 cm, wash it and replace it (parent page). The scraping is 1 m² × 0.02-0.03 m = 20-30 L. The bacteria below recolonise the new surface quickly. Do not disturb the sand otherwise.
Common Mistakes
- Letting the bed stay flooded. Septic water and a dead film.
- Feeding grease-laden water. Fat coats the sand.
- Washing the whole bed. It kills the living layer.
- Reusing the scrapings or wash water on food plants. They are contaminated.
Safety
The sand holds the organisms it removed, and the container can hold foul gas
Scraped sand and the wash water carry faecal organisms: wear gloves, bury the scrapings in a pit away from wells and gardens, and wash hands with soap or ash and water. The output is still greywater: it is not drinkable and must not meet a drinking-water pipe. A sealed filter chamber can contain hydrogen sulphide, which deadens the sense of smell; do not lean into one or enter it.
What’s Next
- Learn next: Constructed Wetlands & Reed Beds — the polishing stage
- Combine with: Grease Trap Design — the stage that protects the sand