A subsurface greywater system buries perforated pipe 15-20 cm below the surface of a garden bed, but a bare pipe with 5 mm holes passes about 59 L/h through a single hole at 10 cm of head, seven times the roughly 8.3 L/h that a household makes at 200 L a day, so it waters only its first few holes. Spread the water along the whole line by laying the pipe level in a gravel-filled trench, or by dividing the garden into short zones, and flush it monthly with clean water.
Part of Greywater Recycling Systems
Subsurface distribution releases treated greywater below the soil surface through perforated pipe, so that plants take it up at the roots and people and animals never touch it. Its weakness is that a gravity-fed pipe has no pressure to push water evenly along its length.
What The Parent Page Specifies
Perforated pipe buried 15-20 cm below the surface of garden beds; greywater seeps through the perforations into the root zone. The holes must be large enough that soap residue does not clog them, 5 mm or more, and the system is flushed monthly with clean water.
Worked Example: Where Does The Water Come Out?
Inputs: one hole of 5 mm diameter, a water head of 0.10 m above it (the water depth in a gravity pipe is small, so this is generous), discharge coefficient 0.6 (the textbook value for a sharp-edged round hole, recalled, not a figure in this wiki).
- Hole area = π × 0.0025² = 1.96 × 10⁻⁵ m².
- Speed through the hole = √(2 × 9.81 × 0.10) = 1.40 m/s.
- Flow = 0.6 × 1.96 × 10⁻⁵ × 1.40 = 1.65 × 10⁻⁵ m³/s = 0.0165 L/s = 59 L/h.
The household’s flow at 200 L/day is 200 ÷ 24 = 8.3 L/h, and a family may release it in bursts of far more. So one hole can pass about seven times (59 ÷ 8.3 = 7.1) the whole average flow. The first holes release all the water, the rest stay dry and the first stretch is flooded. That is a flaw in the plain design, not in the arithmetic. Compare the 1-8 L/h per emitter of the pressure-compensated drip systems in Drip Irrigation: greywater cannot use those, because their fine outlets clog.
Three Ways To Spread The Water Evenly
- A level pipe in a gravel trench. Lay the pipe level in a trench of coarse gravel 15-20 cm deep, with holes facing down. Water leaving the first holes fills the gravel and runs along it to later holes, as in a drain field.
- Short zones. Divide the bed into short runs, each with its own valve or mulch basin fed in turn, so no zone has long dry stretches.
- A tipping dose. Collect a batch in a small chamber and release it quickly through the pipe so the whole length fills before water drains out. This needs a siphon or tipping trough and is more work to build and keep clear.
Each costs more than the parent page’s bare pipe, but each puts water where the plants are.
What You Need
- Perforated pipe with holes of 5 mm or more, or slotted pipe
- Coarse gravel for the trench
- Mulch 10-15 cm over the trench to hold moisture and keep the water out of contact
- A flushing point: a capped end or an access box
- A diverter upstream
Method: Lay And Run
Step 1: Mark the bed and dig a trench 15-20 cm deep.
Step 2: Lay the pipe level, with the holes down, on a 5 cm bed of gravel; cover with gravel to the top.
Step 3: Cover the gravel with soil or mulch, so no water is exposed.
Step 4: Feed the line from the filter, and watch the first week to see where water appears.
Step 5: Flush monthly with clean water through the cap or access point (parent page); more often if the flow slows.
Common Mistakes
- Long runs of bare pipe. Only the first holes water.
- Small holes. Soap and fat clog them quickly.
- Planting roots close to the pipe that grow in and block it.
- Burying it under root crops. Keep to the plants on the approved list.
Safety
The pipe is a greywater line and must never touch the drinking supply
A buried pipe is easily connected by mistake to a hose or tap. Mark it and keep it apart (Plumbing Separation). Flush water and the contents of access boxes are contaminated: wear gloves, wash hands afterwards and keep children out of the bed. Do not enter an access chamber.
What’s Next
- Learn next: Suitable & Unsuitable Plants — what to put above the pipe
- Combine with: Mulch Basin Distribution — the simpler alternative for trees