A mulch basin is a shallow pit 60-90 cm in diameter and 20-30 cm deep dug around a fruit tree or large shrub, filled with wood chips or straw, with greywater piped in beneath the mulch so that it sinks into the soil without surface contact. Such a basin has a volume of about 57 to 191 L, so a family’s 100-200 L a day needs one or several basins, and each must drain within hours, not days.
Part of Greywater Recycling Systems
A mulch basin is the simplest greywater distribution method: a pit full of organic mulch at the root zone of a tree. The mulch holds the water, filters it a little further and keeps it from the surface.
How It Works
Water enters under the mulch and soaks into the soil below and around the pit. The mulch layer keeps flies, animals and children away from the water, shades it, and gives soil life something to feed on. The parent page lists the advantages as simple, low-maintenance and able to handle variable flow, and the disadvantage that it only suits trees and large shrubs.
How Much One Basin Holds
Inputs: diameter 60-90 cm, depth 20-30 cm (parent page). Treat the basin as a straight-sided cylinder.
- Smallest: π × 0.30² × 0.20 = 0.0565 m³ = 57 L.
- Largest: π × 0.45² × 0.30 = 0.191 m³ = 191 L.
Mulch fills some of that space, so the free volume is less. A one-day flow of 100-200 L can exceed one small basin, so use more than one, or feed them in turn. The volume only helps if the soil drains it.
The Real Limit Is How Fast The Soil Drains
A basin that holds water for days becomes a breeding pit and a source of smell. Two figures from this wiki bound it: greywater turns septic in about a day, and mosquitoes need 7 to 12 days from egg to biting adult (Mosquito Prevention). So water must be gone from the mulch well within a day, and ideally in hours.
A quick drain test. Dig a trial basin, pour in a bucket of water, and time how long until the mulch surface is dry. If it takes longer than a few hours, the soil is too tight: make the basin wider so more soil is in contact, use several basins, or use a different route. I give no sourced time threshold; this is a judgement from the figures above.
What You Need
- A fruit tree or large shrub (see Suitable & Unsuitable Plants)
- Mulch: wood chips or straw, enough to fill the basin and heap slightly
- A pipe with a steady fall, discharging beneath the mulch
- A branch pipe or tees to divide the flow among basins
- A diverter to a dry well for wet or frozen weather
Method: Dig, Fill, Pipe
Step 1: Choose the position at the tree’s drip line, not against the trunk. Water against the trunk rots the bark.
Step 2: Dig the basin, 20-30 cm deep and 60-90 cm across, with a level floor.
Step 3: Fill with mulch and heap it a little above ground level so rain runs off the pile.
Step 4: Bring the pipe in so its end opens under the mulch, not on the surface.
Step 5: Divide the flow. With several basins, feed the first, and let an overflow pipe carry surplus to the next, so none floods. Keep the overflow below the mulch too.
Step 6: Rotate and refresh. Top up mulch each season; move the main feed between basins so the soil can rest.
Common Mistakes
- Discharging onto the mulch surface. Contact and smell.
- One basin for the whole flow on heavy soil.
- Planting a vegetable bed inside the basin. Only trees and large shrubs.
- Letting the basin fill with silt and fat. A pre-treatment stage failure.
Safety
Standing greywater is a disease and mosquito hazard
If water stays in a basin for more than a day, stop feeding it and send the flow to a dry well until the soil recovers. Keep children and animals away from the basins; wear gloves when handling the mulch, which carries organisms. Do not locate basins near wells, springs or streams, and never connect the pipe to the drinking supply (Plumbing Separation).
What’s Next
- Learn next: Seasonal Maintenance Schedule — keep basins working through the year
- Combine with: Subsurface Drip Systems — the option for beds