A reed bed is a shallow lined basin of gravel planted with wetland plants such as common reed, cattail, yellow flag iris or bulrush, through which pre-treated greywater flows slowly below the gravel surface so that microbes in the root zone consume what remains. The parent page sizes it at about 1 m² per person, so four people need roughly 4 m², a loading of 25-50 mm of water a day at 100-200 L a day.
Part of Greywater Recycling Systems
A constructed wetland, or reed bed, is a managed imitation of a marsh: plants and the microbes living on their roots treat water that flows through the gravel they grow in. It is the last treatment stage before greywater reaches the garden.
How It Works
Plant roots put air into the gravel around them and give microbes a surface to live on. Close to the roots, oxygen-rich zones feed bacteria that break down soap and organic matter; farther off, oxygen-poor zones support other microbes. Together they remove more than a single aerobic film does. The parent page says these microbes break down soap, organic matter and even some pathogens; it gives no removal figure, and neither does this wiki, so do not treat the output as safe to drink or splash.
The Water Stays Below The Gravel
The parent page fills the bed with gravel and has water enter at one end and flow through the gravel and roots to the exit. Keep the water level below the gravel surface all the way. Exposed water is where mosquitoes breed (Mosquito Prevention) and where people and animals touch it. An adjustable outlet standpipe sets the level.
Sizing: Check The Parent Page’s Numbers Against Each Other
- Excavation: 1-2 m wide, 3-5 m long, 40-60 cm deep: area 1 × 3 = 3 m² to 2 × 5 = 10 m².
- Rule: about 1 m² per person, so four people need 4 m².
- Loading: 100-200 L/day ÷ 4 m² = 25 to 50 L per m² per day = 25 to 50 mm per day.
A bed 1 m × 4 m or 1.3 m × 3 m suits four people, within the parent page’s range. A household of six needs about 6 m². The detailed design is in Bed Construction.
What You Need
- A basin: 40-60 cm deep, shaped to a gentle fall from inlet to outlet
- A liner: clay, plastic or bentonite, so untreated water does not leach into groundwater
- Washed gravel, the planting medium
- Wetland plants (Plant Selection)
- An inlet pipe with a distributor and an outlet pipe with a height-adjustable standpipe
- Pre-treatment, grease trap and sand filter first
Method: Line, Fill, Plant, Run
Step 1: Excavate and shape the basin with the outlet end slightly lower.
Step 2: Line it. Overlap sheets or compact clay; no gap or tear.
Step 3: Fill with gravel to the planned depth.
Step 4: Plant, and water with clean water at first so roots establish before the greywater arrives.
Step 5: Connect the pre-treated greywater to the inlet; set the outlet standpipe so the water surface sits a few centimetres below the gravel.
Step 6: Watch for ponding at the inlet (clogging) and for plant health.
Seasonal Notes
The beds go dormant in winter but survive if the roots lie below the frost line; reduce flow in cold weather to avoid ice build-up (parent page). Harvest and thin the plants each season and compost the cuttings away from food beds.
Common Mistakes
- Feeding it raw or greasy water. The gravel clogs in weeks.
- Open water on the surface. Mosquitoes and contact.
- No liner, leaving the bed to leak into the soil and groundwater.
- Planting the bed with invasive reeds near natural waterways.
- Eating plants from the bed. The parent page calls cattail roots edible; do not eat any plant that has grown in greywater.
Safety
The bed holds partly treated greywater and must be kept apart from drinking water
Do not eat roots or shoots from the bed, do not let children play in it and never connect it to a drinking-water line (Plumbing Separation). Yellow flag iris is poisonous if eaten. Site the bed away from wells and springs, and do not build it where the soil is karst or fractured rock without a measured flow path.
What’s Next
- Learn next: Bed Construction — liner, gravel, inlet and outlet in detail
- Combine with: Plant Selection — which plants for which climate