A reed bed for four people is a lined basin about 1 m wide, 4 m long and 0.5 m deep filled with clean gravel, which needs about 14.6 m² of liner and 2.0 m³ of gravel and holds greywater for 2.8 to 5.6 days at 200 to 100 L a day. The water level must sit below the gravel surface, the bed floor needs a gentle fall to the outlet, and the gravel must be washed and uniform because clogging, not size, is what makes these beds fail.
Part of Greywater Recycling Systems
Reed bed construction is the sequence of excavating, lining, filling, plumbing and planting the gravel basin that forms the last treatment stage of a greywater system. The dimensions follow from the number of users and the flow.
Dimensions From The Parent Page
The parent page: excavate a shallow basin 1-2 m wide, 3-5 m long and 40-60 cm deep, about 1 m² of bed per person, line it with clay, plastic or bentonite, fill it with gravel, plant it, and let water enter at one end and leave through a collection pipe at the other. For four people: 4 m². Worked example below uses 1 m wide, 4 m long, 0.5 m deep.
Worked Example: Quantities
Inputs: length L = 4 m, width W = 1 m, depth d = 0.5 m, anchor margin m = 0.3 m (assumed).
- Excavation = 4 × 1 × 0.5 = 2.0 m³.
- Gravel = about 2.0 m³ (the basin filled with gravel to its rim).
- Liner, treating the sides as vertical and adding the anchor margin on every edge: length = L + 2d + 2m = 4 + 1 + 0.6 = 5.6 m; width = W + 2d + 2m = 1 + 1 + 0.6 = 2.6 m; area = 5.6 × 2.6 = 14.6 m². Sloped sides need more.
Worked Example: How Long The Water Stays
Assume the water surface sits 0.4 m deep in the gravel, leaving 10 cm of dry gravel on top. Taking a porosity of 0.35 for clean gravel, a value I recall from design practice, not a figure in this wiki:
- Wet gravel volume = 4 × 1 × 0.4 = 1.6 m³.
- Pore volume = 1.6 × 0.35 = 0.56 m³ = 560 L.
- Retention = 560 ÷ 200 = 2.8 days at 200 L/day, and 560 ÷ 100 = 5.6 days at 100 L/day.
Several days of slow flow through plant roots is the point of the bed; it is also several days of standing water, which is why the bed must be aerated by roots and never be fed septic water.
Worked Example: Will The Water Get Through The Gravel?
Water in the gravel follows Darcy’s law: Q = K × A × i, with K the hydraulic conductivity of the gravel in metres per day, A the cross-section of flow and i the slope of the water surface.
- Flow cross-section A = width × water depth = 1 × 0.4 = 0.4 m².
- Fall of 1% over 4 m (about 4 cm, a figure I recall as typical practice): i = 0.04 ÷ 4 = 0.01.
- Q = K × 0.4 × 0.01 = 0.004 K cubic metres per day.
- To pass 0.2 m³/day (200 L/day) requires K = 0.2 ÷ 0.004 = 50 m per day.
Clean gravel passes water at many times more than 50 m a day; I recall hundreds to thousands of metres a day, not a figure in this wiki. The unused margin is what clogging by fats, soap film and roots eats up. Once K falls below 50 m/day, the water backs up to the surface. So keep the gravel clean and uniform, keep pre-treatment working, and watch for ponding at the inlet.
What You Need
- A shaped basin with a floor falling gently to the outlet end
- A liner of clay, plastic or bentonite, checked for tears
- Washed uniform gravel with fines removed. The parent page gives no size; too fine clogs, too coarse short-circuits the water
- An inlet distributor: a perforated pipe across the full width at the inlet end, in a short band of coarser stone
- An outlet collector: a perforated pipe along the floor at the outlet end, joined to a vertical standpipe whose height sets the water level
- Plants, cold-hardy where winters are hard (Plant Selection)
Method: Build In Order
Step 1: Excavate to the depth, with a gentle floor fall to the outlet.
Step 2: Line it, with the sheet folded into the anchor margin. No holes; a puncture leaks untreated water into the ground.
Step 3: Place the outlet collector and standpipe, then the gravel, washed and carefully placed.
Step 4: Place the inlet distributor across the inlet end.
Step 5: Test with clean water. Fill the bed and watch the level for 24 hours. Falls beyond evaporation mean a leak.
Step 6: Plant, and run clean water until the roots establish, then connect the pre-treated greywater and set the standpipe so the surface is a few centimetres below the gravel.
Common Mistakes
- Unwashed gravel, whose fines clog the pores from day one.
- A standpipe set too high, leaving open water on the surface.
- No liner or a torn one.
- Feeding it before the roots have grown.
Safety
The bed receives treated greywater, never drinking water, and nothing from it is safe to touch freely
Keep the bed away from wells and springs, never connect it to a drinking-water pipe (Plumbing Separation), and keep children and animals out. Dig the basin with the walls sloped back in loose ground, and keep spoil heaps from the edge. In fractured rock or karst, a liner does not make a site safe; do not build without a measured flow path.
What’s Next
- Learn next: Garden Irrigation from Greywater — the beds that receive the bed’s output
- Combine with: Seasonal Maintenance Schedule — keep the bed working