Constructed Wetlands & Greywater

Constructed wetlands treat household greywater (sink, laundry, and bath water) using plants, gravel, and microbial communities. They turn a waste stream into a productive landscape feature while protecting groundwater.

Key Concepts

• —Subsurface flow wetlands: water passes horizontally or vertically through a gravel bed planted with wetland species; treatment occurs primarily through microbial activity in the gravel biofilm rather than plant uptake.
• —Greywater versus blackwater: greywater (from sinks and bathing) has lower pathogen load than blackwater (toilet waste) and can be treated on-site; blackwater must go to a latrine or composting toilet system.
• —Sizing guidelines: a subsurface flow wetland needs approximately 2-5 square meters of bed area per person served; larger surface areas improve treatment but are not always necessary.
• —Plant selection: cattails (Typha spp.), bulrushes (Scirpus spp.), and phragmites are the most robust treatment wetland plants across temperate and tropical climates; they tolerate waterlogged roots and feed microbial communities.
• —Sequential treatment: a simple grease trap (a covered tank that allows grease to float and settle) before the wetland extends the life of the gravel bed and prevents clogging.

Practical Guide

1. 1.Locate the wetland downslope from the house so greywater flows by gravity; keep it at least 15 meters from any well or water source and outside any vegetable garden irrigation zone.
2. 2.Excavate a bed 60 cm deep, line it with heavy plastic sheeting or puddled clay to prevent groundwater contamination, and fill with washed gravel (20-40 mm diameter) to within 5 cm of the surface.
3. 3.Install an inlet pipe at one end delivering water 30 cm below the gravel surface, and an adjustable outlet pipe at the other end set to maintain the water level just below the gravel surface.
4. 4.Plant wetland species at 30-50 cm spacing across the bed; water regularly until established (two to four weeks), after which the greywater input sustains the plants.
5. 5.Build a simple grease trap upstream: a small covered tank with an inlet pipe entering below the water surface and an outlet pipe also below the surface; clean it every one to three months.
6. 6.After three to six months of operation, collect an effluent sample and test for clarity and odor; cloudy or foul-smelling output indicates the bed is undersized or the grease trap needs cleaning.
7. 7.Harvest wetland plants annually by cutting to ground level; compost the biomass separately from food-crop compost due to possible pathogen uptake.