Household sewage can be divided into two types — Greywater and Blackwater. Greywater comes from kitchens, washing basins, laundry, sinks, and showers. Blackwater comes from toilets and contains urine and feces.
If collected separately, greywater is easy to filter and reuse for irrigation. Separated greywater has a low concentration of pollutants and is far easier to treat than mixed wastewater. This makes it ideal for off-grid areas with no central sewerage system.
The most important criteria for greywater reuse systems are that they be low-cost and simple — as most of those who need greywater reuse are low-income communities.
Natural Greywater Treatment Systems
Natural greywater treatment uses a hybrid system of constructed wetlands filled with sand, gravel, and water-filter plants. Many naturally occurring plants use toxins in greywater as nutrients — such as the Canna plant.
1. Hybrid Constructed Wetland System
Greywater from washing basins, laundry, and kitchen sinks is collected separately and passed through a series of three treatment chambers.
Chamber 1 — A plastic tank (1m long × 1.5m wide × 1m deep) filled with river sand and gravel. Perforated pipes at the bottom distribute greywater at a max 2% slope. Water filter plants such as Canna, Leptochloa fusca, Typhadomengensis, and Cyperus are planted on top. Greywater passes through the sand and gravel, leaving pollutants such as soap particles behind while plant roots absorb harmful nutrients.
Chamber 2 — A shallower plastic tank (1.5m wide × 1m long × 30cm deep) containing only water and floating plants such as water hyacinth, water lettuce, cattail, and water mint. Water hyacinth has very fibrous roots that absorb harmful minerals. After 2–3 weeks, the hyacinth is removed and composted as farm fertilizer.
Chamber 3 — A final plastic tank (1.5m wide × 1.5m long × 30cm deep) filled with water filter plants such as Syngonium, Umbrella Palm, Lotus, and Water Lily. The outlet drain at the top of each tank keeps greywater in the system for sufficient treatment time. Water from this chamber is clean enough for irrigation use. A grease container can also be added to the main sewage pipe to trap grease before it enters the system.
2. Reed Bed System
A reed bed is a natural filtration system that treats and improves water quality before it is discharged. Reeds are tall grasses found around water bodies — their roots break down bacteria and clean water naturally. Reed beds are a low-cost, low-tech, zero-energy solution that also provide a rich wildlife habitat.
Greywater is collected separately in a pipe and directed to a trench dug a few feet deep.
A plastic sheet lines the trench with layers of coarse aggregate, gravel, and sand placed on top. The only opening is the greywater drainage pipe at the bottom.
Reed beds are planted at the top. As greywater passes through, the reed roots break down, decompose, and purify the water as it travels to the other end of the trench.
After a few days, filtered water at the outlet can be collected in a small pond or directed to fields for irrigation. Treated water is not suitable for human consumption — irrigation use only.
