While, for example, French regulations (Order of May 6, 1996) set the limit values for treated effluent at 40 mg/l for BOD5 [1] and 30 mg/l for TSS [2], experimental monitoring conducted over 12 consecutive months on an ecological filtration system showed results of 14 mg/l BOD5 (min = 5 mg/l; max = 33 mg/l) and 4.6 mg/l TSS (min = 2 mg/l; max 10 mg/l) at the system’s output.
Also in France, several municipalities have implemented this type of wastewater treatment and achieved better results than those required by European standards, which are stricter than French standards; to name just two, Honfleur in Calvados with a capacity of 26,000 population equivalent, and Corcoué/Logne in Loire Atlantique with a capacity of 1,600 population equivalent.
General Explanations
Several similar terms refer to this type of treatment: “phyto-purification,” “filter gardens,” “phytoremediation,” “macrophyte beds,” “planted filters.”
To achieve the ecological treatment of wastewater from a greenhouse or household (showers, toilets, kitchens...), wastewater is collected and then transferred, after screening or through a grease trap, to sealed basins using natural clay layers, compacted bottoms, or specific waterproofing installations. These basins, which are embanked and connected, also contain gravel for drainage, as well as bacterial microorganisms and plants. The water sequentially passes through each basin and is completely purified by the end. Note that the purified water must often be discharged via overflow, but discharge into a surface environment (streams, rivers...) requires official authorization from the municipality or landowner where the treated water is released. Analyses must also confirm the treatment’s effectiveness. Maintenance of these installations is essential (regular dredging), but these systems are consistently more effective than traditional systems, including in terms of logistics.
Overall, there is limited documentation, though competent specialists and professionals are becoming increasingly available. The Architecture Service of the NGO Objectif Sciences International provides you with this expertise.
Detailed System Components
The following components describe the implementation developed by the Architecture Service of the NGO Objectif Sciences International. Various alternatives exist, but this version combines all key advantages, though it requires a bit more space than each alternative when used alone. This setup produces the highest possible water quality at system output, resulting in pure water.
Pre-Filtration
While wastewater from toilets is pre-filtered through a screen (either a pit filled with medium-sized rocks or a vertical-flow PVC screen, each with a volume of 0.1 m3 to 0.2 m3 per user and an outlet at the bottom), wastewater from showers, sinks, and kitchens is pre-filtered within a grease trap (0.05 m3 to 0.1 m3 per user—if the grease trap volume is reduced, maintenance cleaning frequency increases and insufficient temperature drop results in improper function. The larger the trap, the less frequent emptying is needed).
These two parallel devices prevent clogging materials from reaching the vegetative filter bed and separately recover 80% of the dry matter in the wastewater.
The emptying of these two pre-filters follows a standard schedule (per the manufacturer’s documentation). The collected dry matter and grease are to be reused conventionally (for fertilizer spreading or compost for soil creation). The volume determines the time between each emptying.
Organic and Phosphate Filtration
A first basin of 5 m2 per user, 1 m deep, is filled with porous gravel, preferably basaltic, where water arrives at the surface, and reeds (Phragmites) are planted. The reeds’ rhizomes (root networks) harbor bacterial colonies. These are aerobic bacteria, meaning they consume oxygen and organic matter.
In this first filtration basin, phosphates in the water are absorbed by the porous gravel, while complex organic matter, which reed roots cannot absorb, is digested by the bacteria, which release simpler organic molecules that the reeds can absorb.
To ensure ongoing phosphate absorption, the gravel should be replaced every 30 years if the basin has a volume of 5 m2 per user at a depth of 1 m. However, we recommend testing the phosphate levels before replacing the gravel, as the period could be longer, which is advantageous for logistics and finances.
Other rhizome plants could replace reeds (essential for hosting bacterial colonies), but the reeds’ natural resistance to wind prevents surface clogging, adding further benefit to this model.
Nitrate Filtration
After the water flows through the first basin’s surface, it is directed into a second basin of half the size and with an elongated surface to increase flow time. Water is injected at the bottom of this second basin, which also contains porous gravel but no reeds. It is covered with sand and topped with clay-rich soil to create an airtight seal. The anaerobic bacteria here don’t consume oxygen (since none is available) but instead consume the nitrates present in the water.
In this second filtration basin, residual phosphates are stored by the gravel, remaining complex organic matter is consumed by the bacteria, as well as all remaining nitrates. The bacteria also consume simpler organic matter.
This second basin’s gravel only needs to be changed every 30 to 40 years or less (based on tests of water composition at the output).
Lagooning
Water exiting the second basin is phosphate- and nitrate-free, nearly devoid of organic matter, and clear. However, it is not drinkable because it still contains harmless bacteria that could cause intestinal issues.
This water can be released into the soil, which completes its natural purification, or collected as a pond or fire water reserve. In a warm country with mosquitoes, this lagoon should be avoided; soil filtration is preferable. In this case, verify that the water percolates deeply into the soil or create a controlled flow, avoiding standing water to prevent mosquito larvae.
Public Access Buildings
Public buildings (ERP) experience variable occupation, leading to fluctuating effluent flow. Conventional basins could dry out and harm the reeds. The NGO Objectif Sciences International’s Architecture Service designed a model for ERP with a riverbed shape to support low, medium, and high flow, allowing reeds and bacteria to thrive even during low-occupancy periods.
Under Snow?
The filtration basins operate well under snow and at sub-zero temperatures. Simply trim and lay reeds flat before snowfall to form an insulating layer, keeping the basin functional with minimal efficiency loss. Once snow melts, remove last year’s reed stalks to prevent unnecessary organic buildup, which can be composted.
Flow Control
Water is not visible in the first basin; it flows horizontally below the gravel surface, then downward. Overflow near the top prevents surface water from becoming exposed. The second basin also keeps water below the surface, avoiding mosquito access.
Reeds
The reeds perform four functions:
- The plants absorb nutrients (nitrates and phosphates), contributing actively to purification.
- The root zone (rhizosphere) supports purifying microorganisms. Oxygen produced in the leaves is returned through roots for aerobic bacteria, which are highly effective in organic matter treatment.
- Roots stabilize the filter structure, preventing clogging and ensuring maximum diffusion.
- During sunny summers, evapotranspiration can reach 10 liters per m² of the bed, reducing discharge when streams are low.
If you are interested in ecological wastewater treatment, please contact us.
