Everything you need to know about greywater phytoremediation in self-build construction

Greywater phytoremediation involves passing water from the shower, sink, or washing machine through a planted gravel bed, where the substrate, bacteria, and roots remove much of its organic load. Some of the work can be done yourself, but domestic wastewater treatment is regulated: the project must first be approved by your local council or the relevant authority.

The subject is attracting more and more gardeners and DIY enthusiasts, drawn by a natural, energy-efficient solution that blends into the landscape. Experts in aquatic environments since 1998, we work every day with filtering plants and pond lagoon systems. Here is what you need to know before you get started, without any unrealistic promises.

Greywater, blackwater: what are we talking about?

In a home, there are two main types of wastewater:

• Greywater comes from the shower, bathtub, sink, kitchen sink, and washing machine. It mainly contains soaps, grease, textile fibres, hair, and food scraps from the kitchen sink.
• Blackwater (or sewage) comes from toilets. It is much more heavily loaded and requires a different treatment process.

Greywater phytoremediation therefore deals with a less heavily loaded flow, making it more accessible. But "less heavily loaded" does not mean "clean": untreated greywater ferments quickly, produces odours, and can clog the soil. This is why careful design is essential.

Also note that water from the kitchen sink, rich in grease and food particles, is the hardest to treat. Many projects route it through a properly sized grease trap or direct it to a different treatment process.

The principle of the planted filter

At the heart of the system is a filter bed filled with gravel or pumice and planted with wetland species. As in a pond lagoon system, three elements work together:

• The substrate retains particles and provides an enormous surface area for microorganisms.
• Bacteria attached to the gravel and around the roots break down organic matter.
• Plants maintain the permeability of the bed with their roots and stems, which move in the wind, deliver some oxygen deep down, and absorb some of the nutrients.

There are two main approaches: vertical flow, where water is distributed over the surface and percolates through the bed, and horizontal flow, where water flows entirely through the bed while remaining saturated. Recognised systems often combine the two. The choice, surface area, and depth of the bed depend on the number of residents and the chosen treatment process: this sizing cannot be left to chance.

Regulations: the step you must not skip

In Belgium as in France, domestic wastewater treatment is regulated. Depending on your region and situation (whether or not you can connect to the sewer system, autonomous treatment zone), an individual treatment system may require declaration, a permit, or approval, and may be subject to inspections.

In practical terms, before you dig:

1. Contact your local council or the relevant wastewater treatment authority to find out the rules applicable to your property.
2. Ask which treatment processes are permitted: some regions only allow approved systems or those sized according to specific rules.
3. Clarify the permitted discharge method for the treated water: infiltration into the soil, discharge into surface water, or another method.
4. Get professional advice from a design office or installer for sizing and documentation.

What you can do yourself

DIY has its place in a well-managed project. Once the treatment process has been approved and sized, much of the work remains within the reach of a careful handyman.

Tasks that can often be carried out by an individual include light earthworks, installing the waterproofing when the system requires it, filling the bed with washed substrate, planting, and routine maintenance. However, connections to the home, choosing the correct slopes, and checking compliance are best left to a professional.

What plants for a greywater filter?

Plants in a planted filter must tolerate alternating periods of water and air, poor substrate, and a regular organic load. The most commonly used species are hardy wetland plants:

• Common reed (Phragmites australis), the benchmark plant for planted filters thanks to its powerful rhizomes.
• Yellow flag iris (Iris pseudacorus), hardy and decorative, and popular in finishing areas.
• Bulrushes, very vigorous, reserved for large areas.
• Rushes, bulrush, or water mint, to diversify the planting.

The exact choice depends on the chosen treatment process and your climate. A diverse planting is more resistant to adverse conditions and more pleasant to look at.

Maintaining a planted filter

A well-designed planted filter requires little intervention, but it is essential:

• Empty and clean the grease trap regularly: neglect is the leading cause of clogging.
• Cut off the aerial parts of the plants at the end of winter and remove them from the bed.
• Check that the water is distributed evenly over the entire surface and does not stagnate in puddles.
• Pull out unwanted plants (woody plants, brambles) that may establish themselves in the bed.
• Use household cleaning products sparingly: excessive detergents and disinfectants hinder bacterial activity.

At home, small steps make the filter's job easier: a hair trap in the shower, a screen in the sink, and using the quantities of laundry detergent recommended by the manufacturer.

Limitations and common mistakes

Phytoremediation has an excellent image, sometimes to the point of overlooking its constraints. The most common mistakes are:

• An undersized bed to save space: it clogs, water stagnates, and it smells bad.
• No pre-treatment: grease and fibres clog the substrate within a few months.
• Unwashed or too-fine substrate, which quickly loses its permeability.
• Unauthorised discharge of treated water, leading to disputes with the local council or neighbours.
• Thoughtless water reuse: water coming from a planted filter is not drinking water. If you are considering using it in the garden, find out what your regulations allow and avoid watering vegetables that are eaten raw.

If your main goal is to have a more natural garden and balanced pond water, a planted lagoon area connected to the pond, without wastewater, is a much simpler project. Our article on the pond filtered by plants details this approach.

Frequently asked questions

What is greywater?

It is wastewater from the shower, bathtub, sink, kitchen sink, and washing machine. Toilet water, called blackwater, is not included.

Can you build a planted filter for your greywater yourself?

Some of the work can be done yourself, but the project must be approved by your local council or the relevant authority, and sizing is best left to a professional.

What plants should you use in a greywater filter?

Common reed is the benchmark plant. Yellow flag iris, rushes, bulrushes on large areas, or water mint are also used.

Is water coming from a planted filter drinkable?

No. It has had much of its organic load removed, but it is not drinkable. Its use and discharge depend on local regulations.

Why is a grease trap necessary?

It retains grease, hair, and fibres that would clog the filter bed. Regular maintenance determines the filter's lifespan.

Read also

• Plants for lagoon systems and phytoremediation
• Creating a natural filter for your pond: building a planted lagoon area
• List of filtering aquatic plants