Reed bed filters: how they purify wastewater in most small communities in France

Rémy Gourdon, Mathieu Gautier, July 2023

The Conversation

In many small towns in France, wastewater is treated using simple techniques, often reed bed filters. Far from being the whim of a few isolated environmentalists, these techniques are used in more than 5,000 wastewater treatment plants across the country.

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Increasingly frequent and severe water shortages are alerting us to the vulnerability of water resources to industrial, agricultural and domestic pollution. To protect these essential resources, we need to adapt our production systems in the long term, avoid waste and reduce pollutant emissions at source (particularly nitrates, phosphates and pharmaceutical and cosmetic residues).

But in the shorter term, we also need to treat the effluents generated by human activities, in particular domestic wastewater from our kitchens, bathrooms and toilets. These treatments are regulated by European and national legislation, accompanied by decrees setting treatment objectives and discharge thresholds. They are implemented in wastewater treatment plants using intensive or more modest processes (also known as ‘extensive’ processes, but whose operating principle is based essentially on the activity of bacteria that ingest many pollutants) depending on the characteristics of the area.

The aim of wastewater treatment is to make it less polluting, but also to regenerate resources: where regulations allow, treated water can be reused for domestic purposes or for irrigation, for example. Other high value-added resources can be extracted, such as nitrates and phosphates, which can be used as fertilisers.

Reed bed filters or intensive treatment plants?

Densely populated urban areas use intensive collective treatment plants with high treatment capacities suited to the large volumes of wastewater they generate. Their efficiency is based on high energy consumption, reagents and relatively sophisticated compact equipment.

Rural and semi-rural areas have fewer space constraints and opt either for non-collective sanitation techniques (septic tanks in particular) or for collective treatment plants with reduced treatment capacity adapted to the number of connected households. Treatment plants with a treatment capacity equivalent to 2,000 inhabitants or less account for more than 80% of the total number of plants. Of these, more than half (59%) are intensive plants based on the same principles as their counterparts in large cities. The rest are so-called extensive plants, which are less sophisticated than intensive plants and have lower investment and operating costs, but still guarantee high treatment performance. Reed bed filters account for more than 50% of these extensive plants, with the remainder consisting of lagoons or other types of technology.

The advantages of reed bed filters

Reed bed treatment systems are ‘nature-based solutions’, i.e. their operating principle is inspired by that of natural ecosystems: marshes. They offer many technical advantages:

Reed bed filters can replicate the ecosystem functions of the natural wetlands they emulate, such as marshes, peat bogs, ponds and mangroves. Thanks to their unique position at the interface between the planet’s three physical environments (water, land and air), they are home to considerable biodiversity, regulate water transfer and temperature through evapotranspiration, and contribute to water purification.

How do planted filters work?

The reed bed filtration system involves a series of simple steps, but their effectiveness requires skilled engineering.

The system consists of one or more filters in series, isolated from the ground by a ‘geomembrane’ and filled with layers of granular material through which the wastewater to be treated flows – the grain size increases with depth. Reeds are planted on the surface of the filters. These fast-growing plants develop a dense root network that facilitates hydraulic flow and oxygen transfer, creating favourable conditions for the activity of the microorganisms responsible for purification.

The wastewater first undergoes simple ‘screening’ to prevent clogging of the filters and protect the pumps used.

Treatment is then carried out by physical filtration on the surface of the porous mass, which retains suspended solids and certain micropollutants, which are captured on the surface of the particles.

A very rich microflora develops in the filter medium, composed mainly of aerobic bacteria, i.e. bacteria that use oxygen to oxidise the substances in the wastewater and mineralise them into carbon dioxide and water. These bacteria are naturally present in wastewater; those best suited to the conditions within the filter predominate, consuming the organic load and pollutants present in the water in order to multiply. In this way, water pollution is transformed into new microbial cells that are retained in the filter medium as the purified water passes through it.

Vertical flow systems are the most commonly used in France. They typically consist of two filtration stages. Each filter stage is made up of several independent cells. The surface of the filters in the first cell is regularly sprayed with a given volume of wastewater for a few days or a week; then the second cell is fed, and finally the third. The alternating saturation and desaturation of the water is particularly favourable to the microorganisms involved in purification. The water leaving the first stage is collected by drains at the bottom of the cells and treated in the second stage following the same protocol.

The purification efficiency of reed bed filters, their moderate level of technical sophistication, their low operating and investment costs, and their excellent integration into the landscape are the major advantages of this extensive purification system, which also offers other ecosystem functions beyond their main purification function. However, these systems will need to evolve to adapt to emerging pollutants and increasing treatment requirements.

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