Plants and microorganisms: how to clean wastewater naturally

Plants and microorganisms
How to Clean Wastewater Naturally

November 19, 2023, 12:27 pm

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Interest in constructed wetlands is increasing. No wonder, as planted soil filters offer several advantages. But there are also challenges. The basins required for this are large and can only clean a limited volume of water at one time.

Tall reeds, dense plants, if you look closely you can see some water in the middle – in fact, they look like large overgrown lakes. But the smell is sometimes unusual. No wonder, because the reed-covered basins in the small Lower Franconian community of Theres in the Haßberge district near Schweinfurt are a sewage treatment plant, or more precisely, a built sewage treatment plant.

In constructed wetlands, wastewater is conveyed to basins with gravel and sand planted with plants such as reeds. Solids such as feces are usually removed beforehand in a preliminary clarification process, for example by depositing them in a so-called settling pond. Water is purified through the interaction of plants, soil materials, air and, above all, microorganisms. There are often several tanks, although they are never all operated at the same time, so that the biology can remineralize during the resting phases.

Planted soil filters bring advantages

Experts talk about planted soil filters rather than constructed wetlands. “Cleanliness does not come from plants, as is commonly thought, but from microorganisms,” says Roland Müller, who has long researched natural sewage treatment plants at the Helmholtz Center for Environmental Research Leipzig (UFZ). Instead, plants ensure the formation of the cleaning biofilm. Once clean again, the water can, for example, be led to the nearest stream or simply run off.

Müller is under the impression that demand for the systems is increasing. Not only because of their ecological image, but also because they have low operating costs. Plant-based wetlands have several advantages. According to the German Association for Water Management (DWA), they require little energy, less sewage sludge is produced and the plants improve the microclimate. The facility in Theres operates entirely without electricity. There is also little need for staff.

Mainly small systems in Germany

Constructed wetlands are therefore found not only among advocates of an ecological lifestyle, such as in the eco-village Sieben Linden in Saxony-Anhalt, but also in rural settlements without connection to larger sewage systems, such as in Theres or at some campsites. Most systems are quite small and only clean the wastewater of up to 50 residents (so-called small sewage treatment plants). Officially we talk about population values ​​– that is, population plus commercial wastewater. But it is also possible to do more. There are even large-scale systems abroad, says Müller. “Probably the largest in the world, with several hundred hectares, is in Oman.” There it cleans contaminated water from oil production.

Plant-based wetlands also appear to be an option for poorer countries. In many areas of the world, toilets and wastewater disposal are not yet available, which can lead to the spread of disease. The so-called World Toilet Day, on November 19th, draws attention to this. “Especially in areas with little money, electricity and know-how, but a lot of space, constructed wetlands are relatively easy to operate”, says Martina Stockbauer, deputy spokeswoman for the “wastewater lagoon systems” working group. of the Bavarian State Office for the Environment (LfU). But constructed wetlands are also relatively widespread in France.

They definitely keep up with the technical systems

“For size class 1 systems, constructed wetland systems can clean as well as a technical system,” says Stockbauer. Size class 1 corresponds to up to 1000 population equivalents. According to Stockbauer, drug residues could also be removed using constructed wetlands in combination with activated carbon. The question of the extent to which wastewater treatment plants need to be equipped with the so-called fourth stage of purification for substances such as pharmaceuticals is currently being discussed in the EU. Although there is still skepticism at the Federal Environment Agency about whether constructed wetlands create such micropollutants, researcher Müller is optimistic. “Some bacteria can also break down exotic things.”

But planted ground filters also have disadvantages: they require a relatively large amount of space – although newer ones have saved more space, water stays in basins for a long time and fluctuating amounts of water are difficult. “In addition, with natural systems you can intervene less and, for example, change the ventilation,” says Stockbauer. Additionally, people should only use biodegradable personal hygiene and cleaning products if the water is sent to a sewage treatment plant.

According to the Bavarian State Water Management Office, all types of agricultural wastewater also have a negative effect on the wastewater plant. The Theres community realizes this. According to Mayor Matthias Schneider, water taken from agricultural land occasionally causes scale. “We are therefore imploring adjacent property owners to divert water from agriculture in a different way,” says Schneider.

Plant-based wetlands are not new

The factory in Theres, Bavaria received an innovation award in 2016. However, constructed wetlands are generally not new. “The first considerations about the precursors of constructed wetlands were probably made around 1870, more concrete scientific approaches emerged around 1960 and, from 1980 onwards, studies on the topic increased”, says Stockbauer. The first municipal facilities were built in Bavaria in 1990. Some are still in operation today.

Neither the Federal Environment Agency nor the German Association for Water Management (DAW) know how many there are currently in Germany. Rhineland-Palatinate reported 31 municipal sewage treatment plants in 2020. Small private sewage treatment plants are not included.

According to its mayor, Matthias Schneider, the pond system in Theres works perfectly and is obviously a model. “Other municipalities keep asking and want to do a tour,” says Schneider. Despite the many advantages, constructed wetlands are not a surefire success, says LfU’s Stockbauer. “Some communities underestimate the fact that even natural facilities need care and maintenance, for example, extraneous growth must be removed.”