Plants and Microorganisms: How to Clean Wastewater Naturally

Plants and microorganisms
How to clean wastewater naturally

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Interest in constructed wetlands is increasing. It is not surprising, because planted soil filters offer several advantages. But there are also challenges. The tanks required for this are large and can only clean a limited volume of water at a time.

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

In constructed wetlands, wastewater is directed into gravel and sand basins planted with plants such as reeds. Solids, such as faeces, are usually removed beforehand in a preliminary clarification process, for example by settling in a so-called settling pond. Water is purified through the interaction of plants, soil materials, air and, above all, microorganisms. There are usually several reservoirs, although they are never all put into operation at the same time so that the biology can remineralize during rest phases.

Planted soil filters provide benefits

Experts talk about planted soil filters instead of artificial wetlands. “Cleaning does not come from plants, as is commonly thought, but from microorganisms,” says Roland Müller, who has been researching natural wastewater treatment plants for a long time at the Helmholtz Center for Environmental Research (UFZ) in Leipzig. Rather, the plants ensure that the cleaning biofilm can form. Once the water is clean again, it can be taken, for example, to the nearest stream or simply filtered.

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

Mostly small facilities in Germany

Artificial wetlands are therefore found not only among supporters of an ecological lifestyle, such as in the ecovillage of Sieben Linden in Saxony-Anhalt, but also in rural settlements without connection to large sewage systems, such as in Theres or in some campsites. Most facilities are quite small and only clean the wastewater of up to 50 inhabitants (so-called small wastewater treatment plants). Officially we talk about population values, that is, population plus commercial wastewater. But it is also possible to do more. Even abroad, there are large installations, says Müller. “Probably the largest country 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. Toilets and sewage disposal are still not available in many areas of the world, which can lead to the spread of disease. This draws attention with the so-called World Toilet Day, which is celebrated on November 19. “Constructed wetlands are relatively easy to manage, especially in areas with little money, electricity and knowledge, but with a lot of space,” says Martina Stockbauer, deputy spokesperson for the working group “Wastewater pond systems” of the State Office of Bavaria for the Environment (LfU). But artificial wetlands are also relatively widespread in France.

They are definitely up to date with the technical systems.

“In 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. There is currently debate in the EU to what extent wastewater treatment plants should be equipped with the so-called fourth stage of purification for substances such as pharmaceuticals. While there is still skepticism at the Federal Environment Agency about whether constructed wetlands generate such micropollutants, researcher Müller is optimistic. “Some bacteria can also break down exotic objects.”

But planted soil filters also have disadvantages: they require a relatively large amount of space; Although the newer ones are now more space-saving, water remains in the containers 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 cleaning and personal care products if the water is going to a wastewater treatment plant.

According to the Bavarian State Water Management Office, all types of agricultural wastewater also have a negative effect on wastewater treatment plants. The Theres community notices it. According to Mayor Matthias Schneider, water washed away from agricultural land occasionally causes scaling. “That’s why we ask the owners of adjacent properties to divert water from agriculture in other ways,” says Schneider.

Plant-based wetlands are not new

The plant in Theres, Bavaria, received an innovation award in 2016. However, constructed wetlands are usually 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 subject intensified,” 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 wastewater treatment plants in 2020. Small private wastewater treatment plants are not included.

According to its mayor Matthias Schneider, the Theres pond system works perfectly and is clearly a model to follow. “Other municipalities continue to ask and want to take a tour,” says Schneider. Despite their many advantages, constructed wetlands are not a sure success, says Stockbauer of the LfU. “Some communities underestimate the fact that even natural facilities need care and maintenance; for example, foreign growths must be removed.”

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