Microplastic particles. Picture Max Hoffmann

Microplastics in the water – an invisible threat

The consequences of microplastics are getting more serious. Even in the remote Arctic sea ice, a record concentration of microplastics was reported last April. Conventional purification plants cannot properly filter all of the small particles, resulting in tons of it ending up in Germany's rivers and lakes every year. So how can we relieve the waters from this plastic pollution?//by Jannik Böhm and Max Hoffmann

Microplastics are an increasingly big problem. The difference to common plastic products, such as bags or packaging, is that you cannot see the small particles submerged in water. Microplastics are often defined as particles that are smaller than 5mm. In most cases they are even fundamentally smaller, having sizes that rang from micrometers to nanometers.
"Unfortunately the particles don’t just disappear by themselves, and to our current knowledge, they cannot get fully decomposed", says Dr. Melanie Bergmann, a marine ecologist at the publicly funded Research Institute Alfred Wegener Institute (AWI). The small particles remain in the environment, spread in the waters and have a pollutive impact.

Record concentration of microplastic in Arctic sea ice

A month ago, AWI reported that they found microplastic particles even in the remote Arctic. A team of AWI researchers took samples from five different Arctic regions between 2014 and 2015. In all samples collected, microscopically small particles were found. "We know that a huge amount of microplastics have already entered remote marine areas like the polar regions or the deep sea", Bergmann notes. All samples contained very high numbers of microplastics, with one having a record concentration of 12,000 microplastic particles per liter. Bergmann notes that "it will likely continue to get worse rather than better since there is currently too little being done to reduce the entry of microplastics into the water."

Microplastics in our cosmetics

Lots of cosmetic peelings and other cosmetic products consist of artificial particles, such as polyethylene, polypropylene and other synthetic plastics. By using these products, the small plastic pellets quickly enter into our public wastewater. This is a known issue. Therefore, the cosmetic producers are being asked, by the government, to perform a voluntary self-control. Several drug store chains and cosmetic producers have already announced that they will remove the microplastic from their products.
However, a 2016 study by the consumer platform Codecheck in collaboration with the non-governmental organisation for environmental protection BUND states that every third face peeling product still contains polyethylene and other microplastic particles.
Furthernmore BUNDS`s  2018 annualy shopping advisor list, states  that hundreds of cosmetic products still contain microplastic particles.

According to a 2017 study by the non-governmental organization IUCN, cosmetics consisting of microplastics make up only a small proportion of the pollution. Tire wear, plastic textile fibers and the decomposition of plastic products play a much bigger role in the cause of pollution. Apparently, more things than you might think are contributing to the pollution of our waters.
Unfortunately, most of the purification plants are unable to effectively filter all of the microplastic particles since the small particles cannot be captured by the conventional sewage cleaning machines.

Conventional purification plants

The most common purification plants remove solids from the sewage using gravity. In the first step, coarse pollutants such as packaging, textiles and hygiene materials are captured with rakes and removed from the wastewater.
By slowing down the water's flow speed in the grid chamber, mineral substances can sink to the bottom. Then the sand, gravel and stones are separated from the water. After that, the flow speed is lowered again in the preliminary settling tank, so that more fine particles can settle as sludge. Small buoyant materials, including microplastics, remain on the water`s surface. In the aeration basins dissolved organic substances in the wastewater are decomposed by using bacteria. What remains is a sludge, that subsequently settles to the bottom after lowering the flow rate again in a final step. Unfortunately, the microplastics remain in the water if the purification plants have no further filters.

Membrane filter technique as an effective alternative

The purification plant "Nordkanal" of the Erftverband Bergheim near Cologne uses a membrane filter that is installed in the aeration basins. "In membrane filtration plants, or more precisely referred to as membrane bioreactors, biomass is not simply separated by sedimentation in separate tanks, but purified wastewater is directly filtered through membranes", says Christoph Brepols, a graduate engineer at the Erftverband Bergheim. Pressure forces the wastewater through a fine membrane, capturing even the finest particles. The membrane filter consists of modules that are equipped with a variety of fine tubular hollow fibers. The maximum pore size of these membranes is less than 0.1 micrometer, resulting in all larger particles being filtered out of the water - including bacteria and viruses. Once the water has flown through the membrane, it can flow directly into lakes, rivers and oceans.
Since the bacteria that is responsible for the aeration are retained by the narrow membrane pores, their concentration can be kept higher than in conventional plants. Consequently, the basins can be much smaller than found in conventional plants. Another advantage is that additional steps are not needed once the water passes through the membrane aeration.

The Nordkanal purification plant that uses membrane treatment to purify water. Picture: Erftverband

Membrane bioreactors, like those of the Erftverband can filter out most particles according to a study by the University of Bremen. The scientists examined water samples from the outlet of selected purification plants. While there are on average, 34 to 54 microplastic particles per liter in conventional sewage treatment plants without filtration, only 4 particles per liter were found for those that use a membrane filter system.

The biggest disadvantage of a membrane filter system is its energy consumption. "Due to the filtration process, the energy consumption is basically a bit higher than in conventional technology that uses sedimentation tanks", explains engineer Brepols. Furthermore, the filters must be maintained regularly and replaced depending on the condition, resulting in significant reinvestments. In addition, the membrane systems cannot remove 100 percent of the particles from the water. Some particles are still too fine.

New filter system for gullies

A research project by Prof. Matthias Barjenbruch and Daniel Venghaus, both employed at the TU Berlin in the department of "Chair of Urban Water Management", prepares to deal with the problem a few steps earlier. They are working together in a publicly funded research project with the companies MeierGuss and Funke Gruppe to improve  gully filter systems. These filters should prevent microplastic particles, for example caused by the rubber abrasion of tires from reaching the water circulation.
The filter systems are directly installed into the gullies. When it rains, the polluted water runs through the filter and the plastic particles get captured.

Testing phase in Berlin

Currently, the gully filters are being tested in Berlin at a public street. The testing phase will end this summer, but scientists can already say that under scientific testing conditions the filters can withhold between 66 to 99 percent of the particles. "We will recommend that the filter systems be installed at hot spots for microplastic and other pollutions", scientist Daniel Venghaus points out. The purchase, installing and preventive maintenance of these filters will likely be a big expensive.

Decomposition of plastics

The favorable qualities of plastics include their persistence and structural stability, which are also the reason for their threat to the environment and its ecosystems. These qualities result in extremely long decomposition and degradation, resulting in plastics having a long-term impact on the environment. According to the German Federal Environment Agency, the decomposition time of plastics is influenced by environmental factors - such as temperature variation, light irradiation, abrasion or wave action. Decomposition can result in the formation of microplastic particles from larger materials, a process that can take up to 450 years. Even then a full degradation is not guaranteed.

What are Microplastics? Short Lesson

In general, one divide microplastics into primary and secondary plastics:

Primary microplastics
These are plastics, that are released directly into the environment. They are purposely added to product by industry (e.g. cosmetic peelings).

Secondary microplastics
These are microplastics that originate from the degradation of larger plastic material into smaller plastic fragments.

Microplastics have different shapes, sizes and origins.

Microplastic shapes:

1. Beads
These particles are spherical or cup-shaped particles with a size between a few nanometers and microm-eters. They are often used in cosmetics and other hygiene products. They predominately get into the water through domestic sewage.

2. Fibers
These microplastics are primaly from fibers of synthetic textiles that are detached during washing, having lengths of microns down to a few millimetres. These particles get into the sewage system from washing the textiles.

3. Fragments
These are small fragments with uneven shapes that comes off of larger plastic products. They are released into the environment or into the sewage system directly through unauthorized or unintentional disposal.

Die Autoren:

Jannik Böhm

Max Hoffmann

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