During his last vacation at the seaside, Felix was thrilled when he discovered the high waves.
Dad, look – those are cool waves, I want to jump right in. But what are those colorful sprinkles in between?
Those aren't sugar sprinkles, they're plastic particles.
Where do they come from?
Plastic in the oceans, rivers, and other bodies of water has many sources. Plastic waste, for example, that is thrown away somewhere instead of in the trash can.
Don't people know that plastic belongs on the right?
On the right?
Yes, just like at home, plastic on the right, residual waste on the left!
Plastic waste is only one aspect of the problem. Lost or abandoned fishing equipment, known as ghost nets, pose a significant danger to marine animals and also to shipping. Fish, whales, turtles, and birds can get tangled up in them, as can ship propellers.
Poor animals!
Approximately 70% to 80% of marine waste can be described as plastic. Microplastics are particularly problematic.
What are they again?
There is no fixed definition yet. Microplastics are usually defined as solid, water-insoluble plastic particles that are smaller than 5 millimeters.
How small is that?
Like the “sprinkles” you can see between the waves. Fish and birds often mistake them for food and swallow them. And when we eat fish, it ends up on our plates.
Yuck, I don't like plastic fish! Is there a lot of plastic in fish?
Various studies have found between 1 and 14 particles per fish. These were all found in the digestive tract and not in the muscle meat. Between one and four particles per mussel were also found in mussels. Not everything that is often identified as microplastic is actually plastic. The detection methods used often do not distinguish between particles made of plastic and other materials such as sand, metal, or glass. Therefore, some results look more dramatic than they actually are. In addition, many everyday objects in the laboratory are also made of plastic. This can mean that some plastic particles do not come from the sea, but from the sample container.
Secondary microplastics make up the majority of microplastics in the oceans. These particles originate from larger plastic parts that have been broken down mechanically or through other processes. One significant source is rubber abrasion from car tires.
There are also primary microplastics.
What are they?
These particles are “micro” from the outset. Powder for various applications, granules for plastic production, or even microparticles for cosmetics.
Kosi-what?
Cosmetics such as creams, shampoos, shower gels, toothpastes. The shower gel with the little colorful beads that you saw in the store once, for example.
It looked so cool, I really wanted it. Don't the little beads stay in the sewage treatment plant? The bacteria could play with them.
But those would be big balls for the bacteria.
In a classic sewage treatment plant, the heavier solids are separated from the treated wastewater in the secondary clarifiers. Plastic is generally rather light, often lighter than water. Such particles therefore float on the surface.
Just like my plastic fish, it doesn't sink either!
It can therefore be assumed that microplastics are not completely retained by sewage treatment plants. Sewage treatment plant effluents examined contained between zero and over 400 particles per liter, with inflows ranging from a few individual particles to over 10,000 particles per liter. Polyester, polyethylene, polyethylene terephthalate, and polyamide were found most frequently, with fibers accounting for the largest proportion.
Fibers?
Fibers from clothing, for example. A fleece sweater can lose as many as 1,900 fibers in a single wash cycle.
I knew it, the washing machine is a clothing-eating monster!
On average, 2.8 particles per day come out of the sewage treatment plant for each inhabitant, so for a plant with a population equivalent of 700,000, that's two million plastic particles every day.
Compared to the inflow, it can be seenthat sewage treatment plants remove a high proportion of microplastics, 95% or more.
What happens to the plastic particles that remain in the sewage treatment plant? Do the bacteria eat them?
No, microplastics are difficult to digest. Bacteria could use them as growth surfaces. Ultimately, they end up in sewage sludge. Up to 600,000 particles and fibers per kilogram of dry matter have been found in sewage sludge in Austria.
How much microplastic is actually in rivers and oceans?
According to a study by the Federal Environment Agency in the Danube, less than 14 tons of microplastics enter Austria each year, and we send less than 41 tons onward. That may sound like a lot, but if you take into account the volume of water in the Danube near Vienna, the concentration of microplastic particles is 0.7 µg/l, which is lower than some trace substances. The input from sewage treatment plants is a maximum of 10%, thanks to the good cleaning performance of the sewage treatment plants, which are already quite capable of doing this – without additional measures or expensive additives. The precipitating agents certainly also make a certain contribution here by promoting good flocculation, in which the plastic particles are then bound. A fourth treatment stage would remove additional particles, as further treatment often requires a filter (sand filter, other filters, activated carbon). This would kill two birds with one stone. But as I said, sewage treatment plants are not the main source of microplastics; they are already doing a good job.
The total amount of plastic in the world's oceans is estimated at 5 to 50 trillion particles larger than 280µm, with samples typically collected using trawl nets covering the upper 10 to 15m of the oceans.
Plastic is part of our lives. Plastics are diverse and have advantages that we can hardly do without. Therefore, we must find ways to use them responsibly. This offers many opportunities for research.
Incidentally, there are also plastics produced by bacteria.
Even by the bacteria in sewage treatment plants?
Yes, sewage treatment plant bacteria also produce polymeric substances. There are even plans to produce technically usable plastics, known as PHA (polyhydroxyalkanoates), which are polyesters. The necessary raw materials could be obtained from wastewater, and the microorganisms would produce and store the PHA.
That's cool!
There's just one catch: the process is still too expensive and not competitive. But now let's jump into the waves...
Dad – your hair looks like a crumble cake now. Wait, I'll quickly get my little bucket and there will be 20 fewer microplastic particles in the sea!
Verweise
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Header image created with the support of ChatGPT (AI-generated).