Microplastics in filtered sewage sludge
Nowadays, our groundwater and wastewater are highly contaminated with microplastics. At least the latter is largely cleaned of the plastic particles in wastewater treatment plants. The microplastic that is filtered out collects in the sewage sludge. About 80 % of the plastic particles from the wastewater are retained in the sewage sludge. Afterwards, the sludge goes through a filtration process – but studies show that significant amounts of microplastics are also found in the filtered sewage sludge. This means that the particles cannot yet be satisfactorily removed during sewage sludge filtration.
We have a new solution for this in the short term. Together with ReTech Resources Technology GmbH, we have developed a special separation filter and an accompanying overall process for sewage sludge treatment, which will contribute to a significant increase in filtration success with regard to microplastics in the future. In the article below, we will outline the problem of microplastic generation and microplastic residue in sewage sludge treatment to present and give you an insight into our innovative solution for pre-filtration.
What is microplastic actually and how is it produced?
As a rule, microplastics are all plastic particles that are smaller than 5 mm in diameter. They are not dissolvable in water or otherwise degradable.
Microplastics are a constant companion in our society today. The particles are often added to products, especially in the cosmetics sector – such as toothpaste, skin creams and shower gel. Here they are supposed to remove dead skin or improve the consistency as a binding agent. This intentionally produced form is called primary microplastic.
However, the plastic particles can also be produced secondarily during the decomposition of larger plastic parts. Car tyre wear on the street, for example, or clothing made of synthetic materials, have a significant share in this process. Large quantities of microplastics are also produced unnoticed from these sources.
How does microplastic get into wastewater and the broader water cycle?
Plastic particles find their way into the water cycle in many different ways. Outdoor microplastics, such as tyre wear or plastics from playgrounds, paints and litter, are washed directly into groundwater with the rainfall. In this way, the plastic particles reach surface waters, and ultimately the soil and the sea. Microplastics from cosmetics enter the wastewater mainly through the body hygiene, the plastic from synthetic clothing through the washing cycle. Due to the residues of plastic particles in filtered wastewater and especially in sewage sludge, which is applied to agricultural land as fertiliser, this plastic also accumulates in soils and seas.
However, this is not the end of the microplastic’s journey. Marine animals absorb the particles with their food – and ultimately we do, through the consumption of seafood and fish. There is currently insufficient research on the effects of plastic consumption on the body.
Current status of the filtration of microplastics from wastewater and sewage sludge
Currently, microplastics are removed from wastewater at various points in the treatment process. Coarser plastic particles can be pre-filtered via the screenings. In the grit trap and grease trap, a differentiation is made according to weight: heavier particles can be separated in the grit trap; light, floating particles are extracted in the grease trap. Microfibres and other very small particles can settle with the help of water-soluble flocculants and be sucked off. So far, these processes remove up to 95% of microplastic particles from wastewater.
More problematic is the residue of microplastics in the sewage sludge produced in the filtration process. According to the current state of the technology, high levels of microplastics remain here even after filtration. Up to now, sewage sludge treatment has consisted of a pre-cleaning stage using a strainer or sieve filter, followed by a decanter, centrifuge and chamber filter press. The sludge is then disposed of or transferred to agricultural use – with the remaining plastic.
New solution for
pre-filtration in sewage sludge treatment
Our sewage sludge treatment process provides for pre-filtration with an innovative separation filter. The filter, which is set up at an angle, is equipped with a motor-driven filter element in the front area. This rotating filter element is equipped with two scrapers. Due to the rotating movement, the resulting filter cake can be separated and discharged into the filter housing. This filter cake contains all components of the sewage sludge that are not organic – including microfibre and microplastic particles. In this way, between
80 and 95 % of the plastic particles can be removed from the sewage sludge.
The filter cake is fed into a press chamber via a spiral screw. Here it is pressed, while the organic substances and liquids are discharged again. Once the filter cake has been dehumidified in this way, it is automatically removed from the system via the spiral conveyor once a certain filling quantity has been reached. While the filter cake can now be disposed of in a controlled manner, the pre-cleaned sewage sludge can be transferred to further treatment.
Innovative overall process
In this further sewage sludge treatment, which was also newly developed, the sludge passes through ultrasonic disintegration, solid-liquid separation and additional filtration. Heavy metals and other pollutants as well as resistant germs and antibiotic residues are separated. The overall process thus combines mechanical, chemical and biological process technologies. In addition, phosphorus and nitrogen are recovered.
Are you interested in the new process for sewage sludge treatment? We will be glad to consult you on the use of the innovative microplastic filtration in your wastewater treatment plant.
Advantages for sewage sludge utilisation – and the environment
The filtration process described makes a significant contribution to the ecological recovery of sewage sludge. In addition to microplastic filtration, other pollutants can also be removed from the system, especially when using the proposed overall process. As a result, when the organic sewage sludge is used for agricultural purposes, no harmful substances and plastics are returned to the environment. There is no loss of carbon needed in agriculture. Instead, a nutrient-rich, humus-like product is obtained.
The water from the sewage treatment process can also be used for agricultural purposes, such as irrigation. In addition, CO₂ can be saved due to dry substances values of about 40 % of the separated, inorganic components, and the dewatering process does not require the use of synthetic polymers at all.
With the help of the newly developed and patented technology for sewage sludge pre-filtration, a major factor in microplastic contamination can be significantly reduced in the future. The innovative separation filter, especially when used in the overall process described, ensures closed material cycles and material recycling of the sewage sludge: microplastics and other pollutants are removed from the cycle and profitable use of the organic sludge with its valuable nutrients is made possible.
In the future, however, stronger measures to prevent microplastics from entering wastewater and groundwater should be implemented to control the primary problem. Together with political-regulatory, planning and infrastructural measures, technical solutions such as our filter process can significantly decrease microplastic pollution in the future – for the protection of the environment and health.