Bewertung von Gärresten als Input für die Kompostierung: Untersuchung von Nähr‑, Ballast- und Schadstoffen

Digestates represent a significant waste stream that can be composted (mixed with structure-rich waste) and achieve product status as compost in accordance with the Austrian Compost Ordinance. Digestate increases the nutrient content of compost, and a significant proportion of the original organic substances are converted into stable humic substances through the rotting process.

However, the input material in biogas plants is often food waste, which can be heavily contaminated with food packaging residues. Plastics represent an inert contamination of biowaste that can lead to problems during recycling. During waste treatment, “large” plastic parts are fragmented into microplastics that can no longer be removed from fermentation residues or composts. This microplastic can enter agriculture via compost or digestate and contaminate the soil over long periods of time and enter the food chain and the groundwater.

In this study, the mass and type of macro- and microplastics > 2 mm in digestate from a municipal biogas plant were investigated. For this purpose, digestate samples were taken at two-month intervals over a period of one year and examined for inert impurities (glass, metal, and plastics), heavy metal, salt and nutrient content. The most common polymers found in the digestate were polyolefins and other types of commodity plastics, suggesting that their sources are common packaging materials.

The results of the study show that municipal digestate severely lowers the quality and usability of the compost produced from it, mainly due to contamination with plastics. Heavy metal and salt contents, on the other hand, allow a proportion of at least 40% (m/m) in the input mixture for the production of compost quality class A.