Fate of microplastics in a centralized biogas plant treating mainly sewage sludge

According to estimates, 20-55 % of microplastics entering wastewater treatment plants is entrapped in sewage sludge. Sewage sludge is often treated by anaerobic digestion in Europe, either in biogas plants directly connected to the wastewater treatment plants, or in centralized biogas plants receiving sludge from multiple sources. The anaerobic digestion is typically followed by dewatering of the digestate, producing solid and liquid fractions. The solid fraction of digestate, particularly, is often recycled to agricultural land due to its high nutrient content, thus introducing also microplastics to soil. In order to develop safer digestate-based recycled fertilizers, it is important to understand the behavior and fate of microplastics across the anaerobic digestion and the subsequent solid-liquid separation process. In this study, we examined the abundance and flow of microplastics (>20 μm) at a centralized biogas plant receiving mainly sewage sludge. Over 99 % of microplastics and over 90 % of phosphorus were retained in the solid fraction of digestate after the dewatering, further emphasizing the need to find new methods to manage sewage sludge (digestate) to enable the safe recycling of nutrients according to the circular economy principles. Polystyrene microbeads of unidentified source composed 65 % of microplastics detected in the solid fraction, indicating that local sludge origins may have a significant impact on the occurrence and types of microplastics observed at centralized biogas plants. The liquid fraction of digestate carried less than 1 % of microplastics and over two-thirds of nitrogen, proposing the potential of this fraction as a cleaner source of nitrogen for recycling.