Polar Micropollutants and Metals in Centrate from Dewatered Sewage Sludge Intended for Reuse in Soilless Horticulture

Abstract

The so-called centrate, the water remaining from digested sludge centrifugation in municipal wastewater treatment, is an untapped resource of both macro- and micronutrients for plant cultivation. However, both organic and inorganic contaminants present in the centrate may affect the quality of the produce if taken up by the plants. Little is known about the micropollutants present in the centrate. The centrate, influent, and effluent of a municipal wastewater treatment plant were analyzed for 27 polar organic micropollutants over a period of 28 days by direct injection liquid chromatography tandem mass spectrometry. The median dissolved concentrations in the centrate exceeded the concentrations in both the influent and effluent for most compounds: highest concentrations were found for benzotriazole (79 μg/L), valsartan (57 μg/L), and ibuprofen (18 μg/L). Micropollutants that are anaerobically degradable were not detected in the centrate. Among the heavy metals analyzed, highest concentrations were detected for Zn (47 μg/L) and Ni (17 μg/L). The increasing turbidity of the centrate led to elevated concentrations of less-polar micropollutants, while higher coagulant dosing increased the concentrations of more polar compounds. This first comprehensive study on polar organic micropollutants and metals in the centrate outlines that nutrient recovery from the centrate has to deal with both organic micropollutants and heavy metals present in the centrate.