Organic Polluants in Swiss Compost and Digestate

Composting (aerobic treatment of organic wastes) and digestion (anaerobic treatment of organicwastes combined with biogas production) are important waste management strategies with increasingsignificance in the European Union and Switzerland. Most of the compost produced is applied toagricultural soils, which hereby recycles nutrients and influences soil properties beneficially. However,compost can contain pollutants that may be hazardous for the soil ecosystem. The problem related toheavy metals had been recognised and measures for reduction were taken. Regarding organicpollutants, the current knowledge is insufficient for quality control and risk assessment.This thesis provides a comprehensive overview on organic contaminates in compost, digestate, andpresswater and describes factors that may influence them. In the beginning an extensive literaturereview was carried out to summarize the current data. Compound classes to be analysed in Swisscomposts and digestates were prioritized and analytical methods established. Polychlorinatedbiphenyl concentrations (Σ of PCB 28, 52, 101, 118, 138, 153, 180) were significantly higher in urban(median: 30 μg/kgdry weight (dw), n=52) than in rural samples (median: 14 μg/kgdw, n=16), which points -together with low concentrations in general to aerial deposition on feedstock material as major inputpathway to compost. Median polycyclic aromatic hydrocarbon (PAH) concentration was 3010 μg/kgdw(Σ of 16 PAH defined by the US EPA except dibenzo[a,h]anthracene, n=69). PAH levels wereinfluenced by the organic matter degradation process (higher levels in digestate than in compost), theseason of input material collection (spring/summer>winter>autumn), the particle size (higherconcentrations in unsieved or sieved >20mm than in sieved to ≤ 20mm products), and maturity (lowerconcentrations in more mature composts). One fourth of the samples exhibited PAH concentrationsabove the Swiss guide value for compost (Ordinance on the Reductions of Risks linked to ChemicalProducts). These elevated concentrations can lead to considerable input of PAH to soil by compostapplication. To assess the major contributors of PAH in compost, characteristic PAH ratios and somemolecular markers were considered, which pointed mainly to combustion origin of these contaminants.Multifactor statistical analysis indicated traffic emission, straw combustion and some asphalt abrasionas potential additional sources. Concentrations of other organic pollutants determined (dibenzo-p-dioxins and –furans, dioxin-like PCB, brominated flame retardants (BFR), perfluorinated alkylsubstances (PFAS), di(2-ethylhexyl)phthalate, nonylphenol and chlorinated paraffins) were mostlyabove levels found in background soil, except for nonylphenol, which was not detected. Out of 269pesticides analysed, 30 fungicides, 14 herbicides, eight insecticides and one acaricide were detected.For the first time, the fate of organic pollutants during full-scale composting and digestion wasassessed. Concentrations of low chlorinated PCB increased during composting (about 30 %), whereasa slight decrease was observed for the higher chlorinated congeners (about 10%). Enantiomeric ratiosof atropisomeric PCB were close to racemic and did not change. Levels of low molecular weight PAHwere reduced during composting (50 to 90% reduction), whereas heavier compounds remainedstable. However, as indicated above, conventional composting does not reduce PAH concentrationssufficiently to comply with Swiss guide values (see above).Further research is needed to i) identify measures to reduce PAH concentrations in digestate andcompost, ii) to monitor organic pollutants that are still increasing in other environmental matrices (e.g.BFR and PFAS) or identify new compounds which have not been detected in compost yet, iii) toevaluate potential risk of compost application to soil by assessing the bioavailable fractions of organicpollutants and iv) to examine possible new input materials and co-substrates for composting anddigestion.