Rahel C. Brändli

Dissipation of pesticides during composting and anaerobic digestion of source-separated organic waste at full-scale plants

In the present study, concentration levels and dissipation of modern pesticides during composting and digestion at full-scale plants were followed. Of the 271 pesticides analyzed, 28 were detected. Within the three windrows studied, total concentrations were between 36 and 101microg per kg of dry matter (d.m.) in input materials and between 8 and 20microg kg d.m.(-1) in composts after 112 days of treatment. Fungicides and among them triazoles clearly dominated over other pesticides. More than two-thirds of all pesticides detected in the input materials showed dissipation rates higher than 50% during composting, whilst levels of most triazoles decreased slightly or remained unchanged. The investigation on semi-dry thermophilic anaerobic digestion suggests that pesticides preferentially end up in presswater after solid-liquid separation.

Organic pollutants in compost and digestate.

In Europe, 9.3 x 10(6) t(dry weight (dw)) of compost and digestate are produced per year. Most of this is applied to agricultural land, which can lead to considerable inputs of organic pollutants, such as polychlorinated biphenyls (PCB) and polycyclic aromatic hydrocarbons (PAH) to soil. This paper presents an inventory of the pollutant situation in source-separated composts, digestates and presswater in Switzerland by a detailed analysis of over 70 samples. PCB concentrations ( summation PCB 28, 52, 101, 118, 138, 153, 180) were significantly higher in urban (median: 30 microg kg(-1)dw, n = 52) than in rural samples (median: 14 microg kg(-1)dw, n = 16). Together with low concentrations in general, this points to aerial deposition on compost input material as the major contamination pathway. Enantiomeric fractions of atropisometric PCB were close to racemic. Median PAH concentration was 3010 microg kg(-1)dw( summation 15PAH, n = 69), and one quarter of the samples exhibited concentrations above the relevant Swiss guide value for compost (4000 microg kg(-1)dw). The levels were influenced by the treatment process (digestate > compost), the season of input material collection (spring-summer > winter > autumn), the particle size (coarse-grained > fine-grained), and maturity (mature > less mature). The main source of PAH in compost was pyrogenic, probably influenced mainly by liquid fossil fuel combustion and some asphalt abrasion, as suggested by multiple linear regression. This study, together with a companion paper reporting on other organic contaminates including emerging compound classes, provides a starting point for a better risk-benefit estimation of the application of compost and digestate to agricultural soil in Switzerland.

Optimised accelerated solvent extraction of PCBs and PAHs from compost

This study is the first thorough method optimisation for accelerated solvent extraction (ASE) of polychlorinated biphenyls (PCBs) and polycyclic aromatic hydrocarbons (PAHs) from chemically dried compost. For PCBs, optimised solvent composition, temperature, pressure, number of static cycles, duration, and flush volume were as follows: toluene/acetone 1 : 3 (v/v), 120°C, 2000 psi, 3 × 5 min, and 50%, respectively. Limits of quantification and method precision were between 0.16 and 2.46 µg kg−1 dw and 6–17% respectively for individual PCBs. Absolute recoveries of isotope-labelled extraction standards used for each of the analytes ranged from 65 to 105% and relative recoveries were between 85 and 99%. The method proofed to be robust and was successfully applied to different compost samples. The optimisation of PAHs extraction was performed and resulted in the following conditions: solvent: hexane/acetone 1/3 (v:v), temperature: 140°C, pressure: 1500 psi, extraction time: 3 × 5 min, and 50% flush volume. Limits of detection and method precision for individual PAHs were between 1.1 and 37.2 µg kg−1 dw and 12–34% respectively. Absolute and relative recoveries ranged from 24 to 68% and from 85 to 99%, respectively. Optimal extraction conditions for PAHs were more difficult to determine due to the inhomogeneous distribution of PAHs in samples. However, the method appeared to be feasible and suggestions for further improvements are presented.