Chemical identification and quantification of volatile organic compounds emitted by sewage sludge

Abstract

The recycling of organic waste products (e.g. sewage sludge, SS) is currently being promoted as a substitute for mineral fertilizers for agricultural lands. The spreading of SS allows the recycling of the nutrients and organic matter it contains. SS contains various pollutants such as volatile organic compounds (VOCs) that adversely affect the ecosystem and human health through ozone production and serve as critical precursors of atmospheric secondary organic aerosols. There are very few studies quantifying the gaseous compounds emitted from SS, and those studies primarily address their odorant properties for identifying suitable odour abatement techniques. There is an urgent need for more comprehensive quantitative information on VOCs emitted from SS as aerosol precursors. In this context, an experimental study was performed on SS samples taken from a wastewater treatment plant located in France. Undigested SS (UDSS), digested SS (DSS) and SS with 30% and 60% dryness were collected from different stages of treatment sequence and analyzed using atmospheric simulation chambers coupled to proton-transfer-reaction quadrupole ion-guide time-of-flight mass spectrometer. Our study revealed that SS samples emitted a large spectrum of VOCs. 380 compounds were detected, quantified and classified into different chemical groups. The VOC emissions increased with the increase in the dryness of the sample; the highest being in SS 60%, followed by SS 30%, UDSS and DSS. OVOCs were dominant in SS 60%. The statistical analysis showed that the anaerobic digestion and the dewatering to 60% of dryness decreased the emissions of sulphuric compounds. Aromatic compounds and indoles (e.g. skatole) were emitted significantly from the UDSS. Some of these VOCs can serve as precursor gases for atmospheric aerosol formation. The experimental dataset obtained in this study provides an accurate inventory reference for the VOC emissions from SS samples and shows the impacts of the treatment on emission characteristics of VOCs.

Keywords: Aerosol precursors; Anaerobic digestion and dewatering; Mass spectrometry; SS; Volatile organic compounds.