Potential agronomic and environmental properties of thermophilic anaerobically digested municipal sewage sludge measured by an unsupervised and a supervised chemometric approach

Anaerobic digestion (AD) is the most widely used method of sewage sludge treatment (SS) before its agricultural use. AD achieves the required “sterilisation” of pathogens and is able to cover the energy required by the process, reducing pre-treatment costs, thanks to the production of biogas. The SS agronomic (fertilizer properties), environmental (pollutants contents) characteristics and nuisance to people (odours and pathogens) need to be evaluated together for the safe and useful deployment of SS in agriculture. To evaluate SS properties an unsupervised (Principal Component Analysis) and a supervised (K nearest neighbours) chemometric approach was applied to rank digested SS for agronomic and environmental properties in comparison with other organic matrices for which the agronomic and/or environmental properties are well known or expected. To do so, complete chemical, biological and “impact on people” characterization was carried out on SS ingestate (SS-ing.) and SS digestate (SS-dig.) and another 10 biomasses. The SS-AD process enhanced the agronomic properties of sewage sludge and did not lead to a substantial concentration of pollutants because of the low degradation of organic matter. The best PCA performances were reached for amendment and fertilizer modules but the results found for the environment and nuisance to inhabitants were not satisfactory. The KNN approach proposed to evaluate the suitability of a biomass for agricultural purposes, represents a win-win approach as it allows one to avoid time-consuming and costly full field studies.