Water losses during household waste characterization: An overlooked issue?

Manual sorting of household waste for composition analysis inevitably exposes samples to ambient air, but potential water losses and associated biases are not quantified in the existing literature or guidelines. This short communication provides preliminary insights on the occurrence and magnitude of moisture loss during waste characterization and aims to stimulate discussion on the relevance of these effects. Since food waste and residual waste account for most of the total water content in municipal household waste collection, these fractions were selected for the experiments. Six food waste and six residual waste samples were repeatedly weighed during 6-8 h of exposure under typical indoor sorting conditions and subsequently oven-dried to determine total water content. Food waste samples lost approx. 2 mass-% of initial water in 8 h, while residual waste showed greater variability (1.6-5 mass-%; up to 14 mass-%). While these water losses seem to be relatively small, they affect not only the accuracy of compositional results but can translate to substantial overestimation of wet-basis performance indicators. The observed moisture losses corresponded to 9-13 % higher estimates of lower heating values and biogas potential, which may exceed uncertainty levels commonly accepted in waste characterization and related assessments. Given the limited number of samples, the reported moisture losses demonstrate the existence of the phenomenon but should be interpreted as indicative for the documented indoor conditions (approx. 20.6 °C and 55.4 % relative humidity). Under warmer or more ventilated conditions, moisture losses and the resulting bias may be even larger than observed here.