Within-Batch and Between-Campaign Variability in the Methane Yield of Separately Collected Kitchen Waste: A Pilot Study on Sampling Representativeness

Separately collected kitchen waste is a common feedstock for anaerobic digestion, but feedstock assessment often relies on a few samples whose representativeness is rarely tested. This pilot study examined the variability of the methane yield of kitchen waste (EWC 20 01 08) delivered to a municipal collection point. Nine samples were taken from three random points of same-day stockpiles in three campaigns (February–April 2026) and analysed for total and volatile solids (TS, VS), macronutrients, elemental composition and methane yield in batch assays (40 °C, inoculum-to-substrate ratio 4.3). VS/TS varied little (80.5–93.1%), whereas the methane yield ranged from 278 to 456 NmL CH₄ g⁻¹ VS (mean 362; coefficient of variation 21%). Samples from different points of the same stockpile differed significantly (nested ANOVA, p = 0.012). The between-campaign component was the largest variance estimate but was not significant with three campaigns (p = 0.070). Kitchen waste of this type was therefore highly heterogeneous when assessed from single grab samples. Neither TS nor VS/TS was related to the methane yield, whereas the composition of the dry matter was: the yield correlated with lipid content (r = 0.95), also within campaigns.