Process-oriented screening of agro-industrial waste streams for co-digestion in WWTP anaerobic digesters

Wastewater utilities require a systematic method for screening agro-industrial co-substrates before testing them in sewage-sludge digesters. This study developed and applied a hierarchical preliminary screening framework based on physicochemical characterisation, biochemical methane potential assays, first-order kinetic modelling, and volumetric methane production. Seven waste streams were evaluated individually: a fat-rich fraction recovered from meat-processing wash water, four meat-processing wash waters, used cooking oil, and confectionery-production wash water. Eight fixed-ratio mixtures were then assessed using selected meat-processing streams, cheese whey, olive mill wastewater, and winery wastewater. Specific methane production of the individual waste streams ranged from 0.087 to 0.969 m³ CH₄ kg^-¹ VS. Across the mixtures, final SMP ranged from 0.390 to 0.855 m³ CH₄ kg^-¹ VS and t₉₀ from 9.0 to 23.3 d. TS, VS, and COD were associated with volumetric methane production at days 10 and 30 but not with final SMP, k, or t₉₀. The COD-VMP10 and TS-VMP30 models explained 87.5% and 86.0% of the variability, showing that volumetric methane output was influenced by feedstock concentration. Biochemical compatibility was classified using final SMP, t₉₀, and evidence of sustained methane suppression; VMP10 and VMP30 then refined priorities within each category. The cheese whey + meat-processing wash water R2 mixture (M1; 1:1 v/v) was prioritised because it combined high SMP (0.792 m³ CH₄ kg^-¹ VS), rapid apparent conversion (t₉₀ = 11.8 d), and the highest volumetric relevance. The framework converts physicochemical and BMP-derived results into a traceable basis for selecting external co-substrates for subsequent controlled validation with municipal sludge.