Co-composting Rugulopteryx okamurae, Alperujo and local organic waste: a sustainable strategy for odour control and compost quality improvement

This study evaluated the co-composting of the invasive phaeophycean seaweed Rugulopteryx okamurae with two organic wastes, Alperujo (AL) and sewage sludge (SS), to determine process performance and compost quality. Raw material characterization revealed a low C/N ratio in the seaweed, corrected by mixing with AL and SS. Five Dewar experiments were conducted: two controls (C-AL and C-SS), each containing 80% AL or SS and 20% wood chips as bulking agent (BA), and three mixtures combining seaweed with AL and SS in different proportions (1, 2 and 5%) plus BA (M1, M2 and M3). C-SS exhibited efficient biodegradation, characterised by a slower pH increase and high metal levels, meeting Spanish regulation Decreto (1310)/1990 for agricultural use. C-AL rapidly reached an alkaline pH but showed lower organic matter degradation; its final C/N ratio of 20 and lower metal content than C-SS, classified it as Category A (RD 999/2017). In contrast, the seaweed mixtures achieved C/N ratios below 20, with metal contents meeting Category C classification. Odour analysis revealed that seaweed reduced emissions compared to controls, which peaked at 60,100 ou E /m 3 in C-AL and 38,970 ou E /m 3 in C-SS, while M1 peaked at 6,890 ou E /m 3 , M2 at 12,635 ou E /m 3 and M3 at 21,250 ou E /m 3 . Experiments exhibited odour peaks during the thermophilic phase. Respirometry (OD 20 and SOUR max ) indicated that none of the mixtures reached full maturity, requiring an additional maturation phase for stabilization. Overall, co-composting R. okamurae with regional wastes improved C/N balance, reduced odour impact and offered a sustainable strategy for managing invasive biomass and local organic waste.