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Nowadays, fermentation of organic wastes for the production of carboxylic acids as precursors of higher-value products has attracted significant attention. In this paper, sewage sludge and food waste were co-fermented to produce carboxylic acids and study the subsequent chain elongation process. The Copenhagen waste stream scenario was taken as a case study. Firstly, design of experiments was used to investigate the overall carboxylic acids and hexanoic acid production in batch, as a function of the co-fermentation ratio, substrate to co-culture ratio and initial pH. Optimal operating conditions for hexanoic acid were obtained with SS/FW 6.61, S/Xo 6.73 and initial pH 6.83. Statistical optimization increased the overall carboxylic acid titer by 41%, while co-fermentation allowed to increase hexanoate annual production up to 77%. Furthermore, a continuous fermentation experiment was performed to study the effect of reactor operating conditions. The overall carboxylates titer was 2 times higher, which also favored chain elongation compared to batch mode. An increasing loading rate did not affect the overall carboxylate titer, however the hexanoic acid titer increased by 44%. A maximum titer of 4.9 g/l of hexanoic acid was produced, achieving a productivity of 2.46 g/l/d of hexanoic acid with a retention time of 2 d and no external electron donor addition. This would correspond to 610 t/y of hexanoic acid and 350 t/y of other carboxylic acids that could be produced, based on the waste availability in Copenhagen. Image 1 • Stable production of hexanoic acid was achieved with 2 days HRT. • Increasing OLR in continuous and S/Xo in batch operations increased chain elongation (hexanoate to carboxylates ratio). • 4.9 g/l hexanoic acid were achieved without external electron donor addition. • Co-fermentation could improve hexanoate annual production by 77%. • Valorization of Copenhagen waste streams would produce 610 t/y hexanoic acid. [ABSTRACT FROM AUTHOR]