Valorizing Primary Digestate via Codigestion with DAF Sludge and Hydrodynamic Cavitation

Anaerobic digestion (AD) efficiently converts biodegradable feedstocks to biogas; however, residual digestate remains challenging to valorize. Digestate from primary digesters retains significant carbon; however, it has a lower-than-desirable carbon-to-nitrogen (C/N) ratio (∼7) and therefore typically yields low methane productivity. In this note, we present two strategies to enhance the methane yield from digestate. The first is to adjust the (C/N) ratio by mixing dissolved air flotation (DAF) sludge from the dairy industry, which has a high (C/N) ratio (∼30). The DAF sludge is rich in fats, oils, and grease and is difficult to digest (operational difficulties, long lag phase, and poor biogas yields). In this study, we demonstrate the synergistic effect of the codigestion of primary digestate and DAF sludge. In this study, BMP tests were conducted on mixtures of primary digestate and DAF sludge to evaluate synergistic effects, defined as the ratio between the experimentally observed BMP of the mixture and the volatile solid (VS) weighted sum of BMPs obtained from the monodigestion of the individual substrates. A 1:1 blend (on a VS basis) exhibited the highest performance, achieving a synergy factor of 1.7 and a BMP of 684 mL CH₄/g VS. We also demonstrate that for cases where measured BMP is much smaller than the theoretical BMP (BMP_th), the HC pretreatment enabled methane recovery up to ∼80% of the BMP_th. In this work, we used a vortex-based HC device for the pretreatment of a 1:1 mixture of digestate and DAF sludge and showed that the pretreated mixture could achieve 82% BMP_th. This HC-assisted codigestion approach provides a sustainable pathway for valorizing two of the most difficult waste streams in the AD and dairy industries, supporting renewable energy recovery and circular bioeconomy goals.