Thermophilic and mesophilic two-phase co-digestion of the organic fraction of municipal solid waste with sewage sludge and equine manure

This study aimed to evaluate the feasibility of treating a unique mixture of substrates through two-phase anaerobic digestion at reduced retention times. Sewage sludge was co-digested with organic fraction of municipal solid waste and straw-rich equine manure in 20 L laboratory-scale anaerobic digesters operated under thermophilic and mesophilic conditions. Despite short hydraulic retention times (HRT) of 2 d in the thermophilic reactor followed by 12 or 15 d in the mesophilic reactor, no apparent process inhibition was observed. The system operated under high organic loading rates in the mesophilic phase (6.7 and 5.3 g VS ·L −1 ·d −1 ). Under these conditions, methane yields of approximately 235–245 NL CH₄ ·kg VS −1 were achieved, corresponding to nearly 80% of the biochemical methane potential of the co-digested substrate (301 NL CH₄ ·kg VS −1 ). The methanogenic reactor also showed good resilience, with gas production improving over the course of the experiment, with the volumetric methane production rate reaching 1.60 NL CH₄ ·L −1 reactor d −1 at an HRT of 12 days and 1.27 NL CH₄ ·L −1 reactor·d −1 ) at 15 days. The addition of strawed equine manure improved the C/N ratio and avoided potential inhibition by ammonia, reducing ammoniacal nitrogen concentrations from 4.7 g·L −1 to 2.6 g·L −1 . The laboratory results indicated a strong potential for scale-up, because the system exhibited consistent performance and adaptability. These results were subsequently used in the design of a pilot plant processing 2–3 t·d −1 , which is currently operational. • Thermophilic-mesophilic TPAD reduced global hydraulic retention time • Stable co-digestion achieved with high OFMSW content (78–80% TS ) • Methane yield reached ~80% of BMP under short retention times. • Acetate-rich thermophilic effluent was fully converted in mesophilic digestion • Equine manure addition reduced ammonia inhibition