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

Abstract

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.