Thermophilic-mesophilic temperature phase anaerobic co-digestion of sewage sludge, wine vinasse and poultry manure: Effect of hydraulic retention time on mesophilic-methanogenic stage

The present study investigated the influence of the hydraulic retention time (HRT) in temperature-phase anaerobic co-digestion (TPAcD) for methane production. The reactors were started-up with a mixing ratio of 49.5:49.5:1 of sewage sludge, wine vinasse and poultry manure. The TPAcD was operated at thermophilic temperatures in the first stage and mesophilic temperatures in the second stage. The thermophilic stage operated with a constant HRT of 5 days, while the methanogenic stage was optimized under the HRT of 15, 12, 10, 8, 5, 4, and 3 days. The best results were obtained for an HRT of 12 days in the methanogenic stage, 56.35 % of volatile solids (VS) biodegradation was achieved, with a methane yield of 391 mL CH4/gVSadded. Regarding the whole TPAcD process (acidogenic following by methanogenic), the vS and total volatile fatty acids reached, respec-tively, 93.13 % and 97.43 % of removal efficiency. The microbial population revealed that Eubacteria was higher than the Archaea at the HRT with the highest methane yield, and the microbial activity increased proportionally to the organic loading rate, which in turn was related to methane production. Due to the strong pathogen reduction in the TPAcD, the digestate obtained can be classified as class A biosolids in all HRT evaluated, being a promising alternative for its application as agricultural fertilizer. Finally, the presented TPAcD process can be an environmeltally friendly alternative for the management of sewage sludge, wine vinasse, and poultry manure in an integrated biorefinery for the recovery of bioenergy and fertilizer, advocating a sustainable approach for the circular economy transition.