High solid temperature phased anaerobic digestion from agricultural wastes: Putting several reactors in sequence

High solid anaerobic digestion of agricultural wastes (manure and grass silage) was tested on temperature phased anaerobic digester (TPAD) at lab-scale. A 4 days hyperthermophilic hydrolysis was followed by a 15 days methanogenesis. The TPAD was divided into 4 or 5 tanks in series without recycling: 2 sectors for hydrolysis (2 days HRT each) and 2 or 3 sectors for methanogenesis (5+10 or 3 x 5 days HRT). Two different combinations of temperatures (65 degrees C-37 degrees C, 65 degrees C-55 degrees C) were investigated to assess the process efficiency. For all tested conditions, the TPAD operated steadily with a methane productivity of 235+/-3 mLCH4(STp) gvs(-1). Using four or five reactors in series enabled to identify that during the two first days of hydrolysis, the hydrolysis yield and acidification yield were much higher (22% and 13%, respectively) than during the next two days (2% and 2% respectively). Moreover, concerning methanogenesis it appeared that the retention time may be shortened by 5 days with little loss in methane production (11%). In the methanogenic sectors, the uptake of volatile fatty acids and the methane production were higher at 55 degrees C than 37 degrees C. The results suggest that the hydrolytic sector could be limited at 2 days HRT, while the methanogenic sector would support a 10 days HRT with the feedstocks used. (C) 2017 Elsevier B.V. All rights reserved.