Comparison of operating strategies for increased biogas production from thin stillage

The effect of increasing organic loading rate (OLR) and simultaneously decreasing hydraulic retentiontime (HRT) during anaerobic digestion of sulphur-and nitrogen-rich thin stillage was investigated duringoperation of continuously stirred tank laboratory reactors at two different temperatures. The operatingstrategies and substrate were set in order to mimic an existing full-scale commercial biogas plant inSweden. The reactors were operated for 554-570 days with a substrate mixture of thin stillage andmilled grain, resulting in high ammonium concentrations (> 4.5 g L-1). Initially, one reactor was operatedat 38. C, as in the full-scale plant, while in the experimental reactor the temperature was raised to 44. C. Both reactors were then subjected to increasing OLR (from 3.2 to 6.0 g VS L-1d-1) and simultaneouslydecreasing HRT (from 45 to 24 days) to evaluate the effects of these operational strategies on processstability, hydrogen sulphide levels and microbial composition. The results showed that operation at44. C was the most successful strategy, resulting in up to 22% higher methane yield compared with themesophilic reactor, despite higher free ammonia concentration. Furthermore, kinetic studies revealedhigher biogas production rate at 44. C compared with 38. C, while the level of hydrogen sulphide wasnot affected. Quantitative PCR analysis of the microbiological population showed that methanogenicarchaea and syntrophic acetate-oxidising bacteria had responded to the new process temperature whilesulphatereducing bacteria were only marginally affected by the temperature-change. (C) 2014 Elsevier B. V. All rights reserved.