Article pubs.acs.org/est
Effects of Temperature and Hydraulic Retention Time on Acetotrophic Pathways and Performance in High-Rate Sludge Digestion Dang Ho,† Paul Jensen,† and Damien Batstone*,† †
Advanced Water Management Centre, The University of Queensland, St Lucia, Queensland 4072, Australia S Supporting Information *
ABSTRACT: High-rate anaerobic digestion of organic solids requires rapid hydrolysis and enhanced methanogenic growth rates, which can be achieved through elevated temperature (>55 °C) at short hydraulic retention times (HRT). This study assesses the effect of temperatures between 55 °C and 65 °C and HRTs between 2 and 4 days on process performance, microbial community structure, microbial capability, and acetotrophic pathways in thermophilic anaerobic reactors. Increasing the temperature did not enhance volatile solids (VS) destruction above the base value of 37% achieved at 55 °C and 4 days HRT. Stable isotopic signatures (δ13C) revealed that elevated temperature promoted syntrophic acetate oxidation, which accounted for 60% of the methane formation at 55 °C, and increasing substantially to 100% at 65 °C. The acetate consumption capacity dropped with increasing temperature (from 0.69−0.81 gCOD gVS−1 d−1 at 55 °C to 0.21−0.35 gCOD gVS−1 d−1 at 65 °C), based on specific activity testing of reactor contents. Community analysis using 16S rRNA pyrosequencing revealed the dominance of Methanosarcina at 55−60 °C. However, a further increase to 65 °C resulted in loss of Methanosarcina, with an accumulation of organic acids and reduced methane production. Similar issues were observed when reducing the HRT to 2 days, indicating that temperature 3 days are critical to operate these systems stably.
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at mesophilic temperatures)5 and is consistent with increased reaction kinetics according to the Arrhenius equation.8 These findings have motivated further studies on sludge digestion at shorter retention time (i.e., higher loading rates)7,9 and its application to temperature phased anaerobic digestion (TPAD) processes5,10 to maximize solids degradation and energy recovery. TPAD has demonstrated benefits as a pretreatment process but still requires a long retention time (>12 days) mesophilic process to enable methanogenesis of hydrolyzed solids. While it is clear that increasing temperature has a positive impact on hydrolysis, the effects on subsequent AD steps have not been fully investigated. Methanogenesis is often regarded as a key step determining digester capacity and stability under thermophilic conditions due to the sensitivity of the methanogenic consortia to environmental changes.11 Methanogens are more susceptible to temperature fluctuation (
