Effects of organic loading rate on reactor performance and archaeal community structure in mesophilic anaerobic digesters treating municipal sewage sludge

Abstract

In this study, the organic loading rate (OLR) of a high-solids anaerobic digestion (HSAD) system was increased from 3.4 to 5.0 gVS L⁻¹ day⁻¹ and reactor stability, performance and microbial community structure were determined. Laboratory simulations (3.5 L) of the full-scale process (500 dry ton year⁻¹) were conducted using continuously stirred-tank mesophilic reactors. OLRs of 3.4 gVS L⁻¹day⁻¹ (equal to the full-scale HSAD), 4.0, 4.5 and 5.0 gVS L⁻¹day⁻¹ were evaluated. Biochemical parameters and archaeal community dynamics were measured over 42 days of steady state operation. Results showed that increasing OLR increased the amount of organic matter conversion and resulted in higher organic matter removal and volumetric methane (CH₄) production (VMP) rates. The highest volatile solids (VS) removal and VMP results of 54 ± 2% and 1.4 ± 0.1 L CH₄ L⁻¹day⁻¹ were observed for 5.0 gVS L⁻¹ day⁻¹. The efficiency of reactor conversion of organic matter to CH₄ was found to be similar in all the treatments with an average value of 0.57 ± 0.07 LCH₄ gVS⁻¹_removed. 16S rRNA gene terminal restriction fragment polymorphism (T-RFLP) analyses revealed that archaeal TRFs remained stable during the experiment accounting for an average relative abundance (RA) of 81 ± 1%. Archaea consistent with multiple terminal restriction fragments (TRFs) included members of the Euryarchaeota and Crenarchaeota phyla, including acetoclastic and hydrogenotrophic groups. In conclusion, this laboratory-scale study suggests that performance and stability as well as the archaeal community structure in this HSAD system was unaffected by increasing the OLR by nearly 50% and that this increase resulted in a similar increase in the amount of CH₄ gas generated.