Impact of Hydraulic Retention Time at Constant Organic Loading Rate in a Two-Stage Expanded Granular Sludge Bed Reactor

Abstract

Equivalent organic loading rates (OLRs) can be achieved by running high strength chemical oxygen demand (COD) slower or low strength COD faster. Comparisons of reactor performance under these conditions have not been characterized. Pilot-scale tests were conducted to assess the impact of hydraulic retention time (HRT) for fixed OLRs on reactor performance and stability in a two-stage anaerobic expanded granular sludge bed reactor. A distillery wastewater was used as the substrate at mesophilic temperature (35°C). Results showed that COD removal efficiency and biogas production rate increased by ∼33–42% and ∼22–32%, respectively, as HRTs increased by approximately five to six times, while maintaining a constant OLR (∼3, 5, 7, or 9 g COD/[L·day]). Results imply that for equivalent OLRs, better reactor performance is achieved when running high-concentration COD at a slower rate compared with a lower concentration COD at a faster rate. This also implies a diffusion limiting process where a higher molecular weight and slowly degrading organics, such as crude proteins and fats, likely are flushed through the reactor faster than they are efficiently able to diffuse into the granular biomass for digestion. The Monod model was employed to demonstrate the stability of the granular biomass behavior. The maximum specific growth rate, the half-saturation coefficient, and the death rate all remained approximately constant, indicating that biomass function and viability remained consistent over the duration of testing.