Media-Induced Hydraulic Behavior and Performance of Upflow Biofilters

Abstract

Laboratory studies were conducted to assess the influence of media-related factors such as porosity, specific surface, and pore size on hydraulic behavior and performance of upflow anaerobic biofilters (ABFs). Three 15-L upflow biofilters, each packed with different support media, were subject to identical synthetic protein-carbohydrate substrate with chemical oxygen demand (COD) concentrations ranging from 2,500 to 10,000 mg/L, and hydraulic retention times from 15 to 30 hours, corresponding to organic loading rates (OLRs) varying from 2 to 16 g COD/L/d. Tracer studies were carried out to characterize hydraulic behavior of the biofilters containing media with and without biomass, designated as dirty-bed and clean-bed, respectively. The results indicate that hydraulic flow regimes in all biofilters were characterized by a plug-flow pattern with a large extent of dispersion under clean-bed conditions. The tracer response curve under dirty-bed conditions operating at an OLR of 16 g COD/L/d reflects more closely the response of a mixed-flow reactor than that of a plug-flow unit, which suggests that there is significant short-circuiting in the ABFs. Waste treatment performance indicates that the biofilter associated with media of the largest pore size and porosity consistently demonstrated the highest COD removal from 96% to 73% at loadings varying from 2 to 16 g COD/L/d. The same reactor exhibited the lowest magnitude of dispersion along with minimum dead space within the bed from the tracer analysis. This implies that the use of support media with larger pore size and porosity may reduce the extent of short-circuiting, leading to better waste treatment performance. Increasing the media specific surface at the expense of media porosity may result in lower treatment performance in upflow anaerobic biofilters.