Anaerobic Digester Optimization using Factorial Design Approach

The object of this study was to optimize operating conditions of anaerobic digesters using factorial design approach. Three operating parameters were of concern; (1) mixing, (2) mean cell residence time (MCRT) and (3) solids loadings. Mixing intensity was applied in two modes; low mixing and high mixing. MCRTs tested in the study were 15 and 25 days. The feed to the bench-scale digesters, collected from the Northeast Water Pollution Control Plant (NEWPCP) operated by Philadelphia Water Department (PWD), consisted of 50% primary and 50% DAF (Dissolved Air Floatation) thickened waste activated sludge (on mass basis). The solids contents of the feed sludge tested during the study were 3.5 and 7 %. Although many researchers have looked into the role of individual parameters, the authors are not aware of a comprehensive work looking at these operating parameters in one study. The unique aspect of this study is the experimental approach that used factorial design to assess the effects of each operating parameter, as well as the interactions between them, on the overall performance of anaerobic digesters. The digester performance was assessed through specific methane production, methane generated per unit mass of volatile solids feed. The results show that solids loading and MCRT have statistically significant effects on specific methane generation. In addition, interactions of MCRT with the two other parameters seem to be statistically significant. Mixing, on the other hand, does not seem to have a significant impact on processes performance. Overall, factorial design approach, compared to traditional experimental design where only one variable is changed per experiment, is highly effective in terms of evaluating several parameters and their interactions in fewer experiments.