Dynamic modeling of a full-scale anaerobic mesophilic digester start-up process for the treatment of primary sludge

Abstract

The current start-up procedures for anaerobic sludge digesters offer limited early biogas production. This increases the cost of heating since natural gas must be purchased until methane can be produced onsite. In this research, three simulations were carried out using the BioWin 5.2 software package to expedite the production of biogas during the start-up of a full-scale anaerobic digester treating primary sludge. The first set of simulations was conducted to assess the predictive capabilities of the software when used to model this start-up process. The second set of simulations was conducted to identify which kinetic factors most significantly affect the aforementioned model. The third set of simulations was conducted to evaluate strategies designed to accelerate biogas production. The start-up strategies were developed for use in wastewater treatment plants with limited availability to seed sludge. These strategies aimed to determine the minimum amount of seed sludge, the initial sludge feed rate, and the daily sludge feed rate (with and without the addition of a pH control agent), for early biogas production. BioWin was able to reliably simulate the start-up of a full-scale digester with a relatively good fit to the plant measured data. The overall mean absolute percentage error was less than 25%, and the overall Willmott index was greater than 0.82. The results of the sensitivity analysis indicated that the BioWin outputs were more sensitive to changes in the hydrolysis rate than the acetoclastic maximum specific growth rate or the acetoclastic anaerobic decay rate.