Analysis of sequencing batch bioreactors in large-scale denitrifying applications

Certain industrial processes (e.g. munitions manufacture) involve the generation of large volumes of waste with high nitrate concentration. Biological denitrification can be a very cost-effective means to deal with these wastes. At the same time, sequencing batch reactors (SBRs), also called fed-batch reactors, have a number of inherent advantages over conventional continuous treatment processes for high strength waste. They are much more flexible in responding to changes in the feed composition and flow, and changes in microbial characteristics. The purpose of this study was to develop a mathematical model of denitrification using a sequencing batch reactor, and apply that model to a scale-up process. Mathematical analysis of biological denitrification in SBRs is complicated by both the cyclical fill and draw operation of the SBR, as well as by generation of a toxic intermediate (nitrite). The Monod expression was used to describe nitrate utilization, while the Andrews (substrate inhibition) expression was used to described nitrite utilization. The key operating parameters included in the model were the fraction of reactor volume emptied at the end of each cycle, the total cycle time and fraction of time devoted to fill, and the hydraulic residence time. The mathematical model was tested experimentally, and the results indicated operating regimes that would minimize nitrite concentration. These operating and design parameters were then incorporated into the specification of a large-scale SBR system, treating 8,000 mg/l of nitrate at a flow rate of 1,022 m³/day, using four 340 m³ reactors.