Denitrification Rates: Sampling, Modeling and Design Considerations

Biological denitrification of wastewater has been a topic of interest for several decades; however, recent nutrient reduction initiatives and the need to achieve limit-of-technology (LOT) effluent quality have renewed interest in denitrification rate research. Since readily-biodegradable chemical oxygen demand (rbCOD) drives rapid denitrification, it is essential for design efforts to include raw influent and primary effluent sampling, for different seasons if possible. However, the influent rbCOD/TKN ratio is not the only factor to consider, since rbCOD can be easily destroyed by non-optimal design and operating conditions. Two case studies are presented, each having very similar influent rbCOD/TKN ratios but very different full-scale denitrification performance. Furthermore, bench-scale tests that were conducted for the two plants yielded similar denitrification rates, which raises the question: how can engineers apply denitrification rate research to full-scale designs? Activated sludge models provide some insight to the unknown (dissolved oxygen in the mixed liquor recycle) but cannot predict rbCOD destruction due to hydraulic conditions, or fermentation due to heterogeneous mixing. This paper discusses all of the factors that affect denitrification and poses questions for future research.