The Effect of Degree of Recycle on the Nitrifier Growth Rate

 Water Environment Research, Volume 83, Number 1, January 2011 , pp. 26-35(10) 

Jimenez, Jose; Melcer, Henryk; Parker, Denny; Bratby, John

Abstract

The nitrifier maximum specific growth rate, μ_A,max, is a critical parameter for the design and performance of nitrifying activated sludge systems. Although many investigations studied μ_A,max, only a few have dealt with the effect of the reactor configuration on this important kinetic parameter. Bench- and full-scale trials were devised to study the effect of the internal mixed-liquor recycle (IMLR) on the nitrifier growth rate constant. The nitrifier growth rate constant for an existing activated sludge plant was determined at different operational conditions using the high food-to-microorganism ratio (F/M) test and by process model calibration. Overall, the results obtained during this study indicate that high IMLR values have a negative effect on μ_A,max. Based on the results obtained during this investigation, a 15% decrease in μ_A,max was observed at an IMLR of 4Q or higher. It is surmised that, at high IMLRs, the reactor behavior shifts from a plug-flow configuration to a “quasi” complete-mix configuration, influencing either the species selection in activated sludge population or at least the adaptation of specific species. These results have a tremendous effect on the design of activated sludge processes that incorporate IMLR for denitrification, such as the Bardenpho, Modified Ludzack-Ettinger (MLE), University of Cape Town (UCT), and Phoredox or anaerobic-anoxic-aerobic (A²/O) processes.

Keywords: nitrification; nitrifier growth rate; high food-to-microorganism ratio method; kinetics; biological nutrient removal