Filamentous organism bulking in nutrient removal activated sludge systems - Paper 8: The effect of nitrate and nitrite

The presence of nitrate (NO3-) and nitrite (NO2-) has a significant effect on the proliferation of low F/M filamentous organisms in nitrogen (N) and n utrient (N and phosphorus-P) removal activated sludge systems. In experimen ts in which the concentrations of NO3- and NO2- were manipulated in the 2nd anoxic reactor of a modified University of Cape Town (MUCT) nutrient remov al system, either by addition of ammonium to the influent or by dosing NO3- or NO2- to the nd anoxic reactor, the extent of low F/M filamentous organi sm proliferation could be controlled, as measured by the diluted sludge vol ume index (DSVI). With a sufficiently high TKN/COD ratio (> 0.10 mgN/mgCOD) in the influent, or by dosing NO3- and NO2- at a level at which the denitr ification potential of the anoxic zone was exceeded by the NO3- or NO2- loa d, the DSVI increased from values less than 100 ml/g to values greater than 150 ml/g in periods of between 3 and 5 sludge ages. it could not be determ ined which of NO3- or NO2- had the most significant effect on filament prol iferation. While the stimulatory effect of the NO3- or NO2- passing into th e aerobic zone on low F/M filamentous organism proliferation was positively identified, the mechanism by which this effect operated could not be estab lished.