Influence of pH over nitrifying biofilm activity in submerged biofilters

The influence of pH over nitrification in submerged biofilters has been studied through the observation of three pH effects over the nitrifying biofilm: activation-deactivation, substrate limitation, and free ammonia inhibition. Within a pH range of 5.0–9.0, a pH increase of one unit produce a 13% increase on the nitrification efficiency. A stoichiometry of 7.1 mg CaCO3/mg N was measured for the process, which became alkalinity limited below pH 5.0. The highest activity of ammonium oxidizers and the highest values of volatile attached solids (VAS) in the filter were obtained at pH 8.2. The concentration of volatile attached solids (VAS) was regulated by free ammonia inhibition, substrate limitation (NH4+ concentration), and reactor hydrodynamics. Inhibition by free ammonia controlled bacteria activity at pH greater than 7.5, observing an increase of microorganisms concentration in the filter as a specific free ammonia concentration (mg NH3free-N gVAS−1) decreased. A nitrite accumulation of up to 80–90% was obtained for specific inhibitory concentrations greater than 1.5 mg NH3free-N gVAS−1 resulting from the selective inhibition of nitrite oxidizers.