Evaluation of substrate and pH effects in a nitrifying biofilm

The effects of substrate and pH on biofilm nitrification were studied using a microelectrode technique and a microslicing technique. Three different kinds of biofilms were cultured using lab-scale, rotating drum biofilm reactors fed with synthetic wastewaters. The measured concentration profiles provide direct experimental evidence of the substrate and pH effects on biofilm nitrification. It was found that pH decreases within the nitrifying biofilm depended on the mole ratio of HCO₃/O₂. When this mole ratio was >5, the maximum pH decrease within the nitrifying biofilm was <0.4- 0.6 pH units. When this ratio was <3, the maximum pH decrease could be 1.4 - 1.h pH units A pH decrease of 1.4-1.6 pH units resulted in an inhibition of the biofilm nitrification process, and therefore stopped a further decrease in pH. No dramatic pH decrease (>2-2.5) was observed during the study. It was found that increases in glucose loading or ammonium loading caused more consumption of oxygen, which resulted in inhibition of the biofilm nitrification. The bacterial trophic population measurement revealed that even in a pure nitrification system, heterotrophs could exist in the nitrifying biofilm. In biofilms cultured with an influent COD of 170 mg/L or higher, the numbers of nitrifiers were always four to five orders lower than those of heterotrophs. This study demonstrates that competition between heterotrophs and nitrifiers for substrate (including O₂) and space greatly affects the degree of pH decreases within nitrifying biofilms.