The effect of alkalinity on nitrifying biofilm activity was determined by collecting twenty-one day old biofilm samples from a full-scale nitrifying trickling filter and evaluating bench-scale nitrate plus nitrite generation rates at (1) various initial carbonate alkalinity concentrations
and (2) with four types of available alkalinity; carbonate only, phosphate only, phosphate plus hydroxide, and phosphate plus carbonate alkalinity. Initial carbonate alkalinity concentrations were varied between 308 and 20 mg/L as CaCO3. Ammonia, nitrite, and nitrate concentrations
were measured at time zero, 90 minutes, 180 minutes, and 270 minutes. Generation rates in g N/m2*day were calculated for each time period and normalized against dry weight biomass. The effect of the initial carbonate alkalinity concentration on generation rate was evaluated
by twotailed ANOVA followed by LSD analysis at a 95% confidence interval. Generation rates were impaired at initial alkalinity concentrations at and below 40 mg/L as CaCO3 and were unaffected at concentrations of 45 mg/L and above. This is consistent with Gujer and
Boller's (1984) proposal and Szwerinski's et al. (1986) data that alkalinity is rate limiting below 1-2 millimoles/liter or 50-100 mg/L as CaCO3. For reactor runs with different alkalinity types, ammonia, nitrite, and nitrate concentrations were measured at time
zero and at 375 minutes. Generation rates were calculated and normalized against dry weight biomass measurements. The effect of alkalinity type on generation rate was evaluated by two-tailed ANOVA followed by Tukey analysis. The type of alkalinity, carbonate versus phosphate, affected nitrification
rates. If the carbonate alkalinity was below 45 mg/L, nitrification rates were impaired regardless of the total alkalinity. This effect seems to be independent of pH for the range of 6.92 to 7.99 evaluated here. This would suggest that in addition to neutralizing the acid generated by
the nitrification process, a minimum level of carbonate alkalinity is necessary to meet the ammoniaoxidizer's inorganic carbon requirement for cellular synthesis and growth.
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