Effect of alkalinity type and concentration on nitrifying biofilm activity

The effect of alkalinity on nitrifying biofilm activity was determined by collecting 21-day-old biofilm samples from the top of 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. Initial carbonate alkalinity concentrations were varied between 308 and 20 mg/L as calcium carbonate (CaCO3). Ammonia, nitrite, and nitrate concentrations were measured at time zero, 90 minutes, 180 minutes, and 270 minutes. Generation rates in grams of nitrogen per square meter per day were calculated for each time period and normalized against dry-weight biomass. Generation rates were impaired at initial carbonate alkalinity concentrations of 40 mg/L and lower (as CaCO3) and were unaffected at concentrations of 45 mg/L and greater. For reactor runs with different alkalinity types, ammonia, nitrite, and nitrate concentrations were measured at time zero and at 375 minutes. The type of alkalinity, carbonate versus phosphate, affected nitrification rates. When the carbonate alkalinity was less than 45 mg/L, nitrification rates were impaired regardless of the total alkalinity concentration. This effect seems to be independent of pH for the range of 6.92 to 7.99 evaluated here. This suggests that in addition to neutralizing the acid generated by the nitrification process, a minimum level of carbonate alkalinity is necessary to meet the ammonia-oxidizer's inorganic carbon requirement for cellular synthesis and growth.