Use of limestone for pH control in autotrophic denitrification: Batch experiments

The autotrophic denitrification process using elemental sulphur consumes about 4 g alkalinity (as CaCO3) per gram nitrate nitrogen reduced and is severely inhibited at pH lower than 5.5. Using limestone as an alkalinity source to control the pH, the autotrophic denitrification of synthetic wastewater with varying initial alkalinity to NO3-N ratios was evaluated in laboratory-scale batch reactors, which contained sulphur and limestone granules in different volumetric ratios. The results demonstrated that limestone supplies effective buffering capacity, if the initial alkalinity is insufficient for complete denitrification. The alkalinity supplied by limestone is a function of the pH, which in turn depends on the initial alkalinity of the wastewater and the extent of the biological denitrification. Given the specific denitrification rates, in mg NO3-N/g S h, and limestone dissolution rates, in mg, CaCO3/g CaCO3 h, as a function of pH, the required volumetric ratio of limestone to sulphur for maintaining a given pH can be quantitatively established. It was found that the optimum volumetric ratio of limestone to sulphur for minimization of the reactor volume equals approximately 1:1. (C) 2002 Elsevier Science Ltd. All rights reserved.