Destruction of concrete by nitrification

Summary Nitrification was studied, using samples of asbestos concrete plates and water, both taken from a cooling tower, in which ammonia-containing water had been circulated for several years. Almost the entire nitrifying activity and the majority of autotrophic nitrifying bacteria were found to be located in the coating, covering the asbestos concrete plates. When sprinkled over these plates, the H⁺-concentration of such ammonia-containing water increased from neutrality to about pH 5.3 and this pH was maintained as long as nitrification was feasible. Death of microorganisms and lack of either ammonia or oxygen caused a rapid decrease of H⁺-concentration, which was obviously due to concrete destruction. The temporary acidification observed in the medium resulted from both acid-producing microbial nitrification and acid consuming concrete dissolution, with both reactions counteracting each other. The amount of nitric acid required to maintain a low pH in presence of concrete was determined by automatic titration. The obtained result indicated that considerable amounts of nitric acid might have been formed in the cooling system, which could well account for the observed damage.