Biochemical nitrification was investigated in a large scale pilot study using aged galvanized steel, unlined cast iron and a hybrid pilot distribution systems consisting of a combination of aged polyvinyl chloride, lined cast iron, unlined cast iron and galvanized steel pipes. Variations of free ammonia, nitrate, nitrite, dissolved oxygen (DO) and chloramine residual in the distribution system water are discussed for each pipe material. Chloramine depletion, DO depletion, free ammonia release, heterotrophic plate count (HPC) growth, NO2–N and NOx–N measured as the sum of nitrite and nitrate N were dependent on pipe material. Galvanized steel pipe had the highest chlorine demand and produced conditions without residuals, which corresponded to the higher HPC proliferation and DO consumption and the lowest nitrification. The greatest production of nitrite and nitrate was observed in unlined cast iron pipe. Denitrification was observed in the unlined cast iron pilot distribution system following residual loss and development of anoxic conditions.