Corrosion in a distribution system: Steady water and its composition

Abstract

Corrosion processes in drinking water distribution systems have been investigated for years. This paper proves the existence of a layer of steady water surrounding and partly filling corrosion scale in corroded water pipes. It is expected that steady water may substantially influence the corrosion. The idea of steady water explains why longer retention times would lead to more turbid waters containing more suspended Fe oxides (hydroxides). During stagnation period stagnant water mixes partially with steady water and the compensation of ions' concentration as well as particle diffusion is observed. Steady water is rich in ions and has reductive properties causing quick disappearance of nitrates and unwanted ammonia formation. Ammonia in turn may be responsible for fast oxygen and chlorine decay in the distribution system due to nitrification. Nitrates may also undergo denitrification in steady water, i.e. they would support bacterial existence in the distribution system. Such an environment makes favourable conditions for sulphate reducing bacteria and in fact in all of the steady water samples we have detected sulphides. Moreover, our results have shown that considerable amounts of assimilable organic carbon (AOC) can be formed in the corroded distribution system.