Influence of water hardness on concrete surface deterioration caused by nitrifying biofilms in wastewater treatment plants

In aeration tanks of wastewater treatment plants (WWTP), biodeterioration can lead to a partial dissolution and ensuing erosion of the concrete due to an acid attack caused by the nitrifying biofilm covering the tank surface. In this study, concrete samples were exposed in four different WWTP. The CaO content of the cement used, the water-to-cement ratio, and curing affect the resistance of the concrete to acid attack. However, both experimental data and thermodynamic modeling show that water hardness is the dominating parameter for surface erosion. In the presence of high water hardness, the protons produced by the nitrifiers are buffered and the calcite precipitation occurring in all concrete samples is increased. This results in a decrease of porosity near the surface and higher mass transfer resistance against carbon dioxide and therefore in lower deterioration rates of the concrete.