Data concerning the manuscript: Small dose of monochloramine increases nitrite formation via two different routes in water distribution Pirjo-Liisa Rantanena, Alejandro Gonzalez-Martinezb, Ilkka Mellinc, Riku Vahalaa, Merja Ahonend, Minna M. Keinänen-Toivolae a Department of Built Environment, Aalto University, PO Box 15200, FI-00076 Aalto, Finland b Department of Microbiology, University of Granada, Campus Universitario de Cartuja, 18071 Granada, Spain c Department of Mathematics and Systems Analysis, Aalto University, PO Box 11100, FI-00076 Aalto, Finland d Faculty of Technology, Satakunta University of Applied Sciences, PO Box 1001, FI-28101 Pori, Finland e Office of vice rector on RDI, Satakunta University of Applied Sciences, PO Box 1001, FI-28101 Pori, Finland Abstract Nitrite is potentially harmful to humans and in drinking water its concentrations have a maximum admissible limit in European legislation (0.5 mgN L‑1). In this study, it was conjectured that monochloramine may affect nitrite formation rates indirectly via other routes than merely constituting an ammonium substrate for nitrification when decomposing. The conjecture was tested in laboratory scale by comparing the following conditions with the same initial ammonium concentration: tests with free ammonium, and with free ammonium and a 20% share of ammonium as monochloramine (monochloramine dose of 0.39 mgCl2 L‑1, as median). The nitrification tests were organized consequently in two parallel simulated distribution systems. Monochloramine increased indirectly the apparent nitrite formation rates (2.4 times) and the maximum nitrite peak concentration (1.6 times). According to the pseudo-first order model interpretation, this was primarily due to enhanced ammonium oxidation. Conceivably, the inactivation of monochloramine was mainly targeted to the heterotrophic surface layer of the biofilm, likely enhancing ammonium transfer to ammonium oxidizing bacteria (AOB) below the surface layer. Both ammonium and nitrite oxidizing bacteria were observed in the biofilms confirming nitrification activity; Nitrosomonas sp. 9%–11% and Nitrospira sp. 9%–22%. These results may explain the highest nitrite concentrations observed in drinking water distribution systems (DWDSs) in USA, and the fast formation of nitrite in Finnish DWDSs.

2021