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In summer 2019, a full-scale trial was carried out to investigate the effects in drinking water quality when tertiary treated wastewater was discharged into the Llobregat River upstream of the intake of one of the major drinking water treatment plants of Barcelona and its metropolitan area. Two scenarios were investigated, i.e. discharging the reclaimed water with and without chemical disinfection with chlorine. This study investigates the concentration of N-nitrosodimethylamine (NDMA) as the specific disinfection conditions employed in this trial may favor its formation. To this aim, both NDMA and NDMA formation potential, were measured. The river contained NDMA at very low concentrations, but the concentration of NDMA precursors was already high. The NDMA concentration was reduced from discharge to the river to drinking water intake probably due to a combined effect of dilution and photolysis. The formation potential was also reduced probably due to dilution and biodegradation. The concentration of NDMA in the drinking water was always low (<7.3 ng/L), although the formation potential was above 10 ng/L in one sample. Dissolved organic matter characterization by high resolution mass spectrometry revealed differences between the nature of the organic matter in the river before and after reclaimed water discharge. ga1 • Disinfection increased NDMA in comparison to the non-disinfected reclaimed water. • NDMA was reduced in the river probably due to both dilution and photolysis. • NDMA precursors were reduced in the river probably due to both dilution and biodegradation. • NDMA net removal at the drinking water treatment plant was 40 ± 20%. • Compounds with low O/C and H/C > 1.5 correlated with NDMA formation potential. [ABSTRACT FROM AUTHOR]