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Both ion exchange (IX) and reverse osmosis (RO) technologies are effective in removing NO3− from drinking water, but the disposal of waste streams and the large amount of salt needed for to prepare fresh brine in IX have become economic and environmental challenges. To overcome these barriers, photocatalytic denitrification (PD) using TiO2 nanoparticles in different anatase/rutile (A/R) ratios was applied to IX brine waste (IXWB) and real RO concentrate (real ROC). The synthesized samples were characterized by XRD, FESEM-EDX, and elemental mapping, BET, and UV-Vis absorption spectra. Experiments design, process optimization, and confirmation of results were performed using CCD-RSM. The study also investigated the use of glycerol, a by-product of biodiesel production, as an economic hole scavenger. The effect of different concentrations of SO4−2 on the removal efficiency of NO3− and the N2 selectivity was also investigated. The anatase phase converts to rutile with increasing calcination temperature, resulting in larger crystallites and particle sizes and narrower optical band gaps of TiO2 nanoparticles. Under optimal conditions, the mixed A (79%)/R (21%) phase of TiO2 with FA showed the highest photoactivity in conversion NO3− (89% and 95%) with N2 selectivity (83% and 85% for IXWB and real ROC, respectively). For real ROC, the use of glycerol as an economical hole scavenger resulted in 100% NO3− reduction. A possible mechanism involving glycerol and FA is discussed. Finally, optimized (A/R) ratios of TiO2 nanoparticles were successfully supported on the surface of GAC (GAC/TiO2). The composite sample can be easily recycled and reused from solution and exhibits high photoactivity even after five cycles. [ABSTRACT FROM AUTHOR]