Photodegradation of pharmaceutical compounds in partially nitritated wastewater during UV irradiation

The first step of an anaerobic ammonia oxidation (anammox) system is typically the formation of nitrite (NO₂^-) via partial nitritation, which can generate hydroxyl radical (OH) when irradiated with ultraviolet (UV) light. This study demonstrated that the presence of nitrite in buffer and wastewater matrices during medium-pressure UV irradiation (at λ ≥ 220 or ≥ 280 nm) enhanced the degradation of select pharmaceutical compounds of different therapeutic classes (atenolol, carbamazepine, fluoxetine, and trimethoprim). Total pharmaceutical removals in a wastewater matrix irradiated at λ ≥ 280 for 120 minutes were 47% for trimethoprim, 50% for carbamazepine, 60% for atenolol, and 57% for fluoxetine at fluences of 58.6 mEi m^-2 (2033.1 mJ cm^-2). When irradiated at λ ≥ 220 for 60 minutes, removals were 52% for trimethoprim, 56% for carbamazepine, 69% for atenolol, and 90% for fluoxetine at fluences of 634.7 mEi m^-2 (23:969.2 mJ cm^-2). Reaction with OH accounted for ∼78-90% of pharmaceutical removal at λ ≥ 280 nm. Although direct photolysis did contribute to target compound removal for irradiation with λ ≥ 220 nm, much of the light was absorbed in the buffer and wastewater matrices, and reaction with OH accounted for ∼70-93% of pharmaceutical removal. Quencher experiments with isopropanol confirmed the importance of reaction with OH as the main contributor to pharmaceutical removal. para-Chlorobenzoic acid was used as a probe to estimate steady-state OH concentrations, which averaged 8.58 × 10^-15 M for both matrices at λ ≥ 280 nm and 3.50 × 10^-14 M for both matrices at λ ≥ 220 nm. Nitrosamines were formed and accumulated during the UV treatment step, however, concomitant with their direct photochemical destruction. Presence of the pharmaceutical micro-pollutants studied, such as the secondary-amine containing atenolol and fluoxetine, did not elevate nitrosamine formation.