Biodegradability of pharmaceutical industrial wastewater and formation of recalcitrant organic compounds during aerobic biological treatment

The biodegradability of different wastewater samples originated from the industrial production of three pharmaceuticals (naproxen, acyclovir, and nalidixic acid) was performed through the standard Zahn–Wellens test. Moreover, the wastewater composition before and during the test was evaluated in terms of parent compounds and main metabolites by LC/MS, and the biodegradability of the parent compounds was also assessed by performing extra Zahn–Wellens tests on synthetic solutions. The results, besides showing the relatively good biodegradability of acyclovir and naproxen, evidenced the masking role of the organic matrices, especially in the case of nalidixic acid. The latter compound showed to be recalcitrant and persistent, despite the apparently good performance of the Zahn–Wellens test. Deeper evaluation evidenced that the biodegradation of high concentrations of organic solvents and other biodegradable compound tended to “hide” the lack of removal of the target compound.