Comparison of photo-Fenton treatment and coupled photo-Fenton and biological treatment for detoxification of pharmaceutical industry contaminants

This paper compares the feasibility of two different treatment approaches, chemical oxidative treatment by the solar photo-Fenton method and a combined approach applying the solar photo-Fenton method as a pre-treatment for aerobic biological wastewater treatment. The targets are two biorecalcitrant substances used as synthesis intermediates in the pharmaceutical industry, 2-(2,4-dichlorophenyl)-2-(1H-imidazol-1-ylmethyl)-1,3-dioxolan-4-ylmethanol (CAS 84682-23-5) (DIDM) and 2-(2,4-dichlorophenyl)-2-(1H-1,2,4-triazol-1-ylmethyl)1,3-dioxo1an-4-y1methanol (CAS 67914-85-6) (DTDM). Solar photo-Fenton treatment was performed in Compound Parabolic Collector pilot-plants, and biological treatment in an immobilized biomass reactor pilot-plant. Analytical determinations included the measurement of dissolved organic carbon, chemical oxygen demand, inorganic compounds (chloride, ammonium, nitrate), formed during degradation, acute toxicity tests applying Vibrio Fischeri luminescence bacteria and the Zahn-Wellens biodegradability test. The synthetic wastewater contained 200 mg/L of each compound, an initial DOC of 200 mg/l and an initial COD of 700 mg/L. The target compounds proved to be susceptible to complete degradation and partial mineralization (95%) by photo-Fenton. Total elimination of both substances occurred after 74 minutes of illumination and required the consumption of 27.5 mM of hydrogen peroxide. Nevertheless, acute toxicity persisted practically to the end of the treatment, and biodegradability could only be slightly enhanced, never rendering the wastewater completely biodegradable at any stage of the chemical oxidative treatment. By Zahn-Wellens test DIDM and DTDM proved to be non-biodegradable and only samples pre-treated by photo-Fenton until achieving a DOC below 98 mg/L showed a significant degree of biodegradability (60% after 11 days of biotreatment). This particular behaviour contrasts with other substances that have been tested before and may be related to the refractory nature of the imidazole and triazole moieties in the molecular structure of DIDM and DTDM