SUPERCRITICAL WATER OXIDATION OF PHARMACEUTICAL WASTEWATER

Abstract

Pharmaceutical industrial wastewater is typical wastewater consisting of complex organic compounds with higher concentration, microbial toxicity, strenuous to deteriorate, and environmental threatening. Pharmaceutics are recalcitrant molecules that demand advanced treatment techniques for their complete elimination in water streams. Supercritical water oxidation (SCWO) has been proved as an effective technology to remove the recalcitrant molecules from wastewater streams. Experiments were carried out in a continuous flow SCWO reactor by altering reaction conditions such as temperature (400-600 °C) and oxidant coefficient (OC 0 to 3) on total organic carbon (TOC) removal efficiency and gas fractions. Temperature and OC showed a positive impact on the TOC removal efficiency. Liquid product analysis indicated the TOC removal efficiency could reach up to 99.5% without catalyst at 600 °C, OC 3, and maximum H2 fraction was attained at 600 °C, OC 0.5. Finally, the degradation mechanism of ACM using SCWO was speculated based on the intermediate compounds obtained in Gas chromatography - Mass Spectroscopy (GC-MS) analysis. Co-fuels are solvents that promote the oxidation rate of the process by producing extra free radicals and heat generation. To reduce the operating conditions, co-fuels such as methanol, ethanol, n-propanol, isopropanol, and glycerol were implemented in acetaminophen degradation using SCWO. Near-complete removal efficiencies (99.7%) were attained with ethanol and n-propanol at an operating temperature of 500 °C and OC3. The gas composition analysis reflects the highest H2 volume fraction for ethanol followed by glycerol and n-propanol at the lowest supercritical temperature 400 °C and OC 1. However, with the rise in temperature 500 °C and OC 3, the CO2 mole fraction increased confirming the acceleration of oxidation reactions. Catalyst plays a prominent role in oxidation reactions. The use of a suitable catalyst enhances the oxidation rate, generates more amount of radicals, and breakdown complex compounds. To study the technical feasibility of highly concentrated ACM, Fe(II) catalyst of 0.5 and 1 ppm was implemented by varying the temperature and OC. The highest TOC removal of 99.99% was obtained with Fe(II) catalyst 1 ppm at500 °C. Maximum H2 fraction was attained without catalyst at 600 °C, OC 0.5, and with the catalyst at 500 °C, respectively, whereas, CO2 tends to rise significantly with both temperature and oxidant concentration. The catalytic process is more efficient and economical in comparison to the non-catalytic process. Real pharmaceutical sample containing a variety of analgesics, antibiotics, antipyretics, antifungal, β-blockers, and drug intermediates contributing to overall TOC of 2017 ± 49 mg/L was collected from a nearby pharmaceutical industry for this research. The treated effluent with TOC removal of 97.8% was within the permissible effluent limits of CPCB at the optimum operating conditions of 550 °C, 23 MPa, OC 2 and 1 min. With the help of ethanol and n-propanol, the TOC removal of 99.2% and 98.9% respectively at a much lower operating temperature of 500 °C. Maximum removal of 99.92% was obtained with the addition of catalyst Fe(II) to the SCWO process at 1 ppm Fe(II), 500 °C, OC2, 23 MPa, 1 min residence time. The addition of Fe(II) to SCWO has the additional benefit of acting as a catalyst, increasing the reaction rate, as well as a promoter for the production of more HO• radicals, finally completely degrading the recalcitrant molecules at moderate reaction conditions. Batch subcritical water oxidation (SubCWO) experiments have been performed to study the mineralization efficiency of industrial pharmaceutical wastewater. The optimum temperature, oxidant coefficient, and reaction time of the process were found to be 250 °C, OC 3, 60 min, which resulted in a TOC conversion of 57.96%. The experimental modeling results conveyed that the influence of process parameters followed the order: temperature > time > oxidant coefficient. To improve the mineralization efficiency, process parameters were changed to attain the near-complete conversion (~99%) of the pharmaceutical wastewater. The qualitative analysis also showed that only a few components were present in the treated effluent. For real effluent, the degradation efficiency of SubCWO after 4 hrs was nearly 90% and unable to destroy the recalcitrant pollutants completely. The catalytic SCWO significantly reduced the operating conditions, with 99.92% conversion in 1 min. Wet oxidation products are not completely degraded because the ionic mechanism dominates, as opposed to supercritical water oxidation, where the free radical mechanism dominates. As a result, catalytic supercritical water oxidation technology can be used to degrade organic wastewater efficiently and safely at higher concentrations in shorter reaction times while producing value-added products.