Electrochemical Processes for Degradation/Removal of Pollutants from Healthcare Wastewater

Emerging pollutants present a new global water quality challenge with potentially dramatic threats to human health and ecosystems. Pharmaceuticals are among these emerging environmental pollutants, and their fate in water bodies is an increasing environmental concern. Hundreds of tons of pharmaceutical compounds are annually dispensed and consumed worldwide. The concentrations of pharmaceuticals in healthcare wastewater measured up to 150 times higher than in urban wastewaters. Studies show that conventional wastewater treatment processes are not an efficient solution for the removal of emerging environmental pollutants such as pharmaceuticals, which are therefore released into the environment. Thus, the inefficiency of conventional treatments in destroying bio-recalcitrant contaminants has prompted the search for more potent methods.

In this PhD thesis, the occurrence and ecotoxicological effects of pharmaceuticals in freshwater aquatic environments in the African and European context were investigated, which helps to identify a manageable and smaller subset of pharmaceuticals of high concern for deployment of experiments on its removal. A literature survey has been performed, resulting in 3024 data points related to the environmental occurrence. The concentration levels of 71 pharmaceuticals were assessed. The top ten most frequently detected and quantified compounds in both continents were sulfamethoxazole, carbamazepine, diclofenac, trimethoprim, ibuprofen, naproxen, paracetamol (acetaminophen), ketoprofen, venlafaxine and clarithromycin. The maximum concentrations of 17β-estradiol, estriol, ciprofloxacin, sulfamethoxazole, paracetamol, naproxen reported in African aquatic environments were ~3140, ~20,000, ~125, ~100, ~215 and ~171 times higher, respectively than the concentrations reported in European based studies. The variation in pharmaceutical consumption, partial removal of pharmaceuticals in wastewater treatment processes, and the direct discharge of livestock animal farm wastewater were identified among the major reasons for the observed differences. Several pharmaceuticals were found in aquatic environments of both continents in concentration levels higher than their ecotoxicity endpoints. In Europe, compounds such as diclofenac, ibuprofen, triclosan, sulfadimidine, carbamazepine and fluoxetine were reported in a concentration higher than the available ecotoxicity endpoints. In Africa, much more compounds reached concentrations more than the ecotoxicity endpoints, including diclofenac, ibuprofen, paracetamol, naproxen, ciprofloxacin, triclosan, trimethoprim, sulfamethoxazole, carbamazepine and fluoxetine, estriol and 17β-estradiol.

Out of the studied 71 pharmaceuticals, lamivudine was selected for electrochemical experiments, as it is the most frequently reported pharmaceutical in African aquatic environments, with recorded concentrations higher than all other surveyed pharmaceuticals. Furthermore, 70 % of the drug is not metabolised in the human body and excreted unaltered via the urine. Moreover, it was demonstrated that it is non-biodegradable during biological treatment and has toxic effects on activated sludge bacteria. Moreover, it is very stable for various forced decomposition conditions of hydrolysis, UV light, low H2O2 concentrations and thermal stress. Thus, in this PhD thesis, the performance of photoelectrocoagulation, peroxi-electrocoagulation and peroxi-photoelectrocoagulation for the removal of the antiviral drug lamivudine formulation from wastewater by a stainless-steel electrode was explored. To investigate matrix effects for this oxidation process, the influence of substrates such as urea and simulated wastewater (SWW) was studied. Moreover, degradation kinetics and energy efficiency are also discussed. The results indicate that the removal efficiency was in the order of peroxi-photoelectrocoagulation > peroxi-photoelectrocoagulation (in the presence of urea) > peroxi-photoelectrocoagulation (in the presence of SWW) > peroxi-electrocoagulation > photoelectrocoagulation. In peroxi-photoelectrocoagulation, the 96% degradation of lamivudine formulation indicates a nearly complete degradation of lamivudine. In this process, the presence of urea and SWW resulted in a substantial reduction of chemical oxygen demand (COD) decay. Kinetic studies using linear pseudo-first and pseudo-second-order reaction kinetics showed that the pseudo-first-order equation effectively described the removal of lamivudine formulation. The highest energy consumption per kg-COD decay (i.e., kWh kgCOD−1) was obtained for the photoelectrocoagulation process, while the lowest energy consumption was obtained for peroxi-photoelectrocoagulation, for all electrolysis times. The peroxi-photoelectrocoagulation process was shown to be an effective and energy-efficient technique for removing the antiviral drug lamivudine formulation from wastewater.

Following the observation that the peroxi-photoelectrocoagulation process is higher in removal efficiency of lamivudine formulation from wastewater, its efficiency in the removal of COD, color and phosphate from the real healthcare wastewater was investigated. A 2-level full factorial design with center points was created to investigate the effect of the process parameters, i.e., initial COD/ initial phosphate, H2O2, pH, reaction time and current density. The total energy consumption and average current efficiency in the system were evaluated. Predictive models for percentage COD, percentage phosphate, percentage color removal and energy consumptions were obtained. The results show that the initial COD and pH were found to be the most significant variables in the reduction of COD and color. Hydrogen peroxide only has a significant effect on the treated wastewater when combined with other input variables in the process like pH, reaction time and current density in the reduction of COD. Current density appears not as a single effect but rather as an interaction effect with H2O2 in reducing COD and color. Lower energy expenditure was observed at higher initial COD, shorter reaction time and lower current density. The average current efficiency was found as low as 13% and as high as 777%. Furthermore, the percentage removal of phosphate ranges from 38% to 98% and the concentration of phosphate dropped up to the level of 0.12 mg L-1. The electric energy consumed in the treatment processes rages 0.27 to 5.94 kWh/g-P. The energy consumption per load of removed phosphate was found to depend on the initial phosphate concentration, pH, RT and current density. Overall, the study showed that this hybrid electrochemical oxidation process can be applied effectively and efficiently for the removal of pollutants from healthcare wastewater.