Degradation studies of carbamazepine and escherichia coli in wastewater using a non-thermal electrical discharge reactor.

Abstract

Over the last 30 years, there has been increasing detection of emerging chemical pollutants and biological contaminants in the aquatic environment. As Wastewater Treatment Plants (WWTP) are considered the primary gateway for chemical and biological contaminants into water bodies, it can be inferred that WWTP in their current configuration are ineffective against the emerging contaminants. Therefore, it is imperative to investigate new technologies capable of removing emerging chemical and biological contaminants. With this background, Advanced Oxidation Processes (AOP) are a candidate technology that can be utilised for wastewater remediation. This study explores the potential application of an AOP electrohydraulic discharge as a non-conventional tertiary stage technology for the inactivation of Escherichia coli (ATCC 25922) (E. coli) and degradation of carbamazepine (CBZ) as representative contaminants in wastewater. A reactor geometry with vertical copper electrodes with a 2 mm electrode gap in a point-to-plane configuration, connected in negative polarity to a high voltage direct current power supply, was utilised. Separate semi-batch studies were conducted for E. coli and CBZ. All the experiments were conducted using synthetic wastewater. For the E. coli studies, the effects of the electrode gap on the electric discharge, the influence of initial bacterial density, copper electrode material on bacterial inactivation were investigated. The target treatment criterion in the study was the total bacterial inactivation of E. coli. For an electrode gap of 2 mm, total inactivation for E. coli cell density ranging from 3.96 × 104 to 2.5 × 107 CFU/mL was achieved in 180 seconds. Short inactivation times can probably be attributed to the synergy of shockwaves, radicals, and Ultraviolet radiation within the reactor. Control reactor experiments confirmed the anti-bacterial properties of the copper ions in solution. However, bacteria inactivation can be attributed primarily to plasma discharges in water due to the observed higher inactivation in plasma-treated cells. For example, at 3.96 × 104 CFU/mL initial E. coli density, 4.5 log reduction by plasma treatment was observed at 180 seconds compared to 1.7 log reduction in 300 seconds for copper control. The bacterial cell walls were damaged by plasma treatment as observed for plasma-treated cells through the Scanning Electron Microscope imaging. Thus, the destruction of the cell wall can be proposed as one of the possibly numerous contributing mechanisms resulting in E. coli inactivation by plasma. In preliminary benchmarking against chlorination and ozonation, the electrohydraulic discharge reactor used in this study had considerably higher treatment costs per cubic metre of wastewater. For CBZ degradation, discharge current, air flow rate, and initial concentration were investigated, with removal efficiency and energy consumption being the response variables. CBZ degradation experiments utilised Liquid-liquid chromatography for extraction followed by GC-FID and GC-MS for quantitative and qualitative analysis, respectively. Discharge current, air flow rate, and initial concentration all influenced the removal efficiency to different degrees. However, for energy consumption, only current and air flow rate were significant variables. The highest removal efficiency obtained was 93% ± 4% for 10 and 40 mg/L initial CBZ concentration after 10 minutes of plasma treatment at a current of 0.45 A and no air flow rate. The high removal efficiency could be attributed to higher currents (0.45 A), resulting in an improved generation of highly reactive and high energy species and UV generation. Additionally, no air could also improve discharge stability resulting in an uninterrupted discharge. The experimental setup and plasma reactor used in this study show a prospect for sectors where the concentration level of the contaminants in the effluent is above 10 mg/L based on the investigations carried out within the context of this research. Treatment cost per cubic metre benchmarking of the reactor against established technologies revealed that the reactor still requires significant optimisation. The research on the reactor demonstrated its capability to treat high chemical and biological contaminants in wastewater with possible applications being in pre-concentrated wastewater remediation. However, there is still room for optimisation. One key area of focus being reducing treatment cost, which may be achieved theoretically, pending further experimental investigation, by the introduction of an AC power supply.