Evaluation of veterinary antibiotics in a swine slaughterhouse wastewaters and their removal using advanced oxidation processes

Abstract

Antibiotics are found in low concentrations in water sources and surrounding environments. Despite their presence in low concentrations in the environment, they are associated with antibiotic resistant bacteria (ARBs) in water sources thus necessitating stringent legislations worldwide. The burden of ARBs has drawn worldwide attention into investigating this rising phenomena to better understand the seriousness of the effects of these contaminants. Studies conducted, however, have mostly been in the developed nations, and the focus has been on human pharmaceuticals. Information on veterinary pharmaceuticals is very limited, even though, the veterinary pharmaceuticals are known to cause as much havoc as human pharmaceuticals. Given the considerable impact that the veterinary antibiotics can have on humans and the environment, there is need for thorough investigations to be done regarding their detection and removal from water sources using biological and other appropriate technologies such as advanced oxidation methods. However, there is very little that has been done to date in this area globally and in South Africa in particular which highlights the novelty of this work. The findings of this study will therefore inform future decision making by policy makers and governments in handling these veterinary antibiotics in water sources. This study was divided into four phases covering the five objectives investigated. The first phase covered the first and second objectives, in which a suitable and sensitive analytical method was developed for the determination of veterinary antibiotics based upon solid phase extraction (SPE), ultrahigh liquid chromatography with photodiode array detectors (PDA) and mass spectrometry (MS) (UHPLC-PDA-MS). Four classes of antibiotics were selected: tetracycline, β-lactam, sulphonamides and fluoroquinolones. The studied antibiotics were extracted from slaughterhouse wastewater samples using strata-X cartridges. The extraction of antibiotics from water matrices was tested at several pH values. The best recoveries were obtained at pH 2. Depending on the nature of antibiotic, the limits of detection (LOD) and limits of quantification (LOQ) were in the range of 0.1–0.3 μg/L and 1.3–2.9 μg/L, respectively. The range of antibiotics detected in the wastewaters in effluents was 0.008 to 4.9 ng/L while in the influent, the range was 1 to 21 ng/L; thus higher concentrations were found in the influents as compared to effluents. This therefore confirmed the presence of these contaminants in the South African slaughterhouse wastewaters. The second phase entailed the investigation of the third objective which was to determine the possible mechanisms of removal of antibiotics from wastewaters using anaerobic digestion and to evaluate the biodegradation kinetics. A laboratory scale upflow anaerobic sludge blanket (UASB) reactor was employed to treat synthetic wastewater to explore the removal efficiencies of five veterinary antibiotics with an initial concentration of 50 µg/L. In a like manner, batch reactors were further used to evaluate the removal routes of the antibiotics. The UASB reactor was operated continuously under mesophilic conditions to evaluate its performance regarding the removal of organics; biogas production was also monitored. Organic loading rate (OLR) was varied from 8 to 9.2 kg.COD.m-3.d-1while keeping the hydraulic retention time (HRT) constant at 12 h. A chemical oxygen demand (COD) removal efficiency higher than 75% was achieved at an OLR of 9 kg.COD.m-3.d-1, with a HRT of 12 hours. About 80% of the antibiotics were removed during the continuous processes, however, a distinctive pattern of removal was not observed. The kinetic studies using a batch process showed that the removal route for the antibiotics was majorly adsorption to the sludge. Biodegradation occurred alongside adsorption but to a lesser degree. The kinetic data showed that the antibiotics degradation followed a first order kinetic model with half-lives that ranged from 6 to 77 days. Given the ineffectiveness of the biological process against the antibiotics, there was need to explore alternative wastewater treatment technologies. In this case adsorption and photocatalysis were investigated. Phase three presents the preparation and characterization of the integrated photocatalyst (IPCA). The adsorption properties of the IPCA, titanium dioxide (TiO2) and activated carbon (AC) were assessed using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD). These adsorbents were used to treat wastewater containing the target antibiotics. The effect of process variables such as adsorbent concentration, contaminant concentration and solution pH was investigated. The IPCA demonstrated good adsorption ability attaining removal efficiencies of over 50% while AC efficiency was over 60%. However, TiO2 demonstrated negligible adsorption performance. In the fourth phase, objective five was evaluated to determine the effectiveness of advanced oxidation processes (in this case photocatalysis) using IPCA and TiO2. The effect of process variables such as photocatalyst concentration and solution pH were investigated. It was found that photocatalysis attained almost 100% degradation of the target contaminants. Maximum removal efficiencies for both AC and IPCA were above 50% for an initial concentration of 100 mg/L. Adsorption using AC and IPCA followed the Langmuir and Freundlich isotherms, however, higher coefficients of correlation were obtained for the Langmuir isotherms for four of the antibiotics viz. AMO, CIP, ENRO and TET. The Freundlich model was the best fit for the SULFA in terms of the coefficient of correlation. With regards to the photodegradation, it was found that photocatalysis attained almost 100% degradation of the target contaminants. Complete degradation was achieved within half-lives of 60 to 102 minutes for all the compounds. Although both photocatalysts effectively degraded the contaminants, the IPCA had the unique advantages of possessing both adsorptive and photocatalytic properties. The activated carbon in the IPCA provided sites for the attachment of the antibiotics and TiO2 thus enhancing the photocatalytic performance. Apart from this, the IPCA can be easily recovered for reuse by decantation unlike the slurry TiO2. Therefore, the study demonstrated the effectiveness of the IPCA as a suitable photocatalytic material for the complete degradation of these antibiotics.