The Treatment of Antibiotic Excess Sludge via Catalytic Wet Oxidation with Cu-Ce/γ-Al₂O₃ and the Production of a Carbon Source

Abstract

In the present study, the effectiveness of catalytic wet oxidation triggered by using Cu-Ce/γ-Al₂O₃ to degrade antibiotic excess sludge was investigated, during which some small molecule carboxylic acids were produced, which are valuable in biological wastewater treatment as an organic carbon source. The influence of reaction parameters on the degradation efficiency was explored through single-factor and orthogonal experiments, including catalyst amount, reaction temperature and time, and oxygen supply amount. The results illustrated that the treatment system can achieve 81.2% COD and 93.8% VSS removal rates under optimized reaction conditions. Carboxylic acids produced after the sludge degradation mainly included acetic acid, propanoic acid, etc. The results of wastewater biological treatment experiments exhibited that the degraded solution after catalytic wet oxidation has potential to be used as a carbon source to meet the demand of biological treatment, which helps the removal of COD and TN. This work confirms the effectiveness of catalyst for enhancing antibiotic excess sludge treatment, which provided a new idea for the rational disposal of antibiotic excess sludge.

1. Introduction

Wastewater treatment plants have been employed throughout the world to treat wastewater and are a key part of the residential water cycle [1]. Due to the increase in the production and consumption of drugs, an increasing concentration of pharmaceutical compounds has gained much attention. However, due to the complex composition of sewage wastewater admitted to sewage treatment plants, antibiotic substances are difficult to be degrade effectively. At the same time, microorganisms in the sewage treatment system develop resistance under the influence of antibiotics, which not only deteriorates its treatment effect, but also may be discharged with the effluent water to cause a risk of polluting the natural water environment [2,3,4]. Pharmaceutical wastewater contains various high toxicity pollutants, including refractory antibiotics. Some antibiotics are adsorbed in pharmaceutical sludge. Therefore, it is important to adopt effective methods to dispose of antibiotic-containing sludge. Usually, the disposal methods of residual sludge include landfilling and incineration, among which incineration has certain economic benefits, while landfilling poses the risk of environmental leakage, which is not suitable for the treatment of hazardous sludge [5,6,7]. With regard to the incineration method, secondary pollutants can be released, for example, dioxins, sulfur, and nitrogen oxides [8]. In recent years, advanced oxidation technology has demonstrated a large application potential in sludge treatment [9,10,11]. Antibiotic sludge safety issues and resource utilization could be solved at the same time by choosing a reasonable disposal method. Catalytic wet oxidation technology has been widely studied due to its strong oxidizing properties for organic pollutants [12,13]. In this process, the solid was transferred to liquid through thermal hydrolysis, which happened during the solubilization process. This process provides high volume reduction efficiency and stabilization of heavy metals. Some small molecule organics, produced after the oxidation process, may be utilized for the growth of microorganisms. Furthermore, compared with expensive incineration technology, this process is cheaper, because it can be conducted with a self-sustaining condition and heat can be resource-utilized. However, there is a lack of detailed study on the treatment and utilization of antibiotic-containing sludge using catalytic wet oxidation, and the degradation mechanism is still unclear. The biochemical performance of the reaction solution after oxidative degradation of sludge and its performance as a carbon source for reuse in wastewater treatment systems are not clear. The antibiotic excess sludge can be degraded efficiently to improve its safety, and, at the same time, the degraded material can be used as a carbon source to replenish the sewage treatment system to improve the biochemical properties of the system, which is extremely favorable to save treatment costs and begin a new process.

Recent studies have focused on the exploitation of new catalysts, which induced a decrease in the operating temperature and an increase in the reaction efficiency [14,15]. For instance, various solid catalysts and homogeneous catalysts exhibited high efficiency, including Cu²⁺, Mn²⁺, and Ni²⁺ [16,17]. In our previous studies, we reported on some catalysts, for example, CuO-CeO₂/γ-Al₂O₃, molecular sieve, etc. [18]. Aluminum oxides are commonly used as catalytic wet oxidation catalysts to enhance oxidation performance [19,20]. Among them, Al₂O₃ is widely used due to its good catalytic performance, high stability, low cost and is easy to obtain. γ-Al₂O₃ has a larger specific surface area than α-Al₂O₃, with excellent heat resistance and high porosity; thus, higher-activity catalytic materials can be obtained with it. Do** modification with rare earth metals can not only obtain well-dispersed composite catalysts, but it also greatly optimizes the reaction conditions and reduces energy consumption [14,21]. For example, a Cu-loaded catalyst exhibited a notable enhancement of the oxidation of acetaldehyde under wet oxidation conditions [22]. In Chou’s study, the oxidation of aromatic compounds was enhanced considerable with a Cu-based catalyst [23]. Taran et al. reported the noticeable effect of Cu in the catalytic wet peroxide oxidation for the treatment of formic acid and phenol [24]. In addition, a Ceria-based catalyst also attracted much attention in the catalytic wet oxidation of pollutants [25,26]. However, the catalytic wet oxidation of real industrial sludge has been rarely studied. Furthermore, the production of useful chemicals in the catalytic wet oxidation is not clear.

Some volatile fatty acids (VFAs), including acetic acid, propanoic acid, etc., can be generated in a considerable amount with in the wet oxidation solution of sludge [27]. These acids have the potential to be utilized as an organic carbon source for the biological treatment of wastewater. Gapes reported on the high yield of VFAs in the catalytic wet oxidation treatment of sludge [28]. Specifically, Chung reported that the concentration of propionic acid could be obtained with 13.5 mg/L at 240 °C in the catalytic wet oxidation of sewage sludge [29]. Up until now, a lot of the available literature has reported that acetic acid and other useful chemicals can be generated after the wet oxidation of organic pollutants [30,31]. Therefore, the study of the production and utilization of VFAs in the catalytic wet oxidation of sludge is very important.

Therefore, in the present study, we used the catalytic wet oxidation technique to treat antibiotic sludge in order to change its physicochemical properties and generate a microbially available organic carbon source. The degradation performance and influencing factors of the Cu-Ce-modified γ-Al₂O₃ catalyst on the antibiotic/sludge mixture were investigated. The organic acids produced during the degradation process were identified. Further, A/O processes were constructed to explore the effects of utilizing the degradation solution as a carbon source on the effluent indexes, and the effects of replacing the degradation solution by adding other carbon sources were compared. These results can provide a reference for the treatment of the antibiotic/sludge mixture and carbon source utilization.

2. Materials and Methods

2.1. Synthesis of Cu-Ce/γ-Al₂O₃

The specific preparation method was as follows: γ-Al₂O₃ was used as a carrier, repeatedly washed with distilled water 3~5 times until the cleaning solution was clear, dried at 105 °C, and then roasted for 3 h at 550 °C. After cooling, it was placed in a mixed solution of Cu and Ce nitrate at a concentration of 2.0 mol/L. The pH value was adjusted to about 10.0 by adding NaOH; then, the solution was filtered and dried. Afterwards, it was baked in a muffle furnace at 700 °C for 3 h and cooled naturally to obtain a Cu-Ce/γ-Al₂O₃-supported catalyst.

2.2. Characterization

The morphology and structure of the prepared catalyst were analyzed via scanning electron microscope (FESEM, SU8020, HITACHI, Tokyo, Japan) measurements with energy-dispersive X-rays spectroscopy (EDS). Brunauer–Emmett–Teller (BET) analysis was used for the acquirement of the surface area and pore size distribution information (America, ** in the removal of COD and TN. This work confirms the effectiveness of this catalyst for enhancing excess antibiotic/sludge treatment, providing new ideas for the rational disposal of excess antibiotic/sludge.