Microalgae for Antibiotic Removal in Wastewater Treatment: Efficacy, Physiological Responses, and Removal Pathways
Authors:   , Frascaroli Gabriele 
Published: June 2024

In recent years, the rise of antimicrobial resistance in pathogenic bacteria, attributed to antibiotic misuse and overuse, has posed a challenge in treating infections effectively. Antibiotics administered during patient treatment are often incompletely metabolised, resulting in their continuous release into wastewater treatment plants. Conventional treatment facilitiesface challenges in effectively eliminating antibiotics from wastewater, necessitating advanced chemical and physical solutions that are both energy-intensive and costly. Consequently, the effluents from these treatment plants contain substantial amounts of antibiotics, exerting selective pressure on pathogens and exacerbating antimicrobial resistance. In response to this pressing issue, recent studies have underscored the inherent efficacy of microalgae in eliminating antibiotics from wastewater, establishing their potential in sustainable wastewater treatment solutions. Microalgae exhibit proficiency in pollutant removal, nutrient recovery, and generating valuable biomass, making them prominent ecofriendly solutions for addressing antibiotic-containing wastewater. However, several knowledge gaps must be addressed to facilitate the application of microalgae-based solutions in real antibiotic-containing wastewater. It is crucial to assess the efficiency of microalgae in removing antibiotics and to understand how they can impact system performance. This work aimed to enhance the utilisation of microalgae in wastewater treatment for antibiotic removal by addressing key knowledge gaps. Two systematic reviews were initially conducted to identify key antibiotics linked to antimicrobial resistance in wastewater treatment plants and evaluate microalgae strains suitable for antibiotic removal, respectively. The first review highlighted global concerns regarding antibiotics such as ciprofloxacin, clarithromycin, erythromycin, metronidazole, ofloxacin, sulfamethoxazole, and trimethoprim, emphasising their widespread presence, resistance to treatment, potential ecological harm and link to antimicrobial resistance. The second review identified benchmark species and pinpointed knowledge gaps for subsequent experimental phases. Following this literature assessment, the physiological responses of three microalgae species (C. acidophila, T. obliquus, and A. protothecoides) to the selected antibiotics (ciprofloxacin, clarithromycin, erythromycin, metronidazole, ofloxacin, sulfamethoxazole, and trimethoprim)were assessed in small-scale batch experiments, using antibiotic mixtures at environmentally relevant concentrations (10, 50, and 100 μg L−1). These experiments revealed the resilience of the microalgae to antibiotics, with minimal inhibition observed, particularly at higher concentrations. A. protothecoides exhibited a 30.6% growth increase at the highest antibiotic concentration due to hormesis. Additionally, all three species showed increased chlorophyll (up to 37%) and carotenoid (up to 32%) production under the highest antibiotic concentration. A. protothecoides displayed a 46.6% increase in ammonium removal and a 44.8% increase in phosphate removal, suggesting a hormetic effect on nutrient removal. Moreover, antioxidant responses in T. obliquus and A. protothecoides indicated a defensive reaction against antibiotic stress. Then, the analysis explored biotic and abiotic factors in antibiotic removal, including hydrolysis, photolysis, biosorption, and active removal. Microalgae efficiently removed antibiotics, primarily through rapid biosorption, with up to 91.5% removal for specific antibiotics like ciprofloxacin. T. obliquus and C. acidophila achieved total adsorption of 37.2%and 49.3%, respectively. A. protothecoides exhibited the highest active removal efficiency, eliminating 22.1% of total antibiotics. Additionally, abiotic degradation processes, including hydrolysis and photolysis, contributed to the removal of antibiotics such as ciprofloxacin, ofloxacin, clarithromycin, and erythromycin, with removal ranging from 34.7% to 96.7%. During a long-term trial, antibiotic removal and its effects on nutrient removal and biomass productivity in one selected species (A. protothecoides) were investigated in a large-scale bioreactor continuously supplied with synthetic wastewater. The microalgae-based system achieved high antibiotic removal efficiencies of 45.8% to 70.1% within 3-4 days. Antibiotics stimulated algal growth by 11.0%, while ammonium removal increased from 78.0% to 86.4%,and phosphate removal rose from 85.1% to 90.3% in the presence of antibiotics. Biomass parameters like pigments, lipids, proteins, and carbohydrates also increased by 12.9%, 20.6%,45.8%, and 50.6%, respectively. In conclusion, this study highlights the importance of incorporating analyses of antibiotics when employing microalgae-based solutions for wastewater treatment due to the relevant influence of the hormesis effect on the treatment system. The varied capabilities of microalgae in antibiotic removal suggest promising approaches to address antibiotic pollution and antimicrobial resistance in wastewater. This opens up possibilities for broader applications of microalgae-based technology in antibiotic-containing wastewater treatment as a potential aid for standard wastewater facilities or a technology directly applicable to smaller-scale environments significantly affected by antibiotic pollution.