## 2 Source of Antibiotic Contaminants  
Antibiotics are considered a critical emerging contaminant in aquatic environments [25]. In order to understand the fate and transport of antibiotic contamination, the primary step is to identify their origins. Aquatic ecosystems receive contaminants in two ways: diffuse/nonpoint and point sources. Point source pollutants are pollution from a particular contaminant source having a specific discharge point. Point sources include pollutant discharges from Waste Water Treatment Plants (WWTPs), industries, commercial outlets, urban drainage, hospitals, and market. Nonpoint source pollutants are pollution from diffuse sources, which do not have a defined discharge pattern and are distributed over an area. These areas include pollutants from agricultural areas, informal settlements, animal husbandry, fish cultivation, pastoral grazing areas, and sewer leakages.  
There has been a high dependence on antibiotics for human and veterinary applications. The average annual consumption rate of pharmaceuticals is estimated to have increased yearly by 5% since 2017 [20]. Antibiotic consumption in 76 countries (Asia, Europe, and North America) from 2000 to 2015 was reported to be 21.1 to 34.8 billion in defined daily doses, with an estimated 200% increase rate by 2030 [26]. Due to incomplete metabolization, approximately 50-80% of antibiotics administered to humans and animals are excreted into the environment as urine and feces and transported with wastewater [27]. WWTPs generate the major antibiotic residual contribution to the aquatic environment [20]. WWTPs receive untreated wastewater from various sources, such as households, commercial outlets, hospitals, and factories. Currently, most WWTPs are operated using conventional treatment techniques, which are less effective in treating organic compounds. Although advanced removal techniques, bioreactors, advanced oxidation, and nanotechnologies are applied, WWTPs cannot completely remove all residues. As a result, high antibiotic residue concentrations are detected in WWTP effluents [28; 29; 30; 31; 32]. Therefore, WWTP sludge and effluents are known to be a significant source of antibiotic pollution in the aquatic environment. Wastewater and sludge generated from pharmaceutical industries contribute to high concentrations of antibiotic contamination in aquatic environments [33]. Further, household usage of antibiotics, unused and patient-utilized antibiotics from hospitals, and unintentional disposal due to the lack of an engineered solid waste collection system are other potential sources for extensive antibiotic release to the environment. Solid waste disposal sites accumulate unused prescriptions from municipal waste, creating diffuse antibiotic pollution from wash-off due to intense rainfall, and are transported into the water environment through surface runoff and infiltration. Furthermore, due to sewer leakage and lack of connection to sewer systems, wastewater from households and industry are disposed of into the river system directly or through runoff and contributes to significant contamination in the environment.  
Additionally, food-processing industries, aquaculture, and livestock farming, generate considerable residual antibiotic contamination [34]. Antibiotics applied for breeding, growth promotion, and infection treatment in aquaculture and livestock farming account for 63-84% of total usage [33]. In livestock, unabsorbed antibiotic residues are excreted as feces and urine, while in aquaculture, antibiotics applied through direct feeding and splashing produce extensive waste directly entering the water environment.