Emissions and transport of pharmaceutical residues from three wastewater treatment plants in Saudi Arabia and the associated risk for the aquatic environment

Pharmaceuticals are inadequately removed by wastewater treatment plants (WWTPs), allowing their residues to contaminate the environment and pose potential risks. This study investigates the occurrence, removal efficiency, and environmental risks of 16 pharmaceuticals in three WWTPs in Riyadh, Saudi Arabia. Seasonal variations and the leaching behavior of these compounds when wastewater is applied to soil were also examined using laboratory soil column experiments.

A total of 144 wastewater samples over 12 months and 80 soil column samples were collected and analyzed. Wastewater samples were processed using solid-phase extraction (SPE) followed by liquid chromatography-tandem mass spectrometry (LC-MS/MS), while soil samples were analyzed using ultrasound-assisted extraction (UAE) with LC-MS/MS. Of the 16 pharmaceuticals, 10 were detected in WWTP influents and 5 in effluents. Caffeine and acetaminophen were the most abundant (>1000 µg/L), followed by ciprofloxacin, metformin, and others (<1000 µg/L). Baclofen, reported for the first time in the environment, was detected in influents at 0.33–2.82 µg/L. Larger WWTPs (H and M) showed higher pharmaceutical levels than the smaller plant (KSU). Effluent concentrations for the 5 detected compounds did not exceed 34 µg/L.

Pharmaceutical concentrations exhibited seasonal variations, particularly in influents during autumn and winter. Mass loadings in larger WWTPs were significantly higher than in the smaller plant (p ≤ 0.5). Average removal efficiencies for pharmaceuticals exceeded 70%, with caffeine and acetaminophen almost completely removed (99%). Baclofen showed a removal efficiency of 81–97%, correlating with ambient air temperature but only weakly with TSS removal. Removal rates were consistent across WWTPs, despite differences in tertiary treatment processes.

Environmental risk assessments revealed high to moderate risks for most detected compounds, particularly antibiotics like ofloxacin. Effluents also posed ecological risks, highlighting the need for better management of pharmaceutical discharges to reduce environmental impacts.

Soil column experiments showed most pharmaceuticals had a high affinity for soil particles, accumulating in the top 5 cm and not migrating to groundwater, except for trace levels of caffeine and cephalexin in leachate. This suggests limited groundwater contamination potential under natural conditions.

This research provides critical insights into the occurrence, behavior, and risks of pharmaceuticals in Saudi Arabia, emphasizing the urgent need for regulations on wastewater quality and emerging contaminants. By identifying key risks and removal inefficiencies, the study supports efforts to minimize pharmaceutical pollution and protect environment and human health.