Characterisation of occupational exposure to benzene, toluene, ethylbenzene and xylene associated with wastewater treatment plant in Gaborone, Botswana

Abstract

Background The treatment of wastewater has become increasingly complex as urbanisation and industrialisation introduce volatile organic compounds (VOCs), particularly BTEX (benzene, toluene, ethylbenzene, and xylenes), into sewage systems. In Gaborone's semi-arid climate, where temperatures frequently exceed 30°C, the volatilisation of these compounds poses potential occupational health risks, including respiratory irritation, neurological effects, and long-term carcinogenic risks for workers, yet systematic characterisation of worker exposure remains limited. Aim This study aimed to characterise occupational exposure to BTEX compounds at the Gaborone Wastewater Treatment Plant (GWWTP) by evaluating air concentrations across different treatment stages and comparing them with international safety standards. Methods Ambient air sampling was conducted over 18 weeks using passive samplers from Scientific Kit Corporation (SKC 575-001) deployed across three treatment zones: night soil discharge area, screening unit, and grit chamber. Samples were analysed at an ISO17025-accredited laboratory using gas chromatography-mass spectroscopy (GC-MS). Environmental parameters such as temperature, humidity, flow rates, and retention times were monitored throughout the sampling period. Statistical analysis included ANOVA and Tukey HSD tests to evaluate spatial and temporal variations in BTEX concentrations. Results The concentrations of BTEX compounds varied significantly across treatment locations (p < 0.001) but remained below international occupational exposure limits. The Night Soil Discharge Area showed the highest mean concentration (0.093 ± 0.077 μg/mL), followed by the Screening Unit (0.032 ± 0.033 μg/mL), and Grit Chamber (0.007 ± 0.013 μg/mL). Toluene emerged as the predominant compound (mean 0.113 ± 0.080 μg/mL), accounting for approximately 48.7% of total measured VOCs. While spatial variations were significant (p < 0.001), temporal variations in daily operations were not statistically significant (p > 0.94). Environmental monitoring revealed that retention time (0.63-2.85 hours) had a stronger v correlation with BTEX concentrations than temperature (20.0-38.0°C), contrary to previous research predictions. The facility achieved an 87% reduction in BTEX concentrations from initial to final treatment stages. Conclusion This study provides a comprehensive assessment of BTEX exposure patterns in Gaborone's semi-arid climate wastewater treatment facility. The findings demonstrate that while BTEX concentrations vary significantly between treatment stages, current facility design and operational protocols maintain exposures below international safety thresholds. The study highlights that while concentrations were consistently below occupational exposure limits, the Night Soil Discharge Area presents heightened risks for worker exposure requiring targeted interventions such as improved ventilation or enhanced personal protective equipment. Furthermore, retention time appears to be a more critical operational parameter for BTEX management than previously recognized, particularly in semi-arid climates. These results provide an evidence-based framework for improving occupational safety protocols in similar facilities across developing countries.