Induced two-phase air-water flows and droplet emissions after flushing a squat toilet

Flushing squat toilets can generate numerous aerosols that facilitate infectious disease transmission. However, the precise release locations, temporal rates, and particle size spectrum of aerosol emissions while flushing a squat toilet remain unclear. This investigation proposes a multiphase computational fluid dynamics (CFD) model to investigate the dynamics of waterflow, airflow, droplets, and their nuclei while flushing a squat toilet. The volume of fluid (VOF) method within the Eulerian framework was employed to capture the bulk waterflow and detachment of millimeter-sized large droplets. A coupled Eulerian-Lagrangian approach was adopted to convert large droplets into airborne particles for the Lagrangian tracking. A simplified droplet impact and splash model was developed to resolve the breaking of large droplets into fine droplets. Evaporation of droplets into nuclei, their spatial transport, and the probable inhalation exposure in both the squatting and standing breathing zones, were investigated. Experimental data of the measured waterflow, airflow, and the particles were adopted for model validation. The results reveal that a single flush of a squat toilet produces approximately 590,000 droplets, among which 58% return to the waterflow while the remaining rapidly evaporate into droplet nuclei within 5 s. The flushing-induced airflow can carry the droplet nuclei to the entire cubicle throughout within 2 min after the toilet flushing. A residence of 300 s in the squatting breathing zone results in an inhalation ratio of 4.5‰ of the emitted droplet nuclei. The deposited droplet nuclei are mainly on the rear wall of the cubicle and on the floor.