Time-Variant Positive Air Pressure in Drainage Stacks as a Pathogen Transmission Pathway of COVID-19

Abstract

Time-variant positive air pressure in a drainage stack poses a risk of pathogenic virus transmission into a habitable space, however, the excessive risk and its significance have not yet been sufficiently addressed for drainage system designs. This study proposes a novel measure for the probable pathogenic virus transmission risk of a high-rise drainage stack with the occurrence of positive air pressure. The proposed approach is based on time-variant positive air pressures measured in a 38 m high drainage stack of a full-scale experimental tower under steady flow conditions of flow rate 1–4 Ls−1 discharging at a height between 15 m to 33 m above the stack base. The maximum pressure and probabilistic positive air pressures in the discharging stack ventilation section with no water (Zone A of the discharging drainage stack) were determined. It was demonstrated that the positive air pressures were lower in frequency as compared with those in other stack zones and could propagate along the upper 1/3 portion of the ventilation pipe (H’ ≥ 0.63) towards the ventilation opening at the rooftop. As the probabilistic positive pressures at a stack height were found to be related to the water discharging height and flow rate, a risk model of positive air pressure is proposed. Taking the 119th, 124th, 140th and 11,547th COVID-19 cases of an epidemiological investigation in Hong Kong as a baseline of concern, excessive risk of system overuse was evaluated. The results showed that for a 20–80% increase in the frequency of discharge flow rate, the number of floors identified at risk increased from 1 to 9 and 1 to 6 in the 34- and 25-storey residential buildings, respectively. The outcome can apply to facilities planning for self-quarantine arrangements in high-rise buildings where pathogenic virus transmission associated with drainage system overuse is a concern.

Vertical outbreak of COVID-19 in high-rise buildings: The role of sewer stacks and prevention measures

“…In response to such a hypothesis, experimental investigations using pilot sewer stacks [ 44 , 47 ], tracer gases studies in buildings [ 35 , 36 , 48 , 49 ] and mathematical simulation [ 50 , 51 ] were conducted to delineate how the vertical outbreak occurred. Figure 1 provided a summary of the vertical outbreak mechanisms showing the possible route of virus-laden airflow, and the pressure profiles of non-flushing periods and flushing periods, respectively.…”

“…Air may also move down the stack if subjected to entrainment by the falling wastewater as in Fig. 1 B [ 50 , 52 ]. Most of the time (non-flushing periods), the aerosolized pathogens travel from horizontal collection drains, through wastewater pipes or the vent stack, toward the cowl on rooftops under natural buoyancy forces ( Fig.…”

“…1 A). Whereas in toilet flushing events, the probability of positive pressure is associated with the wastewater flow rate or discharge height, and the flow-generated pressure can propagate towards the ventilation exit at top of the roof [ 50 ]. In addition, the wastewater volume for toilet flushing has negligible impact on drainage airflow in vertical pipes [ 51 , 53 ].…”

A review of facilities management interventions to mitigate respiratory infections in existing buildings

“…Six publications in this review confirmed that viral aerosols in Amoy Gardens' building plumbing were sucked into unit bathrooms through floor drains, contributing significantly to the outbreak, with case studies [ 110 , 113 ], experimental studies [ 107 ], and simulations [ 108 , 111 , 112 ]. Unsurprisingly, another study cluster emerged during the Covid-19 pandemic, employing similar research approaches, such as case studies [ 117 ], experiments [ 115 , 116 ], and modelling [ 109 , 114 , 118 ]. However, the settings of SARS drainage studies were all high-rise Multi-Unit Residential Buildings (MURB), while Covid-19 studies extended the research on hospitals and general high-rise buildings.…”

Steady-State Hydraulic Analysis of High-Rise Building Wastewater Drainage Networks: Modelling Basis

“…Coronaviruses are capable of surviving in sewerage systems for days to weeks [5], and more recently, the SARS Cov-2 pathogen ('COVID-19') has been found in high concentrations in a WC in a hospital building [6]. There is thus strong anecdotal evidence that the spread of COVID-19 is linked to malfunctioning WDSs; this has recently prompted risk assessments Buildings 2021, 11, 344 2 of 18 to have been carried out high-rise buildings [7,8]. These assessments indicate that risk is most substantial in the upper floors of high-rise buildings, and is non-negligible.…”