Exploring the impact of sewer-derived airflows on the air-pressure dynamics within building drainage systems

Abstract

The performance of a building drainage system (BDS) relies on complex internal airflow and pressure dynamics, governed primarily by the unsteady wastewater flows from randomly discharging appliances such as WCs, sinks and baths. Designers attempt to optimise the safety of the system by including pressure equalisation strategies in the form of ventilation pipes and more active devices such as pressure attenuators and air admittance valves. Failures within these systems can compromise water trap seals, allowing hazardous sewer gases to enter buildings. While these measures can equalise the air pressure within the above ground drainage system, air coming from the sewer can have an effect on the performance also. Traditionally, above and below ground drainage systems are designed in isolation and there is no recognition of the influence of one on the other. This thesis documents the development of a novel model to represent the impact of sewer air on the performance characteristics of a BDS, leading to the development of new conceptual diagrams describing the interaction, that illustrate the correlation between newly introduced terms, such as; modified entrained air and modified air pressures, when the system is exposed to both BDS operation and sewer air. Laboratory experiments were conducted using a full-scale drainage test rig representing both low rise (3 storey) and high rise (34 storey) buildings that together provide empirical insights scalable to real-world applications. This approach bridges the gap between laboratory experiments and real-world dynamics, thereby enhancing the reliability and applicability of the research findings. The research confirmed that the airflow and air pressure regime within the vertical BDS stack is modified by and influenced by the connection to the main sewer in a manner consistent with an interaction analogous to a fan and system loss curve, requiring the solution of simultaneous equations describing both. The findings of this study confirm a direct correlation between pressure fluctuations and building height when exposed to sewer air.