The appliance trap seal has been the primary defence against drainage cross-contamination since its invention in the late 18th century and its introduction into ‘modern’ drainage design from the 1850s.

The Victorian era obsession with the prevention of odor ingress led to complex drainage venting schemes.

These aimed to ensure that local air pressure at the appliance trap was never sufficient to deplete the trap through applied suction pressure or to drive contaminated air though the trap into habitable space by the action of positive system pressures.

Such schemes invariably involved connecting the appliance waste downstream of the trap to vent pipework that eventually led to an open termination above roof level – thus the imposition of atmospheric pressure as a terminal boundary condition.

They were essentially passive in nature, as their operation did not depend on changes in local air pressure conditions.

The ‘if it exists vent it’ passive approach of the Victorian engineer could not survive the growing complexity of buildings, and there had to be modifications to the acceptable level of vent connection.

Design solutions

Current regulations now accept a range of design solutions, from the more conventional ‘one pipe’ system favoured in Europe and the United States to the single-stack approach pioneered in Britain 40 or more years ago but still not universally recognised as a suitable solution.

Technological developments have also added to the interest.

There have been long-running debates on the acceptability of local active control, including air admittance valves and variable volume containment devices being introduced to limit local air pressures, hence protecting the trap seal.

The operation of drainage vent systems is now fully understood and indeed is capable of simulation.

The unsteady flows involved have been recognised as part of a well-known set of flow conditions, including water hammer.

They are also dependent on air pressure transients propagated in the network by changes in the applied water flow following appliance discharge, and possible surcharges of the network due to excessive water flows.

However, it is also clear that an ‘engineering only’ approach to understanding the risks inherent in system operation is insufficient.

Just as important are the maintenance levels applied to the system, the controls possible on occupancy (hence loading), and the control and recording of system modifications that may lead to big changes in system usage – thus a heightened risk of trap seal depletion.

It is difficult, if not impossible, for the designer to allow for such loading changes in the future use of the system.

Although it seems quite reasonable to the Code committee in whatever home country to prescribe allowable maximum flow rates and air pressure excursions, the actuality of operation may be quite different.

The SARS connection

The prevention of contaminated air ingress into habitable space has been a central concern in the design of building drainage and vent systems since the 1850s.

However, failure to provide the necessary protection – primarily through appliance water trap seals – was a contributor in the SARS epidemic in Hong Kong in 2003.

This was particularly relevant to fatalities in the Amoy Gardens housing complex where World Health Organisation and local investigative reports identified dry trap seals as a primary cause.