Seal of approval

The SARS outbreak at the Amoy Gardens apartment complex in Hong Kong in 2003 infected 329 residents, of whom 42 died. The subsequent investigation by the World Health Organisation and the Hong Kong SARS Expert Committee revealed that the rapid transmission of the respiratory virus partly resulted from the open floor drain traps, which provided a path for virus-laden water droplets to enter the indoor environment.

Appliance trap seals, such as those at Amoy Gardens, play a vital role in safeguarding the indoor environment from the foul gases prevalent in a building drainage system. Among the most common are water-based trap seals, whereby the water in the U-bend provides a physical barrier to contain the gases within the drainage system. However, the water can be displaced as pressure waves generated from actions such as flushing toilets surge through the drainage network, causing the eventual loss of the physical water seal between the indoor environment and the drainage system. A clear path is then created for the flow of the foul gases into the indoor environment, providing a route for cross-contamination.

The problem at Amoy Gardens was exacerbated by extractor fans in bathrooms creating a pressure differential across the trap, resulting in forced movement of foul air from the drainage network into the bathrooms. Further contamination occurred as the infected droplets were discharged to the atmosphere, exposing adjacent apartments and neighbouring buildings to the virus (see figure 1).

The high number of casualties emphasised the importance of ensuring that a building’s drainage system is regularly monitored and maintained to minimise risks to public health. This becomes even more critical when considering high-risk buildings, including those with high occupancies, such as hotels and offices, or hospitals, where the concentration of infectious diseases is greatest.

New research

If the floor drain traps at Amoy Gardens had been regularly maintained to ensure they were fully primed, it is reasonable to assume that the number of residents infected with SARS would have been greatly reduced. At present, however, the only way to identify an open trap is through periodic visual inspections, which is impractical and effectively impossible in a large or complex building. Members of the Drainage Research Group at Heriot-Watt University in Scotland have undertaken research funded by the Engineering and Physical Sciences Research Council and international collaborators with the aim of developing a maintenance technique for detecting and identifying open traps using remote and non-invasive means. This would provide a valuable tool for facilities managers to assess regularly (and potentially remotely) the condition of a building’s drainage system, helping to prevent cross-contamination.

The methodology will also help public health designers by highlighting areas with recurring problems that may require modification to ensure compliance.

The proposed method is based on sending a low-amplitude air pressure wave through the drainage system and analysing the response. Similar methods have been developed to identify leaks in large-scale water supply networks. As the pressure wave propagates it will be transmitted and reflected by all the parts of the system including the branch-to-stack junctions, air admittance valves, open terminations and appliance trap seals with each having a specific transmission and reflection coefficient (see figure 2).

Every system component the propagated pressure wave encounters along the length of the pipe will determine a characteristic response signature. If any of these components is altered, the system response will be changed because the time taken for the altered reflection wave to arrive back at the monitoring station will be different. Once this time is known, the position of the altered component can be identified as the wave propagation speed is known.

The loss of a trap seal can be identified by comparing the pressure-time history of a test trace with a benchmark profile obtained while the system was defect free, because an open trap generates a negative reflection coefficient (similar to an open termination) compared with the positive coefficient for a full trap seal (see figure 3). The Heriot-Watt researchers have developed an algorithm that compares the two traces and returns the time at which the test trace diverges from the defect-free benchmark, hence delivering the reflection time and the location of the open trap.

Verification of the proposed technique was provided by initial field trials on a standard single-stack drainage system in an unoccupied 17-storey residential building in Dundee. A low-amplitude pressure pulse, generated by a pneumatic piston, was introduced to the 150mm diameter cast-iron stack via existing access panels.

The system was tested by removing the water seal from each of the toilets in turn and recording the system response using a high scan-rate pressure transducer located on the inlet branch. A sample of the measured system responses, compared with the defect-free benchmark profile, is shown in figure 4.

To avoid the incident pressure wave dividing at the branch-to-stack junction, and a portion of it being lost to the sewer section, a valve was installed at the junction to give control over the direction of the pressure pulse flow. Although the location of each open trap was successfully identified, observations showed that under certain conditions trap seal displacement occurred in response to the pulse generation, which itself posed a threat to trap integrity.

To overcome this, further laboratory development was carried out. This demonstrated that the application of a sinusoidal pressure excitation at frequencies of about 10Hz would produce minimal trap seal movement because of the rapid rate of change of the local conditions at the trap boundary, thus protecting and enforcing trap integrity. The fall in trap oscillation achieved by increasing input frequency from 1Hz to 10Hz is shown in figure 5.

Test run

With the technique now truly non-invasive, the researchers evaluated the methodology in a fully occupied and operational building at Heriot-Watt.

A schematic of the drainage system, which serves the male toilets and shower rooms on levels three and four, with a floor gulley located in the fifth-floor boiler room, is shown in figure 6.

Ensuring all traps were fully primed, the benchmark defect-free profile was first determined. As mentioned previously, the defect-free system response is dependent on all of the connected components and is, therefore, specific to each system. The technique was then tested by removing the water seal from each trap, recording the system response and comparing this with the benchmark profile.

While carrying out these experiments, the researchers discovered that the boiler room floor gulley (trap T21 in figure 6) dried out daily despite being fully primed every morning. The test response in figure 7 clearly indicates a negative reflection returned from trap T21. The recurring drying-out had obvious health risks, which were unlikely to be identified without application of this open trap identification technique. A waterless trap can now be installed to ensure the gulley does not pose a continued health risk.

It is envisaged that, in practice, this short-duration test would be carried out in periods of low usage and so would be programmed to run automatically, for example, during the night following a pre-determined schedule, whether it be daily or weekly, etc.

The compliance factor algorithm can be integrated into a standard BMS to raise an alarm when an open trap is detected, removing the need for maintenance staff to become involved and allowing the system to be controlled remotely.

The researchers are entering the next stage of development and are looking for somebody interested in supporting the research by providing access to an independent operational building. Importantly, the test equipment can be installed readily in newbuild and existing buildings. 

This article is based on a research paper by David Kelly, John Swaffield, Michael Gormley, David Campbell and Lynne Jack of the Drainage Research Group at Heriot-Watt University.