A potential killer beneath our feet

Download PDF version of article

Brian Back, HND, BEng (Hons), CEng, FIET, founder of the Zero Pollution Network – a ‘network of solution providers that aim both to tackle aquatic pollution, climate change, and sustainability issues, and to improve Corporate Social Responsibility and Environmental Sustainable Governance’, who is also CEO and CTO of radio telemetry specialist, Radio Data Networks, discusses the significant health risks to patients, staff, and visitors, from blocked sewers and drains in hospitals and other healthcare facilities. He highlights some of the key technologies for detecting such issues before they become a major problem.

With published papers uncovered dating back to 20031 citing the persistence of SARS and coronavirus in urine and sewage being as long as four days, the current coronavirus (COVID-19) outbreak should make hospital managers in a variety of clinical settings acutely aware that their staff run an increased risk when dealing with blockages, and that a consequential spill of sewage into any public spaces could have fatal consequences. 

Oblivious to the many that work in, visit, or become patients within, the healthcare sector, there are drains and sewers threading their way through the fabric of every building, under pavements and car parks, all carrying the waste stream produced by our healthcare sector – which emanates from toilets, kitchens, drains, and waste macerators.

Out of sight and out of mind the vast majority of the time, people are oblivious to the existence of this intricate network of pipes and stacks until disaster strikes and a blockage occurs. As a minimum, a blockage can lead to a toilet not flushing, a sink failing to empty, or urinals backing up. Irrespective of the cause, the failure to address a majority of blockage quickly can lead to a spill that has the potential to spread infection and disease like a tsunami across a hospital.

Radiological risk factor

The problem, however, doesn’t stop with infection, as there is also a radiological hazard in many sectors of healthcare. This relates to ‘hot stacks’ and ‘hot drains’, i.e. those that carry radioactive isotopes that are a by-product excreted by patients undergoing cancer treatment. It is absolutely true that there is no single ‘magic bullet solution’ to preventing blockages. Even if you have draconian policies against flushing ‘un-flushables’, such as wet wipes and nappies, stop kitchens from using cooking fats, oils, and grease, and outlaw the use of clinical waste macerators, there is still the issue of sedimentation/scaling that can lead to stacks and pipes closing up and blocking. In any drainage network there will be certain locations more prone to blocking than others. Generally, setting aside blockages induced by mechanical failure, if a blockage occurs once, then it is likely to happen again and again, making certain locations become regular ‘hot spots’.

Monitoring for blockages

If anywhere requires an omni-present X-ray or ultrasound machine, then I would argue that it is our sewers, stacks, and drains. Unfortunately, there is no simple or single solution for detecting blockages, and the task is made more challenging through our use of steel/iron stacks and/or stacks hidden in ducts and risers with fire check breaks. 

Having been a pioneer in the art of blockage detection, I have to admit that there is no single technique, and indeed the solution required will vary from location to location. Furthermore, one of the largest issues with any monitoring system/solution is verification – simply how can you demonstrate that a system is working, and will trigger when you need it, without blocking the sewer on a regular basis to test its operation.

Reporting blockages from underground chambers

I would be most surprised if you could find a more inhospitable location for any electronics to survive in than our sewer and drainage systems. Moisture, corrosive gases, detergents, bleaches, rags, tissues, steam, hot and cold water, rodents, and a whole cocktail of chemicals, await the sewage blockage detection system. There is also the chance that a potentially explosive atmosphere could exist, necessitating any electronic devices used complying with the ATEX Directive. 

Were all this not enough, there is then also the issue of how to get the alarm message out of the chamber back to the hospital’s BMS system, without chopping up floors to run cables, changing manhole covers, or compromising cyber security. Finally, whatever is installed needs not only to be robust, but also easy to fit, test, and maintain, without the need for specialist apparatus, or operators with a PhD in computer science. 

The majority of practical systems work by detecting water/sewage levels that are outside normal parameters. Float switches have been used, but with limited success, as FOG (fats, oils, and grease), rags, and congealed pulped cardboard from maceration, can lead them to get stuck in the ‘down’ position, and hence fail to report. In addition, float switches can be difficult to set up, and require a certain amount of free travel to operate; otherwise they can easily get snagged or hit the underside of the cover before they trigger

Dual redundant BDT-based blockage detection

Where we have had a great deal of success is with what we call the bulk dielectric principle. This is where we use a transducer that automatically measures the change in the electrical permittivity of its surrounding media as it transitions from air to water, a technology that we abbreviated to BDT (Bulk Dielectric Transducer), and which, when allied with a radio transmitter head, becomes a very powerful and effective first line of defence in the battle against blockages. The BDT principle also has the very useful trait, since, unlike a float switch, it will ‘fail safe’ (false positive) if covered in rags or sodden debris 

Blocked stack detection

Detecting blockages in stacks is much more of an art, and does require a degree of mechanical intervention. Experience has shown that ultrasonics can be used, but they are difficult to set up, can generate false alarms with scale build-up, and are very difficult to verify/test without deliberately blocking the stack. Further, they need to be coupled using an acoustic grease, and held very securely in place, as the slightest airgap or misalignment can cause the system to fail, ‘false negative’

Three methods proven to work 

In practice there are three methods that have been proven to work, each with its own merits. These include static head pressure, conductivity, and air back pressure. 

Static head pressure is best measured using an atmospherically compensated pressure transducer that is attached to the stack or a lateral via a sealed air nipple. As soon as level backs up the stack beyond the connection point, the alarm message is generated. Typically, this results in a minimum detection head requirement of circa 1 ft (30 cm) above the point of connection of the detector. Reliable detection does require the entire assembly to be both fully air and watertight, plus freely vented to atmosphere, and not reliant on an air admittance valve. Regular inspection is recommended, as is the use of clear air hoses. The latter permits scale/debris build-up, which could prevent detection.

Stack monitoring system using an air pump

The back-pressure technique is more active, and involves a small air pump proactively forcing air into the pipe via a length of smallbore hose, typically 1/4 inch in diameter (6 mm). This technique can be highly effective in awkward locations where space is limited, and is probably the only viable solution that can cater for small waste pipes of below 2 inches (50 mm), in particular those often associated with showers, urinals, and sinks. As with the pressure transducer system, the pipes need to be freely vented to atmosphere. In practice the minimum head for detection is 2 ft (30-60 cm). However, this can be compensated for by feeding the hose down the pipe/stack to a lower level, enabling much earlier detection to be made.

Conductivity-based blocked stack detection system

The third of three aforementioned solutions is conductivity. Numerous parties have tried stainless steel electrodes inserted into the pipe walls, or concentric cable or cylinder assemblies, all with little avail. Sooner, rather than later, they generate false alarms due to damp and scale. In practice the only effective way to use conductivity is to create a side branch or chamber containing the sensors out of the flow. This way, under normal circumstances the electrodes remain dry and free from scale, and only once a blockage arises does the sewage/water enter the chamber and trigger the alarm. 

Alarm panels and secure gateways for connection to BMS systems

The concept of a blocked sewer or stack system is no different to an intruder alarm. Each stack or drain is assigned to a zone, and each has an alarm contact and a fault/tamper circuit.

Typical BMS gateway with air gapped relay contacts

With the ever-present fear of cyberattack, the only certain way to eliminate the risk is to use 100% air gapped technology. Typically called gateways these are – in the case of wireless based sensors – an alarm receiver with an array of volt-free contacts that isolate it from the world, and which thereby avoids the scrutiny of the cyber police.


With strategically placed detectors, it has been demonstrated that many incidents can be detected before they become disasters. Economically, it would never be viable to install a system to monitor every yard (m) of a stack or sewer; hence we recommend that the 80:20 rule is applied, to focus on the ‘hot spots’. These are those sewer or drain locations that have demonstrated a tendency to block previously, or those equally that pose the highest threat, such as those emanating from isolation wards, where the risk of infectious discharges is the greatest. 

Brian Back

By profession a Chartered Engineer, Freeman of the City of London, and Liveryman with the Worshipful Company Engineers, Brian Back HND, BEng (Hons), CEng, FIET, describes himself as a ‘driven corporate and non-executive director’ and technical author, with a career spanning 40 years. He has a ‘proven and unique track record’ dealing with matters ranging from the construction of real-time smart data networks, smart metering, environmental and drainage protection, and smart cities and networks, to social media, CSR, corporate ESG, technological innovation, governmental politics, finance, and the delivery of capital projects to deal with global challenges – from supply chain ethics, and ‘plastics in the oceans’, to sustainable urban development, smart metering, smart water, and wastewater network management. The founder of the Zero Pollution Network, he is also CEO and CTO of radio telemetry specialist, Radio Data Networks.

Reference 1    Sobsey MD, Meschke JS. Virus survival in the environment with special attention to survival in sewage droplets and other environmental media of faecal or respiratory origin, 21 August 2003. Semantic Scholar online (https://tinyurl.com/qsuqbs9).