Combined sewers receive both sewage from buildings and urban runoff. When there is an excess of stormwater, this could potentially cause drains to back-up causing sewage flooding within buildings. To prevent this expensive, unpleasant and dangerous occurrence, sewers are fitted with overflows which allow them to discharge into rivers or the sea, before backing up into buildings. These storm overflows are often simple dams, allowing the wastewater to flow over the top of the dam when a critical level is reached. While this prevents damage to buildings, releasing raw sewage into the environment should be considered an absolute last resort.
When a rare storm even occurs which truly exceeds the capacity of the sewage system, discharges are unavoidable. However, often these discharges occur due to poorly-maintained and blocked sewers, downstream of the storm overflow. This situation currently results in frequent illegal discharges. Liquid level measurement plays a critical role in understanding where these blockages are becoming an issue. Ideally this will be well in advance of an actual discharge occurring. In the worst case, unmonitored storm overflows may discharge continuously for prolonged periods, with blockages going undetected (see also recent feature, via www.is.gd/azepub).
Monitoring will typically be configured at a storm overflow to trigger when the sewage level reaches 80% of the height of the dam, sending an alert that action is required. When the level reaches 100%, this is also logged, so there is a record of when and for how long discharge is occurring, known as event duration monitoring (EDM). The number of store drains equipped with EDM increased from 862 in 2016 to over 12,700 in 2021, 89% of all storm drains. The Environment Agency aims for EDM to be fitted to all storm overflows by the end of 2023. The monitoring data shows that 87% of storm drains have at least one spill in a year, and 5% spill more than 100 times.
“Liquid level measurement is important for two reasons. Once the measured level reaches a certain level, pumps, screens, and valves can operate to redirect sewage. This might not be enough at a very high level, which is why overflows exist. It is then very important to accurately monitor and track these discharges to avoid very high fines,” says Vladislav Snitko, solutions engineer at Emerson.
Sensors are often battery-operated devices that send alerts to a SCADA for remote monitoring. Mobile phone networks are used to transmit this data, typically 3G.
HOW DO LIQUID LEVEL SENSORS WORK?
There are various different sensor technologies used in different applications. The one most people will be familiar is the float used to control the fill valve in a toilet cistern. This is a type of level switch, with electromechanical versions also available. Other types of liquid level measurement sensors use ultrasound and capacitance. Liquid level sensors are used in many applications including monitoring coolant, lubricant and fuel levels in vehicles and machinery and storage tanks. Examples include:
- Level switches use a float that moves up and down with the level of the liquid, this allows them to work with a wide range of different liquids. The float may be magnetic, allowing it to operate a hermetically-sealed reed switch to open and close an electrical circuit. These are simple on/off switches that trigger at one specified height. They are simple, low-maintenance sensors that are resistant to shock, vibration and pressure
- Non-contact ultrasound sensors are time-of-flight measurement sensors that emit regular pulses of ultrasound and detect the echo. The time between emitting the pulse and detecting the echo gives the distance to the surface of the liquid. These are digital sensors that can continuously monitor the liquid level and be programmed to give an average reading for a liquid that may be sloshing around
- Radar is also a time-of-flight measurement, but rather than using ultrasound it uses radiation in radio frequency spectrum. This can be more reliable and accurate
- Contact ultrasound sensors simply detect attenuation as the sensor makes contact with the liquid. These sensors may be mounted in a linear array to measure the level in a number of discrete steps
- Capacitance level sensors use a conductive probe that extends below the surface of the liquid. The probe and the surface of the vessel act as the two plates of a capacitor, with the liquid acting as the dielectric medium. The capacitance increases with the liquid level.
Non-contact ultrasound sensors have been the preferred sensors for liquid level measurement in sewers, although radar is now also being used. These non-contact measurements place the sensor out of the flow of sewage and stormwater. This makes them less susceptible to damage and easer to maintain.
Adds Snitko: “Time of flight measurement using radar waves is based on the time to go to the surface and back. Radar and ultrasonic sensors both use time of flight, but the reliability between the technologies differs quite significantly. Ultrasound is affected by air temperature, gases, wind, and turbulence at the surface. Radar is completely unaffected by the environment and or any turbulence at the surface. Previously radar was significantly more expensive, but is now on a par with ultrasound.
“Hydrostatic pressure transmitters use liquid pressure and density; these submersible pressure transmitters disregard surface conditions, and in applications where dense foam is present, they can sometimes perform better than non-contacting alternatives. However, as these sensors are in direct contact with the media, you will have to maintain, replace, and recalibrate these devices frequently, increasing total cost of ownership significantly.”
REDUCING POLLUTION
Measuring the liquid level as storm overflows can help to reduce pollution in a number of ways. By simply providing information on the number of spills that have occurred, political pressure and financial penalties can be applied to incentivise water companies to improve in this area. Real-time alerts notifying water companies when a spill is occurring enables them to bypass or remove blockages and reduce the duration of the overflow. Continuous monitoring of the liquid level allows predictive maintenance to remove sludge build-ups before they result in actual overflows. Looking at patterns across the sewage network, potentially assisted by AI, can enable improved decisions about maintenance and infrastructure upgrades to make overflows even less likely.
There are essentially two ways that sewer system reliability can be achieved. The traditional way relies on sufficient over-capacity and routine maintenance schedules. The more modern way uses condition monitoring to operate the same physical infrastructure at higher capacity and with less maintenance, responding to blockages and collapses as they occur. The second method has the potential to be more efficient, and with continuous monitoring of liquid levels it could remove blockages before overflows actually occur. However, it is potentially also more vulnerable, with sensors open to cyber attacks. In a conflict, disrupting sanitation could be a major strategic target, allowing a malicious actor to inflict massive casualties without any conventional attack. This highlights the importance of good cyber security in condition monitoring systems.