Using temperature to help measure Inflow and Infiltration

For a number of years, the recognised way to measure Inflow and Infiltration (I&I) into a sewer network has been via area velocity flow measurement. This traditional method takes into account hydraulic flow and rainfall data in relation to both wet & dry weather patterns. The method is certainly effective, but it also has its limitations.

Timing isn’t everything

Infiltration can often be attributed to a significant event. If there is heavy rainfall in a specific location over a period of time, we can use a traditional ‘Time of Concentration’ (Tc) method – the time required for water to flow from the furthest point in a watershed to the watershed outlet. This ‘lag’ allows us to measure flow based on the Tc. However, during large flow events, the smaller inflow & infiltration events can often be masked therefore missing subtle changes in the hydraulic response. This is one of the key limitations of conventional I&I monitoring.

At Detectronic, we’ve been exploring how those responsible for managing the operation of wastewater sewers can potentially use temperature to assist in accurately measuring the true level of I&I within a wastewater sewer network.

Cooling off

During an extensive I&I measurement project Detectronic completed for a holiday park in the southwest of England, we used the inbuilt temperature sensor of our MSFM 2.5T AV flow meter to further measure inflow and infiltration on the site.

Using the inbuilt temperature sensor on the MSFM AV 2.5T, we continuously recorded the temperature of sewage in the local network – ambient sewage temperature usually hovers around 12-14°C. When an abundance of water came into the network, we saw the temperature of the sewage falling by 1.5°C. The cold surface water coming from surrounding run-off cooled the background temperature of the wastewater. From the study it was concluded that even a subtle drop in temperature can be an indication of surface water flowing into the network.

Further studies using temperature

As a result of employing temperature data alongside the traditional method of area velocity flow data, we were able to provide a highly comprehensive report for the holiday park owners who were, in turn, able to implement specific measures to successfully mitigate I&I and prevent historic flooding and pollutions. The outstanding results encouraged Detectronic to establish further studies using the MSFM AV 2.5T AV flow meter connected with our LIDoTT Smart monitor to measure temperatures in other network monitoring projects. Not only does this combination enable us to record any change in effluent temperature, but we can also record the ambient temperature as well. This information together with the millimetric accuracy of both hydraulic level and velocity, allowed us to successfully mine and forge three key datasets into one.

Measuring temperature across the network

By installing this combination of measuring equipment in specific locations within a wastewater sewer network, we can create a precise picture of I&I. For example, if the upstream temperature is higher than downstream, there is clearly more cooling downstream which means more infiltration is coming into the network downstream. Thanks to the advancement in mathematic models, we can now better understand exactly what is happening and why and calculate how much infiltration is entering the network. This enables us to accurately pinpoint the location of infiltration within the network, thus informing engineers on where action is needed.

Discharge of FOGs

The use of temperature to measure I&I also facilitates insight into the discharge of FOGs (fats, oils and greases) into the network. For example, post lockdown 2020, Detectronic fortuitously conducted a flow survey using the combination of AV Flow and temperature to remotely discover that a local public house had started to prepare for opening up its doors again to the public. A fluctuation in temperature coinciding with a peak hydraulic discharge gave an indication that something in the network had changed.

During the busy lunchtime period (after pubs were allowed to open following the second lockdown) at 1pm the data analysts at Detectronic picked up a significant rise in temperature from the diurnal pattern thus indicating an unusual inflow. It was deduced that this rise in temperature together with the peak flow was caused by the increased use of dishwashers, sinks and toilets during the lunch service!

“Infiltration can reach serious proportions in older systems”*. In some cases, I&I can account for over 50% of the Dry Weather Flow (DWF). Therefore, with population growth, urban creepage, and the impacts of climate change on an ever-aging sewer network, which in some cases dates back to the 1800s, it’s time to explore new and innovative ways of monitoring and managing the complex wastewater sewer networks for tomorrow.

Watch out for LIDoTT ® Smart wastewater sewer management, coming to a sewer near you soon.

* Butler & Davies Urban Drainage 2^nd edition