Event Duration Monitoring (EDM) helps ensure sewage treatment works (STW) are sticking to their permitted discharge limits. When combined with flow to full treatment monitoring it provides an accurate picture of where flow is going when a STW is at full capacity. In this guide we explain what it means, why it’s important and how to implement it. 

What is Event Duration Monitoring and why is it important?

When we talk about an “event” in flow measurement terms, we’re referring to an adverse weather event, typically a storm, that increases the amount of flow going through a sewage treatment works.

In simple terms, Event Duration Monitoring allows a treatment works to monitor whether or not an event is happening, and how long it lasts.

This is important because an STW has a maximum limit it is permitted to treat, known as flow to full treatment (FFT). This is based on a calculation, normally three times the maximum dry weather flow (DWF). Anything exceeding this amount when storm event occurs is permitted to overflow to storm management systems, or be discharged to a river.

The problem arises when a works discharges to storm management before its maximum limit is reached, as this is a breach of its permit and illegal.

The Environment Agency’s U_MON3 driver requires water companies to install EDM monitoring by 2025, and more recently the Government announced further measures to ensure STWs are not relying on storm overflows outside of storm events.

What does the Environment Agency say?

In 2018, the Environment Agency announced a series of measures designed to better monitor flow to full treatment, which included mention of Event Duration Monitoring:

“Install EDM on WwTW overflows to storm tanks at those WwTW where we can’t use existing monitors to be confident that the permitted FFT setting is being complied with.” U_MON3 Driver

In a recent report, the EA said it had installed 12,000 monitors by the end of 2020, with installations on remaining sites expected to be completed by 2023.

The EA says EDM will give them an accurate picture of how water companies are using their storm overflows and allow them to identify where improvements can be made, take enforcement action where necessary, and ultimately improve water quality.

Best practice with Event Duration Monitoring

The EDM device is usually situated immediately upstream of the FFT meter so if any excess flow goes into stream it can record it.

Traditionally, the most common way of conducting Event Duration Monitoring is by using a flow measurement device with probes. However, as they are fundamentally contact devices this can cause problems if the probes become damaged or dirty. Careful installation is essential for this type of device because it can be very difficult to reference them against the low point of the weir.

A better solution is a non-contact device such as an ultrasonic or radar, installed on a fixed robust bracket that includes a calibration reference point and correctly referenced against the spill height. With care, this should be able to be undertaken within +/- 2mm.

As with any flow measurement device, EDM needs to be set up and calibrated by a qualified MCERTS inspector to ensure it doesn’t trigger too early or not at all.

Talk to us about Event Duration Monitoring

To find out more about EDM and how we can help, get in touch with our friendly and professional team.

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