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What is Storm Overflow EDM Data?

The release of EDM (Event Duration Monitoring) data through Environmental Information Requests has driven a high level of public interest and debate into the operation of storm overflows as for the first time, information showing the frequency of discharges has been readily available. It has generated significant media coverage and as a result, DEFRA has set up a taskforce with the objective of identifying measures to "eliminate environmental harm" within the upcoming Environment Bill at its core.

In the past, EU directives and the quality obligations which they have imposed have been the major driver of investment into the waste water system. Brexit means that there will be no new EU regulation, but the release of EDM data and its recent impact shows how public sentiment, equipped with open data, could be the new driver of investment in improvements to the quality of our rivers. At Yorkshire Water, we are very focused on good data quality and improving the confidence of our EDM data as this informs what we do next. There are however some limitations to the data which we will explore further.

Event Duration Monitoring (EDM) Purpose

The purpose of installing EDM equipment was part of a long-term plan set out by James Bevan (Chief Exec of the Environment Agency) in 2013 to deliver storm overflow improvements. The EDM data was to be used to trigger investigations into the environmental impact and identify cost beneficial solutions for delivery. By 2025 we will have carried out investigations on 160storm overflows and as part of our next 5 year plan (2025 and 2030) many further investigations and cost beneficial solutions will be identified and delivered.

EDM Regulatory Requirements

Monitoring requirements for EDM have been set out by the Environment Agency on a risk based approach. Depending on the amenity classification of the receiving water course, there is a requirement to carry out spill monitoring at either 2 or 15 minute intervals and to submit an annual return to the Environment. The annual spill data is calculated utilising DEFRA 12/24 methodology - this aggregates multiple spills within 12 and 24 hour time periods. The Environment Agency can also request the detailed data for start and end times of individual spill events.

Environmental Permit Conditions do not contain a requirement for a level of data availability, quality or a maximum spill frequency or duration.

EDM Monitoring Equipment

The EDM technology was deemed appropriate to meet the regulatory expectations at the time of installation. Storm overflows are mostly monitored via level sensing technology and these sensors monitor the level of the wastewater compared to the overflow point.

Wastewater can be an aggressive environment to monitor and factors such as debris, foaming, flooding of chambers and fauna can all impact on monitoring accuracy. Monitoring also relies heavily on mobile network connectivity and can be located in difficult to access environments such as requiring major road closures to access for safe maintenance.

EDM Data

EDM data is big data - over 80 million data points have been collected to complete the annual return for 2020. Data validation has been prioritised where our reporting tool has indicated potential data quality issues requiring further investigation such as missing, negative or flatlining data.

In addition, the 600 overflows with the most frequent spill events as well as overflows with zero spill events have been analysed to understand if the data is representative of performance. As part of our commitment to continual improvement of this data set, follow on work and investigations will continue.

Where data has been investigated and determined as not representative of performance due to an instrumentation issue, the data has been flagged and manually corrected. A full internal audit trail is in place for any corrected data.

As part of our commitment to data transparency, a summary of our approach to data validation has been submitted to the Environment Agency alongside our annual return.

Other Regulatory Data

EDM and other regulatory monitoring requirements are shown on the flow chart below.

Monitoring the volumes of treated sewage effluent discharged to the environment for dry weather flow (DWF) compliance at wastewater treatment works has been a regulatory requirement for a number of years. The requirement is to monitor the daily flows to the environment with an accuracy of 8% or better. Flow values are recorded at 15 minute intervals. The best place to install the monitor is on the final flows after all treatment processes as this reduces uncertainties. Monitoring can be installed at other locations including at the front of the works provided the values recorded represent the flows to the environment to within 8% of the daily volume.

A new monitoring requirement has been set out by the Environment Agency to assess compliance with the permitted minimum pass forward flow for last in line overflows to treatment works. Monitoring composes of two components, a flow monitor to assess flows being passed (U_MON4) forward and an event monitor to record when an overflow operates (U_MON3). Monitors record values at 2 minute intervals. Investigations and installation of monitors is currently taking place with most monitoring to be installed by March 2025.

There is no regulatory definition of flow to full treatment (FFT) or therefore requirement to monitor FFT. DWF compliance monitors could be used to assess if sufficient flows are being passed forward to comply with the conditions for the storm discharge but the monitors may not be in an appropriate location and are not certified for that purpose.

EDM Data Examples - Otley and Pool WwTW

We have included examples of EDM and flow data for Pool and Otley Wastewater Treatment Works (WwTW).

The Analogue folder is readings of tank levels, which is in the units % to demonstrate percentage of the tank that is full. This data is instantaneous values at regular 15 or 5 minute intervals depending on the Site ID.

The Digital folder is data about the state for the sites (overflowing or not).

The FFT folder contains data about the flow to full treatment into a site, which is in the units l/s. This data is in 15 minute averages. There is only one flow inlet metre per site.

These examples highlight a couple of the limitations with the EDM data:

Where WWTW inlet works receive flows that are pumped to site from the upstream network, this can result in short duration increases to flows. Due to the nature of Flow to Full Treatment (FFT) measurement being required to measure treated flows at a 15 minute average, short duration increases to pumped inlet flows can be smoothed within the average.

Accurate monitoring on storm tanks can also be challenging due to the tank remaining full following storm events until flows have dropped sufficiently to return the tank contents. This means that minor changes to the calibration or the levels themselves (e.g. caused by environmental conditions such as wind on a storm tank) can cause under or over reporting of spills.

The warm up session

We'll talk more about the existing data at the warm up session on Wednesday and also at the full event on 20 July 2021. Our aims are to build better understanding of what the existing EDM data tells us and also to identify additional datasets - either water company or open source - which can help us to better understand the impacts of discharges on water courses and help us and others to plan for cleaner rivers.