Wastewater Flow Control

Wastewater flow is one of the key themes in the next Asset Management Period (AMP), as it features in the Environment Agency’s National Environment Programme (WINEP). This programme aims to monitor and control the flow that a site must achieve before going to storm, with storm tanks used only when absolutely necessary.

In order to do this, there are some considerable technical issues that the water industry faces moving forward, and discussions are ongoing as to how these are going to be managed.

Monitoring Flow to Full Treatment (FFT)

One solution to the technical issue of monitoring and controlling flow relating to storm events is to install a device that will monitor when a wastewater treatment works goes to storm, whilst measuring the FFT.  In theory, on a site where flows are precisely controlled, the site goes to storm as the FFT is reached; in reality, this is much harder to achieve.

One of the first challenges that the water companies face is measurement of the FFT, especially under the Monitoring Certification Scheme (MCERTS), which has not been designed for inlet flow control, and retrofitting of flow measurement is potentially very difficult to achieve.

Figure 1: The complications of FFT flow measurement. An example of this can be seen in Figure 1, where flow control on a small rural works is achieved using a simple weir device.  Although very simple, this device ensures that greater than the FFT is achieved at all times.  

Due to lack of space, flow measurement can often be practically impossible and very expensive to achieve.  In these cases, an alternative to FFT needs to be investigated.  There has been ongoing work in the industry by way of an UKWIR study that has been looking into the potential of using flow measurement elsewhere on the treatment works to remotely imply the FFT flow.  The problem with this method is that it will need to use a device that can be certified under the MCERTS scheme, which is normally located on the final effluent to the works, and the temporal hydrodynamics of the treatment works will need to be understood.  In some cases, this might be possible; in others, definitely not.

The Potential in Technology

There are, however, flow measurement technologies that have the potential to make the practically impossible a lot more feasible.

The measurement of MCERTS flow is primarily achieved using open channel flow measurement, which utilises level measurement and flumes (which are also excellent for providing flow control).  Where this is present, some refurbishment may be necessary, but this is by far the most practical approach. 

The second most popular measurement technique is the use of electromagnetic flow measurement, but this method can experience problems relating to the inlet, with fouling affecting the accuracy of flow measurement if not maintained correctly.

In cases where neither of these technologies can be used due to practical issues, the use of Area-Velocity devices is the next option.  Typically, this option has occupied a niche representing 2-5% of the industry’s total installed base.  As a flow measurement device, it offers a simple solution for retrofitting into inlet works, as it does not suffer from the complications associated with flumes, where relatively large heads are required, which in some situations would require substantial changes to the inlets of treatment works.

Figure 2: A typical non-contact Area-Velocity flow measurement device. As with any technology, Area-Velocity devices do have their limitations.  As the devices work on the principle of measuring the area, inlet channels must be kept clean, as any changes due to fouling will affect the accuracy of the measurement.  The devices also work by measuring an average velocity within the channel, so the location of their installation can be relatively complicated.  This issue has been addressed recently with the release of Bulletin 27 of the Environment Agency’s MCERTS scheme, which states that when the minimum distance requirements of 20 upstream diameters and 5 downstream diameters are not met, it is advisable to complete a pre-survey to prove that suitable conditions for the use of the technique are in place.  This evidence should be provided to the MCERTS inspector as part of the inspection process.  The issue here, however, is that there are only a few experts within the industry capable of undertaking such pre-surveys.

The Challenges Ahead

In the next AMP, flow will be a huge challenge.  There are over 2,500 installations to inspect for accuracy of FFT flow measurement, in addition to 3,742 installations that are measured for accuracy of dry weather flow (DWF) measurement.  This is a huge inspection burden for MCERTS inspectors.

In addition to this, there are over 1,000 investigations to complete in the first two years of the next AMP to check flow measurement installations are suitable to use to assess the FFT.  These inspections require specialist skills and knowledge to complete, looking at the potential for flow measurement across the works and implying it to the FFT point.  There is also potential for the installation of over 5,000 event duration monitors to detect when a site weirs over to storm, or weirs over from the storm tanks to the environment.  The issue surrounding event duration monitoring for FFT is the monitoring of data availability, which has historically been a challenge in the industry; this will need to be near-perfect, increasing the management burden of the asset base.

All of this monitoring is done for a purpose - to make sure that all of the treatment works are compliant with both their DWF and FFT flow conditions.  The methodology for managing DWF compliance is well-established; this is, however, not the case for FFT measurement - methodologies have been proposed, but are not yet set.

All of this has resulted in a programme of works in the next AMP (and, realistically, the following one too) that will stretch the technical and delivery capabilities of the water industry as a whole, and increase the need for specialists in the water industry who can help to deliver it.

Moving forward, there needs to be a holistic and systematic approach to water companies’ asset and data management in order to regulate the flow measurement asset base and its data.  The approach would need to ensure that flow installations are routinely maintained and calibrated (including secondary verification), and that data is accurate and available for the assessment of compliance, as flow management becomes a regulated parameter with the same weighting as sanitary compliance.

Oliver Grievson, Z-Tech Control Systems

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