Gauging the open flow gauges

Directives from the EC are sometimes more myth than reality, remember the straight cucumber regulation that never was? But there is one ruling from Brussels that, along with new requirements from the environmental regulator, is creating challenges for the water companies when it comes to measuring effluent flows: the Urban Waste Water Treatment Directive (UWWTD).

The directive regulates the reporting of data on the quality and quantity of effluent discharged to the environment. Most of the discharges come from WwTWs, while some originate from industrial sites, and a significant number of them are through open channels. The prime driver for water companies to self-monitor their own effluent flows is the Environment Agency’s (EA) mandatory policy that pollutant loads should be estimated more accurately, for which both concentration and flow data are needed. In its role as regulator, the EA is asking companies which discharge effluent: ‘exactly how much and what is in it?’

In response the water industry is undertaking major investment to bring effluent flow monitoring up to the levels of accuracy required by the regulator. The operators face a double challenge in meeting the new requirements:

l they have to come up with the data on discharges which the regulator requires, even when flows are low,

l they must also be able to demonstrate the validity of their measurements recorded in open channels, even though many of these structures do not strictly comply with the existing design standards.

When the companies installed the measuring devices they did not face these requirements, and retrofitting is as we say these days ‘challenging’ or in the old days ‘bloody difficult’. Anything can be achieved with enough money and the key as always is to find an acceptable balance. The two challenges are examined below and a possible solution offered.

Monitoring low flows

The usual difficulty arises where there are low flows i.e. at smaller WwTWs. The small works while not having the greatest overall effect are the most numerous, least likely to be manned and require similar instrumentation and monitoring, hence their importance.

To illustrate the difficulty of low flow measurement the EA has told operators continuous monitoring of flow rates is needed where daily discharges are higher than 50m3, and intermittent monitoring where discharges are between 5-50m3. Taking daily variations into account, this means companies will need to have devices capable of accurately measuring flows of less than 0.2l/s. That is the equivalent flow of a household tap with lots of solids in it.

Such low flows are below the range of most of the structures defined in the British Standard (BS) which is relevant for the geometry of open flow devices. If you read the standard you will find the construction tolerances are very precise, so even for relatively high flows it is estimated that 90% of current structures would not meet the specification. In the case of low flows the only exception to this generalisation is the narrow angle V-notch structure, which become blocked with leaves or other debris.

Thus to overcome the difficulty of low flow measurements of wastewater requires novel solutions. Among the innovations being developed are new gauging structures, self-validating instruments that estimate their own error based on flow conditions, and new instruments, some using new technologies. The EA is working with water companies to encourage research and will help promote their take-up.

Validating the accuracy of measurements

The regulator is asking for a measurement uncertainty of ± 8% of flow volume over a 24-hour average period. The water companies have around 6,000 open channel structures which measure flows discharged from WwTWs. These assets are of considerable value in measuring flows at the moment, but little is known of their ability to deliver results within the tolerances now being applied. Because the 8% criterion is new it is highly improbable many of the existing structures will comply.

The reasoning behind this is that most of the structures are flumes that were broadly built to the requirements of the BS and deal with daily discharge quantities well above the 50m3/d minimum. Compliance with the standard would deliver a performance accuracy of about ± 3.0%. Figures 1 and 2 illustrate that the flume is a complex geometrical shape requiring careful manufacture and sizing to the flow, and not just a curved piece of concrete. It is also true the BS requires surfaces should be smooth, however the passage of time and sewage containing solids will ensure most flumes are far from smooth enough.

Below standard

At most sites, however, it is generally accepted the BS dimensional tolerances were not achieved during construction and that no calibration trials were ever carried out. As a result there is considerable doubt about the contribution these installations can make to the utility companies’ obligations – a question of particular significance in view of their asset value or more importantly their replacement cost.

In response to the water companies’ concerns, the EA is setting-up a panel of experts to give guidance on how it can assess flow structures that do not meet the BS but may be substantially compliant with its ± 8% uncertainty criterion. The panel’s aim is not to place unnecessary costs on the operators. In any case it is unlikely any assessment will not require in-situ inspections.

Since the water industry identified a serious gap in current open-channel flow measurement technology, those involved are working in partnership to find solutions. Innovative work is being undertaken by organisations such as Sheffield University/Yorkshire Water and Cranfield University. WRc, is also closely involved in a range of novel approaches that will help to deliver acceptable solutions in the search for accurate measurement. One of our current projects aims to identify the most common gauging structures for effluent monitoring and use a range of techniques to identify critical design parameters. The techniques used may include CFD, physical modelling, laboratory testing, sensitivity analysis and using calibration data from structures in the field.

As part of the project, flow ranges and the effect on total daily flow error, as well as instantaneous flow rate, will be examined using WRc’s Meter Performance Model (PERM) The model, when checked against theoretical conditions, shows good

agreement. The investigation of deficiencies in the flume manufacture and installation using PERM shows constant percentage errors at mod-

erate to high flows.

However, while this is relatively good news in that it may be readily possible to recalibrate the outputs from the flume instrumentation to account for structural problems, ultrasonic depth accuracy measurement may not be good enough at very low flows. As is typical in these cases a wide variety of tests showed a wide variety of results. This is relatively good news in that the modelling can eliminate many devices from needing anything more that a recalibration while banding the others into those that cannot be used in their current state and an intermediate band where further sitework is necessary.

targeted investment

Once a company has this sort of analysis to hand it will be in a strong position to timetable investment so as to meet an agreed program to come into compliance with the EA’s requirements. Quick wins to show progress at low cost, while keeping the more difficult and more expensive sites until later will allow new technology and changing circumstances to assist in defraying the expenditure. What is also encouraging is that the current developments do indicate that such hopes for reducing the cost of meeting new environmental standards are not just a vain hope but a distinct reality provided the necessary investment in R&D is maintained – to save a lot you need to spend a little in the right places