Site surveys in the Highlands

IETG explains the benefits of telemetry controlled flow monitoring

A long-term flow-monitoring network can provide water companies with the necessary data to verify a macro-model, enabling them to predict the effects of storm conditions on existing and proposed wastewater networks and STWs.

Edinburgh is situated on the Firth of Forth on the east-coast of Scotland. The city covers 26,113ha with a population of around .5M. There is very little industry in the area and the city has undergone a number of redevelopments throughout its history.

The city originally discharged all its wastewater into nearby coastal waters via seven or eight outfall sewers, with no pre-treatment. During the 1970s two major trunk sewers were constructed running east to west. These picked up the flows from the outfall sewers and diverted them to a new primary treatment works at Seafield. This scheme was later extended to include flows from East Lothian. The present day network remains much the same with the addition of other trunk sewers conveying flows to Seafield from the city and its outlying areas.

Driving force
In the late 1990s, EoSWA decided to install a long-term flow-monitoring network throughout the Edinburgh catchment area. The requirement for real long-term data was seen as essential in providing increased confidence in the output results from the Edinburgh macro-model.

There were a number of major drivers behind the decision to install the monitoring network. It would enable EoSWA to plan strategically with regard to its capital investment programme, which included amongst others the Almond Valley trunk sewer.
The Urban Wastewater Treatment Directive (UWWTD) required improved treatment capability at Seafield STW. A private finance initiative (PFI) contract was implemented to provide improvements and to maintain and operate the works over the next 30-years. Capital expenditure on the PFI contract is valued at around £100M.

The PFI proposals were based upon the results predicted by the Edinburgh macro-model, so reliable long-term data was required to provide viable proposals and predict future income. The PFI contractor is paid per/m³ of wastewater treated.
Confidence in the results was necessary to obtain finance from the bankers providing the money for the scheme and EoSWA had to be satisfied the scheme would achieve targets laid down by the UWWTD.

Information provided by the macro-model also assisted in the operational aspects of the Edinburgh system, therefore verification of the model results was seen as invaluable.

In addition to the existing macro-model EoSWA had an extensive programme of drainage area studies to be undertaken. These were based upon information from more detailed models. However, the long-term data produced from the flow monitors would verify the output from the individual areas into the main trunk sewers.

Leeds-based IETG was approached to come up with a proposal. The flow-monitoring system required by EoSWA and MWH had to fulfil certain criteria. This included;

  • accuracy better than 10%,
  • long-term life expectancy,
  • low-maintenance costs,
  • high data return,
  • proven technology,
  • intrinsically safe,
  • easily retro-fitted into the existing system,
  • wide varying flow measurement capability,
  • cost benefits,
  • the capability of monitoring reverse flow and surcharge conditions.

As a result IETG presented EoSWA with a proposal for the provision of an ADS system, the proposal covered four options including purchase or hire of the monitors with either manual or telemetry service, and these options were presented with a five-year cost projection.

Having reviewed the options available and the likely length of the monitoring period, the client made the decision to purchase the monitors with the sites being telemetered. This option also ensured data was readily available for EoSWA via the telephone lines, and regular weekly diagnostics could be undertaken to ensure a high-level of data return was guaranteed.

System Specification
The system chosen consisted of 20 ADS 3600 QS flow monitors connected by telemetry to the client's main computer network, the flow meters were supplemented by 12 Casella tipping buckets and Technolog logging modules, also connected via telemetry to the main PC. Data was collected by telephone and analysed using the dedicated ADS Quadrascan Software.

The ADS system measures the flow by utilising three sensors located inside the pipe by an expandable steel ring. The sensors consist of a miniature velocity sensor, a pressure depth sensor and a quad-redundant ultrasonic level sensor.

The project criteria was fulfilled through the technology used by the ADS monitor. The ADS monitors provided accuracy to within ±5% of the true flow conditions and it was possible to obtain within 3% in ideal situations. The quad-redundancy and multiple sensors ensured the chance of losing data because of silting, ragging and pressure problems was minimised and therefore reduced the requirement for expensive site visits. The sensor installation utilised the expandable ring and was easy to install. Special brackets were also made for installation in open channels. The monitors were designed for long-term
reliability and minimal maintenance. Importantly, the initial purchase cost of the equipment was also shown to be recoverable over the first five-year period via the reduction in maintenance and operational costs.

Data obtained from a short-term flow survey, which had been previously undertaken to provide data for the PFI
contractors, enabled EoSWA to select proposed sites at critical points in the system. IETG checked these locations during a pre-site investigation survey to ensure the actual site conditions would provide a suitable flow-monitoring environment.
Of the 20 original sites proposed by the client a number had to be relocated due to the lack of available telephone connection points. Equipment available now, however, would not experience this problem.

The ADS monitor is an intrinsically safe flow monitor and is powered by a 9V battery, with the telemetry connection made via an electronic modem unit (EMU) which is an interface box, placed between the monitor and the modem link to the public switched telephone network (PSTN) lines. This interface had to be situated outside the confined space of the manhole and it was decided to install pavement boxes adjacent to the final locations of the EMU, modem and telephone line connection. The monitor itself was hung inside the manhole and a cable connection between the manhole and the pavement box had to be laid prior to installation.

The installation of the system took four months to complete. The initial proposal submitted by IETG also included a 12-month maintenance contract, this is reviewed on an annual basis and the contract is currently in its fourth year. Data is downloaded on a weekly basis directly to IETG offices, the data is then checked for reliability and a monthly report is provided to EoSWA. Unless there is an urgent requirement, quarterly site visits are made to provide regular maintenance of the equipment such as cleaning of the sensor heads and battery replacements, (monitors could have been connected to mains power but this option was not chosen by the client). The ADS monitor allows remote diagnostics to be undertaken, which may involve the reconfiguring of the ultrasonic sensors should one become greased during surcharge, this means minor alterations can be made prior to the next scheduled maintenance visit.

The system has performed well to date and despite some minor teething problems in the early stages 99.15% uptime has been achieved. All the monitors have produced an average 99% data return annually.

As a result of IETG's flow monitoring programme EoSWA has continually fine-tuned the macro-model, increasing the accuracy from what appeared to be an acceptable 8% to ±3%. When dealing with projects as large as those planned within the catchment area, a 5% improvement becomes very valuable and because the real long-term data has increased the understanding of the macro-model it has led to more accurate proposals for future designs.

A predicted benefit for the future is that MWH and Scottish Water can use highly detailed rainfall models as operational tools with increased confidence and accuracy. The ADS monitors are already utilised in Europe as the basis for real time control and this could be applied within the Edinburgh network to ensure that if the contracted peak flows are exceeded some of the water can be remotely diverted to other parts of the system.


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