Site surveys in the Highlands

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.