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
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.
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
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
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
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.
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
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
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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