FLOW MONITORING AND METERING
Winning the battle for water
quality is easy with Argonauts
Producing unacceptable water can prove costly. Which is why using an effective flow meter is so important
T
he costs that can result from
unacceptable drinking water quality are
potentially huge, so engineers at a major
UK water treatment works have installed a pair of
YSI SonTek Doppler flow meters at the outlets to
two slow sand filters. As a result, they report a
major improvement in purification control that
has substantially reduced operational risk.
Water demand can fluctuate by as much as
30% throughout the day in response to weather
conditions, and this can place pressure on slow
sand filters, so it is important that they are
managed carefully. Slow sand filters are
employed in the purification of surface water.
They use a biological filter as well as the physical
filter provided by the sand itself and need careful
management to ensure that the biological layer,
known as Schmutzdecke, is maintained in
optimal condition. Failure to do so can result in
The Argonaut-SW uses acoustic beams to measure water level, velocity and forwards and reverse flow
the presence of bacteria and algae in the
produced water. length required by most traditional flow meters How it works
If a filter is found to produce water of was not available. It would be necessary to The Argonaut-SW has three acoustic beams. One
unacceptable quality, there can be serious excavate and construct a flow path before a beam points straight up, and the other two point
repercussions. Firstly, the filter will be taken out of traditional flow meter could be installed. The cost up/down stream at a 45-degree angle. The
commission for at least eight weeks. Secondly, was estimated to be £27,000 per filter and the upward-looking beam measures water level. The
this may result in fines by the Drinking Water engineers were not comfortable undertaking two slanted beams measure the water velocity in
Inspectorate and thirdly, a drop in drinking water such work so close to the potable water supply. two dimensions via the acoustic Doppler method
production may mean that it becomes and can measure both forward and reverse flow.
impossible to supply 100% of the distribution The solution Summarising the benefits of Doppler flow
network, which will result in compensation Nick Martin, from Letchworth-based, YSI measurement, even in very clean water, Nick
payments and the costs associated with the Hydrodata, recommended the installation of a Martin says: “If this water company had chosen
provision of an alternative supply. small Doppler flow meter, the Argonaut-SW. As a to install traditional flow meters an extra capital
Problems associated with slow sand filters result, two Argonauts were installed within the cost of over £54,000 would have been incurred
usually relate to flow rates – low flow can result outlets of the two slow sand filters. and the filters would have been out of action for
in reduced levels of dissolved oxygen that would No capital works were necessary for the at least a week. Instead, we were able to install
affect the Schmutzdecke and high flow rates can installation. The flow meters were attached to the Argonauts in less than an hour with minimal
result in excessive turbidity. The target is to blocks made of the same material as the cost and no interruption to supply.
establish a flow rate that is equivalent to a drop channel in which they were located. A diver “With minimal outlay, the water company was
in water level in the filter of at least 2” per hour. placed the meters in the pipes. able to avoid the substantial costs that would
Conscious of the need to be able to measure The Argonauts scan and store data have been incurred if water quality had suffered
flow continuously, engineers at the treatment continuously; however, two-minute average data as a result of a problem with the sand filters.
plant reviewed the flow measurement options is transferred to the water company’s SCADA “This trial demonstrates that the Argonauts can
and were dismayed to discover that their options system, which controls the flow rate through the resolve difficult flow monitoring issues, even
were extremely limited. filter. This has ensured over two years of trouble those with turbulent water and short flow path
free operation for the sand filters. After the initial lengths, so we look forward to hearing from
The problem installation, a ‘drop test’ was conducted in anyone with similar problems.”■■■
Firstly, the feed channel from the filter had a which a known volume of water was passed
square cross-section, secondly the water supply through the beds and compared with the For further information, please contact:
pipe contained a natural venturi and the pipe Argonaut, which demonstrated accuracy of YSI Hydrodata Ltd; T: +44 (0)1462 673581
was only 2m long, so the 10 pipe diameters in better than 99.9%. F: +44 (0)1462 673582
32 Water & Wastewater Treatment September 2009
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