The 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 and report a major improvement in purification control which has substantially reduced operational risk.

Water demand commonly fluctuates throughout the day, and in response to weather conditions, by as much as 30% and this can place considerable pressure on slow sand filters, so it is important that they are managed carefully.

Slow sand filters are commonly employed in the purification of surface water. They employ a biological filter in addition to the physical filter provided by the sand itself and as such require careful management in order to ensure that the biological layer, known as Schmutzdecke, is maintained in optimal condition. Failure to do so can result in the presence of bacteria and algae in the produced water.

If a filter is found to produce water of unacceptable quality, there can be serious repercussions. Firstly, the filter will be taken out of commission for at least 8 weeks. Secondly, this may result in fines by the Drinking Water Inspectorate and thirdly, a drop in drinking water production may mean that it becomes impossible to supply 100% of the distribution network, which in turn will result in compensation payments and the costs associated with the provision of an alternative supply.

Problems associated with slow sand filters usually relate to flow rates -- low flow can result in reduced levels of dissolved oxygen that would affect the Schmutzdecke and high flow rates can result in excessive turbidity. The target is to establish a flow rate that is equivalent to a drop in water level in the filter of at least 2 inches per hour.

Conscious of the need to be able to measure flow continuously, engineers at the treatment plant reviewed the flow measurement options and were dismayed to discover that their options were extremely limited.

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