Network audit and critical mains replacement

Auditing water demand and eliminating UFW

On any communal drinking water supply, it is important for management to answer two questions: firstly, how much water are we pumping into the supply network as a whole and into each branch of the network? In other words, what is the actual water demand? Secondly, how much water should we be pumping in an ideal situation, in which there is no leakage and no excessive usage by individual consumers? This is referred to as theoretical water demand.

It is important to know the answers to these questions as the first step in a water conservation strategy and process that will benefit the environment while reducing pressure on treatment systems and infrastructure as well as operational costs for the scheme and its members.

The first question is easily answered by monitoring flow through the bulk meter leaving the treatment facility. Monitoring flow through district meters on sections of mains, referred to as District Metered Areas (DMA), lets a scheme see if there is disproportionate demand on particular pipelines, possibly indicating leakage on the main or on the consumer side of connections or unauthorised connections.

Assessing ‘theoretical’ water demand is a simple matter of adding up what individual people and livestock units might reasonably be expected to use on a daily basis. This is not as straightforward as it seems, however, as there are times of the year when demand is higher than normal, such as during hot summer days when people and livestock require more water than they do in winter. Factoring in these variables, the NFGWS provides a ready reckoner that helps schemes calculate their theoretical demand.

The objective of the water supplier is to reduce actual water demand to the theoretical figure, or as close as is possible, bearing in mind that the savings from detecting and repairing small leaks might not always justify the cost involved and remember also that the GWS depends on its members to address water loss or excessive usage on their side of the metered connection.

While the GWS will always aim to encourage water conservation where excessive use is identified on individual connections, the first objective is to identify and eliminate Unaccounted for Water (UFW). This is the difference between the volume of water supplied into the network over a fixed period (actual daily demand) and the total volume of water recorded on consumer connections during the same period. For instance, if the total is recorded on all individual meters over 24 hours is 10,000 litres, but the bulk meter at the treatment facility is telling us that 12,000 litres were fed into the supply, then the UFW is 2,000 litres. UFW will comprise water lost through leaks and through unauthorised (and unmetered) connections.

Quantifying UFW requires the completion of a water audit by the GWS. This is generally conducted late at night as this limits disruption to consumers and because the scheme would reasonably expect to detect very little water demand compared to daytime flows.

Taking note of throughflow on the main bulk meter, the GWS compares this figure to the combined figures from district meters on its DMA lines. Using sluice valves, it can then isolate each DMA and compare the district meter reading to the combined demand recorded through individual meters. Armed with this information, the scheme will use various tools to pinpoint where the UFW is being lost. This can be painstaking work, but well worth it if a large leak or unauthorised connection is identified and dealt with. The more sluice valves it has in place, the better, as these can be turned off in sequence, with flows recorded on the district meter after each individual valve and all individual meters have been closed. Once a significant drop in flow is recorded then you know that the leak is located after that sluice valve.