With reference to D. Knol's letter Water wasted, June 22, I would like to address some of the issues regarding water supply operations as they relate to the author's concerns.
In order that drinking water quality for both health safety and palatability be maintained, water distribution system operators are obliged by the Drinking Water Act and the regulation to operate and maintain the system so it meets the bacteriological, chemical and physical standards.
In order to accomplish these goals, annual unidirectional flushing (at minimum) of all water mains is recommended.
It should be understood that even in a disinfected water supply where a small quantity of residual chlorine is present, bacterial growth can occur on the pipe walls. This biofilm may react with or consume the very small concentration of chlorine. When the residual chlorine is depleted by organics, potentially harmful effects can arise. For example, chloro-organic compounds may be produced which can cause taste and odour problems while disease causing bacteria and microscopic protozoans could multiply. One or both of these outcomes could compromise the effectiveness of the chlorine and thus the safety of consumers.
While it's true that flushing requires fairly large volumes of water, the key idea is that each section of pipe must be flushed at sufficient velocity and volume to move the sediments and dislodge the biofilm from the pipe walls.
As a minimum, the entire volume of each run of distribution main has to be discharged to waste and replaced by clean disinfected water. The volume may be quite large, depending on the length of line that can be practically flushed in one go. To a layperson, the sight of a fire hydrant operating fully open may seem to be an awfully large amount of water, and it is, but as a percentage of annual production and consumption, it is a relatively small, though not insignificant quantity – but it is far less than that used for domestic lawn and garden irrigation.
Interestingly, the letter's author was made aware of the concept of dead-ended water mains and commented on the fact that from an operational and engineering perspective these dead-ends should be avoided if at all possible – that is absolutely true, but not always possible. Dead-ended mains should in good practice be flushed more often, but looped or grid systems are always preferred.
Lastly, the author asks whether there might be a better way to clean water mains. Actually there is, it's called foam swabbing. This technique involves launching and retrieving bullet-shaped polyurethane swabs (commonly called pigs) in various sections of pipeline. It is very effective – essentially a squeegee effect. It is however, a complex operation that is more difficult to do properly. Ideally it requires purpose-built launch and recovery points, although hydrants may be used.
It is also much more costly to do. It must be remembered that at least one, and preferably two full-pipe volumes, being swabbed has to go to waste. However, overall the method uses significantly less water.
I hope that this clarifies some of the issues raised by D. Knol.
R. Byrne, CTech,
Water and wastewater operator (retired)
Vernon