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Chlorine residuals in long pipelines

This question came through email from Sarah:

There are a few issues that I have been doing some desk research on, that I would also be interested to hear how the UK Water Industry deals with them, before concluding to make recommendations for a project that I am working on in a low income African country.

The issues relate to:

  1. How they maintain chlorine residuals in very long pipelines – for example 80km long (obviously this is also a very hot climate which adds additional problems, but I’d still be interested to know how the UK Industry deal with this)?
  2. Re-algae: a. Whether the UK Water Industry undertakes specific identification of algae to establish any potential toxin problems? b. If they do identify the algae what methods do they use? c. Or if the identification is too complex because of the wide number, so they just focus on ensuring effective treatment? d. If they do treat which methods do they tend to use? (activated carbon, chlorination, sonic system etc.)
  3. What specifications are used for PPE for handling chlorine and cleaning the inside of water tanks / tankers (particularly masks/respirators)?
  4. What specifications do they use for paints for the inside of water tanks? (I have found some specific makes that are approved by DFID, but I was wondering if there is a standard specification that the industry uses)

Thanks for your help in advance!

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Hi Sarah This is a response from Graham Welland from Thames Water in the UK.

  1. How to maintain chlorine residuals in very long pipes:

Although a pipe may be long the key issue is how long the water takes to flow through the system. The best way would therefore be to minimise this time as far as possible through design of the distribution system. If is not practical and water stagnates for several days then the likelihood of chlorine decay will be substantial . In this case you either have to booster chlorinate at key points to maintain residual or you have to move away from chlorination to chloramination. Chloraminiate water involves the addition of ammonia to the chlorinated water. This produces a more stable compound which will not be as volatile chlorine on its own and so not decay so quickly. The proportion of ammonia to chlorine has to be very carefully controlled however so this may not be the preferred option. I would therefore suggest the introduction of booster chlorination points within the system would be the best option.

2.Re-algae: a. Whether the UK Water Industry undertakes specific identification of algae to establish any potential toxin problems?

Water companies in the UK would either have or contract out the capability to identify and count algae. It is important to understand species as each will challenge treatment in a different way and so impact removal. The species present will also produce different toxins.

b. If they do identify the algae what methods do they use?

Simply by microscopic analysis. If you simply want to identify what species are present than gentle centrifugation of the sample to concentrate the algae and observation under the microscope offers a quick method. Because algae can float or are motile it is usual to stain them with iodine first. This essentially kills them and allows them to separate out be seen more clearly under the microscope. If accurate counts are required then a standard volume of water usually 100mls although the volume will depend on the concentration ie high numbers need a much smaller volume is captured, iodine added as before and the sample allowed to settle over 24 hours before sucking off most of the water to leave behind the small undisturbed volume that settles out say 5 ml. This can then be observed and counted under the microscope.

c. Or if the identification is too complex because of the wide number, so they just focus on ensuring effective treatment?

As mentioned before the type of algae will challenge the treatment in different ways. The shape of the algae is often a good crude way of assessing this. Pin shaped (penate algae) are able to penetrate sand filters and so cause them to block. Centric (round) algae will filter out well but often can cause taste and odour issues. Other algae float and so are easy to remove by sedimentation while others are so small they will pass through filters easily and need ... (more)

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