Designed storage capacity of potable service reservoirs to maintain supply during short term outage events

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Hi all

I am currently doing some work on developing some operational standards for the design, operation and maintenance of water network assets. A question I am getting challnged on a lot is how much capacity do we build into our new service reservoirs to maintain supply to our customers during short term outage events on incoming water mains or at the local treatment source. It is about managing the risks of maintaining supply against being able to manage the overall quality of the water in the reservoir.

We currently are proposing a minimum of 24 hours of available supply made up of either storage and/or alternative supply from rezone etc. if the population serviced by the asset is above a specfic size (44k pop in urban area and 22k pop in rural area) then this increases to 36 hours to allow for our emergency response capabiltiy (alternative water supplies - tankers, static bowsers, bottle water stations etc) and the likely longer duration of any repairs to the treatment plant or feeding trunk main.

I am trying to seek views on how members of the Water Network address this issue within their businesses. Do you have a standard methodology you could share with me or I would also welcome your thoughts. I do not think there is a definitive right or wrong answer but would welcome any thoughts / help on this

Regards

Mick

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6 Answers

  1. Apologies for my late reply, Mick (assumed the system will let me know when there is new activity on this thread).

    I've attached three papers that represent our main work on municipal storage reservoir reliabliity.

    Yes, we assumed a failure to occure whenever the reservoir runs dry.

    Best,

    Kobus

  2. Hi Michael, I was about to reference my experience in NWL, but I guess you would know that more than I these days!

    Globally my expereince backs up your comment re "no right answer". I think it depends on the appetite for risk, ie likelyhood and consequence. Age of network assets, key customers in network, confidence in response to rectify, water quality, and ability to rezone.

    I would suggest putting all these things into the mix for the specific situation, get the Ops guys involved and work through it. Personally I would think that (given the focus on customer experience) 24 hours is excessive unless its impossible to rezone. My starting point "rule of thumb" has always been 18 hours (taking into account likely growth), noting that the risk analysis may push that up, or even knock it down a tad.

  3. Many thanks Kobus

    This is very useful. If you are happy to post your papers this would be appreciated

    can i just confirm that when you talk about failure you are referring to the reservoir running empty - ie unable to meet system demand - peak demand situations etc?

    Thanks

    Mick

  4. I've studied the reliability of municipal storage reservoirs using stochastic analysis (modelling user demand, fire demand and supply pipe failures) and have published a couple of papers detailing our results (can post them if anybody is interested). Here are a couple of our findings:

    The number of failures per annum reduces exponentially with the tank capacity.

    The reservoir failure risk is highly sensitive to the ratio of the weekly average demand to the inflow capacity, i.e. there is a great seasonal variation in reliability with the summer peak being the most critical time for failures.

    We analysed a typical residential area where the reservoir capacity requirement was 52, 59 and 49 hours of annual average demand for the US, France and South Africa respectively. Based on an acceptable failure risk of 1 failure in 10 years under seasonal peak conditions, we found the required capacity to be 27 hours, which is substantially lower than those recommended by the design guidelines.

    The reliability of a storage reservoir is not only a function of the size of the storage reservoir, but also of the capacity and configuration of the pipe system supplying it. For gravity systems, the optimal solution seems to be near a supply capacity of 1.3 times the seasonal peak demand with a reservoir capacity of 24 hours (but varies according to local conditions).

    I hope this helps,

    Kobus

  5. Thanks Iouri

    we always allow for potential population growth within any long term investment decisions including new service reservoirs - usually 25 years forward as this is the data we can usually attain. My question is more about the resilience capabiltiy of the reservoir to continue to supply customers downstream when the inlet source is interrupted - either through a burst main or a failure at a treatment source. It is always a question of how much of a risk are you prepared to take and how you are able to manage the impact from within your normal business emergency planning capability. Obvioulsy if we have over capacity it costs more to build and makes managing the water quality of the stored water that little bit more difficult, too small and its ability to provide customers with resilient supply of water is compromised!

    I really dont think there is a definitive answer but a compromise position hence the question to others as to how they reach this decison

    Thanks

    Mick

  6. Hi Mick, Understand that you are talking about the potable water supply here, however the increase in population would also lead to the increased waste water flows as well. Are you talking about gradual natural increase in population over say 5 -20 years or a seasonal one like in the tourist areas where influx of transient population arrive at certain times every year? In any case - you should look at both storage and treatment as one, rather than separate items. Some treatment design can accommodate 10-25% variance in flow rate, some can not. There is also a min retention time required for the raw water depending on its source, however it also can be (temporarily) reduced with some adjustments to the primary/secondary treatment at your TP. In the worst case scenario - you might need to upgrade some parts of you TP to accomodate the increase in peak flows. That should be balanced with the increase in the storage lagoons volume - the best solution will be determined by the land availability/costs, TP design, operational rational, seasonality, population projections etc. Not much of the answer - I know - bit still my 5 c worth... Regards, Iouri