Water Age and Tank Mixing - Evaluating the Benefit of a Mixing System

I am trying to use WaterCAD to evaluate the benefit of having a tank mixing system vs. not, all else being equal. However, my results show the same water age whether the tank is "completely mixed" or "LIFO". Tanks are ground storage/standpipe. Should WaterCAD be able to compare the two options? If a mixing system is used, do I still input the same inactive volume for the portion of the tank that does not fill/drain?

  • Hello Laura,

    A good place to start is by first understanding which tank mixing model to use. Since you are working with standpipes (tanks) LIFO / 2-compartment could me appropriate to use. See this article which discusses how to select the correct mixing method as per your requirements: Which "Tank Mixing Model" attribute should I use?

    As far as age calculations are considered see the below article which discusses how WaterCAD evaluates water age for LIFO mixing models;

    How does WaterCAD/GEMS determine the age of the tanks using the LIFO or 2-compartment method?

    Hope this helps.


    Regards,

    Yashodhan Joshi

  • In addition to Yashodhan's reply - since you mentioned that you are considering implementing a mixing system, you can certainly try the "completely mixed" mixing model option. Calculated water age results would normally be different with different tank mixing models, but it may also depend on what is happening with your tank. Are you graphing the calculated age over time? Is the tank experiencing good turnover or do you see it only drain or only fill? (try graphing the percent full attribute).

    More water age troubleshooting tips can be found here: Troubleshooting water age results

    If this does not help, please send a copy of the model for review: Sharing Hydraulic Model Files on the OpenFlows Forum


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • Jesse- Let me know if you have another way for me to share the model files. I am not going to share them publicly. It does seem now that ground storage is has higher water age when completely mixed than when LIFO, all other things being equal. Is that what you typically see?

  • Hi Laura,

    Jesse- Let me know if you have another way for me to share the model files. I am not going to share them publicly.

    In the link about sharing model files, you will see an option to share it privately with Bentley only, via a secure "ShareFile" link. If your organization has a preferred file sharing method, you can send me a link via private message, However, you may not need to share the model if the below information helps.

    It does seem now that ground storage is has higher water age when completely mixed than when LIFO, all other things being equal. Is that what you typically see?

    Water age behavior can be different for every system, so there may not be a "typical" trend for one mixing method vs. another that applies to all systems.

    If the water age tends to be higher when set to completely mixed, it could mean that relatively "new" water is entering the tank and it is filling and emptying without covering a large elevation range. So, with LIFO, the newer water enters and quickly leaves, and since the age reported in such case is at the inlet of the tank, it may show a relatively new age. On the other hand with the "completely mixed" option, the new water entering the tank mixes with the rest of the "dead" volume, yielding an overall average (mixed) age that is higher. This is just a guess but hopefully it helps you consider the kinds of things that can happen with a water quality analysis.

    If you are seeing rapid changes in the model EPS run, you may need to try using a smaller water quality timestep to see if the age results are sensitive.

    Note that if you have not done so already, you can use scenarios and alternatives to quickly compare the different mixing methods. Simply use a different alternative in two scenarios, set the tank mixing method, switch to the other scenario, change it to the other mixing method, compute both scenarios, graph age and select both scenarios in the graph series options. You will then see the calculated age line for each scenario in the same graph. Note that the tank mixing model attribute is actually stored in the Constituent alternative, so you will need to use a different constituent alternative between the two scenarios even though you are doing an Age analysis. See: Viewing graph data for multiple scenarios


    Regards,

    Jesse Dringoli
    Technical Support Manager, OpenFlows
    Bentley Communities Site Administrator
    Bentley Systems, Inc.

  • So are you saying the water age results are reporting the water age in the active volume only? That would make my results make more sense. The mixed water age is older, and the LIFO is always newer, but shows high ages when the tank is drawn down during some extreme demand or unusual circumstance. In this case it seems difficult to use these results to quantify the benefit of mixing, which I was hoping I could do.