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Tank Turnover Rates

Discussion in 'General Discussion' started by TbyZ, Mar 12, 2018.

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  1. TbyZ

    TbyZ Member

    I understand that over the years most particulars in regards to scrubber design have been well & truly nailed. Like screen size per food fed - illumination intensity needed per square inch of screen - flow rate over screen.
    But what about tank turnover rate - the number of times a tanks volume needs to be cycled per hour through a scrubber to effectively removed the inorganic nitrogen & phosphate accumulated during a 24 hour period?

    Tank volume would come into the equation, as would the effectiveness & efficiency of the scrubber.

    I can't say I've ever seen any specs for this. If a scrubber is fed by its own pump in a sump, rather than by the aquarium overflow into the sump, that would make the calculation more complex.

    For example;
    Consider two seperate tanks. Both with identicle fish stocks, fed two cubs a day. Use the same size scrubber (6" x 4" screen), with the same flow rate over their screens - 210g/h, fed by the tank overflow.

    One tank is 50 gal, so has a turnover rate of 4 times/hour.

    The other tank is 400 gal, and has a turnover rate of 0.5.

    How do we know if this slower rate of turnover is enough to filter the water efficiently over a 24 hour period?

  2. Turbo

    Turbo Does not really look like Johnny Carson Staff Member Site Owner Multiple Units! Customer

    Over time I've found that a few of these need some adjustment or comment actually
    This one seems pretty accurate, but the construction of the scrubber does seem to affect things. For instance, an open-air scrubber, or one that is enclosed but in an overly large chamber (more than 1" or so from the screen to the chamber wall) does not really hit the 3D growth stage, or if it does, it's not nearly as early as one that has a smaller growth chamber. That's not me bragging about my design and trying to upsell it, that's just the way it is based on what I've seen. Now if I do have to brag, I'll say this: my Rev 1 had about 2.5" of width in the growth chamber, so 1.25" from screen to panel. In Rev 2 & 3 this was I think 2.25" and the 3D growth kicked in faster. For Rev 4 I made this 2", roughly (sloped walls on growth chamber due to injection part) and the 3D growth improved (speed to get there and density of growth). So IMO, construction makes a big difference.
    This still seems pretty accurate but I continue to learn more, I'm still experimenting with a few things
    This one is one that I can say that I've partially debunked. Not really "me" that did, but observation of what works and what doesn't work. Generally speaking, you want to start with only enough flow to coat the screen evenly. This usually ends up being somewhere in the range of 20 GPH/in, not 35 GPH/in - 35 seems to be overly high on a new screen, so going with a lower flow (and intensity) seems to allow algae to attach better initially.

    Once that has started, you can increase the flow, but again this might not actually be necessary. Maintaining even flow is somewhat important, and what drives this is the slot cut. Having a straight slot makes quite a bit of difference. Increasing flow can start to fall into the category of "if you need to". The lower flow rate might work just fine for a specific setup. So I've taken the "35 GPH/in flow rate rule" out of the equation when troubleshooting, especially on start-up, because that generally is not a root cause of issues during startup.

    Similarly, going much over 35 GPH/in does not seem to help in most situations. In fact, I've had a couple people report a reversal of growth quality when the push the flow up to 50 GPH/in, which is why I stopped pushing the "High Flow" capability after I rolled out Rev 3 - it just turned out, that wasn't really important, so that's another flow rate guideline that appeared to be based on theory (not my theory) and logic (it did seem to be logical) but with no real world proof or testing (which seems to be a repeating pattern)

    I just happen to have this
    I just wrote that up off the cuff, and if you read that carefully you'll notice it's based on "rated flow" of 35 GPH/in, which I don't push anymore, but that doesn't mean it's "wrong" to run at that flow rate, just not "critical".

    However, one reason I didn't really make a big deal about this is because it is only a calculation that I use as a secondary factor to determine sizing. Also, this factor is dependent on screen width, so it's more difficult to apply. You could set a target TTR that you want and then use that to reverse-calculate your screen width, and that's fine.

    Scrubber work over a long term, multiple pass scenario - versus a one-pass scenario where you can measure nutrient differences in and out. So while the total flow compared to the total volume will definitely affect the effectiveness of the scrubber in that system, the more primary factors like feeding vs screen size, lighting factors, slot pipe cut, flow rate being used, growth chamber - those will make more of a difference.

    Granted, my paragraph above is tailored to my products, but you could extrapolate that TTR > 1.0 to be a general target, something to consider when designing a system. But if you look at an extreme situation where you have a scrubber on a large tank that is only 6" wide but is 48" tall, that's a long-pass design (where you probably could see a measurable drop in nutrients in/out) which would probably work just fine, but the TTR would be extremely low. Thus, it's not a critical/primary factor.

    At least, that's my take on it!
  3. TbyZ

    TbyZ Member

    Thanks for the reply.
    there's a few things I want to bring up, but for now I'll just comment on a couple of things you bought up-

    the value of the fraction of inorganics removed from the water with each tank turnover through the scrubber,

    and the volume of water flowing over the screen.

    Increasing the flow over the scrubber's screen will increase the tank turnover rate, but I don’t believe that increasing the rate of flow over the scrubber’s screen, above the minimum required to completely immerse the algae with no channelling, will increase the fraction of inorganics removed by the algae with each tank turnover. The algae only removes a fraction of the available inorganics, and a faster flow should not change the value of this fraction.

    Although, the flow rate needed to completely cover a bare screen would be less I'd imagine that the flow rate needed to totally immerse all the algae growing on a screen.

    The amount of algae growing on a screen (screen surface area), and the width of the screen (volume of water flowing over it) and the duration period of illumination, and intensity/ spectrum of light all determine the value of the fraction of inorganics removed with each tank turnover.

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