Thread: Bubbles in return with two pumps but not with one

Hello everyone
I'd like to understand the following behaviour which I find very strange.
I have a Draco C30/340, two 110mm outlets, two variable 22,000LPH pumps to underwater returns. Assuming perfect efficiency, which I know is not a valid assumption, I may be getting up to 22,000LPH throughput from one pump. I have two pumps for failover, and for the occasion that I need a brief super powerful blast to purge drains.


With just one pump turned up to full, and the other off and its isolation valve closed, the system works well. The pump is not sucking air bubbles from the bio chamber, which Triman described in https://www.koiforum.uk/pond-constru...d-returns.html
At any power setting, the returned water is smooth and free of air.


But when I turn on both pumps, even with both set to their minimum 30% level, both returns spit out bubbles of air. I know that these bubbles are form the aerated moving bed bio chamber, because when I turn off the air, the bubbles from the returns cease. I can hear in the pumps that they don't like trying to move froth instead of water.


The water exit from the C30/340's bio chamber looks like part K of the photo on page 6 of https://dracodrum.com/pdfs/2022/m-se...de-ver-2.1.pdf






Why does a pair of pumps each moving circa 6,000LPH suck far more bubbles than one pump at 22,000LPH? Even considering the total flow of circa 12,000LPH it's still less than a single pump on full.




Please can you help me: What can I do to avoid this, because although the bubbles are attractive I know that it's not great for the pump.






Thanks

Just to get my head round this are you saying:

With the air turned off you can run either of the pumps or both of the pumps at full power and not get any air bubbles?

But with the air on and both pumps running you get bubbles?

Are the bubbles always coming from the same pump or is it from both?

Is it possible with both pumps on you are creating a vortex in the bio chamber that's pulling air down and through the pumps? That's all I can think of if you are certain the bubbles are 100% coming through from the bio chamber.

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You could try an internal baffle wall within the biochamber as a bubble trap? Only bottom water (less likely to have bubbles) will feed the pumps.

I doubt a small volume of air will harm your pumps. On very hot summer days, I push some air by air pump and a check valve directly into the pump impellar to create a fine mist of micro bubbles. This lifts my dissolved oxygen level dramatically.

Internal baffle wall is open at bottom, higher than water level at top. Many aquarium sumps use this to stop bubbles getting pumped back into fish tank.

Hi
Thanks for replies.


I don't think there is a leak letting air in. Otherwise, bubbles would be present all the time. Bubbles are only present in both halves when both pumps are on.

Yes, when I turn off the aeration air, bubbles cease.

>So the issue here is the bubbles from the aerated biofilter are getting sucked into the pond returns when the flow is too high.
Yes, I agree. But when I don't understand is why this happens only when both pumps are on even at their lowest setting. A flow of 30% per pump, times two pumps, should be less than one pump maxed out.


A baffle in the bio chamber might work, but I'm rather loathed to install one given that this commercial off-the-shelf filter is supposed to be able to handle two pumps and 30,000 LPH. Shouldn't the filter exits be designed to deal with this?

An intermediate chamber would probably work. Probably not simply a stand pipe, the water would be rushing through the T too fast for many bubbles to escape, I should think. But a large chamber (barrel etc) would surely do the trick. Shame there's not a lot of space for one in my filter house.


Micro bubbles unlikely to affect the pump, agree, but these are relatively chunky bubbles, and the noise the pumps make certainly implies they don't like it.

Let me take a video over the next few days.


Cheers

Perhaps with the single pump at high flow the bubbles still get sucked in but are greatly pulverised and less noticable?

A second work around is to remove the air stir completely and switch to a submersible water pump to move your media. Although the media would still rotate and move under the pond pumps alone, adding the submersible is best to give multiple grabs /media circulations per water pass through bio chamber.