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.