How to prevent backwash from pond filters

I recently got a message from someone asking about using a venturi to prevent backwash occuring (dirty water being syphoned back from the filter into the pond) and thought other people might find it useful......

Hey Jim: printed out your pages on the Skippy and venturi and wound up with a question. I have been concerned with power failures in our area and need to prevent backwash from the filter. I have had this happen twice and don't want to risk it again. I have been fitting together various connectors to see which is more efficient and since a venturi would take up so little space in the plumbing I was wondering if it would be enough to kill the siphon effect. Of course the P connection would probably work as well. Before I start with the glue I wanted to check with you to see if my idea is something you tried. At the point where the tubing would take a 90 degree turn I would use a tee rather than an elbow, place a cap on the vertical opening, drill a hole for the air tubing and pack with expandable foam. I hope I have made this clear enough. I find I am not always clearto people who aren't used to me. Thanks, Sandy

Hi Sandy,

I'm glad you found my pages interesting.

I'm not dead clear on what you mean, but anyway here goes! Whenever I turn off the pump then the venturi makes quite a nice rude sound as the air is drawn in, so preventing any syphoning back from the filter into the pond. My bio-filter stays full of water. [During normal operation with the pump running, the venturi makes a constant quiet sucking-glooping sound as it takes air in and down into the bio-filter.]

Air will always move faster through an opening than water, so despite the air intake being quite small it is enough to prevent the syphoning backwash. This is quite handy because I know I can turn off the pump and it won't drain the bio-filter dry. Its also worth mentioning that before I fitted the venturi the syphoning would COMPLETELY empty the bio-filter! If this were to happen during the day, un-noticed due to a power failure, not only would it backwash into the pond, but also the bacteria on the media in the bio-filter might die - I think they can only stand a couple of hours without water, and then of course you're back to stage one with maturing your bio-filter all over again, which depending on the amount of dead bacteria could take a few weeks again!! Not good.

When you talk about replacing the 90 degree elbow with a T-piece, do you mean the bend is turning downwards? Because the venturi effect won't work going round a bend (I presume you do want the venturi to suck air in during normal operation, while air is drawn backwards to prevent backwash in the event of a power failure). The water flow must be straight through to create the venturi vacuum to suck the air in, as seen in the photos and the design diagram of my venturi.

Does this answer your question?

Your point about backwashing is a good one, and I will add something about it to my web pages.

Cheers
Jim