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Siphon During Power Outage

Joe S. asked about Erik O's comment re reverse siphoning of a trickle
filter during a power outage:  " To keep the water returns from
back-siphoning everything out of the tank during a power outage, I have
small holes drilled near the top that will break the siphon.  I know
you're out there Erik. How do you prevent water from squirting out these
holes when the pumps are one? If you are using a siphon to take water
out of the overflow, how do you get the siphon restarted after the power
goes out? I seems that you would pump the sump dry and ruin your pump."

I'm pretty sure I know what Erik has in mind, so I'll respond.  In the
sump at the bottom of a trickle filter Erik (and all the rest of us with
trickle filters and sumps) has a pump to return water in the sump to the
tank.  The standard off-the-shelf setup returns the water near the
surface of the tank.  Mine has a couple of elbows with a small piece of
plastic in a fan shape to distribute the water broadly near the surface
of the tank.  Those of us with supplemental CO2 have learned that
returning water near the surface causes considerable outgassing of CO2.
To reduce this problem we extend the sump water return pipe several
inches further down or even to the bottom of the tank.

Now suppose the power goes out.  The return pump and hose will reverse
flow and act like a siphon draining water out of the tank into the sump.
In the off-the-shelf setup this is not a problem because the water is
returned near the surface and the reverse siphon will 'break' before
more water has been drained than the sump can hold.  I tried this when I
first set up my 125 gal. tank and the sump filled up to just under 10
gallons before the siphon broke at the water level where the sump return
hose exited.  

If your return pipe goes to the bottom of the tank as mine does, you'll
more or less drain the entire tank before the siphon breaks.  In my case
that's about 100 gallons of water on my new oak floor!  Happily, I
spotted the problem when I extended my return pipe.  Unhappily, I came
up with a $50 solution instead of the $0.05 solution that Erik smartly
selected.  I bought a very expensive European check valve and installed
it in the return pipe.  When the power goes off and the water reverses
flow, the check valve stops the reverse flow.  Every so often I shut off
the power just to be sure the check valve is still working!  It would be
a catastrophe if it didn't!

Erik simply drilled a few small holes in the return pipe presumably an
inch or two below the water line.  True, a very small amount of water
will flow out those holes when the system is in normal operation.  But
when the power goes out, as soon as the water level reaches those holes,
air will rush in and break the siphon.  The next time I do maintenance
on my tank I will drill a few small holes in that location as a backup
to the check valve.

Regards, Steve Dixon in San Francisco where it's so hot I don't even
think its funny!