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Re: CO2 and trickle filters

On Fri, 14 Jan 2000, Ole Larsen wrote:
> The reason for my posting is, that as far as I am informed, the whole
> idea of a trickle -/Wet-dry filter is to keep the filtermedia in close
> contact with the air, of which ~20% is oxygen, fascititating a better
> environtment for the beneficial (nitrificating)aerobic bacteria,
> resulting in a better nitrification.

I think you're right that keeping the water in close contact with air is a
major focus in the design of trickle filters.  It isn't the whole idea;
they also provide a large contact area between the water and solid surface
where bacteria can grow.

> By keeping the media in some kind
> of enclosure not well ventilated to the surrounding air, I _believe_
> this function gets compromised.

Again, I think you're right.  Trickle filters used in sewage treatment
plants are large circular beds layered with coarse stone and wide open to
the air.  Good aeration of the trickle bed is fairly well assured.
Aquarium trickle filters sealed away in boxes aren't assured of aeration.
That's why some people advise pumping air into the filter box.

> By letting the enclosure getting
> filled with CO2, the concentrations of oxygen (and nitrogen)
> must-unlike in water- decrease further, causing even inferior
> environtment for the process hoped for (nitrification). I may have
> missed a answer, but I do not think so, and I do hope for a
> verification/correction.

The water flowing into the filter from the tank contains oxygen, nitrogen,
carbon dioxide and a few other gases.  All of those gases escape from the
water to fill the open space in the tank.  The filter won't be filled just
with CO2, but with a mixture of gases that reflects the amount and kind of
gases that are dissolved in the aquarium water. Just the same, it would
seem that a sealed trickle filter could never *add* oxygen to the water
because it's only supply of oxygen is already in the water.

A sealed filter wouldn't add oxygen to the water but it would provide for
a lot of contact between the water and the gases in the filter.  Usually
the water coming in and the gases in the filter provide enough oxygen to
keep the filter from getting anaerobic.

I doubt that a sealed trickle filter is any worse than a canister filter
for nitrification, but it wouldn't have the advantage of an open trickle
filter, either.  Of course, if the filter is open to provide aeration then
your CO2 should go bye-bye.

Roger Miller