Media Beds increase oxygen demand?!

[Adam Dusen]

Someone told me this the other day (not from a forum but from an email) and it doesn't sound accurate to me:

[media beds] are great for nitrification, but bad as clarifiers.  Generically they become clogged and need frequent mixing, and/or backwashing.  Fixed or movable media beds used as clarifiers also increase the Oxygen demand of the system, as many of the solids wind up being digested in the system consuming oxygen.  In general as much as 80% of the oxygen demand goes towards solids digestion, so the more of the solids one can remove the less oxygen one needs to generate.

If media beds are flood and drain, isn't the digestion of all solids in the media beds taking place aerobically in the draining sequence? How is this using up O2 in the system, wouldn't it just be using up O2 in the air itself? Also, to what degree do worms keep the system from getting clogged? I have heard many people who say they work great, but do they eliminate the need to wash and clean out the media completely? Because using worms would seem to refute the first claim as well.

[Glenn]

"Many factors affect the dissolved oxygen concentration in pond water. It’s not just the fish load in your pond. Atmospheric temperatures as well as weather conditions affect dissolved oxygen. Fish waste and decaying organic matter have a draining effect on dissolved oxygen levels because oxygen is used up during decomposition.  Aerobic bacteria (good bacteria) consume a lot of oxygen. Algae blooms and submerged plants also have an adverse effect on oxygen level because they consume a lot of oxygen during the nighttime hours, through photosynthesis. This is why you may discover that the dissolved oxygen concentration of your pond is quite low in the early morning hours.  If the demand for oxygen by plants, bacteria and fish is greater than the dissolved oxygen level of the pond, your fish are most likely to suffer."

From: http://www.koihealth.info/oxygen.html

It's easy to find myself thinking along lines that plants like or even need solids. But they don't. Plants  need and ONLY uptake the nutrients that are fully dissolved in water. In our media beds there is a very large amount of surface area for the solids AND bacteria to become attached and for the oxygen demand to be high. The elimination of un-dissolved solids outside of our media beds doesn't reduce the  available nutrients to the plants nor the oxygen available to the fish, to the plants, or to the bacteria our AP systems all rely on.

I hope this helps.

[Adam Dusen]

I fully understand that decomposition of organic matter uses up oxygen... That''s a no-brainer. But wouldn't that oxygen be coming from the abundant atmospheric oxygen during a drain cycle? And not as much the DO in the water during a flood cycle? Am I making sense here?

Glenn said:

"Many factors affect the dissolved oxygen concentration in pond water. It’s not just the fish load in your pond. Atmospheric temperatures as well as weather conditions affect dissolved oxygen. Fish waste and decaying organic matter have a draining effect on dissolved oxygen levels because oxygen is used up during decomposition.  Aerobic bacteria (good bacteria) consume a lot of oxygen. Algae blooms and submerged plants also have an adverse effect on oxygen level because they consume a lot of oxygen during the nighttime hours, through photosynthesis. This is why you may discover that the dissolved oxygen concentration of your pond is quite low in the early morning hours.  If the demand for oxygen by plants, bacteria and fish is greater than the dissolved oxygen level of the pond, your fish are most likely to suffer."

From: http://www.koihealth.info/oxygen.html

It's easy to find myself thinking along lines that plants like or even need solids. But they don't. Plants  need and ONLY uptake the nutrients that are fully dissolved in water. In our media beds there is a very large amount of surface area for the solids AND bacteria to become attached and for the oxygen demand to be high. The elimination of un-dissolved solids outside of our media beds doesn't reduce the  available nutrients to the plants nor the oxygen available to the fish, to the plants, or to the bacteria our AP systems all rely on.

I hope this helps.

[George]

Perhaps not in an immediate sense but aren't you removing nutrients which would have been available later, after being dissolved?  I don't understand why you think it better to remove solids instead of increasing oxygen  On the other hand, I suppose if you want dense fish stocking with lots of feeding, then you must remove solids.

Glenn said:

The elimination of un-dissolved solids outside of our media beds doesn't reduce the  available nutrients to the plants nor the oxygen available to the fish, to the plants, or to the bacteria our AP systems all rely on.

I hope this helps.

[Carl B]

Sounds more like an aquaculture problem.  In high fish density, oxygen needs to be artificially supplied to the system, and anything other than a fish that uses oxygen would be a waste.  In aquaponics, where functioning bacteria are a key necessity, "too much oxygen being consumed by bacteria" sounds a bit like "my fish are eating too much food and getting too big."  So, the observation may be correct, but the implication that it's a bacteria problem isn't.

I agree, the media bed should be adequately oxygenated during the drain phase.  

[Jim Fisk]

If this is really a concern perhaps you should look into ozonation. Used in everything from sewage treatment plants (for exactly the same reasons as AP) to swimming pools these days. The extra oxygen molecules boost oxygen in the system as they go from H2O2 (some as high as H2O8) back to H2O. I have been using a small one, intended to purify drinking water, in my 2500 gal system for a couple of years now as an O booster. Others use pure oxygen bled into the system on a regular basis and based upon dissolved O readings.

As far as media beds are concerned the ebb cycle certainly adds O to the system. I remove the solids, both sinkers AND floaters before the media beds now. An air line in the filter can also help break down those solids right in the filter. A full blown bio-filter will also break down the solids into nitrates before entering any of the beds. There are many ways to address the O2 levels in the system and thereby the breakdown of solids.