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Hello, I'm confused. I can see that there are advantages  (especially in hot climates) of Deep Water Cultures but, I'm unsure of how they work. As I see it, there are two types of bacteria on the growing gravel that changes the ammonia to the nitrites and Nitrates. My question I guess is " What changes the ammonia if there are no rocks"?  Am I right on anything yet? I'm sure that the problems I am encountering is the balance of fish to water to grow beds.

I have found that using shellrock as a medium is not good. You need a IV drip of muriatic acid just to keep the ph balanced. I need to change the medium.

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Comment by Chris Smith on July 15, 2011 at 1:23am

In a raft system the bulk of the beneficial bacteria is actually living in the water column. It also lives on all other surfaces. There is so much of it in the water it actually colors the water a tea color. The fact that the bacteria is living in the water and moving in the water column is what allows us to run very long troughs with no nutrient drop from one end to the other. I know of several systems with hundreds of lineal feet of daisy chained troughs with no nutrient drop from end to end. The bacteria living in the water column and large volume of water allows us to kick start a system with full fish load and have nitrates in about a week. The biggest benefit of the raft method is mobility. Roots are not planted which allows for moving plants. 

I have been using large media beds as a filter for my raft troughs since last year. I now prefer to grow long term plants in media and short term crops on rafts. My media beds have celery, chard, kale, tomatoes. I can harvest off the same plants for many months and I find it beneficial to have the roots planted in media for stability and root aeration. My raft systems mostly have short term crops that are completely harvested at one time. Once a raft is harvested I replant it with young plants that are started in nursery rafts with very dence spacing. Once a grow-out raft is replanted it is replaced in the trough at the opposite end. All the rafts are pushed down the line to let in the newly planted raft. I always harvest out of one end and small plants go in at the other. These plants never stop moving throughout their life.

Comment by Green Acre Aquaponics on July 5, 2011 at 12:56pm

Our micro system has no additional biofiltration and our commercial system has minimal additional biofiltration in the form of a net tank just as TC said.  Don't forget however that all surfaces in DWC or any system for that matter can provide a home for bacteria and in DWC, aside from the rafts or trough surfaces, the plant roots provide immense surface area where the bacteria can do their magic.  We are however, incorporating 250sqft of media bed into our commercial system for the benefit of additional metabolization, nutrient generation and crop diversity.

Comment by TCLynx on July 4, 2011 at 2:37pm

Ah yes, sorry that you got a non pH inert media.  However, what is the natural pH that your media buffers to?  If it is 7.6, don't give up all hope.  I've got a system full of shells that is buffered to 7.6 and I'm still able to grow many things in it without constantly adding acid.  Watercress loves alkali water by the way.


As to how the heck DWC manages to have a bio-filter.  As David said, all surfaces in a system provide some surface area for the bio-filter bacteria so for a very lightly stocked raft system one might actually get away without any sort of supplementary bio-filteration as in the Friendlies micro systems.  However, most DWC or raft systems will add some form of solids collection and supplemental bio-filteration in addition to their raft beds to provide extra area for the bacteria to do their job.  Often this is in a "net tank" just a tank with extra aeration and bird netting stuffed in it.  Or it might be some form of fluidized or moving media filter.  Then there are duckweed tanks that can use up lots of ammonia directly without even needing the bacteria to convert it though this will also mean less nutrients for other plants in the system.  And there are many many other ways to add a place for bio-filteration to take place.

Comment by David Hart on July 4, 2011 at 12:20pm

Your right Matt, the media does have a lot of surface area for the bacteria to grow on.  In any system, the bacteria also grows on all surfaces, as long as it's not exposed to sunlight.....bottom of rafts, the surface of the troughs, inside pipes, ect. In DWC, lots of folks have a net filter, this has a lot of surface area too.

In my DWC system, I use a swirl filter, some gravel beds and a net filter. They all help to clean the water of solids before it gets to my plants. They also provide 'home' for lots of bacteria.

Sorry to hear about the shell rock.

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