Aquaponic Media Bed Effluent – Organic and Inorganic Sludge

Warning!  Some of the images in this video may cause nausea, light headedness…

When using media beds either by themselves or in combination with floating rafts in aquaponics, it is very important to understand they generate a large amount of organic and inorganic sludge.

The video above is an example of 3 months of accumulated sludge, most of which the bed itself generates through dead bacteria and other organics breaking down.

It is a good example of what happens in a media grow bed, especially for those thinking about using them as a primary bio and solids filter in some type of commercial “hybrid” aquaponics.

Don’t get me wrong, media beds work very well on a small system with a low feed rate.  It is once you step up to the commercial arena where your expected feed rates and biomass should be much higher not being able to control or manage this effluent effectively will spell disaster for your production.

This becomes especially important after about 12 months.   It will usually take at least 12 to 15 months for your system to reach its required fish bio mass for good plant growth.  Problems will start to appear after that and they will be difficult to manage if you do not plan for it.

Media Grow Bed Sludge

What you see being pumped out of the media bed in the video at the beginning of this article is what accumulates in the bed itself and it does not break down or mineralize.

Some may say “obviously he does not have a balanced eco system” or “he has too many fish” or “feeding too much”.   They would be very wrong.  Here is why.

In this system very little of the solid fish wastes have any chance to make it onto the media bed for the following reasons:

1. The primary bulk of the solids are taken out by the radial flow filter.  These solids are mineralized in a separate, aerated drum on the end of the wicking bed.  For which that water becomes the primary water source for that wicking bed.
2. Further after the solids filter and bio filtration, any solids that make it out to the plant integrated system goes first into the end of a 7 meter raft (as shown in the video).  Most of the remaining settleable solids will and do settle out in the raft.
3. Water containing solids are not pumped in the grow bed so the effluent sludge is being created primarily by the bacteria in the bed.

It is also good to note the gravel bed has a very large and healthy population of worms, so don’t be fooled by their capacity to reduce solids in your media bed system.  I certainly would not be relying on that to happen effectively when you have money invested in a farming enterprise.

Aquaponic System Break Down

There is 4000 litres of aquaculture.  The biomass of fish is 3 of the 1000 litre fish tanks currently has 100 x 200 gram Jade Perch (yes I said 100…. per 1000 litres).  At this point in time there is approximately 20kg of fish in each of the three tanks. The other tank still has some 25 x 500 gram Silver Perch.  These are runts left over from the last batch.

Just to clarify the huge number of fish.  Once the Silvers are removed the stock will be split between each tank.  When the first harvest of the Jades comes out they will be sitting at about 30kg/m3  when the main harvest will occur.  Because the system is designed to handle at least 160kg of fish being fed at 1.5% per day, there is no concern for running at that density.  Avoid trying that at home though if you are just starting out.

To be perfectly clear for those that may think this system is overstocked or underfiltered.  The aquaculture part of the design has absolutely no need for the plant system in terms of filtration solids or biological.  The fish system can run completely independant up to the bio mass it is designed for, as mentioned above.

Moving along….

As I said earlier the fish water goes through a radial flow filter which will catch at least 45% of the settleable solids before the water enters the floating raft.  At the current biomass the fish are being fed about 600 grams per day.  The waste from that will be something in the order of 180 grams per day.  The solids filter will catch about 90 grams of that each day.

I expect the other 90 grams per day is making it to the floating raft which is shown in the video where the bottom of the raft is covered in 50mm of sludge which does not break down and will continue to accumulate.  What you see in the bottom of the raft is about 9 months of collected sludge.

The reason the water runs through the raft before it is air lifted into the media gravel bed is we can easily drain and wash the raft and it acts as a big settling chamber which will catch the remaining settleable solids.

Also, I like easy to clean and do not fancy digging out 4 cubic meters of gravel and risk damaging the grow bed in the process of all that work.  If you are thinking about building a big grow bed, give some thought as to how you are going to clean it…

The water is then air lifted into the media bed.  It is not pumped into it so there is very little chance of putting any of the solids into the grow bed.

The drum you see us pumping out the sludge is there specifically to act as a sludge trap.  There is very good reason for this.  You want to avoid the gravel bed contributing to the suspended solids and very much want to eliminate resuspending any of that very nasty sludge as it will negatively impact the productivity of the entire system.

At the end of the day, if you are building a commercial system you really want to avoid building in labour intensive tasks, especially ones that will interrupt your production cycle.

If you must use media beds in a commercial integrated aquaculture or aquaponic system, it is much simpler to have some effective solids capture before the media grow beds and something to capture the solids they generate.

It must be designed it in a way that reduces or eliminates maintenance labour and yet still gives you the capacity to manage the nutrient dynamics in the system.

This particular quote from Michael Timmons speaks volumes to solids removal:

It cannot be overemphasize the importance of rapid and complete solids removal from the culture vessel. All other unit processes will fail if this primary function is poorly performed.

If you do not apply some solids control and a media bed as seen in this video is not suitable for that task, you will wish you had given it some thought.  This sort of accumulated sludge is not a matter of will it happen or if it will happen.  It will and it is just a matter of time and when it will happen.  Either pay the price when you start with good planning or you will pay for it later.

Regards
Paul