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Aquaponic Media Bed – 7 Tips to Prevent Disaster

The nitrogen cycle is often referenced when starting up an aquaponic system where bacteria convert fish metabolic wastes (ammonia) to plant food (nitrate).  Is it really a cycle where the nitrogen is recirculated with in the engineered eco system we create with aquaponics?  It is not.  This is purely because the nitrogen we take out of the system in terms of fish and plants does not get put back in by us (that would be a little disgusting).  We use an external source of nitrogen (fish feed) which means an aquaponic system is not a natural eco system but an engineered one. Nitrification is the basic process of bacteria converting organic nitrogen from animals into inorganic wastes for plants, which you, me and other animals eat, then put back into the ecosystem as organic nitrogen and away it goes again. A missing part in the nitrogen cycle is denitrification by a different set of micro-organisms (heterotrophs) and this happens in aquaponic media beds in the presents of lots of nitrogen (nitrate) and low to no oxygen (anoxic). All living things require three main things to live, oxygen, nitrogen and carbon.  This set of micro-organisms, which includes some bacteria and all fungi, are just like us they get their nitrogen and carbon from other living organisms.  In other words, from organic nitrogen (proteins) and carbon (carbohydrates etc.).  They also get oxygen from the water.  But there is a unique thing about them, they can live in aerobic and anaerobic environments, a condition an aquaponic media bed will suffer from over time. Aquaponic media bed recipe for disaster In any media bed prefiltered or not, a layer of organic sludge gathers on the bottom layer of the bed.  This happens much faster in unfiltered media beds than it does in pre-filtered and this is a very important difference.  There are two sources of organic wastes in a media bed, the first and most obvious is the uneated feed and feces (not prefiltered) and the other is dead bio cell mass.  This cell mass comes from all micro-organisms in your media bed. It makes great sense that the more organic solids you have in your media bed the more microflora you will have living and dieing.  So in an unfiltered media bed the issue of organic build up is amplified by the increased contribution to the bio load from dead mass of microflora. As this organic mass grows beyond the capacity for the micro guys to process, the environmental conditions in the bed can change very suddenly.  Changes in the environment like unexplained and sudden increases in both ammonia and nitrite along with fish gasping at the surface indicating very low oxygen and/or nitrite poisoning. Then disaster strikes almost overnight, fish die for no clear or understandable reason.  The day before everything was well and everyone happy and this leaves the aquaponic gardener a little confused.  Now you know why… But what else is wrong with this picture aside from dead fish? There is another negative to this anoxic (low oxygen high nitrogen) condition; denitrification starts to happen.  In other words you start to lose a higher amount of precious nitrogen to the atmosphere, poof it is gone… Keep in mind nitrogen is the primary and often the limiting nutrient when growing plants.  Nitrogen derived from protein in the fish feed is very expensive to produce in aquaponics compared to other nitrogen sources eg; synthetic fertilizers.  So why would we waste it? These denitrifying bacteria and fungi start to get a little tricky as the oxygen available in your system gets less and less.  They start to take oxygen from your fertilizer nitrate!  In other words your precious plant food becomes the electron donor and give up oxygen to the little guys.  So instead of growing food you are growing bacteria and fungi! To simplify this process it looks like this.  NH4 -> NO2 -> NO3 – no oxygen in the substrate -> NO3 -> NO2 -> NO -> grabs more nitrogen -> N2O – then loses all oxygen -> N2 gas to atmosphere.  You can see where the microflora start to take the oxygen from our nitrate, convert it to nitrite (there is the spike) then right back to nitrogen gas and it is gone. The other spike you will encounter with media beds is the ammonia.  This is produced two ways.. The same group of bacteria breaking down the organic wastes produce ammonia and with that remember the heterotrophic guys eat other organisms, this includes your nitrifying bacteria which reduces the biofiltration capacity of the growbed.  Further to that the heterotrophs also smother the outside layer of the chemoautotrophs which reduces the substrate ammonia for the bacteria and wellah they do not nitrify very well because limited access to the food they need. On top of all that this lowering of oxygen level and increased biological oxygen demand (BOD) will not let go very easily because the conditions favor these heterotrophic micro-organisms and they can double their population in 20 minutes which puts more pressure on the system.  A very slippery slope indeed. How do you prevent this from happening to your system? Prefilter your media beds regardless of the fish load Use a uniform size media to avoid filling the void spaces with smaller media Use a larger media to prevent blocking the void spaces with organics Clean your media before use Routinely clean your media beds Harvest the roots of the plant, don’t leave them in the bed Be vigilant with your water testing keeping an eye out for sudden spikes It is fair to say that some denitrification happens (up to 10%) in any system aquaponic or aquaculture regardless of its design.  However, an aquaponic media bed is prone to this loss of valuable nitrogen at a much greater level. Keep in mind the cost of fish feed is based on the protein cost, the higher the protein the higher the cost and the more nitrogen you are buying.  In other words the nitrogen is a key cost in the operating cost of your system.  If you chose to work with an aquaponic media bed at a commercial level, be sure to limit these losses.  If you are in the back yard, those costs add to the family food budget so this is very important to you as well. Regards Paul…

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