Water chemistry aspects to consider in AP design (recovered blog)

[Kobus Jooste]

In the previous post on water chemistry for aquaponic beginners, I tried to post the bare details of the nitrogen cycle, and why we end up with ammonia, nitrite and nitrate after feeding our fish.  There is obviously much to expand on, but then it would not have helped being in “basic and useful info” then would it.  I could bore people with valence electrons and how interesting hydrogen and oxygen really is but who cares.  Truth is I figured most people want the bare basics of why they are testing for stuff in the water or why they are being advised to do so.  People seldom ask for a brief rundown of the internal combustion reaction as a refresher when they are looking at buying a new car.  They ask a few questions to disguise the fact that they are really only concerned about looks or price, and off they go. 

 

So in the spirit of this minimalist approach, here is another instalment of aquaponic water chemistry basics for newcomers.  In the previous one, you got a glimpse of why we test for Ammonia, Nitrite and Nitrate.  There were also hints thrown in that pH and temperature play a role in some of the reactions, particularly the availability of unionized ammonia that could kill your fish before you know it.  The three items left that were not covered in the nitrogen cycle, but which you are advised to test for, is DO (Dissolved Oxygen), Alkalinity and pH.  Before you even read the rest, you may want to know if there is anything specific about your water source that may cause you to start with issues.  High/Low pH, hard/soft water all play a role in how your system would act up.  This section aims at the run of the mill set-up.

 

Now on pH you are told time and time again that plants prefer pH of X, bacteria prefer pH of Y and that fish float somewhere around Y + X / 2 (just kidding on the formula but it does kinda work!!). So why does the pH in your system swing around so much?  Not having inert media/beds/tanks is one option.  Not being adequately buffered is another.  You can test every day and adjust with a base or an acid, but if the system is not buffered, the agent of swing will return and you will be cursing again.  Typically the system would like to go acidic, thus the best buffer for those conditions is CaCO₃ (Lime, shells, coral sand). If you are trying to get to pH 7 and your system leaks a base, it will go up, and down if you have some acidic substance being released.  Apart from this, your biggest swings in the system come from not being able to buffer the influence of plants (pulling pH down) the Nitrification process (pulling pH down) or gas exchange processes (generally releasing CO₂ and having a likely downward influence too). I do not want to dwell to much on the chemistry here, but when you have CO₂ (data is sparse, but some authors put CO₂ production from fish at 28% of feed input) and H being dumped in the water all the time, you need to buffer this with alkalinity.  Few people test for that, kind of the foster child of the bunch.  For the nitrification process alone to be buffered, you will be requiring 7.14 g of alkalinity (typically CaCO₃) to deal with the breakdown of 1 gram of Ammonia.  While not a pollutant really, for those who are interested, total phosphae (TP) release from your food is typically under 1% of food input.

 

The next major issue is DO.  Again, the most likely reason people focus on this is because fish come floating to the surface when it is too low.  Plants like it too, and as seen from the nitrification process, so does the nitrification bacteria.  You need a whopping 4.57 g of oxygen to take care of 1 gram of ammonia nitrogen.  Another factor to keep in mind in media bed systems is the fact that fish do not just excrete nitrogen based wastes.  Along with Nitrosomonas fodder comes a nasty called BOD or biological oxygen demand.  This is basically the lumping of decomposition processes that target Carbon based (organic) substances.  Just looking at fish BOD, you can safely assume that around 30% of your feed input will become BOD.  The amount of oxygen needed by fish on a daily basis varies according to size, age species etc., but you can expect that around 30% (can be up to 50% for something like a salmon) of your food input will be required as DO in the water.  Thus if you have just added a kilogram of food, you will be seeing 300 g of oxygen (at least) going out of the system).  That is quite a bit.  In my location, saturation point is 8 mg/L, which means that I will need 38 000 liters at saturation to deal with that food input.

 

There plants aren’t useful either, as they can ooze some of their photosynthate into the system too, upping BOD.  You therefore have to work hard at keeping your DO as high as you can (but do not super-saturate beyond a safe level).  Typical warm water safe limits is around 5 mg/L, but getting to 8 is always nice.  Flood and drain media beds with cascades can work wonders, as do bubblers and venturi agitation.  There is much more information available of typical DO levels, but it gets complicated as it all depends on your height above sea level and the temperature of the water.

 

Hope this helps a bit.  Here is a diagram to put some of the important stuff in a visual format.

 

 

[Kobus Jooste]

Thanks for the complement.  Most people are picture guys and gals, they just do not know it!  Our brains cannot memorise words half as well as diagrams and mind maps.  The icon I use is the album cover for a South African band, the Parlotones called "The world next door to yours".

[Aaron]

Hi Kobus, As a science teacher, and avid student of AP, I appreciate your concise description of how an AP system interacts.  I also loved the diagram, as I am a picture guy, and have just one off-topic question for you.  What is your profile photo?  I've been trying to figure it out.  It looks like a knight holding a person and a megaphone. Ha ha! Just curious.