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This post first appeared on Cold Weather Aquaponics.Keeping up our trek through the list of aquaponics winterization techniques (in no particular order), this week we turn to water chemistry. This post will provide an explanation for why nitrification in cold water offers us a challenge.  It assumes you've digested the basics of nitrification.  If you haven't, read up here.  The next technical post will give you a guide for sizing your biofilter. As you read this, keep in mind that I'm not a chemist, a biologist, or a bio-chemist, and this is not my area of expertise.  I'd recommend taking this post with about 500 ppm of salt. I'm only going into this issue because it's essential for cold weather operation.

  • Passive Solar Greenhouse Design
  • Insulated and Air-Sealed Fish Tanks and Grow Beds
  • Insulated Piping
  • Multiple Layers of Thermal Protection for Plants
  • Fish Selection for Cold Hardiness
  • Plant Selection for Cold Hardiness and Freeze/Thaw Tolerance
  • Efficient Water (Not Air) Heating
  • Programmable Temperature-Dependent Pumping Controls
  • Strategies for Maximizing Nitrification in Cold Water
  • Aquaponics-Integrated Hot Tubs (Seriously)

Cold Water Nitrification

In some ways this post puts the cart before the horse because we haven't gotten into seasonal water temperatures and the reasons for them. For the meantime, I hope you'll just trust me that keeping warm water in a backyard aquaponics greenhouse in a climate like mine is foolish. Lots of people do it. But then again, lots of people buy huge off-road vehicles and never go off road. Go figure. 

Anyhow, assuming that you'll be lowering your water temperatures down to 50°F or lower in winter, you'll need to start thinking about bacteria.

If you're not familiar with the bacteria that live in your aquaponics system, welcome to the club! Some may try and pretend that we understand the bacteria in our systems, but in reality it's 99.9% mystery. We know that some things work, and that's what we do. But in truth, humans know more about the farthest reaches of space than we do about our own soil bacteria.  For more on this, read Teaming with Microbes: A Gardener's Guid to the Soil Food Web. But I digress.  

For the purposes of nitrification (converting ammonia to nitrite then to nitrate), there are two types that do the work.  Or, rather, we group bacteria into these two types depending on the work that they do. Nitrosomonas bacteria convert ammonia (toxic to fish) to nitrites (even more toxic to fish).  Once that step is done, Nitrobacter then convert nitrites to nitrates (not very toxic to fish).  

By the way, "convert" as it's used here is a euphemism for "eat the one thing and poop out the other," in the same way that we "convert" Cracklin' Oat Bran into little rocks. There are two things that get otherwise squeamish people comfortable talking about poop. One is gardening. The other is raising children.

This beautiful, magical process works wonderfully and (usually) reliably in our aquaponics systems. Mine has gotten so reliable that I only check my ammonia and nitrate levels once a month (or whenever the neighbor kids want something to do).

However, like most things, this slows down when it gets cold. I can relate to this. When it's 10°F outside, it takes a lot to get me off the couch and out the door for a run. I have to put on a jacket, a hat, gloves, maybe a mask, and the whole darned thing is uncomfortable. Bacteria is the same way. Converting ammonia is such a bother. I need a nap.

Fortunately, this cold-weather slowdown is predictable, as the following charts show us. I've copied a bunch of them because the results don't all agree with one-another and I don't have a clear way of determining which is most accurate. 

Nitrification Rates by Temperature in Soil[/caption] [caption id="" align="alignnone"

Various Nitrifying Bacteria Growth vs. Temperature in Aquaculture

Nitrification Rates by Temperature in Aquariums

According to the US EPA, most strains of nitrifiers grow optimally at temperatures between 77 and 86°F, but nitrification has occurred over a wide range of temperatures (46-79°F).

This article suggests that nitrifying bacteria can operate at even lower temperatures than the studies listed above.

What this all means for you is that the rules of thumb for biofilter and/or grow bed sizing proposed by most of the sites and books about aquaponics may not serve you well when your water temperatures dip below about 50°F.  Stay tuned for the next technical post on how to size your biofilter.

What do you think - have you had issues with cold weather nitrification?

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Comment by Rob Nash on September 21, 2014 at 9:43pm

look into Archea is said to work into the negatives,,, and into the extreme heat..

awesome stuff for plants.. have a look and let us know what you think.

Comment by Vlad Jovanovic on September 14, 2014 at 12:01am

I do not know if my experience is 'generalizable' in every and all instances...but definitely feel that nitrification at lower temps, is better than what most charts/books would lead one to believe. Sort of how a number of years back, it was difficult to convince people (who weren't doing it) that nitrification can, and does occur at pH levels in the mid or low 6's. Because all the studies and charts said that it would not. These studies mostly came from the world of waste water treatment, and one had to look at the methodologies employed, and how those studies were actually performed to see why it was that they were saying you could not have nitrification taking place at pH 6.5...yet it certainly does take place as many of us can attest to. ( fresh water systems it's mostly Nitro spira that oxidizes NO2 to NO3...not that it really matters to most floks what they are called, but...just sayin')

Sure, with the right design and some due diligence, most anything is actuated "arm" to jar the grate every couple of hours for you might help to keep the ashes from building up...

My/Jim's stove basically acts like a big rocket stove (which it is) but does not enslave you to feed the thing puny pieces of thin wood every 30-45 minutes. For a 'normal' rocket stove to work properly, certain design proportions must be adhered to...not so with Jim's design. Once proper operating temps were achieved, I could load that dragon up with big 'ol logs and walk away from it. It might be worth checking out. Here's a video of one of his builds...

and here's one of the threads with some good info and pics strewn through it...

Comment by Jeremiah Robinson on September 13, 2014 at 9:02pm

Fascinating.  So if your experience is generalizable the conventional wisdom about bacteria going to sleep is wrong.

I'm also intrigued by your dehumidification strategy.  I spent about a year trying to build a small (1000 btuh) biochar gasification stove based on the WorldStove design that would take like 10 lbs of wood pellets and burn all night.  I got it to burn at the right rate, but couldn't get it to go all night because ash started to pile up and changed the airflow characteristics after about 3 hours.

I'm not so into biochar at the moment, but do you think a stove like that is possible?  Would it need a fan?  Is that what you use?

Comment by Vlad Jovanovic on September 13, 2014 at 6:08pm
Sure thing...towards the end of winter, as the thermal mass of the water was exploited, the water temps were as low as 4-6 degrees Celsius at times. I did heat the air with a homemade pyrolytic stove (a slight modification of Jim Fisk's design...just to keep things from freezing and to help dry the air out a bit at night (since venting at night would have been silly)...
Comment by Jeremiah Robinson on September 13, 2014 at 5:43pm

Thanks for sharing your experience.  I've heard that elsewhere, but had a hard time believing it when the data seems to suggest otherwise.

Can I ask how cold your water got or gets?

Comment by Vlad Jovanovic on September 13, 2014 at 2:11pm
I cycled my big system (2,100 sq.ft) a few years back,in the fall (October) and grew out my first couple thousand plants that winter. I fretted and worried about ammonia oxidation because all the charts and books painted a rather bleak picture of biological activity of our microbes at such low worked fine. Certainly not nitrifying at optimal capacity, to be sure...but truth be told, those grow bed ratios and common 'Rules of thumbs' probably have as much, if not more to do with mechanical filtration, than nitrification. So everything worked better than than predicted (based on charts, books and incomplete data :) Insulate, conserve the daytime heat (oh, and the almost 5,000 gallons of water i.e. thermal mass certainly helped) and do what you can to help conserve heat...and don't fret too much :)

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