Sunday, August 4, 2013

Aquaponics Water

**This post continues our series on the aquaponic system we set up in our apartment last year.**

The lifeblood of an aquaponic system is the water.  The fish live in it, the plants need it to survive, and it carries nutrients from the fish tank (where they're a liability) to the grow bed (where they're an asset).  There are a number of important parameters for the water, including temperature, pH, chlorination, salinity, dissolved oxygen content, and level in the fish tank.  Generally the fish are more sensitive to extremes and rapid changes in these parameters than the plants are, but both do better when the parameters are optimized.  The pH in our system turned out to be pretty self-regulating (high at first before adding fish, then decreasing to a steady 6.5 after a month or two with fish), and the water level was just something we had to keep an eye on.  A surprising amount evaporated, especially when we had a bubble bar going to increase the oxygen content, and we needed to add almost 5 gallons per week to keep the tank topped up. The salinity was the default salinity of our tap water (after the dechlorination discussed below), which is the factor to which the fish are generally the least sensitive.  That is to say, most of the parameters required only minimal effort to optimize or standard aquarium solutions.  Therefore, we want to focus here on our experiences with temperature and chlorination, which required a little creativity (temperature) or were particularly challenging (chlorination).

Plants can do well over a wide range of water temperatures, but fish often times require fairly narrow ranges.  For example, tilapia do best between 72 and 90 °F, and grow much slower when the temperature is below 70 °F.

We controlled our temperature with an old aquarium heater graciously donated by Matt and Elise.  It was a heater designed for an aquarium with constant water level, so we had to make some adjustments for our slightly fluctuating levels.  It turns out that three or four styrofoam cups with a heater-sized hole cut in the bottom (we were looking for a way to reuse them!) work well, as long as we added a few pieces of gravel to keep the center of gravity low.  Then the heater could float around the tank like a bobber.
We monitored the water temperature with a meat thermometer near the outlet into the grow bed.  Within a few days of minor adjustments on the heater, we were at a steady 75 °F.  Homeostasis restored.

If you have well water, congratulations.  You can probably skip this part, unless you're interested in water chemistry problems experienced by most folks on municipal (city) water systems.  For the rest of you, let's do some chemistry!  Municipal water is almost universally chlorinated as a final treatment step to sterilize the water before releasing into the pipelines that eventually end at shower heads and kitchen sink faucets.  Generally it's a good thing because, hey, who wants to get a life-threatening bacterial infection from their drinking water?  The problem with chlorination in aquaponics is twofold: the chlorine can kill the fish, and the chlorine can kill the nitrifying bacteria that convert the fish waste into harmless nitrates (at least harmless for the fish, to a certain extent).  For a long time, the primary chlorinating agent was hypochlorous acid (HOCl), typically added as sodium hypochlorite (NaOCl), or bleach.  Hypochlorous acid is very effective at killing bacteria, but is relatively volatile and will evaporate away within a few days.  Chlorine in this form is called 'free chlorine' and itself doesn't pose a huge problem for aquaponists because the water can be dechlorinated just by letting it stand (or circulating it through the system) for a few days.  And if water is added to the system in small amounts (e.g., to offset evaporation), the concentration is low enough that the fish don't have too much trouble.

The evil older brother of free chlorine, however, is combined chlorine, or the chloramines (especially NH2Cl, but also NHCl2 and NCl3).  Recently, many municipalities (including our former town) have begun ammoniating their chlorinated water to convert hypochlorous acid into monochloramine (NH2Cl), which is not as good as HOCl at killing germs, but sticks around a lot longer.  For municipal water suppliers, monochloramine means the water stays safe for longer, which is a good thing.  For aquarists and aquaponists, monochloramine means that water stays toxic to fish for longer, which is a bad thing.  (Chloramines are quite toxic to fish and likely people, especially after reacting with organic matter).  Fortunately, through the magic of chemistry, there are a number of ways to get rid of even the chloramines without too much trouble.  We were gearing up to do a first-of-its-kind series of experiments here at the Homestead Laboratory to figure out the best way to get rid of these dastardly devils, but as is so often the case in research, someone had already done it.  (And did a better job than we were probably going to do!)

While the chloramines will eventually evaporate away if given enough time (see here and here), they are likely to kill your fish and friendly bacteria in the meantime.  Thus, other methods of chloramine removal are necessary.  Two ways that seem to work well and that are the especially practical are 'equilibrium shifting' and 'reactive adsorption,' which we want to discuss in more detail partly because the chemistry is super cool and partly because it's what we did in our system.

'Equilibrium shifting'
Hypochlorous acid and all three of the chloramines exist in equilibrium with ammonia (NH3) in solution, which means that the solution has the ability to convert these compounds into each other in order to give itself the lowest possible energy. (Nature is kind of lazy this way--always trying to minimize it's energy and stuff.) 

Equilibria of ammonia (NH3), hypochlorous acid (HOCl) and the chloramines.  The higher volatility compounds are easier for aquaponists to deal with because they just float away into thin air.  Adding bleach (or hypochlorous acid, if you can find it) shifts the equilibrium toward the more volatile compounds.  Of course, you still have to let the bleach evaporate before adding the water to your aquaponic system.

Le Chatelier's principle says that if we add more of one component, the concentrations of all the other components will adjust to reestablish the equilibrium.  Since we've got a limited amount of NH3 (essentially what was added at the water treatment plant), if we add more HOCl, we will shift the distribution of the chloramines to the more chlorinated species (NHCl2 and NCl3).  Such a shift is good because the more chlorinated chloramines are more volatile, so they will evaporate and take the chlorine with them.  This approach is similar to what's called 'breakpoint chlorination,' in which free chlorine is added to the point where the chloramines are converted to NHCl2 and NCl3 and evaporate away.

The salient point here is that the equilibrium can be shifted the same way by adding bleach, which is something many folks already have on-hand.  (For this purpose, however, use only the stuff that doesn't have additional fragrances or other additives.) Alternatively, instead of adding bleach, it would also work to lower the pH (e.g., by adding the aquarium product pH-Down), presumably by converting OCl- into HOCl, which essentially shifts the chloramine equilibrium in the same direction.  On the other hand, if you're going to add external chemicals to the system, you might as well just add a campden tablet (sodium or potassium metabisulfite, Na2S2O5 or K2S2O5, respectively, which reacts with essentially all of the chlorine (both free and combined, as long as the proper ratio is campden/chlorine is observed) within a minute.  Campden tablets are available at most homebrew stores (both online and in real life).

'Reactive adsorption'
The other way that can readily be used to dechloraminate the water is with a carbon filter.  Depending on the distribution of chlorine species present in the water, the carbon acts either as a catalyst or as a reagent through the following set of reactions:

Reactions of the chlorinated species with a carbon filter.  C* represents an active site on the filter, CO* is an oxidized active site.  Since monochloramine can react with both the active sites and the oxidized active sites, it doesn't consume the filter material.  Since the other two species will be present to some extent, the filter will eventually need to be replaced.  Even if the filter material is still visible, it's a good idea to test the water for chlorine!  Testing kits are easy and not that expensive (e.g., here).

The pitcher-type water filters (e.g. Brita or Pur) can be used to remove the chlorine from the water, but they don't do the job completely, and are pretty slow.  That's why we ended up investing in a water deionizer from an aquarium supply company.  We didn't need the deionization, but the system comes with two parts--the ion exchange resin (IER) and the carbon filter.  The IER is aimed at extracting metal cations from the water (Ca2+, Fe3+, etc.), and is typically exhausted after about 50 gallons, according to the packaging.  (The pH of the water is considerably lower after the fact, so to some extent the ions must be exchanged for H+.)  However, the dechlorination part should continue to work as long as there is solid carbon filter material present, with the added bonus (in a sense) that the pH won't be artificially low after the ion exchange resin is exhausted.  We've been using the same filter cartridge for well over 100 gallons and the chlorine removal is still complete.  Plus we can hook it up to the sink and let it run until the tank is full.  (Normally we let it fill a container in the bathtub so that when we inevitably forget about it, we don't flood anything.)

There you have it--chemical equilibria, multiphase reactions, and heterogeneous catalysis in a real-life amateur science project.  Isn't chemistry cool?

Do you have any tricks you've used to control your aquaponic water parameters?  Do you have any other ideas for chlorine removal?  Let us know in the comments section below!

(We don't get any kickbacks from the companies linked above, so if you know of a better deal, please tell us!)

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