The Science Behind DIY CO2 I see a lot of questions asked about do-it-yourself CO2 injection in home aquaria, and a lot of false information gets propagated. I’d like to clear some of that up by explaining the chemistry of why and how DIY CO2 works. In addition to aquaria, another one of my hobbies is home brewing beer and wine, so this process is of some interest to me. Theory First off, CO2 is a byproduct of fermentation. Fermentation is the process of any of a number of yeast species consuming a sugar source as food. The yeast that is most commonly used is bakers yeast, Saccharomyces cerevisiae. It is the same species that is used in brewing most beer, and is known as a top-fermenting (ale) yeast strain. This means it ferments at the top of the liquid, and does best at room temperatures or slightly warmer. Yeast goes through 2 basic stages – aerobic and anaerobic. In the first stage, the yeast is in aerobic mode; when there is dissolved oxygen in the solution, the yeast will use this to multiply. Yeast needs oxygen to synthesize sterols, which are used in building cell walls. Without oxygen, their ability to do this is drastically reduced; therefore, the majority of their energy is then put into metabolizing glucose. When yeast is aerobic, they actually produce more CO2 than when they are anaerobic – twice as much, in fact. However, there is no practical way for us to keep the yeast in aerobic mode and still harvest the CO2. Once all the oxygen is used up, they enter anaerobic mode, where they start to consume sugars and release ethanol and CO2 as byproducts. This is the actual fermentation. Here is a visual of the reaction: And here is the actual reaction: C6H12O6 + 2Pi + 2ADP- → 2CH3CH2OH + 2CO2 + 2 ATP (energy released: 118 kJ/mol) So, as you can see, when fermentation begins, each molecule of glucose results in 2 molecules of ethanol and 2 molecules of CO2. Application I’ve seen a number of different “recipes” for DIY CO2, and most will work as well as any other. However, there are a few points to take into consideration. Dumping a solution of white sugar and water onto a yeast bed is kind of like asking you to live on a cup of white rice every day. Could you do it? Sure, for a little while, but you’d be much healthier with a balanced diet with vitamins and minerals. The same thing goes for yeast. It will do alright with just sugar and water, but does much better with some sort of vitamin source. This can be almost anything – orange juice, molasses, tomato paste, spaghetti sauce all work fine. If you’re using a 2 liter bottle for your CO2, a teaspoon or so of some sort of nutrient is all you need. Even dropping in a multivitamin tablet will work. Yeast is most active in solutions with a neutral to acidic pH. There are many recipes out there that recommend the addition of a buffer (commonly baking soda) to help regulate the reaction. While it is not something I recommend based on theory and experience, I can see the basic logic behind it. The idea is that by creating a non-ideal environment for the yeast, the fermentation will be slowed and will last longer. The problem with recommending it as a basic starting recipe is that everyone has different water. Some people will try starting with bottled water, with RO water, with straight tap water (city or well). Some people will have a pH of 6 out of the tap; some will have a pH of 8. If you are starting with a pH of 8 and are adding additional buffer to the water, chances are, your ferment is going nowhere fast. Adding a buffer to pH of 6 might slow down the reaction some, but you can get the same result by adding less yeast in the beginning. Now, to talk measurement. Much like saltwater, the sugar content of your solution is measured in specific gravity. Since we’re not getting overly exact here, most people don’t need to run out and buy a hydrometer. It’s good enough to use some basic ratios and go with that. Now, different yeasts have different alcohol tolerances. As the fermentation proceeds, the sugar is consumed, the CO2 is gassed off, and ethanol is produced in solution. This ethanol is what will eventually limit the yeast’s capacity to continue the fermentation. Normal strains of brewers yeast are active to about 8-10% abv (alcohol by volume). Bakers yeast is active up to ~14%, champagne yeasts to 16-18% and there are some turbo yeasts on the market and in development that tolerate upwards of 20-22+%. For most people, bakers yeast is the most economical option. It’s readily available at most grocery stores, and very cheap. So, the CO2 output is pretty much directly proportional to the amount of sugar the yeast has available, up to the tolerance cut-off. BUT – this does not mean you can dump 5 lbs of sugar into 2L of water, and let the yeast use what it wants. Not only is that wasteful, it also puts unnecessary strain on the yeast. When the yeast is put into a solution with a high specific gravity, it puts a lot of osmotic pressure on the cell walls. This causes the yeast to work at far less than their optimal performance. For this reason, you want to calculate about how much sugar it will take for the yeast to run out of food at about the same time the alcohol gets high enough to cause the cells to shut down anyway. That way, the yeast works at optimal performance, and you’re not dumping excess sugar down the drain every time you change your mix. So, what is this magic number? A good rule of thumb is to use 1kg of sugar for every gallon of water. This solution should have a potential alcohol of about 13-14%, which is the upper limit for bakers yeast. For those not on the metric system, one cup of sugar weighs approximately 200g. So, 1kg of sugar should be about 5 cups. If you’re using a 2L soda bottle for your CO2 solution, 2-2.5 cups of sugar should be the right amount to maximize the production, without creating waste. Now, there are lots of variations on the CO2 solution recipes, using sugar, yeast, frozen grape juice concentrate, etc. Basically, anything with sugar in it will ferment to some degree. Using white sugar and tossing in some nutrients (whatever happens to be on the refrigerator door that day) is probably the most economic approach for most people. So hopefully that will lay out some of the basic (and maybe, not-so-basic) elements of yeast fermentation and CO2 production, and clear up some of the common misconceptions and assumptions that people make when putting together a solution. With a bit of planning and measurement, it is pretty simple to make it work and get on track to growing some plants. References: Campbell, Neil A. and Jane B. Reece. Biology. 6th ed. San Francisco: Benjamin Cummings, 2002. Palmer, John. How to Brew. 2 May 2008. <http://howtobrew.com/>. Silva, Prof. Doutor Pedro. The chemical logic behind... Fermentation and Respiration. 4 Apr 2008. <http://www2.ufp.pt/~pedros/bq/respi.htm>.