For those of you less concerned about methods and graphs feel free to cut out the below summary, tape it to the side of your tank and ignore the rest.
The major challenges of DIY injecton stem from the short lifespan of the yeast culture and the associated variability of CO2 output over the productive life of the culture. "Typical" DIY recipes tend to produce an exponential output growth phase (boom) of less than two days followed by a decline (bust) that fizzles out completely within two to three weeks. The injection rate in week one is up to six times that of week three, causing a risk of initial pH crash and making it very hard to achieve consistent results and stable water chemistry.
The output profile for a typical recipe was measured and is provided in the figure below, together with the output profile of the proposed Standard Recipe. This typical recipe is representative of the many thousands available on the web, generally involving variations to “2 liters of water, 1 teaspoon of dry yeast, 2 cups of sugar, 1 teaspoon of tomato paste and a pinch of baking soda”.
It is clear from the graph that the Standard Recipe produces a much more stable gas output over a longer period of time. By using this recipe in conjunction two interconnected fermentation bottles that are renewed alternately every three weeks or so you can achieve a very consistent and predictable output of gas.
The Standard Recipe is based on cheap and readily available ingredients and simple methods. Providing that you have the ingredients and basic equipment at hand the total replacement time for a bottle of brew is less than 5 minutes, equivalent to an hour or so of effort per year and tank. And the results can be equally spectacular to those of far more expensive and by no means maintenance-free pressurized gas systems.
The approach is based on four principles:
1. Use Only Sugar, Yeast and Baking Soda
DIY CO2 requires a bit of attention so don’t make it more difficult than it has to be. Stick to widely available, low-cost ingredients and use them consistently. Key findings from the study include:
▪ Yeast nutrients cause boom & bust and will shorten rather than extend the productive life of the culture.
▪ Champagne yeasts and other exotic strains deliver a limited or no improvement. Use commonly available, cheap baker’s yeast.
▪ Don’t re-use old yeast from batch to batch. Fermentation will be very unpredictable and the cost saving negligible. You too can afford a few millilitres of fresh baker’s yeast every month.
▪ Jelly is for kids and the toothless. Did you know that stuff is made of hoofs?
▪ Shaking the brew during fermentation will only produce a temporary burst of dissolved CO2 followed by a reversal back to trend output. Once fermentation is underway leave the bottle alone.
In short - use only plain white sugar, plain dry baker’s yeast and baking soda. Most large supermarkets stock “big packs” that will give you a very low cost per batch. Stick with the basic recipe, the same manufacturers and brands and be consistent in everything you do. Set up the bottles, leave them alone and get into a routine of renewing a bottle every three weeks or so.
2. Be Precise
If you use too little sugar the fermentation will terminate prematurely. Use too much and your bottles will be clogged with yeasty fudge. Overdo the yeast and you will have boom & bust. Insufficient yeast will yield little or no CO2.
The experiments show conclusively that the most important factor for culture performance is the initial amount of yeast added. The longevity and output of the culture is directly proportional to the initial concentration of yeast. Two ml of yeast will result in double the rate of CO2 output for about half the duration compared to one ml of yeast.
The reason is (chiefly) that yeast reproduces mainly in aerobic conditions, which in your closed container will be present only for the first few hours. As the culture goes anaerobic the cell reproduction ceases and the population goes into irreversible decline. The initial concentration of yeast will therefore limit the cell concentration and CO2 output for the duration of the culture’s productive life. The experiments also showed that the concentration of sugar has a comparatively insignificant impact on gas production over and above a minimum concentration.
Precision is absolutely critical when adding the yeast. A teaspoon won’t cut it but a plastic 1 ml measuring spoon will serve you well.
3. Use a High Concentration of Baking Soda
The experiments showed that falling pH is the second constraining factor for CO2 output, not increasing alcohol concentration as commonly believed. The pH level of the brew is buffered by the addition of baking soda (sodium bicarbonate) and the results show a direct relationship between the concentration of baking soda and culture longevity, even at very high concentrations of soda. Only small toxic effects could be detected within the experimental range. Don’t worry about using too much baking soda, and don’t think that “a pinch” is sufficient.
4. Stick to the Recipe
I recommend that you adjust the total CO2 output rate by varying the volume of brew, leaving the proportion of ingredients unchanged. You may decide to increase the yeast concentration to achieve a higher gas output per litre of culture, but beware that this will increase output variability and reduce the longevity of the brew. It is basically a choice between using large containers with low yeast concentrations renewed infrequently, or small containers with high yeast concentrations renewed frequently. Once you have settled on your particular version of the Standard Recipe it is recommended that you stick with it and change only the volume of culture to suit your requirements.
A good way to increase volume is to use multiple fermentation containers. Again I recommend the use of two containers which are renewed alternately.
Thoughts on Injection
Fluctuations in CO2 production are unavoidable in the DIY approach, whatever the recipe. Even the Standard Recipe will produce the highest output of gas in the first 24-48 hrs followed by an exponential decline, although the decline is much less severe than for other recipes. The culture will also be subject to temperature fluctuations and other environmental factors and will respond to them through changing metabolism and CO2 output.
The last step of the DIY process is therefore to ensure a constant injection rate of CO2 into your water whilst minimizing gas waste. I have found the diffusion bell to be a very cheap, safe and reliable method which is illustrated below:
Benefits of the diffusion bell include:
CO2 has a high water solubility and any gas in contact with the water will dissolve very quickly. As the actual uptake of gas into the water is governed by the area of the gas/water interface, any excess gas will simply bubble over the side of the bell, thereby eliminating the risk of over-injection. The fixed size of a diffusion bell provides a very steady injection rate and a predictable dissolved concentration of gas. If you want to increase or reduce the injection rate, simply change the diameter of the bell.
The bell allows you to monitor gas output by the traditional bubble counting method.
A bell can be easily constructed using any old transparent container, a drill and standard silicone tubing.
A transparent bell is relatively inconspicuous. Place it loosely under the tank cover with the tube passing to the side of the cover. The bell pops out whenever the cover is removed but so what? Just put it back again and it will fill up with gas in no time.
For the sake of simplicity you should leave the bell in place overnight. The water concentration of dissolved carbon will build up and reach a maximum just before the light is switched on, but this is generally not a problem. You can easily avoid pH crash by starting with a low injection rate (small bell and/or slow gas injection) and work your way up over a few weeks or months while monitoring the effect on plants and fish.
As you increase injection rate over time you will soon discover “pearling” of gaseous oxygen from the plants when the light is on. The sight of swirling clouds of tiny oxygen bubbles and explosive plant growth is a marvellous thing, and an indication that the CO2 concentration is about right.
Methods
Appendix A contains an illustration of the experimental rig. The yeast cultures were incubated in nine 1.5 l plastic PET bottles, immersed in a water bath to moderate ambient temperature fluctuations. Prior to mixing, all equipment and bottles were sterilized in hot water and handled using sterile methods.
The caps of all bottles were drilled and lengths of 4 mm Poly tubing inserted. The diameter of the tubing was slightly larger than that of the hole to ensure a tight fit. Nine 1 ml syringes were inverted and glued to the inside of a transparent plastic chamber. The chamber was filled with water to displace the gas up to the 0 ml mark, which corresponded with the water level mark.
The tubes from the bottles were attached to the spout of each syringe. A T-connection and gas release valve was spliced into each connecting tube. The valve allowed gas to escape and water to fill the syringe. Upon closing the valve the time for displacement of 1 ml water from each syringe was measured with a stop watch. The displacement rate was converted to ml/hr output.
Appendix B and C contain the results of the two most important experiments; the relative importance of sugar and yeast concentration and the effect of yeast nutrient and baking soda additions. The proposed Standard Recipe corresponds to the “best profile” result highlighted in Appendix C.
Appendix A - The Experimental Rig
Appendix B - Gas Output as a Function of Yeast and Sugar Concentration
Appendix C - Cumulative Gas Output as a Function of Yeast and Sugar Concentration
Appendix D - Gas Output as a Function of Nutrient and Baking Soda Concentration
Appendix E - Cumulative Gas Output as a Function of Nutrient and Baking Soda Concentration
