Kitchen Waste Biogas Digesters
My final project in Colombia was to co-create a simple biogas digester for processing kitchen waste. Using a couple of DIY videos from Kathy Puffer, we succeeded pretty well despite lacking a hole saw, Uniseals, and a few other parts. Many thanks to Rafa, Ezekiel, Miguel, Margarita, Dan, Nubia, Isabel, Monica, and all who helped!
Why bother with biogas, when you can just compost kitchen scraps?
1. It’s a free source of methane for cooking.
2. All the same nutrients are in the effluent as in compost, in a form that is more beneficial for plants and the environment. This valuable digestate can be sold and can replace chemical fertilizers that damage the soil and run off into rivers.
3. A compost pile off-gases methane to the atmosphere. By capturing and burning the methane for cooking, you prevent greenhouse gases.
4. Biogas will replace either propane (which is a fossil fuel, often from fracking, which is very polluting) or firewood (which means deforestation and also lung and eye problems for the cooks). How much longer do you want to rely on fossil fuel?
How is biogas produced?
Methane is formed when several strains of bacteria, all found in cow manure, act together in an anaerobic environment, as in a cow’s gut — first producing acetic acid, then a mixture of methane, CO2 and H2S. The kitchen digester is primed with 10% manure for microbes.
To concentrate the methane percentage, filters are added to remove CO2 and H2S. Temperature also affects methane production; keeping the digester warm is best. That’s one reason why biogas is a good option in a warm climate like Barichara.
1 kg of kitchen waste plus 2 L water makes as much biogas as 35 kg of cow dung (200–300 L of gas). Fruit waste is high in sugars, which the microbes love. Starch & fats are also great. Do NOT add: meat, bones, eggshells, fruit pits, anything that won’t grind in a food grinder. Almost half of solids become biogas. Remaining solids can be added to a compost pile with good results.
Another calculation is: 25 L. (6.6 gallons) of ground food-waste mixed with water in a slurry produces 1000 L (1 cubic meter) of biogas. (For a barrel digester, you will not add more than 2 Liters a day.)
1 cubic meter (1000 Liters) of gas allows 1.5–2 hours of cooking on 1 burner. One cubic meter of biogas could power a gas refrigerator for 12 hours, or power a gas lantern for 15 hours. A regular gas stove burner can be modified by making the holes larger.
If continuous feeding is desired, to achieve a 25-day Hydraulic Retention Time in a 50-gallon barrel biodigester, you can add approximately 2 gallons (7.5 Liters) of ground kitchen waste mixed with water, per day. This ensures that the system is not overloaded and operates efficiently. Once it fills, digestate will come out through the effluent tube every time more liquefied food waste is added. If the digestate has an odor, that can be managed by adding Effective Microorganisms (EM).
Ideal particle size is <5 cm, achieved with 1 hp waste food grinder. Some people just use an old blender.
Monitor:
Keep pH 6.5-7.5 (neutral). No woody lignin waste. Overfeeding causes too much acid (pH 6 or less); add lime to raise pH to neutral. Test the effluent pH.
C:N ratio (carbon to nitrogen)– 20-30:1 is best. Different organic wastes have varying C:N ratios. For instance, cow dung has a C:N ratio of 24:1, vegetable wastes range from 11-19:1, while rice wastes can have a high ratio of 90-120:1. A mixture of cooked and raw kitchen waste ranges from C:N of 17-52:1. Coffee grounds are high in nitrogen. If too little carbon is in the mix, you get ammonia, which is alkaline and inhibits methane production. Add carbon, add water. (Not vinegar!) Dry leaves ground up or grass clippings can be added to increase carbon.
Filtration
To get a higher concentration of methane by removing CO2 and H2S, run the gas through a tube filled with steel wool. H2S is very corrosive and will degrade any metal, such as the stove, so it should be removed, not only for the bad smell. CO2 can be removed just by bubbling through water. Water vapor is present in biogas and will condense as it cools. Therefore, a water trap should be added before the gas goes to the steel wool filter to prevent blockage of tubing and rusting of the steel wool. The steel wool will need to be replaced after 3-4 months (or if you get a whiff of H2S).
Digester effluent: liquid fertilizer
The effluent improves seed germination, disease resistance, and crop yield; it increases feed value of fodder for animals; improves soil structure and fertility, with less soil erosion. Effluent can also be treated with EM to minimize odors and remove any pathogens. Before applying to plants, it should be diluted 10:1 with water.
Gas storage options:
- Two-barrel system consists of a 55 gallon barrel without a lid, and a smaller barrel without a lid turned upside down that fits inside it. The large barrel is filled 50-60% with water. Gas is introduced into the upside-down barrel at the top, via tubing from the digester. Placing weights on top of the smaller barrel helps add more pressure, giving a stronger flame so cooking time is cut down. This pressurized gas then goes directly to the stove. Strips of rubber such as from bike tire tubes can be used to anchor the top barrel to the rim of the bottom barrel, to prevent the top one from accidentally popping out as gas pressure builds up inside.
- A tractor tube of 252 cm circumference can hold 1 cubic meter of gas; it can be pressurized with weights. Smaller, truck inner tubes can be used too.
Safety
Methane is of course flammable and it is very important to prevent leakage, so make sure all connections are tight. Adding hose clamps to fittings, using PVC primer and then PVC glue to secure pieces together, and sealing any connections with silicone can help. Pay good attention to the valves and turn off after cooking is completed. Sometimes you may run out of gas before cooking is complete; be sure to turn off the gas valve because new gas is being generated all the time.
It's best to use your biogas at least every 2 days, so that there is not more biogas generated than the barrel can hold. Think of your digester as alive and needing food and care, because indeed the microorganisms are alive!
Bigger & better
A 275-gallon IBC (International Bulk Container) can accommodate 5 times more waste than a 200 Liter barrel and provide 5x more biogas. It should be painted black to hold in heat (digesters function best when warm) and prevent algae growth.