Biogas consists mainly of methane (about 60% to 80%) and carbon dioxide (about 20% to 40%) with some other gases, such as hydrogen sulfide, ammonia, and water vapor, in trace amounts. It is highly flammable and is produced through the anaerobic (without oxygen) decomposition of organic materials from plants and animals. It is similar in most respect to Natural gas (obtained from fossil fuel) used for heating and cooking at homes and industries. Natural gas, just like biogas consists mainly of methane. Therefore, after undergoing some purification, biogas can be used just the same way we use natural gas to produce heating and cooking at homes and offices.
Biogas is also produced, artificially, using biogas generators/digesters. Biogas digesters are large air-tight tanks used to simulate the natural processes that produce biogas, by allowing the digestion of organic matters from plants and animals under anaerobic conditions. The processes are as follows:
- Plant materials and animal wastes (feedstock) are shredded and placed inside the biogas digester.
- Water is added and the tank is closed and properly sealed to allow no air into the tank.
- After several days, biogas begins to form at the top of the tank due to the activities of some bacteria usually termed “methanogenic” bacteria (i.e. methane forming bacteria).
- The biogas that is formed is piped into a storage location where it can be used as needed.
- As the production of biogas in the generator slows down, old feeds of organic matters are taken out and new feeds of organic matters and water added to the generator.
- The old feeds can be dried and used as soil manure or fertilizer.
The common feedstocks for biogas include:
- Livestock Manure (e.g. cow dungs, pig dungs/hog, poultry dungs etc),
- Food processing (by-products of meat processing, potato, dairy, cheese whey, sugar beet, pea hulls, and vegetables); and
- Energy crops cut as silage (wheat, barley, triticale, clover, alfalfa, ryegrass, turnips and corn).
Unlike natural gas, biogas is renewable; it can be replaced in a life time. It is environmentally friendly; it reduces greenhouse gases. Methane gas from Swamps, Landfill sites and Sewage Treatment sites, that could have been released directly to the atmosphere are redirected for power production. In cases where plants are planted to provide feedstocks to Biogas generators or digesters, the plants serve as sinks to reduce the amount of carbon dioxide released to the atmosphere.
Design Overview of BioDigester (by Gerardo P. Baron)
- Flexible: May be used alternately for continuous flow (valve 1 open) or plug flow digestion (valve 2 open as needed.)
- Small, Compact & Inexpensive: Separate digester allows full use of its contents for digestion; results in high gas yield versus digester volume; and, costs less.
- Clean & Sanitary: Exposed digester liquids are kept at minimum compared to digesters with telescoping compartments.
- Versatile: By raising the top section of the bladder, a suction (vacuum) effect may be created to extract gas. Conversely, by pressing down or applying weight on the top of the bladder, gas pressure is increased or adjusted.
- Simple & Functional: Containers like 55-gallon metal or plastic drums can be easily made into digesters with just minor modifications. Bladder (under test) is made of inexpensive tarpaulin which is tougher, more durable and safer than PE used for TPED or PBD. It is sealed and shaped like an inflatable pillow. Moisture Traps are maintenance free (i.e. overflow when full.) A check valve using a ping-pong ball is being designed.
The pillow shaped (oval cross-section) tarpaulin has also been used effectively as a 2 cubic meter (cbm) biodigester tank. Same concept was also used to make two 10 m3 digesters and one 10 cbm bladder using 1.5 mm HDPE material.
For more information: Email Gerardo P. Baron at firstname.lastname@example.org and Call or SMS 0927-4071142.
Building a Methane Digester
To produce 1 cbm of BIOGAS, to cook 3 meals daily for a small family of 4 to 6, you need at least 5 liters of pig manure/day. Eight sows will produce 5+ liters/day easily and your 36 heads total will probably give you at least 15 liters
A 2 cbm digester will produce 1 cbm of BIOGAS/day — good for experimental purposes. I suggest your first digester be at least 5 cbm. This will produce more BIOGAS when needed and accommodate your farm’s growth.
The China Fixed Dome, India Floating Cover and DOST-PSTC designs are the most popular here in the Philippines. Download drawings from the internet but I suggest you get professional help as the building process is not easy. Here are people who can help:
Trained at Asia-Pacific Biogas Research and Training Center, Chengdu City , Sichuan , China.
Provincial Agrarian Reform Officer, Cebu City
Cell No: 0920-9236930
Engr. Orlando Anselmo
Has installed 35+ DOST-PSTC digesters in Aurora Province
DOST Officer, Baler, Aurora Province
Cell No: 0915-5699631
Construction of Home Biogas System (HBS)
Advantages of the HBS
- Easier to build, less expensive, and simpler to operate and maintain (clean & repair)
- Does not need a concrete dome that is difficult to build, expensive and prone to leaks.
- Does not need a floating (metal) cover that corrodes, is expensive and difficult to operate.
- Does not need a stirring system that corrodes, is laborious and prone to leakage.
- The HBS has a simple sediment removal process that is easy and convenient to operate.
- The HBS can be located closer to the kitchen or place where the gas will be used to minimize piping problems like clogging and leaks.
- It can be built as a Do-It-Yourself (DIY) project where expert masonry skills are not required using common and inexpensive materials available anywhere.
Biogas production from pig manure where 1-2 pigs excrete 1L (L=liter)
- 20L/day = 250 Pesos biogas/month
- 40L/day = 500 Pesos biogas/month
- 60L/day = 750 Pesos biogas/month
The plan described here is for an 8 cubic meter digester. It can produce up to 1,000 Pesos/month of biogas from 80L/day of pig manure.
- Inoculant Preparation: Three weeks before construction, place 100L of manure in a 200L drum and mix thoroughly with 50L to 100L of water. Preparing more is better if possible.
- Upon completion of HBS (all surfaces are fully dried), pour all the innoculant prepared into the HBS.
- Keep adding manure (thoroughly dissolved in water with 1:1 ratio) into the HBS until the outlet compartment is half full.
- With biogas outlet valve closed, after a few days, the HBS cover must begin to bulge (indicating it is filling up with gas.
- Try lighting or burning the gas produced. It should not ignite during the first few days or weeks as the gas produced is CO2 and not biogas. CO2 will actually extinguish flames or fire.
- Keep adding manure daily as needed. Feeding may be skipped occasionally, with no adverse effect.
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