Biogas Digesters

Biogas Digesters

What are Biogas digesters

Biogas digesters are devices that carry out anaerobic fermentation on organic waste to release methane gas and produce biogas. The most common types of biogas digesters are wet biodigesters. This is because they digest liquefied manure on farms. Most people prefer wet bio-digesters in removing methane from manure so they can use the methane to power their appliances. The reason is that it allows optimal mixing leading to high biogas production.

The bio-digester does not require maintenance or sewage system. The bacteria used procreate and do not require repeated dosing.

Methane Digester

Also called anaerobic manure bio-digesters. They breakdown manure to release methane used to produce biogas which is a renewable source of energy. If organic waste is mishandled, it emits methane which creates a warming effect 34 times stronger than carbon dioxide.

To control this, organic waste is sealed in tanks called anaerobic bio-digesters. These digester tanks are made of 14-inch-thick concrete walls and thick floors with heating coils. Inside the bio-digester, the thick slurry is stirred by an agitator while being heated to 101 degrees.

Agricultural, industrial, and human digestion processes create an ongoing supply of organic waste. They use microbes to transform scraps and sludge and produce biogas. They also produce residual solids called digestate which are great fertilizers.

The resulting residue is normally separated. This is to give enough water to be used in forming the slurry at the beginning of the process. When the methane reaches the top of the tank it is captured in an expandable rubber bladder. It is cooled and dried then moved in pipes to the combined heat and power unit to produce biogas.

Plants absorb this slurry better than that of artificial fertilizers.

Types of Biogas Digesters

Fixed dome biogas plants: It has an immovable gas holder and a displacement pit. The upper part of the digester stores the gas while the waste is displaced in the displacement pit. Pressure increases with an increase in gas.

Floating-drum biogas plants: Consists of underground bio-digesters and movable gas holders. The gas is collected in the drum above the digester. This moves up and down according to the gas collected.

Balloon plants: Has a rubber bag or balloon and it combines the bio-digester and gas holder. The skin of the rubber bag is connected to the input and output.

Biogas Digester Construction

A biogas digester consists of a sealed tank that holds the bio-matter and a means to collect the produced gases. For these parts to work you obviously need the provision to mix the slurry. You also need to pipe off the gas and dry off the effluent.

Be sure to consider how much organic waste you can collect a day for feeding the tank. Also, determine the model of bio-digester you want as they are all not built the same. You will also need other things like adhesives and tools. Finally, make sure you know what you’re doing.

 Home Biogas Digester

example of kitchen refuse

It can be used to process kitchen waste, animal refuse and even human waste. It may be connected to a sewage system or a manually fed bucket in the yard. The gas produced can be directly used to cook or heat water. It can also be stored. However, it is safer to use the gas immediately it is produced as it is highly flammable. Lack of proper handling could cause fatal results.

The system needs close attention. The slurry from the mashed materials should be fed into the bio-digester as close to 95 degrees Fahrenheit as possible.

 Commercial Biogas Digester

Most commercially operating bio-digesters operate at high temperatures and complete-mix reactors. The type of digester used varies with consistency and solids content of the feedstock.

 Before processing the fresh animal manure is stored in a collection tank. The tank has a mixer for homogenization of the waste. The mixed waste is passed through a macerator for uniform particle size then pumped into suitable capacity bio-digesters for fermentation.

 During anaerobic digestion, the biogas produced contains hydrogen sulfide (H2S) gas. It is removed in a biological desulphurization unit in which the hydrogen sulphide is oxidized into elemental sulfur. The gas is then dried and vented into a generator to produce electricity and heat.

  Can Biogas be Made from Human Waste

Sewage system containing human waste.

Compared to animal waste, human waste gives a limited amount of gas. This is because human waste does not contain methanogens. Research by David House says that 1000lbs of human waste can produce 0.6 cubic meters of biogas. This is fuel enough for only two people.

Humans have bacteria in the digestive system which produces farts. These farts contain methane while cows have rumen which increases the production of methane.

Commonly, human waste is used in a slurry batch reactor under anaerobic condition. Since human waste does not contain methanogens, sometimes cow dung is also used.

Bacteria Used in Biogas Production

For biogas to be produced, a process called anaerobic digestion must take place. This is digestion that takes place in the absence of oxygen with the assistance of certain bacteria. Anaerobic bacteria decompose organic matter into methane, carbon dioxide and sludge. In the beginning, acidogens break down the waste into simple fatty acids. This is called primary fermentation.

In secondary fermentation, anaerobic bacteria convert them into acetate, hydrogen and C1 compounds. Then methanogens consume these products and produce methane and biogas as a byproduct. The two organisms must be in balance if biogas is to be produced.

 How Much Do Biogas Digesters Cost

A ready-made basic domestic biogas unit costs between Ksh. 50,000 and Ksh. 80,000. However, if you want to construct your own, the price may vary.

The construction cost of an 8m3 can range between Sh80,000 and Sh100,000 depending on availability of materials. It can also be less or higher depending on the experience of the technician.

A farmer with four cows each producing 15kg of waste per day can construct( 15 x 4 x 2×60)/1,000 = 7.2m3. Recent studies, however, have shown that 75 per cent of the waste is digested within 25-30 days.

At BlueFlame Energy Solutions, we design and construct biogas digesters. For enquiries, you can reach us by using the contact details in our Contact us page.


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