Biogas Production

Living Offgrid

'Living Offgrid or on country side', is it not romantic? Every romance comes with some challenges and also thrills.At off grid continuous supply of fuel is really a challenging matter. No body wants to run after fuel, particularity when he wants to sip his favorite brand coffee in moonlight. Under this scenario bio gas is an excellent choice.

What is Biogas


Biogas is a combustible gas which produces when bacteria, in the absence of air, decomposes organic material. This process is called fermentation. Biogas comprises primarily methane and carbon dioxide.

Organic material (biodegradable material) includes biomass, manures(pig, dairy, beef and poultry), 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, clover, alfalfa, ryegrass, turnips and corn), sewage, municipal waste, green waste.

Why Biogas


Biogas is high-energy, renewable, green power fuel that can replace fossil fuel energy. Raw material for production of biogas is very cheap because most of required material is considered as waste and junk in society.

Biogas can be used as a low-cost fuel for heating purpose, such as cooking. It can also be used in modern waste management facilities where it can be used to run any type of heat engine, to generate either mechanical or electrical power. Biogas can be compressed, much like natural gas, and used to power motor vehicles.

1m3 of biogas can give as much light as a 60-100 Watt bulb for 6 hours

1m3 of biogas can cook 3 meals for a family of 5 - 6

1m3 of biogas can replace 0.7 kg of petrol

1m3 of biogas can generate1.25 kilowatt hours of electricity

How to produce biogas

Biogas can be produced in a closedairtightvassel, this vassel is called digester.

Two simple biogas digester designs have been developed; fixed dome digester and floating cover biogas digester. The digestion process (fermentation) is the same in both digesters but the gas collection method is different in each. In the floating cover type, the water sealed cover of the digester rises as gas is produced and acts as a storage chamber, whereas the fixed dome type has a lower gas storage capacity and requires good sealing if gas leakage is to be prevented.

The raw material mixed with mater, is fed into the digester via the inlet pipe and undergoes digestion in the digestion chamber. The temperature of the process is quite important because methane-producing bacteria do their work best at temperatures between 30 – 40oC or 50 – 60oC. It takes from 2 to 8 weeks to digest a load of waste, depending on the temperature.The biogas, which is mainly methane, accumulates at the top of the tank where it is collected and taken by pipe to the user.The left-over slurry is removed at the outlet for use as a fertilizer.

The size of a digester can vary from a small household system to a large commercial plant of several thousand cubic meters. This depends on how much biogas is required and the availability and amount of livestock manure and water (water, number of cows, goats or other livestock), and the materials available on site (bricks, etc) for construction of the digester. A regular supply of water is essential for operation of biogas plants.

In Biogas production plant following seven ‘S’ are mile stones.

1. Substrate: The organic material that you will be feeding your digester. This can be manure, plant waste, paper pulp, or other biodegradable material.

2. Slurry: The slurry is the homogeneous mush that you will be passing through the digester. In

order for efficient digestion to take place, the substrate material should be mashed or ground

and mixed with water to make a uniform substance with 15–40% solids, depending on your

digester design.

3. Stratifi cation: As your slurry breaks down in the digester, it will separate into layers. These layers or “strata”—are biogas, scum, supernatant, sludge and solids. Mixing of the slurry prevents

excessive stratifi cation, but some stratification will always occur, especially in “batch” digesters.

4. Scum: The scum layer fl oats on the top of the material in the digester, just below the gas level.

The scum level is formed by hard-to-digest material like coarse straw and grease.

5. Supernatant: The spent liquid of the slurry. The Supernatant has a high content of solids, making it of high value as fertilizer.

6. Sludge: Below the liquid supernatant is the sludge layer. The sludge is the digested and semi-digested organic solids. This sludge can provide excellent composted fertilizer, but depending on the feedstock, it may need to be dried in the sun to kill any surviving pathogens. This is more of an issue with manure-based systems.

7. (Inorganic) Solids or Sand: The bottom layer consists of those non-digestible solids that fi nd

their way into the digester. These could include dirt, sand, small rocks, plastic or metal—any

inorganic solid that may inadvertently be introduced into the system.

A biogas plant visited by Author

Uses


Biogas burns very clean with a flame comparable to that of liquefied petroleum gas (LPG) and can be used directly in a simple low-pressure gas burner. Biogas can be used for a variety of other purposes, such as lighting, refrigeration, engine operation, or generation of electricity. The digester effluent (the residue of the digestion process) is a good-quality fertilizer.

Advantages


  • It is a renewable fuel that is ‘carbon negative’: creating and burning biogas releases less greenhouse gases than if the dung was left on the ground to decompose naturally
  • Biogas burns very cleanly, and produces less pollutants during cooking than any other fuel except electricity
  • Depending on the type of gas stove used, the handling of biogas for cooking is easy and allows for strong heat as well as for small simmering heat.
  • It can be used for lighting as well.
  • The by-product from the digester is a good fertiliser
  • Every domestic biogas plant can save about four tonnes of fuelwood per year and thus the 120 000 biogas plants can reduce the fuelwood consumption of the country by about 3.5 million tonnes. The annual CO2 savings vary between 13 000 tonnes and 157 000 tonnes, depending on whether the fuelwood is harvested in a sustainable manner or non-sustainably, and on the amount of kerosene replaced by biogas.
  • Harness methane, the highly destructive green house gas, so it does not go into the atmosphere and contribute to global warming.
  • Provide clean- burning fuel for stoves and lamps instead of wood or paraffin.
  • Reduce respiratory disorders caused by smoke from cooking with firewood, which especially affect women and small children. Improve village hygiene as attached toilets lead to the pathogen–destroying digestion tanks of the biogas system. That is, germs are not released to the environment.
  • Encourage farmers to zero–graze livestock for dung collection, which protects ground cover and forest regeneration from grazing animals.
  • Savings from the substitution of the energy recourses (cooking and lighting with biogas
  • Savings from the substitution of purchased mineral fertilizers (slurry)
  • Increased crop yield by using bio–fertilizer
  • Income from the sale of bio-fertilizer
  • Savings in the cost of disposal and treatment of waste and wastewater
  • Time previously spent collecting fuel wood can now be used to attend school, generate income or engage in productive activities.
  • Improvement of sanitation and hygiene
  • Conservation of trees and forest reserves and a reduction in soil erosion
  • Improvement in health from reduced ground, water and air pollution
  • Recycling of local resources to clean up the environment
  • Establishment of a new industry skills enhancement and employment for people in rural areas.

Disadvantages


  • Investment costs: digester, gas stove and installations for the gas to get into the kitchen.
  • It can increase the workload, that is feeding raw materials into the plant.
  • Installations (depending on material and location) must be protected against theft and damages.
  • Cultural rules might limit the acceptance of handling facies and their use for cooking
  • The complex fermentation process in the biogas plant needs a continuous supply of suitable feedstock (preferably dung of other animal waste). This requires an appropriate farming system which may be a problem especially for poorer families:
  • It requires a continuous supply of water, which is a problem in arid areas.
  • The need for a continuous supply of dung requires, in most cases, that livestock is kep confined for at least some of the time.
  • Tonnes of slurry fertiliser may need to be transported before use.

Precautions


Biogas is not poisonous. The only danger is from explosion if it is mixed with air and lit by fire. However, it only explodes if mixed with air or oxygen with a lighted match or fire very close by. For example, you shouldn’t ever lean into the tank with a lighted match! If you have a biogas leak in the house, and have good ventilation (windows, fresh air blowing through the house), it should not pose much danger of explosion although you would lose your stored biogas. This is why good maintenance is important. Remember, at all stages of construction, operation and maintenance of a digester, you are exposing yourself to potential hazards. Some feedstocks, especially manures, contain pathogens or parasites. You can be exposed to them not only in the loading phase, but also in clean-out, depending on retention times. The digestion process may not kill them all. In a dung-based system, special care should be taken to keep yourself, your tools and your work area clean. Methane and the other gases produced can be deadly. If you are a rural resident, you have probably heard stories of individuals who have been overcome by “septic gases” when cleaning a septic tank. The same can hold true for an anaerobic digester. Remember, the digester is designed to be an oxygen-free environment good for anaerobic bacteria, bad for humans! Finally, of course, biogas is explosive. Never smoke or use a torch or lantern in the presence of biogas, and use soapy water NOT a flame to test for the presence of biogas, unless it is fed to a properly regulated burner.

Last but not Least


Complex mathematical and design calculations are skipped intentionally. So that reader may not loss his interest. However If somebody is interested in technical details and design calculations, then feel free to ask. I will assure you that you will be satisfied.

Comments 5 comments

Munawar Zaman 5 years ago

What is the production rate of biogas any mathematical proof


Saeeduallah 5 years ago

What an interesting technology

what is the cost of single unit


mussafer profile image

mussafer 5 years ago from Islamabad Author

Saeeduallah this technology is not to much costly. you can build a biogas plant in just 250$. the plant which you can see in the hub, is operative since 1992. only one time maintenance is required after every 8 to 10 years.

Any how thanks for your interest.

feel free to ask if any question is there in your mind.


Uche 5 years ago

Hi, I am a prospective livestock farmer from Nigeria. Can you please assist me with information on the technical details of constructing a biogas digester using poultry and pig manure. This technology is yet to be exploited in Nigeria. I hope to hear from you soon. My email is obih_uchenna@yahoo.com


Rasa 9 months ago

Hi

If we have a cattle (100 cows), in a cold land (Norway), how much biogass can we produce with cowdung?

Thanks

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