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Biogas in China | Documentary Video

An IFAD-supported project in China's Guangxi province encourages poor farmers to produce their own biogas as a means of reducing poverty while improving local environmental conditions


Details of the Flexi bag biogas system

I’ve been inundated with requests for more information about the biogas system after my good friend Jagi told all his friends and they told theirs so on … I love social networking but in this case it’s socialnetWORKING.
I got a comment from Stephen Kamau who says

“EYE WITTNESS-This system really works.I bought the system through M-pesa after visiting a home in Transmara where i found Dominic stiring cow shit, and when he explained how it works,i decided to purchase. I installed it myself with my wife giving hand when i needed help and, after three weeks there was more gas than what our kitchen requires.
Thank you for the easiest technology.”

And here’s his proud construction

This blog is for all those people who want a share of Stephens joy.

Everyone who has seen my biogas system above, has asked how to get one. Loads of people want to buy one for their mother back in the village. Yes you can have one!
If you’d like to order it, simply call Dominic 0722 700 530 to place your order  or email him Dominic Wanjihia dwanjihia@yahoo.com.  The costs  vary but range from Ksh 45,000 to 55,000.  Specifics can all be found here on the Simply Logic official website  http://www.biogas.co.ke/

Start with basics. What is biogas?
Biogas is the gas produced by the biological breakdown of organic matter through fermentation. The gas is made up of methane (CH4) and carbon dioxide (CO2). Methane is highly combustible with oxygen when in contact with air. This occurs in anaerobic conditions which is in the absence of oxygen.
Biogas can be produced from a variety of materials such as manure, sewage, garden and kitchen wastes, and yes, your own dung.
Methane biogas burns hotter than butane with a beautiful blue hue – it helps to have someone beautiful to cook for you too (no that’s not me).

Thus, biogas is a low-cost fuel which can be used for any heating purpose, such as cooking and boiling water.
Justification for biogas use in Kenya
Everyone knows that Kenyas forests are disappearing because of charcoal and even though it’s leading to climate change, water shortages and power cuts, yet everyone still uses charcoal!
Some scary facts

  1. It is estimated that wood makes up 95% of domestic energy use in Kenya.
  2. The per capita use of wood is 1 ton per year! Crikey there are more than 40 million of us now!What about next year and the next and on and on….
  3. Natural replacement rates of wood fuel are estimated to be less than 60% – ie wood supplies are disappearing by 40 % each year in Kenya.
  4. Given the above, there will be no (affordable) charcoal in the near future. We need alternative affordable energy.
Most Kenyans have no alternative fuel source to wood fuel. 75% of the population, lives in rural areas with no access to, or money to afford liquid gas or Kerosene. But the good news is that nearly everyone has a cow – or access to one.
Actually biogas can be produced from any biological waste so rural farmers and pastoral communities can produce more than enough energy though biogas if they just had digesters… that’s where we come in

The miracle of Biogas
Before you get too excited let me remind you that biogas is not a new concept. Ancient Persians observed that rotting vegetables produce flammable gas and in 1859 Indians built the first sewage plant in Bombay. Biogas has been used in China for 2,000–3,000 years. It’s just that we in Kenya are ridiculously slow in adopting appropriate and obvious technology.
Two types of biogas digesters have been previously promoted in Kenya. The floating dome type from India and fixed dome types from China. Both are used widely in their home countries but haven’t done all that well here.
Studies have been done to find out why Kenyans don’t take to biogas and have come up with several reasons -
  1. Fixed dome and floating top systems are very expensive to install with costs ranging from Kes 150,000 and upwards for domestic systems. Ie. it’s just too expensive.
  2. They require technical expertise in construction which requires masonry stones and cement. It can take several days or weeks to construct. This adds to the cost and sheer scaryness of it.
  3. Metal parts of the dome are prone to rusting requiring regular repair or replacement
  4. Since it’s a construction, the user must own the land due to the construction requirements
  5. Parts are bulky requiring expensive truck transportation – we all know what a rip off it is to hire a pick up or lorry
  6. Some of the parts are imported and are not locally produced or widely available

Enter the (Dominic) FLEXIGAB biogas digester
Dominic Wanjihia has been scratching his head about the energy problems in rural parts of Kenya. Just think about it, if you are reading this blog you probably have an obvious source of energy, electricity.
All you need is one energy source to provide you with all the conveniences  you need – fridge, stove, hot water, lights. Without electricity your life would be severely compromised – no way to cool and store food, no way to work at night, watch TV, listen to radio, have a hot shower … and most of all, no way to work! But that’s how many Kenyans live. No wonder we are losing so much  productivity.
After seeing piles of dung wasting away at Maasai Manyatta’s, and the owners lamenting the flies, Dominic wondered why they weren’t making and using biogas – afterall, the women spend hours fetching firewood. The simple reason – they didn’t know anything about it.
Building a fixed dome by a manyatta didnt’ seem all that acceptable or convenient, especially if the family moved, and getting materials into remote areas was another challenge.  Then there’s the cost – the underground dome systems cost too many cattle. So started the flow of ideas – the Maasai needed something transportable, cheap and easy to operate,
That’s the true story of how the Flexibag biogas digester was born.
Dominic scratched his head some more, toured the juakali sector for materials, parts, ideas, pipes, and started sewing things together… testing what works and what doesn’t. We did a lot of research on the internet – bags have been used before but they failed due to light weight materials used. After many experiments and failures we now have something that works incredibly well.
The system Dom finally settled on is so simple it made him laugh. It comprises a heavy duty rubber bag as the digester. It sits mostly above ground and uses PVC pipes for inputs and outputs. Plastic gas pipes tap the gas from the digester and transfer it to the point of use. If the volume of gas is low the pressure can be increased by simply adding pressure to the biogas bag by placing 4 – 6 jerrycans full of sand on top (nothing with sharp edges should be used).
Some simple facts about WHY YOU SHOULD GET ONE OF THESE.
  1. It is cheap – Ksh 28,000 (USD 400) Ksh 45,000 (sorry folks I’ve had to update this today for the new and much improved digesters (March 2nd 2011) – check Simply Logic Website for all updates on costings) for the bag and pipes making it affordable for domestic and small businesses (appliances are a separate cost).
  2. Cooking appliances can run directly from the biogas after a slight modification.
  3. It is made from locally available and affordable materials. Ie there is no need for imported parts
  4. Durable – the envelope is made of a very strong rubberized textile which is tear resistant materials. To protected from sunlight with a layer of grass, and from livestock by surrounding with a small fence
  5. It is light weight and easily portable – this system can be quickly transported into rural areas weighing 10 kg and packs small enough to be carried on a bicycle or motorbike.
  6. If you decide to move it, it is easily transferrable. There is no masonry construction involved in setting up this system so the bag can be emptied, rolled up and moved to a new location.
  7. Installation is quick  – the only requirement is to level a patch of ground. the time taken to install build the system is only a few minutes for roll out the envelope and connect pipes.
  8. In the event of rips, tears (vandalism) or broken pipes, repair of the system is easy, quick and cheap (no digging and masonry works)
  9. Biogas production is rapid – being above ground promotes rapid gas production attributed to the high temperatures achievable by direct sun exposure
Let me explain how it works. You add fresh cow dung that has been mashed with water til it’s smooth like porridge into the poop pipe in the foreground. Once in the bag it begins to ferment and move slowly across the digester till it’s “exhausted”. Gas that is formed presses down on the poop and pushes the exhausted  stuff out of the orange pipe in the background. If the gas pressure rises because you aren’t using enough gas, it will simply push more poop out the other end. No, it can’t explode and shower you with wet dung.
We found that two big buckets of dung were enough to keep a household of 3 plus 4 dogs going continuously.
The pipe in the middle is the gas pipe – simply connect that to your gas stove* Note that this sytem will not work on those fancy Hotpoint cooker jobbies, we’re talking about 1 or 2 ring Mekko  type stoves which are specially adapted for biogas which does not come under as much pressure as methane in a tank. Once modified the stove cannot be restored to a methane stove. We have also made special burners for ovens – special is an overstatement, they are so simple you will cry.
10.  The envelope digester will produce and hold up to 5 cubic meters of gas which is adequate for one family cooking needs for 2 days.  As long as dung is added daily the gas production can be maintained.
11.  The capacity of the system is easily expanded – to increase gas capacity (eg for several houses, a village or school), simply add more flexi bags beside the initial one and connect with pipes.
12.  The exhausted dung is emitted by the system automatically and has no bad odour (dung will need to remain in the bag for up to 3 months to be fully digested). It is channeled directly to the farm or vegetable patch where it can be used immediately (no further composting required).
13.  This system can be adapted to include human wastes as well as kitchen wastes.
14.  The cost of Flexibag system will be offset within 2 years afterwhich the cost of gas is free.
I did a simple calculation and worked it out
Options Cost (US$ ) Time to install (days) Labour Maintenance Durability
Fixed dome 1,500 – 2000 21 5 people Low Decades
Floating top 2,000 – 3,500 21 5 people Low Decades
Flexi bag envelope 400 1 1 person Low 10 – 15 years
Fuelwood  or LPG cylinders 200 (per year) 0 0
I bet you want to ask, but does the system REALLY work?
Yes it does. I’ve installed one at home and my gas cylinder is now idle. So many people want to know more about thow it works, and especially how to get one. Well, hopefully it’s all in this blog post

Source : http://wildaboutafrica.wordpress.com/2010/07/27/details-of-the-flexi-bag-biogas-system/



Anaerobic digestion is the natural biological process which stabilises organic waste in the absence of air and transforms it into bio fertiliser and biogas.  It is a 4-stage process: hydrolysis, acidification, acetogenesis and methanogenesis.

In the first phase anaerobic bacteria use enzymes to decompose high molecular organic substances such as proteins, carbohydrates, cellulose and fats into low molecular compounds. During the second phase acid forming bacteria continue the decomposition process into organic acids, carbon dioxide, hydrogen sulphide and ammonia.  Acid bacteria form acetate, carbon dioxide and hydrogen during the acetogenesis phase.  The methanogenesis phase involves methane forming bacteria producing methane, carbon dioxide and alkaline water.

The process of anaerobic digestion takes place in the intestines of humans and animals and in a landfill site, the latter in an uncontrolled manner.  Anaerobic digestion has been widely used in Germany, Sweden, Austria and Denmark and the technology is well proven and established.

1.      Biogas

                The valuable component of Biogas is methane (CH4) which typically makes up 60%, with the balance being carbon dioxide (CO2) and small percentages of other gases.  The proportion of methane depends on the feedstock and the efficiency of the process, with the range for methane content being 40% to 70%.  Biogas is saturated and contains H2S, and the simplest use is in a boiler to produce hot water or steam.  The most common use is where the biogas fuels an internal combustion gas engine in a Combined Heat and Power (CHP) unit to produce electricity and heat.  In Sweden the compressed gas is used as a vehicle fuel and there are a number of biogas filling stations for cars and buses.  The gas can also be upgraded and used in gas supply networks.  The use of biogas in a solid oxide fuel cells is being researched


1.1    Make up of Biogas

Percentage content of gas
Carbon dioxide
Carbon monoxide
Hydrogen sulphide
H2 S

2.      Production of Biogas

2.1    Feed Stocks

                A wide range of materials can be used in a biogas plant.  Energy crops can be grown specifically as a feedstock or by products and waste materials used.  The most important differentiation for waste materials is those that include animal by-product (ABP) that require pasteurisation, and those that do not. Mesophilic and thermophilic bacteria responsible for the process are only able to work and multiply if their substrates are sufficiently dilute (they cannot exist on solids) making slurry an ideal substrate for the process.

2.2    Types of Feedstock

                Source separated bio waste
                Mechanically-separated municipal waste (ABP).
                Food processing and abattoir waste (ABP).
                Commercial catering waste (ABP).
                Sewage sludge.
                Animal slurry.
                Vegetable and pack house waste.
                Energy crops – maize/grass silage, whole crop wheat, whole grain maize.
                Mixtures of the above.

2.3    Process Mechanics

         Substrate from different feed stocks is mixed in a fermentation tank or biogas digester; a warmed, sealed airless container (the digester) which creates the ideal conditions for the bacteria to ferment the organic material in oxygen-free conditions.  The digestion tank needs to be warmed and mixed thoroughly to create the ideal conditions for the bacteria to convert organic matter into biogas.  Fermentation tanks allow production and collection of methane from the anaerobic process.

         Gas is dried and vented into a gas engine connected to a generator to produce electricity. Heat can be taken off the engine to give combined heat and power.  The slurry by-product is virtually odourless and is a uniform high value fertiliser.  Heat produced is recycled to warm the substrate in the digesters.

2.4    Yields of different Feed Stocks

         Production of electricity per tonne of raw material feedstock
         Maize silage                    402 kWh/t
         Vegetable wastes   122 kWh/t 
         Grass silage                    256 kWh/t
         Slurry/manure                  47 kWh/t

         Typical gas production level and residue production (per tonne raw material)
         Maize silage                    = 200m3 gas (500kg liquid output)
         Corn Cob Mix                  = 500m3 gas (350kg liquid output)
         Whole crop rye       = 600m3 gas (300kg liquid output)
         Slurry                              = 20m3   gas (978kg liquid output)

2.5    Different types of Anaerobic Digestion

         There are two types of anaerobic digestion:-

         Mesophilic Digestion:  The digester is heated to 30 - 35oC and the feedstock remains in the digester typically for 15-30 days.  Mesophilic digestion tends to be more robust and tolerant than the thermophilic process but gas production is less, larger digestion tanks are required and sanitisation, if required, is a separate process stage.

         Thermophilic Digestion: The digester is heated to 55oC and the residence time is typically 12-14 days.  Thermophilic digestion systems offer higher methane production, faster throughput, better pathogen and virus ‘kill’, but require more expensive technology, greater energy input and a higher degree of operation and monitoring.  During this process 30-60% of the digestible solids are converted into biogas.  

Mesophilic digestion is the most common approach since it is more reliable and plant management is easier.

3.      Advantages of Anaerobic Digestion and Production of Biogas

         Processing organic waste anaerobically to create Biogas is a sustainable, renewable waste to energy solution.  The process offers numerous advantages over conventional technologies and if waste materials are used in the process, can reduce greenhouse gas emissions in 4 ways:-

    1.      Preventing the uncontrolled emissions of CH4 (22 times more powerful than CO2) from landfill.

2.       The Bio fertiliser produced can displace mineral fertilisers.  Nutrients are conserved with more than 90% of nutrients entering anaerobic digesters conserved through the digestion process.  By conserving nitrogen during digestion the N:P ratio of the treated manure is more favourable for plant growth[1].

3.       Reducing the transport of waste.

4.       Renewable electricity and heat can be produced, reducing greenhouse gas (GHG) emissions.  Since anaerobic digestion operates in a closed system, substantial reductions in greenhouse gas emissions are achieved.  Ammonia losses, while not of direct GHG concern, are also reduced.

3.1    Other Benefits of the Process

    1.      Odour levels are greatly reduced during manure processing, creating a relatively odour-free end product (closed vessel processing confines odorous compounds which are converted to other chemicals).  The product of digestion has no more odour than compost.

    2.      Improvement in slurry characteristics such as: fluidity, crop compatibility, homogeneity, reduction of weed germs.

    3.      Anaerobic digestion greatly reduces pathogen levels. Pre- or post-digester technologies can ensure pathogen-free end products.

    4.      Production of electricity and heat provides valuable income.

    5.      Reduced land fill tax and climate change levy charges.

    6.      Income from Renewable Obligation Certificates (ROCs).[2]

7.       Positive use of organic waste materials reduces land and water pollution.

4.      Economics

         The viability of an anaerobic digestion plant will depend on:-

§  The availability of waste to give a feedstock of zero cost, and if a gate fee is charged for the waste received.
§  Whether the electricity generated is displacing existing demand and the type of contract.
§  The value at which the ROCs are traded.
§  Whether value is derived from surplus heat and the biofertiliser produced.
§  Scale and location of the AD plant

4.1    Arable crop feedstock

         In Germany and Austria energy crops are grown specifically as a feedstock for Biogas production.  In Germany legislation was introduced in 2001 whereby electric generated from renewable resources is directly subsidised by the government.  The subsidy is guaranteed to generators for 15 years.  The wholesale price for electricity is supported with top ups for use of energy crops and combined heat and power.  The compensation is increased by 1 Cent/kWh if fuel cells, micro gas turbines or stirling engines are used for electricity production as opposed to an internal combustion engine.  As a result of this support 72,000 ha of maize was grown for biogas production in Germany in 2005, and plant numbers in Germany increased in the period  2004 to 2005 by 690 plants with 2700 plants operational at the end of 2005.

Compensation paid for electricity [€-Cent/kWh]
Electric Capacity
Farm Substrates
Non-Farm Organic Wastes
< 150 KW
< 500 kW
< 5 MW
> 5 MW

There is no direct subsidy for renewable electricity generated in the UK.  Renewable electricity generators rely on the value of renewable obligation certificates to supplement the wholesale electricity price that they receive 

5.      Plant Costs

A rough guide to plant costs, and the potential income stream if waste materials and slurry are used as a feedstock, is summarised below.  Dependent on construction specifications and site  €1.5 – 2 M is the capital investment required for a green field site biogas plant using energy crops as a feedstock in Germany.

Plant Size
Capital Cost

Operating Surplus
(per annum)
250 kw
£400 - £500k
£75 - £125k
250 kw ABP compliant
£800k - £1.2M
£150 - £300k
1 Mw
£1.2 - £1.6M
£400 - £600k
1 Mw ABP compliant
£2.5 – £3.0M
£800k  - £1.2M

6.      Biogas Plants in the UK

Anaerobic digestion plant development in the UK has been very slow with the most notable agricultural operation in Devon.  Holsworthy Biogas Ltd was established in 1998 and is owned by Farmatic Biotech Energy, an entrepreneur, the local community and the supplying farmers.  The plant has the capacity to process 146,000 tonnes per annum of cattle, pig and poultry manure plus organic food waste (20%).  Generating 14.4 million kWh of electricity at approximately 6 p/kWh and eventually 15 million kWth of heat energy will be used for a district heating network.

Greenfinch have recently built 7 on-farm biogas plants built in Southwest Scotland for the Scottish Executive for research into diffuse pollution from agriculture.

Bedfordia Farms have a biogas plant using pig slurry and food waste near Bedford.

A DEFRA supported initiative between Greenfinch and South Shropshire District Council has seen the construction of a Biowaste digester recycling 5000 tonnes per annum of source-segregated kitchen & garden waste into bio fertiliser and renewable electricity in Shropshire.

7.      Opportunities

Opportunities for biogas investment in the UK are likely to be in a rural business that has access to slurry and waste material or by products such as pack house waste.  With greater capital investment and the ability to use animal by products, gate fees for food waste can provide a useful income stream.  It is very unlikely with the wholesale electricity price in the UK and the support for renewable electricity through ROCs mechanism that it will be economically viable to grow crops specifically as a feedstock. However, crop grown potentially as a feedstock could help underpin the variability in availability of waste materials.   The crop could be harvested and used if insufficient supplies of waste were available or to make a buffer stock of material.

Utilising the heat energy generated through a district heating network would significantly improve the economic case, so there is potential scope for biogas facilities in the proximity of new housing and development areas, particularly if a recycling scheme could be put in place to utilise kitchen and green waste from the housing as a supplement to other feed stocks.


Download : http://www.face-online.org.uk/resources/factsheets/discovering/anerobic%20digestion%20and%20biogas.doc

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