The Biogas Digester Expert: Small scale farm and rural community biogas  

Biogas digesters take any biodegradable feedstock, and convert it into two useful products: gas and digestate. In this section we will concentrate on small scale farm and rural community biogas applications.

A number of popular simple designs of digester have been developed; the Chinese fixed dome digester and the Indian floating cover biogas digester .  Development has taken place over many years and each nation tends to have its own preferred design. While international collaboration over the next few years can probably develop and improve these designs, by incorporating the best worldwise experiences, local designs will no doubt be well optimized for local farming practices and the types of waste available.

At the end of the 1990s, probably about five and a quarter million farmer households had biogas digesters, worldwide.  However, most national governments would like to see many more installed as after the operation of biogas digester, it is no longer a significant source of pollution. The economic viability of these plants is rising fast as oil prices soar ever higher and the date of peak oil production for the globe gets closer.

Cost

Operation and Maintenance Operational requirements are low, due to automatic influent feeding and mixing of animal and toilets wastes Limited operator skill required (but household members need training to understand the system) Needs checking for gas leaks, especially distribution pipes Desludging occasionally necessary    

Advantages Provides source of biogas, this results in less dependence on fossil fuels, which may not be readily available to households Improves the household overall sanitation by treating blackwater, organic wastes, and manure Effluent is a nutrient rich fertilizer, and more hygienic than untreated human waste Less frequent/ almost no desludging required compared to septic tanks and can be built locally.

Disadvantages:

• Requires good design. Skilled, trained labour is required for the construction of the biogas digester Requires availability of animal excrements for optimal biogas production There are sometimes cultural prejudices against using gas from human waste    
• Medium to high capital costs.
• The main constraints for installing a digester, however, are the initial investment cost and the competition over kerosene. 

Advantages:

• High revenue by saving of energy costs and higher agricultural yields     Suitable for SIDS and low income coastal countries.  In areas where the cost is high, the "sausage" or bag digester appears to be ideal. 
• In developing countries biogas can be used as a low-cost fuel for cooking.  Paraffin fuel is currently used in many of these same areas for heating and cooking, but the cost of transportation is relatively high. 
• The simple household biogas digester (apart from initial installation) costs them an affordable invetsment which once installed costs nothing but the effort to maintain it.
• In terms of cost, biogas is cheaper, on a life cycle basis, than conventional biomass fuels (dung, fuelwood, crop wastes, etc. 
• An ideal plant should be as low-cost as possible (in terms of the production cost per unit volume of biogas) both to the user as well as to the society. 
• Also, if easily available biodegradable wastesare used as inputs, then the benefits could be of two folds: (a) economicvalue of biogas and its slurry; and (b) environmental cost avoided in dealingwith the biodegradable waste in some other ways such as disposal in landfill.  Thus the total cost of the biogas plant is paid back within one to two years. 

Example Projects

The Ibadan plant will be one of the larger biogas installations in Africa, providing gas to 5,400 families a month at around a quarter the cost of liquefied natural gas. 

In 1982 Tanzania started distributing concrete-and-steel digesters that cost about US$1,400; by 1991 there were only 200 functioning biogas units in the country, according to an article by Innocent Rutamu in the July 1999 issue of Livestock Research for Rural Development. 

In those environments, says Austin, the cost per unit of energy over a digester's 15- to 20-year life cycle is lower than both solar electrification and the cost of extending a conventional electrical grid. 

Some family biogas digesters even support small-scale enterprises,by providing electricity for agricultural and cottage industries. 

The advantages of using a biogas digester to convert human waste and cow dung into an easily accessible alternative energy source do not stop at cheaper power and free time. 

In addition, there are added health benefits to the use of the biogas digester.  In regions where there is already a mature electrical grid, there is limited incentive to use simple biogas digesters because they are not easily scaled up to produce energy comparable to hydropower and coal.