The STEPs research project explores the relative benefits and dis-benefits of larger centralised biogas systems at a village scale versus smaller family-scale systems. It also investigates the economic and financing factors (centralisation brings economies of scale but can only really be implemented by organisations/governments, family-scale systems may be out of reach of user capital without financing arrangements), environmental factors, and social and behavioural considerations (do users want to collectively cook, issues with economics of pipe gas supply meaning necessity of group facilities etc) inherent in biodigester development.
Biogas digesters can be a valuable solution to providing thermal energy services to rural and urban households in the developing world. The technology is particularly applicable in rural areas, where access to feed stock for the digesting chamber in the form of agricultural wastes and other organic wastes is greater. In general, digesters fall into two broad categories: household-scale biodigesters, and larger, centralised biodigesters.
Dome-type biodigester in Arusha, Tanzania 
Household-scale biodigesters are often seen as the most viable option for rural communities and households. These are generally small, with digesting chambers of volumes in the 4 to 13 cubic metres range. These installations will support the cooking needs of a rural household, as well as providing biogas for heating or lighting if required. Tailoring the size of the biogas system to the availability of feedstock for the household is critical for successful functioning of the system: studies have suggested 4-6 heads of cattle is a sustainable target if using agricultural wastes for feedstock, for an average-sized family of five. Individual biodigesters can produce sufficient gas for a single person on as little as 1 kg/day of feedstock.
However, one of the primary limiting factors in the adoption of household biodigesters is financing and end-user capital constraints. Household-scale systems are still relatively expensive for the majority of rural developing-world users, and experience has shown that without the provision of credit facilities in biodigester programs, or government subsidies, adoption rates remain low.
Centralised biodigester systems offer a different set of benefits and challenges. Economies of scale are the major advantage: one centralised system can serve a medium-scale settlement or several small settlements, with a reduced burden for upfront capital costs and maintenance compared to the same service with household-scale systems, in the range of US$100 – 500 per household. The Chinese National Biogas Program [which will be the subject of a later blog in this series], has been the major implementer of centralised systems, however experience also exists in other South-East Asian countries. Examples of this can be found in the centralised digesters built near Beijing to service rural villages. For an upfront cost of ~US$1 million, 1900 households are serviced through each centralised digester, with biogas available at a 20% discount compared to market LPG prices, and the additional benefit of organic effluent being made available for sale to the local farms feeding the digester. The major constraint, however, to wider dissemination of centralised systems is the significantly higher up-front capital costs. This puts the systems out of reach for private users in the majority of cases, government-scale implementation is more common.
Socio-political conditions are another factor that has proved a constraint in biogas implementation projects in developing countries. Centralised biogas digesters can have difficulty with biogas supply to end-users, particularly given the poor economics of installing piped gas supply in small rural communities. Communal cooking facilities have been a solution to this problem in theory, however experience from India suggests that collective cooking is not desired by the rural population, and this has impacted upon the success of centralised digester installations. As with dissemination programs for clean cookstoves, biogas installations need to take into account the end-users needs and desires in design and installation for product use and performance.
The other posts in this series will cover the question of why biogas hasn’t succeeded in Sub-Saharan Africa as it has in South-East Asia, the maintenance question for biogas services, and lessons from the Chinese National Biogas Program.
 Hojnacki et al, MIT (2011) Biodigester Global Case Studies. Available at: https://colab.mit.edu/sites/default/files/D_Lab_Waste_Biodigester_Case_Studies_Report.pdf