People in developing countries regularly lack access to energy or their energy source is not reliable. Low cost anaerobic digestion systems have the potential to provide methane to be used in a variety of end uses. Unfortunately, many low cost systems are not evaluated and it is unclear if they are living up to the expectations of the end users or those that are promoting or financially supporting their installation.
We have evaluated multiple small scale anaerobic digestion systems in Uganda and Bolivia to assess their energy production potential, impact of digestate as a fertilizer (using plot studies), pathogen reduction through the digester, and impact to kitchen air quality when biogas stoves replace firewood. Based on feedback we have also designed, tested and implemented a low cost separation system for handling digestate to recycle separated liquids and improve handling of solids. We have also modified an absorption chiller to run on biogas and are in the process of wider spread adoption and evaluation.
Throughout this assessment we have learned that many institutional level digestion systems in developing countries are not meeting the biogas demands of the end users. While they like the improved cooking time and reduced air quality impacts in the kitchen, only small households are producing enough gas to realize many of these benefits. Biogas poses a reduction in PM2.5 (fine particulates) within kitchens when compared to firewood stoves. However, when any amount of firewood is used in the kitchens (when there is not enough biogas), much of this benefit is lost. Therefore it is critical to improve the biogas production of these systems.
Maize plot trials show that compared to control plots digestate applied in any form (slurry or separated solids) significantly improves yields. When compared to inorganic fertilizer applications the grain yields are statistically similar but the stover yields increase significantly. End users show a preference for using the separated solids and the reduction in water needed to operate the systems. While these benefits seem appealing, there may be concern for the risks associated with pathogens in the digestate when applied to food crops. While digesters showed a significant reduction in pathogen related to the system retention time, pathogen remained in the effluent and must be handled properly to limit transfer to food and the human health risks after ingestion.
Increasing the end use of biogas beyond cooking to chillers has shown great potential for implementation and has high demand for end users. Systems have been able to provide cooling at multiple locations for extended periods with low biogas demands. Additional materials are needed to provide end users with guidance on troubleshooting and operation.
Based on the results of these studies we are moving forward with farmer trials of the digestate to assess end user issues and motivations. In addition, we are currently designing a low cost heating system to improve biogas production efficiency in order to meet end user needs or decrease the size of digesters. Finally we are working on an evaluation of chiller biogas needs and providing training on all aspects of the digestion systems.
Rebecca Larson, Assistant Professor at the University of Wisconsin-Madison
A. McCord, Associate Director at University of Wisconsin-Madison, Vianney Tumwesige, CEO at GreenHeat Uganda, Dorothy Lsoto at W2E Uganda
McCord, A.I., S.A. Stefanos, V. Tumwesige, D. Lsoto, A. Meding, A. Adong, J.J. Schauer, and R.A. Larson. 2017. Biogas and the impacts of fuel choice on institutional kitchen air quality in Kampala, Uganda. Indoor Air. In Review, revisions requested.
McCord, A.I., S.A. Stefanos, V. Tumwesige, D.T. Lsoto, M. Kawala, J. Mutebi, I. Nansubuga, and R.A. Larson. 2017. Anaerobic digestion and public sanitation in Kampala: risks and opportunities. In Review.