This study, conducted in Uganda, addressed 3 key questions:
1. What is the potential of flexible-balloon digesters in SSA?
2. What changes are needed in farmer attitudes, equipment and design of farming systems?
3. What is the value of a biogas digester in terms of energy, organic fertiliser, reduction in deforestation, improved sanitation and improved household air quality?
Key findings and recommendations:
- Installation of a flexible balloon digester is simple; installation takes only a few days and is easily learnt by householders
- Payback time in Uganda is under 4 years; this estimate only accounts for reductions in wood fuel and compost use, including health impacts would further reduce payback time
- Bioslurry is an effective fertiliser; yields of crops treated with bioslurry are significantly higher than the control and not significantly different to crops treated with urea or compost
- Bioslurry reduces greenhouse gas emissions; losses of nitrous oxide from crops treated with bioslurry are significantly lower that from crops treated with urea
- Aerobic pathogens in bioslurry are significantly reduced by anaerobic digestion; the mean reduction in total coliforms in the digestate was 4.58 log CFU / g sample
- Household air quality is improved on conversion to biogas; in the households trialled, carbon monoxide and particulate matter with a diameter of less than 2.5m were reduced by an average of 25% on conversion from woody biomass fuel to biogas
- Digesters are too expensive; cost of flexible balloon digesters in Uganda is over 5 times the cost reported in Asia, which is more than farmers are willing to pay
- Digesters are susceptible to damage; protection is needed from sharp objects and sunlight
- Labour is increased by conversion to biogas; this is due to extra work needed for mixing and handling organic wastes
- Anaerobic pathogens in bioslurry are not significantly reduced during anaerobic digestion
- Both anaerobic and aerobic pathogens are increased in the local environment; this is due to spillage during handling of organic wastes to feed the digesters
- Conversion to biogas is only recommended for households with access to 20 – 30 dm3 water each day per person (either by re-use of household wastewater and additional water collection), and manure from over 0.5 cows, 2 pigs, 5 goats or 5 sheep for each person in the household
- Householders should consider the impact of the digester on household labour; labour will only be decreased by the installation if (distance to wood) > (2 x distance to water) plus 1000m (10m3), 1200m (8m3), 1600m (6m3) and 2400m (4m3 digester)
- Flexible balloon digesters can provide payback within 4 years, but installation should avoid sharp objects and construct a fence and shelter to protect the digester from damage
- Spillage and excessive handling of manures should be avoided when feeding the digester
- To bring household air quality within safe limits for CO (WHO = 6 ppm) and PM2.5 (EPA = 250 g m-3), biogas use should be sufficient to reduce firewood use to less than 10 kg day-1
Key issues needing further research:
- How can we reduce the cost of digesters to less than £65/digester?
- How should we adapt the design/layout to minimise handling of organic wastes?
- What is the optimum rate of bioslurry application to different crops?
- How does composition of bioslurry change with different treatment conditions and feedstocks?
- Can combining anaerobic digestion and composting further reduce pathogens in organic wastes?
Smith, J.U. The potential of small-scale biogas digesters to improve livelihoods and long term sustainability of ecosystem services in Sub-Saharan Africa. Final report. University of Aberdeen, Aberdeen, UK (2013) 188 pp.