Evaluation of the performance of “biodigester toilets” in Kampala.

Abstract

The installation of biodigester toilet system has become popular in the recent years as an alternative to septic tank systems based on biodigester manufacturer’s premise that they are cost friendly, easy to maintain and do not smell as compared to septic tanks. This report examines the performance of biodigester toilets in Kampala, focusing on their design variations, operational practices, and comparative assessment with traditional septic tanks. The study aims to ascertain the types of biodigester toilets installed, evaluate the impact of current operational practices on their performance, and compare their effectiveness to septic tanks. This study employed a variety of methods to achieve its objectives. Field visits were conducted to identify biodigester toilet users, installers, and septic tank users, facilitating data collection. A convenient sampling approach was used to select biodigester and septic tank users based on willingness to participate and operational duration. Questionnaire-based interviews were conducted to gather quantitative and qualitative data, and pilot testing ensured questionnaire effectiveness. Laboratory analysis of collected samples involved determining Total Suspended Solids (TSS), COD, and faecal coliforms (FC) for evaluating removal efficiency. Two types of biodigester toilet designs were identified: type 1 with a separate soak pit and type 2 with the soak pit surrounding the tank. Most users (90%) had the type 1 design, while 10% had the type 2 design. Design distribution varied among user categories, with schools and rentals primarily using type 1 designs. The study found variations in construction practices and lack of standardization for biodigester components such as tanks, covers, and soak pits. The allocation of users per tank size lacked a clear criterion, leading to inconsistent capacity estimations. Pricing of biodigester systems varied based on tank size, complexity of the piping system, and individual pricing strategies. Enzyme treatments for odour management were commonly used by users and had a significant impact on satisfaction. From the laboratory analysis tests, the biodigester achieved an average efficiency of 52.1±3.3%, 23.5±9.7% and 97.5±0.4% for COD, TSS and e coli respectively. Similarly, for the septic tank the mean values were 43.7±5.1%, 33.9 ± 12.7% and 78.6± 4%. Biodigester toilet systems recorded an average of 4.40 ou/m3 of air and the Septic tank recorded an average of 5.75 ou/m3 of air. Any readings of ou/m3 ranging from 15ou/m3 and above was considered to be offensive (Afful, 2017). This implies that both systems were considered not offensive to users in terms of smell. Biodigester toilet systems have become popular as an affordable and odorless alternative to septic tanks. This study reveals design variations, construction practices, and pricing differences in biodigester systems. It also shows their impressive removal efficiency and user satisfaction, positioning them as equal to or better than septic tanks. In conclusion, to improve the performance and sustainability of biodigester toilet systems, it is recommended to standardize construction practices and components, establish clear guidelines for user allocation and pricing transparency, and conduct further research on improving enzyme treatments. These steps will reinforce biodigester toilets as a dependable alternative to septic tanks.