Assessing seasonal rainfall variability impact on carbon sequestration in Uganda’s natural forests: Budongo, Bwindi, and Mabira

Abstract

In Sub-Saharan African (SSA) cities, “septic tanks” that are poorly designed and maintained are widespread. This poses significant risks of pathogen contamination to the surrounding environment. When in the environment, these pathogens put people at risk of contracting sanitation related diseases. The factors influencing pathogen concentrations in “septic tank” effluent, as well as the capacity of the surrounding environment to safely attenuate discharged effluent, remain poorly understood. This study investigated the potential for environmental contamination by pathogens released from household “septic tanks” in Kampala. The study was conducted in Bukoto I and Mulago III parishes, where the existing “septic tanks” were classified and evaluated against 13 key design criteria. Indicator pathogen presence in “septic tank” effluent was assessed by enumerating Escherichia coli (E. coli) in samples collected from 54 containments. A bivariate risk classification framework was then developed to categorize the risk of pathogen contamination to the environment by “septic tanks”. Four main typologies were identified: (1) fully lined tanks with an effluent pipe to a soak away pit; (2) fully lined tanks with an effluent pipe to an open drain; (3) fully lined tanks without an effluent pipe; and (4) lined tanks open at the bottom. The study revealed that none of the “septic tanks” met all the 13 key design criteria, and about 74% satisfied at least seven of the assessed design criteria. This raises concerns over the broad classification of such systems as septic tanks. Mean E. coli concentration in “septic tank” effluent was 6.52 ±1.83 log10 CFU/100mL. There were significant correlations between E. coli in “septic tank” effluent with the “septic tank” volume per capita (ρ = -0.386, p = 0.005), total volatile solids (TVS) (ρ = 0.463, p = 0.000), years of operation (ρ = -0.369, p = 0.023) and detention time (ρ = -0.397, p = 0.0036). “Septic tanks” receiving combined black and grey water exhibited significantly higher effluent E. coli concentrations (1.13 log 10 difference, p = 0.025) compared to those treating black water alone. The sampled “septic tanks” were later categorized into four risk zones based on their potential to contaminate their surrounding environment with pathogens. These were critical (30.2%), latent (28.3%), localized (26.4%), and minimal risk (15.1%). The study shows that many household “septic tanks” do not meet recommended design, sizing, or siting standards. This increases the likelihood of poorly treated effluent entering the environment, especially in areas with shallow groundwater, poor soils, or close to streams. Improving septic tank design and construction practices is therefore necessary to enhance treatment effectiveness. High-risk systems should be prioritized for upgrades, such as secondary treatment units, retrofitting or extension of centralized sewer networks where applicable.