Assessment of faecal sludge quality, heavy metal contamination, and ecological risk: implications for sustainable agriculture

Pit latrines represent the predominant form of on-site sanitation in Botswana, posing unique challenges in faecal sludge (FS) management. The key concerns revolve around FS extraction, treatment, and safe disposal. Currently, co-treatment with wastewater is the primary approach, but it strains wastewater treatment plants (WWTPs) and compromises effluent quality. This study comprehensively assesses FS quality from pit latrines and evaluates the potential health risks when used in agriculture for soil improvement/amendment. Systematic sampling of soils at various depth intervals, approximately 30 cm apart, was conducted, followed by extensive laboratory analysis, including determination of heavy metals (copper, iron, lead, cadmium, zinc, manganese, and arsenic) using inductively coupled plasma atomic emission spectroscopy (ICP-OES) and nutrient analysis using ion chromatography (IC). The findings unequivocally demonstrate that FS from VIP (ventilated improved pit) latrines poses no significant health risks due to heavy metal content. Specifically, Geo-accumulation Index (I_geo) values for nickel (Ni), chromium (Cr), and arsenic (As) were consistently negative, indicating negligible risk of environmental contamination. Copper (Cu) concentrations averaged 40.36 mg/kg in samples collected from Mogoditshane and 591.61 mg/kg in those collected from Broadhurst (Gaborone, Botswana) with Igeo values indicating a moderate pollution risk. Nutrient analysis showed high levels of nitrogen (NO₃^-), with concentrations reaching 4.47 × 10³ mg/kg in some samples, and phosphorus (PO₄^3-) levels as high as 3.9 × 10⁴ mg/kg. These findings highlight its agricultural potential for soil amendment, though prudent management is needed to mitigate eutrophication. The study advocates for separate FS treatment, resolving co-treatment operational challenges and enhancing sustainability. Implementing these recommendations promises to address FS management issues, bolster food security, and enhance Botswana's ecological well-being.