Sedimentation dynamics in Waste Stabilization Ponds and implications for helminth eggs removal
Izdori, Fides John
Helminthic infections pose significant worldwide public health challenges, and standard therapy includes mass drug administration (for de-worming) and three monthly vaccinations. However, research suggests that improved sanitation, especially efficient wastewater treatment, is a potential methodology for eradicating helminth infections. In low and middle income countries, waste stabilization ponds (WSP) are considered a cost-effective and highest ranked wastewater treatment system, capable of removing helminth eggs through sedimentation. However, eggs are still recovered in some ponds effluents, resulting into environmental contamination. Understanding sedimentation processes, and causative factors in WSP may improve system design for helminth eggs removal, hence reduce infections. Therefore, this research explores the sedimentation processes in the Buguruni WSP- Tanzania, concentrating in particle modifications inside the pond, settlement patterns at the bottom and properties of sedimented particles at different locations inside the pond. The methodology employed covered analysis for particle size distribution (PSD), wastewater helminth eggs composition, and 3D hydraulic and sedimentation modelling in Delft3D. Data shows that, incoming particles have unimodal distribution with sizes ranging from 1 to more than 1000 μm, with almost half-half composition by supra-colloid and settleable particles. Inside the pond, there are settling and non-settling PSDs, characterized by unimodal and bimodal distributions respectively. Sedimentation takes place along the hydrodynamic path-line, up to about 100 m from the inlet, where flocs are deposited in order of decreasing size; a phenomenon known as hydraulic sorting. Wastewater flocs fall under two categories according to their densities; lighter particles that are majority with a mode around 1000.45 kgm-3 and denser particles with a mode around 1020 kgm-3 for Buguruni facultative pond. Analysis shows that, the individual densities of helminth eggs do not play any role in their sedimentation, the eggs sediments immediately, within few meters of the inlet if incorporated into denser flocs, otherwise when incorporated into the lighter flocs, they have high probability of being transported to the outlet. Modelling and analysis of wastewater samples showed that, helminth eggs are mostly deposited close to the inlet, although a few eggs were recovered close to the outlet. The presence of high densities of the algae A. fusiformis observed is linked to high volume of particles with sizes between 1 and 100 μm, as well as lighter flocs in the pond. These algae, although useful for biological treatment of wastewater, they result into formation of buoyant flocs, hence poor sedimentation. This implies that, there is only a possibility of optimizing one process in primary facultative ponds, either biological action or sedimentation. However, research has shown that a large percent of BOD is contained in the supra-colloids range, hence may be sedimented and later anaerobically digested in sludge. Therefore, a significant area can be saved by constructing a system that favours sedimentation, followed by a sludge digester. This research also showed that, wind and discharge from rainfall, play an important role in sludge accumulation patterns, and that the increased discharge from rainfall may be resulting into pond 'self cleaning'. Therefore, neglecting maintenance of the pond, as well as the sewer system has potential severe health and environmental effects, impacting communities downstream of the WSP. It's proposed that, maintenance of the pond and sewer system and addition of flocculants, especially locally available and natural may improve the sedimentation process and hence helminth egg removal in WSP, and therefore future research should focus on this.