Designing Very Shallow Water Bodies for Disinfection: Impact of Daily Stratification/Destratification

Very shallow fresh water bodies are very common throughout the world. They occur as both natural and constructed systems, with the latter being used extensively in waste water treatment systems in the form of maturation ponds. These ponds, which are normally 0.9 to 1.5 m deep are used primarily for pathogen inactivation/removal by various mechanisms, including naturally supplied sunlight that results in ultraviolet disinfection, predation and high pH levels (Stratton et al., 2015). The treatment efficiency, with regard to pathogen removal is, however, extremely variable between maturation ponds, which in part is due to a lack in the understanding of the flow and mixing dynamics in these ponds.
While sunlight disinfection is an important component of the treatment, the vertical movement of the pathogens is also critical. This movement is affected by the diurnal stratification cycle, which is prevalent in these common, very shallow water systems. Turbulent and thermal convection play a major role in transporting pathogens into the near-surface region to be affected by the ultraviolet component of sunlight. This study analyses a slice of a maturation pond and simulates E. coli moving within the slice and being affected by the stratification, sunlight attenuation and mixing driven by wind shear and natural convection. Importantly, the results found here are readily applicable to our understanding of how pathogens can decay in natural shallow water systems.