Author(s): F. Van Eeden; M. Retief; R. Van Ballegooyen; G. Jacobs; A. Malan
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Keywords: Saint Helena Island; Numerical modelling; Marine outfall; Effluent dispersion; Cost analysis; Wastewater treatment
Abstract: This study eva luates a mar ine outfa ll as a lternat ive to land based treatment opt ions for the island of Sa int Helena in the South Atlantic Ocean. At present, port ions of the ageing sewer networks need upgrading while poorly treated sewage discharges direct ly into natural valleys, as well as into the marine environment. In addit ion, uncontro lled ingress o f storm water into the sewer system causes overflowing of sewers and an overload of the current treatment facilit ies (septic tanks). Various treatment options were evaluated: membrane bioreactor, activated sludge, rotating biocontactor, trickling filter, oxidation ponds and disposal via marine outfall pipeline. The feasibility of the various options was init ially evaluated in terms of their respective capital cost (CAPEX) and operational cost (OPEX) and it was found that the marine outfall warrants an in-depth invest igat ion due to the potential lower maintenance costs when compared to the land-based treatment options. To this end, near- and far-field plume dispersion modelling was undertaken in order to investigate the fate and transportation o f the discharged effluent by calibrat ing the numerical models with data collected during a comprehensive measurement campaign. The optimal placement of the outfall was invest igated in order to satisfy the environmental object ives and determine a cost effect ive construct ion solut ion. The CAPEX and OPEX (to 30% accuracy) of the outfall are compared to the land based treatment options. It was found that a marine outfall is preferred due to its ability to accommodate high variations in flow (added storm water flows), a reasonable construction cost and exhibit ing very low maintenance and operational costs when compared to land based treatment options.
Year: 2016