Wastewater Collection and Transportation through Drainage Network- a Potential Threat to Biodiversity & Local Eco-system in Zanzibar Stone-town & Surroundings

Waste water effluents voided in the coastal areas of Zanzibar are highly polluted posing a public health crisis. This cross-sectional study, carried out in September 2017, investigated bacterial load and physicochemical characteristics of the effluents discharged into coastal marine waters of the Zanzibar stone town and peri-urban areas. Ten drains, seven of which were impervious and three pervious, were sampled twice per day in mornings and afternoons. Each drain was sampled at three sites; the out-pour site close to the sea and two sites upstream 100 meters apart. Physicochemical parameters including Dissolved Oxygen (DO), pH and turbidity were determined immediately after arrival in the laboratory. Colony forming units/ ml were also determined.

Bacterial counts were higher in impervious drains, in out-pour sites and in the afternoons. Conversely counts were lower in the out-pour sites compared to upstream sites in the pervious drains. DO was significantly higher in the pervious compared to impervious drains (p = 0.004). Turbidity was high in the mornings, but one drain had significantly high turbidity (p=0.0009). In all drains pH was high in the afternoon than mornings. Thirteen bacterial species, mostly gram-negative Enterobacteriaceae were isolated. Vibrio cholerae was isolated from 5 drains. Ground water close to pervious drains had coliforms beyond WHO guidelines. Waste water drains in Zanzibar urban and peri-urban areas are vastly contaminated with organic matter and pathogenic bacteria leading to high turbidity and oxygen depletion. It is recommended to increase awareness to the public and introduce waste water treatment strategies both physically and chemically.