The discharge of sewage can affect water quality by locally increasing the biochemical oxygen demand (BOD) and therefore reducing the available oxygen required by aquatic species for their survival. Although the quantity of material discharged from boats is small, it is untreated.

Raw sewage introduces microbial pathogens into the environment and can pose a health hazard for swimmers and those consuming shell fish from the area. In addition, excessive quantities will raise the nutrient levels of the water which can trigger algal ‘blooms’. Portable toilet effluent and some holding tank systems contain chemical additives such as chlorine, formaldehyde, ammonium and zinc compounds.

In sufficient quantities all these chemicals are toxic to marine life. The most immediate impacts of boat sewage are those which are visual and odour-related, both of which are unwelcome to boaters and non-boaters alike.

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