Making space for waste

In the past 15 to 20 years, onboard wastewater treatment has shifted from an auxiliary service to a core ship system. More stringent discharge requirements and treatment of multiple wastewater streams have increased process volumes, making structural tanks, machinery space and system integration significant design considerations.

The influence begins early, often during concept and general arrangement development. Biological treatment processes depend on substantial treatment, buffering and storage volumes, so tank allocation and location must be established while there is flexibility in the vessel design.

Sufficient upstream capacity is needed for black and grey water collection, as well as downstream capacity for treated effluent and sewage sludge. Large structural tanks may be subdivided into process compartments, or multiple tanks hydraulically linked through balance connections. Such arrangements must satisfy stability requirements while minimising free surface effects, as process tanks are often partially filled.

A balancing act

Designers must balance treatment capacity, redundancy and operational flexibility against available machinery space. Redundant treatment arrangements, increasingly specified by owners, can add significant piping complexity and valve counts.

With environmental requirements likely to evolve over a vessel’s life, flexibility in