Holding Tank

A tank that retains sewage or other wastewater aboard until it can be discharged through an authorised route.

Marine holding tanks most commonly receive toilet waste, but greywater may be stored separately or in a system designed for it. Typical components include inlet and vent lines, level indication, inspection access, a deck pump-out connection and, where legal, a controlled overboard discharge arrangement.

Discharge restrictions vary by location and vessel, and a seacock or valve may need to be secured against unlawful use. Vent blockage, overfilling, permeated hoses, leaks and gas accumulation create sanitation and safety problems, so the system requires compatible materials, ventilation, monitoring and maintenance.