Sewage Treatment System Retrofit for Baltic Sea Passenger Ferry

A passenger ferry operating between Sweden and Poland cross some of the most tightly regulated waters in the world. The Baltic Sea is designated a Special Area under MARPOL Annex IV, meaning ships carrying passengers must meet strict nitrogen and phosphorus discharge limits when operating within the region — a standard far more demanding than the global baseline.

To meet these requirements, I worked in a team to complete a retrofit of a sewage treatment system aboard a passenger vessel serving this route. Working within the constraints of an operating ferry — limited engine room space, tight dry-dock windows, and the need to keep guest-facing operations running smoothly — the project replaced outdated treatment technology with a modern biological treatment system engineered to consistently meet Baltic Sea nutrient removal standards.

Key aspects of the retrofit included space-efficient system design to fit within the existing footprint of the vessel's technical spaces, avoiding costly structural modifications; upgraded biological and disinfection stages to reliably achieve the nitrogen and phosphorus reduction levels required in the Baltic Special Area; and minimized vessel downtime, with pre-fabrication and modular installation planned to fit the available dry-dock schedule.

The result is a vessel that can continue serving one of the busiest passenger corridors in the Baltic while meeting some of the strictest marine sanitation discharge standards in the world — protecting the sensitive waters it sails through, voyage after voyage.