On-Board Sewage Treatment
While the global application of Annex IV focuses on limiting hygienic nuisance and organic pollution measured as Bio-chemical Oxygen Demand (BOD) and does not regulate nutrient content of treated sewage, the MARPOL Annex IV special area regulations sets requirements also for nutrient discharges.
The Baltic Sea was designated as such a special area in 2010, based on a joint submission by HELCOM countries. Applicable dates of enforcement were decided on by IMO in 2016 to be 1 June 2021 for existing IMO registered passenger ships, 1 June 2019 for new passenger ships. For direct passages between St. Petersburg and the North Sea there is an extension until 1 June 2023.
For comparison with treatment requirements on land, the HELCOM Recommendation 28E/5 sets reduction targets of Nitrogen 30% & Phosphorus 80% for cities with 2,000-10,000 inhabitants which are situated in the Baltic Sea coastline or catchment area. Average number of passengers and crew on board each of the 85 cruise ships operating in the Baltic in 2015 was 2,000 persons.
Main types of on-board sewage treatment equipment
Treatment systems can be classified into three main types in terms of their capacity to reduce nutrients:
Marine Sanitation Device (MSD) which disinfects the waste water (with e.g. Chlorine) but are not designed to remove any nutrients.
Advanced Waste-water Purification (AWP) systems which do not meet the IMO MARPOL Annex IV Baltic Sea Special Area requirements. These have higher treatment level in general compared to MSD and in addition remove nutrients to varying degrees (depending on equipment).
AWP which fulfil the Baltic Sea Special Area Requirements- for Nitrogen 20 Mg/L or 70% reduction and for Phosphorus 1 mg/L or 80% reduction.
All above systems decompose organic sewage to disinfected sludge.
AWP equipment meeting Annex IV special area requirements
By April 2017, 52 different AWTS models from 5 manufacturers (Aco Marine, Evac, MARTIN Membrane Systems, Rochem Technical Services & Scanship) are approved to fulfil the requirements of Passenger Ships & Ferries operating in MARPOL Annex IV special areas (including the Baltic Sea) according to the IMO MEPC resolution 227(64). Details on the available systems are available after public login from the IMO GISIS Website (tab Pollution Prevention Equipment and Anti-fouling Systems).