1.3 Sewage treatment units
Traditional Type II Marine Sanitation Devices
On most cargo and cruise ships with traditional Type II Marine Sanitation Devices
(MSD), sewage is treated using biological treatment and chlorination. Some cruise ships do
not treat their sewage biologically, but instead use maceration and chlorination. The treatment
system typically includes aerobic biological treatment to remove biochemical oxygen demand
and some nutrients, clarification and filtration to remove solids, and final chlorine disinfection
to destroy pathogens. The system also may include screening to remove grit and debris.
Cruise ships typically install up to four systems, allowing one or two to be placed off-line for
maintenance at any one time. Cargo ships use one unit only.
Advanced Wastewater Treatment Systems (AWTS)
To improve environmental performance, cruise lines are testing and installing
wastewater purification systems that utilise advanced technologies. These onboard wastewater treatment systems are designed to result in effluent discharges that are of a high
quality and purity; for example, meeting or surpassing standards for secondary and tertiary
effluents and reclaimed water.
Effluents meeting these high standards would not be subjected to the strict discharge
limitations. AWTS systems are still at the development stage. Generally advanced treatment
systems use enhanced aerobic digestion with physical filtration to clean shipboard wastewater.
On some cruise vessels, sewage and often grey waters are treated using AWTS. AWTSs
generally provide improved screening, biological treatment, solids separation (using filtration
or floatation), and disinfection (using ultraviolet light) as compared to traditional Type II
MSDs.