Sewage collection and treatment technologies onboard ships
There are three methods with which sewage is processed onboard ships; mechanical, biological and chemical. Usually the process of treatment is a combination of the three methods meaning that biological treatment can be combined with chemical and mechanical or mechanical treatment can be used together with chemical in order to achieve the highest quality of effluent. Sewage treatment has different stages onboard ships:
Pre-treatment of wastewater – before pre-treatment, black water, gray water and galley water are collected in holding tanks. The pre-treatment process reduces the amount of solids and the need for oxidation. Mechanical pre-treatment makes use of sieving and sedimentation units with large particles passing through a shredding pump before sieving.
Oxidation of wastewater – after the mechanical process of filtering at least 50% of organic load is reduced. The remaining organic compounds will have to be oxidized either chemically or biologically. During the process of chemical oxidation, chemicals like ozone, chlorine or hydrogen peroxide are added to oxidize organic impurities in the sewage water. If biological treatment is used, sewage becomes the source of nourishment for microorganisms. Biological filters and biorotors are also used so that bacteria that destroy impurities will attach to the filtering material. However, the success of this process depends on the amount of active biomass and the living conditions for bacteria.
Clarification and filtration of wastewater – the most common processes used in ships are membrane filtration, dissolved air flotation and settling. In dissolved air flotation, the process works through the injection of microscopic air bubbles into the feed water stream so that particles will float on the surface of a basin to be skimmed off and removed from the wastewater stream.
Disinfection of wastewater – this is the last phase of the wastewater purification process. The success of this phase is dependent on the previous treatment methods used. Disinfection is used to enhance the quality of wastewater which is an important phase in the purification process. When membrane clarification and filtration is used, disinfection is done with UV light. On the other hand, if water is too turbid, UV light may not be suitable for disinfection so that chlorine, radicals or ozone are used.
Treatment of sludge – also depends of the previous treatment processes. When effective pre-treatment before the bioreactor has reduced the production of sludge, the process of sludge drying will be enhanced. Sludge that comes directly from this process is centrifuged meaning that dry substance concentration has been increased to 17-27% if compared to before treatment dry substance concentration of 2-3%. The common sludge handling techniques include holding, incinerator, steam drying, filter press or an alternative sludge conditioning process that makes combustion possible.