Composition of sewage from passenger ships
MARPOL defines sewage as all wastewater from toilets and urinals and also when mixed with drainage from medical premises through wash basins, wash tubs and scuppers and from spaces containing living animals. In contrast to household sewage, sewage from ships do not only cover black water that contains fecal and urinal waste but mixtures of black water, grey water that is generated from laundry, dishwashing and bathing and food stuffs that may include mixtures of oil and substances like chlorine. For ships the use of advanced sewage collection and treatment systems will be determined by several factors that include ship type, number of passengers and length of the voyage.
Chemical sewage treatment
The chemical treatment system is basically a storage tank that collects solid waste for disposal in onshore collection and treatment facilities. The system treats the waste collected and retains it until the ship reaches land where the onshore facility is available to receive the collected sewage. Sewage must be treated so that is will be acceptable in terms of smell and appearance. Treatment is done through the use of various chemicals for odor and color removal and to assist in its breakdown. Solid materials are physically broken up using a comminutor to assist in the chemical breakdown process. These solid materials will settle at the bottom of the tank and stored prior to its discharge to the sullage tank where liquid can be recycled for flushing purposes. Tests are usually performed daily to ensure the right chemical levels to prevent odor from developing and to avoid corrosion that may result from high levels of alkalinity.
Biological sewage treatment
The biological sewage collection and treatment system utilizes bacteria to breakdown sewage into acceptable substances before it is discharged into the waters. The system also makes use of an extended aeration process that will provide a right environment for the growth of oxygen-loving bacteria. Aerobic bacteria will digest sewage and convert it into sludge. The treatment plant makes use of three watertight compartments for aeration, settling and chlorine application. Sewage enters the aeration chamber where it will be digested by aerobic bacteria and microorganisms. The existence of aerobic bacteria is enhanced by the presence of atmospheric oxygen which is consistently pumped it. Sewage will flow into the settling compartment where the activated sludge will settle out while clear liquid will flow into the chlorinator and after treatment. The activated sludge contained in the settling tank will be continuously recycled and build up so that it can be partially discharged in a decontrolled area.