Raw sewage discharged into restricted waters will eventually overwhelm the self-purification ability of the limited quantity of water. In a closed dock the effect can be seen in a black sludgy water which when disturbed gives off an unpleasant smell possibly Hydrogen Sulphide.
When the quantity of sludge is reasonable aerobic bacteria digest the sewage breaking it down to simple compounds and Carbon dioxide using up Oxygen in the process. These compounds and Carbon dioxide promote plant life which returns oxygen to the water.
When the quantity of Oxygen becomes so depleted that the aerobic bacteria can no longer function, anaerobic or bacteria not requiring Oxygen to function will take over. The breakdown of the sludge is then associated with the same process of decay with foul smelling and dangerous gasses being produced. Therefore the principal means of sludge conditioning on board is that of aerobic action, Types of sewage disposal
There are four main types of sewage disposal systems fitted to ships;
Discharge from the toilet bowl into a common drain leading to overboard via storm valves
As above except common drain leads to a storage tank with or without aeration. Contents discharged ashore or at sea when appropriate.
Sewage treatment systems with sewage being collected and treated to produce an effluent suitable to discharge without effect on environment.
Vacuum collecting system where the drains are kept at a slightly negative absolute pressure , on flushing water, sewage and air are drawn into the drains being led to a collecting or treatment tank which is kept at atmospheric pressure.
Aerobic and anaerobic bacterial action
When the sewage enters the drainage system it is acted upon by aerobic bacteria and is broken down, during this process the naturally occurring Aerobic Bacteria strip the water of oxygen and produce; more water, Carbon Dioxide, and more bacteria.
If, however, there is insufficient oxygen for these bacteria then alternative bacteria dominates. These Anaerobic Bacteria produce Hydrogen Sulphide, Methane and Ammonia. These gasses are either highly toxic or flammable or both. In particular Hydrogen Sulphide is toxic to humans in concentrations down to 10ppm and its flammable vapours are heavier than air so may build up in lethal pockets in enclosed spaces.
The generation by anaerobic bacteria these toxic and flammable gasses is present in all types of systems to some degree. The possibility of anaerobic action within a sewage treatment plant should be reduced as far as possible.
Should these gasses be generated and allowed to enter the accommodation could lead to disaster.
The following are some methods which may help to reduce the risks;
– The fitting of proper ventilation in toilet spaces and the fitting of water traps can only be seen as secondary measures to reducing the risk. The primary concern is to eliminate the possibility of generating the gasses in the first place.
– Where sewage is stored in tanks for discharge, some method of maintaining an adequate level of oxygen in the water must be in place. Examples of these may be by direct air injection or by air entraining into the liquid whilst pumping through a nozzle.
– Where active aeration is not fitted then the contents of the storage tank should be changed within a maximum of a 24 hour period unless some other means of treatment is used.. The conditions in the tank should be closely monitored
– Where aerobic treatment plants are used then manufacturers operating instructions should be closely adhered to. A system of maintenance should be in place.
Maintenance of Aerobic treatment units.
Thorough , regular cleaning and inspection with particular attention being paid to areas behind internal division plates.
Checks on alarms and trips
Checks on aeration equipment
Checks on transfer systems in the tanks
It is recommended that a low air pressure switch rather than a motor failed alarm be fitted to the air blower motor hence eliminating the danger of the fan belts snapping and going undetected.
Tank Ventilation arrangements.
Ventilation pipes should be in good condition and free from obstructions. They should be of a size to minimise pressure drop and ensure good gas clearance. They should be self draining to prevent blockage by water.
Any flame gauze’s or other fittings should be checked for cleanliness.
Toilets, showers, washbasins, etc.
The condition of drainage pipes should be checked regularly, as should the operation of the water seal or other fitted arrangements to prevent the back flow of gasses.
Accommodation ventilation arrangements
The ventilation should be sufficient to ensure proper balance allowing each compartment to be correctly supplied. The ventilation system should be correctly maintained and checked for cleanliness.
Air extraction is of vital importance and the cleanliness of grills should be checked, the opening under doors should not be blocked, vent louvers should be correctly position to ensure all spaces are properly vented.
The forced ventilation equipment should be regularly checked and maintained.
Only approved toilet cleaning agents should be used, the use of excessive quantities of bleach should be avoided as this may kill the bacteria.
Complaints of foul or musty smells should be dealt with immediately as these may indicate anaerobic action. The dangers of these gasses should be explained to all crew.
The quantity of solid waste in the effluent is weighed. After drying on an asbestos mat filter element.
Biological Oxygen demand (B.O.D.)
Aerobic bacteria use Oxygen in the process of breaking down the sewage. At the end of the process the action of the bacteria reduces and so does the Oxygen demand. The effectiveness of a sewage treatment plant may be gauged by taking a one litre sample and incubating it for 5 days at 20oC. The amount of Oxygen consumed in milligrams per litre or ppm is termed the B.O.D.
It is possible that the effluent contain bacteria and viruses hazardous to health if it has not been properly treated at the final stage. An indication of this is a count of the Coliform bacteria which are found in the intestine.
A coliform count in a 100ml sample incubated for 48 hrs at 35oC. Another test at the same temperature but over a 24 hour period produces a colony of bacteria.
Annex IV of MARPOL 73/78 (IMO) regulates the disposal of waste from ships internationally. In addition certain countries have their own national and regional controls.
In general this means that untreated sewage can only be dumped outside 12 miles offshore, and treated disinfected waste outside 4 miles.
For further information see m-notice M.1548