Sewage Treatment

Sewage systems


Legislation preventing the discharge of untreated waste overboard has been in place for some time with a requirement that it should be retrofitted where not already in use. American legislation defines three types of sewage treatment units.

Type 1  A device capable of discharging effluent having no floating solids and a coliform count of less than 1000 per 100ml of effluent.  


Type II  A device capable of discharging effluent with suspended solids not in excess of 150mg/litre and a coliform count of less than 200 per 100ml  


Type III  A device to prevent the discharge overboard of treated or untreated waste.  

Ventilation systems are to be kept independent of other vents A log is to be kept of any discharge overboard from a holding tank

Aerobic (Biological) Treatment plant (Flow through system)


Biological system require a steady and relatively constant flow of solid sewage so the bacteria can exist in sufficient quantity to maintain effluent discharge at the correct quality. sludge build up is a possible problem although extended residence in the aeration chamber greatly reduces the amount. For example, sewage with 80% solid waste is reduced to 20% of its original weight after 12 hours in the aeration tank.

The process of aerobicity strips oxygen from the water and creates more water, carbon dioxide and bacteria.


The Trident sewage treatment unit shown above consists of three chambers.
Sewage enters the aeration chamber via a coarse mesh filter where large solids are broken down. The aeration chamber is where the main biological action takes place. Here air blowers mounted on the outside of the unit oxygenate and stir the effluent and bacteria mix via a series of pipes and nozzles. The sewage remains in this aeration tank for some time.

Incoming sewage displaces some effluent of the settling tank (or hopper) where under inactive conditions biological floc, activated sludge and bacteria, settle out and is returned to the aeration chamber via air lift pumps also driven by the blowers. A second transfer pipe scum's the surface of the settling tank and returns it back to the aeration chamber. This returned sludge contains the bacteria to digest the incoming sewage. Thus the importance of this floc return can be seen

Authors note:
This is a common question in orals

Effluent passing over from this chamber should be clean and ready for disinfecting in the chlorinating chamber. The level in this chamber is controlled by a pump and float switch arrangement. typical chlorine levels at discharge is 5ppm.

Valves are fitted to the aeration and primary chambers to allow them to be pumped out and back flushed as necessary.

The bacteria are susceptible to water conditions including temperature and the presence of toilet cleaning agents. In this way the system is fitted with by-pass valves so passing contaminated water overboard. Should the bacteria be killed it takes some time before a new colony forms. There are special 'feeds' which promote the reestablishment of these colonies.

Physical-Chemical Sewage system

This is based on the separation of the liquid element from the sewage flow. This is disinfected in a 5% chlorine for 30 minutes to kill off coliform bacteria and then discharged overboard in full MARPOL compliance.

One problem with this system is the required space, Only a finite amount of space can be set aside for the storage of the solid part of the waste which can only be discharged in port or outside territorial waters when allowed. If these facilities are unavailable the system become inoperative.

There is also the need to carry quantities of Calcium Hypochlorite for conversion to Sodium HypoChlorite for the disinfection of sewage flow. Calcium Hypochlorite requires very careful handling.

Electrocatalytic Oxidation

Sewage is collected, macerated and passed through a electrolytic cell.

Electrolysis produces Sodium Hypochlorite which is used to oxidise organic material before discharge. Alternately dosing by chlorine may be used. The effluent passes on through to a settling tank were the oxidation process is completed

These type of plants can be 50% smaller than biological types, this and the fact that pass through times are extremely short-typically 30 minutes compared to the several hours of the biological unit- are the main advantages of this system.The discharge contains no solids and is totally free of coliform bacteria.

A disadvantage of this system is due to the short exposure time in the oxidiser relatively high levels of chlorine are required to ensure destruction of the coliform bacteria. It is possible that this chlorine level can be present to some degree in the discharge. Dechlorination plant may be fitted

Vacuum sewage systems


Liquid flows from the aeration tank of an aerobic sewage tank to a coarse impeller centrifugal pump. This delivers the liquid under pressure via an eductor and back to the tank. The eductor reduces the pressure in the sewage system pipework to a set point after which the pump is stopped. When the pressure in the pipework rises above a set value it is restarted.

The pipework consists of a network of mainly pvc pipes connected into separate zones- typically by deck- and brought down to a common manifold via isolating valves. These valves allow work on sections of the system whilst still maintaining others in use.

The toilets are connect to the system via a vacuum operated foot valve. Vacuum timers are also fitted which allow measured quantities of flushing water to be applied.

Where toilets are connected in the same zone but exist at different heights non-returning valves may be fitted. In addition filter boxes may be fitted along with additional isolating valves to improve operation.

Advantages and disadvantages

Very little flushing water is required and the volume of sewage dealt with can be much reduced with the downsizing of relevant equipment and cost saving.This has made them very popular for passenger vessels.Lloyds regulations state that the capacity of a sewage system for flushing water with conventional plant is 115 litres/ person/ day and 15 litres for vacuum systems.

The main disadvantage is blockage due to drying and crystalisation of urea. Over a period of time this can be so severe as to completely close the pipes. Chemicals are on the market which can be added in very small doses which help remove and prevent this deposits but there success is not guarenteed.

In the event of vacuum failure a method must be in place to prevent dangerous gasses passing back into the accommodation.

The Hazards and regulations regarding the Sewage Systems

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.

Safety Parameters

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;

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.

Operational aspects

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.

Suspended solids

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.

Coliform count

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