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Sewage systems - Ship service systems
The exact amount of sewage and waste water flow generated on board ship is difficult to quantify. European designers tend to work on the basis of 70 litres/person/day of toilet waste (including flushing water) and about 130-150 litres/person/day of washing water (including baths, laundries, etc.). US authorities suggest that the flow from toilet discharges is as high as 114 litres/person/day with twice this amount of washing water.
The breakdown of raw sewage in water is effected by aerobic bacteria if there is a relatively ample presence of oxygen, but by anaerobic bacteria if the oxygen has been depleted. When the amount of sewage relative to water is small, dissolved oxygen in the water will assist a bio-chemical (aerobic) action which breaks down the sewage into simple, clean components and carbon dioxide. This type of action is produced in biological sewage treatment plant in which air (containing 21% oxygen) is bubbled through to sustain the aerobic bacteria. The final discharge from an aerobic treatment plant has a clean and clear appearance.
The discharge of large quantities of raw sewage into restricted waters such as those of inland waterways and enclosed docks, will cause rapid depletion of any oxygen in the water so that aerobic bacteria are unable to survive. When the self-purification ability of the limited quantity of water is overwhelmed in this way, breakdown by putrefaction occurs. Anaerobic bacteria, not reliant on oxygen for survival are associated with this action which results in the production of black, turgid water and gases which are toxic and flammable. The process is used deliberately in some shore sewage treatment works to produce gas which is then used as fuel for internal combustion engines on the site.
The very obvious effects of sewage discharge in waterways and enclosed docks prompted the Port of London Authority and others to establish regulations concerning sewage discharge and to provide facilities ashore for ships' crews. The lavatories were vandalized and the scheme was found to be impractical. Legislation imposed nationally by the USA (through the Coast Guard) and the Canadian Government was more effective and together with the anticipation of the ratification of Annexe IV of the 1973 IMCO Conference on Marine Pollution was probably more responsible for the development of holding tanks and on board sewage treatment plant.
Some plants are designed so that the effluent is retained in the vessel for discharge well away from land, or to a receiving facility ashore; others are designed to produce an effluent which is acceptable to port authorities for discharge inshore. In the former type, the plant consists of holding tanks which receive all lavatory and urinal emptyings, including flushing water, while
wash-basins, showers and baths are permitted to discharge overboard. Some are designed to minimize the amount of liquid retained by flushing with recycled effluent. It is claimed that such a system only requires about 1% of the retaining capacity of a conventional retention system.
Effluent quality standards
To discharge sewage in territorial waters the effluent quality may have to be within certain standards laid down by the local or national authorities. These will usually be based on one or more of three factors, namely the bio-chemical oxygen demand (BOD), suspended solids content and e-coliform count of the discharge.
Bio-chemical oxygen demand
The bio-chemical oxygen demand (BOD) is determined by incubating at 20UC, a sample of sewage effluent which has been well-oxygenated. The amount of oxygen absorbed over a five-day period is then measured. The test is used in this context to evaluate the effectiveness of treatment as it measures the total amount of oxygen taken up as final and complete breakdown of organic matter by aerobic bacteria in the effluent occurs. The quantity of oxygen used equates to the amount of further breakdown required.
Suspended solids
Suspended solids are unsightly and over a period of time can give rise to silting problems. They are usually a sign of a malfunctioning sewage plant and when very high will be accompanied by a high BOD. Suspended solids are measured by filtering a sample through a pre-weighed pad which is then dried and re-weighed.
Coliform count
The e-coliform is a family of bacteria which live in the human intestine. They can be quantified easily in a laboratory test the result of which is indicative of the amount of human waste present in a particular sewage sample. The result of this test is called the e-coli. count and is expressed per 100ml.
Holding tanks
Simple holding tanks may be acceptable for ships which are in port for only a very brief period. The capacity would need to be excessively large for long stays because of the amount of flushing water. They require a vent, with the
outlet suitably and safely positioned because of gas emissions. A flame trap reduces risk. Inhibiting internal corrosion implies some form of coating and, for washing through of the tank and pump after discharge of the contents at sea, a fresh water connection is required.
Figure : Biological sewage treatment plant (Hamworthy)
Summarized below some of the basic procedure of machinery service systems and equipment :
- Ballast arrangements
The ballasting of a vessel which is to proceed without cargo to the loading port is necessary for a safe voyage, sometimes in heavy weather conditions. On arrival at the port the large amount of ballast must be discharged rapidly in readiness for loading....
- Cargo ships bilge systems
The essential purpose of a bilge system, is to clear water from the ship's 'dry' compartments, in emergency. The major uses of the system, are for clearing water and oil which accumulates in machinery space bilges as the result of leakage or draining, and when washing down dry cargo holds. The bilge main in the engine room, has connections from dry cargo holds, tunnel and machinery spaces.....
- Bilge system layout details
All bilge suctions have screw down non-return valves with strainers or mud boxes at the bilge wells. Oily bilges and purifier sludge tanks have suitable connections for discharge to the oily water separator or ashore. The system is tailored to suit the particular ship......
- Domestic water system
Systems using gravity tanks to provide a head for domestic fresh and sanitary water, have long been superseded by schemes where supply pressure is maintained by a cushion of compressed air in the service tanks....
- Reverse osmosis
Osmosis is the term used to describe the natural migration of water from one side of a semi-permeable membrane into a solution on the other side. The
phenomenon occurs when moisture from the soil passes through the membrane covering of the roots of plants,....
- Salinometer features
The condensate or product, if of acceptable quality, is delivered to the appropriate tanks by the distilled water pump. Quality is continuously tested by the salinometer both at start up and during operation. If the device registers an excess of salinity it will dump the product and activate the alarm using its solenoid valves. The product is recirculated in some installations......
- Sewage systems
The exact amount of sewage and waste water flow generated on board ship is difficult to quantify. European designers tend to work on the basis of 70 litres/person/day of toilet waste (including flushing water) and about 130-150 litres/person/day of washing water (including baths, laundries, etc.). US authorities suggest that the flow from toilet discharges is as high as 114 litres/person/day with twice this amount of washing water......
- Sewage zero discharge system
A retention or holding tank is required where no discharge of treated or untreated sewage is allowed in a port area. The sewage is pumped out to shore reception facilities or overboard when the vessel is proceeding on passage at sea, usually beyond the 12 nautical mile limit. ...
- Biological sewage treatment
A number of biological sewage treatment plant types are in use at sea but nearly all work on what is called the extended aeration process. Basically this consists of oxygenating by bubbling air through or by agitating the surface. ....
- Sterilization system
Sterilization by the addition of chlorine, is recommended in Merchant Shipping Notice M1214. A later notice, M1401, states that the Electro-Katadyn process in use since the 1960s, has also been approved. Another problem with distilled water is that having none of the dissolved solids common in fresh water it tastes flat. It also tends to be slightly acidic due to its ready absorption of carbon dioxide (CO2). .....
- Treatment of water from shore
There is a risk that water supplied from ashore may contain harmful organisms which can multiply and infect drinking or washing water storage tanks. All water from ashore, whether for drinking or washing purposes, is to be sterilized. When chlorine is used, the dose must be such as to give a concentration of 0.2 ppm....
- Water production low pressure evaporator
A considerable amount of fresh water is consumed in a ship. The crew uses on average about 70 litre/person/day and in a passenger ship, consumption can be as high as 225 litre/person/day. Water used in the machinery spaces as make up for cooling system losses may be fresh or distilled but distilled water is essential for steam plant where there is a water tube boiler.
Steamship consumption for the propulsion plant and hotel services can be as high as 50 tonnes/day.....
- Flash evaporator system
The evaporator , boils sea water at the saturation temperature corresponding to the uniform pressure through the evaporation and condensing chambers. With flash evaporators the water is heated in one compartment before being released into a second chamber in which the pressure is substantially lower......
- Oil content monitor system
In the past, an inspection glass, fitted in the overboard discharge pipe of the oil/water separator permitted sighting of the flow. The discharge was illuminated by a light bulb fitted on the outside of the glass port opposite the viewer......
- Oily water separator
Oil/water separators are necessary aboard vessels to prevent the discharge of oil overboard mainly when pumping out bilges. They also find service when deballasting or when cleaning oil tanks. The requirement to fit such devices is the result of international legislation....
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