Home || Diesel engines
||Steam turbines ||Fuel treatment ||Pumps ||Valves ||Refrigeration ||
Sewage treatment on board- biological and chemical sewage treatment plant working principles
The discharge of untreated sewage in controlled or territorial waters is
usually banned by legislation. International legislation is in force to
cover any sewage discharges within specified distances from land. As a
result, and in order to meet certain standards all new ships have sewage
treatment plants installed.
Untreated sewage as a suspended solid is unsightly. In order to break
down naturally, raw sewage must absorb oxygen. In excessive amounts it
could reduce the oxygen content of the water to the point where fish and
plant life would die.
Pungent smells are also associated with sewage as a
result of bacteria which produce hydrogen sulphide gas. Particular
bacteria present in the human intestine known as E, coli are also to be
found in sewage. The E. coli count in a measured sample of water
indicates the amount of sewage present.
Two particular types of sewage treatment plant are in use, employing
either chemical or biological methods. The chemical method is basically
a storage tank which collects solid material for disposal in permitted
areas or to a shore collection facility. The biological method treats the
sewage so that it is acceptable for discharge inshore.
Chemical sewage treatment
This system minimises the collected sewage, treats it and retains it until it
can be discharged in a decontrolled area, usually well out to sea. Shore
receiving facilities may be available in some ports to take this retained
This system must therefore collect and store sewage produced while
the ship is in a controlled area. The liquid content of the system is
reduced, where legislation permits, by discharging wash basins, bath and
shower drains straight overboard. Any liquid from water closets is
treated and used as flushing water for toilets. The liquid must be treated
such that it is acceptable in terms of smell and appearance.
chemicals are added at different points for odour and colour removal
and also to assist breakdown and sterilisation. A comminutor is used to
physically break up the sewage and assist the chemical breakdown
Solid material settles out in the tank and is stored prior to
discharge into the sullage tank: the liquid is recycled for flushing use.
Tests must be performed daily to check the chemical dosage rates.
This is to prevent odours developing and also to avoid corrosion as a
result of high levels of alkalinity.
Fig:Biological sewage treatment plant
Biological sewage treatment
The biological system utilises bacteria to completely break down the
sewage into an acceptable substance for discharge into any waters. The
extended aeration process provides a climate in which oxygen-loving
bacteria multiply and digest the sewage, converting it into a sludge.
These oxygen-loving bacteria are known as aerobic.
The treatment plant uses a tank which is divided into three watertight
compartments: an aeration compartment, settling compartment and a
chlorine contact compartment .
The sewage enters the
aeration compartment where it is digested by aerobic bacteria and
micro-organisms, whose existence is aided by atmospheric oxygen which
is pumped in. The sewage then flows into the settling compartment
where the activated sludge is settled out. The clear liquid flows to the
chlorinator and after treatment to kill any remaining bacteria it is
Tablets are placed in the chlorinator and require
replacement as they are used up. The activated sludge in the settling
tank is continuously recycled and builds up, so that every two to three
months it must be partially removed. This sludge must be discharged
only in a decontrolled area.
- Compressed Air Systems for various shipboard operations
The main aim of a compressor, as the name suggests, is to compress air or any fluid in order to reduce its volume. Some of the main applications of compressors onboard ships are main air compressor, deck air compressor, AC compressor and refrigeration compressor. Failure to start or control air compressor can be inconvenient, costly and can carry risks, which need to be managed.....
- Marine air compressors working principles
Control or instrument air supplies have particular requirements with regard to being moisture and oil free and without impurities. A special type of oil-free compressor may be used to supply control air or it may be treated after delivery from an ordinary air compressor. This treatment results in the air being filtered and dried in order to remove virtually all traces of oil, moisture and any atmospheric impurities.....
- Coolers at sea- Shell and tube type coolers and plate type coolers Heat exchangers on board ship are mainly coolers where a hot liquid is cooled by sea water. There are some instances where liquid heating is required, such as heavy fuel oil heaters and sea water heaters for tank cleaning. Although being heat exchangers, the main condenser for a steam ship and the evaporator/distiller are dealt with separately .....
- Distillation system- Production of distilled water for ships use - The evaporation process
Distillation is the production of pure water from sea water by evaporation and re-condensing. Distilled water is produced as a result of evaporating sea water either by a boiling or a flash process. This evaporation enables the reduction of the 32000 parts per million of dissolved solids in sea water down to the one or two present in distilled water. The machine used is called an 'evaporator', although the word 'distiller' is also used.....
- Oily water separator and filter unit for 15 parts per million purity
Oil/water separators are used to ensure that ships do not discharge oil when pumping out bilges, oil tanks or any oil-contaminated space. International legislation relating to oil pollution is becoming more and more stringent in the limits set for oil discharge.....
Sewage treatment on board- biological and chemical sewage treatment plantThe discharge of untreated sewage in controlled or territorial waters is usually banned by legislation. International legislation is in force to cover any sewage discharges within specified distances from land. As a result, and in order to meet certain standards all new ships have sewage treatment plants installed.....
- Incinerator for ships Stricter legislation with regard to pollution of the sea, limits and, in some instances, completely bans the discharge of untreated waste water, sewage, waste oil and sludge. The ultimate situation of no discharge can be achieved by the use of a suitable incinerator. When used in conjunction with a sewage plant and with facilities for burning oil sludges, the incinerator forms a complete waste disposal package.....
Marine machineries - Useful tags
Marine diesel engines ||Steam generating plant ||Air conditioning system ||Compressed air ||Marine batteries ||Cargo refrigeration ||Centrifugal pump ||Various coolers ||Emergency power supply ||Exhaust gas heat exchangers ||Feed system ||Feed extraction pump ||
Flow measurement || Four stroke engines || Fuel injector || Fuel oil system || Fuel oil treatment ||Gearboxes || Governor ||
Marine incinerator ||
Lub oil filters ||
MAN B&W engine ||
Marine condensers ||
Oily water separator ||
Overspeed protection devices ||
Piston & piston rings ||
Crankshaft deflection ||
Marine pumps ||
Various refrigerants ||
Sewage treatment plant ||
Starting air system ||
Steam turbines ||
Steering gear ||
Sulzer engine ||
Turbine gearing ||
Two stroke engines ||
UMS operations ||
Drydocking & major repairs ||
Critical machinery ||
Deck machineries & cargo gears
|| Control and instrumentation
||Engine room safety ||
Machinery Spaces.com is about working principles, construction and operation of all the machinery
items in a ship intended primarily for engineers working on board and those who working ashore . For any remarks please
Copyright © 2010-2016 Machinery Spaces.com All rights reserved.
Terms and conditions of use