MARINE SEWAGE TREATMENT PLANT

Principle of sewage treatment

• 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 oxygen has been depleted.
• Aerobic bacteria require free oxygen to survive.They breakdown the organic matter to produce safe products such as water, carbon dioxide. Final discharge has a clean and clear appearance.
• Anaerobic bacteria can only multiply in the absence of free oxygen.They breakdown the organic matter into water, carbon dioxide,methane, hydrogen sulphide and ammonia which are noxious and toxic.

Pic: Sewage treatment plant

Components/Chambers/Tanks

• Wire mesh
• Primary tank
• Aeration chamber
• Settlement tank
• Chlorination and collection tank
• Blowers
• Chlorinator
• Low level, high level alarm

Operation

• Sewage water is introduced into a primary vessel through a wire mesh.
• It is then passed through a wire mesh which grinds the sewage and breakdown into smaller particles.
• A plate is provided which makes the sewage to rise and flow into the aeration chamber.
• Air is forced through the diffuser into the air chamber which helps the aerobic bacteria to grow and attack the sewage.
• Constant pressure of around 0.3-0.4 bar is kept for proper agitation and formation of bubbles.
• The sewage is decomposed into carbon dioxide, water and inorganic sewage.
• The mixture of liquid and sludge is passed to the settling tank from the aeration chamber.
• The sludge settle at the bottom and  clear liquid on the top, sludge is not kept in settling tank as it may produce anaerobic bacteria and foul gases are produced.
• The sludge formed is recycled by sending it back to the aeration chamber.
• The clear liquid produced from the settling tank is overflown and is disinfected with the help of chlorine.
• This is done due to the presence of E-coli present in liquid which has to be eliminated.
• The liquid during chlorination is kept for a period of at least 60 min.
• The collected liquid is discharged overboard when the high level alarm raises and the vessel is in proper geological location. 

Effluent quality standards

• The bio-chemical oxygen demand (BOD) is determined by incubating at 200 degree Celsius, 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 measures the total amount of oxygen used for complete breakdown of organic matter. This indicates the strength of the sewage. IMO recommends BOD of less than 50 mg/L after treatment through sewage treatment plant.
• Suspended solids - this can give rise to silting problems. Suspended solids are measured by filtering a sample through a pre-weighed pad which is then dried and re-weighed. IMO recommends 50 mg/ litre after treatment.
•  Coliform count - The e-coliform is a family of bacteria which live in the human intestine. The result of this test is called the e-coli count and is expressed per 100ml. Presence of these organisms in water is an indication of pathogen (disease causing bacteria responsible for cholera, dysentery, typhoid). IMO recommends fecal coliform count of less than 250/100 ml. of affluent after treatment.

The rules which has to be followed for the discharge of sewage is given in annex IV as below.

Marpol Annex IV: -

• Vessel should be equipped with a certified sewage treatment system or holding tank.
• Within 3 miles of nearest land, sewage discharges are to be treated by a certified marine sanitation that is a sewage treatment plant device prior discharge.
• Between 3 miles and 12 miles from shore, sewage discharges must be treated by no less than maceration or chlorination.
• Sewage discharge beyond 12 miles from shore is unrestricted.
• The discharge of sewage should be such that the ship is en route and is proceeding not less than 4 nautical miles.