BIOLOGICAL TREATMENT SYSTEM

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Many systems in use at sea are of the aerobic type, where the bacteria generated require a feed of oxygen to thrive. Raw sewage enters the first stage (aeration) chamber where it is broken down by micro-organisms, with the products being water, carbon dioxide and an inert residue. Note in the absence of oxygen, due perhaps to a restriction to the oxygen supply, anaerobic bacteria thrive and the products include hydrogen ­sulphide and methane, which are flammable and poisonous, see MGN 33.

The tank is sized for the ship to give a dwell time in normal use of about 24 hours. The liquor is then transferred to the settlement hopper, where precipitation takes place leaving a clear colourless liquid, and bacteria rich "activated sludge" at the bottom of the hopper. This sludge is periodically returned to the first stage to help maintain the bacteria colony.

The liquid effluent overflows, via a chlorination unit, into a third stage tank where it is diluted with "Grey Water" (drains from sinks, baths etc.). A float operated pump periodically discharges the third stage tank overboard.

A macerator may sometimes be fitted to the sewage (black water) inlet to the first stage, and some authorities prefer the use of Ultra Violet fight to kill off bacteria in the third stage, rather than chlorine which can damage marine life.

The maintenance for these plants generally consists of regularly removing the sludge build up, maintaining clear passageways (including ventilation) and pump and compressor overhauls. The plant should also be inspected internally on an annual basis and any coatings reapplied.

When working on the plant regard to the risks is paramount. Risks include the fact that it is an enclosed space, possible presence of anaerobic conditions and biological hazard from waste products – including hospital drains.

THE AEROBIC PROCESS

Aerobic bacteria require free oxygen to survive, and break down the organic matter to

            H₂O + CO₂ + Inert Residue

Anaerobic bacteria produce toxic and flammable gasses in a process known as putrefaction, the products being             H₂O + CO₂ + H₂S + CH₄ ( Methane )

BIOCHEMICAL OXYGEN DEMAND (B.O.D.)

Gives a measure of the strength of sewage, since if oxygen is being used then there are still bacteria present and they are still actively digesting the sewage. The test expresses the amount of oxygen taken by a one litre sample incubated at 20°C for five days. The B.O.D. of raw sewage is 300‑600 mg/litre. IMO recommend 50mg/litre after treatment

COLIFORM COUNT

Coliform Bacteria are present in the human intestine and their presence in sewage is an indicator of the level of pollution. They indicate the presence of pathogens which may cause Typhoid, Dysentery etc.

Each person contributes 125 to 400 billion bacteria per day, IMO recommend a count of 250/100 ml of treated sewage.

SUSPENDED SOLIDS

These are solids which are physically suspended in solution. They can be removed by filtration through an asbestos mat, which is then dried and weighed Level in Raw Sewage is 300‑400 mg/litre. IMO recommend a final level of 50 mg/litre

RESIDUAL DISINFECTANT

Should be as low as possible after final treatment ‑ Canadian restriction is between 0.5 to 1.0 mg/litre IMO prefer Ultra‑Violet exposure to chlorination

MGN33

When not equipped with active aeration features, sewage tanks should be emptied and flushed through every 24 hours unless some treatment is used which maintains satisfactory conditions

When active aeration features are fitted, manufacturers' recommendations with respect to air blowers, distribution systems etc. should be followed and the plant maintained annually