SEWAGE PLANTS AND SYSTEM

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Sewage.:-   Is a term used to describe human body waste, normally defined as "Black Water". This is the only liquid that is treated with a "Marine Sanitation Device"

Grey Water:-  Is defined as water from baths, showers, wash basins, sinks, laundries  etc. At present there is no requirement for treatment of Grey Water prior to its discharge overboard.

Terms used in waste treatment

Biological Oxygen Demand (B.O.D.)

Is used to give a measure of the strength of sewage, it identifies the biologically decomposable substances and is a test that depends on the activity of bacteria, which in the presence of oxygen feed on and consume organic matter.

Results of the test are expressed as the amount of oxygen taken by a one litre sample (diluted with aerated water) when incubated at 20^o for five days. Can be defined as the amount of oxygen utilised by micro-organisms in the stabilisation of organic matter,

B.O.D. of raw sewage is 300 to 600 mg/litre

I.M.O. recommend a B.O.B. of 50 mg/litre after treatment.

Coliform Count:-

Coliform organisms are recognised as the Indicator Organisms of sewage pollution. The numbers present in sewage are large each person contributing between 125 billion in winter to 400 billion in summer.

Coliform are present in the human intestine and their presence in water is taken as an indication of the Pathogen count.

The Pathogens are disease causing organisms responsible for Typhoid, Dysentery, Poliomyelitis, Cholera etc.

I.M.O. recommend a coliform count of 250/100 ml. of effluent after treatment.

Solids:-

Dissolved:- Solids which are in solution.

Suspended:- Solids physically suspended in sewage that can be removed by

            laboratory filtration. Are relatively high in organic matter.

Settleable:- Suspended solids that will subside in quiescent liquid in a

             reasonable period (usually taken as one hour)

The Suspended Solids level of raw sewage is 300 to 400 mg./litre.

I.M.O. recommend a level, after treatment of 50 mg./litre.

Glossary cont'd/

Residual Disinfectant:-

After treatment the residual disinfectant should be as low as possible.

Canadian restriction is between 0.5 and 1.0 mg./litre.

I.M.O. prefer the use of Ultra-Violet exposure to Chlorination.

         Biological Digestion Of Sewage

Anaerobic Process:- Anaerobic bacteria can only multiply in the absence of free oxygen, they use chemically bound oxygen to survive.

In the anaerobic process the bacteria break down the organic matter into, water, carbon dioxide, methane, hydrogen sulphide and ammonia.

The process is also called Putrefaction. The gasses produced are both noxious and toxic. The effluent produced is of poor quality and other by-products are highly corrosive.

Aerobic Process:- Aerobic bacteria require free oxygen to survive. In the aerobic process the bacteria break down the organic matter safely.

The Aerobic Process has end products of :-

                H₂.O + C.O₂ + Inert Residue + Energy To Synthesise New Bacteria

Note!

Disposal of waste from ships was discussed internationally as long ago as 1973, but there was little initial progress on legislation. This led to certain countries introducing legislation unilaterally, among these were Canada, Japan, Sweden, U.S.S.R. and U.S.A. all with their own variations.

Other nations have since followed, but the variation between different countries still causes confusion for visiting ships.

Treatment plants can be broadly divided into:-

Physical Processes:-  Comminutation, Filtration, Ultrasonic Degradation and Incineration.

Chemical Processes.

Biological Processes:- Degradation by micro-organisms.                                           BIOLOGICAL TREATMENT SYSTEM

The principle used involves "Activated Sludge" and has been used ashore for many years. Raw sewage is introduced into an aeration chamber, where "Biolysis" of the faecal matter is achieved by micro-organisms in the presence of free oxygen.

After vigorous agitation the liquor is transferred to the settlement hopper, where activated sludge falls to the bottom.

The process results in a light brown precipitate, that settles, leaving a clear odourless liquid.

Periodically some of the sludge is returned to the aeration chamber to help maintain the biolysis.

Liquid effluent overflows into the contact tank, where it is usually diluted with "Grey

Water" before its eventual discharge overboard.

The operation is never as successful on board ship as it is ashore. mainly because of variations in load and the effect of sea water. The main disadvantage of this plant is that micro-organisms developed in salt water may die off in fresh water, during the time required to develop new organisms the discharge effluent level will be high.

After a shut down it may take up to five days until the growth of new organisms is sufficiently developed to maintain the low effluent levels required.

Also heavily chlorinated water is a danger to marine life, hence there is a problem with residual disinfectant, Canada is particularly strict on this, mainly as a protection for the great lakes.

Either"Sodium Hyperchlorate" or Calcium Hyperchlorate is used for chlorination.

NOTE!:

If sewage is not adequately oxygenated, putrefaction sets in. Anaerobic bacteria multiply and produce Urea, Ammonia, Hydrogen Sulphide, and Methane. From this it is clear that if sewage is not inactivated and is stored for more than a few hours it MUST be thoroughly aerated.

A typical plant gives:-

    B.O.D.            < 40 mg/litre

    Suspended Solids  < 50 mg/litre

    Coliform          < 200 per 100 ml.

It is due to the difficulties with the activated sludge type at sea, that many owners stick with the physical/chemical types of treatment.

 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