GROUP ASSIGNMENT SEWAGE TREATMENT PLANT

PREPARED BY: ABDUL JABBAR KHAN PANDIARAJ A/L RETENAM

SEWAGE TREATMENT PLANT ON A SHIP EXPLAINED

Discarding sewage produced onboard on a ship is one of the few tasks on a ship which should be taken utmost care of if one wants to same him and his shipping company from heavy fine. The sewage generated on the ship cannot be stored on the ship for a very long time and it for this reason it has to be discharged into the sea. Though sewage can be discharged into the sea, we cannot discharge it directly overboard as there are some regulations regarding discharging of sewage that needs to be followed. Sewage on sea is generally the waste produced from toilets, urinals and WC scuppers. The rules say that the sewage can be discharged into the sea water only after it is treated and the distance of the ship is 4 nautical miles from the nearest land.

But if the sewage is not treated this can be discharged 12 nautical miles away from the nearest land. Also the discharged sewage should not produce any visible floating solids nor should it cause any discoloration of surrounding water. There are different methods of treating sewage available in the market, but the most common of them is the biological type for it

Working of a Biological Sewage Plant

The basic principle of the working of a biological treatment plant is decomposition of the raw sewage. This process is done by aerating the sewage chamber with fresh air. The aerobic bacteria survive on this fresh air and decompose the raw sewage which can be disposed off in the sea. Air is a very important criterion in the functioning of the biological sewage plant because if air is not present, it will lead to growth of anaerobic bacteria, which produces toxic gases that are hazardous to health.Also, after decomposition of the sewage with anaerobic bacteria, a dark black liquid causes discoloration of water which is not accepted for discharging. Thus in a biological sewage treatment plant the main aim is to maintain the flow of fresh air.

Division of Processes

The biological sewage plant is divides into three chambers:-   Aeration chamber  This chamber is fed with raw sewage which has been grinded

to form small particles. The advantage of breaking sewage in small particles is that it increases the area and high number of bacteria can attack simultaneously to decompose the sewage. The sewage is decomposed into carbon dioxide, water and inorganic sewage. The air is forced through diffuser into the air chamber. If pressure is kept high then the mixture of air and sewage will not take place properly and it will escape without doing any work required for decomposition. It is for this reason; controlled pressure is important inside the sewage treatment plant as this will help in proper mixing and decomposition by the agitation caused by air bubbles. Generally the pressure is kept around 0.3-0.4 bars

Settling tank The mixture of liquid and sludge is passed

to settling tank from the aeration chamber. In the settling tank the sludge settles at the bottom and clear liquid on the top. The sludge present at the bottom is not allowed to be kept inside the settling tank as this will lead to growth of anaerobic bacteria and foul gases will be produced.The sludge formed is recycled with the incoming sludge where it will mixes with the later and assist in the breakdown of sewage.

Chlorination and Collection In this chamber the clear liquid produced from the

settling tank is over flown and the liquid is disinfected with the help of chlorine. This is done because of the presence of the e-coli bacteria present in the liquid. To reduce these bacteria to acceptable level chlorination is done. Moreover, to reduce the e-coli, the treated liquid is kept for a period of at least 60 minutes. In some plants disinfection is also done with the help of ultra violet radiation. The collected liquid is discharged to overboard or settling tank depending on the geological position of the ship. If the ship is in restricted or near coastline then the sewage will be discharged into the holding tank; otherwise, the sewage is discharged directly into the sea when high level is reached and is disposed automatically until low level switch activates.

4 Important Terms Related to Sewage Treatment Plant on Ships

Biochemical Oxygen Demand (BOD) Coliform Count Recommended levels of pumping out

solids Bio-chemical digestion of sewage

1. Biochemical Oxygen Demand

Biochemical oxygen demand is a test to identify biological decomposable substances and to test the strength of the sewage. BOD depends on the activity of bacteria in the sewage. These bacteria feed on and consume organic matter in the presence of oxygen.

BOD of raw sewage is 300-600 mg/litre. IMO recommends BOD of less than 50 mg/litre after treatment through sewage treatment plant.

2. Coliform Count Coliform is a type of organism which is present in

human intestine and is recognized as indicator organisms of sewage pollution. Presence of these organisms in water is an indication of pathogen (pathogen count), which are diseases causing bacteria responsible for cholera, dysentery, typhoid etc.

The number of coliform organisms present in sewage on ship is very large, with each person contributing around 125 billion in winters and 400 billion in summer.

IMO recommends faecal coliform count of less than 250 faecal/100 ml. of affluent after treatment.

3. Recommended levels of pumping out solids Dissolved solids – Solids which are dissolved in the

solution Suspended solids – Solids physically suspended in

sewage that can be removed by laboratory filtration and are relatively high in organic matter.

Settle able solids – Suspended solids that will subside in quiescent liquid in a reasonable period of time (usually around an hour)

Suspended level of raw sewage – Around 300-400 mg/litre; IMO recommends 50 mg/ litre after treatment.

Residual disinfectant – After treatment residual disinfectant should be as low as possible. IMO recommends use of ultra violet exposure for chlorination method.

4. Biochemical digestion of sewage: Anaerobic process Anaerobic bacteria can only multiply in the

absence of free oxygen as they utilize chemically bound oxygen to survive. Anaerobic bacteria break down the organic matter into water, carbon dioxide, methane, hydrogen sulphide and ammonia. This process is called putrefaction.

Aerobic process Aerobic bacteria require free oxygen to survive.

They break down the organic matter to produce safe products such as water, carbon dioxide, inert residue, and energy to synthesize new bacteria.

Physical-Chemical System Operation •Grey water collected in tank and chlorine

added for disinfecting & sodium hypochlorite to aid flocculation

•Sewage is circulated from dosing tank to comminutor and to chemical treatment tank –converted to slurry.

•Slurry, by action of chemical, separate into sludge and clear liquid in settling tank (flocculation).

•Clear liquid is recirculated for toilet use.

Physical-Chemical System

Sewage Treatment-Operation

•The system collect and store sewage in controlled areas. Where legislation permits, wash basins, bath and shower drains is discharged straight overboard.

•Various chemicals are added at different points for odour, colour removal and to assist breakdown and sterilisation

•A communitor is used to physically break up the sewage and assist the chemical breakdown process.

•Solid material settles out in the tank and is stored prior to discharge into the sullage tank.

•The liquid is recycled for flushing use.

Maintenance In case of a blockage of the sewage line there is a

connection for back flushing which uses sea water. This is to be used to unclog the sewage pipelines; however, it is to note that all valves are closed and only the necessary valves are open, for it might back flush from WC of the cabins.

Generally, stewards are instructed for using chemicals provided by various manufacturers such as Drew Marine and Unitor during washing so that no blockages of lines are caused. However, there shouldn’t be any overuse of these chemical as it would lead to killing of aerobic bacteria which will decrease the efficiency of the plant and other problems.  The amount of chemicals is to be as per manufacturer recommendation.

THANK YOU END OF SLIDE SHOW