SEWAGE TREATMENT PLANT
4 Important Terms Related to Sewage Treatment Plant on Ships
Sewage on board ships needs to be treated before it is discharged to the sea. Sewage treatment plant is used to treat the sewage and make it less harmful for the sea.
However, apart from the above mentioned aspects, marine engineers should also know four important terms while dealing with sewage treatment plants on ships. They are:
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 can also be defined as the amount of oxygen required by the micro-organisms in the stabilization of organic matter. The results are generally expressed as the amount of oxygen taken by one litre sample (diluted with aerated water) when incubated at 20 degree for five days.
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.
The products thus produced out of this process are noxious and toxic. The effluent is of poor quality and by-products are highly corrosive.
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.
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.
Generally, ships prefer treating sewage before discharging to save themselves from any type of embarrassment. There are different methods of treating sewage available in the market, but the most common of them is the biological type for it occupies less space for holding tank, unlike those of the other methods. Moreover, the discharge generated from this plant is eco friendly. It is to not that each sewage treatment system installed onboard has to be certified by classification society and should perform as per their requirement and regulations.
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. The pressure of air flow also plays an important role in decomposition of the sewage. 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.
Procedure for Starting and Stopping of Sewage Treatment Plant on a Ship
Any machine on the ship requires a proper procedure to be followed for starting and stopping it. Failure to follow this step-by-step procedure will lead to either failure in starting or stopping the machine or lead to some fault.
Sewage treatment plant is one such equipment on the ship which requires a step-by-step procedure to be followed for starting and stopping it. In this article we will go through the procedure of starting and stopping a sewage treatment plant.
Starting of a Sewage Plant
Sewage plant is generally running all the time during sailing, but it might need to be started when the ship is installed with a new sewage treat plant which needs to be stopped at regular interval of time for improving its performance and maintenance procedures. Below are the points that need to be followed for starting a sewage treatment plant.
1. Make sure if any maintenance is carried out on the sewage treatment system, all the openings have been closed properly before starting.
2. The sewage plant is be filled with fresh water inside the chamber.
3. At this stage, there are no aerobic bacteria inside the chamber, but the sewage has started coming to the plant. Thus, in order to increase efficiency and starting rate of the plant bio pac is added to the plant by flushing the amount specified in the manual. This bio pac is mixed with warm water which helps in growth of these bacteria and also efficient functioning of the plant.
4. If the bio pac is not added, the plant might take up to 5 to 7 days to be completely functional. However, with the bio pac it becomes functional within 24 hours.
5. Start the air compressor or open the air valve as per the design of the plant. The pressure is maintained as per the manual. Generally 0.3-0.4 bars.
6. Open the sewage overboard valve and close holding tank valve when the ship is out of restricted waters.
7. The plant is continuously monitored and checked for the flow through the transparent plastic tubes.
8. The sample is taken for checking for suspended solids and chlorine content.
Stopping of the plant
Stopping of the sewage treatment plant is generally done either before entering the dry dock or in case some maintenance has to be carried out inside the treatment plant.
1. For stopping the system, close the inlet valve to the sewage plant and close the overboard valve and let the sewage go overboard.
2. Empty all the three chambers of the plant i.e. aeration, settling and chlorination chambers. If the chambers are not emptied, it will lead to growth of anaerobic bacteria which forms the toxic H2S gas.
3. If entry has to be made inside the tank, the later should be checked for hydrogen sulphide gas H2S with the help of dragor tube by taking a continuous sample from the plant. Entry is made with the help of mask and rubber gloves should be put on.
4. In case the ship is going to dry dock the overboard should be connected to shore reception facilities.
Maintenance and Checks for Sewage Treatment Plant on Ship
An efficient running of a sewage treatment plant on a ship requires periodic maintenance and daily checks of the system. Failure to do so can lead to an output that cannot be discharged into the sea, blockage of pipelines, and even failure of some parts.
There are several factors that results in smooth working of a sewage treatment plant on a ship and this article enumerates all the maintenance and checks for that smooth running.
Routine Checks
1. During daily rounds the pressure of the system should be checked and should be within the limits.
2. The air lift return should be checked to make sure the system is working properly. This is usually checked by the flow through the clear plastic pipe present on the installation. A clear sludge can be seen flowing through the tubes back to the aeration chamber.
3. Over a period of time, the sludge content in the aeration tank due to the recycling of the sludge from settling tank and fresh sewage increases. This sludge content or suspended solid particle is measured in mg/liter. The method of checking it is to take sample in a conical flask provided by the manufacturer and filling it up to 1000ml mark. The sample is then allowed to be settled and reading of sludge content is checked.
The sludge content should not be above the 200 mark, but if it is above the 200 mark, the tank has to be emptied for increasing the performance. In some ships this is checked by filtering the sample through a pre-weighed pad which is dried and re-weighed. This also depends from manufacturer to manufacturer, but is done every week.
4. Also the bio-pac is added every week to the plant to increase efficiency. The bio-pac contains aerobic bacteria which get activated when mixed with hot water.
5. The chlorination of the sample should be between 1-5 ppm and accordingly the dosing has to be increased or decreased.
6. The level of biological oxygen demand (BOD) is also checked and it should not be above 50 mg/liter. The sample is checked by incubating the sample at 20 degrees and well oxygenating the same. The amount of oxygen absorbed over a period of five days is measured. This is done to check the oxygen required for full breakdown of sewage after it has been treated by aerobic bacteria.
7. The internal coating of the sewage treatment plant should be checked for cracking and blistering. If any kind of damage is found then we first need to empty the tanks and then necessary repairs to be performed. Special precautions should be taken before entering the tank as it may contain toxic gases that cause suffocation. The gases should be checked by dragor tube, a special tube in which samples of various gases are taken before entering.
When it is made sure of the absence of toxic gases, entry is made with the mask and gloves. After completion of work the area has to be disinfected. Later, hands should be properly scrubbed and overalls be thoroughly washed.
8. If the sewage treatment plant is fitted with UV disinfectant system instead of the chlorination system, the UV lamp has to be changed as recommended by the manufacturer.
9. High and low level limit switches should be checked for auto cut-in and cut-out of the discharge to over-board pump.
10. Make sure the stand-by sewage discharge pump is put on auto during the working of the sewage treatment plant.
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.