Last Updated on August 19, 2020 by Amit Abhishek
A sewage treatment plant is designed to treat and process raw sewage over different steps involving breaking, filtering, settling, controlled aerobic decomposition and chemical treatment.
One of the most common things that come in our mind regarding human waste; is to dump it to the sewers and let the government take care of it. With an average person producing more than 90 litters of sewage per day; it’s obvious to wonder how sewage is managed and treated on ships.
Now since we no longer have the luxury to discharge untreated human waste at sea and have limited space to store; the sewage produced is first treated and then discharged. The process used to systematically break the sewage into small parts; using biological and chemical method is known as sewage treatment.
While the system used to perform sewage treatment is called sewage treatment plant. A sewage treatment plant on ship consists of a screen filter, primary chamber, aeration chamber, demisters, blowers, settlement and chlorination chamber. Together they treat and discharge waste to comply with Marpol regulation IV.
But Why All The Trouble – Can’t We Let Them Break On Their Own ?
AHH ! The good old days before Marpol regulation 4 when we could literally dump untreated human waste at sea. But why did it was forced to stop ? In the absence of sewage treatment plant when we discharge the waste in open water; the waste starts to attract aerobic bacteria and decompose on its own.
Not just it suck up the necessary oxygen from the water but also lead to widespread risks of health epidemic if discharged near port. Moreover not all parts of sewage is biodegradable or require same time to break down; thus making the situation even more complex. Many a time the traces of nitrates, phosphate and other organic matter that untreated sewage contains; leads to contamination of surrounding waters.
So to protect marine environment and spread of any epidemic the international maritime community ask for a governing law under Marpol. This is how Marpol regulation 4 came into existence on 27th September 2003; which deals with prevention of pollution due to discharge of sewage at sea.
Thus to avoid heavy fine and possible embarrassment of the ship and the company; every ship is equipped with dedicated sewage treatment plant.
Design And Construction Of Sewage Treatment Plant
A sewage treatment plant is designed and constructed to best suit the need of the facility; whether be industry, ships or regular sewers in urban cities. The sewer stored can be treated with mainly two major ways; chemically or biologically. Although many new plants came with hybrid models. Let us learn about the design characteristics of these prominent types with detailed discussion on each one of them.
1 ) Chemical Sewage Treatment Plant
The chemical sewage treatment plant consist of preliminary treatment, primary treatment and secondary treatment chambers. The preliminary chamber is equipped with coarse and fine mesh of screen as filters to remove large solid particles from getting into the system. In many designs it stay set at the top of the primary chamber with flow measurement device recording and filtering waste water inlet at the same time.
The primary chamber is nothing but a large storage tank much like your in house septic tank. It accounts to hold all the wastes flushed off your toilet, wash basin and bathrooms. The Chamber is equipped with float switches to control the sewage level. Furthermore they also acts as sedimentation tanks as the solid particles settled to the bottom; allowing large volume of solid waste to be stored and latter discharged off shore to designated authorities.
The secondary chamber first chlorinate the non solid water waste and store them for further chemical treatment. This is done by adding a 5 % solution of chlorine to kill of bacteria within a period of 30 minutes. Further chemical treatment is done to remove the smell and get rid of the pale colour.
The treated water is then either discharged to the sea, to shore facilities or used in toilets for flushing. De-Chlorination is required to remove or reduce the amount of chlorine compound present; prior to the discharge at sea.
This is done by using de-chlorinate solution such as ammonia, sulphur compounds and activated carbon. The solid waste is stored in a sludge tank latter to be discharged to shore collection facilities.
2 ) Biological Sewage Treatment Plant
Unlike chemicals used in chemical sewage treatment plant biological plants use aerobic and aerobic bacteria to break down the sewage in simpler form and decompose. The process by which the sewage is treated in biological sewage treatment plants by using aerobic bacteria is known as aeration.
The plant consists of six major parts; the fine mesh filter, primary chamber, aeration chamber, demister, settlement chamber and air blowers. The fine mesh of screen is fitted at the inlet to the primary chamber filtering unwanted solids and debris fall into the system. The mesh screen also helps in breaking the solid particle of organic waste ( sewage ) to break into smaller particles.
The primary chamber acts as the collection and holding tank for the raw sewage. These tanks are equipped with level sensors and float switch to detect sewage level and avoid overflow. This is the place where the heaviest of the particle settles down and the rest moves to the aeration chamber.
Aeration chamber is the place where the magic happens. It’s basically a hybrid bioreactors with air blows installed to provide favorable condition for aerobic bacteria to reproduce and grow.
The sewage after bacterial action is then passed on to the next chamber called settling tank. It consists of a set of separating channels with slop slides. The heavier sludge and other sewage particle is separated of the light water; which is then discharged overboard.
Design Of Sewage Plants Used On Board Ship
Due to strict rules and regulations regarding the quality of water allowed to be discharged from sewage treatment plant. The ship’s are generally equipped with hybrid treatment plants; with biological process as their primary method of treatment and chemicals as secondary.
The basic principle of the plant ( i.e to decompose raw sewage ) is taken up by the biological units; and the killing of involved bacteria before final discharged is handled by chemical units.
Just like the biological sewage treatment plant the system consist of; primary chamber, aeration chamber and settling chamber with an addition of chlorination and activated carbon chamber for chemical treatment.
1 ) Primary Chamber
The raw waste water originating from toilet, wash basins and bathrooms; with a concentration of 0.1% solid waste by weight is stored in the primary chamber. The sewage is fed into the chamber with special macerator pumps that reduce human waste to slurry using blending and grinding techniques.
The blended raw sewage is then passed through a series of coarse screen mesh which acts as a filter; stopping any unwanted solids such as parts of metal scraps, plastics and raw toilet paper. Primary chamber then then stores the raw sewage for some time before passing it on to aeration chamber.
2 ) Aeration Chamber
Aerobic bacteria needs food and warmth along with oxygen to survive. While there is plenty of food stuff in the form of raw sewage; which they can break and utilize for energy but there is very little oxygen content. This is why aeration chambers are equipped with one set of two air blowers with one in service and another on stand by.
The air is pumped from the blew of the aeration chamber using a controlled air diffuser thus forming air bubbles. This thus increase the surface area and helps in the growth and reproduction of aerobic bacteria. The proper ventilation system and exhaust fitted in the chamber allow for the NH3 and CO2 Produced to escape.
3 ) Settling Chamber
The waste water after biological treatment went to the settling chamber where the heavier solid particles settles down by effect of gravity. To further support the process and nullify effects of flow of sewage; the waste water is inserted into the chamber from chamber and exit from top to the next chamber.
The chamber is sometimes also called as the hopper tank due to the presence of sloping slides one after another to increase the separation efficiency of the settling tanks / chamber. To reduce the need to add fresh set of bacteria and increase plant efficiency; many new designs came with air driven ejector pump.
They pumps 1/4th of the sludge back to aeration chamber for further treatment and growth of bacteria in the fresh lot of sewage.
4 ) Activated Carbon Addition
Activated carbon is added on purpose to the biological treated sewage to remove the foul color and bad smell. It get on to absorb all the organic molecules associated with the smell and distinct colour. In many design the activated carbon sets are filled just after the settling chamber; thus allowing waste water to be treated before moved to next chamber.
This is achieved by the process called adsorption. Many a times they not just used to remove odor and color; but also unwanted bi-product of biological treatment. The short explanation for why only activated carbon is used for the purpose is that; it provide large surface area with many bonding sites to trap targeted molecules or compound.
5 ) Chlorination Chamber
Just like everything that glitters is not gold; water that don’t smell or appears unhealthy is allowed to be dumped at sea. In fact even after passing through the activated carbon; the waste water still contains the aerobic bacteria that we used in first place. So the treated waste water is disinfected killing all germs ( Salmonella & Pseudomonas ) and bacteria prior to final discharge.
The chlorination chamber thus provides the space and means to hold and disinfectant the waste water using chlorine solution. While some ships use chemical injection method; most prefer the old but solid method of tablet dosing.
The chlorine reacts with their inter cellular enzymes and cell membrane; making them incapable to reproduce, die or nonfunctional.
Rules And Regulation Governing Sewage Treatment Plant
While most authorities whether at sea or land prefer to limit the oxygen demand of the bio-chemical compound; to be as less as 20. But at sea there are special set of rules stated under Marpol regulation 4; governing the operation and discharge criteria of sewage treatment plant.
The law requires all ships and water vessels above 4000 Gross tonnage dead weight or carrying more than 15 crew / personal in international waters is required to have dedicated sewage treatment plant or sludge tank to hold sewage for appropriate time. The provision of these rules are nor applicable to territorial waters under state control.
Under regulation 9.1.02 of Marpol Annex IV the discharge of sewage is permitted if the ship proves to have approved sewage treatment plant installed on board. The law required the ships to have a log of sewage produced, discharged, treated and transferred to shore facility.
The law require the ships to discharge disinfected treated waste at a distance of at least 3 nautical miles from the nearest land. Furthermore in case of normal biological sewage treatment plant i.e waste water untreated with chlorine and active carbon; is required to discharged at at least 12 nautical miles from the nearest land.
Under Resolution MEPC 62 adopted on 2013 all passenger ships; are restricted from discharging sewage in special area ( i.e Baltic Sea ). They are only required to discharge if they provide evidence that; their sewage treatment plants are of approved type and capable to meet the standards for nitrogen and phosphorus removal.
S.T.P ( Sewage Treatment Plant ) Working Principle
The main aim of sewage treatment plant is to treat the raw sewage to acceptable levels so can be dumper or discharged at sea. To perform task the raw sewage had to undergo several process; that break, disintegrate, treat and disinfect the waste water. These process start with the introduction of raw sewage into the primary chamber through a coarse filter.
This is the place where the heavy grit settles down on its own weight leaving the mixture of fine solid waste and water. This mixture is then introduced to the aeration chamber; where either fresh set of aerobic bacteria is added or reproduced from a part of sludge and waste left from last treatment.
These microorganisms use the available oxygen and organic waste to reproduce and grow. In the process they convert organic matter into carbon dioxide, ammonia and other byproducts including other inorganic and organic residue. While there are various factors that can play hand in the time required in the process; but the most important are, air supply ( Oxygen ), temperature, P.H, surface area and sewage quality.
The treated waste is then passed through sedimentation tank; where the slurry and heavy solids get separated of waste water. It is then sent to the chlorination chamber through; an activated carbon channel.
These substances with their large surface area and targeted bonding hands; adsorb molecules and compounds leading to distinct colour and smell of waste water. The non stinky clean waste water is then added with chlorine solution to kill off any residual bacteria; before the final discharge to the open sea.
Answering Questions You Have but Feel Hesitate To Ask
Everyone have question but its thought to us whole life not to ask. But here at Shipfever.com we know the importance of doubts and curiosity. So whether how silly it may be ! We try to answers some of the most common doubts and question we all have at some point of time; regarding sewage treatment plant and its operation on ship.
Q. Does Sewage Treatment Plant On Ship Smells ?
For everyone who has nor been on ship and watch ship’s sewage treatment plant working; its a very obvious question. So yes all sewage treatment plant smell but the smell produced is contained within its wall; to ensure convenience and safety of the operating crew in the engine room.
But in case you be require to open the manhole above the screen filter; be prepared for the sudden burst of foul smell. So it is advised to wear appropriate masks before such inspection and cleaning. It will smell much like rotten eggs with strong nauseating feel due to ammonia and traces of hydrogen sulfide.
The complete plant is properly covered and airtight to secure and hold all the by product even the gas produce with proper ventilation ducts. So if you feel the strong feeling of foul smell near S.T.P its probably due to improper locking of access hatches or manholes.
Q. Does Sewage Treatment Plants Work Similar To The Septic Tanks ?
Although both sewage treatment plant and septic tank seems to work in a similar way; they are two different thing. The septic tank on one hand is a closed chamber placed deep down in the soil; allowing the sewage to decompose on its own. On another hand sewage treatment plant use dedicated system and chamber to treat the sewage.
The one let the nature to do its job either aerobically or anaerobically while the other regulates the production of aerobic bacteria and their consumption of human waste. In short a septic tank is a low cost alternative to sewage treatment plant which are generally complex and require frequent maintenance and watch keeping.
While the septic tank is good for your house or camping needs; sewage treatment plant is for the cities, ships and industries. The S.T.P require a regular dose of bacteria, chlorine and air; while septic tanks require one dose of live bacteria packets per year.
Furthermore the end product of the septic tanks can be used as biomass for agriculture; but are still hazardous to health and so require properer care while utilization. On other hand the output of sewage treatment plant are clean and hazard free due to no live bacteria in it.
Q. How And When Treated Water is Discharged On Ship ?
The treated waste water is discharged by the ships sewage treatment plant by using the level switch; activated by the level sensors on the final chlorination chamber. When the level is high the treated waste water is set to flow through the discharge line to; dedicated sewage discharge pump.
The operating crew adjust the position of valves to discharge overboard or to the sewage holding tanks; based on location of ship from nearest land. However in any situation the crew or operating personnel must always comply to the rules and guidelines of Marpol regulation IV.
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