Sewage treatment plant is an important installation on ships since so many crew members are on board and continuous sewage is generated due to them. This sewage cannot be directly dumped overboard but needs to be treated and this is where the sewage treatment plants come in the picture.

This will ensure that the sewage that goes out in the ship causes less harm to the sea and the marine life in it. Every ship has a sewage treatment plant that treats the sewage before it is let out into the water.

Although it is mainly the duty of marine engineers to look after sewage plant maintenance yet it is better if everyone is familiar with the working principle and starting stopping procedures of the plant, so let us learn some theory and terms associated with a sewage treatment plant on board a marine vessel.

(Biochemical Oxygen Demand) BOD:

BOD is the amount of oxygen that is required by the micro-organisms in order to stabilize an organic matter. The IMO (international maritime organisation) states that the BOD of the sewage that is disposed by a ship should be less than 50 mg per litre.

This is possible only after the sewage has been treated in the sewage treatment plant that is present in every ship. These treatments generally go on for about five days to make the sewage less harmful for the sea water.

The sewage treatment plant provides extra oxygen to the sewage. This increases the rate of decomposition of the sewage as the bacteria can work better in presence of more oxygen. The incubation temperature is kept at twenty degrees.

Coliform Count:

Another term that is commonly used in sewage treatment plant is coliform count. Coliform is an organism that is found in human intestines. If the count of this organism in high in waters, it can cause diseases like cholera and typhoid.

The IMO clearly states that the count of the coliform should be as less as 250 faecal per hundred mili litre. This level is very difficult to achieve as the crew in the ship can contribute very high amount of coliform.

The normal rate would be 125 billion per person in winters. The same figure can go up to 400 billion in the summers. This is exactly why the sewage has to be treated in the sewage treatment plant well so that the count of coliform can be bought down to a level that is acceptable by the IMO and safe for the sea water. If such high amount of coliform is let into the water it can cause great damage to the marine life.

The IMO also states the recommended level of solids that can be pumped out into the sea water. The different types of solids would be like dissolved solids which can be dissolved in a solution. Suspended solid are high in organic matter this is exactly why they can be filtered in a laboratory. The third type of solids is the settle able solids which can subside in a liquid over a period of time. The time taken to subside can be even an hour. The suspended level of raw sewage can be 50mg per litre as specified by the IMO. Residual disinfectant is very harmful for the sea water. The IMO states that if required even ultra-violet rays can be used to treat this. We have to ensure that the residual disinfectant is as low in the sewage as possible.

Biochemical Digestion:

Biochemical digestion of the sewage is another term that is used in sewage treatment plants. They have two type of processes one is aerobic and the other is anaerobic.

In the aerobic process oxygen is very essential. In this process the sewage is broken down and the results are safe. For example the breakage of the sewage in aerobic process will produce water, carbon dioxide, etc. This can also produce energy that synthesizes new bacteria.

Anaerobic process is one in which there is no need for free oxygen. This process is also called putrefaction. Breaking down of wastage in this process can create water, ammonia, hydrogen sulphide, etc. The by products that are produced in this process are highly toxic. The by products are also corrosive.

IMO Regulations:

We have been talking about IMO regulations but what exactly are they.

• Well basically IMO says that you can discharge sewage directly overboard if your distance from the nearest land is more than 12 nautical miles, provided it does not discolour the sea water or leads to any floating debris.
• Between 4 to 12 nautical miles from land, sewage can only be disposed off after treatment
• Anything less than 4 nautical miles you cannot dispose overboard.

An Actual Sewage Treatment Plant:

There are several types of sewage plants used on ships which vary as per their usage and working principle and one of the most common type is based on the principle of bio-decomposition of sewage and its working can be understood with the help of the following diagram

A typical biological sewage plant on a ship can be divided into three main chambers

• Aeration Chamber

Here the bacteria decompose the sewage and break it down

• Settling Chamber

Here the heavy particles settle down which are then later discharged to shore facilities while liquid passes to next stage

• Chlorine treatment Chamber

Here the liquid is purified by chlorination and then passed on for discharge as per conditions of previous section

Chemical Sewage Treatment Plant

There are other types of sewage treatment plants which work on the chemical decomposition of sewage rather than the use of bacteria. We will cover them separately in another post.

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