Important Terms

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 microorganisms in the stabilization of organic matter.


The results are generally expressed as the amount of oxygen taken by a one-litre sample (diluted with aerated water) when incubated at 20 degrees for five days. BOD of raw sewage is 300-600 mg/litre. IMO recommends BOD of less than 25 Qi/Qe mg/l (Updated by MEPC159) and the chemical oxygen demand (COD) does not exceed 125 Qi/Qe mg/l. The test method standard should be ISO 5815 1:2003 for BOD5 without nitrification and ISO 15705:2002 for COD, or other internationally accepted equivalent test standards. after treatment through the sewage treatment plant.


Coliform Count Coliform is a type of organism which is present in the human intestine and is recognized as an indicator organism of sewage pollution. The presence of these organisms in water is an indication of the pathogen (pathogen count), which are diseases causing bacteria responsible for cholera, dysentery, typhoid, etc.


The number of coliform organisms present in sewage on ships is very large, with each person contributing around 125 billion in winter and 400 billion in summer. IMO recommends a faecal coliform count of less than 100 thermotolerant coliforms / 100 ml (Updated by MEPC159) of effluent after treatment.


Dissolved solids Solids that are dissolved in the solution


Suspended solids Solids are physically suspended in sewage that can be removed by laboratory filtration and are relatively high in organic matter.


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


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 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.

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