There are two methods used onboard to treat the sewage, chemical or biological methods. The chemical method is basically a storage tank which collects solid material for disposal in permitted areas or to a shore collection facility. The biological method treats the sewage so that it is acceptable for discharge inshore.
The most preferred sewage treatment in ship is biological sewage treatment i.e., using aerobic bacteria. So we will discuss only biological treatment plants here.
Principle
The basic principle of a biological sewage treatment plant is decomposition of raw sewage with the help of aerobic bacteria. This is done by aerating the chamber with fresh air. The extended aeration process provides a climate in which oxygen-loving bacteria multiply and digest the sewage, converting it into a sludge. These oxygen-loving bacteria are known as aerobic bacteria.
Working
A typical ship STP is having 4 chambers
> Primary Chamber
> Aeration Chamber
> Settling Chamber
> Chlorination Chamber
Primary Chamber
The row sewage enters the primary chamber via a coarse mesh filter where large solids are broken down. The advantage of breaking sewage in small particles is that it increases the area and a high number of bacteria can attack simultaneously to decompose the sewage.
Aeration Chamber
The sewage enters the aeration compartment where it is digested by aerobic bacteria and microorganisms. The aeration chamber is where the main biological action takes place. Here air blowers mounted on the outside of the unit oxygenate and stir the effluent and bacteria mix via a series of pipes and nozzles. The sewage remains in this aeration tank for some time.
The sewage is decomposed into carbon dioxide, water, and inorganic sewage
Settling Chamber
The sewage then flows into the settling compartment where the activated sludge is settled out. Any solids that settle out are returned via an air lift to the aeration chamber which ensures that they are fully broken down. This returned sludge contains the bacteria to digest the incoming sewage.
Chlorination Chamber
The clear liquid then overflows from the settling tank to the chlorination chamber, and the chlorinator disinfects the liquid. The chamber has float switches, which control the discharge pump, and a high level alarm.