A sewage system is used for draining sewage from bathrooms, laundries, showers, toilets, etc., to gray water tanks(sanitary tanks) or to treatment units. According to European designers, 70 liters/person/day is used for toilet waste (including flushing), and about 130 - 150 liters/person/day of washing water (baths, laundries, etc). According to US authorities, the flow from the toilet is 114 liters/person/day and twice the amount in case of washing water. 

Sewage treatment plant efficiency is based upon BOD(94%), suspended solids TDS(92%) and coli count. 


Water system onboard


 The onboard domestic water system comprises freshwater and saltwater systems. Fresh water is used for washing and drinking purposes. While salt water is used for sanitary purposes only. Both systems work by the same principle whereas the freshwater system has an additional calorifier(provides hot water). 


There are mainly two types of sewage treatment systems used onboard, which are: 

  1. Biological Treatment plant 

  2. Chemical treatment plant


Operation of Biological Treatment Plant


Working Principle 

Biological treatment plants work on the principle of aerobic breakdown process. There is a supply of oxygen by bubbling air through the water. Air contains Oxygen which helps in the multiplication of bacteria that decompose waste. This decomposed waste will be converted to clean effluent liquid which will be disinfected (using Chlorination) and discharged overboard.


Sewage system onboard ship



Operation 


The sewage treatment system usually has 4 compartments where the incoming waste coming from the toilet is passed through a coarse screen into the primary collection tank. The waste will remain in the primary collection tank until it is overflowed to the aeration section. There is a connection provided in the primary collection tank to pump out directly.


The aerobic bacteria break down the waste in the aeration compartment. This whole process happens in the presence of oxygen. Root-type compressors/blowers are used for providing oxygen supply. The air enters the aeration section through a fine bubble diffuser at the bottom. The diffuser is of porous material so that clean air is needed to prevent blockage. The bubbles also create turbulence which prevent settlement and good mixing. After prolonged aeration, the mixed liquid is displaced into the settlement tank which is in a conical shape. In the settlement tank the biological floc is formed. Activated sludge gravitates to the bottom. This sludge is withdrawn and returned to the aeration chamber to mix with the incoming wastes. 

From settling tank clean effluent is collected in the last compartment. In the last compartment disinfection using chlorine will occur. Discharge to overboard is done using discharge pump which are normally activated using 2 float switches. As the tank fills, the top switch starts the discharge pump. As the tank level decrease, low level switch gets activated which stops the pump and simultaneously opens a solenoid valve in the water supply line to the hypochlorinator. After passing through the chlorinator the clean effluent is completely sterilized and ready for pumping overboard.