STP

1. Sewage Treatment Plant
Prepared by
P.Mukherjee/ Rajan Bhandari

2. Tutorial Assignment
Q1. Describe with a simple sketch the operation of an aerobic
type sewage treatment plant commonly found onboard ship.
Q2. Explain the effect, if air supply into the aeration tank is
stopped.
Q3. Describe with a simple sketch the operation of zero
discharge type sewage treatment plant.
Q4. State the important international regulations regarding
shipboard sewage discharge.

3. Sewage Treatment Plant
Overview
 All modern ships are provided with sewage treatment plant to
comply with the international regulation of MARPOL 73/78
(Annex IV) to prevent pollution of sea by discharging human waste
 The lesson will provide the details of sewage treatment plants fitted
onboard
 Procedure of treating sewage prior disposal overboard

4. Learning Objectives
 Purpose
 Various types of sewage treatment plants
 Operating Principle of aerobic type
 Components & operation of aerobic type
 Components & operation of “No Discharge” or
“Zero Discharge” type
 International regulations

5. Sewage Treatment Plant
Why do we need sewage treatment plant ?
Raw sewage, if left untreated……
 is unsightly & smells
 becomes decomposed and source of diseases like
 dysentery
 typhoid
 diarrhoea etc……
due to the presence of coliform bacteria found in the intestines.
According to regulation coliform count in the effluent discharge in
restricted water should not exceed 1000 per 100 ml
Hence, sewage need to be treated and disinfected as well.

6. Types of Sewage Plant
 Biological or Aerobic type
 Chemical or Zero discharge type
 Retention system
 normal collection
 vacuum collection

7. Aerobic Type Sewage Plant
Operating Principle
 Bacteria existing naturally in water is cultivated
by supplying air continuously
 Upon digestion of raw sewage, bacteria
excretes inert sludge, having no detrimental
effect on the environment

8. Aerobic Type Sewage Plant
What happens if air supply is stopped ?
 Aerobic bacteria will die
 Anaerobic bacteria that does not require oxygen will strive
 It will also breakdown the raw sewage
 But liberates stinking H2S gas as by product
 Anaerobic bacteria is harmful for environment
 Can cause various diseases

9. Aerobic Type Sewage Plant
Major Components
 Aeration tank
 Settling tank
 Holding tank
 Air compressor
 Comminutor
 Discharge pump

10. Zero Discharge Sewage Plant
Discharging effluent overboard,
after disinfecting with chlorine,
is not environmental friendly as
it causes pollution by adding
chemicals to sea.
Zero discharge sewage plant
overcomes this problem by
retaining sewage onboard.

11. Shipboard Sewage Discharge
International Regulations
 Sewage plant must be fitted with a system to comminute and
disinfect the sewage
 The holding tank must be sufficient to meet the number of
crew onboard
 There must be an external line for shore discharge
 For commuted & treated sewage, it must be discharged at
more than 4 nautical miles from nearest land
 Speed of vessel during discharge must be more than 4 knots