Sewage Treatment Plant on Ships: Working Principle, Construction, Operation, Maintenance, Troubleshooting and MARPOL Requirements
A Sewage Treatment Plant (STP) is a critical pollution-prevention system installed onboard ships to collect, treat and discharge sewage in accordance with applicable environmental regulations. Unlike a simple sewage holding tank, an STP uses a combination of mechanical, biological and disinfection processes to reduce organic matter, suspended solids and harmful microorganisms before the treated effluent is discharged or transferred ashore.
On many modern vessels, the STP is integrated with a vacuum toilet and sewage collection system. A typical arrangement consists of a vacuum collecting unit followed by a biological treatment plant incorporating aeration, settling and disinfection.
The IMO regulates sewage pollution from ships under MARPOL Annex IV, which covers sewage-treatment equipment, discharge requirements, surveys, certification and reception facilities.
What is a Sewage Treatment Plant?
A sewage treatment plant is a shipboard system designed to receive sewage and wastewater, biologically treat the organic matter, separate activated sludge, disinfect the clarified water and produce an effluent suitable for discharge where permitted.
In a conventional biological marine STP, the basic treatment sequence is:
Collection → Aeration → Biological treatment → Settling → Disinfection → Discharge
The biological stage is particularly important because microorganisms use oxygen to break down biodegradable organic matter.
A useful engineering representation is:
Organic matter + Oxygen + Microorganisms → Carbon dioxide + Water + Biomass
The exact process and equipment arrangement varies between manufacturers and approved plant designs.
What are the Main Sources of Sewage and Wastewater on a Ship?
Sewage generated onboard can originate from accommodation spaces, sanitary facilities, medical facilities and other domestic services.
Black Water
Black water is wastewater containing human excreta. Typical sources include:
Toilets
Urinals
Medical or hospital sanitary facilities
Grey Water
Grey water is wastewater generated from sources such as:
Showers
Wash basins
Baths
Laundry
Galley sinks
Dishwashing facilities
The precise streams connected to an STP depend on the ship's design and the approved sewage system.
IMO's MEPC.227(64) defines grey water separately from drainage originating from toilets, urinals and hospitals.
Why is a Sewage Treatment Plant Required on Ships?
Untreated sewage can introduce pathogens and organic pollutants into the marine environment. It can create health hazards, consume dissolved oxygen and produce visible pollution, particularly in coastal waters.
Therefore, international regulations control when and how sewage may be discharged from ships.
Under MARPOL Annex IV, ships within its scope must have an approved sewage treatment plant, an approved comminuting and disinfecting system, or an approved sewage holding tank, as applicable.
Basic Arrangement of a Marine Sewage Treatment System
A typical biological arrangement can be understood as:
Vacuum Toilets
↓
Vacuum Piping
↓
Sewage Collection Tank
↓
Transfer / Ejector System
↓
Aeration Chamber I
↓
Aeration Chamber II
↓
Settling Chamber
↓
Disinfection Chamber
↓
Discharge Pump
↓
Permitted Overboard Discharge / Shore Reception Facility
At the same time, settled activated sludge is returned to the biological treatment section.
What is a Vacuum Toilet System?
Vacuum toilets transport sewage using a pressure differential rather than relying entirely on gravity.
This arrangement offers several advantages for ships:
Low flushing-water consumption
Smaller sewage piping
Greater flexibility in pipe routing
Effective sewage transport over relatively long pipe runs
Suitability for ships where conventional gravity drainage is difficult
The vacuum system is particularly useful because ships have limited space and extensive accommodation areas distributed throughout the vessel.
What is the Vacuum Collecting Unit?
The vacuum collecting unit is responsible for collecting sewage and maintaining the vacuum required for the toilet system.
Its principal functions are:
Generating vacuum for vacuum toilets
Discharging sewage from the collection tank
Controlling automatic operation
Providing alarms and indications
Providing signals to the engine monitoring system
A typical arrangement includes:
Sewage atmospheric tank
Ejector
Ejector pump
Vacuum/pressure switch
Vacuum gauge
Motor-operated discharge valve
High-level switch
Low-level switch
Shut-off valve
Control panel
How is Vacuum Generated?
The vacuum is produced using an ejector and ejector pump.
The ejector pump circulates motive water through the ejector. The resulting flow creates a low-pressure region, producing vacuum in the connected vacuum piping.
The vacuum exists in the piping while the sewage collection tank itself remains at atmospheric pressure.
This is an important oral-examination point.
Is the sewage collection tank itself under vacuum?
Not necessarily. In this arrangement, the vacuum is maintained in the vacuum piping, while the sewage collection tank is atmospheric.
How Does the Vacuum Switch Control the Pump?
The vacuum switch automatically starts and stops the ejector pump according to the vacuum level.
In the LER3D arrangement described in the supplied material:
Pump starts at approximately –35 kPa
Pump stops at approximately –60 kPa
Therefore:
Vacuum decreases → Pump starts
↓
Vacuum increases → Pump continues operating
↓
Required vacuum reached → Pump stops
The actual set points must always be checked against the manufacturer's manual because they can vary between systems.
What Happens During Sewage Discharge?
The ejector pump and motor-operated discharge valve are used for the discharge operation.
The discharge can be controlled:
Manually
Automatically by tank level
Automatically by timer
In automatic level control, the high- and low-level switches determine when the discharge operation starts and stops.
How Does Level-Controlled Discharge Work?
When the sewage level rises and activates the high-level switch, the discharge operation begins.
The pump continues transferring sewage until the level falls sufficiently to operate the low-level switch.
The sequence can therefore be remembered as:
High level → Discharge starts
Low level → Discharge stops
This prevents tank overflow while avoiding unnecessary continuous operation of the discharge equipment.
What is the Biological Sewage Treatment Plant?
The biological section is the heart of the STP.
It consists of:
Aeration Chamber I
Aeration Chamber II
Settling Chamber
Disinfection Chamber
The complete biological treatment sequence is:
Aeration → Aeration → Settling → Disinfection
What Happens in the Aeration Chambers?
Aeration supplies oxygen required by aerobic microorganisms.
Incoming sewage contains biodegradable organic matter. Microorganisms use this material as a source of energy and carbon while oxygen supports their metabolism.
Air is supplied by an air blower and distributed through air distributors or diffusers.
The aeration process therefore provides:
Oxygen
Mixing
Biological activity
Improved breakdown of organic matter
Why are Two Aeration Chambers Used?
The first aeration chamber initiates the biological treatment process.
The wastewater then enters the second aeration chamber, where biological treatment continues.
The two-stage arrangement provides additional biological contact time and allows the microorganisms to continue reducing biodegradable organic matter before the wastewater reaches the settling stage.
What is Activated Sludge?
Activated sludge is a mixture of microorganisms and suspended biological solids formed during the aerobic treatment process.
It contains the microbial population responsible for degrading organic matter.
The microorganisms need:
Organic substrate
Oxygen
Suitable environmental conditions
Adequate retention time
A portion of the activated sludge is returned to the aeration chamber to maintain the required biological population.
What is the Function of the Air Blower?
The air blower is one of the most important machines in a biological STP.
Its main function is to supply air to the aeration chambers.
The supplied air provides oxygen to microorganisms and also helps maintain proper mixing of the biological solids.
If aeration is lost for a prolonged period:
Dissolved oxygen decreases
Aerobic biological activity decreases
Treatment efficiency deteriorates
Sludge characteristics may change
Odour may develop
Final effluent quality may deteriorate
Therefore, blower operation should be monitored carefully.
What is the Function of the Settling Chamber?
After biological treatment, the mixed liquor enters the settling chamber.
Here, the activated sludge is separated from the treated water primarily through gravity settling.
The heavier biological solids settle toward the bottom.
The clarified water remains above the settled sludge and flows toward the disinfection chamber.
The settling process is therefore essential for producing clear effluent.
Why is Sludge Returned to the Aeration Chamber?
The sludge contains microorganisms that are required for continued biological treatment.
Therefore, a portion of the settled sludge is returned to the aeration chamber.
In the described arrangement, an air-driven ejector is used for sludge return.
The basic process is:
Settling chamber → Activated sludge → Air ejector → Aeration chamber
This maintains an adequate biological population in the treatment system.
What Happens in the Disinfection Chamber?
The clarified water leaving the settling chamber may still contain microorganisms that must be reduced before discharge.
It therefore enters the disinfection chamber.
Depending on the STP design, disinfection may be achieved using:
Chlorine-based chemicals
Ultraviolet radiation
Other approved technologies
In the system described in the supplied LER3D material, a chlorine-based disinfectant is introduced using a dosing pump.
How Does Chlorine Disinfection Work?
The dosing pump injects disinfectant into the clarified water to reduce harmful microorganisms, including coliform bacteria.
Correct dosage is important.
Under-dosing can result in inadequate disinfection.
Over-dosing can result in excessive chemical consumption and may affect effluent characteristics.
The chemical concentration and dosing rate should always be maintained according to the approved system and manufacturer's instructions.
Can UV be Used Instead of Chlorine?
Yes.
Some marine STPs use UV disinfection.
UV radiation damages the genetic material of microorganisms, preventing them from reproducing effectively.
The advantage is that UV treatment does not require conventional chlorine dosing.
However, UV performance depends on factors such as:
Water clarity
Suspended solids
UV intensity
Lamp condition
Quartz-sleeve cleanliness
Therefore, a UV system also requires proper maintenance.
What Happens to the Treated Effluent?
After disinfection, the treated effluent is transferred by the discharge pump.
Depending on the vessel's approved arrangement and applicable regulations, it can be:
Discharged overboard where permitted
Retained onboard
Transferred to a shore reception facility
The discharge must comply with MARPOL Annex IV, flag-state requirements, coastal-state requirements, port regulations and the vessel's SMS.
What happens to the sludge produced by the STP?
Biological treatment produces sludge.
Part of it is returned to the aeration chamber to maintain the biological population.
Excess or mineralized sludge must be managed according to the approved shipboard arrangement. It may be transferred to:
A sludge holding tank
Shore reception facilities
Another approved disposal arrangement
The actual disposal route must be based on the vessel's approved system and applicable regulations.
What is MARPOL Annex IV?
MARPOL Annex IV is the section of MARPOL dealing specifically with prevention of pollution by sewage from ships.
It covers:
Sewage discharge requirements
Sewage treatment equipment
Holding arrangements
Port reception facilities
Surveys
Certification
The current IMO framework requires ships within the scope of Annex IV to have an approved sewage treatment plant, approved comminuting and disinfecting system, or sewage holding tank, as applicable.
What are the Sewage Discharge Requirements?
No.
MARPOL Annex IV places restrictions on sewage discharge.
According to the IMO, untreated sewage may generally be discharged only when the applicable conditions are met, including the ship being en route and being more than 12 nautical miles from the nearest land, with the applicable requirements for discharge rate and ship speed. Comminuted and disinfected sewage has different requirements, including a distance of more than 3 nautical miles under the specified conditions.
An approved STP provides a separate regulatory route for discharge, but the ship must use an approved system and comply with applicable operational and local requirements.
Never treat a generic distance figure as permission to discharge in every location. Port, coastal-state, flag-state and Special Area restrictions may apply.
What is MEPC.227(64)?
IMO Resolution MEPC.227(64) contains the 2012 Guidelines on Implementation of Effluent Standards and Performance Tests for Sewage Treatment Plants.
The guidelines address the design, installation, performance and testing of sewage treatment plants required under MARPOL Annex IV.
The guidelines provide a standardized framework for evaluating the performance of approved sewage treatment plants.
They also require appropriate familiarization of ship personnel and emphasize that routine and repair maintenance should be defined by the manufacturer and recorded.
What Effluent Parameters are Important?
Important parameters include:
Biochemical Oxygen Demand
Chemical Oxygen Demand
Suspended solids
Thermotolerant coliform bacteria
pH
Where applicable, nitrogen and phosphorus
The MEPC.227(64) guidelines establish effluent standards for approved plants.
For passenger ships intending to discharge treated sewage in the Baltic Sea Special Area, additional nitrogen and phosphorus removal requirements apply.
What is the Baltic Sea Special Area Requirement?
The Baltic Sea is currently the only Special Area designated under MARPOL Annex IV.
Passenger ships operating in the Special Area are subject to stricter sewage discharge requirements. An approved sewage treatment plant must meet the applicable requirements, including additional nutrient-removal standards where required.
This is particularly important because nitrogen and phosphorus can contribute to eutrophication in sensitive marine environments.
What Parameters Should a Marine Engineer Monitor?
A marine engineer should regularly monitor:
Vacuum
Ensures proper operation of vacuum toilets and sewage piping.
Tank Level
Confirms correct collection and discharge operation.
Blower Operation
Ensures continuous oxygen supply.
Air Supply
Indicates whether adequate aeration is available.
Sludge Condition
Provides an indication of biological and settling performance.
Chemical Dosing
Ensures adequate disinfection.
Pump Performance
Abnormal pressure, current, noise or vibration can indicate developing faults.
Effluent Appearance
Cloudy, excessively coloured or abnormal effluent can indicate treatment problems.
What are the Common Causes of Low Vacuum?
Possible causes include:
Leakage in vacuum piping
Faulty toilet valve
Ejector problem
Ejector pump problem
Insufficient motive water
Faulty vacuum switch
Damaged piping
Incorrect valve position
A systematic approach should begin with checking the vacuum gauge, pump operation, valve positions and visible leakage before dismantling equipment.
Why Does an STP Produce Bad Odour?
A healthy aerobic biological process should not normally produce severe foul odours.
Possible causes include:
Insufficient aeration
Blower failure
Excessive sludge accumulation
Anaerobic conditions
Excessive retention time
Biological imbalance
Poor ventilation
The first engineering checks should include blower operation, air supply, sludge condition and tank condition.
What Causes Poor Effluent Quality?
Poor effluent quality can result from:
Insufficient aeration
Low biological activity
Excessive hydraulic loading
Excessive sludge
Poor settling
Sludge carryover
Chemical dosing failure
UV system failure
Incorrect sludge return
Excessive cleaning chemicals entering the plant
The engineer should determine whether the problem originates in the biological stage, settling stage, disinfection stage or discharge system.
What Causes Excessive Foaming?
Foaming can occur because of:
Changes in wastewater composition
Surfactants
Biological imbalance
Excessive biological growth
Aeration conditions
Sludge problems
The correct approach is to identify and eliminate the underlying cause rather than simply attempting to remove the foam.
What Happens if the STP is Left Without Aeration?
Without sufficient oxygen, aerobic microorganisms cannot maintain normal biological activity.
The result can be:
Blower failure → Reduced oxygen → Reduced biological activity → Poor treatment → Poor effluent quality
If the plant remains without aeration for an extended period, the biological condition may deteriorate significantly.
The manufacturer's instructions should be followed when restoring a plant after prolonged shutdown.
What are the Important STP Maintenance Activities?
Routine maintenance should include inspection and testing of:
Air blower
Air filters
Air diffusers
Pumps
Ejectors
Vacuum switch
Level switches
Motor-operated valves
Chemical dosing pump
UV system where fitted
Control panel
Alarm system
Pipes and valves
Sludge system
IMO's MEPC.227(64) guidelines specifically emphasize that routine maintenance should be defined by the manufacturer and that maintenance and repair activities should be recorded.
What Daily Checks Should an Engineer Carry Out?
A practical daily inspection can include:
Check vacuum level
Check sewage tank level
Check blower operation
Check air distribution
Check pump operation
Check chemical level
Check dosing pump
Check alarms
Check for leakage
Observe effluent condition
Check abnormal noise and vibration
Verify that automatic sequences are functioning correctly
These checks should be incorporated into the vessel's PMS and operating procedures.
What Safety Precautions are Required?
Sewage systems present both biological and mechanical hazards.
Appropriate PPE should be used when working around sewage equipment.
Particular care is required when handling disinfecting chemicals.
Most importantly, sewage tanks and treatment chambers can constitute enclosed spaces.
Potential hazards include:
Oxygen deficiency
Hydrogen sulphide
Methane
Other toxic gases
Engulfment
Slipping hazards
Can an engineer enter a sewage tank simply because it appears empty?
Answer: No.
An apparently empty sewage tank can still contain a hazardous atmosphere.
Entry must only take place under the vessel's approved enclosed-space entry procedure, including atmosphere testing, ventilation, communication, standby arrangements and other required controls.
Why is Hydrogen Sulphide Dangerous?
Hydrogen sulphide can be generated when organic material undergoes anaerobic decomposition.
It is highly toxic and can rapidly impair the sense of smell, meaning that smell cannot be relied upon as a warning method.
Therefore, atmosphere testing is essential before entering sewage tanks or other potentially hazardous enclosed spaces.
What Happens if the Sewage Treatment Plant Fails?
The response depends on the vessel's approved sewage arrangement, but generally the crew should:
Stop unauthorized sewage discharge
Isolate the defective equipment where necessary
Use the approved alternative sewage arrangement
Retain sewage in the designated holding system where available
Inform the responsible engineer/officer
Carry out troubleshooting and repair
Record the failure and corrective action
Follow company SMS and reporting requirements
The vessel must not simply bypass the treatment system and discharge sewage without confirming that such discharge is legally permitted.
How is a Marine STP Different from a Conventional Gravity Sewage System?
A vacuum sewage system uses pressure differential to transport sewage.
A conventional gravity system relies primarily on gravitational flow.
Parameter | Vacuum System | Gravity System |
Sewage transport | Vacuum/pressure differential | Gravity |
Flush water | Generally low | Generally higher |
Pipe size | Generally smaller | Generally larger |
Routing flexibility | High | More dependent on gravity |
Vacuum equipment | Required | Not normally required |
Suitability onboard | Very useful | Also used depending on design |
The exact arrangement depends on the ship's design.
How Should the STP be Operated During Port Stay?
No.
Port and coastal regulations can impose restrictions beyond the general MARPOL framework.
During port stay, sewage may need to be:
Retained onboard
Stored in a holding tank
Delivered to a shore reception facility
Treated and discharged only when specifically permitted
The vessel's SMS, port requirements, flag-state requirements and local environmental regulations must always be consulted.
Important Oral Examination Questions
What is the purpose of an STP?
To treat ship-generated sewage so that the resulting effluent meets the applicable environmental and regulatory requirements before permitted discharge.
What is the principle of biological sewage treatment?
Aerobic microorganisms consume biodegradable organic matter in the presence of oxygen.
What are the main chambers of the described biological STP?
Two aeration chambers, one settling chamber and one disinfection chamber.
What is the function of the aeration chambers?
To supply oxygen and provide conditions for aerobic biological treatment.
Why is activated sludge returned?
To maintain an adequate population of microorganisms in the biological treatment process.
What is the function of the settling chamber?
To separate activated sludge from clarified treated water.
What is the function of the disinfection chamber?
To reduce harmful microorganisms in the clarified effluent before permitted discharge.
What is used for chemical disinfection?
A disinfectant may be injected using a dosing pump; chlorine-based systems are one common arrangement.
Can UV be used?
Yes, depending on the approved STP design.
What controls the vacuum pump?
A vacuum/pressure switch according to the system's preset vacuum levels.
What starts level-controlled discharge?
The high-level switch.
What stops level-controlled discharge?
The low-level switch.
What is MARPOL Annex IV?
The MARPOL annex dealing with prevention of pollution by sewage from ships.
What is MEPC.227(64)?
The IMO 2012 guidelines covering effluent standards and performance testing for sewage treatment plants.
What is the most important factor for aerobic biological treatment?
Adequate oxygen supply and healthy biological activity.
What happens if the blower fails?
Aeration decreases, biological activity deteriorates and treatment performance can fall.
Why is sludge recirculated?
To maintain the required concentration of active microorganisms in the biological treatment process.
Why is sewage tank entry dangerous?
Because the tank can contain toxic gases, oxygen-deficient atmospheres and other enclosed-space hazards.
