APPLICATION
Sewage Treatment Effluent
As with all discharge effluents we need to confirm what we are discharging is within environmental limits. A measure for sewage effluent is turbidity.
Marine vessels must treat sewage before it can be discharged overboard. As with all regulated discharge streams, the effluent must be confirmed to be within the environmental limits set by MARPOL Annex IV before discharge can proceed. For sewage effluent, the primary compliance measurement is turbidity — a measure of the clarity of the treated effluent. Rivertrace’s SMART Turbidity monitor is used to measure sewage treatment plant effluent turbidity in real time, confirming compliance before discharge.
| Key thing to remember
Turbidity is the primary performance indicator for marine sewage treatment plant effluent. Under MEPC.227(64), a type-approved sewage treatment plant must produce effluent that meets defined turbidity standards. The SMART Turbidity monitor measures this continuously at the treatment plant outlet — providing the real-time compliance data that confirms effluent meets the standard before it is discharged overboard. |
What Is Marine Sewage Treatment Effluent?
Marine sewage treatment effluent is the treated output from a ship’s sewage treatment plant (STP). Ships generate sewage from crew and passenger accommodation, galleys and laundry facilities. This sewage must be treated before it can be discharged overboard — discharging untreated or inadequately treated sewage is a MARPOL Annex IV violation.
A marine sewage treatment plant processes sewage through biological and physical treatment stages to remove solids, reduce biological oxygen demand and disinfect the effluent. The treated effluent must meet the performance standards set by MEPC.227(64) before it can be discharged. Turbidity — the cloudiness of the effluent caused by suspended solids and particles — is one of the key performance indicators measured to confirm that the treatment plant is functioning correctly.
Marine sewage discharge is regulated separately from bilge water discharge under a different MARPOL Annex. For an overview of how MARPOL regulates different discharge streams on marine vessels, see the MARPOL regulation review.
What Regulations Govern Marine Sewage Discharge?
Marine sewage discharge is governed by MARPOL Annex IV and the associated resolutions adopted by the IMO Marine Environment Protection Committee. The key regulatory instruments are:
MARPOL Annex IV
MARPOL Annex IV sets the conditions under which sewage may be discharged overboard from ships. It prohibits the discharge of untreated sewage within specified distances from land and in MARPOL special areas, and requires ships of 400 gross tonnage and above — and smaller ships certified to carry more than 15 persons — to be equipped with either an approved sewage treatment plant, a sewage comminuting and disinfecting system, or a sewage holding tank.
MEPC.227(64) Performance Standards
IMO resolution MEPC.227(64) sets the performance standards that type-approved sewage treatment plants must meet. These standards specify limits for biochemical oxygen demand, suspended solids, pH, total coliforms and turbidity in the treated effluent. A sewage treatment plant must be certified to MEPC.227(64) standards to be accepted for use under MARPOL Annex IV.
MARPOL special areas
In MARPOL special areas — currently the Baltic Sea and potentially expanding to other regions — stricter restrictions on sewage discharge apply. In the Baltic Sea Special Area, passenger ships must meet enhanced nitrogen and phosphorus standards in addition to the standard effluent quality requirements, and must not discharge sewage unless equipped with an approved treatment plant that meets the enhanced standards.
What Are the Discharge Conditions Under MARPOL Annex IV?
The conditions under which treated sewage may be discharged overboard under MARPOL Annex IV are set out in the table below.
| Discharge condition | Requirement |
| Distance from land | More than 3 nautical miles from the nearest land (for comminuted and disinfected sewage); more than 12 nautical miles for untreated sewage |
| Vessel condition | Proceeding en route at a speed of at least 4 knots |
| Treatment requirement | Sewage must be comminuted and disinfected using an approved system; or treated using an approved sewage treatment plant |
| Discharge rate | At a moderate rate — not in one large concentrated release |
| Turbidity limit | Effluent from a type-approved sewage treatment plant must meet a turbidity standard — measured in NTU — as part of the type approval performance criteria |
| Applicable standard | MARPOL Annex IV and MEPC.227(64) — Performance Standards for Sewage Treatment Plants |
| Sewage discharge in special areas
In MARPOL special areas — including the Baltic Sea — the discharge of sewage is subject to stricter requirements than in open sea areas. Passenger ships operating in the Baltic Sea Special Area must use type-approved sewage treatment plants meeting enhanced nutrient reduction standards. Operators should confirm the applicable requirements for each operating area before the voyage. |
Why Is Turbidity Used to Monitor Sewage Treatment Effluent?
Turbidity is a measure of the cloudiness or haziness of a liquid caused by suspended particles. In sewage treatment effluent, turbidity reflects the residual suspended solids content of the treated water — including biological matter, colloidal particles and other undissolved material that was not fully removed during treatment.
Turbidity is used as a primary performance indicator for sewage treatment plant effluent because:
- It is a reliable proxy for the overall quality of the treated effluent — high turbidity indicates that the treatment process is not performing correctly and that suspended solids content is elevated
- It can be measured continuously and in real time using an inline sensor — unlike biological oxygen demand or total coliforms, which require laboratory analysis
- It is specified as one of the performance parameters in MEPC.227(64) — turbidity in the treated effluent must be below the standard to confirm that the treatment plant is meeting its type approval specification
- It provides an early warning of treatment plant malfunction — a sudden increase in turbidity at the effluent point indicates a problem with the biological or filtration stage of the treatment process before it escalates
Measuring turbidity at the outlet of the sewage treatment plant provides a continuous, real-time check that the plant is functioning within its approved performance parameters before the effluent is released overboard.
How Does the Rivertrace SMART Turbidity Monitor Work?
The SMART Turbidity monitor accurately analyses the cloudiness of the treated effluent and outputs the turbidity value in Nephelometric Turbidity Units (NTU) in real time. It is installed at the outlet of the sewage treatment plant and continuously measures the turbidity of the effluent as it passes through the measurement cell.
The monitoring system works as follows:
- A sample of the treated sewage effluent flows continuously through the measurement cell of the SMART Turbidity monitor
- The monitor measures the amount of light scattered by suspended particles in the effluent — higher turbidity means more scattering and a higher NTU reading
- The turbidity value is displayed in real time and transmitted to the vessel’s monitoring system
- If turbidity rises above the alarm set point — indicating that the treatment plant effluent quality has deteriorated — the monitor activates an alarm to alert the crew
- Where connected to the sewage treatment plant control system, a high turbidity alarm can prevent discharge from proceeding until the treatment plant performance is restored
- All readings and alarm events are logged for the vessel’s environmental compliance record
Which Vessels Are Required to Monitor Sewage Treatment Effluent?
The requirement to fit a sewage treatment plant and monitor its effluent applies broadly to commercial vessels under MARPOL Annex IV. The table below summarises the vessel types to which the requirements most commonly apply.
| Vessel type | When MARPOL Annex IV applies |
| Passenger vessels | All passenger ships above 400 GT or certified to carry more than 15 persons; particularly large cruise ships and ferries operating in coastal and inland waters |
| Cargo vessels | All ships of 400 GT and above on international voyages; vessels of any size in MARPOL special areas |
| Offshore support vessels | Vessels of 400 GT and above operating in offshore fields where discharge regulations apply |
| Naval and government vessels | Subject to flag state requirements which typically mirror MARPOL Annex IV for vessels of equivalent size |
| Yachts and recreational vessels | Vessels of any size in MARPOL special areas — the Baltic Sea, Black Sea and Antarctic area — where sewage discharge is prohibited |
For the full range of Rivertrace products used across marine vessel discharge monitoring applications — including bilge water, produced water, exhaust scrubbers and sewage effluent — see the marine industry page.
What Is the Difference Between Sewage Effluent Monitoring and Wastewater Discharge Monitoring?
Sewage effluent monitoring and wastewater discharge monitoring are related but distinct applications. Sewage effluent from a marine sewage treatment plant is monitored for turbidity under MARPOL Annex IV and MEPC.227(64). Wastewater discharge — which may include grey water from sinks, showers and galleys, and other process water streams — is subject to different regulatory requirements depending on the vessel type and operating location. For more on wastewater discharge monitoring, see the wastewater discharge application page.
The key difference is the measurement parameter. Sewage treatment effluent is primarily monitored by turbidity, which confirms that suspended solids have been adequately removed by the treatment process. Oil-in-water monitoring — used for bilge water, cooling water and produced water discharge — measures oil concentration rather than turbidity.
What Should Ship Operators Check to Maintain Sewage Effluent Monitoring Compliance?
For ship operators and technical superintendents responsible for sewage treatment plant compliance, maintaining effluent monitoring requires attention to:
- Confirm the sewage treatment plant is type-approved to MEPC.227(64) and that the approval certificate is held onboard
- Confirm the SMART Turbidity monitor is installed at the correct point in the effluent line and functioning correctly
- Verify that the turbidity alarm set point is correctly configured for the treatment plant type approval standard
- Confirm that turbidity readings and alarm events are being logged correctly in the vessel’s compliance record
- Carry out routine maintenance of the measurement cell — fouling of the optical surfaces by biological material in the effluent can cause inaccurate readings if the cell is not cleaned at the intervals specified in the manufacturer’s maintenance schedule
- Ensure crew responsible for the sewage treatment plant understand the turbidity alarm response procedure and the conditions under which discharge must be suspended
Frequently Asked Questions
What is marine sewage treatment effluent?
Marine sewage treatment effluent is the treated output from a ship’s sewage treatment plant. Raw sewage from crew and passenger accommodation is processed through biological and physical treatment stages to remove solids, reduce biological oxygen demand and disinfect the water before it is discharged overboard. The effluent must meet the performance standards set by MEPC.227(64) to comply with MARPOL Annex IV.
Why is turbidity the key measurement for sewage effluent monitoring?
Turbidity reflects the residual suspended solids content of the treated effluent — a reliable indicator of overall treatment plant performance. Unlike biological oxygen demand or coliform count, turbidity can be measured continuously and in real time using an inline sensor. It is also specified as a performance parameter in MEPC.227(64), making it the primary continuous monitoring measurement for sewage treatment plant effluent compliance.
What regulations govern marine sewage discharge?
Marine sewage discharge is governed by MARPOL Annex IV and the performance standards set out in MEPC.227(64). Ships of 400 gross tonnage and above on international voyages must be fitted with an approved sewage treatment plant, comminuting and disinfecting system or holding tank. The treated effluent must meet turbidity, suspended solids, BOD and coliform standards before discharge.
Which Rivertrace product monitors sewage treatment effluent?
The SMART Turbidity monitor measures turbidity in NTU at the sewage treatment plant outlet in real time. It provides continuous monitoring of effluent quality, alarms when turbidity exceeds the set point, and logs all readings and alarm events for the compliance record.
Does the Baltic Sea Special Area impose stricter sewage discharge requirements?
Yes. In the Baltic Sea Special Area, passenger ships are subject to enhanced sewage discharge requirements under MARPOL Annex IV, including limits on nitrogen and phosphorus content in addition to the standard MEPC.227(64) effluent quality parameters. Passenger ships operating in the Baltic Sea must be fitted with a type-approved treatment plant that meets these enhanced standards.