Sewage Treatment Plant (STP) onboard Ships Explained

Sewage treatment plants on ships are essential for managing and treating wastewater generated by the crew and passengers while at sea. These systems are designed to ensure that sewage is properly treated and does not harm the marine environment.

Discarding sewage produced onboard on a ship is one of the few tasks on a ship which should be taken utmost care of if one wants to save him and his shipping company from heavy fine. The sewage generated on the ship cannot be stored on the ship for a very long time and it, for this reason, it has to be discharged into the sea.

Though sewage can be discharged into the sea, we cannot discharge it directly overboard as there are some regulations regarding discharging of sewage that needs to be followed. Sewage on the sea is generally the waste produced from toilets, urinals, and WC scuppers. The rules say that the sewage can be discharged into the seawater only after it is treated and the distance of the ship is 4 nautical miles from the nearest land.

But if the sewage is not treated this can be discharged 12 nautical miles away from the nearest land. Also, the discharged sewage should not produce any visible floating solids nor should it cause any discoloration of surrounding water. The details of the sewage discharge regulations can be found in MARPOL Annex IV.

Marine Sewage Treatment Plant On Ship Principle and Working

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

1.  Primary Chamber
2. Aeration Chamber
3. Settling Chamber
4. Chlorination Chamber

1. Primary Chamber

The raw 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. 

2. 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

3. 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.

4. 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.

A Quick Explanation Regarding Sewage Treatment Plant Onboard Ships

Why Are Sewage Treatment Plants Required?

In the simple context, sewage treatment plants are required not just on ships but should be on any establishments near the sea or on the sea.

STP is required to prevent pollution and destruction of marine life.

Sewage disposal on board is governed by several regulations internationally, that intend to protect the environment and marine life loss.

Sewage treatment plant components

• Screen Filter – A screen filter is often installed on the first chamber to prevent solid waste such as tissues, napkins, cigarette butts and etc, which people usually dump on urinals, toilets, and shower drains.
• Macerator pump – The macerator pump physically breaks the solid entering the first chamber into smaller pieces so the biological bacteria could easily decompose the organic components of the wastewater.
• Chlorinator – The chlorinator pumps chlorine to the final chamber at a preset volume and chlorine percentage.
• Discharge pump – The discharge pump sucks the treated water and releases it to the sea.
• Blower/air compressor – The blower usually a rotary vane pump provides oxygen to the second chamber to help the organic bacteria to do its job of breaking carbon components into water and carbon dioxide
• Primary chamber – The foremost tank on a sewage treatment plant where it collects the raw wastewater and comminuted solid wastes
• Aeration chamber – The tank where decomposition takes place by the reaction caused by the organic eating bacteria and oxygen introduction
• Settling chamber – Is where the separation of comminuted solid waste is separated from the partially treated wastewater
• Collection chamber – The final stage of treatment where wastewater is chlorinated and dechlorinated before flushing into the ocean

Regulation Governing Sewage Treatment Plant

The main body which governs sewage disposal at sea is the MARPOL Annex IV: Prevention of Pollution by Sewage from Ships.

Enforced by the International Maritime Organization (IMO) the annex entered into force on September 27, 2003. A revision is adopted on April 1, 2004, which has been fully enforced by August 1, 2005.

Mainly, the discharge of sewage shall be;

• Comminuted and disinfected sewage: at least 3 nautical miles from the nearest land, or
• Sewage not comminuted or disinfected: at least 12 nautical miles from the nearest land. Sewage that has been stored in holding tanks shall be discharged at a moderate rate when the ship is en route and proceeding at not less than 4 knots (the discharge rate shall be approved by the Administration based upon standards developed by the Organization).
• The ship has in operation an approved sewage treatment plant which has been certified by the Administration

Annex 4 of the MARPOL Convention has tons of rules and regulations, a law book, that has to be followed by ships to uniformly protect the environment from the hazards of sewage disposal at ports, open sea, and even special areas.  Violation of the said rules has it’s on weigh of consequence and punishment to the offender.

Here’s an overview of sewage treatment plants on ships:

1. Collection and Segregation: Sewage generated on ships is collected from various sources, including toilets, showers, and kitchen sinks. The sewage may be segregated into black water (from toilets) and grey water (from showers, sinks, and other non-toilet sources) for more efficient treatment.
2. Primary Treatment: In the primary treatment stage, coarse screening and sedimentation processes are used to remove larger solids, such as toilet paper and debris, from the sewage. This helps prevent clogging and damage to the treatment equipment.
3. Biological Treatment: After primary treatment, the sewage is typically subjected to biological treatment. This involves the use of aerobic bacteria (similar to the aerobic bacteria used in municipal wastewater treatment) to break down organic matter in the sewage. Oxygen is supplied to support the growth of these bacteria. Biological treatment helps reduce the concentration of organic pollutants in the sewage.
4. Secondary Treatment: Depending on the ship’s design and capacity, a secondary treatment step may be employed to further treat the sewage effluent. Secondary treatment can include additional biological processes or chemical treatments to enhance the removal of contaminants.
5. Disinfection: To ensure that the treated sewage effluent is free from harmful microorganisms, disinfection is often employed. Common methods of disinfection include chlorination or ultraviolet (UV) radiation. This step helps reduce the risk of disease transmission and minimizes the impact on marine ecosystems.
6. Monitoring and Compliance: Shipboard sewage treatment plants are equipped with monitoring systems to ensure that treated effluent meets regulatory standards and environmental requirements. Compliance with international regulations such as the International Maritime Organization’s (IMO) MARPOL Annex IV is essential.
7. Effluent Discharge: Once sewage is properly treated and disinfected, it can be safely discharged overboard in accordance with regulations. The discharge must occur at a specified distance from the shore and sensitive marine areas to minimize the impact on aquatic ecosystems.
8. Record-Keeping: Ships are required to maintain detailed records of sewage treatment activities and discharge events. These records help demonstrate compliance with regulations and are subject to inspection by maritime authorities.

Sewage treatment plants on ships are crucial for environmental protection and compliance with maritime regulations. They ensure that sewage is treated to a level that minimizes its impact on the marine environment and reduces the risk of contamination and the spread of diseases.

Regular maintenance and crew training are essential to ensure the proper functioning of these systems during voyages at sea.

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