The aft peak tank – ballast water challenges

May 20 2021

Tanker operators are struggling to treat ballast water from the aft peak tank, as the water cannot be mixed with the main ballast cargo, so it needs a separate system and there is not much space.

Handling ballast water in aft peak ballast tanks on tankers is a challenge. By regulation, the water can’t be mixed with the cargo body ballast water, due to the risk it gets contaminated by oil from an adjacent leaking cargo tank. This would be an explosion risk. So it needs a separate ballast water treatment system.


Aft peak ballast tanks typically hold just 5 per cent of the total ballast water volume on tankers and have a volume flow rates of around 200 cubic meters per hour. 


This tank is very different than the cargo body ballast tanks that are filled and emptied in coordination with cargo operations. 


The aft peak tank water is used for purposes such as cooling the stern tube bearing, dampening propeller vibration in the stern, ensuring propeller immersion. More recently it has been used as salty feedwater for electrolytic based ballast water treatment systems, where the salt is used to generate sodium hypochlorite as the ballast water disinfection chemical. 


To comply with ballast water regulations, tanker operators face a choice of decommissioning this ballast water tank and losing these important functions, or installing a ballast water treatment system dedicated to the aft peak tank.


There is not usually much space available for a system near the aft peak tank. Companies will often select a smaller version of the same ballast treatment technology selected for the cargo body ballast tanks.  This is not often the optimal decision in terms of arrangement, operations, or cost.


Such systems have features that are not needed when treating only the aft peak tank, for example filters for pre-treatment, which then require backflushing pumps, flow monitoring, and flow regulation. 


If the disinfection uses UV bulbs, the system additionally needs protective sleeves, careful cooling considerations, and a means of wiping or acid cleaning the sleeves. 


If it uses electrolytic systems, the cells require cleaning, and the generated hydrogen gas requires dilution and venting to a safe place. 


Both approaches also require a final step, either a second UV disinfection stage on discharge or a neutralization stage for the electrolytic systems.


A smaller system

My company, oneTank, has developed a low-cost, compact, easy-to-install system that is IMO BWMS certified and US Coast Guard type approved. It is designed specifically for the aft peak ballast water tank, treating tank volumes of up to 4,000m3.  oneTank is intended as an add-on, complementary to the cargo body ballast water treatment system, but a smaller, simpler, and lower-cost solution. 


oneTank was designed to keep the aft peak tank installation simple.  There is just one assembly to install which minimizes the number of foundations and components to handle.  The unit itself has a small 559 x 604 mm footprint which is very helpful in a busy and crowded engine room, particularly the lower aft engine room space near the aft peak tank.


The onboard computer performs all operations automatically, including regulatory required records. 


The unit requires a single 120 VAC feed at 15 amps, or 220 VAC feed at 7.5 amps, to power the onboard computer and dosing pumps.


The circulation pump creates a circulation loop from the aft peak tank through the oneTank unit and back to the aft peak tank.  oneTank automatically calculates the amount of bleach needed, applies it, and measures the result. 


A twenty-four hour hold period is required between the disinfection and discharge to the sea.  A neutralisation process is also required before discharge to the sea, with a one hour cycle of adding sodium thiosulfate into the ballast tank. Afterwards, the system verifies that the water is ready to discharge.


The typical tanker aft peak installation requires a 120 m3/hr circulation pump.  Usually, an existing general service or fire pump can perform this.  Alternatively, oneTank can supply a new dedicated circulation pump. 


The system is offered in a standard, off-the-shelf package for USD 65,000, which can be shipped anywhere in the world.


With some variation depending on labour rates, the installed cost of a oneTank system is estimated at USD 95,800 if using an existing pump and USD 118,000 if installing a new, dedicated pump.  These estimates are as much as USD 100,000 less than some recent installations we have seen for tankers with a filter and disinfection solution.


One barrel of bleach typically costs USD200 and can treat more than 1,000m3 of ballast water, adequate for most aftpeak tanks.  A 20-litre pail of neutralizer chemical typically costs USD25 and will neutralize more than 1,000m3 of ballast water.


oneTank announced in December 2020 that it had won a contract with Overseas Shipholding Group (OSG) to provide an aft peak ballast water treatment system for a tanker.


oneTank was founded by marine engineers. It builds its treatment system in its main facility in Seattle, and offers applications engineering, installation and commissioning support, and a robust spare parts and service program.  The company is a wholly-owned subsidiary of marine consulting firm, Glosten.


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