Eductors have long been used to remove the final contents of ballast tanks. David Smith of The Ballast Water Centre at PML Applications in the UK, considers how they interact with treatment systems
What do eductors do?
Ballast tanks often consist of a labyrinth of chambers with openings and channels allowing water to enter all areas of the tank during ballasting and to drain towards the pump suction during deballasting.
During tank emptying, the flow of ballast water to the suction pipe can become restricted by the bottom structure; consequently, the main ballast pump will often lose suction before all the water has been removed.
To remove most of the remaining ballast water, a process known as ‘stripping’ may be undertaken using an eductor. This uses local sea water at a high pressure to create a vacuum which is applied to the ballast pipework and suction point in the ballast tank.
Eductors may affect the correct functioning of a ballast water management system (BWMS) that uses chemical additives. This treats the ballast water on board with a disinfecting compound and will incorporate a control mechanism to apply a neutralising compound to condition the discharged ballast for environmental acceptability.
This control function incorporates a sensor for total residual oxidants (TROs) which is fitted to the discharge line and governs a unit that injects a chemical neutraliser to the outgoing ballast water to reduce the oxidant content to tolerable limits.
An eductor’s discharge will be a mixture of local harbour water and that from the ship’s ballast tanks. Its characteristics will be that of extremely aerated water, often discoloured with tank sediment. This may affect the ability of a sensor to operate correctly and result in incomplete neutralisation of disinfected water.
What are the issues for TRO sensors
TRO sensors available for use with a BWMS operate on two basic principles: colorimetric or amperometric. Both types have limitations when used to process eductor generated water.
For colorimetric sensors, air bubbles and sediment may result in discoloration that could give false readings or render the unit inoperable. Amperometric monitors can also be adversely affected by this and be prone to fouling by iron and manganese in the water.
How can this affect the use of a BWMS?
The measurement of TRO becomes problematic when eductors are employed. Without reliable information being fed back into its control system, a BWMS employing chemicals cannot function correctly and may fail to properly counteract the adverse environmental effects of its discharge.
What does IMO say about this?
IMO document PPR 1/WP.6 was produced in February 2014 at MEPC 66. It included an annex that proposed some draft guidance relating to stripping operations using eductors, including a paragraph about eductor use with BWMSs using chemical neutralisers.
It proposed that if monitoring shows that the bulk of the ballast discharged through the main system does not need neutralising, “it is accepted that the remainder of the ballast water in the tanks will also be compliant and may be discharged via an eductor system using local water as motive water without additional monitoring.”
This was considered during MEPC 67, which recommended that ballast water sampling should not be performed during stripping operations.
Why does this matter?
Some aspects of using eductors with treated ballast water remain unclear and may be the subject of debate again in the future.
While the case for not sampling eductor discharges is accepted when no neutraliser has been required during the bulk discharge, what should occur if the BWMS had to apply significant quantities of neutraliser during this period?
Further, it may have been assumed that eductors only handle a very small quantity of water, say less than 1 per cent of the total discharge. The reality may sometimes be different with eductors being engaged at an earlier stage of the discharge process.
One solution is that the BWMS could record the average neutraliser concentrations applied during the bulk of the ballast discharge and continue to inject similar concentrations into the eductor discharge water.