6 Questions and 6 answers about Ballast Water Management

Ships are carrying ballast water around the world to ensure stability, trim and structural integrity and the ballast water often originates from environmental ecosystems with different organisms than the ecosystem in which the ballast water is discharged. Ballast water management is needed in order to protect marine ecosystems around the globe.

Having a full understanding of the different BWM technologies, their benefits and limitations, sets the foundation for making the right decision when selecting a BWM system. 

Below is highlighted 6 questions followed by answers to explain and elaborate on the area of Ballast Water Management.  

1. How do I deal with the difference in IMO and USCG regulations?

USCG versus IMO


A known challenge for UV-based BWM systems is the different regulations and efficacy testing methods adopted by the two regulatory bodies; USCG and the IMO.


The USCG evaluates BWM system performance using a test which measures the number of living organisms after treatment (FDA/CMFDA method). On the other hand, the IMO evaluates UV-based BWM systems by determining whether the treated organisms are viable or non-viable, i.e. are they capable of reproducing.



If they cannot re-produce IMO determines that the organisms do not pose a threat of becoming invasive species.


The organisms’ ability to reproduce is measured with the MPN (most probable number) method. It is a fact that a significant larger UV-dose, approx. 2 – 3 times higher, is required to kill organisms rather than rendering them non-viable.

Switching between codes and modes


This disparity between the USCG and the IMO’s testing methods implies that many UV systems increase UV dose levels when treating ballast water to be discharged in US-regulated waters. This is typically achieved by reducing the water flow with 50% or more while using maximum power settings.


Therefore, these systems have two different operating modes that the operator must choose between before starting a ballast operation: IMO and US. The US mode is essentially an operation mode with max power settings combined with reduced flow.


Numerous complications exist for ship operators using a BWM system that must switch operation modes between US- and IMO-regulated waters. Already during ballast uptake operation, careful attention needs to be paid to whether the ballast water will be discharged in US- or IMO regulated waters. This determines which operation mode of the system must be selected.

The consequence is that the operator of the system needs to know where the ballast water he is taking on board is going to be discharged. If the vessel is taking on ballast water in e.g. Shanghai, but knows that it will be discharged in the US, the US operation mode must be selected, even though the vessel is in China, far from US regulated waters.


Naturally, the selection of either IMO or US mode becomes quite complicated if the operator does not know where the ballast water he is taking on board is going to be discharged. It will be tempting to choose the IMO mode, because US mode will typically mean reduced flow, which leads to longer ballast operation and hence longer port stay.


However, if the IMO mode is used during ballast operation, but then later on it is determined that the ballast water shall be discharged in the US, the ballast water will be non-compliant and must be treated as such.

This means the local port state authorities in the US must be contacted and an agreement about what to do must be reached. This could be a requirement that the vessel must conduct a full ballast water exchange at least 200 nautical miles from shore in water at least 200 metres in depth.


A similar situation will occur if the ship’s next port of call is changed from a non-US port to a US port: All the ballast water on board treated in the IMO mode will then be non-compliant.

The CompactClean does not need a special mode to comply with USCG requirements, but has just one global approved mode for worldwide operations.

The advantage of using a single operation mode globally is that it removes the need to know the de-ballast location at the time of ballast uptake.

This means the ship can never get into a situation where the ballast water on board is compliant for discharge in one location, but not in another.

- No need for switching between codes and modes when loading and discharging your ballast water. No need for knowing the discharge location at the time of loading the ballast water.


With only ONE global operation mode your compliance is ensured – also when changes in route occurs. 


Your choice of ballast water management system can make a huge difference to your daily operation and lead to lost income. This story from the field will show you why operating with just one global operation mode instead of two, is what you want for your vessel.

2. Do I need to worry about "Hold time"?

Hold times take time


The USCG requires UV systems to have a minimum hold time, which is defined as the shortest allowable time between ballast water uptake and ballast water discharge.


If the minimum required hold time is not achieved before a de-ballast operation, it could result in serious operational issues such as e.g. inability to start cargo operations because ballast water cannot be discharged yet.


To complicate matters further, hold time requirements are for many BWM systems dependent on UV-I and salinities (for example fresh, brackish or sea water).



The required hold time can also be affected when UV-I drops suddenly during ballast water uptake. This makes operation and planning even more complicated, as the required hold time is not known until the ballast operation has been completed.


For some systems, a hold time of up to 72 hours in USCG-regulated waters should be observed. In US regulated waters the CompactClean only requires a holding time of two hours for all salinities irrespective of UV-I levels. In IMO waters the CompactClean has no hold time requirement.

With CompactClean planning is easy, and your operational flexibility is ensured.

3. What about Ultraviolet transmittance (UVT)?

UV based treatment


BWM systems based on UV treatment typically applies two treatment steps: filtration and UV treatment. During ballasting operations, both filtration and UV treatment is applied. After filtration, the ballast water is routed via UV chamber(s) to the ballast tanks.


To avoid potential non-compliant ballast water discharge due to organism regrowth in the tanks, the ballast water is during de-ballasting treated with UV light again.



The water by-passes the filter and gets pumped through the UV chamber before it is discharged. 


Understanding UV

The UV treatment uses either low pressure or medium pressure UV lamps to break down cell membranes and/or damage their DNA, which respectively kills the organisms or destroys their ability to reproduce, making them non-viable.

The percentage of non-viable or killed organisms in the water after treatment depends on the applied UV dose.

What is UV dose?


The UV dose depends on the UV intensity (UV-I) and exposure time, and is simply defined as the product between these two parameters. UV-I measures how much light (or energy) reaches a given measurement point.


Most UV systems measure the UV-I, but the systems can vary considerably in terms of the UV lamp being used and the sensor setup. In particular, the distance between the sensor and the UV lamp influences the measured UV-I. For this reason, UV-I values shouldn’t be used to compare systems.


Fortunately, another measurement called UV-transmission (UV-T), can be used to compare systems.

A more reliable UV performance measure


UV-T measures the capability of UV light to penetrate water. When the UV-T is high, that is, close to 100%, the water is very clear. This means the UV light can penetrate deep into the water. When the UV-T is low, the water is not very clear, and the UV light can only penetrate the water for a limited distance.


This means that the lower the UV-T a BWM system can treat and still meet the IMO and USCG discharge standards, the better the performance of the system. CompactClean can treat ballast water and effectively kill organisms down to 42% UV-T, which means even extremely challenging water can be treated to meet both IMO and USCG discharge standards. 

4. Power consumption for BWMS – Is it a big deal?

Power consumption is often an important consideration when selecting BWM equipment. Yet, while power consumption makes up a proportion of a ship’s operational expenditures (OPEX) over time, BWM systems are only operated a few percent of the time in a year. 

So when comparing BWM systems, the difference in annual OPEX is actually insignificant. The following case example illustrates this point.

Bulk Carrier Operating Profile:

Consider a bulk carrier with a total ballast pump capacity of 1,000 m3/h with following operating profile:


l ballast tank capacity 16,000 m3
Number of ballasting/ de-ballasting operations per year 24 (ballast water passes through the treatment system 48 times per year)
Hours per ballasting/de-ballasting operation 16 hours
Estimated specific fuel oil consumption of ship’s gensets 0.224 kg/kWh

According to the operating profile, the total ballasting and de-ballasting operating hours per year is 768 hours. If the vessel is operating only in IMO-regulated waters, the total fuel oil consumption per year for one CompactClean-1000 system (assuming an average power consumption of 86 kW) is 14.8 tons. In comparison, a system with a slightly lower average power consumption (e.g., 60 kW) consumes 10.3 tons of fuel oil per year.


From a cost perspective, if the price of fuel oil is 390.00 EUR per ton, the total fuel cost for operating the CompactClean-1000 system for one year is approximately 5,800.00 EUR, while the 60 kW system would cost 4,100.00 EUR, i.e. a difference of 1,700.00 EUR.


It should be noted that this case example does not consider holding time or flowrate restrictions in US mode, and as illustrated below the small fuel saving that can be obtained in IMO mode with a lower power consuming system is quickly converted into an additional cost when the same system occasionally has to be operated in its USCG mode.

CompactClean does not apply flow reduction to meet USCG discharge standards. But other, slightly lower power-consuming BWM systems as the one in this example, typically apply a 50% reduction in flowrate when ballasting and de-ballasting in US mode. In this example, it means the time required for ballasting and de-ballasting in one year would increase from 768 hours to 1,536 hours.


This increases the total fuel cost from approximately 4,100.00 EUR to 8,200.00 EUR, which is 2.400 EUR higher OPEX in a year than with the CompactClean.


Assuming the maximum power draw of a BWM system is available on the ship during cargo operation, it is hard to argue that a BWM system with low power consumption should be prioritised without carefully examining the system’s limitations. This is especially true in terms of hold time, flow restrictions and operating system complexity, which can easily create much more cost than the small saving obtained due to slightly lower power consumption.

5. How do I prepare my BWM-operation for the future?

Navigating the BWM convention

The Ballast Water Management (BWM) Convention administered by IMO sets the international standard for ballast water treatment.

On 28 October 2020, most of the world’s ports will close to new ballast water treatment systems installed on or after that date without meeting IMO BWMS CODE requirements. The BWMS Code (resolution MEPC.300(72)) entered into force in April 2019 and supersedes - the 2016 Guidelines for approval of ballast water management systems (G8).

To ensure compliance and operability of your vessel you should choose a BWMS system having the IMO BWMS Code type approval.

Before 28. October 2020
Systems approved under the former G8 guidelines (MEPC.174(58)) not later than 28 October 2018 can still be installed ("installed" means the contractual/actual date of delivery of the ballast water management system to the ship). No upgrades to these systems will be necessary. 


28. October 2020 and after
If the installation of a ballast water treatment system falls on or after 28 October 2020, the system to be installed must be type approved according to the BWMS Code requirements.


Installation of a system without revised G8 or BWMS Code type approval will not be permitted for global use.


6. Can I get a completely customized BWMS solution?

Making the right choice

Selecting a BWM system entails much more than just selecting technology and equipment. Finding the right supplier is extremely important as long-term service and support is crucial to keeping the BWM system operational throughout a vessel’s lifetime and compliant in both USCG- and IMO-regulated waters.


Continued support, including service and after-sales, is important for ship owners and operators. Adding to the support is extensive training for BWM system operators, so that they can be confident in their ability to handle any given situation.

DESMI - more than an equipment supplier

At DESMI we know that selecting and installing a ballast water treatment system is about much more than equipment supply and requires planning well in advance. Throughout the selection process DESMI delivers deep knowledge and we understand that not all type of technologies, systems etc. with their limitations and specifications will work acceptably on all vessels.


In addition, DESMI offers on-board inspection and engineering support for the installation. The projects are followed closely by one of our project managers who understand the entire process and what it takes for a successful installation and commissioning.

Ballast water treatment systems are generally standardized equipment. It is however important to choose a supplier that has the capability as well as flexibility to assist if the installation somehow requires customization apart from the standardized equipment. In DESMI we are ready to provide customized solutions e.g. valve control, customized interfaces etc. to ensure successful projects.


Last, but not least, DESMI is a trustworthy partner with more than 185 years of history and today supplier of a range of equipment to the Marine and other markets. You can therefore count on DESMI being by your side for many years to come.

Global Contacts

DESMI is a global company

Find address and contact information of our global locations


Sign up for our newsletter and we'll send you the latest headlines directly to your inbox.


* indicates required