Closed Loop Scrubbers – An Overview

With the IMO’s 2020 global sulphur cap deadline getting closer, owners and operators are faced with having to make a significant operational and commercial decision – whether to continue operating on low sulphur fuel, post 2020, or whether to finance the retrofitting of a scrubber system to the vessel.

Depending on the type of scrubber selected, retrofitting the system to an existing vessel can be a complex and expensive process. Setting aside the significant capital cost of the technology itself, the engineering required to integrate the scrubber system, supporting structure, pipework and all required modifications to the vessel can be vast.

However, perhaps the most critical decision to be made by owners and operators is the type of scrubber system itself. With three different types of scrubbers available, namely open loop, closed loop and hybrid systems, each with their own inherent benefits and drawbacks, the selection of the most appropriate system for the vessel’s trading patterns is highly important.

Sticking to our commitment to being owners and operators trusted advocate in achieving environmental compliance, Cleanship Solutions has developed a three part deep-dive series on each of these scrubber types, highlighting critical considerations of each option.

Part two is the closed loop scrubber.

Please note that this article does not discuss the physical retrofit practicalities of the open loop scrubber option – as this will be covered in detail in future scrubber related articles. Rather, this article has been developed to identify critical commercial considerations of the open loop scrubber option, and their potential impact on the vessel’s performance.


Closed loop scrubber systems generally use chemically controlled sea or fresh water as the scrubbing liquid. Unlike open loop scrubber systems, which pump water directly from the sea chest, through the scrubber, and directly overboard, closed loop systems re-circulate the scrubbing liquid, with only a small bleed off discharged overboard. Once passed through the exhaust gas, the wash water is typically held in a process tank, treated, and then re-circulated.

It is a common misconception that closed loop scrubber systems have no overboard discharges. The chemical dosing of the scrubbing liquid has a limited efficacy, and in order to maintain sufficient efficacy, and prevent the build up of the chemical by-products within the wash water, the system requires top up with fresh water, with the existing water gradually bled off. Some closed loop systems can temporarily prevent overboard discharge by storing the bleed off in designated tanks.

Primary Benefits of Closed Loop Scrubbers

Closed loop scrubbers offer a number of benefits over open loop and hybrid systems.

The main benefit is their immunity to the alkalinity of the surrounding sea water. In the first part of our three part guide to scrubber technologies LINK HERE we discussed the importance of the scrubbing liquid alkalinity to scrubber efficacy, and the fact that open loop scrubbers are limited by the alkalinity of the water in which they trade. Closed loop scrubbers, by virtue of their chemical dosing, can artificially control the alkalinity of the scrubbing liquid.

Another significant benefit of closed loop systems is their vastly reduced wash water flow requirements compared to open loop scrubbers. With existing wash water being recirculated, closed loop systems generally require 50% of the water volume compared to open loop systems – resulting in considerably smaller pumps, and, intuitively, lower power consumption from said pumps.

Closed loop scrubbers are also immune to some of the other drawbacks of open loop scrubbers – including potential fouling of scrubbing nozzles and wear caused by intake of sand or sedimentation. Since the closed loop system is not drawing water directly from the sea, they generally do not face these issues.

Critical Considerations

Closed loop scrubber systems do have a number of critical considerations that owners and operators are well advised to examine when determining their suitability for specific vessels. As outlined in the first part of this three part series, Cleanship Solutions take great care to analyse these aspects when conducting scrubber feasibility studies on behalf of our ship owner and operators clients, and feel it important to share these considerations with the wider community.


System Complexity

Compared to the open loop system, the closed loop system is considerably more complex, and requires a number of additional components. The entire wash water system requires a number of storage and holding tanks, as well as additional water treatment equipment that the open loop system does not require. With the closed loop systems also having to treat the bleed off water and discharge it overboard, albeit on a smaller scale compared to open loop systems, the issues and challenges associated with discharging scrubber discharge water overboard also remain.



Chemical Handling & Storage

Closed loop systems require artificial adjustment of scrubbing liquid alkalinity levels – this is typically achieved using chemical dosing. In many cases, the chemical of choice is caustic soda (NaOH), which is considered readily available in most countries, however, sodium hydroxide is also used by some systems.

Regardless of the chemical or compound used for alkalinity adjustment, additional storage tanks will be required onboard, to safely store the chemicals for use, and may, in many cases, require careful temperature and humidity control to ensure chemical stability.

Finally, vessel crews will also be required to become familiar with the chemical’s storage and handling requirements to ensure safe handling, use and bunkering throughout the vessel’s operation.

Fresh Water Consumption

With closed loop scrubbers requiring bleed off of the wash water, to maintain chemical composition and efficacy of the wash water, there is a very real, and comparatively significant, need for fresh water top up. In some cases, this fresh water top up can be as high as 2.5m3 per day, per MW. Even for modestly powered vessels, this additional fresh water consumption can add up.



In more extreme cases, owners and operators may be faced with upgrading the existing fresh water production plants onboard to meet these requirements, adding not only to the upfront investment costs in the scrubber retrofit, but also ongoing operational costs of the vessel.

Impact on Waste Heat Recovery and SCR Systems

As discussed in part one of our three part series, one of the most detrimental effects of scrubber systems is their impact on the temperature of the exhaust gas. Scrubber systems, due to using low temperature wash water, can significantly lower the temperature of the exhaust gas to the extent that it has a negative impact on any existing exhaust based systems – such as waste heat recovery systems or selective catalytic reduction systems (SCR). Whilst one may intuitively relocate such existing technologies upstream of the scrubber, this too can have negative impacts on the temperature of the exhaust gas, and cause detrimental performance issues on the scrubber. Therefore this process is somewhat iterative and requires close discussions between the scrubber manufacturer and the manufacturer of the waste heat recovery or SCR systems, to ensure both are operating optimally, or, at the very least, within an acceptable compromise.



The closed loop scrubber system is an attractive option for vessel owner and operators – particularly those with vessels trading in areas with low alkalinity levels – making open loop scrubber options unsuitable. However, with a number of critical considerations and technical implications, closed loop scrubbers should ultimately be analysed with other scrubber options, as part of a detailed feasibility analysis.

Cleanship Solutions offer comprehensive scrubber feasibility studies for owners, analysing the various options available and objectively determining the most suitable system for any given vessel. We have no commercial ties to any particular manufacturer, meaning we can offer vessel specific recommendations free of any subjective preference…