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Giovanni Romagnoli* frequents this beach most during hot summer days, with his own booth and parasol. But he had never seen anything like it before in August last year. 'When I noticed that black substance around us, I panicked. I immediately wanted to get everyone out of the water as quickly as possible,” he says.
Having worked on cruise ships in the past, he now works as a chief engineer on a tugboat in Civitavecchia, the port of Rome, where he lives. The city is one of the busiest and most polluted ports in the Mediterranean, alongside Barcelona and Marseille. He is convinced that what he saw in the water on that summer's day was “scrubber water”'.
Scrubbing is a technology comparable to a filter that cuts off polluting emissions when applied to ships, cargo ships and cruise ships. It's a great invention, given the huge amounts of exhaust gases emitted by the less refined fuel oils used in shipping when burned. However, scrubbers – technically known as exhaust gas cleaning systems (EGCS) – have a dirty secret: the wastewater produced by their cleaning process is dumped into the sea. Scientists have warned that this practice has aggressive environmental consequences.
As the whole Mediterranean region grapples with the effects of an escalating climate crisis and pressure from the tourism industry, coastal cities now have to deal with this new threat to environmental health, despite the technology being presented as a way to improve air quality.
Scrubbers: a growing threat to an already polluted sea
Under pressure to address the public health risks posed by air emissions from marine traffic, the International Maritime Organisation (IMO) introduced a global limit on the sulphur content of fuel on 1 January 2020.
Since then, the maximum permitted sulphur content of marine fuels is 0.5%, while in Sulphur Emission Control Areas (SECAs), the limit is 0.1%. From May 2025, the Mediterranean also becomes a SECA zone. However, the IMO's Marine Environment Protection Committee (MEPC) has left open the possibility of an alternative to cleaner fuels: installing an exhaust gas cleaning system to reduce sulphur emissions.
Since the global sulphur cap came into force, the adoption of scrubbers has multiplied worldwide. According to ICCT estimates, the number of ships using scrubbers has grown from less than 800 in 2018 to over 5,000 in 2025, accounting for around 5% of the international merchant fleet.
“We're talking about thousands of tonnes of discharged wash water per hour: on the biggest ships, this can reach 6,000 tonnes in just 60 minutes,” says Guillaume Picard, a former captain turned campaigner against the cruise industry and scrubbers. Nearly 300 million tonnes of wash water are expected to be discharged in major ports worldwide by the end of the year, with the top 10 ports accounting for 28% of this total — about 83 million tonnes.
Despite representing only 5% of all ships operating with scrubbers globally, cruise ships' high energy consumption in some ports leads to them accounting for over 70% of wastewater discharges by volume in the top ports worldwide, according to ICCT estimates. Barcelona and Civitavecchia are the only European and Mediterranean ports on the list, with discharges of 3.7 and 3.5 million tonnes respectively. However, Barcelona has recently decided to put a halt to this new source of pollution by banning open-loop discharges in port waters.
As the Mediterranean is a closed basin, the toxic water produced by the scrubbing process does not easily dilute, which amplifies the effect.
The IMO's decision to permit scrubbers as a means of complying with sulphur caps was highly controversial and criticised by numerous marine scientists and environmental organisations. Carlos Bravo, an environmental consultant for the NGO OceanCare who has over thirty years' experience working with organisations such as WWF and Greenpeace, says that the IMO “made a mistake by allowing the continued use of high-sulphur heavy fuel oil with scrubbers”.
Like many others, the expert specifically accuses the allowance of this technology of contradicting Article 195 of the United Nations Convention on the Law of the Sea (UNCLOS). This article of the so-called "Ocean Constitution" stipulates that states must ensure that measures taken to prevent, reduce and control pollution of the marine environment do not transfer damage or hazards from one area to another, or transform one type of pollution into another.
The environmental impact of scrubbers
“There is a scientific consensus that scrubber discharges are dangerous and that scrubbers should not be used on a large scale,” states Tronczynski from the French Institute for Ocean Science (IFREMER). The pollutants in scrubber wash water originate from two sources: the chemical process that converts sulphur dioxide (SO₂) in the exhaust into sulphates (a reaction that makes the water acidic), and the contaminants in the fuel or produced during combustion, such as heavy metals, hydrocarbons and nitrogen compounds.
Once discharged into the water, these substances can harm marine life. Some, such as nitrates, can lead to algae blooms that reduce the water's oxygen content. Others, such as PAHs (Polycyclic aromatic hydrocarbons, a group of over 100 chemicals that are formed when organic materials are burned or heated) and heavy metals, are toxic, persistent contaminants that can accumulate in organisms and pose a risk to the entire food web, including humans.
Indeed, an increasing amount of scientific research suggests that these releases harm marine life, pollute the sea with heavy metals like zinc, vanadium, and iron, and increase acidification. This field of research has recently been boosted by the EMERGE Project, a European consortium uniting 18 partners to comprehensively analyse the marine and atmospheric impacts of shipping.
This consortium collected 15 different scrubber effluent samples from diverse sources, including on-board two operating ships equipped with open-loop EGCS systems (from DANAOS Shipping co. LTD) sailing across the Mediterranean, and a laboratory-scale pilot system at Chalmers University of Technology (Sweden) and analysed them chemically.
This is no small feat, since independently studying the contents and impacts of scrubber wastewater discharges in real-life conditions has been challenging for researchers, because many shipping companies have been reluctant to allow them to board the ships and sample, as researchers like Nicolas Briant (IFREMER) have told Voxeurop.
One of the main outcomes of the EMERGE research project was that, while current legislation (based on the 2021 IMO Guidelines and the US Environmental Protection Agency criteria) is focused on just a few components – 16 types of polyaromatic hydrocarbons and selected heavy metals – washwater contains many more pollutants that are ignored but which are equally or even more concerning in terms of environmental harm. Most notably, alkylated PAHs.
"This is a very large group of pollutants that are not considered in risk assessments, but our study shows that they account for more than 85% of the toxicity of samples", explains Mira Petrović, Research Professor at the Catalan Institution for Research and Advanced Studies (ICREA) and EMERGE Consortium specialist in the study of emerging contaminants (ECs) in environmental samples.
The EMERGE consortium also carried out The EMERGE consortium also carried out experiments on the effects of pollutants on living organisms and the environment at five research laboratories. The findings indicate that EGCS effluents are toxic at significantly lower concentrations than previously documented.
The invisible victims
Marine microalgae, fitoplancton, copepods and invertebrates such as blue mussels and sea urchins: although we might not see them, they are essential to the ocean's ecosystem. They are among the first to be affected when exposed directly to scrubber water diluted in the sea. Their biological and reproductive functions can be negatively impacted by acute, early-life stage and long-term exposures. For example, larval development in mussels (specifically Mytilus galloprovincialis) was reduced by 50% when exposed to diluted water samples containing concentrations of scrubber wastewater as low as 5%.
Despite all this growing evidence, some industry representatives minimise these impacts and disparage the scientific research behind them.
“There is absolutely no impact on marine life,” commented Chris Millman, Vice President of Corporate Marine Technology at Carnival Corporation, one of the main adopters of this technology among cruise liners, when reached by Voxeurop. “But it’s true that it’s slightly acidic; it’s below the neutral point on the pH scale.”
In fact, it’s all a matter of money. Research by the EMERGE Consortium found that shipowners have turned to exhaust gas scrubbers as a cost-effective solution, enabling them to burn cheap, sulphur-rich heavy fuel oil (HFO) instead of pricier, lower-sulphur alternatives such as marine gas oil (MGO) or very low-sulphur fuel oil (VLSFO).
Despite the urgency of ocean health and the risks of scrubber pollution, a unified Mediterranean policy remains elusive. The issue has yet to gain further traction at the International Maritime Organisation (IMO).
According to Víctor Jiménez Fernández, Spanish representative at the IMO who participated in the most recent session of the Pollution Prevention and Response (PPR) subcommittee in January 2025, what appears to be beyond the scope of the current work stream in IMO is the consideration of a total global ban on the use of scrubbers. Similarly, the industry favours strengthening the existing regulatory framework over introducing such a ban. “I feel fooled,” Romagnoli explains, as he stands on the deck of a tugboat in the port of Civitavecchia. In the distance, almost 15 cruise ships break the horizon. “I understand progress and technology. But this is not a real solution,” he continues, sounding frustrated.
In the northern Mediterranean, countries are struggling to find common ground. Since 2022, France has banned the use of open-loop systems in all coastal and port waters within the three-nautical-mile coastal zone (approximately 5.5 kilometres or 10–15 minutes' navigation time). Spain has also made some localised progress: by the end of 2024, seven ports – including major ones such as Algeciras, Huelva and Barcelona – had enacted local regulations restricting the use of scrubbers.
A national ban on discharges in ports and inland waters is also being developed. However, in Italy, ships are permitted to discharge scrubber wastewater even within port areas. In December 2024, the NGO Cittadini per l’Aria sent a formal request to Italy’s Ministry of the Environment, advocating a nationwide ban on scrubbers.
Research and watchdog groups warn that continued reliance on scrubbers for cost savings, without policy changes, could lead to the widespread discharge of harmful effluents and pose a serious risk to already stressed marine ecosystems such as the Mediterranean.
*A pseudonym was used to protect his identity.
This article is supported by Journalismfund Europe and the Earth Journalism Network
This article is published in partnership with Irpi.media, Climatica and Reporterre.
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