A global effort to curb pollution from the heavy fuel oil burned by most big ships appears to be encouraging water pollution instead. A 2020 regulation aimed at cutting sulfur emissions from ship exhaust is prompting many owners to install scrubbing systems that capture pollutants in water and then dump some or all of the waste into the sea.

Some 4 300 scrubber-equipped ships are already releasing at least 10 gigatons of such wastewater each year, often in ports and sometimes near sensitive coral reefs…. The shipping industry says pollutants in the waste don’t exceed national and international limits, and that there’s no evidence of harm. But some researchers fear scrubber water, which includes toxic metals such as copper and carcinogenic compounds called polycyclic aromatic hydrocarbons, poses a rapidly growing threat, and they want to see such systems outlawed.

The emerging debate is the result of a 2020 regulation put into place by the International Maritime Organization (IMO), an arm of the United Nations that works with 174 member states to develop common rules for international shipping. By banning the use of sulfur-heavy fuel oil, the rule intended to reduce pollutants that contribute to acid rain and smog. IMO estimated the rule would slash sulfur emissions by 77% and prevent tens of thousands of premature deaths from air pollution in ports and coastal communities.

But cleaner fuel can cost up to 50% more than the sulfur-rich kind, and the rule allows ship owners to continue to burn the cheaper fuel if they install scrubbers. In 2015, fewer than 250 ships had scrubbers (often to comply with local regulations); last year, that number grew to more than 4300, according to industry figures.

A scrubber system sends exhaust through a meters-tall metal cylinder, where it is sprayed with seawater or freshwater, depending on the type, at rates comparable to gushing fire hydrants, to capture pollutants. In the most popular systems, called open loop scrubbers, seawater is discharged to the ocean after little or no treatment. Other systems retain sludge for disposal on land and release much smaller (but more concentrated) amounts while at sea….Researchers are particularly worried about discharges in areas that IMO has designated as ecologically sensitive. The Great Barrier Reef, for example, receives about 32 million tons of scrubber effluent per year because it’s near a major shipping route for coal. Ships also release scrubber water around the Galápagos Islands….

Ports see substantial discharges, too. Cruise ships dominate those releases, contributing some 96% of discharges in seven of the 10 most discharge-rich ports. Cruise ships typically need to burn fuel in port to continue to operate their casinos, heated pools, air conditioning, and other amenities. Most ports have shallow water, so pollutants are less diluted and can accumulate more rapidly….

Researchers, who are participating in a €7.5 million European effort to study shipping pollution called EMERGE, would like to study how scrubber water affects fish larvae.

But shippers have become hesitant to share samples and data with scientists. “We’re reluctant to give it to organizations which we know have already an established agenda,” says Mike Kaczmarek, chairman of the Clean Shipping Alliance 2020

The ultimate solution is to require ships to use the cleanest fuel, called marine gas oil. In the meantime, 16 countries as well as some localities have banned the most common scrubbers.

Excerpts from Erik StokstadShipping rule cleans the air but dirties the water, Science, May 13, 2021

The International Council on Clean Transportation (ICCT) study, released on April 9, 2021