Wash water from scrubbers: the acid test

Malcolm Latarche
Malcolm Latarche

18 June 2018


Scrubbers are permitted as a means of meeting the SOX exhaust requirements under MARPOL Annex VI, but their use is not a free choice for shipowners as it must be approved by flag states. Whatever reasons there are for some owners to adopt exhaust gas cleaning systems and for others to shun their use, probably the most common argument advanced against them is the matter of wash water discharge into the sea.

Critics of scrubbers including some environmental NGOs are of the opinion that instead of SOx being released into the atmosphere, scrubbers merely push the pollutants into the sea. It is easy to understand why such a claim might be made. Mixing water with sulphur oxide compounds does indeed produce an acidic mixture that also contains other pollutants including nitrates and heavy metals, but the wash water should be treated before discharge in two different ways to remove any oily waste and to reduce the acidity levels.

If a scrubber is used the only requirement of it is that it must treat the exhaust so as to ensure that regardless of the sulphur content of the fuel, the exhaust gas contains no more sulphur than it would have if compliant fuel was used. The IMO guidelines for scrubbers detail recommendations for wash water treatment but it is the flag states that must determine if the IMO guidelines for wash water will apply or if a more or less stringent regime is in order. Whatever they choose, ships will not be permitted to discharge the wash water into the sea without first removing any hydrocarbons because that is mandated by other aspects of MARPOL. Port states are free to set limits of their own if they so wish but few have.

The fact that the IMO guidelines are not mandatory comes as a surprise to many not least because there is an expectation that they will be at some point and because there is a genuine belief that they must be. As Don Gregory said at the 2020 Sulphur cap Conference, “Too often, a throw away comment such as, “open loop scrubbers are banned in Europe” becomes the factual position”.

The facts are that open loops scrubbers are used in all of Europe except Belgium where government legislation imposed a ban on all water discharges long before scrubbers came to the market. Germany also has a partial ban in some of its rivers but not everywhere. Under the US VGP, similar standards to the IMOs are detailed for scrubber wash water but they are mandatory in US waters.

The acidity of wash water will depend on the treatment it is subjected to. In an open loop system, the wash water is mixed with more seawater and effectively neutralised. In a closed loop system, the addition of chemicals is used instead of seawater. Operation in brackish or fresh water which are less alkaline, could mean that discharge from an open loop scrubber may be more acidic than when operated in typical seawater.

Most scrubber system makers say that so long as the wash water is properly monitored and treated, it need not be so acidic as cause problems. However, some recent announcements by Belgian repair specialist Hydrex and its sister company Subsea Industries suggest that some damages to ships have occurred and been blamed on washwater discharges.

The announcements suggest that pipework associated with discharge of washwater can become severely corroded. “We have noted an increase in repairs to corroded pipework and outlets, which does appear to correspond with the increase in the number of ships fitted with scrubbers,” said Dave Bleyenberg, Hydrex Production Executive. Previously, it was announced that Subsea Industries’ Ecospeed coating was applied to scrubber outlets on three vessels in China to prevent similar corrosion problems.

There is no disputing that the work involved was necessary or that the scrubber discharge was the main cause, but too little other information is available. It could be that the scrubber was being operated wrongly, was faulty in some way, the sulphur level in the fuel was higher than declared leading to insufficient wash water being used or that the seawater chemistry was not adequately considered. The problem may also have been exacerbated by the choice of materials for the pipework and associated fittings.

Monitor market growing

Monitoring the exhaust gas to ensure the scrubber is performing correctly is essential to prove compliance with Annex VI SOx rules and monitoring wash water as well is a sensible precaution as under some flag state rules it will be necessary to prove the wash water meets the IMO or other guidelines.

The effect of wash water on local environments is itself being monitored in some regions and the IMO guidelines are likely to join the US VGP requirements in being made mandatory. As the number of scrubber installations grows so monitoring systems are being developed and released on to the market. Green Instruments G6100 and Chelsea Technologies Group’ Sea Sentry were early pioneers. The most recent entry is the Smart ESM monitor from UK-based Rivertrace

The Smart ESM is suitable for both the inlet and outlet of a wet exhaust gas cleaning system, measuring and recording PAH, turbidity, temperature and pH, on open-loop, closed-loop and hybrid scrubber systems. It is fully compliant with MEPC 259(68) and has a large touch screen display which provides a flexible user interface.

The monitor provides onscreen graphs showing either live data or historic hourly, daily or weekly figures. Data can be emailed automatically anywhere in the world for accurate record keeping. The monitor also provides automatic cleaning of the optical path to ensure continual accuracy. The Smart ESM has been developed to support all scrubber systems and include a broad selection of inbuilt analogue and digital outputs.