There are a number of sources for metals to enter the wash water:

• System materials – typically iron, copper and zinc
• System inlet water from electrochemical protection to prevent fouling of seawater pipes and from antifouling paints- typically copper
• The combustion of fuel and lubricant- typically vanadium, nickel, calcium and zinc.

The reduced pH of wash water, which can be as low as pH 3 within the system will increase the solubility of metal ions and therefore the choice of materials is very carefully considered by exhaust gas cleaning system designers. The correct materials will prevent corrosion and resultant component issues, as well as entry of metals into the wash water.

Electrochemical protection is sometimes fitted at the inlet to seawater pumping systems to prevent the parasitic growth of organisms that may foul pipework, filters and coolers of ship’s machinery. An exhaust gas cleaning systems may take a supply from a point with electrochemical protection but will not have a direct influence on it.

Although it makes up a relatively small amount of the overall particulate matter, ash represents the incombustible residue of burning fuel oil and, to a much lesser extent, lubricant. The majority of fuel oil ash content consists of metal compounds that occur naturally in petroleum; principally vanadium and nickel, which are oil soluble and so cannot be removed by onboard pre-treatment such as filtration and centrifugal purification [3]. As such these metals can enter ecosystems from unscrubbed exhaust emissions into the atmosphere.

In-field studies of wash water samples taken from various points within an exhaust gas cleaning system have shown that that the majority of metals are bound to particles or are particulates themselves and are effectively removed by the wash water treatment plant, thus having a beneficial effect [4].