In 2021, the IMO updated its guidelines for scrubbers, setting stricter limits for open-loop scrubbers on acidity and discharge of polycyclic aromatic hydrocarbons and nitrates. In June of 2022, the IMO’s Marine Environment Protection Committee approved additional guidelines on scrubbers. PAH sensors, used previously in connection with oil rigs and drinking water reservoirs, are used to fingerprint the offender in an oil spill, and can also identify the type of oily substance, thanks to a statistically significant correlation between the substance type and the PAH constituents. PAH monitoring can be used as a proxy for oil monitoring. This is why PAH sensors are used with scrubber discharge systems, as they can detect very low “oil” content.
The different PAH species can be identified by their fluorescence emission when exposed to and excited by UV light. Naphthalene is the lightest of the PAH species, but it is also quite water-soluble, which makes it more vulnerable to degradation, which is why phenanthrene, which is more persistent with regard to combustion and degradation, has been chosen as the reference for “oil” content in discharge water.
IMO’s new guide 340(77) in 2022 looks set to turn PAH sensors in scrubber emission measurement on its head, as it rewards manufacturers such as Chelsea Instruments which have always opted for actual calibration against equivalence rather than a calculated approximation. Under pressure to issue clearer guidelines, IMO’s new guide 340(77) has clarified that in the instrument divisor of 6.2 is no longer acceptable and PAH sensors in washwater monitors must calibrate against equivalence moving forwards. Chelsea’s PAH+ sensor is calibrated against equivalence.