The IMO Pollution Prevention and Response sub-committee (PPR) has set up a correspondence group to provide guidance in the event of accidental breakdown, instrumentation malfunction or perceived non-compliance of exhaust gas cleaning systems. Part of the group’s brief is to clarify some of the language used in the scrubber guidelines (MEPC.258(68)) including the term PAH (polycyclic aromatic hydrocarbons) monitoring.
So, what are PAH and why are PAH monitored? Well, quite simply these are a group of compounds found in oil that can be measured in water at parts per billion concentrations. This enables monitoring of the extremely low levels of oil in scrubber wash water, which is well beyond the measurement capability of traditional instruments associated with 15ppm or even 5ppm bilge water separators.
PAH fluoresce when exposed to ultra violet light energy. Monitors are tuned to emit UV light around a wavelength where it is absorbed by one PAH compound in particular – phenanthrene, the second most common of the PAH in petroleum, with detection at a higher range of wavelengths where phenanthrene fluoresces.
In real world water monitoring the fluorescence from other PAH overlap with that from phenanthrene and the overall response can be used to report the concentration of PAH as phenanthrene equivalents or PAHphe.
Online monitoring by UV fluorescence cannot be related to laboratory analysis, for example for the US EPA Vessel General Permit, as the individual PAH compounds are not identified nor quantified. However, EGCSA is keen that the reason for monitoring PAH is not forgotten – it is an effective means of monitoring very low concentrations of oil in scrubber discharges.
In terms of direct online measurement of emissions to air and water, exhaust gas cleaning systems are perhaps the most closely monitored of all marine equipment. It is believed to be unique in terms of compliance requirements that the scrubber guidelines contain a table, which not only has limits for the concentration of PAH in scrubber wash water, but also links these to the water flow rates in order to strictly limit the quantity of oil discharged.
Through knowledge transfer and its code of conduct, a key EGCSA objective is to develop and promote a high level of integrity and standards in exhaust gas cleaning. As part of this effort EGCSA has set up a working group to consider the standards for PAH monitoring.
With expert input from instrument suppliers who are EGCSA Associate members, the group met recently in London to discuss a robust product neutral definition of PAH monitor and consider how the requirements for installation and verification in service can be reinforced in order to promote compliance assurance.
This work will feed into the review being undertaken by the IMO PPR correspondence group to ensure a consistency of reporting, that instruments and their installation can be readily surveyed, and that monitoring standards are maintained at a high level, which any new market entrant must meet.