Low pH water can adversely impact marine organisms such as shellfish; hence the International Maritime Organisation has set pH limits for wash water discharged to sea [10]. Generally, no environmental harm is considered to arise from short-term exposure of seawater down to pH 6.5[1] and this limit must be applied using one of the following two standards.

The pH must be continuously measured online and data securely logged to confirm compliance with the IMO EGCS Guidelines:

• 1. The discharge wash water should have a pH of no less than 6.5 measured at the ship’s overboard discharge with the exception that during manoeuvring and transit, the maximum difference between inlet and outlet of 2 pH units is allowed measured at the ship’s inlet and overboard discharge.

Figure: pH measurement method 1

Under method 1, the two different pH discharge criteria, for when the vessel is stationary and moving allow the adoption of a more stringent criterion for stationary ships in ports when the main engine is not running, whilst still having an appropriate criterion for moving ships. For vessels underway there is a highly effective mixing of the discharged wash water with fresh seawater by the turbulence of the ship’s wake so that recovery of the pH to that of the surrounding water is very rapidly achieved.

Or:

• 2. The pH discharge limit for the exhaust gas cleaning system at the overboard monitoring position, is the value that will achieve as a minimum pH 6.5 at 4 m from the overboard discharge point with the ship stationary, and which is to be recorded as the overboard pH discharge limit in the ETM-A or ETM-B.

Figure: ph measurement method 2

This alternative compliance method of measuring the pH is as per the CORMIX principle which is used for example by United States EPA, where 4m is considered the boundary of the initial mixing zone between water discharged overboard and water surrounding the vessel.

The overboard pH discharge limit by method 2 can be determined either by means of direct measurement, or by using a calculation-based methodology (computational fluid dynamics or other equally scientifically established empirical formulae) to be left to the approval by the Administration.