Under MARPOL Annex VI and the 2015 Guidelines for exhaust gas cleaning systems (EGCS), a scrubber system must be monitored to prove it keeps the SOx emissions equivalent to burning fuel of the required sulphur content. The parameters monitored are:

1. SOx (sulphur dioxide) concentration in the exhaust gas - measured by a gas analyser (SO2 sensor) in the exhaust duct downstream of the scrubber (and sometimes upstream) to confirm the emission ratio is below the limit.
2. CO2 concentration - measured to allow the SO2/CO2 ratio to be computed (the emission ratio method), so the SOx emission is expressed relative to CO2, independent of dilution.
3. Wash water parameters (for the discharge water): pH, temperature, PAH (polycyclic aromatic hydrocarbons), turbidity/particulate matter, and the flow rate of the wash water - measured in the wash water discharge line.
4. The scrubber operating parameters: the wash water flow, the pressure drop across the scrubber, the scrubbing medium (sea water or caustic) supply, and the engine load/fuel flow.
5. The fuel sulphur content (from the bunker delivery note / fuel analysis) used to set the required SOx limit.

Location of monitoring devices: the SO2/CO2 analysers are fitted in the exhaust gas duct (sampling probes) downstream of the scrubber; the wash water sensors (pH, PAH, turbidity, temperature, flow) are fitted in the wash water discharge line; the scrubber pressure/flow sensors on the scrubber; the engine load/fuel flow from the engine control.

Data storage: the monitoring data is recorded continuously by a data recording system (EGCS data logger) and stored electronically (on a computer/PLC) for a minimum period (typically 18 months) as required by the guidelines. The data includes the SO2/CO2 ratio, the wash water parameters, and the operating conditions, with time stamps.

Availability to regulators: the data is made available to the port state/flag state authorities on request, either by displaying it on the monitoring screen, by providing the recorded data (downloadable/printable), or by the ship's record book (EGCS logbook) in which the operation and any exceedances are recorded. The system must be approved (type-approved) and the ship carries the appropriate documentation; the authorities can inspect the data logger and the logbook during port state control.

Pollution of sea water can be caused by the discharge of the wash water from the scrubber, which contains the absorbed SOx (as sulphates/sulphurous acid), PAH (from the fuel), heavy metals, and particulate matter/soot. If discharged untreated, this acidic, contaminated water would lower the pH of the sea water locally and introduce toxic PAH and metals, harming marine life.

Prevention:

1. The wash water is treated before discharge: it is passed through a water treatment system (e.g. a water treatment unit with aeration, neutralisation, and a separator) to remove the solids/PAH and to neutralise the acidity (raise the pH) before discharge.
2. The discharge is monitored (pH, PAH, turbidity, temperature) to ensure it meets the discharge criteria set by the guidelines (e.g. pH not more than a certain value below the ambient, PAH below the limit, turbidity below the limit).
3. In closed-loop systems the wash water is recirculated and treated with an alkaline additive (caustic soda) to neutralise the acid, and the sludge (from the neutralisation) is collected and disposed of ashore (not discharged).
4. The discharge is prohibited in certain areas (e.g. within ports/harbours and in some waters) where open-loop discharge is banned; the ship must switch to closed-loop or low-sulphur fuel in those areas.
5. The sludge produced is stored in a sludge tank and disposed of to a reception facility ashore, not overboard.

These measures ensure the wash water discharge does not pollute the sea.