Some stakeholders express: “Scrubbers generate scrubber sludge.” This sounds like there would be a by-product due to scrubber operation, where actually the contrary is true. Due to the closed-loop operation mode, scrubbers are able to absorb combustion by-products in the scrubbing process. Through a cleaning process, these by-products can then be separated out as waste – scrubber sludge. Scrubber sludge thus contains soot and oil compounds emitted by the combustion unit(s). Let me emphasize that there is no material from the scrubber system itself that forms part of the scrubber sludge, as some mistakenly believe.
Now we know where scrubber sludge originates from, but what is it like as a substance? Depending on the cleaning process, it can be liquid like water or solid and spreadable, or of course anything in between. A liquid sludge typically contains 5% solids (dry matter) while a spreadable sludge contains around 50% dry matter. The dry matter consists of the combustion-released soot and oil, but might also contain cleaning agents that are sometimes used in the cleaning process (flocculants and/or coagulants). The latter will always be separated out as sludge. The watery part of the sludge, being 50–95% of it, contains water and water-soluble substances from the scrubbing process itself, i.e. sulphates (due to absorption of SOx from the exhaust gas) and salts from the alkali additive needed to sustain the SOx-cleaning efficiency, which typically contains natrium or magnesium. The scrubber sludge will not contain a higher concentration of sulphates and salts than other process streams – it will contain the same share as the bleed-off water discharged to sea, and the same share as remains in the system’s uncleaned process water (washwater) until the latter is topped up with fresh water containing no or little salt.
If you are curious about an exact scrubber sludge analysis, one can be found in the Danish scrubber impact assessment study on page 60: https://www2.mst.dk/Udgiv/publications/2012/06/978-87-92903-30-3.pdf Loads of other information relevant to scrubbers can be found this same report.