How many different types of sludge are there?

3 types:

Sewage sludge

Septic tank sludge

Industrial sludge

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The term "sludge" is used in different disciplines to mean different things. For example, there is

sewage sludge

industrial sludge

metal sludge

gallbladder sludge

engine sludge

the muddy gunk on the bottom of a stream or river

slushy ice on the ocean

an agglutination or aggregation of blood cells