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What does "digest" mean in the following?

The remaining sewage then enters the secondary tanks for the third stage of treatment. The solids that were not treated in the primary tanks are removed here through decomposition, which digests the material. Then, the liquid sewage is filtered through sand.

If it means "break down," isn't the meaning superfluous since it is already conveyed by "decomposition"?

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  • I agree that it is redundant. I also think that it is confused: it is not the decomposition that digests the material, any more than it is our eating which digests our food: we, or our stomachs, digest the food, and the tanks or their contents which digest the material,.
    – Colin Fine
    Aug 29, 2020 at 13:26

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You're right about the meaning and that it might seem redundant here.

The general sense of the word is exactly the same as in "to digest food." Merriam-Webster gives a few applicable definitions that all have to do with decomposition or breaking things down:

  • 2 : to convert (food) into absorbable form
  • 4 a : to soften, decompose, or break down by heat and moisture or chemical action

Wikipedia describes the scientific term anaerobic digestion:

Anaerobic digestion is a sequence of processes by which microorganisms break down biodegradable material in the absence of oxygen. The process is used for industrial or domestic purposes to manage waste or to produce fuels. Much of the fermentation used industrially to produce food and drink products, as well as home fermentation, uses anaerobic digestion.

...

Anaerobic digestion is used as part of the process to treat biodegradable waste and sewage sludge. As part of an integrated waste management system, anaerobic digestion reduces the emission of landfill gas into the atmosphere. Anaerobic digesters can also be fed with purpose-grown energy crops, such as maize.

While the casual reader (which includes me) might see this as redundant, a technical reader (such as a sanitation engineer, biologist or other scientist) might see digest[ion] as a more technically-specific reference to the process behind the decomposition.

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  • If the author meant to explain something, he or she should have switched "decomposition" and "digestion" around. Digestion as a specialized term in chemistry refers to a type of decomposition, but not vice versa. I'd have expected: The solids that were not treated in the primary tanks are removed here through digestion, which ecomposes the material.
    – Apollyon
    Aug 29, 2020 at 14:02
  • @Apollyon Yeah, I think I agree. But I'm not an expert in this field so I can't be sure I'm not missing some aspect of the writer's thinking.
    – TypeIA
    Aug 29, 2020 at 18:49

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