Indigo and heat pump: what they have in common

1. Step 1. Indigo — is a kind of — Dye. A dye.
2. Step 2. Dye — uses — Fermentation. Some dyes (indigo) were obtained by fermenting plants.
3. Step 3. Fermentation — is used in — Sewage treatment plant. Activated sludge is microbes.
4. Step 4. Sewage treatment plant — has part — Compressor. Blowers supply oxygen to the microbes.
5. Step 5. Compressor — is part of — Heat pump. A compressor.

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