汰

I couldn't find any reference to this kanji as meaning cleanse. It seems to be most often used in the following compound 沙汰（さた）to mean affair. The other meanings seem to be closer to 'select' so maybe Heisig meant cleanse in the ethnic sense here.. I'm going to use the keyword "selection", as in natural selection. Story: natural selection first began in the water, and those that were too plump were selected out.

Source: Kanji Koohii community