The process of cleaning a filter by running water backwards through it, or the action of water flowing backwards in a pipe or channel.
From 'back' (Old English 'bæc') combined with 'washing' (Old English 'wæscan'). The term emerged in the 20th century as swimming pool and water filtration technology became common.
This humble pool maintenance term reveals how swimming pools revolutionized suburban life—backwashing is essential because filters trap tiny particles that would otherwise cloud the water, which is why pools became possible as a convenient luxury rather than just public bathhouses.
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