Wastewater from toilets and sewage systems, called 'black' because of its dark contaminated color; also used for murky water in swamps or rivers.
A compound of 'black' (Old English 'blæc,' meaning dark) and 'water.' The term became standard in sanitation and wastewater management to distinguish it from 'greywater' (from showers and sinks).
Blackwater management is crucial for public health—a single liter of human sewage can contain up to 200 trillion harmful bacteria, which is why modern treatment plants use a combination of settling, bacterial digestion, and UV light to make it safe again before returning it to rivers.
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