Blackwater is cheaper and easier to process than greywater.

The question seems to be incomplete, but both types of water are involved in the sewage systems in society.

Blackwater is basically wastewater that contains materials flushed from toilets (e. g. feces, urine). Greywater on the other hand, is also known as sullage. They are wastewater that people generate from sources that are not coming from toilets (e. g. food washing, dishwashing, bathing, clothes washing).