Coal processing had been a dominant industry since 1869. By the late 19th century, there were 22 coal plants with frontage on the canal. Coal plants along the upper canal used large amounts of water in the conversion of coal to coke, liquids and gases. Coal gas was soon used for heating, light, and factory power. Coke was used to make steel. Wastewater and coal tar (now known to be a source of carcinogens) were dumped back into the canal. Brooklyn's slaughterhouses dumped blood and other wastes into the canals.