By this time next year, Defianceâs water quality may be improved, or at least well on its way to becoming better. Thatâs because a huge project is about to take place at the cityâs Baltimore Road water treatment plant â installation of a new granulated-activated carbon (GAC) filtration system. This will remove trihalomethanes (TTHMs) â a treatment bi-product which Ohio EPA says can be harmful to certain segments of the population â and other impurities, and address taste and odor issues with the drinking water. TTHM levels require the city to issue regular notices to water customers noting the potential health hazards.