Durham (like most other triangle cities) uses a process called chloramination to treat its drinking water. Chloramine offers purported benefits over the other allowed option (chlorine) and many municipalities have been switching to it in recent decades.
One concern with chloramination is controlling nitrification. This can be a delicate thing to deal with, and the state of NC requires plants to annually perform these “burn out” periods, during which time only chlorine (and not ammonia) is added to the system. By doing so they can ensure that nitrification is avoided.