Water Quality Q&A


What is temporary change in disinfectant (TCD)?

TCD is a proactive effort by the NTMWD and its customer cities to assist in cleaning city water distribution systems. This annual maintenance is performed once per year for a period of 28 days. During the rest of the year, NTMWD uses a mixture of chlorine and ammonia, called chloramine, for continued disinfection in city distribution systems. During chlorine maintenance, pure chlorine is used instead. It is a stronger disinfectant than chloramine, that effectively eliminates potentially harmful microorganisms that may exist inside distribution pipes. By performing TCD in early spring, NTMWD can reduce the growth of potentially harmful microorganisms, which can flourish during summer heat. NTMWD maintains the same concentration of chlorine in drinking water during TCD that it does of chloramines the rest of the year. By its nature, the chlorine presents a stronger taste and odor than chloramines.

Cities may supplement chlorine maintenance with aggressive distribution line flushing.  During flushing, treated water is released from fire hydrants to allow microorganisms to be pulled out by high-velocity water. The downside of flushing is that it uses a lot of water and works against water conservation goals.

What is a "chlorine burn" and is it different than "chlorine maintenance" or "TCD"?

NTMWD and its customer cities call the process chlorine maintenance because they do not increase the chlorine concentration in the chlorine maintenance period.  Some utilities may actually increase chlorine concentrations during this period, hence use of the “chlorine burn” terminology.

Can you reduce the amount of chlorine in the water so the taste and odor isn't as noticeable and disinfection byproducts (DBPs) are reduced?

Residual disinfectant, whether chlorine or chloramine, must remain in the distribution system of all customers to assure the continued ability to destroy microorganisms that could impact human health. Disinfectant concentrations decrease with increased time in the distribution system. This means customers further from the Wylie water treatment plant will experience concentrations lower than customers near the plant. NTMWD must assure every retail water recipient, such as Allen, is able to maintain the required minimum disinfectant concentrations, regardless of their distance from the water plant. The minimum concentration level for chlorine disinfectant is 0.2 parts per million; for chloramines it is 0.5 parts per million. This minimum is a point-in-time minimum, not a rolling annual average (RAA).  Dropping below either minimum level means the system is unable to safeguard human health.  This is a significant water quality violation and can cause the issuance of a boil water notice. NTMWD's position is that producing either chloramine or chlorine-only water at the plant in the 3.8 to 4.0 ppm range is necessary to assure adequate residual disinfectant levels throughout the entire regional water distribution system.

The other means to reduce Disinfection By Product (DBP) production in the distribution system is by reducing the organics, termed Total Organic Carbon (TOC), coming out of the water plant.  The organics are the contaminants that interact with the disinfectant to create DBPs.  In 2020, NTMWD will complete construction of Biologically Active Filters (BAF).  These filters contain live beneficial microorganisms that consume TOC and reduce the levels of contaminants available to form DBPs in treated water.