Water Quality Q&A

Citizens posed a number of questions and concerns following annual chlorine maintenance by the North Texas Municipal Water District (NTMWD) and the City of Allen which occurred in March 2018. The following is a summary of the key citizen concerns with responses from the City of Allen and/or NTMWD.


What is chlorine maintenance?

Chlorine maintenance 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, breaking down a very thin layer of microorganisms called “biofilm” that form inside distribution pipes. By performing chlorine maintenance in early spring, NTMWD can reduce the growth of potentially harmful microorganisms, which can flourish during summer heat.

Cities may supplement chlorine maintenance with aggressive distribution line flushing.  During flushing, treated water is released from fire hydrants to allow biofilm 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.

If chlorine maintenance is done every year, why did so many people notice a stronger chlorine taste and odor in 2018?

NTMWD has performed the same state-approved system maintenance process for eleven consecutive years, including in 2018. In fact, NTMWD provided a comparison of chlorine concentrations, counted in parts per million (ppm), from 2017 to 2018. The concentrations of chlorine were very close. (See page 5 in the NTMWD FAQ document.)

This does not help explain why so many more water customers sensed a greater chlorine smell and taste in 2018. The water district is looking for answers to the question of why the 2018 chlorine maintenance period was more noticeable.  Hopefully, that answer can help us avoid the repeat of the increased odor and taste of chlorine in the future.  

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

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 do 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.   

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. After these filters are established, NTMWD indicates that it will be feasible to study the adjustment of the level of disinfectants introduced to provide for residual disinfection.