Chlorine residuals in Flint: Continued improvements

Flint residents have been asking, “Since the automated flushing stopped in November, have chlorine levels dropped because the water is not moving through the system as quickly?” 

The expectation is “No.”  Cold weather improves nearly all aspects of water quality, because lower temperatures reduce corrosion rates, slow bacteria growth, and stabilize chlorine disinfectant levels in water. On the basis of our experience with dozens of water systems in cold northern states, chlorine residuals tend to be highest in the winter and bacteria levels lowest.

To examine this question for Flint we provide two datasets.

The first is my own chlorine data collected at 3 am from my home away from home in Flint (i.e., LeeAnne Walters’ house).  Yes, I also take baths in Flint water while I am there — thankfully I have never had a problem with the rashes that afflict some residents. This house has one of the worst problems getting a chlorine residual that I have ever seen.  In summer 2015 to spring 2016, we could not get any detectable chlorine in this house, even if I ran the water continuously (Figure 1).  But in August 2016 (with flushing), or January 18, 2017 (without flushing), chlorine levels were in a satisfactory range, especially considering that the data is from a 3 am sample.  In our experience, 3 am is the worst case because that is the time of least water use across a city. This house had an automated flusher right next to it in August 2016, so it would be expected to show amongst the greatest differences with and without flushing.

Figure 1. Free chlorine at Flint house at 3 am. (Note: For the Feb 2016, chlorine was non-detectable at both midnight and 6 am, and was therefore assumed to be zero at 3 am).

Heterotrophic aerobic bacteria levels have also plummeted at this house.  The recent levels in August 2016 and January 2017 were undetectable to ≈ 500 cfu/mL, compared to the very high levels we found in August 2015 of ≈ 500,000 cfu/mL or moderately high levels in February 2016 of ≈ 7,000 cfu/mL.

The second set of data was collected by EPA from all of their standard distribution system monitoring sites located around Flint, and they were kind enough to share it with me. I made a histogram graph (Figure 2) to compare a hot month (Aug 2016) with the automated flushing on (red line) versus early December 2016 after flushing had been turned off for several weeks.  Put simply, the recent data from December shows much higher chlorine without flushing, compared to chlorine residual data from August with automated flushing. The average chlorine has increased from 0.83 mg/L in August 2016 (with automated flushing) to 1.3 mg/L in December 2016 (without automated flushing). The key reason is that temperature dropped from 23 down to 12° C.  Because temperatures have been getting even colder since,  things should continue to improve.

 

Figure 2. Free chlorine monitoring data from the Flint water system in August 2016 (n = 161) with automated flushing, compared to December 2016 (Dec 1-20) when automated flushing was turned off (n = 54). Chlorine levels are much higher in cold weather, even when automated flushing is turned off.

The EPA and the State will keep monitoring the situation closely. Bacteria levels were dramatically improved in Summer 2016 versus Summer 2015 based on our monitoring. If automated flushing is adopted again in summer 2017, even more improvements in chlorine and bacteria levels are possible.

In Flint MI and elsewhere, the “good news” that higher chlorine brings in terms of controlling potentially harmful bacteria, also brings “bad news” in the form of aesthetic problems.  Chlorine can irritate skin and smells like….well, chlorine. Given the trends in data above, Flint residents now perceive that chlorine levels are higher than they have been historically. That is not just a perception– that is a reality.  In general, chlorine in Flint, has historically been lower than is desired, and also lower than is common in many other cities. Now that chlorine levels are returning to levels considered normal and desirable for bacteria control, Flint residents are noting the change.

What can be done to reduce the irritation due to chlorine, which is one of the most common consumer complaints about drinking water nationally? The state provided lead filters do remove chlorine taste from the water which is used for cooking or drinking. If the chlorine is causing irritation in the shower, inexpensive shower filters do exist that can help although we do not officially recommend such filters, because they could potentially grow some harmful bacteria in a shower device designed to create aerosols that could increase human health risks. We consider purchase of such shower filters to be a personal choice.  Whole house filters can also remove chlorine, but this potentially allows harmful bacteria to grow throughout the entire volume of the building plumbing system.

Primary Author: Dr. Marc Edwards

Acknowledgements: Siddhartha Roy

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