A Chlorine Demand

Have you been tasting and smelling chlorine when you shower or run water for your coffee this week? Blame it on the weather and the government. Due to work related to the new Center Street pedestrian bridge, water flow out of Saylorville Lake had been reduced. In turn, that meant Des Moines Water Works (DMWW) had to pull water from the Raccoon River to meet demand within the city of Des Moines. The 54 degrees (Fahrenheit) temperature of Monday quickly melted snow and ice. This rapid runoff could create a musty odor and taste. To counter that, DMWW increased the chlorine content in the water.

Chris Jones, the laboratory supervisor at DMWW, answered some questions for this post. Chlorine is added to the water supply as the last step before it is sent running through pipes to flow from household faucets. Usually there is 1 part per million (ppm), which dissipates the farther the water travels. On a typical day, the chlorine content for a customer in Des Moines is between 0.3 and 1 ppm. On Wednesday and Thursday, the amount being added was increased to 2 ppm. Powdered activated carbon, which absorbs odors, also was increased.

DMWW monitors the amount of bacteria in the river. When there is a snow melt, the amount of coliform bacteria can increase, but no such increase was noted this week. Thus, the chlorine was needed only to neutralize the musky characteristics of the river water. As the ice began to break up on the Raccoon River, DMWW was able to switch back to drawing water from the Des Moines River. Some ice had formed on the Des Moines but it disappeared once the Corps of Engineers increased the amount of water released from Saylorville.

The average daily demand for water from DMWW is 46 million gallons. It is rare to have a chlorine demand above 1 ppm. In addition to a rapid snow melt, more chlorine may be added during very hot weather when the molecules dissipate at a faster rate than normal. The current aroma of chlorine should work its way through the system in about another day.

The Army Corps of Engineers received a request from the City of Des Moines to reduce water flow from January 22nd through the 24th for the bridge-related work. The water flow had been 1120 cubic feet per second (cfs). It was reduced to 750 cfs. This week it was increased to 1200 cfs and reduced again today (February 1, 2008) to 960 cfs.

Jon Fussell in the city’s engineering department answered questions about the bridge work. The bridge abutments are going in close to the abutments of the dam, which required water to be routed around the dam for an inspection. In addition, Cramer & Associates, the bridge contractor, wanted to drive pilings instead of putting in caissons. Fussell said that was approved with some restrictions on vibrations. He also said the dam looked good, especially since it is over 100 years old.

M.R. Field is editor of Leading Voices: Iowa.