Marshfield Utilities Demonstrates Hydrant Flushing Process
Hydrant flushing is an important task for Marshfield Utilities. Not only does it ensure that a hydrant is working correctly, it also helps ensure the cleanest City water possible.
“The purpose of flushing is two-fold,” said John Richmond, Water Department Manager. “We want to exercise the hydrant to make sure that it works properly. This is per the Wisconsin Administrative NR 810.13(2)(b): A hydrant must be exercised every two years.”
In some areas of the City, Marshfield Utilities flushes once every year to ensure water quality.
“The second reason is that back in 2010 Marshfield Utilities did a water study with Process Research Solutions to determine best practices in optimizing water quality,” he added.“One of the methods that came out of that study was the concept of Unidirectional Flushing. Unidirectional flushing is getting the water to flow in one direction through a series of water mains at high rates of flow velocity. This high rate of velocity scours the inside of the pipes and cleans out the sediment that has developed inside the pipes due to corrosion.”
While flushing, Marshfield Utilities crew members take samples of the water coming out every 15 minutes and measure the turbidity of the sample. (Turbidity is essentially the amount of sediment in the water sample.).
They keep flowing the hydrant at that high rate of speed until the turbidity is measured to be a unit of 1.0 NTU (Nephelometric Turbidity Units).
“During really warm times such as a normal July and August, we will flush what we refer to as dead end mains (mains that are connected to other mains by only one end while the other main is not connected, but ends in a hydrant),” said Richmond. “We do this to maintain a detectable chlorine level for proper disinfection of the system.”
As water warms and stays stagnate, the amount of residual chlorine decreases, so the old water is flushed out and allows for new fresh water to enter the area.
“Proper disinfection of the system is to make sure there is no bad bacteriological growth in the system that would make people sick,” said Richmond.
After implementing Unidirectional Flushing after the 2010 report, water quality has improved.
“The initial NTU’s are not as high as they once were but still way above the 1.0 NTU that we shoot for,” said Richmond. “It is our hope that as time goes on the amount of system area that will need Unidirectionally Flushed every year will decrease.”
Sometimes during the Unidirectional Flushing process, users will report discolored water.
“This happens when they consume water while we are flushing and they bring some high NTU water into their piping,” said Richmond. “This water is completely safe to drink. It is water that is high in iron which is what turns the water from the clear liquid that they are normally used to seeing into the yellow or orange tinged water that they see when this happens.”
Discoloration can also occur in other circumstances, such as during a main break, when an insurance company is doing a fire flow test at a hydrant, or if the fire department has to use the hydrant during a fire mission.
Richmond also explained that the Utility has received accusations of “wasting water” during the flushing process, which is not the case.
“No, it is not a waste of water,” said Richmond. “This water is dispersed to the environment because of a specific intent of keeping the water distribution system as clean as possible.”
Another misperception surrounding hydrant flushing pertains to staff time being used.
“We hear a lot of complaints about seeing the crew member either sitting in the truck or standing by the hydrant to watch it,” said Richmond. “That crew member is actually busy recording data every 15 minutes. They are there with a purpose.”