One of the many devices that takes Concord’s sewage and turns it into clean water will be getting an upgrade next year, with help from a federal grant.
The $3.8 million project will upgrade the wastewater treatment plant’s three secondary clarifiers, which were built in 1982.
“They’ve been running nearly continuously since then with nothing more than preventive maintenance,” said Dan Driscoll, Wastewater Treatment Plant superintendent.
Work probably won’t start until the spring because of the delay in getting steel components and will last most of next year. But don’t worry: Upgrades will be done to one clarifier at a time so the treatment plant can keep operating while it’s being done. Concord definitely does not want the plant to stop running.
“We can only shut down for about two hours before we start backing people up,” Driscoll said.
The cost will be partly covered by a $750,000 grant from the American Rescue Plan Act of 2021. The rest will be covered locally.
The clarifiers, like three concrete swimming pools that you don’t want to swim in, are part of a complex system of physical, chemical and biological methods in the treatment plant on Hall Street before cleaned water returns to the Merrimack River.
The watery mix comes into the plant from a huge series of pipes and is carried the final step by three enormous Archimedes screws, a design that’s been used to raise water around the world for more than 2,500 years.
It gets pumped through screens and a grit chamber to remove the objects that get flushed or washed down the drain – “sand, chicken bones, heavy debris” and a lot more – then goes into a primary clarifier. That’s a large settling tank where more material settles out through gravity, creating a sludge that is removed.
Then it goes through a biotower, where algae grown on filters removes contaminants, then a basin where it is aerated to add oxygen, then the second clarifiers for where contaminants – “mainly bacteria at that point” – settle to the bottom.
The cleaned water flows over top to a building where chlorine is added as a disinfectant, then sodium bisulfate to neutralize the chlorine, and then it’s out to the river.
The plant processes 2 ½ million to 3 million gallons of water a day.
