Engineers made a mistake when they designed the city's $9 million water plant upgrade, Mike Rothberg told the Craig City Council at its Tuesday meeting.
Their mistake does not amount to a design flaw or negligence, added Rothberg, senior vice president for Tetra Tech RTW, the Denver-based firm hired to design the plant upgrade.
The engineers made an error in their math when they designed the plant's raw water pump system, he said.
They planned a pump system that could operate during the Yampa River's lowest water levels because those are the toughest conditions on water pumps, Rothberg said. The engineers did not consider high water levels in their design, which is why the pumps are failing.
"There's no question there is an issue we had related to the treatment plant and the raw water pumps," Rothberg told the council. "The facility needs to be able to deal with the range of operating conditions it's going to face."
But, he has a solution to meet that need, one that should cost about $20,000.
The three raw water pumps, each valued at about $50,000, have been failing since the new plant came online last year.
Officials have sent two of them to Denver to be rebuilt at a cost of $10,000 each. In those cases, the general contractor hired for the project, Cortez-based Southwest Contracting, paid for the repairs.
The third pump also will have to be rebuilt soon, city staff said, though it's unclear who will pay the cost.
The pumps move water taken directly from the Yampa River into the plant's treatment cycle. River water flows into a belowground storage chamber underneath the pumps before being moved into the first treatment stage.
If the water level inside the storage chamber is high, then the pumps don't have to push the water as far up the chamber wall to get into the treatment cycle.
The problem is, the pumps were designed to push water from a lower elevation. Since the water is too high, the pumps are working too hard and shaking themselves apart.
To overcome this, Rothberg and city officials agree the pumps need more backpressure put on them, which essentially will equalize how hard the pumps are working with how hard they need to work.
The water plant's operators developed a temporary fix to use a plug valve to partially close the pipe going into the first treatment chamber. This puts more backpressure into the line and keeps the pumps operating stably.
Rothberg intends to make this fix permanent.
He recommended the city install a computer system to automatically adjust the plug valve based on how much backpressure the system needs.
"We believe this is the best solution," Rothberg said. "In my opinion, if we had thought about this high level situation, we would have put (a computer system) on at the design time."
Southwest Contracting plans to provide the system, Rothberg added, and gave a preliminary price of $20,000.
Rothberg could not give a timeline for when the system could be installed. In the meantime, the city's water plant operators will continue to manually adjust the plug valve.
City Engineer Bill Earley told the council that Rothberg's plan has "merit," but concerns remain over specific details.
The water plant operators also attended the meeting Tuesday and said they have "serious" concerns about future operations because of the mistakes Tetra Tech made in evaluating water levels.
Mayor Don Jones said he appreciated city staff's involvement but that the council had to trust Rothberg's advice.
"It's nice to hear the concerns of the staff, but at the same time we have to answer the concerns of our constituents about if we're going to have a water plant that's ever going to be functional," he said. "We have to listen to (Rothberg) on how to fix that. We have to give him that chance."
The mayor said later the city likely would pay for the new computer system.
Every engineer makes mistakes, Rothberg said to the council.
"The reality is, I never produced a perfect set of plans, and I don't know anybody who has," he said.
But, the consequences are small in this case, he added.
"It's a $9 million plant and we're worrying about $20,000 or $30,000 worth of stuff, and that still leaves us well within the contingency limit we recommended," Rothberg said.
Tetra Tech recommended between a 4 and 4.5 percent contingency fund, however city officials went forward on the project with a contingency equal to about 2.5 percent - or $200,000 - of the plant's $8 million construction cost.
Earley said the city didn't have much choice after construction bids came in about $1 million more than budgeted.
"With the water plant at the time, we barely had enough water coming out to meet demand," Earley said. "We had to do the project or we'd have had to start rationing water."
The city has spent $331,140 in contingency, or about 4.1 percent of the plant's construction cost. The cost of a new computer system to regulate the plug valve would bring the total contingency to about 4.4 percent.
The final contingency could surpass Tetra Tech's recommended 4.5 percent once city officials address problems with the plant's backwash pumps, problems Earley said are "very similar" to those affecting the raw water pumps.
Backwash pumps clean filters at the end of the plant's treatment cycle by pushing water backward through the system.
Those pumps have a computerized valve to create backpressure, as well as variable internal drives like the raw water pumps.
However, plant operators haven't been able to turn them on because they can't get the valve's programming to work with the pumps.
Until that situation is resolved, the plant has a temporary fix.
Operators can push clean water back through city water lines to clean the filters, Earley said, but doing so is inefficient and could possibly damage the pipes.
The plant's ultraviolet and dissolved-air floatation filtration systems also have not been fully tested because problems with the raw water pumps have prevented the plant from operating at full capacity.
Collin Smith can be reached at 875-1794 or firstname.lastname@example.org.