Sean C. Morgan
Sweet Home’s proposed Wastewater Treatment Plant upgrade will cost as little as half of earlier estimates, according to Murraysmith, the firm designing the upgrade.
Previously, Brown and Caldwell had estimated the cost at up to $42 million. Murraysmith representatives think it can be done for less, as low as $21 million, and the new plant could save up to $300,000 in operational costs annually.
Murraysmith Project Manager Preston Van Meter presented initial schematic design overview to the City Council during its regular meeting on Feb. 13.
“We need to look at all our infrastructure, salvage what we can salvage, improve what we can improve, replace what we have to replace,” said Public Works Director Greg Springman. Murraysmith has gone through and taken a detailed look at the plant to get to this proposal.
Preston said the design team thought it “may be better to use some of the existing assets. You already own it.”
Just about any engineer can figure out how to build a facility, Preston said. What he likes to do is rebuild them.
His goal is to help keep rates affordable for the community, he said.
He said Sweet Home’s treatment costs are 30 to 60 percent higher than the industry average, and improvements can help reduce that while increasing plant capacity to handle 11 to 12 million gallons per day, enough to handle current peak flows of around 11.5 million gallons per day. Currently, the plant has a capacity of about 7 million gallons per day.
That’s the amount of water that can reach the Treatment Plant during a heavy storm through inflow and infiltration, which is storm water that enters the sewer system through deteriorating pipes and cross connections to storm drainage.
Four I&I reduction projects have been completed, which replaced sewer mains and lateral service lines throughout large parts of the city. Prior to the work, the plant could receive wastewater flows of up to 22 million gallons per day, forcing the plant to bypass highly diluted but untreated wastewater into the South Santiam River.
The Oregon Department of Environmental Quality entered into an agreement in 2000 allowing Sweet Home to avoid fines by decreasing I&I.
Each I&I reduction project cost more per gallon of I&I reduced, from 47 cents per gallon in the first two phases to $2.86 per gallon in the fourth phase, according to Murraysmith figures. The city has spent some $12.1 million on the four projects.
It’s been one of the more successful I&I reduction projects, Van Meter said.
Sweet Home is a model of how to do I&I reduction right, said Rob Lee, Murraysmith I&I abatement lead, but the city will need to continue to work in its collection system to offset the additional flows as the city grows.
The city can make up some more ground in the plant, handling the remaining excess, by focusing on rehabilitating the plant, which can half the cost of new construction, he said.
“What I actually see is some concrete that’s not in really bad shape,” Van Meter said. Rehabilitation is cost-effective as long as it’s early enough. “What I see is a pretty structurally sound facility that needs a little work. Our opinion is you could recover the original service life.”
Some concrete structures are still functional after 100 years, he said, and several techniques are available to rehabilitate them.
Preston outlined the basic steps the city will need to take at the plant. With upgrades and rehabilitation, the influent pump station to handle 11 to 12 million gallons per day.
“We’ve done lots of pump stations, and this is a pretty straightforward rehab,” Preston said.
The headworks needs an upgrade, he said, which really means replacing the screen that removes wipes and other materials residents flush down their toilets that cannot be processed.
That’s one of the most critical improvements, Preston said. It takes the trash out to help avoid impacting the microbes in the aeration basin that eat and break down waste in the water.
He said he was surprised the screen wasn’t replaced in the early 1990s when the plant last had an upgrade.
The plant’s aeration basin can be reconfigured with “plug flow” system to inject air into the basin, Preston said. The current surface aeration system is the least efficient oxygenation method.
Those upgrades could improve efficiency by 25 to 30 percent within the same footprint, Preston said, and that can be adjusted easily for use in summer or winter conditions.
In the disinfection basin, one of the challenges is the long contact time necessary for the bleach, Preston said. The current contact chamber is undersized looking out 20 to 40 years.
Ultraviolet disinfection can use the existing structure and provide plenty of capacity, he said.
For solids digestion, he proposes installing a new cover on the basin and converting the system from an aerobic process, using microorganisms such as bacteria, and protozoa for the decomposition of organic waste matter, to an anaerobic process that uses bacteria that do not require oxygen.
The anaerobic process is much more efficient, requiring a smaller volume to do the work, Preston said. It also uses less energy and has the ability to produce energy through the production of methane gas.
The plant will need new dewatering equipment to handle the biosolids taken from the bottom of the clarifiers, Preston said, and the city may need to rebuild the structure housing the equipment.
The city can take immediate steps to improve operation and maintenance at the plant by replacing the influent screening and in the aerobic digester, Preston said.
At the end of the project, Preston would like to keep the cost to ratepayers in the range of $60 to $70 per month per equivalent dwelling unit, approximately the current cost, he said.
Noting that the first I&I reduction project loan would be paid off in 2025 and the second and third would be paid off in 2032, the city has some options to pay for the project, Preston said. A 40-year U.S. Department of Agriculture loan may keep rates about where they are now.
The schematic design process will take the remainder of 2018, Preston said. Regulatory review and funding agreements would take place next year, with final design complete in the first quarter of 2020. That will be followed by additional regulatory review and approval, with bidding and contracts in the third quarter of the year and construction beginning in the final quarter, with the project complete in October 2022.