From the Sterling Journal-Advocate (Sara Waite):
Rob Demis of Hatch Mott MacDonald didn’t have good news when he gave an update on his company’s review of Sterling’s wastewater treatment plant during Tuesday’s Sterling City Council meeting.
The engineering firm, which was contracted by the city to look at what improvements will be needed at the plant, has been working on the review for three months, but they are still looking at the data and developing preliminary alternatives.
The company has identified two primary issues facing the city’s system: flooding and improvements necessitated by upcoming changes to regulations.
Demis noted that the system has experienced multiple flooding events at the headworks facility in the last five years, from flooding of the river and heavy rainfall events. The flooding damages to equipment and pumping, overloads the pumping system and treatment plant, overflows into the river and leads to violations of the city’s wastewater permit. Each event can cost around $50,000 to $75,000 to replace the damaged equipment.
Inflow and infiltration are the factors that lead to flooding. The water comes from leaky pipe joints, roof drain connections, leaky manholes, missing manhole covers — storm water coming into the waste water system — as well as leaky customer sewer lines and sump pumps, Demis said. He added that Sterling’s system is relatively old; the design life of pipes is about 50 years, so pipes that have been in the ground since 1966 or earlier are at the end of their useful life. “It’s now time to start thinking about fixing them, or at least trying to slow down the amount of leaks,” he said.
He showed a graph that looked at a significant rain event. Prior to the storm, the typical influent flow to the system was averaging about 1.7 million gallons per day. The rain event exceeded the system’s pumping capacity, so the total amount of inflow isn’t known, but Demis noted that for two weeks following the storm, inflow remained above average. He said that is due to infiltration from groundwater leaking into the sewer system.
Sterling needs a larger pumping capacity, and with it a larger pipe to carry the waste water to the treatment plant. Demis said the city also needs to put in 30 million gallons of storage so when there is above average flow, that water can be fed slowly into the treatment plant and allow the biological processes to occur, which prevents violations.
Changes to regulations as soon as next year will require additional processes at the treatment plant. In November 2017, Sterling will have to meet a Total Inorganic Nitrogen (TIN) limit of 10 mg/L of Nitrogen when discharging to the recharge basins, which it cannot do. By 2022, the city will face limits on nitrogen and phosphorus that it also cannot meet.
The city will need additional tanks and chemical systems for nutrient removal as well as new process equipment, and new clarifiers as the existing ones are at the end of their life. The new equipment will necessitate upgrades to the electrical system, and they will need to implement a process control system to ensure they are meeting the requirements.
Demis noted that nitrogen and phosphorus are popular fertilizers, and they promote the growth of algae, which can kill fish.
Demis told the council he would make further presentations as they complete analysis of the data and the options available to the city.