DRIGGS, Idaho — On Tuesday, Nov. 7, Driggs’ City Council accepted the draft analysis for a new wastewater treatment plant that will utilize a more efficient living biological process in order to mitigate ammonia discharge issues.
Dave Noel, an engineer for Forsgren Associates, presented an in-depth facility plan for the updated wastewater treatment plant to the city council. The draft was unanimously accepted and the city now begins the long process of securing Idaho Department of Environmental Quality (IDEQ) approval, applying for funding and drafting plans for the design and construction.
Driggs’ current wastewater plant, located southwest of town along Bates Road, was built and became operational only about 10 years ago. While the facility is currently in E. coli and harmful bacteria compliance, it has had a rocky history of failing to meet ammonia discharge level standards, according to Driggs’ Mayor August Christensen. After years of ammonia non-compliance, Forsgren Associates was hired to help the city figure out how to best address the problem.
The effluent ammonia criteria was briefly met in 2017 and was in compliance from mid August 2018 to the end of the year. Photo: Forsgren
According to Forsgren’s study a quick fix was not possible and compiled data recommended a completely different wastewater treatment in order to be compliant with the IDEQ.
Noel outlined several key issues plaguing the already-existing plant. He found that the current wastewater treatment plant exceeded its permit limits for flow and load. Flow is the rate at which wastewater is traveling through the system of sewage piping and load is the mass of the organic waste per unit volume of wastewater.
Noel’s growth curve chart averaged Driggs’ population growth from the past 30 years and calculated an approximate 4% gain in human impact per year. The bottom line was, the plant’s current design was maxed out and lacking the hydraulic capacity for the higher demand in the summer, with increased tourism and water runoff. When one of the system’s filtering chambers gets too loaded, it sequentially impacts the rest of the chambers and the ammonia eating bacteria is unable to do its work. The study also looked into ways to keep non-waste water, such as irrigation, rain and snow melt runoff, from entering the system through leaky pipes and manholes.
A graph depicting seasonal flow rate statistics for the time period between 2015-2018. Summer flow rates have increased across the board for average daily flow, maximum daily flow, minimum daily flow and length of the high flow season. The high season data is attributed to a seasonal population increase during the months of May through September. Photo: Forsgren Associates
The questions were, how big does the plant need to be and what treatment system will most efficiently allow the bacteria to break down the nitrogen and hydrogen in ammonia. A collection system for non-waste water sources and larger hydraulic holding capacity was also needed in the plant’s new design.
Noel then explained the engineering firm’s best solution to the city council, a traditional activated sludge process that re-circulates the bacteria biology over a course of 20 days, creating more time to metabolize contaminants, such as ammonia, E. coli and other “bad” bacteria. A pump would be installed to move everything back around in a circle, versus the current linear system where ammonia fighting bacteria only has 12 hours to do its work. Backwash pumps would be installed to clean filters and additional membranes would be more efficient in separating water. Forsgren Associates recommended using the already existing plant and expanding it to house the new system.
According Christensen, the estimated cost for the new treatment system is $25.5 million. If the plan is fully approved by the IDEQ, the design and construction process would take two to three years. Until full completion, an interim process is proposed to be in place in about 15 months, costing $6 million.
Mayor Christensen looks forward to the new facility. “I’m excited for the city to be able to take the next steps on our wastewater treatment plant for the wellbeing of our community, natural environment and to be in compliance of our permit,” Christensen said.
