Marvin Fielding with Keller Associates met with the Rigby City Council to update them on the wastewater treatment plant study and alternatives for the city to consider when deciding how they plan to meet upcoming Department of Environmental Quality demands and the anticipated growth of the city.
Fielding said the city currently has a population of 4,000 people. In the study they calculated what the growth would be between now and 2040 with a 3.25 percent growth which equaled more than 8,000 people. He indicated that the city currently produces 1,200 pounds of biochemical oxygen demand each day. With the projected growth, the study figured 3,165 pounds would be produced each day.
“We’re at the point where we’re evaluating alternatives,” he said. “This is where we start to see the costs.”
The first set of alternatives is related to discharge. The current method of discharge is into the Dry Bed Creek. The first alternative was combining wastewater systems with the cities of Lewisville and Menan.
Fielding said Menan currently use land application and feels like it is in “pretty good shape,” while Lewisville does not have a community sewer system and would be interested.
The second alternative Keller Associates looked at was reuse. Fielding said this would involve a larger lagoon plus 300 acres for land applying.
“That’s a very, very expensive option,” he said. “It’s not really a feasible option.”
Because of the cost, it was recommended that the city continue to discharge to the Dry Bed Creek.
Fielding indicated that the DEQ will begin enforcing ammonia limits by 2023, which the current plant is currently unable to meet.
To help meet the future requirements, Keller Associates looked at four different options; build additional oxidization ditches like the city currently has, a new oxidization ditch configuration (only change the type of aeration and mixing), enhanced oxidization ditches using Integrated Fixed Film Activated Sludge (IFAS) and enhanced oxidization ditches utilizing Nuvoda Mobile Organic Biofilm.
According to the plan study, Keller Associates stated the enhanced oxidization ditch treatment has the lowest 20-year life cycle cost.
“The ability to grow and address our EPA (Environmental Protection Agency) requirements, kind of come together with that IFAS one,” Mayor Jason Richardson said.
Keller Associates also looked at disinfection alternatives.
According to the existing ultra violet disinfection system manufacturer, the city’s current system is obsolete and spare parts will likely be unavailable in the next five to seven years. Therefore the first alternative would be to look at a similar horizontal ultra violet system. The second option discussed was an inclined vertical ultra violet system.
Despite the report recommending a horizontal system, Fielding informed the council that the vertical is more efficient and eliminates the need to manually lift it.
“Incline vertical are now more efficient,” Fielding said.
Lastly, the study looked at solids thickening and dewatering alternatives. The existing gravity belt thickener and belt filter press combination unit will be unable keep up with the additional solids associated with ammonia removal and planned growth.
Three alternatives were chosen for evaluation; continue to utilize the gravity belt section of the belt filter press for thickening and hen purchase a screw press for dewatering; purchase a second combination unit for redundancy; or purchase a new rotary drum thickener for dewatering and a screw press for dewatering.
The recommended alternative is a new screw press as it has the lowest 20-year life cycle cost and would provide dewatering redundancy.
The estimated cost if the council decided to proceed with each of the recommended alternatives was $17 million which would in turn increase user’s monthly utility bills by at least $70.
Before making any final decisions on which alternatives to pursue, the council decided to hold a work meeting at the treatment plant to see the operation May 29 at 7 p.m.