Minot is outgrowing the lagoon and wetlands built to handle its sewage, members of the Minot City Council were informed this week. A study presented to the council's Public Works and Safety Committee Wednesday estimates the cost of a new wastewater treatment facility at $77.4 million.
"Building additional cells isn't going to work for us anymore," public works director Dan Jonasson had told the Finance and Improvements Committee Tuesday. "We are getting to a large enough population that we won't be able to clean our sewage with a lagoon and wetlands anymore. We will have to go to a full treatment system."
The trigger point for building a wastewater treatment plan is around 55,000 to 57,000 people. The state also could impose stricter limits on final discharges that necessitate mechanical treatment.
If the council gives its approval in finalizing the 2015 budget, plant design work could begin next year. The proposed 2015 budget has a capital improvements plan that includes $25 million worth of construction on a wastewater treatment plant in each 2017 and 2018.
A study by Apex Engineering suggested a phased construction approach, with completion by December 2019. The approach assumes Minot's service population will reach 55,000 to 57,000 people in five years. The sewer system serves some residents outside city limits. City officials estimate Minot's resident population at about 50,000. Faster or slower growth in coming would affect how quickly a completed plant is needed.
North Dakota's Clean Water State Revolving Fund has a low-interest loan program available with terms up to 20 years. Assuming no grant or other state assistance, the study estimated a user fee increase of $25.70 a month to pay off a loan and cover operation and maintenance expenses on a mechanical sewage treatment plant.
The average user of 1,200 cubic feet of water a month, or about 10,000 gallons, currently pays $94.31 for water, sewer and storm sewer maintenance and development. A budget proposal to raise the sewer fee would increase the monthly cost to $101.57.
Minot's existing wastewater treatment was constructed in phases going back to the 1960s. The treatment facility consists of two eight-acre aeration cells followed by five stabilization ponds and constructed wetlands. The current 850 acres of ponds can store 799 million gallons.
The key factor is the hydraulic capacity of the system, which engineers estimate at 12 million gallons per day. At a service population of 56,500, Minot likely will exceed capacity, the study reported.
Apex Engineering determined that Minot has a couple of options for the type of wastewater facility that would work for its needs. The options have similar construction and operation costs.
A mechanical wastewater treatment facility requires about 40 acres for process tanks, buildings and room for future expansion, according to the report. The city owns adequate land to the east of the lagoon area.
The facility would have gravity piping from the existing aerated cells. A pretreatment facility would remove debris, and the remaining wastewater would enter the main biological treatment process. It would end up in two final clarifiers to separate solids before entering a disinfection building and finally the constructed wetlands. Treated wastewater ultimately enters the Souris River.
The sludge that is separated would be broken down by bacteria and thickened before going into holding tanks for further mixing and aeration. In the spring and fall, the city would empty the tanks' liquid contents into semi-tankers for application as agricultural fertilizer.
The city is budgeting for a study next year that would investigate whether leakage is occurring from the existing wetlands system. The city has been paying damages to a neighboring farmer based on suspected seepage. Jonasson said the problem of increased alkalinity in the soil may have more to do with the higher than normal precipitation than with lagoon seepage.