Though well past its boom times as a supplier of iron ore, the Iron Range area of northeast Minnesota enjoys a rich history and a fairly stable economy.
Three small Iron Range communities rely on the 0.5 mgd (design) wastewater treatment plant that bears their communities' names. The Coleraine-Bovey-Taconite Wastewater Treatment Facility nears its 25th anniversary having completed an expansion that includes a shift from Class B to Class A biosolids using a lime stabilization process.
The new process resolved a long-standing issue with meeting U.S. EPA standards for volatile solids reduction during the region's long, often frigid winters.
History of beneficial use
The CBT plant takes in wastewater at a main lift station with three Hydromatic pumps (Pentair) plus an overflow/bypass lift station with a Flygt pump (Xylem). From there, the flow enters an automatic bar screen (Parkson) and an aerated grit chamber that removes screenings and debris via a grit pump and classifier (Weir Specialty Pumps/WEMCO Pump).
Wastewater then passes through a Milltronics OCM III ultrasonic flowmeter (Siemens) to secondary treatment in two activated sludge tanks with Sanitaire fine-bubble diffusers (Xylem), and on to two secondary clarifiers (Walker Process). The waste activated sludge from the clarifiers is pumped to an aerobic digester with fine-bubble diffusers (also Sanitaire).
Secondary effluent is sent through a chlorine contact tank (Wallace & Tiernan), where chlorine and sulfur dioxide doses are fed automatically based on flow proportion. After dechlorination, the effluent travels to an effluent lift station (Hydromatic/Pentair) that pumps it 3.5 miles to the Swan River.
After a 40-day retention time, the solids are pumped to a belt filter press (Parkson) for dewatering and then to a reactor (Schwing Bioset) to produce Class A biosolids.
Before the recent expansion, dewatered biosolids were simply land-applied on area farms. "That was what we did from the time we opened in 1987 until about 1997, when the Minnesota Pollution Control Agency (MPCA) adopted the U.S. EPA regulations and changed the way we managed our biosolids," says Vernard Hawkinson, plant supervisor.
Iron Range winter temperatures can be challenging: In and around the town of Coleraine, average lows between November and February are near zero degrees F, and the thermometer once hit a record 51 below zero. For treatment plant operators like Hawkinson, that can be a logistical nightmare.
"The changes to federal EPA guidelines essentially made our whole biosolids process non-compliant during winter months," he says. "That process depends upon bacteria doing their job to reduce the volatile solids content in the biosolids to the level needed for producing a Class B biosolid.
"They do that just fine, except in extremely cold temperatures. When the rules were adopted, we had to meet the minimum 38 percent volatile solids reduction, or other options that were acceptable to meet the Vector Attraction Reduction rule. We were no longer in compliance during the winter, so we had to start taking steps to correct that."
One alternative approach CBT took was to stockpile the dewatered material during winter and then, when it thawed in spring, make it available to area farmers, who had to apply it to their land immediately and incorporate it into the soil. That too, had its problems, says Hawkinson.
"For one thing, because of the climate here, sometimes the biosolids piles would not thaw until the end of May, and most farmers wanted their planting to be done by then, not just starting," he says. "We did that for a few years before deciding it would be better to simply haul the liquid material to the Grand Rapids wastewater plant, or take the dewatered material to their landfill. Unfortunately, they were less-than-receptive to those plans on a long-term basis and we found ourselves back at square one."
Toward Class A
Hawkinson made other efforts to comply with the MPCA and EPA rules. Those included achieving the 38 percent volatile solids reduction using the approximate mass balance equation, and conducting a Bench Scale Analysis and Specific Oxygen Uptake Rate Analysis — all to no avail.
One solution that came to the forefront was to add a second digester. "That seemed like the only recourse we had at the time," says Hawkinson. "Adding a second tank would afford us the extra digestion time we would need to reduce the volatile solids. So we found an engineering firm to tackle the project.
"Their initial estimate came in at about $700,000 for the second digester, and the design process was started. Unfortunately, as the project progressed, additional site work was determined to be necessary which, through no fault of theirs, escalated the cost to roughly twice the original estimate."
During that same time, Hawkinson attended a seminar put on by the Minnesota Rural Water Association at which representatives from Schwing Bioset presented their biosolids treatment process. Intrigued, Hawkinson spoke to a company representative at the show. After back-and-forth talks, Schwing Bioset estimated an installation at CBT would cost about $700,000 — the same as the original estimate for the digester — and the end product would be Class A biosolids.
"That made good sense to us on many levels, so we cancelled the digester design project and committed to Bioset," says Hawkinson.
Just add lime
The Bioset process mixes biosolids exiting the dewatering belt press (with a solids content of about 15 percent) with quicklime and sulfamic acid using a twin screw feeder. Then, using a Schwing KSP-5 pump, the mix is delivered under pressure through an insulated reactor. The enclosed process contains dust and odors while maintaining a constant temperature of 158 degrees F for at least 30 minutes to ensure that all pathogens are neutralized.
Working with the U.S. EPA Pathogen Equivalency Committee (PEC) through the Process to Further Reduce Pathogens (PFRP) treatment process in the 503 regulations, Schwing Bioset has obtained approval for the Bioset process to operate at 131 degrees F, provided the ammonium concentration within the reactor is above 0.5 mg/g dry weight. This offers a significant reduction in chemical usage from the standard 158 degrees F operating regime and translates into an approximate 30 percent reduction in operating costs.
The system at CBT also includes a recirculation feature in which biosolids that have not achieved the necessary temperature on startup are returned to the hopper for reprocessing. Upon exiting the Bioset process, now with a solids content in the 35 to 38 percent range, the Class A biosolids are discharged to a 48- by 75-foot storage building, constructed at the time of the biosolids upgrade. The plant produces about 35 dry tons of biosolids per year.
Popular with farmers
"We now have an excellent product with an effective acid neutralizing power of approximately 64 percent of pure lime," says Hawkinson. "Farmers in the area, who see the product as a great way to raise the pH levels of their soil, have agreed to take as much as we can produce.
"That's a far cry from having to actively seek a place where we could take it. Right now, we only need to rely upon the Bioset process for half the year — we get the volatile solids reduction we need in warmer weather. But it is an excellent alternative, and it has allowed us to create a Class A biosolid with the same outlay we would have incurred for the Class B product. It has really worked out well for us."