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A treatment plant on Lake Superior manages biosolids with a flexible process and a beneficial use program that responds to seasonal demands.
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The staff at the Marquette facility includes, from left, Neil Traye, plant operator; Lyle Michaels, lab technician; Curt Goodman, superintendent; Mark O’Neill, plant supervisor; Neil Hayward, maintenance mechanic; and Bernie Stanaway, plant operator. Not pictured: Dan Johnston.

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When farmland is scarce, growing seasons short and winters cold, as in the northern rim of Michigan’s Upper Peninsula, what’s to be done with a growing volume of biosolids?

The Marquette (Mich.) Area Wastewater Treatment Facility answered the question with a flexible solids process that yields either Class B liquid or cake, and with diverse options for applying material in the field.

“I’m all for beneficial use of biosolids,” says Curt Goodman, the city’s water and wastewater superintendent since 1994. “I refuse to landfill it. We’ve always got that option as a last resort, but it’s something I just hate to do.”

Therefore, the city’s biosolids go to fertilize hay ground, to help reclaim and vegetate dikes around iron mine tailings basins and, most recently, to support growth of trees on a brownfield site once used for taconite storage. It all comes down to what is the most cost-effective option at a given time of year — and Goodman has pegged those costs carefully on a per-dry-ton basis.

Part of the process

Marquette, a city of 21,000, lies on the shore of Lake Superior, in Upper Michigan’s snow belt. Winters are long, and the land application season is short. That makes it necessary to store a significant volume of biosolids during the long wait for spring. And even in season, a scarcity of farmland can mean long hauls to the hay fields.

In view of all this, the biosolids process is not an afterthought. It was an essential component of a major plant upgrade completed in 2008. For 30 years previously, the plant had used rotating biological contactors (RBCs) for secondary treatment. However, a new permit for discharge to Lake Superior tightened limits on ammonia, which the RBCs were not designed to remove.

“We started planning for the upgrade in 2000 and were very fortunate to have good backing from the city to do the project right,” Goodman recalls. “We didn’t want to piecemeal it. We evaluated activated sludge and a couple of other processes and did a lot of work in-house. Before we brought an engineer on board in 2006, we had a good idea what direction we wanted to go.”

The upgrade saved substantial money by using and repurposing nearly all existing concrete structures, including tanks that held the six RBC trains. The new plant has a three-basin conventional activated sludge process with high-efficiency magnetic-bearing blowers (ABS) outfitted with variable-frequency drives, along with fine-bubble aeration (Environmental Dynamics International). Dissolved oxygen is controlled on a feedback loop (Hach oxygen sensor).

Between primary and secondary treatment, a bio-P (biological phosphorus removal) process supports permit compliance and achieved a 3 1/2-year payback by sharply reducing use of sodium aluminate for phosphorus reduction.

Right from the start

Biosolids preparation actually starts at the headworks with a Step Screen (Huber Technology) for fine screening, retrofitted to the existing influent channels in 2000. “We wanted a system where the operators would not have to physically handle the material,” says Goodman. “It was one of the most important projects we did for solids handling. The last thing you want is all the plastic items in your biosolids.

“That was a huge problem we had in the 1990s. Operators had to go out into the field to deal with complaints. The public perception was awful. Of course, our digesters suffered from the plastics — we had to clean them on a yearly basis. Since we installed the fine screening system, we have a much more efficient operation. It’s all automated. Once a day, the operators basically come up and take out the garbage. In 13 years of operation, we’ve never needed major maintenance on it.”

The plant has four primary settling tanks, one of them converted to a sidestream equalization tank. Filtrate from the dewatering press, decant from the digesters and biosolids storage tanks, and other sidestreams are conveyed to that tank and delivered to the influent stream at an even rate around the clock, avoiding slug loads of highly concentrated wastewater that could upset the treatment process.

Making biosolids

The plant has two digesters for primary sludge, each fed around the clock by an air diaphragm pump (Gorman-Rupp Co.). “The best way to feed a digester is 24/7,” says Goodman. “You get thicker material, less volume and less solids to deal with.” A digester for waste activated sludge also serves as a gas holding system, and a clarifier from the old plant now provides 400,000 gallons of waste activated sludge storage.

“You can’t beat anaerobic digestion because of the methane gas byproduct,” Goodman says. “We have two redundant dual-feed boilers [Bryan Steam] that burn our methane to heat the plant and heat the digesters. We’ve been doing that since 1978.” A system of heat exchangers and recirculation pumps maintains digesters at 94 to 95 degrees F. Digestion reduces volatile solids by up to 60 percent.

A combination gravity belt thickener and belt filter press (Komline-Sanderson) provides dewatering. The gravity belt thickener mode raises the solids content of sludges to 4.8 percent before delivery to the digesters. Digested liquid material is transferred to two 400,000-gallon storage tanks, and biosolids are drawn directly from them for spring and fall liquid application to mine sites. Before winter arrives, the tanks are empty.

The belt filter press mode dewaters digested biosolids to produce cake at 18 to 19 percent solids. “In winter, we have storage space for 1,200 cubic yards of cake,” says Goodman. “We’ll fill it up starting in November. Come April, as soon as weather permits, we can have it all emptied. The cake really helps lower our costs. And doing both liquid and cake gives us flexibility to fit the time of year and the fields available.”

Tackling the problem

The combination of liquid and cake Class B products gives the plant significant flexibility for beneficial use. Farms near the treatment plant are hard to come by, especially since many are excluded because their soils are high in phosphorus from years of commercial fertilizer applications.

“We have 400 permitted acres 50 miles away, so it’s cheaper to do the cake application out there,” says Goodman. “We deliver the material, and the farmer does surface application using a manure spreader. We are also working to secure some land about 20 miles from here; we’re working with the farmer and plan to open that site up soon.” The long-term goal is to have farmers pick up the material and do their own hauling.

Meanwhile, the plant continues a mine reclamation project, launched in 1994, when biosolids storage was extremely limited and reaching a breaking point. “At the time, we had just enough storage to make it to the spring,” says Goodman. “The mine project enabled us to do land application throughout the year, except in winter. That probably helped keep our old plant going for another 10 years.

“It’s a liquid application program, and it has been very successful. That’s a site-specific project where we worked with regulators to get approval. It required a lot of research on the front end. You just don’t take the material out there and dump it. However, the application is not limited to agronomic rates.

“If we’re starting from scratch on a tailings basin dike with no vegetation, we apply about 8 to 10 dry tons per acre, incorporate it, seed and mulch it, and pray for rain. If we get vegetation started, we do a secondary application of 4 to 5 dry tons per acre. Once vegetation is established, we do a maintenance application of 1 to 2 dry tons per acre. The application contractor has specialized equipment to control the amount applied. We find that applying liquid at about 6 to 7 percent solids works best.

“We developed the program with a local contractor who is a genius. He grew up on a farm and can do anything. He designed a machine that extends out onto the dike slopes and incorporates the biosolids into the sand.

“I call it a process. We’re building soil essentially from beach sand. It takes about three years to get vegetation established. When the mining company started to see good results, they took it upon themselves to start planting the trees. So when they leave the mine area, it’s all going to be reclaimed as a very nice forest.”

Getting creative

Recently, Goodman found another use for liquid biosolids: brownfield restoration. He contacted the owner of a barren 3,000-acre property, about 12 miles away in neighboring Negaunee Township, next to an iron mine and worked with him through the state permitting process. “We did a presentation to educate the township in what we were proposing,” says Goodman. “The better people understand what biosolids are, the more at ease they are.

“This is a demonstration. We only asked approval for 22 acres. We wanted something manageable to start with because we want to prove to the owner and the Department of Environmental Quality that it’s going to work. We’ve worked on that project for three years, doing a couple of applications a year.

“What I want to do now is go into silviculture. I want to plant some hybrid poplars and willows. Northern Michigan University here just built a new biomass-fueled heating plant. If I can grow the trees and sell them to the university as biomass, that takes reclamation and beneficial use full circle.”


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