School to Work

A close relationship with the University of Wisconsin helps Madison’s wastewater treatment plant find solutions to challenging problems

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At a treatment plant that has been practicingbiological nutrient removal (BNR) for more than 20 years and enjoys a unique relationship with the engineering and soils departments at a major university, siloxanes and struvite probably don’t have a chance.

A hub of water management research and innovations, the Nine Springs Wastewater Treatment plant in Madison, Wis., is well prepared to overcome the challenges of these troublesome residuals, as well as future issues.

“We enjoy a great relationship with the staff and students in the University of Wisconsin’s civil and environmental engineering programs, and more recently, the university’s soils department,” says Paul Nehm, director of operations and maintenance. “We sit down regularly with the faculty and plan graduate-level research projects. We help the students conduct their studies, and the students in turn help us find solutions to problems.”

Perhaps the most significant achievement of this applied research approach was work that showed that single-stage nitrification could perform well in Madison’s cold climate. The discovery saved the plant’s owner, the Madison Metropolitan Sewerage District, at least $2 million versus the cost of a two-stage nitrification system that had been proposed.

That was more than 25 years ago. Today, the focus is on removing siloxanes from the plant’s digester gas so that the chemicals, found in shampoos and other personal care products, don’t deposit in the engines that burn the gas.

Another issue is preventing struvite (ammonium magnesium phosphate) from scaling and clogging pipes and pumps. The plant’s operations and maintenance staffs, and a number of graduate students, have been working on the problems. Solutions are at hand.

80 years of service

The Nine Springs plant lies on the southeast side of Madison, Wisconsin’s capital. A regional facility since the 1930s, it treats about 42 mgd from a district that encompasses a number of cities, villages and townships. About two-thirds of the flow emanates from Madison. The plant also accepts septage from non-sewered areas in the district.

The flow enters the plant through a system with five interceptor sewers and 17 pumping stations and passes through a trio of rotating band screens (Brackett-Green, now Ovivo). Screen openings are 6 mm, and the screens operate with variable-speed drives that control influent wet well levels and maintain a minimum level above the influent flowmeters.

Three vortex-type grit chambers remove sand and grit, and the screenings and grit are deposited in containers for landfilling. Nineteen rectangular primary settling tanks (Siemens clarifier with Rexnord Industries’ non-metallic chain) remove floatable and settleable solids, which are pumped to the solids handling portion of the plant.

Secondary treatment consists of what Nehm and his staff call a “UCT variation” system. The activated sludge process is configured into anaerobic, anoxic, and aerobic zones, equipped with fine-bubble diffusers (ITT Water & Wastewater – Sanitaire). The process achieves phosphorus and ammonia removal in a single tank. For all of 2009, effluent averages were 0.09 mg/l for ammonia and 0.29 mg/l for phosphorus.

Treated water then settles in a combination of center feed/peripheral draw and peripheral feed/peripheral draw Envirex secondary clarifiers (Siemens Water Technologies). The overflow passes to a UV disinfection system which operates between April 15 and Oct. 15. Relief operator Dianne Krewald cleans all UV lamp banks with phosphoric acid solution during winter and prepares the units for the next disinfection season.

Plant effluent is discharged to two watersheds. A 5-mile pipe carries the greater portion to Badfish Creek, which empties into the Yahara River several miles south of the plant. A lesser flow is directed by pipe to Badger Mill Creek in the Sugar River watershed.

Useful biosolids

Biosolids are no afterthought here. In fact, a complex treatment train captures digester gas, turns it into fuel, and provides power and hot water for the plant. Gravity thickeners bring the primary sludge to about 4.9 percent solids, and dissolved air flotation units thicken the waste activated sludge to about 4.5 percent solids ahead of the anaerobic digesters.

The digesters operate as a single-stage mesophilic system, reaching temperatures of about 100 degrees F. They are equipped with Ovivo draft tube mixers and Infilco Degremont cannon gas mixers. Augmented with a liquid emulsion polymer, Ashbrook gravity belts thicken the digested biosolids to an average concentration of 5.4 percent solids. Using district-owned trailers, private contractors and district staff haul the material wet to about 60 area farms and apply it with district-owned Ag-Chem injectors (AGCO). The product is known commercially as Metrogro.

Gas produced in digestion is about 60 percent methane. Some gas feeds boilers for plant heating and runs an 800 hp Dresser Waukesha engine powering one of the aeration tank blowers. The rest fuels two 450 kW Dresser Waukesha engine-generators to produce electricity. In the most recent full year of operation, the total system produced a daily average of 10,440 kWh, and the engine blower saved the purchase of about 8,900 kWh per day.

There’s more: Heat from the engines is recovered to heat the digesters and most of the plant buildings. “I think we’re getting the most efficient use of the gas,” says operations supervisor Jeff Woerpel.

Smooth organization

The 85 district employees work daily to ensure that wastewater is transported safely to the treatment plant, that the treatment plant meets the discharge limits required by the Wisconsin Department of Natural Resources, and that the biosolids are safely recycled. “This requires the commitment of a talented staff at all levels of the organization,” says Nehm.

The plant’s engineering department works with various consulting firms, acting as project managers for collection and treatment systems. The special projects group includes a state-certified laboratory that analyzes samples from the plant and from surrounding communities in the district.

Two employees staff the purchasing department, and the collections team includes a five-person crew responsible for the sewer interceptors. The district maintains a squad of electricians, maintenance mechanics, and building and grounds personnel. All electricians and mechanics receive training through an apprentice program that leads to journeyman status.

Plant operations are the responsibility of eight operators, half working 12-hour shifts, covering the plant 24/7. The other four operators work 8-hour shifts and fill in for vacationing 12-hour operators. Five operators are certified Grade 4, the state’s highest classification.

Solving problems

The Nine Springs plant has had its share of challenges and, using the resources of its professional staff and the support from the university, it has compiled an impressive record of practical solutions.

In the past year, the plant staff has rebuilt a number of pumps and pump stations and revamped headworks, thickener and sludge circulation pumps. The electrical staff performed preventive maintenance under a computerized maintenance management system, calibrated electrical and instrumentation equipment, did thermographic testing of switches and motors, repaired and tested portable gas detectors, and inspected and cleaned all electrical components.

But the projects to control siloxanes and prevent struvite buildup have taken most of the attention in recent months. Siloxanes are on Woerpel’s front burner because they can end up as harmful deposits in the plant’s digester gas handling system.

“We had a catastrophic failure in one of the engines in 2006,” he recalls. “The operator heard a tremendous noise, and the machine simply quit. We discovered a chunk out of the engine crankshaft, and the pistons were full of siloxane deposits — not a good thing. We’ve seen a tenfold increase in siloxanes in recent years.” Moisture in the digester gas contributed to the problem.

To address those issues, the staff worked with Applied Filter Technology, a company in Snohomish, Wash., that had developed a solution in use at a number of landfills and at some treatment plants. The company provided a three-stage gas cleaning system. In the first stage, a large vessel containing wood chips removes sulfides. The final two stages use segmented activated graphite, a carbon based media, to remove the siloxanes so they won’t precipitate as a silicate and foul the engines.

Since the treatment equipment is located outside on a skid, senior maintenance supervisor Joe Lynch, and others devised a shelter to shield workers and the machinery from cold winds. Applied Filter provided basic training in the technology, but the operators essentially learned along the way.

One lesson involved media cleaning. “We cleaned the media in the sulfide removal tank and the first siloxane removal tank after a year and a half,” Woerpel says. “The sulfide removal media was really hard stuff. We had to call in the sewer department and use their Vactor unit to break it up and vacuum it out.”

The effort has been worth it. The siloxane levels are back to normal, and the plant has seen no further engine damage. “It’s been a bit labor intensive,” says Woerpel, “but we’ve been able to manage it.”

Struvite struggles

Even as the district wins the battle with siloxanes, struggles with struvite continue. Steve Reusser, operations engineer, shows a plastic bag full of pieces of struvite, looking like chunks of the White Cliffs of Dover. “This is uncontrolled struvite removal,” he says. Then, showing a clear vial full of small white beads, he adds, “This is controlled struvite removal.”

The beads are the product of a new process developed by Ostara Nutrient Recovery Technologies Inc. of Vancouver, B.C. It consists of an upflow column that forms the struvite beads, which can then be withdrawn from the process and marketed as a fertilizer. The Nine Springs staff plans to install the technology within the next two years.

“Our water is hard here, and we have a high level of magnesium in the wastewater,” says Reusser. “Our digesters produce a fair amount of ammonia, and ever since we installed our BNR process, a lot of phosphorus is released in the digesters.” As a result, struvite forms in the piping coming out of the digesters.

To make more phosphorus available for a process such as Ostara, the district worked with professors and students at the university to research ways to optimize phosphorus release from the waste activated sludge before digestion. This research has been going on for more than 10 years. Plant research engineer Alan Grooms has been further refining the process so it can be included in the construction of updated digestion facilities.

Innovative spirit

After more than 80 years of treating wastewater in the Madison area, the Nine Springs facility remains on the leading edge of process improvements and creative solutions. It’s a spirit of innovation embodied in the plant staff and in the professors and students who do research there. They feed on each other.

Last summer, graduate student Amanda Boyce was dismantling her successful experiment to remove excess phosphorus at the organic acid digester phase and recover it as calcium phosphate, which could be used for fertilizer. “It could be beneficial because it can greatly reduce the amount of phosphorus in biosolids, preventing phosphorus buildup in soil and the pollution of local waterways,” she says.

Boyce, who recently completed her master’s degree and has taken a job with the DNR, says the Nine Springs staff and facilities strongly supported her research. “They’ve done everything from analyzing samples to collecting waste samples for me,” she says. “They also allowed me to use several pieces of equipment, and since my project required daily attention, they gave me access to the plant on weekends and even holidays.”

Retired chief engineer and director Jim Nemke sums up the relationship: “The district has been extremely fortunate to have a great university in its backyard. The multiple projects completed cooperatively have resulted in some great decisions, development of talented engineers, and the advancement of the profession.

“Why wouldn’t anyone with the opportunity and resources get involved with applied research at the local level?”


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