To say that the Olmsted County Environmental Resources Department has incredible foresight is an understatement. Located just southeast of the Minneapolis-St. Paul metropolis, solid waste manager Denny Siems and his team decided more than six years ago to begin separating their waste streams at the landfill with the future expectation of recovering fuel for a yet-to-be-constructed waste-to-energy plant expansion. So with little more than conviction, they started pulling their clean MSW from contaminated industrial waste, and for years, ran two working faces at the landfill.
“We’ve had a two-boiler WTE plant here since 1987, but it is quite small and we had to bypass it and send our MSW to be buried in the landfill,” explained Siems. “But about six years ago, we got a permit to build a third burn unit at the plant. We knew that when that came online, we would be able to start sending our clean MSW for use as fuel, and decided to plan ahead so we could maximize our landfill space and the energy of our waste stream.”
Fast forward to 2011, and sure enough, the new burn unit was built and brought online. With it, the plant’s capacity has doubled: the initial two boilers are 100 ton-per-day units, and the new third boiler is a 200 ton-per-day unit. Combined, the Olmsted County WTE plant now has 400TPD of capacity, which currently exceeds the available fuel in the marketplace, providing the ability to accommodate the region’s expected growth. But for Siems and the landfill, the opportunity to capitalize on their vision had arrived.
The landfill was at a crossroads. Buried in the ground was roughly 60,000 tons of clean, separated MSW that was put there with the specific intention of being used as fuel when the capacity at their burn plant was ready. The landfill was also facing a capacity shortfall … the current space was near full, and a new cell would have to be opened. In addition, their compactor was quickly approaching the end of its useful service, and it too would have to be replaced. Siems and his team got to work, crunched the numbers, built a convincing case and set out to get the community to agree.
Basically, they demonstrated that the County faced two options. The first was to open a new cell and purchase a new compactor, a path that would have required a capital investment of nearly $1.7 million. The second option was to purchase a slow-speed shredder, trommel screen, loader and some roll-off containers at a cost of roughly $1.5 million. The huge advantage of the second option (aside from the obvious $200,000 savings in expenditures), was the reclamation of all the extra landfill space combined with the additional revenue that could be generated from the value of the recovered waste as fuel.
Working with Hayden-Murphy Equipment Company, Siems invited DoppstadtUS to bring on site a slow-speed shredder and trommel screen to run a two-week feasibility test on the clean MSW face to determine with certainty the economics involved with mining that material. Armed with the positive results of that analysis, the landfill team had what they needed to present a strong case and secure the opportunity.
After what Siems described as a 10-month roller coaster, they succeeded in convincing local authorities to sign on to their vision and cleared all the regulatory hurdles required to put the plan in action. “It was extremely satisfying to get this plan enacted,” shared Siems. “Nothing like this had really been done before, and whenever you’re thinking outside the box to come up with a new idea, you’re going to have some extra burden of proof to convince everyone that the idea has merit.”
Olmsted County took delivery of a new Doppstadt DW 3060K slow-speed shredder and Doppstadt SM 726 trommel screen in early 2012. Designed to operate at a low 34 rpm, the DW shredder delivers strong reduction power through a high-torque, single tooth-and-comb shaft design. Material is shredded through the combs as the teeth rotate through, generating a rough, 10″ minus product. Pressure-sensitive accumulators on the comb are engineered to open at a user-defined threshold, harmlessly dumping their contents in the presence of harsh, unshreddable material. This durable engineering fits perfectly in this application, providing protection against unpredictable contents from the landfill.
The established operation begins with digging the landfill, pulling out the buried material and feeding it directly into the DW shredder. The material passes through the shredder, which discharges directly into the SM trommel screen. After screening, the overs are collected and sent directly to the WTE plant to burn as fuel, with the fines being salvaged for employment on site as ADC. After combustion in the WTE mass-burn boiler, the ash is recovered, spread on a pad, and run through an excavator with a magnet on the bucket to recover any ferrous metals. The metals are stockpiled to be processed (also using the shredder/trommel combination), and the ash is buried in a dedicated landfill cell.
Nearly a year into operation now, the results clearly show that the concept has been proven. “We only have a half-year of data to work with so far,” admits Siems. “But in little more than six months last year, we generated and sold more than $200K in excess energy, recovered nearly 10,000 yards of new air space and earned about $300K in revenue from the ferrous metals we collected from the ash.” Extrapolating those results to a full-year run, the County can expect to earn close to $1M in revenue from this operation and open 20,000 yards of additional landfill capacity. Siems estimates that the landfill has roughly 85K recoverable yards of space that contain close to 60K tons of waste, or about 4 years worth of burn fuel.
At the conclusion of this phase of the plan, the DW shredder and SM trommel can continue to be used in processing incoming material, further diverting the waste that comes to the landfill and extending the life of their existing space even more. The County has also taken advantage of this equipment to process bulky waste, such as mattresses and furniture, making it more efficient to dispose of effectively. This newfound capability has created numerous benefits for the community and the additional generation of revenue is always a welcome advantage in these challenging economic times.
With Rochester, Minnesota, home of the world-famous Mayo Clinic, centered in their County, the demand for energy and the need to effectively process waste will only continue to grow. Because of the vision Siems and his team provided more than a half decade ago, the County has found themselves well-positioned to handle this coming energy and waste management demand. By addressing both the economic and environmental concerns, their solution has been a clear win-win opportunity for the County and all its residents.
