BioCycle November 2010, Vol. 51, No. 11, p. 21
After a fire in 2007 destroyed the plant, Sevier Solid Waste, Inc. has constructed a new and improved facility that reflects lessons learned from years of composting mixed waste and biosolids.
THE small subset of the composting industry that uses rotary drum reactors (RDR) gets together for two days each year at an operating facility to share experiences with rotary drum in-vessel composting. In September 2010, Sevier Solid Waste, Inc. (SSWI), which operates a mixed solid waste/biosolids cocomposting plant with five RDRs, hosted the Rotary In-Vessel Users Group meeting in Pigeon Forge, Tennessee. The group votes each year to recognize the best or most improved rotary drum facility, and in 2010 the Eweson Award For Facility Excellence went to SSWI.
Phil Hayes of the Pinetop-Lakeside Sanitary District in Arizona, one of the primary organizers of the annual meeting, presented the award to Tom Leonard, manager of SSWI, saying: “The award this year to SSWI and Tom Leonard is certainly appropriate given the devastating fire, and the fact that the largest MSW cocomposting plant in the U.S is again successfully running.” Daily flow to the facility in Sevierville is 275 tons of mixed MSW, and 60 wet tons of biosolids.
An article in the November 2007 issue of BioCycle, “Tennessee Composting Facility Rises From The Ashes,” recounts the story of the devastating fire on Memorial Day 2007 that destroyed almost the entire plant, built in 1992. Except for the five, 185-foot long rotary drums, which are not wholly contained within a building, the fire demolished over 100,000 square feet of buildings, including waste receiving, digester discharge and aeration floor, plus most of the associated equipment. The facility was insured for approximately $10.5 million. The plant was constructed at a cost of $6.5 million, and over a period of eight years, improvements were made, including the addition of two rotary drums, that increased total capital costs to $12.5 million.
Although SSWI could have issued bonds for a more expensive facility (the previous bonds had been repaid), SSWI’s Board of Directors handed Leonard a tall order: Rebuild a 100,000 tons/year facility for no more than the insured amount, and keep operating costs low enough to compete with area transfer stations (hauling to regional landfills) with tipping fees of $35 to $40/ton. While staying within the insured coverage was a challenge, SSWI just about met the Board’s capital cost mandate, coming in at a price tag of $10,547,267 (see Table 1). “Major aspects of rebuilding the 335 tons/ day cocomposting plant were things I learned from people I met at the annual RDR meetings and on various facility tours,” Leonard notes.
Since Sevier County owns the lined landfill adjacent to the composting plant, SSWI was able to dispose of its MSW there after the fire and while the facility was being rebuilt. “Our trash used up seven acres,” he says. “When the plant is operating we have approval to put the process residue in an unlined area so it’s a big incentive to keep the composting facility running.”
NEW DESIGN FEATURES
The rebuild provided an opportunity to address known challenges to facility operations. A review of the challenges and how they were addressed are below:
Traffic Flow: One of the major operational changes at the plant was to redesign the entrance road and scale house so that there is now one-way traffic and more sufficient space for garbage trucks to line up on the site. This virtually eliminated parking on the county road.
Aeration Trenches To Windrow Turner: Like many composting facility operators with aeration trenches embedded in concrete, Leonard and his staff had virtually given up on keeping the grates in the trenches from clogging with packed compost as they turned piles with front-end-loaders. Prior to the fire, SSWI had purchased a Backhus windrow turner and had stopped running the blowers. That turner was destroyed in the fire, and another Backhus Model 17.50 was purchased.
With a rebuild of the plant, the two new compost buildings do not contain aeration trenches, but do have leachate drains. Instead, windrows are turned an average of two times per day during the initial month of composting, aerating the piles and driving off moisture. When higher proportions of biosolids are being added to the MSW and moisture levels increase, the piles may be turned three times each day.
Tipping Floor Coating: In order to make the tipping building concrete more resistant to corrosion and damage, major portions of it were coated with a special product from Delta Pacific.
Recycle Screen Overs: Final screen overs greater than three-eighth inch from the Liwell screen are conveyed back to the tipping floor through an opening in the wall, making them convenient to use as inoculant for incoming MSW. This also enables recovery of the oversized organics in the final screen.
Hair Ball Grapple: One of Leonard’s major design enhancements, which all RDR operators who process mixed MSW can relate to, is a grapple to pull large balls off the discharge conveyor belt under the drums and load them into an adjacent dumpster. Such heavy “hair balls” are typically wrestled off the conveyor belt with loaders and manpower, sometimes even cutting them into pieces – all very time consuming tasks. Furthermore, a hairball that made its way up a conveyor headed to the primary trommel screen would exceed the motor capacity of the conveyor, or the trommel screen, shutting them down, and requiring more brutal wrestling matches.
Fabric Buildings: SSWI purchased four Coverall brand (now Norseman Structures) fabric buildings to house the tipping floor, digester discharge and two composting structures. One disadvantage of the fabric building for the Sevierville plant’s two composting buildings was that the aluminum frames could not be used to span more than the 200 feet needed (one structure is 200-ft wide; the other is 220-ft wide). Instead, steel frames were used, and Leonard pointed out some surface rust on the frames, lamented he had not coated them, and said he is evaluating coatings that can be applied to the frames in place.
Final Screen: Compost is screened after approximately 60 days of active windrow composting. The previous final screen, a trommel, was destroyed by the fire. The replacement is a Liwell, selected because of its widespread use at other MSW composting plants, and its ability to screen relatively wet material compared to other screens. “The maximum moisture level going into the screen is 45 percent, but we try to hit 38 to 40 percent,” says Leonard. The effective screening is attributed to a trampoline-like movement of polyurethane screen mats that rapidly alternate from loose to tight, stretching the mat panels and preventing blinding of the screen panels. All of the compost is sold to a broker who then markets it to farmers, soil blenders, landscapers and contractors.
Air Handling: An entirely new air handling system was installed in the reconstructed plant, something Leonard acknowledges has been the biggest challenge. Two, 94,000 cfm fans pull air out of the buildings and to the adjacent biofilter. “We were having trouble getting sufficient air out of the buildings once the new fans were operating,” he says. “Eventually we determined that the existing air ducts to the biofilter were too small to handle the 188,000 cfm coming through new, larger ducts from the building, so we will be replacing those ducts.” The tipping building is designed to provide eight air changes per hour; the digester discharge building is 10 per hour, and the two composting buildings are three per hour. To help control odors in the tipping building, two high-speed roll-up doors are used for trucks to enter and exit the building.
Biofilter: One of the few components of the previous plant that survived the fire was the biofilter, located far enough from the buildings to not catch fire. Despite the constrained air flow to the biofilter, Leonard says that so far there have been no odor complaints. The nearest occupied building is a National Guard center directly across the street.
One design improvement – which Leonard credits to participating in the annual RDR user group meeting – is a specially designed pretreatment biofilter to scrub out fine particles from the air stream prior to the main biofilter. The previous facility utilized conventional water spray scrubber towers. The new “soil scrubber” is basically a covered biofilter in a box where exhaust air from the buildings is pushed down through the media in a very short retention time. The filter is kept saturated to scrub out particulates prior to the conventional biofilter. The media can be easily replaced as needed.
Of the five existing RDRs, two had to be refurbished due to heat damage, and one is still not operating since it requires a major replacement of the drive gear. The drums are discharged early each morning, making room for loading of fresh material later in the morning. Front-end loaders push MSW and biosolids to an open pit serving each of the drums, then a hydraulic ram pushes the material in. Addition of biosolids helps achieve a moisture content of approximately 55 to 60 percent, ideal for aerobic composting.
Positive displacement blowers push air into the RDRs from the discharge end, counter to the flow of material through the drum, accelerating the degradation process and removing heat and moisture. The mix of 3-day old compost and inorganic residue is conveyed into a 35-foot long Doppstadt SST 1025E trommel screen, where the raw compost passes through the 1-1/4-inch holes. Overs from the trommel screen drop into a roll-off container, and are hauled to the adjacent unlined landfill. A moisture content of 55 percent is optimal for screening, but SSWI runs as high as 65 percent. “You just have to clean it more often,” notes Leonard.
SOURCE SEPARATED STREAMS
A fairly new occurrence in Sevierville is the increasing number of companies in other counties that are starting to send source separated organics (SSO) to the facility. “Our plant provides a recycling alternative,” explains Leonard, acknowledging that SSO can be advantageous to his facility since there is a smaller percentage of inorganic residue for landfill disposal. “I’d like to get more SSO, and since we don’t have to have 100 percent organic waste, I do not have to be stringent about contaminants.”
One SSO suitor has been Walmart, which has one store in Sevier County. Walmart stores in more distant counties have also sent their organics to the Sevierville facility until they find closer options. Another company using the plant is Green Mountain Coffee Roasters, which has a processing facility in Forks in the River, Tennessee. It sends about one load a day of coffee hulls to the Sevierville plant. Another new generator is the University of Tennessee in Knoxville. “The students decided to start a food waste separation program, and the University now hauls it to our plant, which is about an hour away,” explains Leonard.
Over almost 20 years of operation, SSWI has made local education a priority, with tours to school groups, an educational video shown in the community and played on local cable television, as well as radio announcements. “There are not many students in Sevier County that have not had a tour of our compost plant,” says Leonard. “It’s one of the more enjoyable parts of my job, to show kids how we turn trash into something useful.”
Robert Spencer, an environmental planning consultant based in Vernon, Vermont, is a Contributing Editor to BioCycle.