RAPID CITY CLOSES THE LOOP ON MSW MANAGEMENT

Nora Goldstein

BioCycle November 2003, Vol. 44, No. 11, p. 34

WHEN Jerry Wright joined the Rapid City, South Dakota Department of Public Works in 1988, he asked the landfill supervisor how much capacity was left in the landfill. "He said there were six to seven years left," recalls Wright, "and I replied, we are in deep trouble if we don’t start acting quickly to plan for the near and long-term future."

One of the first steps taken was to redo the grading of the landfill, which bumped its life up to 28 years. Another was to analyze the rate structure at the landfill. "In 1988,the tipping fee was $2.10/ton — about one-third of what it should have been just to cover operations and maintenance (O&M) costs," says Wright, who is superintendent of solid waste operations. "We also had the Subtitle D landfill regulations coming along, and it was expected that 40 percent would be added to landfill O&M costs for environmental controls in order to be in compliance."

The city hired Alliance Architects and Engineers (Alliance) and HDR Engineering to work with city staff to study both landfill rates and Subtitle D compliance. It was decided to raise the tip fee to $10/ton. Almost immediately, the flow of certain materials — particularly from industrial and commercial sources — stopped coming. "A press board plant nearby that uses wood as a feedstock would bring its wood waste to the landfill when the fee was $2/ton," Wright notes. "At $10/ton, it was more economical for them to use it as a fuel." Similarly, businesses started recycling and baling corrugated.

While changing the landfill grading and raising the tip fee were a good first start, it was clear that longer term planning for solid waste management, both for the city and the region, was necessary. The city council decided to form a citizen’s committee, comprised primarily of residents, businesses and elected officials. Meetings began in May 1990, and were held monthly. Issues discussed included Subtitle D compliance at the landfill, solid waste management alternatives to landfilling, and forthcoming recycling requirements by the state. Options were far-ranging, and included a bottle bill, building a new landfill, yard trimmings composting and waste reduction measures to lower the amount of waste being landfilled.

The committee issued a report in July 1991. It recommended doing yard trimmings composting, establishing a recycling program, and evaluating solid waste composting and refuse derived fuel. It also cited the need for a comprehensive solid waste management plan, which the mayor’s office already was considering. Part of the plan would advise the city how to comply with state legislation called the Second Century Environmental Act that banned certain materials from being landfilled. These included yard trimmings, containers, white goods and tires. Many community meetings were held to discuss alternatives, costs and projected results. The population of the city and surrounding region, which includes three counties and several small communities, at the time was about 80,000. (It currently is around 85,000, with Rapid City’s population at about 64,000.)

Alliance and HDR were hired to work with city staff to develop a solid waste plan. "One of the primary issues we studied was the cost to update the current landfill versus siting and building a new landfill," recalls Dallas Wasserburger of Alliance. "Buying comparable acreage and building the landfill would cost $10 million to $15 million and the siting and permitting process could take as long as five to ten years, given the experience of other communities. We also would have had the closure and postclosure costs of the current landfill, plus we would have had to cover the cost of new road construction and maintenance." Adds Wright: "I figured that even if the new landfill were only three miles out further, it would add $150,000/year in hauling costs as we get over 50,000 vehicles/year coming into the landfill. When all those costs and considerations were added up, the plan recommended working with the solid waste utility and acreage we had at the existing site. And the burden would be on us to be a better neighbor as the community expands and moves closer to our site."

The Solid Waste Management Plan ultimately approved had three main elements: Implement a yard trimmings collection and composting program; Build a Materials Recovery Facility (MRF) to process and ship traditional recyclables; and Build a MSW organics composting facility. The MRF and both composting facilities were to be located at the site of the current landfill. There is a total of 345 acres, with the landfill using about 250 acres for storage and disposal of garbage. Site neighbors include a detention center, state highway patrol and several industrial facilities. The nearest residential neighbors are about one-quarter mile away.

Implementation of Rapid City’s solid waste management plan has taken place over a ten year period, starting with yard trimmings composting in 1993 and ending with start-up of the MSW composting plant in 2003. The current tipping fee is $45/ton, enough to support — along with revenues from sale of recyclables and compost — the entire operation. This article relates the evolution of a forward-thinking approach to maximizing recovery in a region with available land for disposal, but a mindset of reduction and recycling.

YARD TRIMMINGS COMPOSTING

Composting of yard trimmings began in 1993. The first year, diversion was voluntary; the next year (1994) no landfilling of yard trimmings was allowed. "A waste composition study done in 1993 showed that yard waste was 10 to 15 percent of the total MSW generated," says Wright. "And it is the easiest and cheapest to pull out of the waste stream." The city established three dropoff sites, and placed 20 cubic yard rolloff containers at each for collection. Curbside collection is available to these receiving city service (about 16,500 households); citizens must place yard trimmings in kraft paper bags, which are collected the same day as garbage and recyclables (during the season when yard trimmings are primarily generated). Yard trimmings also can be brought to the landfill at no charge. About half of the annual tonnage received (about 13,000 tons in 2002) comes from the dropoff sites; the remainder arrives either through the curbside program or self-hauling to the landfill.

Leaves, grass, brush, tree limbs and some manures are accepted. All materials are ground in a Peterson Pacific 5400 horizontal grinder before being placed into windrows. A Scarab turner mixes and aerates the piles once a week initially, then as need after that based on pile temperatures. Average time for the process is five months. Finished compost is screened to either three-eighth or three-quarter inches, depending on the end use. The facility has both a Fuel Harvester and a Portec trommel screen. The three-eighth inch compost is sold for $30/ton; three-quarter inch sells for $25/ton, and the three-eighth inch rejects, mostly wood chips, sell for $10/ton. "Right now, we are producing about 4,000 tons/year of product," says Wright. In 2003, revenues from sale of yard trimmings compost and screened overs are expected to be $75,000. In the first year of the program, sales were $10,000.

RECYCLABLES COLLECTION AND PROCESSING

The next step in the plan was to implement the recycling. Citizens voted against a mandatory program. The city started with a blue bag collection system. Only containers are collected, including #1 (PET) and #2 (HDPE) plastic, steel cans, aluminum beverage containers, and glass jars and bottles. There are dropoff locations in the city — targeted for use by residents in apartments and those not on city collection routes — for those same materials, as well as newspaper and uncontaminated cardboard. "When we were designing the program and looked at the paper markets, we felt they were too volatile," says Wright. "We decided we would handle that material in the MSW composting part of the plant. The paper and corrugated that does come in from the dropoff sites is baled and sold."

Initially, the blue bags were cocollected with the trash, however that created the perception that the recyclables citizens had carefully separated were being thrown away. In addition, the bags would break and their contents would get commingled with the garbage, making it difficult to recycle them. As a result, the solid waste department opted to dedicate one of its six collection trucks to recyclables only.

Construction of the MRF began in 1995, and it opened in 1996. Design, construction and equipment costs were $13 million. In anticipation of the MSW composting facility, the MRF was designed with one sorting line for recyclables and a second one for the solid waste fraction. A movable feed conveyor is put in place to bypass the recycling sort line when MSW is being processed. All Heil equipment was used in the MRF. The same staff is used to sort recyclables and MSW.

In 2003, Rapid City switched to automated collection of trash and recyclables. The 16,500 households serviced by the city (about half of all households) received two carts — a 35-gallon blue cart for recyclables and a green cart for trash. Residents choose between three different sized trash carts — 35-, 65- and 95-gallon and pay a variable rate, depending on the sized cart selected. Table 1 shows the rates/cart, as well as other fees charged to households on a monthly basis. The recycling containers are provided at no additional cost to customers. The switch to automated collection, which required a capital investment of about $2 million in eight new trucks and 33,000 garbage and recycling carts, is expected to result in a net savings of $2 million during the next ten years as a result of reduced labor costs and improved collection route efficiencies.

Trucks enter the 40,000 sq. ft. receiving building through quick opening and closing bi-fold doors that open to the side (versus an overhead door). Collected recyclables are emptied onto the tip floor and pushed onto a feed conveyor. A bag opener is at the top of the feed conveyor; materials then go into an enclosed sorting station. Originally, that picking station was designed to remove the blue bags, and any other nonprocessible materials. Now that the cart-based collection system is in place, the first sort station is used to pick nonrecyclable items. Remaining materials pass under a magnetic separator (capturing steel cans) and then drop onto a finger screen, which is designed to separate out broken glass. They then proceed to an incline conveyor that passes under an air knife to separate the remaining glass from the plastic and aluminum fraction. (All recovered glass is conveyed to a pulverizer.) A second sorting station is in place to pull off #1 and #2 plastic. The conveyor then proceeds to an eddy current separator that captures the aluminum. Any material left on the conveyor (primarily nonrecyclable plastic) falls into a reject bin.

Staff designed a pneumatic system that blows recovered cans and plastics to cages in a room with the baler. Separated materials are then conveyed to the Mosley Gorilla baler. In addition to baling recyclables, the machine can be used for recovered scrap metal as well. Table 2 shows the tonnages of materials shipped from the MRF from 1996 (the first year of operation) through 2002. The decline in recyclables shipped between 2001 and 2002 is attributed to shutting down the plant for construction of the cocomposting facility.

MSW PROCESSING

The sorting line for MSW was installed in anticipation of eventual composting. It was designed to remove oversized materials that could damage the preconditioning and composting equipment. A feed shuttle conveyor is put in place to bypass the recycling line and move materials to the MSW sort line.

At about the time that the MRF was getting ready to go into operation, Rapid City learned about and proceeded to acquire two Dano rotating drums that had been used at a mixed waste composting facility in Portland, Oregon. That plant operated briefly from 1991 to 1992, until the owner went into bankruptcy and the creditors repossessed the equipment. The total cost of the drums, the support equipment and transportation from Portland to Rapid City was $1.26 million. "While we didn’t have a complete design for the composting facility at that time, we knew we did not want to shred the MSW prior to composting," says Wright. "We installed the drums and began operating them in 1997, using them for volume reduction prior to landfilling the MSW."

Each drum is 79 feet long and 12.5 feet in diameter and can process 18.75 tons/hour, or a total of 90 tons/batch. Retention time in the drums is six to eight hours. Water is added as MSW is loaded in.

A two-level trommel-type screen is attached to the end of each drum. Material falls onto the first level, with 5-inch openings for the initial separation. The five-inch plus material is conveyed to a waiting 100 cubic yard walking floor semi-trailer. The five-inch minus falls through onto a screen with 1-3/4-inch openings. The 1-3/4-inch minus fraction goes to the composting hall. Overs also are conveyed to the semi-trailer. About three to four loads/day of rejects are taken to the landfill.

The two-level screen helps to maximize the fraction that can be "captured" for composting. "If there were just one screen, the larger sized pieces would block the screen openings, so more of the organic material that has been broken down inside the drum would go out with the rejects," says Mike Oyler, facility manager. "We have found that the speed at which the drums are unloaded makes a significant difference in the amount of material captured for composting. If too much comes out of the drum at one time, it gets pushed out with the 5-inch screenings because there isn’t enough space for it to fall through the holes. How many loads of rejects per day go to the landfill depends on how the operator is unloading the drum. Therefore that is a key area for staff training."

Currently, water is added to the MSW as it is being loaded in, to achieve an optimum moisture content going into composting of 55 percent. Drums are filled to about 70 percent capacity in order to facilitate air flow and give adequate space for the tumbling action. There is no forced aeration of the drums, however cross ventilation occurs from openings at both ends. In addition, air is pulled off both ends of the drum and directed to a small biofilter, which creates a slight negative pressure of air through the drum.

WIN-WIN WITH BIOSOLIDS

In the very near future, anaerobically digested and primary clarifier biosolids from Rapid City’s Water Reclamation Plant will start being delivered to the recycling and composting facility. Biosolids — at eight percent solids — will become the primary moisture source in the drums. A vertical holding tank outside of the sorting building (and where the drums are loaded) will receive the biosolids from a tanker truck. The holding tank has an air operated diaphragm pump that moves the biosolids into the drum. Biosolids deliveries are expected to start in November.

It was a need by the Water Reclamation Plant for biosolids management that enabled the solid waste division to move forward with the last phase of its MSW processing facility. The initial design concept for MSW composting was to use open windrows and a turner. However, not long after the MRF and drums were fully operational, officials in Rapid City had decided to hold off on construction of the MSW composting plant for economic reasons. Then, in late 1999, Rapid City was faced with expanding its Water Reclamation Facility. Biosolids production would be 25,000 wet tons/year (at 9.5 percent solids) from the 15 mgd treatment plant. A part of the wastewater study was to review the city’s practice of land farming its biosolids. During the course of the study, it was determined the city would need to purchase an additional 1,100 acres to continue land farming.

As an alternative, the city asked Alliance to determine the possibility of cocomposting the biosolids with the MSW. Alliance recommended that approach and worked with the city to prepare a Request for Proposals. The first round of bids that came in exceeded the project budget, so in early 2002, during a second round of bidding, the project was awarded to U.S. Filter, for installation of its IPS agitated bay composting system.

THE FINAL PHASE: MSW COMPOSTING

Construction of the MSW composting portion of the plant got underway in March 2002, and the facility went into start-up in May 2003. A total of nine bays (each 10-feet wide by 8-feet high by 280-feet long) were installed in a 47,000 sq. ft. building. Design capacity is 213 tons/day (five-day week basis), based on a 29-day retention time in the bays. Preprocessed organics account for 157 tons/day and biosolids accounts for 56 wet tons/day. (Maximum plant capacity — going into the drums — is a total of 192 tons/day of MSW and 9 dry tons/day of biosolids.)

Now that the composting facility is operating, sorters on the MSW picking line are pulling items that will create wads of plastic, commonly called "pigs," notes Wright. (Oversized items are pulled on the tipping floor.) The compostable fraction from the drums screen falls onto a covered (but not totally enclosed) conveyor that moves material to the composting hall. Material piles up, and a screw auger continuously moves material from the top of the pile over, so that the pile gradually builds at the base. By the end of each day, all material is loaded into one of the bays so that nothing is left on the floor at night.

Each bay is divided into four zones; each zone is positively aerated with a 5-hp blower. These fans operated on a temperature feedback mode. Air is pulled out of the building by four 100-hp fans (each with 25,000 cfm capacity) for a total of 100,000 cfm. There also are two fresh air units blowing 50,000 cfm each into the building (located at each end of the building, where operators load and empty the bays). These units are equipped with burners to heat the make-up air when the outside temperature falls below 40° to 45°F (i.e., the burner kicks on when the intake air temperature is below 40° to 45°F). This helps minimize fogging in the building during cold weather.

Air from the first section of the bays is pulled by one of the 25,000 cfm fans into an acid scrubber, and then through a biofilter. The other three fans pull air directly into the biofilter. "We are treating the air from the freshest material through the chemical scrubber," says Dallas Wasserburger of Alliance. "That is the freshest material with the highest amount of ammonia."

The biofilter is 24,000 sq.ft. and is made of wood chips. Coarser chips are at the base, which has an HDPE liner for leachate collection. The leachate is discharged into the sewer system (although Wright would like to evaluate the possibility of using the leachate for moisture addition in the bays). Finer chips are on the surface. There is an irrigation system for the biofilter, which is 5-feet deep.

Plastic curtains over the front and back side of the bays (each about 10-feet from the end of the bays) hang from the ceiling, concentrating the air from the composting process in that section of the building. That assists with directing air flow to the biofilter, and keeps each end of the building — where the fresh air intakes are — comfortable for workers.
There are two agitators that turn material in the bays; each pass moves material forward an average of 12.5 feet, or in a range of 9.5 to 14 feet. (The shortest "throw" is at the end of the bays.) The agitators are equipped with a leveling bed device that enables aging material to be stacked higher, thus maximizing the capacity of the bay. Currently, two of the bays are being used on a trial basis for composting yard trimmings. "We have about 100 yards in each bay, and from what we can tell visually, after one month, the yard trimmings have composted to the point where it would have taken five months in the outdoor windrows," says Wright. Samples of the compost are being analyzed for stability, and compared to samples from the outdoor method.

Rapid City and its engineers were very aware of ensuring that the composting building be protected to the greatest extent possible to minimize corrosion from emissions and moisture. The building is constructed from galvanized steel. The sides of the building and the ceiling are covered with insulation and plastic sheeting, which in turn is covered by fiberglass panels. All seams of the panel are caulked. Any exposed steel surfaces were sandblasted and then treated with an epoxy coating. The building has a sprinkler system, as required by the local fire department.

CURING AND PRODUCT FINISHING

After about 30 days in the bays, material is moved with a Volvo C-90 loader with a 6-yard tipping bucket to an adjacent, open-sided, 20,000 sq. ft. curing building. Curing is done under positive aeration with no treatment of the air. Compost remains in the curing area for a minimum of one month.

Product finishing takes place in a 2,000 sq.ft. building that houses a Bivitec screen and a Triple S destoner with a dust collection system. An operator takes a load of compost from the curing area and drives it to a rolloff box equipped with a Keith walking floor. (The box is in a covered area.) The walking floor then meters the compost into the screening/destoning building. "The idea behind this design is that the loader operator can put the compost in and leave — as opposed to having to continually feed compost into the screen," says Wright. "It’s a more efficient use of our labor force."

The Bivitec has a three-eighth inch screen size. Engineers opted for a longer screen deck (4.3-feet by 19.7-feet) to allow more opportunity for contaminants to separate from the compost. Screened material then is conveyed to the destoner, which splits the light and heavy fraction. Key to maximizing separation and minimizing the loss of compost product is having the compost at the right moisture content (about 30 to 40 percent). Initial tests show the destoner is removing about 90 to 95 percent of the heavy inerts. The finished product appears to be free of glass particles, but still contains a significant amount of small pieces of film plastic. Tests are being done by Rapid City on both the chemical (including nutrient) quality parameters of the compost, and the percent inerts. That information wasn’t available at the time this article was written, but can be requested from the facility.

Given the amount of paper (e.g., mixed paper from households, soiled corrugated) going into the compost feedstocks — roughly 70 to 75 percent paper and 30 percent other organics — the C:N ratio is fairly high, says Oyler. It is anticipated that the addition of biosolids will bring that C:N ratio down, and add beneficial nutrients to the compost. "We will test the compost to comply with the EPA Part 503 regulations for Class A compost," adds Wright, noting that Rapid City also will be seeking to participate in the U.S. Composting Council’s Seal of Testing Assurance program.

Markets for the solid waste compost are in development, with land reclamation, agriculture and erosion control and storm water management being targeted. Growth trials have been conducted and additional ones are planned.

ECONOMIC SELF-SUFFICIENCY

The solid waste management facility is city-owned and operated, and paid for out of residential garbage fees, tipping fees and sale of recovered materials (no tax dollars are used). Initially, it was assumed that operating costs would be covered by tipping fees alone. "In fact, selling the end products has ended up being an extremely good rate stabilizer," notes Wright. "Even factoring in three percent inflation, revenues have that leveling effect. We were at a tipping fee of $47/ton in 2001, but dropped it to the current $45/ton and we expect it to remain at that average for many years to come, even factoring in equipment replacement. Basically, we have secured a long-term utility that is cost-effective. "

Wright, Oyler and plant staff are focusing on how to reduce the amount of plastic in the solid waste compost, so that it can contribute to the revenue stream in the near future. Biosolids addition may help to separate out the organic and inorganic fractions as material is screened after the drums. "We seem to get better material breakdown with the biosolids in the mix, which improves our capture rate of the compostable fraction," says Wright.

Stepping back and looking at the past 16 years, Wright believes the pace at which this solid waste management "package" evolved was a good one, allowing time to make corrections and fine-tune the operations. "In the end, the key to our plan was to take the time to find out what we wanted to do, then go into that with realistic costs and expectations."

Inquiries and requests for additional data should be sent to: Jerry Wright, Rapid City Department of Public Works, 300 Sixth St., Rapid City, SD 57701.

Copyright 2003, The JG Press, Inc.