November 22, 2006 | General

Composting Mixed MSW And Biosolids To Extend Landfill Life

BioCycle November 2006, Vol. 47, No. 11, p. 23
A totally enclosed composting facility designed to process 35,000 tons/year of MSW plus 6,700 tons/year of biosolids features state-of-the-art odor control and final product refinement systems.
Robert Spencer

A RURAL New York State county is hosting visitors from around the world to its one-year-old cocomposting plant in the farming community of Walton. The picturesque site, located within the watershed of New York City’s drinking water supply, is geographically isolated from major population centers. In fact, the nature and character of the surrounding communities were two major influences on Delaware County’s decision to compost mixed MSW and biosolids, and become more self-sufficient in solid waste management.
It takes sustained political commitment to build a $20 million composting facility for 50,000 residents in this period of lower cost landfill disposal options. Planning for the entire solid waste management center goes back at least 15 years, considering it has been ten years since Delaware County issued its RFP for the composting facility in 1996. Earlier work included a RFP for hauling waste out-of-county, and waste-to-energy plants. A landfill study was contentious, with very limited locations because the county is in the New York City watershed. In addition, many seasonal homes in the county are owned by New York City residents, who generally value their views of the Catskill Mountain range more than a new landfill and its associated environmental impacts. Watershed regulations are simultaneously increasing the level of treatment required at publicly owned wastewater treatment facilities (POTWs), increasing quantities of biosolids generated in Delaware County. Other project drivers included New York State’s recycling goal of 60 percent, as embodied in the county’s state-approved Solid Waste Management Plan.
The Delaware County Department of Public Works (DPW) is fiscally blessed with annual funding of its $2.5 million/year solid waste management program by one percent of the county’s sales tax. “The county’s solid waste management program started in the early 1970s and over those years, one tradition has been to pay for capital and equipment costs from current dollars,” explains Susan McIntyre, Solid Waste Director. “We are in solid waste services ‘forever’ and our 45-acre property has limited landfill capacity which plays very heavily into the county’s choice of solid waste management practices. Our goal is to provide a full level of service at a reasonable rate.”
The county estimates its entire solid waste program costs are in the “mid $50/ton range,” says McIntyre, not including curbside collection costs by private haulers since none of the municipalities provide municipal waste collection. The county does not charge a tip fee for MSW or recyclables (and it only accepts waste generated within the county). There is a tip fee for C&D and contaminated soils. “As for dewatered biosolids, most are accepted at no charge,” she explains. “We do have a tip fee on a percentage of biosolids coming from those POTWs that New York City required to be significantly upgraded. Still, the vast majority of biosolids, and all those from POTWs outside the New York City watershed, have no tip fee. We recognize that this facility would not be cost-effective as a stand alone merchant plant, but we believe it is competitive when viewed against future costs of managing all of the wastes we currently handle.”
Selection of the composting facility developer, Conporec/S&W Services, Inc., in 1998 triggered the start of a public/private partnership that has culminated in one of North America’s newest mixed waste composting facilities. (The newest facility is in Mariposa County, California – see accompanying article in this report.) The county contributed the land, along with millions of dollars of construction work. Under the direction of John Cammer, Deputy Commissioner of the DPW, the bridge construction crew did all of the compost facility concrete construction, the nonprocess electrical wiring, and installed all processing equipment except the bioreactor. In addition, the highway department did the site work, which includes the roads, septic and storm water systems, and preparation of the 3-acre building pad.
Financing of the approximately $20 million in total costs came from several sources. Delaware County received a $2 million recycling grant from the New York State Department of Environmental Conservation. The remaining $18 million was paid for with a $11.5 million bond from the NYS Environmental Facilities Corporation, and $7.5 million from county solid waste funds.
As shown in the process flow diagram (Figure 1), one 3-acre structure contains the tip floor, the Conporec bioreactor (rotary drum), primary screening trommel, secondary sorting line, the maturation area with the Siemens/IPS Composting System, final screening system, aerated final product storage area, recycling and residue roll-off containers, and a compost load-out truck bay.
Design capacity for the facility is 35,000 tons/year of MSW, plus 6,700 tons/year of biosolids, for an average of 120 tons/day. The county’s solid waste program attempts to keep construction and demolition debris separate as well as bulky items such as furniture. A number of the private haulers serving the county offer curbside collection of recyclables. On the tip floor of the composting facility, a 5-ton capacity grapple on an overhead crane is controlled by an operator in an elevated booth, and is used to pick out large bulky items, which are dropped into a roll-off container inside the building.
Biosolids from the nine municipal wastewater treatment plants in the county, and one industrial food processor, are handled in two ways on the tip floor. For biosolids less than 10 percent solids content, the liquid is discharged into a 30,000 gallon holding tank. Liquid biosolids are metered into the bioreactor as a moisture and nitrogen source. Leachate from the biofilter and other locations in the facility also flows to that tank. For higher solids content biosolids, typically 18 percent, trucks dump into a hopper below the tipping floor that feeds an “Eleveayor” – two conveyor belts that compress sludge and lift it up to a tripping conveyor that feeds the biosolids directly into the bays of the maturation area on the other side of the tipping building wall.
On a recent tour, Jeff Heath, Vice President of Stearns & Wheler, LLC, project partner and design engineers in the joint venture partnership with Conporec, described the facility in terms of five distinct processing areas: tipping, bioreactor, maturation, refining, and curing (see Figure 1). The waste receiving area is a combination tip floor and waste pit with sufficient height and length for large roll-off trucks and dump trailers to tip their loads into the waste pit while the overhead doors remain closed.
The pit has capacity to store about one week’s worth of MSW, or 500 tons. The operator in the control booth then loads the MSW into the hopper, which feeds a conveyor into the bioreactor at the rate of about 10 to 12 tons/hour for approximately eight to 10 hours/day. “We don’t like to have employees sorting raw garbage so we rely on the grapple operator to remove bulky items from the waste pit,” Heath explains. As a backup to the grapple, a front-end-loader can also load the feed conveyor. If the plant should be down for more than a week, MSW and sludge can be diverted to the adjacent landfill.
The operator typically feeds liquid sludge, or well water from a separate tank, into the drum to achieve 53 percent moisture content. That recipe varies depending on the season, and will be adjusted based on moisture content of the product coming out of the drum. “At that moisture content, we find that temperatures in the drum quickly get to 120° to 140°F,” adds Heath. “We are careful to not exceed that moisture content, which is one reason we do not feed the biosolids (18 percent solids) into the drum.”
The 157-foot long by 14-foot diameter rotary drum bioreactor is constructed from rolled steel, and is charged by a hydraulic ram. It rotates seven days/week, 24 hours/ day, and is powered by a 200 hp variable speed electric motor. Heath emphasized one major advantage of the rotary drum is that the MSW does not have to be shredded since knives in the drum open bags, leaving dry cell batteries and other potential contaminants intact, to be screened out “postdrum.”
Design of the Delaware County drum includes several new features that are not on Conporec’s Sorel-Tracy, Quebec drum – the only other Conporec unit in North America (there about 20 facilities in France that use the same drum). Claude Marmen, Vice President of Conporec, says the company constructed a more robust bioreactor shell out of thicker steel – three-quarters to 1.5 inches – in order to better protect against cracks in the shell and end walls of the bioreactor. Another change is that the hydraulic ram feeding MSW into the drum is constructed on a downward slope to gain some help from gravity in pushing MSW into the drum. Conporec also added temperature probes at the front and rear of the reactor, and two blowers instead of one to move air through the vessel, keeping an aerobic environment. “We want to be able to better control the process in the bioreactor and keep it as close to neutral pH as possible in order to minimize corrosion of the shell,” explains Marmen. “If pH drops to 4.5, acidity attacks the shell.” So far, pH of the compost coming out of the Delaware County reactor has been around 7, adds Marmen. (An article in the December 2006 issue of BioCycle looks at rotary drum design issues.)
The drum rotates at a maximum of one revolution per minute and is driven by a ring gear that automatically is lubricated with a spray system. The tire supporting the drum is not welded to the drum, but is free to move, or “float” on the shell; it also has an automatic lubrication system. Since the vessel is inside a building, there is no foam insulation on the drum, allowing for easy inspection of the shell for cracks or other signs of fatigue.
Waste is kept in the bioreactor for three days, with longer retention times leading to a higher occurrence of “hairballs” or “monsters” that can clog the discharge doors. The drum is discharged simultaneously with the loading process, over an eight to 10 hour period. A screen with six-inch diameter holes is attached to the discharge end of the drum to more evenly distribute the discharged material onto the heavy duty, mining grade discharge conveyor, which feeds a covered inclined conveyor going to a covered primary trommel screen.
A 1-1/2 inch diameter screen in the primary trommel allows raw compost from the drum to fall through, with materials over 1-1/2 inches going to the landfill as process residue. The plant has additional hand sorting stations and an overhead magnet to further process overs from the primary trommel. The quality of recyclable plastic and metal containers is poor due to contamination with compost while in the bioreactor, so that sorting step has been temporarily discontinued.
Raw compost from the primary trommel is conveyed to the maturation area with a conveyor system and tripper car to feed 14 aerated, IPS windrow bays, each 235-feet long by 8-feet high by 10-feet wide. Dewatered biosolids are introduced into the bays. Raw compost and biosolids are mixed by the Siemens/IPS agitator, and start their 56-day compost journey to the other end of the bays.
The 56-day detention time is much greater than other composting plants, which typically use 21 to 28 days for biosolids composting. “The decision to have a longer maturation period is based on lessons learned by Delaware County in touring other plants and talking to operators, and our own experience at Conporec’s Sorel-Tracy facility,” explains Heath. “They have a 42-day maturation process that uses a turning machine on wheels (not IPS) but the compost is not fully mature. With the IPS system, there are three aeration zones that allow for better process control with respect to mixing and water addition. It also allows us to effectively cure MSW compost with a relatively high carbon content. To its credit, the county also understood this critical step in the process and specified the 56-day maturation period in their procurement documents.” The targeted moisture content after the third aeration zone is about 35 percent to allow the secondary screening process to be most effective.
Barbara Petroff of Siemens Water Technologies, manufacturer of the IPS Composting System, notes that the Delaware County facility is the sixth application of IPS technology to MSW and source separated organics, starting with Rikers Island, New York City (food residuals); Kewu, Switzerland (green waste); East Hampton, NY (green waste and manure); Halifax, Nova Scotia (mechanical biological treatment); and Rapid City, South Dakota (MSW/biosolids). Delaware County’s facility is similar to Rapid City’s in that it uses a rotary drum vessel, but only for about a six hour mixing process. At both facilities, the majority of the composting process takes place in the bays, which is also where regulatory requirements for pathogen reduction are accomplished.
“Like other components of the Delaware County facility, Siemens has made improvements to its IPS composting system, including automated discharge of compost from the bays to a conveyor,” says Petroff. The system processes a total of about 200 cubic yards/day, moving material down the bays about 13-feet for every pass of the automated agitators. Each pass takes from 45 to 55 minutes to traverse the 235-foot distance to the end of the bay where material is discharged. The agitator is then automatically transferred to another bay.
The entire maturation system with its feed and discharge conveyors and 14 bays, is housed in a frameless, preengineered steel (Corr-Span) structure (supplied by Behlen Buildings) lined with stainless steel panels in order to minimize corrosion of the building. This area of the plant experiences high concentrations of corrosive gases and condensation coming off the positively aerated compost bays.
The secondary screening system is located in the same portion of the building as the primary screening system. A high-speed inclined conveyor with a leveling bar is used to spread out the material in order to separate pieces of glass, bottle caps, rock, and other heavier items that roll backwards down the conveyor (including some chunks of compost). The compost that stays on the conveyor passes through a drum magnet at the top of the conveyor to pull off metal fragments. From there, the compost goes through a trommel with a one-quarter inch screen size, followed by a roller crusher to pulverize remaining glass fragments. A leveler bar spreads the compost out prior to the crusher so that glass particles are not shielded by clumps of compost, something that is taking more work to fine-tune. To control dust, a water spray bar can be used to moisten the compost as it goes up the inclined conveyor; however, this can adversely impact the ability of the inclined conveyor to separate contaminants if the compost gets too wet.
During facility design, it was anticipated that clumps of compost might pass over the secondary trommel, but with further processing could be recirculated through the composting system. So, initially, the facility used a granulator to chop up the overs from the secondary trommel. However, during start-up, it was noted that such a small amount of organics were passing over the secondary trommel that use of the granulator was discontinued.
Following the roller crusher, a conveyor moves the compost to the curing building, which has room for 10 large windrow piles with two aeration trenches per pile to keep the compost aerobic and assist with curing. There is also an overhead spray system in this area in order to control moisture content of the final compost product.
According to McIntyre, the county is very pleased with the compost product quality, particularly the lack of glass, which has been the major contaminant of concern at other mixed waste composting facilities. “We are still in our start-up phase and will be working to further reduce glass in the final product, primarily through adjustments to the roller crusher,” she explains. To date, the county has used some of the compost in its own projects, but has not yet implemented a full-scale marketing program.
The compost has to meet the NYS Department of Environmental Conservation (DEC) Part 360 regulations, and most samples have met the standards for Class A material. McIntyre is investigating the source of possible lead contamination since two out of 15 tests have indicated lead as high as 440 ppm, exceeding the 300 ppm state standard (the other samples were between 150 ppm and 300 ppm). The DEC allows for spikes in discreet samples as long as the average is within the standards, however Delaware County intends to reduce the occurrence of such spikes – including installing a magnetic head pulley on the primary refining system, and increasing education for residents to separate batteries and electronic equipment, items the county recycles. McIntyre thinks there may be improved product quality and consistency once the county increases the percentage of biosolids incorporated with the MSW.
One of the major design features of the facility, which has worked very well to date, is the air handling and odor treatment systems. The closest residence is about one-quarter mile away. The author, who has been in many composting facilities, observed during his tour that there was less odor inside the plant than at any other MSW and/or biosolids composting plant he had worked at or visited. Heath attributes that to the large number of air changes per hour throughout the plant (as many as eight in the maturation building), point source capture of odors from the bioreactor, conveyors and trommel screens, and the aerobic nature of the IPS composting system.
Exhaust air from the maturation room, which includes air from other portions of the facility, is pushed to a 0.5 acre biofilter located outside the building. The biofilter is designed to process 78,000 cubic feet/minute of air, and is constructed in two cells that can be independently operated. A 4.5-foot high gravel berm contains Conporec’s proprietary biofilter media (wood, compost, peat, and other materials) on top of 8-inch diameter corrugated pipe buried in two feet of stone to distribute air across the base of the filter.
Measurements of pressure and air flow through the biofilter have indicated good distribution of the air, notes Heath, which can be adjusted through a series of dampers. A fine water spray humidification system is used in the main header system, and a dust collection system is in the refining areas, but no scrubber system is used to pretreat air prior to the biofilter.
Delaware County’s contract with the joint venture of Conporec/ S&W Services, Inc. calls for one year of operational supervision. McIntyre expects the required acceptance tests to be concluded in the near future. “We want the fine-tuning of the system to continue throughout this start-up period,” she says. Performance guarantees include compost product quality meeting NYS Part 360, no nuisance odor, operating at design throughput, and a 70 percent diversion rate. A future article will address the performance test standards and results.
Robert Spencer is a Contributing Editor to BioCycle. He also works as an Environmental Planning Consultant, based in Massachusetts.

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