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From BioCycle Journal of Composting &Organics Recycling October 2001, Page 54 |
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COMPOST OPERATORS FORUM
Air Handling Holds Key To Odor Management
The Cobb County, Georgia cocomposting facility processes over 400 tons/day of mixed MSW and biosolids. Closed by two fires in 1996, the facility was reopened with a new strategy for odor control.
Joseph Accortt, Richard Krause and Robert Rynk
IT ISN’T easy to do what the operators of the Cobb County, Georgia composting facility must do – compost over 400 tons/day of municipal solid waste (MSW) and biosolids within a few hundred feet of neighboring homes and businesses. Even with enclosed operations and considerable engineering, it is a challenge to prevent the escape of odors from such a large mass of such aromatic feedstocks. The facility’s operating permit quantifies the challenge, requiring it to meet an odor performance standard of 10 dilutions to threshold units (DTUs) at the site boundary.
Early experience running the facility warned of the difficulty. During the facility’s brief start up in the summer of 1996, odor complaints from neighbors were frequent. However, just as operators began to make headway, the facility was hit by a one-two punch, a fire in late August and then another fire in December of 1996 (see “Fires at Composting Facilities: Causes and Concerns,” January 2000). The fires were devastating, shutting down the facility for over a year and half. In the ensuing downtime, Bedminster Bioconversion Corp (BBC), the facility’s original operator, and Cobb County had an opportunity to rethink and reengineer the facility design – with the specific goal of odor control.
The changes at the Cobb County plant resulted in a tighter facility with a ventilation system designed to contain, direct and treat air moving through the building. Odor emissions within the building are captured at critical points and incorporated in the air handling system. Nearly all of the air entering the building leaves through the facility’s biofilter, which also was substantially redesigned. In March and April of 1998, the facility passed a Performance Acceptance Test. Cobb County instituted the test as a condition for the county to assume operation of the facility. In June of the same year, the county took over operation from BBC. The facility has been operating at near capacity since then without significant odor problems.
Materials Flow
The Cobb County facility is located on a 20-acre site. The enclosed processing area encompasses about seven acres. The facility composts a combination of MSW and biosolids generated by the Cobb County wastewater system. Currently, it is operating at full capacity of 300 tons/day (tpd) of MSW and100 tpd of biosolids. The Cobb County facility uses rotating drum digesters for the first stage of composting and finishes composting in turned aerated piles.
The flow of materials is typical of an MSW composting facility. Collection vehicles deposit MSW onto a large tipping floor. Except for very large items unsuitable for composting (and picked out by front end loaders), the MSW is pushed onto the picking line conveyor. At the picking station, large objects (e.g. cables, pipe) that pose a problem for the digesters and other equipment are manually picked off the line along with potentially hazardous items (e.g. propane tanks, pesticide containers). The remaining MSW, including unopened plastic bags, continues on the conveyor belt to the inlet hoppers of the digesters, also inside the tipping area.
Biosolids are delivered to the facility in trucks that dump their load into a covered concrete holding pit. From the holding pit, biosolids are pumped directly into the digester, at an adjustable rate that matches the load of MSW entering the digester. Except during the unloading of the trucks, the biosolids are continually contained. The biosolids handling system was one of the major renovations to the facility. Previously, biosolids were metered onto the conveyor belt moving MSW to the digester. The exposed biosolids substantially added to the odor inside the tipping area.
The facility has five rotating drum digesters, each 13 feet in diameter and 280 feet long. The digesters have three compartments that each hold the materials for one day, providing a total retention time of three days. Inside the drums, the rotation mixes and agitates the feedstocks, breaks apart particles and opens bags while the organic materials start to decompose. Temperatures are maintained at 140° to 150° F by controlling the rate of forced airflow through the digester and the addition of process water. After three days in a digester, the partially composted materials are unloaded from the drum onto a conveyor that leads to a trommel screen. The trommel screen removes the large particles, dominated by plastics, that are sent to a landfill for disposal. At the discharge conveyor for the trommel screen, magnets pick off ferrous metal items. The plastics and other rejects collected in the trommel overs represents one-quarter to one-third of the MSW delivered to the facility. The fines, or unders, from the trommel are composted further in the aeration bays.
The Cobb County facility uses a turned aerated pile system for the second stage of composting. Front-end loaders stack the unders from the trommel screen onto the floor in one of three aeration “bays.” Each bay is essentially an open floor with twelve –foot high concrete partitions running the length of the bay. Aeration ducts, covered with a perforated plate, are imbedded in the concrete floor of the bays. Air is forced from the ducts up through the pile. Each bay houses one continuous pile that is approximately six feet high, 35 feet wide and 360 feet long. Temperatures in the aerated piles are roughly 130° to 160 °F.
Periodically, a turning machine agitates the pile and also shifts it approximately 20 feet toward the discharge end of the bay. When the pile is shifted, it makes space to pile another batch of unders from the screen. The turner is carried across the bay and down the length by a bridge crane that rides on top of the concrete partitions. Each bay has its own bridge crane and turner. The material spends at least 28 days in the aeration bays. The finished compost, at the discharge end, is loaded on conveyors which take the compost to a finished screening plant where vibrating deck screens remove additional inert contaminants.
Handling Air to Handle Odors
A major emphasis of the reengineering at the Cobb County facility concerned the movement of air into, out of and through the building. The entire processing area is enclosed within a large building, 354 feet wide by 700 feet long, including the outside digester yard. The MSW tipping area and the finished screening area are isolated by interior walls but otherwise all materials processing takes place in one building. Only the biofilters are outside. Inside this processing space, odors can be strong, simply due to the concentration and movement of organic materials. An important part of the odor control strategy is keeping the odor laden air from leaking out of the enclosed space and into the neighborhood. Overall, the strategy is to contain, capture and treat all air in the building.
The objective of the air handling system is to direct the air flow to the biofilter in a controlled manner, minimizing leakage of untreated air from the building. The air movement roughly follows the movement of composting materials. Figure 1 illustrates the general pattern and direction of air movement. The entire building is under slight negative pressure created by large exhaust fans along the south wall. Inlets for fresh air, or make-up air, are located near the ceiling in the tipping area, digester output bay and the final screening area. Air is brought into the building by fans and distributed through flexible polyethylene tubes with holes regularly spaced along its length. Air flows at a low velocity from these inlets through the building towards the exhaust fans, carrying the odors released within the building.
There are particular points within the building where odor emissions are captured immediately. Partially composted material discharged from the digesters empties onto a series of conveyors. After passing the discharge of the last digester, the conveyors are hooded. An exhaust fan pulls air from the conveyor hoods and delivers this air to the ducts beneath the compost piles in the aerated bays. Thus the exhaust from the conveyors hoods provides the process air for the aerated bays. In the same manner, odorous air in the headspace of the biosolids pit is captured by a blower and exhausted beneath piles in the aeration bays. After passing through the aerated piles, the process air simply enters the building headspace above the aerated bays. From there, it follows the path to the exhaust fans feeding the biofilter.
Maintaining air in this pattern of airflow requires control of the air inlets and outlets. Open doors provide a potential short circuit in the flow and a means for odors to escape. The problem is complicated by the fact that trucks constantly enter the building to deliver feedstocks. One improvement made in the redesign was the installation of rapidly closing doors to the tipping floor and biosolids delivery areas. The skin of the doors is a high strength fabric that can be repaired quickly in case equipment gets too close. To further minimize leakage, suction fans above doors collect fugitive air when the doors are open.
The process air feeding the digesters also is connected with the building air system. Each digester has a dedicated fan that pulls in air from outside the building and pushes the air through the digester. Air flows against the movement of materials from the discharge end to the digester loading end. The digester exhaust gas is captured in hoods at the loading end of the digester and conveyed in a common plenum to the floor of the aeration bays.
The entire air handling system requires some oversight as the materials and air flows vary among the different components of the facility. On at least a daily basis, operators use a computer monitoring and control system to adjust the air flow rates, intakes and channels to balance the system and maintain the desired negative pressure within the building. Obtaining an even distribution of air to the biofilter is also critical.
Treating Air – Biofilters
The odors from the composting process and the building meet their ultimate fate in the facility’s biofiltration system, an essential element of the facility. Managing the biofilters is at least as challenging as managing the composting process itself. Media, moisture and airflow are all important factors in making the biofilters work. The system includes six humidification towers or scrubbers and seven biofilter beds, covering a total area of about one acre.
The primary purpose of the scrubbers is to cool and humidify the air prior to the biofilters. The exhaust air from the building and process passes through a water spray chamber in the scrubbers. In the process, some of the odor compounds, like ammonia, also are removed. Water is recirculated within the scrubbers, although some portion is removed and pumped to the wastewater treatment plant and replaced with fresh make-up water. The cooled, moistened, but still odorous air leaving the scrubbers is distributed to the base of the biofilters.
The biofilters are arranged in seven beds, each 40 inches deep, 20 feet wide and 280 feet long. Each bed contains three independent zones with 17 lateral air distribution ducts running the length of the zone. Air from the building exhaust fans is dispersed among the biofilter beds after passing the scrubbers. Two fans, together running 80,000 cubic feet per minute (cfm), feed three beds. Five fans, operating at a total flow rate of 150,000 cfm, serve the four remaining bays.
The biofilter media consists of wood chips, with a diverse particle size distribution, mixed with compost. Compost comprises ten percent of the mix. Over time, the material decomposes and becomes more compact. The media is replaced with new material when the airflow pressure drop through a biofilter bed exceeds eight to ten inches of water. This occurs approximately every 18 months. The air distribution and moisture drainage systems also are cleaned at that time. Even with the humidification in the scrubbers, maintaining proper moisture in the biofilter media is a challenge. The biofilter system has sprinklers to add water if necessary but the larger problem is too much water from heavy rains that overwhelm the drainage system. For this reason Cobb County is planning to cover the biofilter.
Odors In Check/Product In Use
The Cobb County cocomposting facility has been back in business for three years now. While an occasional odor complaint might still be heard, the facility has basically kept odors in check. Air samples collected monthly at the property line have shown that the facility has consistently met the odor performance standard of 10 DTUs. Although part of the success is due to management experience, the facility redesign is also a big factor. When participants at the 2001 BioCycle Southeast conference toured the facility in August, inside the building, the dust masks and disposable coveralls were much appreciated. Outside the building, nothing in the air suggested that MSW and biosolids were being composted nearby.
The next challenge for the facility is to expand the uses and markets for the compost produced — locally known as “Bio-Blend.” Currently, most of the compost is used for establishing vegetation, capturing sediment and controlling erosion at the county’s landfills. Experience has shown that the compost is a great soil product, especially for establishing grass cover. Increasingly, the compost also is being sold to contractors, given to county residents and used by transportation departments for establishing vegetation and controlling erosion on highway roadsides and medians.
Joseph Accortt, Director of Solid Waste, and Richard Krause are with Cobb County (Georgia) Solid Waste.
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