August 15, 2004 | General

EXPANDING THE POTENTIAL OF IN-VESSEL COCOMPOSTING


BioCycle August 2004, Vol. 45, No. 8, p. 38
Arizona’s Pinetop-Lakeside facility enjoys community support because it takes two waste streams and combines them to create a marketable product.
Phil Hayes

LOCATED in the eastern mountains of Arizona, the Pinetop-Lakeside Sanitary District has been expanding its cocomposting operations for both municipal solid waste and biosolids. Our management challenge in the late 1980s arose when closing landfills presented the District with a biosolids disposal dilemma. Hauling dewatered biosolids three time a day – more than 80 miles – was not financially viable, so we explored the possibility of in-vessel cocomposting.
Pinetop-Lakeside operates a 2.0 MGD activated wastewater treatment plant, which generates a sludge as a by-product. The sludge removed from the wastewater system is sent to a 36,000-gallon aerobic digester, dewatered to approximately 15 to 18 percent solids and then pumped into the composting vessel. Approximately five to six wet tons of sludge are produced daily. Disposal of wastewater sludge is an industry-wide concern, as it is primarily organic, nutrient-rich and biologically active. It is, however, just what is needed as a water and nutrient source to promote the composting of MSW.
Working with the now defunct Bedminster Bioconversion Corporation, the District would build the premier reference facility in the United States and become the center of focus for entities around the world interested in this process. Utilizing municipal solid waste (garbage) to supply the carbon side of the equation and dewatered domestic wastewater sludge for the nitrogen part of the equation, the first facility only handled ten tons of municipal solid waste (MSW) and five wet tons of sludge per day. This system was intended to serve as a five-year pilot project. However, politics brought about tighter budgets, and operators were forced to extend the five-year project to a “dangerous” 13 years. Dangerous, because the machine used by the District was built in the early 1970s to perform experiments using chicken manure in Arkansas. The District modified the lightweight machine, which was 70 feet long and ten feet in diameter, to process MSW and dewatered sewage sludge on a daily basis.
When the machine was shut down and dismantled in the summer of 2003 to make way for a new biomass mixer, it was found to have holes in the outer shell beneath the foam insulation. The structural integrity would soon have been compromised to a point of complete failure.
Our decision to build a larger facility had come none too soon in January 2003. The District was on the move once again. A new machine was ordered from A/C Equipment Services of Milwaukee, Wisconsin. They would build, deliver and install a 125 foot long, ten feet in diameter, 40 ton per day biomass mixer. An 18,000 square foot metal building would be added to the existing facility to house the new machine bringing the total under roof area to 34,000 sq ft.
This building is totally enclosed with large sliding doors to allow the entry of the packer trucks and the exit of reject material. The building was designed to house an additional four bay aerated static pile floor with positive forced aeration feeding air to this material. Overhead fiber mesh tents were constructed to capture escaping water vapor with large blowers collecting and transporting the foul air to the biofilter. Part of the interior roof of the building is covered with a one-inch thick insulation coating to curtail condensation within the building. District personnel would design and construct the remaining ancillary structures and equipment for the new cocomposting plant and start the process in early November 2003.
GREATER FLEXIBILITY IN CONTROLLING SOLIDS
The technology remains the same, however; the added capacity allows the District greater flexibility in controlling the solids inventory in its wastewater treatment plant. In composting, as in wastewater treatment, bacteria are formed and given optimum conditions in which to perform their work. In order to make an objective evaluation of the metabolic status (or health) of any microbial community, one must consider all of the organisms present under conditions as close to those of their natural environment as possible. Thus, nonselective techniques (those not involving the culturing of individual strains) are preferred for the analysis of both microbial activity and biomass.
Temperature is a critical parameter. Small variations in temperature can affect microbial activity and biomass in composting sewage sludge much more dramatically than small changes in moisture, pH, organics, or C:N ratios. Thus, temperature control during the composting process is highly desirable, if not mandatory, for an efficient and consistent process.
The biomass mixer is coated with approximately two inches (5 cm) of foam insulation to retain heat. Operational temperatures are achieved and maintained by the heat of respiration – no external heat is added. Given the proper moisture, oxygen and nutrients, heat producing biological activity will occur. Two operator-controlled factors influencing temperatures are airflow and residual raw compost to serve as inoculum. Increasing the airflow through the vessel will dry the composting material and remove heated air from the chamber. Cooling and drying the material will significantly slow the respiration rate of the microorganisms, allowing temperatures to decrease. Conversely, slowing airflow to the minimum amount of air necessary to maintain respiration will maximize heat retention.
OPTIMUM MOISTURE
Optimum moisture range in composting operation is between 40 to 60 percent. If the moisture content falls below 40 percent, nutrient availability is reduced and organisms begin to desiccate. When moisture exceeds 60 percent, the structural stability is reduced, the material tends to compact, oxygen transfer is inhibited and anaerobic conditions may occur within the compost pile. Incoming MSW is normally 30 to 40 percent water and sludge is 85 to 90 percent water. Based on these percentages, the approximate weight of MSW and sludge can be calculated for loading; however, experience on the part of the operator is necessary to mix the proper proportions to achieve the correct moisture balance.
During the operational cycle, water can be added to the mixture. Moisture can be removed by increasing the airflow through the biomass mixer; however, a high airflow tends to lower the temperature.
The carbon to nitrogen ratio is important to maintain proper nutrients for the microorganisms. Nitrogen is found in the wastewater sludge, and paper products in MSW are the primary carbon source. Carbon and nitrogen should be present at a 20:1 to 30:1 ratio in a composting operation. The higher the carbon to nitrogen ratio, the lower the amount of nitrogen available and if the carbon to nitrogen ratio falls below 15:1, nitrogen is lost as ammonia causing adverse odors. Operationally, an MSW to wet sludge of 2:1 provides the necessary carbon to nitrogen ratio.
Unfortunately, the lack of recycling programs in this area results in a 35 to 40 percent reject stream leaving our facility on route to the transfer station. The 4,500 residents place all household refuse in wheeled carts to be collected. Most of this material is placed in plastic bags prior to being placed in the wheeled containers and is loaded directly into the cocomposting vessel. Film plastic causes no problem to the system as it passes harmlessly into the reject stream leaving the facility.
Waste Management currently provides municipal solid waste from residential sources gathered from its collection routes within Pinetop-Lakeside. Waste Management has altered collection routes and schedules to accommodate the requirements of the composting operation. The composting process requires delivery of the first truckload of MSW at 8:00 a.m. and delivery of a second truckload about 10:00 a.m., and a final load around 3:00 p.m. to complete loading and off loading in two eight-hour shifts.
Residential MSW is used in the composting process because there is a lesser chance of contamination from metals, solvents, organic chemicals and other commercial by-products. The small size of this project makes the compost vulnerable to contamination from hazardous material in the feedstock. The combined weight of each load of MSW and sludge today is approximately 30 tons at 60 percent moisture content. This machine was built for future capacity of up to 60 tons per day.
Daily operation of the facility begins with the recording of the operational parameters (temperature, moisture and oxygen). A portion of the material contained in the biomass mixer is then off loaded onto a conveyer and the material is sent to an Amadas trommel screen with 1.5-inch openings. Materials greater than 1.5-inches are separated and placed in a compactor to be sent to the transfer station. This reject stream represents about 30 to 35 percent of the original feedstock load and contains mostly film plastics, plastic bottles, clothing, metal cans and assorted junk. Many studies have been done over the years on percentages or breakdowns of materials in municipal solid waste but from where we sit, it’s plastic, plastic, plastic.
ODOR MANAGEMENT
One of the noticeable features of the biomass mixing process at the Pinetop-Lakeside Sanitary District is the lack of objectionable odor. Because the process is aerobic and the composting environment is controlled, only a light earth aroma is detected from the composting process. The tipping floor can have strong odors when the packer trucks unload. MSW is loaded into the biomass mixer soon after arrival to the site.
The entire facility is swept daily and the tipping floor is washed on a daily basis. Good housekeeping is essential to contain odors. A ventilation system and biofilter have been constructed to remove odors form the tipping floor and to ventilate the composting facility. Although odors are not noticeable even a short distance from the facility, ventilation is necessary to reduce humidity in the building and eliminate tipping floor odors.
Material less than 1.5-inches is raw compost and is placed in aerated static piles, which are turned each week for 16 weeks using a small-articulated John Deere loader with a large bucket. Raw compost is a coarse mixture of composting organic material and recognizable particles of glass, plastic, metal and other items. These particles play an important role in the remaining process. The raw compost is approximately 50 to 55 percent water and would tend to settle into a gelatinous mass if it were not for these particles. In the aerated static piles, temperatures of 135 to 155°F are maintained and growth of fungi is encouraged to break down cellulose while the composting process continues. Air is supplied by perforated pipes under the piles fed by blowers that are controlled by timers. Again, too much air dries the material lowering the temperature of the pile and slowing composting activity. Each day newly screened material from the biomass mixer is added to the first aerated static pile. When this number one pile is completed (by week’s end), it begins a 16-week journey down the aerated header floor to the secondary trommel screen. This is also an Amadas trommel.
The secondary trommel screen has .25-inch diameter openings. Once the material passes through these openings, it is considered to be final product or finished compost. Material, which fails to pass through the .25-inch openings, is collected and examined visually to see if it contains mostly noncompostable material such as glass. If so, it is placed into the compactor to be hauled to the landfill. Approximately seven percent of the original feedstock fails to pass through the secondary trommel screen. If this reject material contains mostly compost, it will be reintroduced to the biomass mixer via the following days’ MSW load.
END PRODUCT MARKETS
The district has always enjoyed the luxury of having soil blenders seeking to purchase the final compost to use as an amendment to their products. It is important to remember that the Southwest is by and large a soil poor region with deserts of sand and mountains that are basically clay. These conditions make importing or creating topsoils for growing a necessity. Our compost fits well into the scheme of things but still had to be proven.
In the early years, we launched high profile projects to promote the compost. For instance, landscaping at the post office, city hall, the parks, baseball and soccer fields. One project was allowing private citizens two cubic yards each to use in their landscaping efforts. They were then to fill out a form to return to the district on their successes or failures. Fifty residents joined in, 30 sent in forms, 29 elated, one disappointed. (He was beaten senseless.)
The price of our final material, after the bidding process, is around $7/cubic yard. This helps defray costs, but by no means supports the project. Our savings are on the avoided cost side ie: dewatering equipment to pass the paint filter test, trucking 160 miles round trip, tipping fees, etc. We would control none of this and be at the mercy of others. Cocomposting keeps us in control of our own destiny.
The cost to the Pinetop-Lakeside Sanitary District to produce compost (avoided costs not withstanding) are broken down as follows:
Currently the District does not charge a tipping fee. However, given current operating costs and compost revenues of $14/ton, the District would need to collect about $30.50/ton in tipping fees to fully cover all costs. This is based on; salaries, electrical costs, repair and maintenance, testing and hauling off reject material. It is important to note that the current tipping fees at the local transfer station are over $40/ton. If the District charged a $40/ton tipping fee, this would yield an additional $100,000/year of profit or money to be used for future expansion and equipment replacement. We expect that the new facility has at least a 20-year life cycle.
“TECHNOLOGY IS THERE”
I was recently asked about the future of composting mixed municipal solid waste. Can we reach high-end markets? What role do tipping fees play; how do you consider these issues? Consider auto racing; speed costs money, how fast do you want to go? Composting costs money; how clean do you want to be; how clean do you need to be? The technology is there to take this industry wherever it wants to go. I am sure as this technology becomes more prevalent, it will also become cheaper to recreate much like all of today’s technology. The plasma television that I want is $5,000 today; in two years, it will be $1,500. I’ll buy it then.
Composting wastewater sludge has proven to be an acceptable method of disposal for the Pinetop-Lakeside Sanitary District. All of the sludge is composted which means that no sludge is transported, handled or landfilled off-site. This eliminates these types of costs.
The project enjoys the support of the community because it takes two waste streams and combines them to create a beneficial and marketable product. The production of compost from organic wastes is a very basic form of recycling of materials not generally suited for other manufacturing processes.
The tiny facility “way out west” high in the mountains of Arizona is but one of a larger and hopefully growing family of composters utilizing this in-vessel technology. Elegant in its simplicity, this style of cocomposting is very likely to be run of the mill someday. Larger facilities in the East like Nantucket Island; Sumter County, Florida; Sevierville, Tennessee; Marlborough, Massachusetts; Cobb County, Georgia and the giant plant in Edmonton, Alberta, Canada have diverted a tremendous amount of solid waste and biosolids from being landfilled.
The father of this technology, Dr. Eric Eweson, would be pleased to see that his idea has taken on a life of its own and has evolved into today’s modern facilities. A user’s meeting will be held this fall in Arizona to solidify this composting community’s commitment to the success of this technology.
Phil Hayes is with the Pinetop-Lakeside Sanitary District in Lakeside Arizona.


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