Small-Scale Composting: An Operator's View

R. D. Nordhagen, S. L. Crawford and G. E. Johnson

Minnesota is a Mecca for a variety of alternative technologies and innovative waste management policies. For the past several years, the state has attracted industry specialists and public officials to study its successful programs.

One example of a thriving operation in Minnesota is a small but successful plant located at Thief River Falls in Pennington County, 300 miles northwest of Minneapolis-St. Paul. The facility is the first of its size to combine the processes of recycling, composting and densified refuse-derived fuel (DRDF) production.

The Pennington County plant, which was designed by Future Fuel Inc. and a local engineering firm, was the first to receive a grant from the Minnesota Office of Waste Management (OWM). The plant has operated for nearly a decade and continues to face the ongoing challenges of changing standards and technologies.

Its history, which shows a process of modifications that have increased productivity over a relatively short time, can provide constructive insights for other small-scale operators in similar situations.

Predicting Waste Supply Pennington County generates approximately 10,900 tons per year of residential and light commercial and industrial waste. All waste collected in the county - approximately 40 tons per day - is delivered to the facility at a tipping fee of $45 per ton. The remainder is recycled or deposited at rural landfills. Efforts to implement a two-bag collection system for compostable material and the remainder of the waste have been only somewhat successful, since residents are unaccustomed to separating wet and dry garbage.

The facility serves as the county recycling drop-off center and also receives some waste from neighboring Marshall County. Both the Minnesota OWM and the county expect waste generation rates to remain nearly the same for at least the next 10 years. The facility currently serves a population of more than 13,000. Any expected population increase will be offset by countywide waste reduction and increases in recycling.

Improving Efficiency DRDF is produced in the facility's primary process. Materials suitable for recycling are manually and mechanically separated from the line before the pelletizing process begins. Yard waste and organic process residues from fuel production are composted. The remaining, non-processible wastes are landfilled in nearby Grand Forks, N.D. Recovered materials include aluminum, scrap metals, some plastics and corrugated cardboard. Operating five days per week, the Pennington County facility has 18 full-time employees.

The facility has undergone numerous changes since its design and construction. Due to equipment failure, improper design or changes in waste stream or marketing infrastructure and political/regulatory climate, many pieces of equipment have been modified or eliminated altogether. Since its inception, the operation has replaced nine maintenance workers.

The process begins when incoming waste is received in an enclosed staging area where large, non-processible items are manually removed. A grapple crane is used to place the processible waste on the feedline, where it is moved into the processing train.

The original design had a high-speed bag breaker at the beginning of the process, which proved to be both noisy and dangerous. The unit was removed, since most bags were already manually opened to remove potentially hazardous materials. Bags are now opened manually and the contents visually examined for non-processible or potentially hazardous items.

Post-Separation Changes Using a magnetic unit, ferrous is separated following debagging. In the original design, most of the removed ferrous consisted of food containers, which were badly contaminated by paper and dirt. This made the ferrous fraction difficult to market. An air classification system has been added to the ferrous processing train to help clean the scrap before the ferrous is marketed.

Following the ferrous metal removal, the waste progresses to a trapezoid disc screen unit to separate the lighter fraction - mostly paper and film plastics - from the denser, smaller components such as food wastes, dirt and wet materials. The light fraction travels along the surface of the rotating discs while the denser material falls through the screen to a separate conveyor system that feeds the compostable feedstock line.

Because of operational problems with the disc screen, the length has been shortened by nearly 50 percent. The operator noted that about 90 percent of the separation occurred over the first half of the screen. The removal of the last half of the screen has decreased the amount of maintenance required, lessened the overall processing time and added to the available space within the facility.

The operator also has modified the operations of the disc screen spinner sections to prevent locking up due to grime accumulation. Although these spinners are pressure washed three times a week, they are still encountering some problems.

The light waste fraction is passed to a sorting table for the manual removal of non-fuel quality items. Remaining high density polyethylene (HDPE) and aluminum are also removed during this stage. The light fraction then moves to an air knife classifier and shredder/drier, and then into a densifier/pelletizer.

The air knife was not included in the original design or equipment, but was added later to reduce the amount of metals passing to the shredder. Metal entering the shredder created a dilemma when it began to cause problems with the pelletizers. The pellet feedstock is dried to produce the proper moisture content.

Other Solutions To eliminate the problems of storing wet materials, a propane drier is traditionally added to precede fuel pelletizing. The original design included a prepelletizing propane drier in the processing train, which was expensive to operate and a source of fires. This has been replaced with a drier that uses fuel pellets manufactured at the facility. It performs better than the propane unit and is significantly cheaper to operate.

The pre-composting operation has also changed considerably since start-up. The original design included a compost shredder to reduce the size of the incoming feedstock. Although the unit worked well, it was noisy, high-speed and maintenance intensive. The shredder has been re-placed by a four-auger mixing bin that eliminates problems inherent with the shredder. The bin also furnishes better mixing and moisture to the feedstock before composting.

Finally, nearly all the original drag chain conveyors have been converted to rubber belt conveyors. The original conveyors required much higher maintenance and were more costly than the retrofit rubber conveyors.

Politics And Regulations During the planning and development stages, the Pennington County facility received overwhelming support from the county, state and the Minnesota OWM. Changes in the federal and state regulatory climate and perceptions about refuse incineration and DRDF led to difficulties after the facility's start-up.

In the original plan, the county would own the facility, which would be operated by Future Fuel under a lease-purchase arrangement. The lease payments would cover the debt-service on the facility. Problems were encountered meeting the lease payment schedule because of difficulties with the equipment and marketing the fuel pellets. The original lease-purchase contract is no longer in effect, although the facility remains under the operation of Future Fuel.

The facility's initial focus was the production of DRDF. When the plant was originally designed and constructed, a large percentage of the waste was intended to be converted to fuel pellets. Markets for these pellets were cultivated and approved by Minnesota's regulatory agencies. At that time, the nation was embroiled in an energy crisis and any alternative to imported fossil fuels was welcome.

Later, lower fuel prices and increasing concern over dioxins and metals emissions from refuse incinerators seriously affected the operator's ability to continue to market the fuel pellets. Furthermore, Minnesota passed legislation that required all DRDF users, no matter what size, to test the emissions of their individual stacks.

One pellet customer, Northwest Medical Center in Thief River Falls, has used 100 percent DRDF from the Pennington County facility for eight years. The medical center has thoroughly tested and passed their emissions for all regulated pollutants. The hospital has adequate, although only rudimentary, emissions control equipment. In spite of these data, the state still requires other potential fuel markets to perform their own expensive stack analysis. However, new or existing solid fuel users can now burn as much as 30 percent of this fuel without additional testing.

Both the facility operators and the state found composting to be cheaper and more environmentally sound than continuing to landfill the material. Also, because of the high moisture levels of the residual organics, the material tended to freeze to the bottom of the transport trailers during the winter months, making unloading awkward and messy.

The original facility plan, regarded by the state as the second phase of the operation, was to compost the residual wet organic matter on the ground behind the facility. Composting began in 1986 and was immediately plagued with problems. Although the compost piles were capable of wicking small quantities of moisture from the bare ground, extreme rainfalls made the area impassable for vehicles and windrow turning machines.

The lack of turning and high moisture levels on the piles generated malodorous offgases and resulted in numerous complaints. High moisture levels in the compost also made product screening difficult or impossible.

Today, the composting operation is being improved by the addition of a 100' x 400' roofed composting pad and compost screen and destoner. The pad is a retrofit due to changes in state regulatory mandates and concessions to mud.

With the help of a local manufacturing company and Lake of the Woods County, the facility operator has designed and tested a proprietary compost trommel screen. Although the half-inch screen works well, it leaves many small inorganics and glass shards in the compost.

The operators are now installing a locally fabricated destoning unit. The glass shards and inert materials will be used by the county as an aggregate in road beds. The only odor complaints the facility has received occur during wet weather and uncontrolled composting. These conditions are expected to be eliminated by using the newly constructed compost pad and equipment.

The compost from the facility reportedly meets all Minnesota Class I requirements. The operators are encountering no problems with the marketing of either their recyclables or compost.

Despite the many changes that have been made to update equipment and operations, the facility continues to have minor problems. But the operators have learned what to expect and how to plan ahead for possible contingencies.

Maintaining a good sense of humor is crucial to running a materials recovery and composting facility, as the Pennington County facility's motto illustrates: "Dedicated to a landfill-free America."