How To Transform Into A Wood Processing Powerhouse

Kevin P. McCarthy

Nearly six years ago, in August 1991, a landscaper trucked a load of brush to a local transfer station in San Leandro, Calif. This insignificant pile of yard trimmings, however turned out to be the first of tens of thousands of tons of wood and yard waste headed to this facility over the next several years.

In fact, only five months later, the Davis Street Transfer Station, operated by Waste Management of Alameda County, a subsidiary of Oak Brook, Ill.-based Waste Management Inc., had recycled 1,630 tons of this material. By 1992, that figure had skyrocketed to 8,721 tons. Because of additional yard material collected curbside in Alameda and Contra Costa counties, tonnage tripled in 1993. Operationally, this meant separate dropoff areas for curbside loads and new markets for the material.

As in the past, self-haul loads were transferred offsite for grinding into fuel, while the new curbside material was shipped 60 miles to a compost facility in California's central valley. The cost for transfer and tipping fees at the compost plant was $38 per ton. By 1994, Davis Street was able to lower its cost to about $32 per ton by using mulch markets. This price allowed the 53-acre facility that handles more than 750,000 tons of refuse yearly to "remain competitive, while eliminating the risks of operating its own mulch or compost facility," said general manager Jack Isola.

By 1996, wood and yard material recycling alone accounted for 103,030 tons and this year, because of the increasing interest in yard waste, the facility expects to receive as much as 130,000 tons.

How Davis Street Did It After visiting six processing operations on the West Coast and completing a four-day tour of composting operations in the Netherlands, Davis Street staff arrived at a final design concept in late 1995. "We needed a system that could produce multiple products from an incoming stream of 450 tons per day of self-haul and curbside material," said project manager Jon Benner.

The staff found that a modified flat-deck screen was more effective and versatile than traditional trommel screening systems for handling large volumes of material from varying feedstocks. High production and diverse products were essential because of the expected daily incoming stream of 450 tons.

By the spring of 1996, the staff began selecting the components of the system, using Bulk Handling Systems, and P&R Machinery Inc., both based in Eugene, Ore., to design, manufacture and install the screening and grinding equipment.

To minimize "overs" from the screening process, they bought a horizontally-fed 500 horsepower electric hog for integration with the screening system. This grinder can handle yard materials and also clean wood waste such as milled wood, lumber, and stumps.

In September 1996, Davis Street installed the new 50-ton - or 250-cubic-yard per hour wood and yard material processing system. The $800,000 system adjoins a 125' x 100' concrete tipping pad which allows for all-weather access for curbside trucks and self-haul customers. The equipment itself was installed on a 150' x 125' pad with an aggregate base. The larger pad also serves as a secondary tipping area and for transfer trailer loading with processed material.

Curbside and self-haul loads are tipped in separate areas on the pad because of contract limitations on end uses. Curbside yard material is processed separately for agricultural, mulch and compost markets. Although contracts prohibit processed material to be used for alternative daily cover (ADC), some allow its use as biomass fuel. Self haul material is mixed with wood waste to produce a screened product for ADC and a ground product for biomass fuel.

Davis Street's on-site mini-MRF supplies the majority of the clean wood waste delivered to the new system. The mini-MRF is a small scale line with eight sorters to recover wood, metal and cardboard from rolloff loads. Approximately 50 tons per day of wood waste are recovered.

Once offloaded, the feedstock is inspected on the ground and any obvious contaminants such as plastics, papers and metals are removed.

A front-end loader pushes the material into an in-floor conveyor which leads to an infeed conveyor that waterfalls into a 20-cubic yard hopper. The hopper feeds an incline conveyor leading to a four-person sort line for additional contaminant removal.

The material is processed after it has left the sort line belt. The feedstock waterwalls onto a BHS debris roll screen (DRS) for removing two-inch material before grinding. This primary prescreen serves a valuable function of reducing hammer wear resulting from the abrasive characteristics of fines.

"Overs" (+ 2") are fed into the hammermill and outfed to a conveyor leading to a fines screen (.75" DRS). Unders (2" -) from the primary prescreen pass over a fines screen (.75" DRS). Yard materials passing over the "fines" screen can be discharged from the system or conveyed and rejoined with material exiting the hammermill. All product exiting the system is conveyed to storage bunkers for transport.

This system is designed to separate the feedstock into five different fractions or sizes depending on its end market use. Three screens separate fines or small material from the incoming stream, while the hammermill produces a ground product. "Different applications require different products," noted Linda Cushman, the organic products specialist.

Biomass fuel is made from the self-haul/wood feedstock. The feedstock follows the same general flow as above with one exception: 2" (-) material is discharged from the system without mixing with ground material exiting the hammermill, which re-duces the potential for high ash levels at the biomass plant. Belt and discharge chute magnets are used to remove metal from the fuel product.

Challenges "Implementing this new system has posed a number of challenges for us," Isola said. "We have had some startup problems due to the complexity of the system and the need to retrain our workforce." The real challenge for Davis Street management has been training and integrating the work of three different unions: quality control, transportation and maintenance.

This workforce historically has focused on disposal related tasks, not processing material to make a product. "Our workers are used to handling, moving, compacting and transferring solid waste with heavy equipment," remarked Bob Biasotti, operations manager.

Davis Street is continuing on a track of "training, training and more training" to maximize the capability of the new equipment, said Benner. These efforts have paid off since the system is exceeding its production specifications. In the future, they will begin matching equipment production with the most cost-effective end markets.

With six months' experience, Davis Street is planning operational im-provements. For example, it will add overhead silos for product storage to improve cycle time, and the primary tipping pad will be increased to handle anticipated wood and yard trimming tonnage growth. Peak daily tonnages are expected to reach nearly 800 tons in spring 1997.

Davis Street's marketing program also will continue to expand. Retail sales of fresh organic products will begin this spring.

"It's gratifying to see our wood and yard waste recycling program grow over the past five years," remarked Isola. "It's rare to set up a recycling program that achieves such significant diversion."

Davis Street Station for Material Recycling and Transfer is one of the nation's largest re-cycling facilities.

Processing equipment: * one integrated wood and yard material screening and grinding system (3BHS Debris Roll Screen by Bulk Handling Systems, a 500-horsepower electronic hog grinder by Processing and Recycling Machinery and various conveyors, sorting platforms and hoppers);

* one automated container sorting system (includes various conveyors, sorting platforms, a finger screen, overhead magnet, air classifier and eddy current from Ptarmigan Machinery and Karl W. Schmidt;

* one 300-horsepower horizontal baler, 2-ram from Enterprise Baler Company; and

* one mini MRF (includes an infeed conveyor, sorting platform and sorting belt) that was constructed in-house using existing equipment from other recycling operations.

Refuse processed in 1996: * transferred: 741,760.71 tons;

* refuse capacity allowed under permit: 1,300,000 tons.

Sources of waste: private, city and county haulers, residential, commerical, industrial, construction and demolition.

Employees: 185 people serving departments for transportation, material recovery, maintenance, container repair and administration.

Service area: Northern and Central Alameda county communities for refuse. Recycles for all these cities, plus Contra Costa county.

Local tipping fees per ton: * city of Berkeley transfer station, $60.10;

* Sanitary Fill, San Francisco, $60.13;

* Pleasanton transfer station, $42.50.