The authority currently collects and/or accepts approximately 200,000 tons of curbside collected yard trimmings and 60,000 tons of biosolids from local wastewater treatment facilities each year. For the past seven years, the authority has been operating a cocomposting facility as part of the program for managing these two residuals. The Compost Facility was highlighted in an article that appeared in BioCycle in October, 1995. The Compost Facility is permitted to handle up to 120,000 tons/year and has been operating at approximately 80 percent of its capacity. A mixture of biosolids and shredded yard trimmings provide the feedstock. This report explains the improvements that have occurred in preprocessing yard trimmings utilized as feedstock and postprocessing or screening the compost produced by the Compost Facility.
Yard trimmings discarded at the curb are picked up once per week and delivered to either the authority’s transfer stations or directly to the authority’s new Woody Waste Recycling Facility (WWRF). The WWRF is located in the same 1,400-acre North County Regional Resource Recovery complex as the Compost Facility. Approximately 100,000 tons of the total yard trimmings collected are delivered to the WWRF for processing. The remainder is delivered to another private facility the authority contracts with to produce a boiler fuel.
Past methods to produce the feedstock amendment for the Compost Facility were both labor and equipment intensive. This included receiving, spreading, manually removing contamination, stockpiling, drying (four to six weeks), grinding, screening, stockpiling finished products, and loading products for distribution.
There were several disadvantages. The spreading operation was mechanically intensive and, once spread out, the removal of contaminants was very labor intensive. Grinding and screening operations were not matched for production, which either slowed down the grinder or produced too much blow-by in the trommel screen. Stockpiling of finished products required an additional loader and operator. All of the equipment was mobile and pretty much mismatched as far as production capability.
A new stationary (we call it PAC Man) system, which went into operation in October, 2000, either has reduced operating costs or improved the quality of the products.
NEW PROCESSING SYSTEM REDUCES COSTS
Processing now involves receiving, stockpiling, drying (four to six weeks), prescreening, use of picking station for removal of contaminants, direct loading into a horizontal wood hog, postscreening, and use of stacking conveyors for the screened mulch. All equipment is matched to provide for efficient use of manpower and equipment. Previously, three loaders with operators, one grapple and operator, and eight temporary manpower employees were required to process materials. Now, one loader with operator, one grapple and operator, and four temporary manpower employees are used. The reduction in manpower and machinery has greatly reduced the operating costs. It is anticipated that the savings in operational expenses should be enough to recover the capital expenditures within a five-year period.
The research, procurement and installation of the new stationary yard trimmings processing system was rather involved. The authority took proposals and selected RRT Design and Construction to: Evaluate the authority’s current operation; Define new system performance requirements; Provide system equipment and construction specifications; Estimate construction and operating costs; and Develop conceptual design drawings.
At this point, the authority and RRT made a collective decision to go with a single source equipment supplier who could provide the whole equipment package as a turnkey operation. RRT then was tasked with selecting a vendor and coordinating the design, construction, and acceptance of a turnkey system that would meet the authority’s needs. The selected vendor, Continental Biomass Industries, supplied all the equipment for this system, provided guarantees on production capacity, and provided a warranty covering the entire system. This system consists of four major components: Prescreener, Picking Station, Magnum 4800 Grinder, and Stardeck System.
The Prescreener also doubles as a feed hopper where material is metered into the system. The screen deck consists of heavy-duty discs to screen out a three-inch material and bypass the Picking Station. This material may be conveyed away from the system or reintroduced.
The Picking Station is in line to remove any foreign material (plastic, rock, metal, etc.) from the flow of yard trimmings prior to grinding. Drop chutes were not designed in, as the brush would hang up on the chutes. Metal is removed with a 30-inch magnetic head drum built in a specially designed stainless steel section. This system may be manned with two to eight personnel depending on the level of contamination.
The 4800 grinder is powered by an 860 horsepower Caterpillar engine and has been set up on stationary stands in line with the system. It has the new pocket rotor that is ideal for the yard trimmings material and is a down swing mill, turning at 700 rpm. The slower speed helps in reducing wear on the tips and passing any steel that may get by the picking station. On the discharge belt, there is a cross belt magnet to remove the tramp steel so that it does not go into the end product. In the event that the overs from the Stardeck System need to be reground, the material can be reintroduced.
The multideck Stardeck System allows the product to “waterfall” over each of the four ten-foot decks. This helps spread the palm fronds and be more efficient in separating the fines at 1-1/4 inch from the boiler fuel. The system is capable of changing sizes of screened material by only changing the speed of the stars and not having to change out screens. This is the most efficient means of screening the authority’s high volumes of ground yard trimmings.
After screening, the two resulting products are stockpiled for future use. The unders portion is utilized as an amendment for the Compost Facility. The overs can be used as boiler fuel, erosion control on landfill side slopes or given away to the general public.
CAPABILITIES OF SYSTEM
In operation since October, 2000, production capacity of the new system averages between 50 to 60 tons/hour of input material. Downtime is kept to a minimum because of easy access to alternative loading points in the system. It is necessary to keep an inventory of spare parts on hand to facilitate repair when necessary. The equipment is extremely heavy duty and designed for a 20-year life span.
Screened yard trimmings from the WWRF are delivered to the authority’s Compost Facility and mixed with biosolids from local wastewater treatment plants. The authority utilizes the IPS system supplied by U.S. Filter to cocompost these residuals. The Compost Facility consists of a 36 reactor agitated bay system. The aerobic composting process is computer controlled and the compost mass is mechanically agitated during the minimum 14-day detention time in the facility. Biofilters are used for odor control.
Once the composting process is complete, the unscreened compost is delivered back to the WWRF for screening. A new stationary compost screening system was completed in May, 2000. This new system was designed to match the output of the Compost Facility and to process unscreened compost into various product grades. The authority again contracted with RRT Design and Construction for evaluation and design and construction management of a new compost screening system. The equipment chosen was the bivi-Tec screen supplied by Aggregates Equipment, Inc. It is especially effective at very small sizes below half-inch in size. Currently, the authority can effectively screen compost from 1/6 inch to 1-1/2-inch in size.
The screen design uses a dual vibration from a single drive. Two weights, vibrating at the same frequency, move relative to each other, which tenses and relaxes the screen mats. The linear momentum of both vibrating movements is adjustable. The screen mats can receive up to 50 “g”s of force during the screening process, which assures that the mats are self-cleaning. This prevents the high moisture compost from clogging the screen openings. It also provides a complete release of sticky organic material from larger particles, thereby providing maximum efficiency of fines recovery.
STATIONARY SCREENING SYSTEM COMPONENTS
Components of the authority’s stationary system include infeed hopper, double deck screen box, and various incline and stacking conveyors, which are described as follows:
The infeed hopper consists of a 20 cubic yard chain belt feeder that incorporates a 15 horsepower speed reducer with a mechanical variable speed control. It has a manual gate for regulating feed to the incline conveyor.
Power to the double deck screen box is supplied by a 40 horsepower electric motor that drives the two vibrating weights. The two screen decks have long life polyurethane screen panels with a patented wedge fastening system. The deck has a 15-degree decline from the infeed to discharge area and weighs approximately 32,000 pounds. The double deck screen allows for production of three independent splits with one pass.
There are three stacking conveyors for various material splits, which are 48 inches wide and 60 inches in length. They are single speed and powered by 10 horsepower electric motors. They are manually adjusted for height and use pin retainers. Production capacity to yield 1/2-inch material is approximately 150 cubic yards/hour. The efficiency of recovery is from 96 to 99 percent. The unit has very few moving parts, provides very quiet operation, and is all electric.
MOBILE SUPPORT EQUIPMENT
In addition to installing a new stationary yard trimmings processing system and a compost screening system, the authority also purchased several new pieces of mobile support equipment. These included a log loader; five rubber tired loaders; and a telescoping man lift.
The log loader is manufactured by John Deere and utilizes a 360° rotating grapple rake. A log loader is similar to a standard bucket excavator with three exceptions. First, the boom and stick are designed for lifting instead of digging. Second, the undercarriage is heavier in design and weight. Third, the cab is elevated to increase operator visibility. One other addition to the log loader was to install a tilt cylinder on the grapple rake that allows for controlled movement of the rake. Total weight with grapple is 62,000 pounds.
Rubber tired loaders were supplied by Case Equipment and are equipped with quick disconnect attachments. As mentioned in a previous issue of BioCycle, the authority utilizes various high tip light material buckets and timber rakes as attachments for these loaders. The buckets come in two sizes — ten cubic yards and seven cubic yards. The choice of which bucket to use is dependent upon the weight of material. The timber rakes are utilized to stockpile and move unprocessed yard trimmings.
A telescoping lift was supplied by JLG Industries. This unit is essential for the mechanics that have to work on this equipment. It features a 40-foot reach and a 1000-pound bucket capacity. It is also equipped with welding lead connections extending from ground level to the bucket; a two-wheel drive all terrain unit provides for a safe work platform.