Pre-Processing Operations

Yard trimmings arriving at the Z-Best site are weighed and recorded in the facility’s computer system. The system records gross and net tonnage, place of origin, type of material, date, time and truck information. These records are used for monthly reporting and invoicing, and are maintained on-site indefinitely. The trucks are then directed to the grinding area for offloading.

Z-Best uses a portable grinding system that allows the yard trimmings to be processed directly into windrows, so there is no centralized tipping area. The trucks unload material directly adjacent to the current grinding area. The material is then inspected for contaminants by load checking personnel, and any hazardous materials are either returned to the truck or brought to the site’s designated hazardous materials storage area for appropriate disposal. Non-hazardous contaminants are sorted and placed in trash receptacles for landfilling.

Processing and Composting Operations

After sorting, the yard trimmings are loaded into a horizontal grinder that reduces the material to approximately 3” minus. Horizontal grinders have a significant advantage in safety and operating cost when compared to traditional tub grinders. Additionally, the grinder has a throughput of 120 tons per hour.

Ground material is placed directly into windrows which are trapezoidal in shape, approximately 20’ wide at the base, 12’ high, and 400’ long.

During the 14-18 week composting period, windrows are monitored daily for temperature and moisture. Records are maintained on site and include daily temperature readings, turnings, and documentation of the fifteen-day pathogen reduction period as required by state law. Moisture is adjustedby adding water with one of the on-site trucks as necessary to maintain 40% to 50% moisture content.

Windrows are turned with a Scarab Windrow Turner 1-2 times per week to aerate and control temperature. Throughout the composting process, the desired temperature of 140-150 degrees is maintained through this system of watering and turning. Additionally, laborers walk the windrows after each turning to remove any exposed contaminants.

Finished Products Processing

When a windrow is fully composted, it is brought to the trommel screen for final processing. The trommel screen separates the composted yard trimmings into 1/4 inchfines and overs. The 1/4 inch fines are the finished compost, which is placed into large curing piles. Samples of finished compost are taken monthly, and include testing for metals and pathogen reduction in accordance with Title 14, Chapter 3.1. The finished product is also tested for organic content, nutrient value, Carbon/Nitrogen ratio, and other items useful for marketing purposes. Compost is generally cured for 30-60 days before sale to end-users. Overs from the screening process generally consist of wood chips, fibrous material, and plastic contaminants. Contaminants are removed for disposal and the remaining overs are re-ground. The material is then re-screened, resulting in additional finished compost and wood chips suitable for the mulch and co-generation markets.

Municipal Solid Waste Composting (MSW) Composting

In January 2001, Z-Best began its MSW composting program after applying for and obtaining a full solid waste facility permit from the California Integrated Waste Management Board.  Currently the facility accepts up to 250 tons per day of MSW compostable wastes from commercial establishments in San Jose, Town of Woodside and Portola Valley, County of Santa Clara and from the Sunnyvale SMaRT Station. Z-Best’s solid waste facility permit enables them to take a peak of 700 tons/day of MSW.

At the Z-Best facility, all materials are processed in an enclosed 20,000 square foot tipping building that accommodates new processing equipment. It is designed as follows: An excavator extracts large items such as shopping carts, matrices and pallets and then loads the feed system. Materials pass in front of two sorting stations where recyclables (plastic containers, metals, glass, etc.) are recovered; non-recyclable materials such as trash are removed. Remaining materials pass through a Bulk Handling System debagger, which slices the bags and larger pieces of cardboard. The processed materials pass in front of anther sorting station where glass containers are removed, then go to a BHS 48-inch by 15-foot disc screen to remove the three-inch minus fraction. The remaining materials pass through several hand sorting stations and a magnet. Ferrous and nonferrous metals, and plastic containers are removed and recycled. Plastic bags also are removed for disposal. These clean materials than fall into a shredder where they are sized reduced to 3-inch minus. These processed materials and the 3-inch minus from the disc screen are combined and stockpiled. These stockpiled materials are then loaded in to one of the modified manure spreader, then discharged onto a conveyor that feeds the Ag Bag Bagger.

The bagger ejects the mixed compostable waste into a 350 foot long bag that houses all the compostable wastes. Besides the mixed compostable wastes the bagger also injects two-4 inch reusable PVC pipes which are used to aerate the compostable materials. When the bag has been completely filled, the PVC pipes are connected to a blower which provides the oxygen to the compostable materials.

Each bag has the capability of holding about 500 tons. Retention time in the bags is about four months. As the bags are opened, the contents of several are combined to form a windrow. These windrows are turned for several weeks to facilitate the composting process. The materials are than transported to a screening system that is used to remove any particle larger than 3/4 inches. These larger pieces are than disposed. The 3/4 inch minus materials are stockpiled and cured for an additional four weeks before being screened to 1/8 inch minus. (i.e. the size of coffee grounds). Compost produced using this system is not sold to farmers or materials yards since it does not meet the definition of organic as defined by the Organic Materials Research Institute (OMRI).

Finished compost is sent monthly to an independent laboratory to be tested for nutrient value, contamination and pathogen reduction. The food residuals compost will only be marketed to landscapers and golf courses under the name of Z-Best Landscape Compost to comply with OMRI certification standards.