A study conducted for the Environmental Protection Agency found that 136 million tons of debris was generated in 1996 by construction and demolition (C&D) waste. This remains the most exhaustive national study available. C&D waste in this study was defined as debris from the following structures; both residential and non-residential buildings, roads, and bridges (Franklin Associates, 1998:ES-2). The residential component of those 136 million tons of debris was 43% of the waste or (58 million tons per year). The fact that the non-residential waste stream is larger than the residential should not come as a surprise due to the scale of America's buildings. The surprise may come in the distribution of waste across the categories of activity. Demolitions account for 48% of the waste stream and renovation activity accounts for 44%. This leaves a paltry 8% of the waste stream attributable to new construction. In the residential only realm the renovation activity accounts for 55% of the total residential waste stream, demolition 34%, and new construction 11% (Franklin Associates, 1998:ES-3). Figure 1 illustrates the residential and non-residential contributions to the waste stream, Figure 2 the category contributions, Figure 3 the residential marketplace, and Figure 4 the nonresidential sector.
Figure 1. Sources of C&D debris.
Source: Franklin Associates, 1998:ES-2
Figure 2. Sources of C&D debris by category.
Source: Franklin Associates, 1998:ES-2
Figure 3. Sources of Residential C&D debris.
Source: Franklin Associates, 1998:ES-3
Figure 4. Source of Nonresidential C&D debris.
Source: Franklin Associates, 1998:ES
Estimates are that up to 90% of C&D waste is potentially reusable or recyclable, depending on project type and the local market for waste materials (Triangle, 1995:1).
The National Association of Home Builders (NAHB), in a study done for the EPA in 1997, characterizes the waste stream generated by the new construction of a 2,000 square foot home as being constituted of four tons, or 8,000 pounds of debris (NAHB Research Center, 1997). Their characterization of the materials follows in Figure 5, the National Typical House.
Figure 5. Construction Waste by Weight, NAHB 1997
Waste equals food
There are two main approaches for systematically operating the C&D debris cycle, and an emergent third alternative. The first approach is to transport mixed C&D debris to a central processing facility or materials recovery facility where the debris is sorted. The high value material is high graded out and re-enters the materials supply stream or the material enters a scrap or recycling stream. The low value materials are processed further following their rejection from the material supply stream or the scrap and/or recycling stream and transported for interment into a landfill. The second approach is to separate select materials at the job site and then transport the different materials directly to the markets for those materials (FDEP, 2001). The third alternative is the practice of processing selected materials for end of life use at the job site.
The notion of waste equals food is a biological concept. The basic science of ecology knows that waste equals food, and when it does not the natural flow materials becomes imbalanced and ecosystems become compromised. To the extent that systems designed by man mimic this natural law the degree of success enjoyed by the system is much enhanced. Thus, this tactic is relatively new on the national scene and very rare in the H-GAC study area. This systems approach can result in a high degree of waste being turned into food on site and elimination of up to 83% of the typical homes waste stream by weight's contribution to the landfill.
Central processing facility
The centralized facility approach is the most common facility arrangement. Typically, mixed C&D debris is tipped at a central facility, and the materials with a high market value, such as large pieces of sawn lumber, are removed. The remaining mixed C&D materials are then processed using one of two primary methods. The mechanized size reduction method uses a crusher, a dozer, or a compactor. The materials are then passed through a series of screens, magnets, and other separation equipment. The manual labor method relies on human sorters to pick out materials and place them in specific containers. Screens and magnets may also be employed with the human labor method, but the materials are left in their original form rather than crushed so that they can be easily distinguished and sorted (FDEP, 2001:18-19). The most common approach is a blend of the mechanized size reduction and the human sorter methods.
A primary success for a C&D recycling operation hinges on the degree of contamination of the C&D materials by other types of waste such as nails, paint, foil, oil or plastic. Some processing facilities that aggressively handle a mixed waste stream may cause contamination of the C&D materials, thereby limiting their potential to be recycled.
Table 1 summarizes the methods employed by central processing facilities and also includes the estimated volume of material that is rejected from the recycling stream for each method.
|Table 1. Sorting methods used by C&D central processing facilities. Source: FDEP, 2001:20
|Manual separation only
||Waste is tipped. Large identifiable materials with ready markets are removed by hand. The remaining material is land filled.
|Combination manual and mechanical separation (most common approach)
||Waste is tipped and screened. Manual labor is used to remove the components on a conveyor belt.
|Heavy mechanical processing and separation
||Waste is tipped and processed (often crushed) and sent through a complex train of mechanical equipment for separating the materials.
Job site material recovery
The practice of sorting and processing materials at the job site can result in a higher degree of material recovery but is less commonly used in residential C&D practice. To sort C&D materials onsite, contractors need to either arrange for C&D debris haulers to visit the site during the different stages of C&D activity and waste generation or set out different containers for the different waste materials. Some of the factors that have limited this approach are a lack of experience with job site material recovery, a lack of space for different containers on the job site, and the need for rapid completion of many C&D projects (FDEP, 2001:22).
Various types of equipment are available for C&D processing and recycling, either at a central processing facility or at the job site. Table 2 describes some common types of C&D recycling equipment.
|Table 2. Common types of C&D recycling equipment. Source: FDEP, 2001:39
||Drums, pallets, cranes, bulky waste
||Compacts waste with over 65,000 lbs. of force & displaces over 175 cubic yards per hour
||Gypsum, industrial trash, soft metals
||Punctured pieces of materials are dropped between rotating high teeth then screened
||Steel, C&D debris, land-clearing debris
||Used for the removal of debris in logging operations and construction sites
||All C&D debris
||Material is fed onto a vibrating screen in which the trommel sorts and discharges waste
||Cardboard, metal, paper, plastic
||Like separation systems, but designed with side-fed units
||Plastics, rubber, foam, crates, bins
||Materials are broken up into pieces by rotors then reduced into pellets by rollers' teeth
||C&D debris, land-clearing debris
||Grinds materials from 120-320 cubic yards per hour from a top feeder with dual auger discharge
||Yard waste, wood chips, sludge
||From a conveyor belt the fed material is screened and dispersed evenly, then outfed and stacked
||Solid waste, scrap, bulky materials
||Hauls solid waste materials up to 44,000 lbs.
|Trailers (transfer, roll-off, and walking floor)
||Solid waste, scrap, sludge
||Hauls solid waste materials up to 80,000 lbs.
On-site material processing
The third approach on-site processing of materials presupposes to take tactics from the two main approaches and deploy the technology to the site and to do what makes sense in terms of processing. In the On-site Grinding of Residential Construction Debris: the Indiana Grinder Pilot conducted by the NAHB Research Center in 1999, concluded that 90% of the waste stream is potentially recyclable or reusable on-site.
On-site processing has been evaluated in numerous states and has found to be highly successful in all of the studies conducted regardless of geographic location. The likelihood that this will become the dominant method of processing east of the Mississippi River is very high. The primary element leading to the successful deployment of residential scale on-site materials processing is a portable residential scale grinder capable of handling wood, shingles, drywall, nails, concrete, cardboard, and brick.