March/April 1995
A staggering amount of debris is generated in the course of construction and demolition projects. While the lion’s share of this material is relegated to disposal today, actual and potential markets exist for many components.
By Robert H. Brickner
Robert H. Brickner is senior vice president of Gershman, Brickner & Bratton Inc. (Falls Church, VA), a consulting firm specializing in waste and resource management.
Every year, construction and demolition (C&D) projects in the United States generate about 100 million tons of debris. That makes C&D debris the country's second largest waste stream, behind only municipal solid waste, estimates Mike Taylor, executive director of the National Association of Demolition Contractors (Doylestown, Pa.).
Not all of this material must be handled as waste, however, though much of it is. Indeed, most C&D debris generated in the United States ends up in special C&D landfills, sometimes called "demofills" or "rubblefills," which are typically less stringently regulated than municipal solid waste landfills. This makes disposal in them easier and cheaper than in typical municipal fills, thereby perhaps providing a somewhat lowered incentive for recycling these materials.
But, of course, there are some valuable commodities found in the rough of C&D debris-materials no one wants to bury. Demolition, for instance, brings to mind steel rebar tom from concrete, and C&D debris also contains other metals. In addition to structural steel, these include such items as aluminum siding, appliances, copper and brass piping and fixtures, and ferrous pipes, sheet, and plate. Furthermore, demolition projects frequently uncover old underground storage tanks, which may be acceptable for reuse or processed as scrap.
Another common conventional C&D debris component is corrugated containers, which may be acceptable to paper processors or mills, and, if not, may be suitable for conversion to fuel pellets.
Beyond these materials, what kinds of recyclables are generated by C&D projects and what are their market prospects?
Categorizing C&D Debris
In order to answer that question, it helps to take a step back and consider what the term "C&D debris" really encompasses. Although it is often referred to as if it were a homogeneous stream of material, in fact its characteristics vary widely depending on the nature of the project that generates the debris.
While some constituents of C&D debris are likely to be generated by nearly any kind of project, others may result from only certain types of jobs. And in regulatory terms, there are almost as many formal definitions of C&D debris as there are states. Still, for simplicity's sake, materials can be conveniently broken down into these five major categories based on the types of activities that add them to the C&D stream:
Road Work. Road construction, repair, and removal generates mostly asphalt, earth fill, and concrete, with and without rebar. Bridge demolition may also provide heavy structural steel.
Site Clearance. Preparing open sites for construction or landscaping contributes primarily trees, brush, and earth to the C&D debris stream, but also may add concrete, rubble, sand, and steel as well as extraneous paper, plastics, and miscellaneous wastes.
Excavation. Excavation activities result in generation of earth, sand, and stones. This debris may also include extraneous site clearance wastes and unearthed pipes.
Building Demolition. The C&D debris generated by demolition consists mostly of mixed rubble, including concrete, steel beams and pipes, brick, and timber, but it also may include a wide variety of materials in the form of fixtures and fittings.
Construction/Renovation. Construction activities generate a wide variety of debris, including wood, roofing materials, fixtures, wallboard, insulation, parts of ducts and pipes, and carpet remnants.
Although some of these materials may be less than attractive from a recycling markets standpoint, many components have some positive prospects. Here's a closer look at the major constituents (except metals and paperboard) and their actual or potential uses.
Collecting Concrete and Asphalt
Block concrete, typically a voluminous and heavy constituent of C&D debris, can be crushed and screened and sold as an aggregate for use in new concrete, road sub-base,asphaltic concrete, cement block, or miscellaneous fill. Old asphalt can be crushed and mixed with new asphalt, in either hot or cold mix processes.
In order to be used in these applications, concrete and asphalt are typically processed using jaw crushers, impactors, or cone crushers assisted by feeding equipment,magnetic separators, vibrating screens for product sizing, and stacking conveyors. It is common for concrete and asphalt recyclers to charge minimal or no "tipping" fees for accepting these materials, which are subject to many-if low value-- end uses, including remanufacture by pavement makers and use by local and state transportation departments. In many cases, recycling these materials is economically advantageous.
Converting Wood Materials
The percentage of wood materials in the overall C&D debris stream in a given area varies widely, but wood is generally a significant component of debris from most C&D activities. In fact, in the table on page 190, which shows a sample breakdown of the C&D debris generated during a single week in Des Moines, Iowa, wood products were the biggest constituent of the C&D sample, contributing well over a third of the total weight. And for certain kinds of projects, such as a land-clearing operation, woody debris could approach 100 percent of the material generated.
Untreated wood can be reused in a number of ways, including simple resale as timber--sometimes a high-value market.
Woody debris can also be shredded into chips or ground into powder. Shear shredders assisted by screening systems for product sizing are generally used for processing, though tub grinder systems are also employed to turn wood into mulch. The primary use of wood chips is in landscaping, for applications such as weed suppression and soil conditioning.
Wood chips can also be used in sewage treatment. Many cities today-- Washington, D.C., and Philadelphia, for example--are composting their sewage sludge, and wood chips are part of the process. The chips are added to adjust the carbon/nitrogen ratio of, absorb moisture from, and improve aeration of the sludge. Compost produced from the sewage can be sold as a soil or fill constituent to customers like nurseries, golf courses, and land reclamation projects.
The volume of wood chips needed in the process depends on the moisture of the sludge and the initial characteristics of the chips. For a sludge with 20-percent solids content, for example, the effective ratio for wood chips to sludge will lie somewhere between 1:1 and 4:1 on a loose-volume basis-potentially a substantial amount of chips. The chips are gradually consumed over time at a rate dependent on their original size, but generally approximately 20 to 25 percent per cycle.
In addition, a number of companies are using wood chips as animal bedding, and at least one C&D debris processor is selling shaved wood for use as turkey bedding. Acting as a potential impetus to this market is the fact that straw, used traditionally for animal bedding, is in short supply in some areas of the United States as a result of changes in farming practices. On the other hand, sawdust and wood shavings must also compete with shredded newspaper, an increasingly popular animal bedding material.
Mixed wood debris can also be processed into wood pulp, fuel, or new building products. Pallets, another common C&D constituent, can be used in pallet remanufacture, as well as many of the other end uses listed.
Potential contamination is a serious concern for processors handling woody debris. Old lead-based paint on wood from demolition of an old home, for example, or treated wood from railroad ties or a disassembled patio deck may be cause for the rejection of the shredded or ground wood product in many of these applications, especially those involving plant or animal contact.
Dealing With the Rest Of a Very Mixed Bag
There are a whole bunch of other C&D debris components present in varying amounts and facing varying degrees of potential for reuse or recycling. Here's a rundown of the materials and some possible applications for them:
- Dirt may see reuse as soil, as landscaping fill, or as landfill cover.
- Bricks can be salvaged as usables or crushed for use as ornamental stone.
- Glass may be recycled into new glass, and it may be suitable as a raw material in the production of fiberglass insulation, asphalt, blasting media, and even reflective beads.
- Gypsum and wallboard may be used in soil amendment and new gypsum board or absorbent media. Due to problems associated with older wallboard products, however, new construction activities may be the only source of potentially recyclable wallboard.
- Plastics, including polyethylene, polystyrene, polyvinyl chloride, and acrylonitrile butadiene styrene (better known as ABS ), may be sold through processors or brokers to manufacturers of plastic lumber, highway barriers, and traffic cones. These plastics may also find applications in new insulation.
- Asphalt shingles can be recycled into asphalt paving materials, though this is a very limited recycling activity in the United States today. The probable reason: One company's experience handling source-separated roofing materials suggests that a high volume of shingles--250 to 300 tons per day--may be required for economic processing.
- Carpeting may be used for landfill cover.
- Two other common constituents, fiberglass insulation and linoleum, do not currently appear to have any practical reuse or recycling markets.
Because a large portion of the overall C&D debris stream-like asphalt, concrete, dirt, soil, rock, brick, glass, gypsum board, porcelain--is or can easily be converted to inert granular products, a lot of the materials can also be used in secondary and more general applications than those listed above. These might include use as daily cover in landfills; fill in land reclamation; fill for roads, airports, and port embankments; sub-base and base for roads, parking lots, airports, and buildings; aggregate for filter layers in landfills, general construction, and other drainage systems; and material to retard soil erosion control.
Hey, a Market's a Market
In many parts of the country, little or no C&D debris is processed for recycling. But even where markets may not currently exist, potential markets for products manufactured from C&D debris may be found--though the reader should keep in mind that "markets" in this sense means outlets that provide productive uses for products manufactured from C&D debris, whether or not payment is received or "tipping" fees are charged.
In many cases-covering the bulk of the C&D debris stream-constituent material may not actually have positive value, but can be used as filler in place of other materials, saving money in disposal and purchase of the alternatives.
A staggering amount of debris is generated in the course of construction and demolition projects. While the lion’s share of this material is relegated to disposal today, actual and potential markets exist for many components.