Moving forward: recyclers and generators of scrap gypsum are attempting to widen end markets - Cover Story

C&D Recycler, May-June, 2003 by Timothy G. Townsend

Gypsum drywall is a major component of modern buildings, yet it is often one of the least likely components of debris from building construction, demolition or renovation to be recycled.

Gypsum drywall typically makes up 5 percent to 25 percent of the weight of debris from building-related C&D activities. A typical rule of thumb for drywall generation from construction activities is one pound of drywall per cubic foot of construction.

The amount of post-consumer drywall generated in Florida C&D debris in 2000 was estimated at nearly 500,000 tons. Most of this was disposed in unlined landfills. Recovery and recycling is always desired in place of land-filling when feasible, but several environmental factors (odor, groundwater contamination) create added incentive for recycling drywall.

While several gypsum drywall recycling operations exist in North America at the current time, in many areas (Florida for example) drywall recycling is relatively nonexistent. This paper describes some of the issues facing the C&D debris industry and summarizes the current State of gypsum drywall recycling in North America.

DRYWALL BASICS

Gypsum drywall, often referred to as gypsum wallboard or sheet rock, replaced gypsum plaster as the major interior wall surface because of its ease of installation and its fire resistant properties.

Gypsum drywall consists of approximately 90 percent gypsum and 10 percent paper facing and backing. Gypsum is a mineral composed of calcium sulfate (Ca[SO.sup.4]) and water ([H.sup.2]O). Drywall comes in many different types and sizes to meet specific construction needs. Several specialty products are manufactured including moisture resistant drywall (greenboard) and Type X drywall. Type X drywall contains small glass fibers designed to increase the board's ability to withstand high temperatures from fires for a longer period of time.

Drywall enters the waste stream in several different locations, including the manufacturing facility, construction sites, renovation activities, building demolitions and manufactured housing plants. At the construction site, drywall is sometimes mixed in with the rest of the debris. In other cases, the drywall subcontractors are required to manage the debris themselves. Most drywall from construction is produced during a very short period of time, a positive trait from a separation and recovery standpoint. Because of its friable nature, gypsum wallboard is very difficult to recover from mixed debris resulting from standard demolition practices. It is certainly possible (and sometimes practiced) to first strip a structure of drywall (and other building materials) prior to the demolition of structural components. Drywall is in many cases the largest component of the waste stream at a manufactured housing plant; several drywall recycling companies have developed in the vicinity of these facilities.

AT THE LANDFILL

When gypsum drywall is disposed in landfills, a series of biological and chemical reactions can occur that have the potential for adverse environmental impacts. When drywall in a landfill gets wet, some of the sulfate from the gypsum dissolves into the water. If this "leachate" reaches the groundwater, contamination with sulfate may result. Granted, sulfate is not a risk on the same magnitude as chemicals such as lead or arsenic, but high sulfate concentrations do impart negative qualities to the water.

The U.S. federal secondary drinking water standard for sulfate is 250 mg/L. Concentrations above this level have been observed in the groundwater at unlined C&D debris landfills. The sulfate also contributes to the high total dissolved solids (TDS) concentrations observed in groundwater at many C&D debris landfills.

Another issue results form the biological conversion of dissolved sulfate to hydrogen sulfide ([H.sup.2]S). [H.sup.2]S is a foul-smelling gas (often compared to rotten eggs). It is produced under wet, anaerobic conditions, such as those that often occur in landfills.

The presence of organic matter such as yard trash or cardboard is needed for the microorganisms to thrive, but even the paper on the drywall itself provides enough organic matter for the biological reactions to occur. [H.sup.2]S has been observed over a tremendously large concentration range at C&D debris landfills.

The human nose can detect [H.sup.2]S at relatively low concentrations (<0.1 part per million or ppm). [H.sup.2]S concentrations have commonly been measured in this range in the air above and surrounding C&D debris landfills, thus odor complaints are a common problem. Concentrations in pockets within the landfill have been measured at lethal levels (>250 ppm).

Usually by the time the gas reaches the atmosphere, it is diluted to below most currently accepted safety concentrations. Landfill operators should exercise extreme caution, however, any time undiluted C&D debris landfill gases might come in contact with a worker (e.g. excavation activities, confined spaces).

The odor problem alone has been enough to result in strict actions being required on the part of some landfill operators. Some C&D debris landfill operators have been required to install gas collection and recovery systems, and others have resorted to placing odor masking agents around their sites. In the Vancouver area, gypsum drywall was banned from disposal in landfills as a result of odor problems.