Drywall remains one of the components that poses problems to construction-and-demolition landfill operators.

By Kim Cochran

In addition to the sheer volume of the construction-and-demolition (C&D) debris stream that drywall represents, bad odors resulting from decomposition of the material are causing operators to look at ways to remove the material from the wastestream and turn it into a marketable product.

Fishing From the Wastestream

Drywall can be removed at one of two locations: the job site or a solid waste management facility. Both have pros and consówith job-site removal being the option with the most pros.

Job-site removal requires that a separate pile, containment area, or rolloff box be placed on the job site for drywall only. This might require a change in the way the contractor or subcontractor manages waste. Separation at the job site does mean, however, that there will be little contamination in the drywall material and less labor will be required to remove drywall from the debris stream.

Disposal-site removal means that drywall is removed from loads of C&D debris after they are delivered to the solid waste management facility, whether it is a landfill, a recycling center, or some other waste management facility. Because drywall waste is generated in only one phase of construction, loads generally either are at least 50% drywall or contain little or no drywall. Loads that are 50% drywall or more can be sent to a staging area where contamination can be extracted from the loads by hand or by a skid-steer (see photo 1). In addition, drywall can be manually or mechanically extracted from the face of the landfill. Although this method requires less change in the current waste management system, drywall recovery rates are less and contamination can be greater.

Know Your Market

The most important aspect of investigating the recycling potential of a commodity is market availability. Once the end markets have been determined, it is important to know end-market specifications so that processing can be assessed.

Recycled drywall can be used as an alternative to mined or synthetic gypsum in a number of applications, which makes it more marketable. End markets explored in Florida included agriculture, cement, and new drywall. These are not the only possible end markets for recycled drywall, but they also represent some of the largest. The table summarizes the specifications and contamination tolerance of typical end markets.

Making a Product

Skid-steer pulling drywall from a waste pile

Preprocessing performed by a front-end loader

Loading drywall into a trommel screen for processing

There are a number of technologies that have been utilized throughout the nation to process drywall into a saleable feedstock. These technologies include trommel screens, tub grinders, horizontal end grinders, portable grinders, and various other types of equipment.

Trommel Screens

For most of the pilot drywall recycling projects in Florida, the project teams elected to use a trommel screen because the facilities already owned trommel screens (primarily for use in woodwaste and/or yardwaste processing operations) that were not fully utilized and trommel screens had proven to be an effective processing technology. Trommel screens are effective because they reduce the size of the material and remove almost all of the paper.

The drywall should be preprocessed prior to being loaded into the trommel screen. Preprocessing allows for a more effective separation and a greater recovery of gypsum in the trommel screen (see photo 2). It involves simply running over and tossing the drywall pile with a front-end loader in order to break the larger sheets into smaller pieces. Doing so allows for better separation of the paper backing from the gypsum during processing in the trommel screen.

Once the material has been preprocessed, it is loaded into a trommel screen. A trommel screen is a large cylindrical screen that rotates. As the drywall material enters the screen, the rotation of the screen causes the material to tumble, breaking it apart by sheer gravity. The small gypsum pieces fall through the screen, and the paper and other large pieces continue to tumble out the end of the screen, where they are caught on a conveyor belt. The gypsum pieces are caught on another conveyor belt, where they can be transported to a container. The size of the gypsum pieces is determined by the screen size. Some markets prefer small sizes, while others do not care how large or small the gypsum is. It is important to know the specifications of the end market prior to processing. The effectiveness of the separation is dependent on the speed of the trommel and the size of the screen. The faster the trommel and the smaller the desired end product, the more gypsum will end up in the discard pile and will not be removed from the paper. If a smaller-size end product is desired, the speed of the trommel will have to be slower.

Tub and Horizontal End Grinders

Tub and horizontal end grinders do not allow separation of paper or other contaminants from the gypsum material, but they do provide a fast, effective way of reducing the size of the material. No preprocessing is involved. Extra precaution, however, should be taken to remove contaminants, as all of the material will be ground together. This processing method is best for markets that do not require paper separation, such as agriculture. Similar to trommel screens, grinders are frequently found at solid waste management facilities to process yard trash and wood debris. Thus, no additional equipment would have to be purchased.

Portable Grinders

Portable grinders are slowly becoming more popular as states are exploring the idea of grinding C&D debris at the job site and applying it below the sod layer. These grinders are small enough to be towed by a truck and can be moved from job site to job site. Drywall can be easily ground in these machines. Paper separation is not necessary, as the paper will biodegrade in the ground. Some states, however, do not allow for drywall to be placed on the job site in this fashion due to the fact that gypsum alters the pH of soil, which in some cases is already particularly alkaline.

Other Processing Equipment

Several companies manufacture equipment geared specifically for recycling drywall. The equipment removes the paper from the gypsum and reduces the size of the material. In addition, many of the machines have coverings that prevent dust from escaping, as processing drywall can create a great amount of dust. These are all excellent machines for large drywall recycling operations or locations that plan on processing a lot of drywall. Because they cannot be used for processing other materials, they are not ideal for locations with smaller quantities of drywall.

Money Talks

There are a number of stakeholders and factors that affect the economics of drywall recycling. In order for drywall recycling to be sustainable, the long-term economics must be favorable.

While many factors impact long-term sustainability, three in particular truly drive the economic viability of drywall recycling:

• Tipping fees
• Transportation costs
• Cost of virgin material

Processed gypsum coming off a trommel screen conveyor

Close-up of processed gypsum

For any type of wastestream (including vegetative waste), the most significant challenge to recycling is low tipping fees. With low tipping fees, the cost for disposal might be less than the cost to recycle, depending on location and any surcharge for contamination. This cost differential creates a significant disincentive to recycling.

Hauling costs are proportional to distance. As the distance increases, the cost to haul the material will also increase. Thus, if the recycling facility is too far away, the material will not be recycled due to cost. Similarly, if the distance to the end market is too far, recycling becomes uneconomical.

Future Forecast

Processing drywall is an easy operation that can be performed by equipment that many solid waste managers might already own. The tougher steps in drywall recycling are removing the drywall from the wastestream and finding a market suitable for the end product. Once these steps have been overcome, a successful drywall recycling program could be viable for any area.

Guest author Kim Cochranfis an environmental engineer for R.W. Beck's Infrastructure Services Group in Orlando, FL.

MSW - November/December 2003