A fines challenge: the C&D recycling industry looks for solutions to the problem of H2S generation in ADC made from C&D fines

Construction & Demolition Recycling, March-April, 2006 by John S. Blaisdell

Construction and demolition debris (C&D) Confines, also known as recovered screened material (RSM), have gotten a lot of attention recently due to an apparent link between the use of fines as alternative dally cover (ADO) and the generation of hydrogen sulfide (H2S) gas in a landfill environment. C&D recyclers nationwide are now faced with the challenge of overcoming the barriers to marketing C&D fines for use as ADO, which is an essential component of the mix of C&D end products.

BACKGROUND

C&D fines are the small fraction of mixed construction and demolition debris that are screened during the recycling process. C&D fines range in size, but are typically 3 inches or smaller. The fines are accepted at landfills and used as ADC.

As mentioned previously; C&D fines are an important component in the mix of end products produced by C&D recyclers. Manufacturing and marketing C&D fines is often mischaracterized as the goal of most C&D recyclers. However, C&D fines are really just a necessity given that C&D materials from a mixed material stream are fractured, broken and size-reduced during typical handling procedures prior to being accepted by the processors. Thus, the fines are already there and the processors are now handed a product that they have to manage. The goal of the C&D processor then is to manage the fines product in a cost-effective manner and produce as high a value product as possible with the other portions of the C&D stream (wood, metals, aggregates, etc.).

Given that C&D fines are as much as 30 percent or more of the total mixed C&D material stream by weight, the successful marketing of this material is essential for the overall economics of C&D processing. If the fines product cannot be cost-effectively managed, then the mixed C&D stream may not be viably processed to remove the other higher value products (wood, metals, aggregates, etc.). A lack of markets for C&D fines will restrict the development of higher value products being manufactured by mixed C&D processors.

BEST MANAGEMENT PRACTICES

ADC made from C&D fines is a useful product for landfills because it saves resources and landfill space by replacing virgin soil. However, due to the generation of H2S by gypsum-containing ADC, many individual landfills have stopped taking the product from C&D processors. Additionally; some local and state regulatory entities have greatly restricted or completely prohibited the use of C&D fines as ADC. Seeing the potential damage to the existing and future C&D recycling infrastructure, the CMRA hired Green Seal Environmental Inc. (GSE) to develop a Best Management Practices (BMPs) document for the production and use of C&D fines as ADC.

The BMPs document has been created in draft form and is currently being revised based on the comments from industry representatives. The BMP document focuses on the following points:

* Background information on how and why H2S is generated

* Other sources of H2S

* Gypsum removal

* Mixing C&D fines with other APC or virgin cover materials

* Chemical additives

* Biological additives

* Proper handling procedures at the landfill

* Gas collection and treatment methods

* Issues with leachate re-circulation.

The BMPs document makes several main points, which are discussed below.

The manufacturing methods of C&D fines and thus the make-up of the product varies depending on the facility producing it and the specific state or local regulations enacting or enforcing the specifications. In some cases, the C&D fines product mainly consists of the smaller (typically less than 1-inch) screened "dirt" portion of mixed C&D debris. In other cases, the C&D fines product consists of a combination of larger ground C&D debris materials (typically less than 3-inches) and smaller "dirt" fines (less than 1-inch). The variances in production methods and the resulting product may create variances in the methods for managing H2S generation.

C&D fines are used for different purposes in a landfill environment. In some cases, fines are used as ADC. In other cases, fines are used as shaping and grading material and/or capping material. The varying uses and environments within the landfill that the products are subject to may create variances in the methods for managing H2S generation.

The removal of gypsum prior to processing will provide a reduction in the amount of gypsum available for H2S production. However, markets for gypsum are largely in their infancy, and markets for gypsum from demolition debris (versus virgin construction scrap) are limited currently due to the potential for contamination from paints or treatments to gypsum wallboard.

The removal of additional organic materials (wood, paper) from C&D fines products will provide a reduction in the amount of organics available for H2S production. However, it appears that the paper backing of gypsum drywall alone may be an adequate source of the organics required to create the condition where H2S is produced. Thus the removal of additional sources of organics will not by itself eliminate H2S production.