Heavy metals in recovered fines from construction and demolition debris recycling facilities in Florida

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Abstract

A major product recovered from the processing and recycling of construction and demolition (C&D) debris is screened soil, also referred to as fines. A proposed reuse option for C&D debris fines is fill material, typically in construction projects as a substitute for natural soil. Waste material that is reused in a manner similar to soil must first be characterized to examine potential risks to human health and the environment. In Florida, samples of C&D debris fines from 13 C&D debris recycling facilities were characterized for 11 total and leachable heavy metal concentrations. Total metal concentration results (mg/kg) were compared to existing data on background Florida soil concentrations and to Florida's risk-based soil cleanup target levels (SCTLs). All of the detected metals were found to be elevated with respect to background. The 95% upper confidence level of arsenic from 99 samples was 3.2 mg/kg; arsenic presented the greatest limitation to reuse when compared to the SCTLs. Lead was not found to pose a major problem, likely because of the relatively new building infrastructure in Florida, which results in less demolition debris and less material impacted by lead-based paint. The results of batch leaching tests conducted using simulated rainwater (mg/l) were compared directly to risk-based groundwater levels for Florida and were found not to pose a risk using existing risk assessment policies.

Introduction

Construction and demolition (C&D) debris represents one of the largest components of solid waste generated from municipal activities. Major components of C&D debris include wood, concrete (including masonry products), asphalt (pavement and roofing), gypsum wallboard, cardboard, metal, soil, rock and vegetative debris (from land clearing). A US Environmental Protection Agency (EPA) sponsored study estimated that 127 million metric tons of building-related C&D debris were generated in the US in 1996 (US EPA, 1998a). While C&D debris has often been managed by landfill disposal, recovery, processing and recycling of C&D debris components have become standard practice in some locations. In Florida, where construction continues to be very active, C&D debris comprised almost one-fourth of the weight of the municipal solid waste (MSW) stream in 1998, with a reported 5.3 million metric tons collected (FDEP, 2000). Since C&D debris represents a significant portion of the waste stream, it is often targeted for waste reduction and recycling. Several factors dictate the success of C&D debris recycling including landfill tipping fees, the magnitude of C&D activity occurring in an area, and viability of reuse markets for recovered C&D debris components.

Although some separation of C&D debris components may occur at the construction or demolition site, most mixed C&D debris that is recycled is separated and recovered at central processing facilities (Townsend, 1998). Central processing operations utilize a combination of mechanical equipment (e.g. conveyor belts, float tanks, trommel screens, magnets) and manual methods (e.g. hand picking of specific materials) to separate mixed C&D debris into a number of reusable commodities. Typical operations utilize a screen to remove soil and other fine materials prior to subsequent processing. While wood, concrete, and metal are common C&D debris materials targeted for recovery, a large percentage of the recovered mass is comprised of fines.

While C&D debris for the most part is inert, it may contain some materials with the potential to harm human health and the environment (US EPA, 1998a). Because of concerns over possible risks posed by C&D debris fines when recycled through land application, a study was initiated to characterize and assess the magnitude and range of concentrations of chemicals in recovered C&D debris fines. Organic compounds (Jang and Townsend, 2001a), sulfate leachability (Jang and Townsend, 2001b), and heavy metals were examined. This paper reports the results of the heavy metal characterization of the C&D debris fines in Florida. The total and leachable concentrations of 11 metals were measured. The total metal concentrations (mg/kg) were compared to Florida's SCTLs that are regularly used for the assessment of contaminated sites. The leachable metal concentrations (μg/l) were compared directly to Florida's risk-based groundwater cleanup target levels (GWCTLs) (Saranko et al., 1999). The results are not only of interest to those dealing with C&D debris processing facilities, but also provide an example of the application of generic risk-based target levels or guidance concentrations for waste materials reused in a beneficial manner.

Section snippets

Recovered fines from C&D debris recycling

The processing of C&D debris has evolved over recent years. Demolition contractors have historically processed waste materials for the recovery of concrete and metal, but modern C&D debris recyclers accept commingled and mixed loads of C&D debris for processing and recovery. A variety of approaches and unit operations are employed at C&D debris processing facilities. Some facilities process waste up-front using compactors or impactors to reduce particle size, which allows for efficient

Methodology

Recovered C&D debris fines were sampled from 13 C&D debris processing facilities in Florida over a period of 14 months. Researchers conducted five sampling events; different facilities were sampled in each sampling event, though some facilities were visited separately during two different events [see Townsend et al. (1998) for more details]. The total recoverable concentrations of selected heavy metals (mg/kg) were measured in every sample; leachable metal concentrations (mg/l) were only

Total metals

Nine of the eleven metals analyzed (aluminum, arsenic, cadmium, chromium, copper, mercury, nickel, lead and zinc) for total recoverable concentrations (mg/kg) were detected in some samples above the detection limit. Silver and selenium were not detected in any of the samples above detection limits. For purposes of summarizing the data and statistical analysis, sample measurements that were below detection limit were treated as equal to one-half of the detection limit. To determine whether the

Acknowledgements

This work was sponsored under a grant from the Florida Center for Solid and Hazardous Waste Management, Gainesville, Florida. The authors thank the operators of the construction and demolition waste recycling facilities sampled during this study.

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