Evaluation of Tire Derived Rubber Particles for Biofiltration Media

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ABSTRACT
Currently, on average, one tire is discarded in the U.S. for every man woman and child every year. While the reuse market for used tires has increased over the years to approximately 75%, there are still an estimated 2-3 billion used tires stockpiled in the U.S. Finding economical and sustainable end uses for this tire material is an ongoing challenge to environmental engineers and others. In this study, three different uses of tire rubber were evaluated as biofilm attachment media in bioreactors for wastewater treatment: in an aerobic biofilter, in anoxic bioreactor, and in a hybrid anaerobic static granular bed reactor (SGBR). Size distribution, chemical composition, scanning electron microscopy, and whole effluent toxicity analyses were performed. These tests demonstrated that the tire rubber media was non-toxic and provided good surface area for biofilm attachment. The trickling filter system using chunk rubber (average diameter of approximately 3 cm) achieved 79.6-90.1% COD removal efficiency at organic loading rates ranging from 0.12 kg COD/m3·d to 0.34 kg COD/m3·d. In the hybrid SGBR, anaerobic granular sludge was augmented with fine rubber particles (average particle diameter of approximately 0.2 mm) and achieved greater than 90% COD removal at hydraulic retention times of 48 to 20 h resulting in organic loading increases from 0.44 to 2.7 kg/m3·d. The anoxic TDRP filter system achieved nitrate-nitrogen removal efficiencies greater than 97% at influent concentrations ranging from 52 to 94 mg NO3-N/L. This research demonstrated the utility of TDRP media in multiple biofiltration applications.

INTRODUCTION
Currently, on average, one tire is discarded every year in the U.S. for every man woman and child living. While the reuse market for used tires has increased over the years to approximately 75%, there are still an estimated 2-3 billion used tires stockpiled in the U.S. The largest demand (33% of the tire reuse market) for used tires is in tire derived fuel (TDF), primarily for use in cement kilns (Sunthonpagasit and Duffey, 2004). Other current markets for used tires include civil engineering (CE) applications (15%) and crumb rubber (12%). CE applications for used tires include leachate collection and recovery systems (see Phaneuf and Glander, 2003) and highway embankments. Approximately one third of crumb rubber produced is used for asphalt modification (e.g., crumb rubber asphalt concrete, see Azizian et al., 2003 and crumb tire rubber bitumens, see Navarro, et al., 2004). Another third of the crumb rubber is used for molded products (e.g., using crumb rubber in lieu of virgin rubber). Additional uses for crumb rubber include sports and horse arena surfaces, automotive products, and landscaping mulch (Sunthonpagasit and Duffey, 2004).

Environmental applications for tire rubber have mainly been in adsorption systems. Manchón-Vizuete et al. (2005), for instance, tested chemically and heat treated tire rubber for its ability to adsorb mercury. Entezari et al. (2006) used ground tire rubber, preconditioned with ultrasonic vibrations, to remove cadmium from aqueous solutions. A review by Mui et al. (2004) suggested that activated carbon material made from waste tire rubber could result in porosities in excess of 40% of pore volume and surface areas over 1000 m2/g. Other experimental applications for crumb rubber include its use as a ballast water filtration media (Xie and Chen, 2004, Tang et al. 2006), subsurface drainage for nutrient mitigation (Lisi et al., 2004), and septic tank liners.

The objective of this study was to evaluate TDRP as a suitable media for biological growth and biofilm development in anaerobic, aerobic, and anoxic environments. In this study, three different types of reactors were constructed and operated; a trickling filter with effluent recycle, a denitrification filter with fixed media for attached growth, and a hybrid-static granular bed reactor with anaerobic granular sludge and tire derived rubber particles. Each of the systems was typical of what might be used in the field with the exception of the hybrid-SGBR.

The SGBR is a simple downflow anaerobic system developed at Iowa State University. It utilizes a bed of active anaerobic granules in a downward flow regime (see Figure 1). The innovation in this reactor configuration is that it uses the highly active anaerobic granules (just as in a UASB system), but it operates in a downflow mode. The advantage of a downflow configuration is that the biogas that is generated rises and is easily separated from the granules and the liquid at the top of the reactor. Granule buoyancy is not a detriment to process performance in the SGBR as it is in the UASB. In contrast to the UASB, there is no need for a sophisticated three phase solids, gas, and liquid separator. Neither is there a need for recirculation pumps, timers, mixers, or other ancillary equipment that are required for the UASB systems. Consequently, the effluent quality of the SGBR is improved in comparison to the UASB. The biomass granules are retained within the reactor by the use of a gravel underdrain. Consequently, temperature and hydraulic loading changes are not expected to significantly affect effluent quality.

The technological innovation of the SGBR is that it uses highly active anaerobic granular biomass in a downflow configuration. Other reactor configurations use a downward flow regime (e.g., the anaerobic filter), but the SGBR is the first granular sludge system to operate in a downflow mode. This configuration allows for exceptional effluent quality, simple operation, and reduced volume requirements. The performance of the system was demonstrated in numerous laboratory and pilot studies on a variety of wastewaters (Mach and Ellis, 2000, 2001, Roth and Ellis, 2004, Evans and Ellis, 2005, Debik et al., 2005). The addition of TDRP to the granule bed was evaluated in this study to determine its suitability as a media and to offset the high cost of anaerobic granules (which traditionally have sold for approximately $66/m3).

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