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Weekly UpdatesABSTRACT
Disposal practices for bottom ash and fly ash from waste-to-energy (WTE) facilities include emplacement in ash monofills or co-disposal with municipal solid waste (MSW) and residues from water and wastewater treatment facilities. In some cases, WTE residues are used as daily cover in landfills that receive MSW. A recurring problem in many landfills is the development of calcium-based precipitates in leachate collection systems. Although MSW contains varying levels of calcium, WTE residues and treatment plant sludges have the potential to contribute concentrated sources of leachable minerals into landfill leachates. This study was conducted to evaluate the leachability of calcium and other minerals from residues generated by WTE combustion using residues obtained from three WTE facilities in Florida (two mass-burn and one refuse-derived fuel). Leaching potential was quantified as a function of contact time and liquid-to-solid ratios with batch tests and longer term leaching tests using laboratory lysimeters to simulate an ash monofill containing fly ash and bottom ash. The leachate generated as a result of these tests had total dissolved solid (TDS) levels ranging from 5 to 320 mg TDS/g ash. Calcium was a major contributor to the TDS values, contributing from 20 to 105 g calcium/kg ash. Fly ash was a major contributor of leachable calcium. Precipitate formation in leachates from WTE combustion residues could be induced by adding mineral acids or through gas dissolution (carbon dioxide or air). Stabilization of residual calcium in fly ashes that are landfilled and/or the use of less leachable neutralization reagents during processing of acidic gases from WTE facilities could help to decrease the calcium levels in leachates and help to prevent precipitate formation in leachate collection systems.
INTRODUCTION
Waste-to-Energy (WTE) facilities provide a means to reduce the volume of municipal solid wastes (MSW) that require landfilling while yielding energy and recoverable ferrous and nonferrous metals. Byproducts from WTE facilities include combustible gases and noncombustible residues (fly ash and bottom ash). Although options have been developed for using ash residues in different construction applications, (1,2) the majority of WTE residues produced in the United States are either disposed in ash monofills or co-disposed with MSW and other materials such as residues from water and wastewater treatment facilities (3-5)
The leaching characteristics of WTE residues have been studied extensively, particularly with respect to the leachability of toxic materials from fly ashes and bottom ashes. (6,7) Less information is available on the leachability of dominant minerals that may participate in precipitation reactions. For example, a recurring problem in many landfills is the development of calcium-based deposits that can clog leachate collection systems. (8-11) The purpose of this study was to assess the leachability of precipitatable minerals from WTE residues and evaluate the potential to form mineral deposits in leachate collection systems.
BACKGROUND
The characteristics of WTE combustion residues vary depending on the MSW characteristics, degree of preprocessing (mass-burn, refuse-derived fuel, material recovery), the combustion efficiency, emission control systems, and ash management practices. (12-14) Typically, bottom ash consists of solid waste that is not completely burned or is uncombustible, such as organic char, metal, glass, and ceramic constituents. Fly ash is derived from neutralization and collection of gaseous and particulate material though emission control devices and consists of varying concentrations of metals, organics, and acids sequestered on particulate material. Typical treatment steps include: selective noncatalytic reduction (SNCR) through application of ammonia or urea to control nitrogen oxides, alkaline scrubbers or absorbers (wet or dry) to neutralize acid gases, activated carbon injection to control mercury and volatile organics, and entrapment of particulates using electrostatic precipitators and/or baghouses. Reagents that are used to process combustion gases include water, lime, sodium hydroxide, activated carbon, sodium sulfide, ammonia, urea, and other proprietary agents. (4)
The suite of gas and ash processing techniques and their relative efficiency can affect the composition, stability, and leachability of WTE residues. Typically WTE residues have been reported to have relatively low organic contents and the major elements include Al, C, Ca, C1, Fe, K, Na, and O, whereas minor elements include Cr, Cu, Mg, Mn, Pb, and Zn. (5,15)
Leaching patterns associated with WTE combustion residues have been evaluated by several researchers. (7,16-22) Key factors that impact the potential for mobilization of minerals from WTE residues are leachate pH, biological activity, redox conditions, ionic strength, complexing inorganic ions and organics, and the liquid-to-solid ratio. A comparison of leachate characteristics associated with ash monofills is given in Table 1. (15,23-25) As shown, there is wide variability among and within facilities due to differences in waste sources and processing of the ash and local hydrology; however, in all cases, the dominant dissolved constituents are sodium, potassium, calcium, chloride, and sulfate.
The extent to which WTE residues are co-disposed with MSW varies with local waste management practices and the practicability of ash utilization for alternative purposes. For example, Florida counties that have active WTE facilities typically burn an average of 29%, landfill 45%, and recycle 26% of the MSW (26). Since 1994, when the Supreme Court ruled that ash from MSW combustion must be treated as other hazardous wastes in City of Chicago versus Environmental Defense Fund, all WTE combustion residues must be tested in accordance with Resource Conservation and Recovery Act requirements for hazardous waste before disposal in lined landfills. (26) However, the leachability of nontoxic precipitatable minerals is rarely addressed. In this project, the feasibility of using batch tests to assess the precipitation potential of WTE residues was evaluated.
MATERIALS AND METHODS
Laboratory lysimeters and batch tests were used to assess the leaching potential of WTE combustion residues as a function of contact time and liquid-to-solid ratios. WTE residues were obtained from three facilities in Florida. One facility bums refuse-derived-fuel (RDF) and the other two facilities are mass-burn (MB) facilities that combust MSW "as received". Bottom ash (BA) and fly ash (FA) from the RDF facility were tested separately, whereas WTE residues from the MB facilities consisted of a mixture of BA (70-90%) and FA (10-30%). Lime use for processing of waste gases ranged from 10 to 20 lb of lime per ton of waste resulting in calcium levels in ash of 1-10% (by mass).
Lysimeter tests were conducted in 1.4 m tall, 30.5 cm diameter schedule 40 polyvinyl chloride (PVC) pipes each with a volume of 0.42 [m.sup.3] and a surface area of 0.30 [m.sup.2]. The lysimeters contained a drainage layer, a leachate collection system (32 mm diameter PVC pipe with 9.5 mm diameter perforations, which were spaced at intervals of 15 cm with two staggered rows separated by 120 [degrees]), and a geonet. A 0.73-m layer of WTE residues consisting of a mixture of 159 kg (350.54 lb) of BA and 22 kg (48.5 lb) of FA was placed above the geonet in each lysimeter. To initiate the leaching reactions, the contents of each lysimeter were saturated to field capacity using distilled water. An additional 4 L of distilled water was applied to each lysimeter to generate leachate and the lysimeters were capped and sealed. The lysimeters were operated in a flood and drain mode whereby each lysimeter was flooded with 4 cm of leachate daily by pumping 3 L of leachate to the upper reservoir and distributing it across the top of each lysimeter through a 30.5-cm horizontal perforated plate. Leachate samples were collected routinely over a 7-month period by which time the leaching potential of the waste achieved steady state and analyzed for the parameters shown in Table 2. The volume of leachate that was withdrawn for each sampling event was replaced with an equal amount of distilled water to maintain a constant volume of liquid within each lysimeter. (27)
Two different types of batch tests were used in this project: contact time (CT) and sequential extraction (SE). The CT batch test was conducted over a 21-day leaching period and three replicates were used for each WTE residual. The SE batch tests were developed to evaluate leaching patterns of WTE combustion residues on the basis of liquid-to-solid ratios ranging from 10 g/g to 100 g/g. Batch testing methodology was adapted from the Method for Accelerated Leaching of Solidified Waste, (28) as shown in Figure 1. All tests were conducted in Nalgene amber high-density polyethylene wide mouth bottles. Distilled water was used as a leachant to mimic the chemical composition of rainwater. In each case, ash was homogenized and weighed into the batch reactor. Liquid was added to yield an initial liquid-to-solid ratio of 10. To simulate internal landfill temperatures, batch tests were incubated at 35 [degrees]C. Each test was conducted in triplicate. For the CT tests, three samples were removed at each time interval. Tests were conducted for a 21-day period, however, in most cases equilibrium was observed (dC/dt = 0) within the first 2 days of incubation. For the sequential extraction test, from each of the three samples a measured volume of leachate was removed daily and replaced with an equal amount of fresh leachant. Leachates from batch tests were analyzed in parallel with lysimeter leachates for the parameters listed in Table 2. (29)
Supplemental tests were conducted on leachate samples from batch tests and lysimeters to evaluate the effects of addition of acid or dissolution of gases (carbon dioxide or air) on the potential to form mineral precipitates. For the acid addition tests, a controlled volume of leachate was amended with a fixed increment of hydrochloric, nitric, or sulfuric acid. For the gas dissolution tests, a 1-L volume of leachate was placed in a batch reactor. Gas was diffused into the reactor at a constant rate (0.3 mL/sec). The pH and conductivity were monitored continuously. After a 30-min reaction period, gas addition was discontinued. Particulate material was allowed to settle and the supernatant was tested for the parameters listed in Table 2. Precipitates that formed were collected on 47-mm nylon filters (pore size 0.2 [micro]m), dehydrated, and evaluated using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS) for elemental analysis.
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