Envlron. Sei. Technol. 1984, 18, 668-673
Comparison of Batch and Column Methods for Assessing Leachability of Hazardous Waste Danny R. Jackson,* Benjamin C. Garrett, and Thomas A. Bishop
Analytical Chemistry and Applied Statistics Sectlons, Battelle-Columbus Inorganic and organic analytes were leached from four waste samples by using batch and column leaching methods. Leachate concentration profiles were constructed from sequential leaching of waste with distilled deionized water using a combined solution to waste ratio of 40:l. Leachate profiles produced by the batch and column methods were compared in terms of a fitted leaching profile function and variation of the experimental data. Leachate profiles produced by the two methods were significantly different for 12 of the 16 reported analytes. Variation of experimental data, as represented by relative standard deviation, was over twice as great for the column method than for the batch method. This result was attributed to varying degrees of channeling that may have occurred during leaching of waste constituents using the column method. The batch extraction method offers advantages through its greater reproducibility and simplistic design, while the column method is more realistic in simulating leaching processes which occur under field conditions. Introduction Management practices and regulatory requirements associated with proper disposal of solid hazardous waste have grown in sophistication in recent years (1). These advances have occurred through the realization that inadequate closure of landfill disposal sites and the mixing of incompatible wastes can lead to contamination of surface and underground water resources. Technological developments in synthetic liners and leachate collection and treatment systems have increased the potential safety factor for landfilling hazardous waste. In addition to these developments, accurate characterization of leachable contaminants in solid hazardous waste prior to landfill disposal can contribute greatly to an overall waste management strategy. Laboratory leaching tests which generate leachate from waste have been used to evaluate leachability of hazardous constituents (2). Information derived from a leaching test can provide guidance in conducting a performance evaluation of a landfill prior to disposal of a given waste. A performance evaluation of this type might include assessing the compatibility of various wastes for codisposal, assessing the adequacy of a leachate collection and treatment system, or predicting the impact of waste leachate on groundwater resources should a breach in the landfill liner system occur. The development of standardized leaching tests is needed to facilitate regulatory functions, routine waste analysis, and interlaboratory comparison of results (3). Laboratory leaching methods should work well on a variety of waste types and should provide useful data for the prediction of leachate quality produced by the waste in a landfill environment. Characteristic features of an ideal laboratory leaching method include (1)use of a leaching medium likely to be encountered in a landfill, (2) procedures to estimate the intensity and capacity of leachable constituents, (3) procedures which minimize sample particle size alteration, (4)an optimized liquid to solid ratio which would minimize leachate dilution and experimental 688
Environ, Sci. Technoi., Vol. 18, No. 9, 1984
Laboratories, Columbus, Ohio 43201
variability, and ( 5 ) procedures which are conveniently performed by laboratory personnel (4). Various laboratory techniques have been reported for generating and characterizing leachate from hazardous waste and are generally grouped into batch and column extraction methods (2). A batch extraction method involves the mechanical mixing of a unit volume of water, or an alternative solution, with a unit mass of hazardous waste. This method has been noted for its ease of operation and low experimental variation (5). A column extraction method involves the continuous flow of liquid through a fixed bed of solid waste. Leachate generated by the column method is reportedly more representative of leachate derived from a disposal site than is the leachate from the batch method (6). Column procedures generally include a liquid to solid ratio which more closely represents a field situation. The mechanism of contacting a fixed body of waste with a transient liquid resembles the leaching mechanism imparted by gravity flow of liquid through a waste disposal site. Any proposed laboratory method to generate and characterize leachate from a hazardous waste must be subjected to rigorous experimental evaluation. Such an evaluation involves generating data which can be used to characterize the leachable constituents in the waste and the sources of experimental variation affecting the data. The success of this type of evaluation depends on using well-designed laboratory procedures and a statistical model which describes the laboratory data in detail. The objective of this study was to compare differences in concentration of leachate constituents and experimental variation derived from batch and column extraction methods using samples of four representative wastes. The column leaching method was designed to be comparable to the batch method in terms of operational parameters such as liquid to waste ratio and leaching time. This standardization allowed comparisons to be based primarily on differences in waste constituents dissolved by (1) flowing water through a static bed of waste (column method) and (2) constantly agitating a wastewater suspension (batch method). Leachate profiles were generated on the basis of multiple sequential extractions of waste samples, and results were compared by using a statistical model. Materials and Methods Four waste samples having diverse physicochemical characteristics, summarized in Tables I and 11, were selected for the leaching experiments. These wastes are representative of those considered to land disposal in controlled disposal sites. Electroplating sludge and fly ash were selected as representatives of inorganic solid wastes. A filter cake sludge and a liquid (polystill bottoms) were selected as representatives of organic wastes. Solid content was determined for electroplating sludge and fly ash by oven drying at 105 OC for 24 h. Solid content was determined for the filter cake and polystill bottoms by airdrying for 96 and 24 h, respectively, to minimize possible loss of weight from volatile organics. The characteristic pH of each waste was obtained by using small subsamples of leachate from the first leaching fraction derived from the batch extraction method. Dried
0013-936X/84/0918-0668$01.50/0
0 1984 American Chemical Society
Table I. Physicochemical Characteristics of Four Hazardous Wastes solid primary content, composition % pH
waste type electroplating sludge fly ash filter cake polystill bottoms
inorganic inorganic organic organic
15.6 99.8 65.8 2.6
A1
elemental content, pg/g Cu Fe Ni Pb Se Sn Ti Zn 38 1590 23 1230 a 3740 2450 21400 3600 1420 6.2 51 C0.6 4500 18 5.4 58 22 27 27600 250 170 6.2 C0.6 110 26 4.2 2.9 7 12 2390 4 23 7 a 160 62 310
