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UBC Theses and Dissertations

Chemical and physical characterization of bottom ash from the Burnaby incinerator Miller, Robert


A ten month field and laboratory study was undertaken to chemically and physically describe desifted bottom ash (i.e. bottom ash without grate sittings) from the Burnaby, British Columbia incinerator. The main purpose of the research was to determine if the removal of grate sittings had any effect on leachable metal concentrations in the residual bottom ash stream. Chemical and physical results from a 1992-93 study of bottom ash that had the sittings removed were compared to a 1991 study that contained the grate sittings. Field work consisted of sampling bottom ash from September 17,1992 to March 25, 1993. In total, 252 bottom ash samples were obtained on twelve different days. Each sample consisted of fine fractions of bottom ash which were less than 9.5 mm (3/8") and coarse fractions which were greater than 9.5 mm (3/8"). During the same time period 180 grate sittings samples were collected with particle sizes less than 12.5 mm (1/2"). American Society for Testing and Materials (ASTM) methods were used to evaluate the physical characteristics of the desifted bottom ash. Desifted bottom ash was determined to be a well graded material with consistent physical properties. Compaction densities were constant over the long term. Optimum Proctor densities ranged between 1.87 to 1.91 g/cm3 at water contents of approximately 12%. Physical testing included sorting the coarse fractions of ash into ten different categories to analyze its composition. The three coarser fractions of bottom ash examined consisted primarily of glass and glass mixtures, averaging approximately 35% by weight, followed by ferrous materials (25%). The chemical characteristics and leaching potential of the finer fractions of desifted bottom ash and grate sittings less than a 9.5 mm sieve were tested. Samples were subjected to a Leachate Extraction Procedure (LEP), as outlined in the Special Waste Regulations governed by British Columbia's Ministry of the Environment, Lands and Parks (MOELP). It was determined that lead is the only metal of regulatory concern. Other metals tested for were: cadmium, chromium, copper, iron, manganese, nickel and zinc. It was determined that the desifted bottom ash without grate sittings is not a Special Waste. However, when the ash was graded into different size fractions, leachable heavy metals were observed at higher concentrations in the finer fractions of the ash. The lead concentrations were high enough in the > 4.75 mm and > 2.36 mm fractions to be of regulatory concern. On several occasions, all three fractions had samples or grabs which failed the Leachate Extraction Procedure (LEP). If the desifted bottom ash stream is separated into different size fractions and small volumes are sampled from individual fractions there are Special Waste implications. Certain fractions of grate sittings are of regulatory concern and do not pass the (LEP). Sixty, seventy, and eighty percent reductions in leachable lead were observed in the > 9.5 mm , > 4.75 mm , and > 2.36 mm fractions of bottom ash respectively when comparing the 1991 and 1992-93 LEP data. Mass balance calculations show that the removal of grate sittings from the bottom ash cannot fully account for the reduced lead levels because only 38% of the leachable lead can be attributed to the grate sittings. What is significant is that 38% of the leachable lead originates from the grate sittings which are approximately 6.1% by weight of the bottom ash stream. This suggests that lead partitions to the grate sittings. Copper and zinc also partition to the grate sittings. The grate sittings are not a significant source of leachable copper because approximately 2% of the leachable copper present in bottom ash can be attributed to the grate sittings. The grate sittings are a "concentrated" source of leachable zinc because approximately 45% of it comes from the sittings.

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