Clogging of tire shreds and gravel permeated with landfill leachate

The clogging of tire shreds and gravel is based on four column tests permeated with landfill leachate for up to 2 years. Two different types of tire shred (G shred: 100 mm x 50 mm x 10 mm; and P shred: 125 mm x 40 mm x 10 mm with many exposed wires) and a uniformly graded 38 mm gravel were examined. The compressibility of the G and P shreds at 150 kPa were reported to be 48 and 44%, respectively while the initial hydraulic conductivities were 0.007 and 0.02 m/s, respectively (compared to 0.8 m/s for the gravel). The gravel maintained a hydraulic conductivity greater than 10(-5) m/s for about three times longer than a similar thickness of compressed (at 150 kPa) tire shreds. The tests were conducted at an accelerated flow rate of 0.4 m(3)/m(2)/day. At termination of the rubber shred columns after about I year the hydraulic conductivity at the influent end of the columns had dropped to between 10(-7) and 10(-8) m/s. At termination of the gravel columns after 2 years the corresponding range was 10(-6)-10(-7) m/s. The clog was predominantly calcium carbonate, with calcium making up 29-34% of the total clog material. Aluminum, zinc, iron, and copper leached from the P and G shreds when exposed to typical municipal solid waste leachate, however they were not detected in the effluent leachate. The highest concentration of metals was found in the P-shred clog and this is attributed to the greater abundance of exposed steel in these shreds. It is inferred that gravel should continue to be used in critical zones where there is a high mass loading. The results suggest that an increased thickness of compressed tire shred may be used to give a service life similar to that of a given thickness of gravel in noncritical zones.