The primary goal of this research was to begin an understanding of the differences in using leachate removed at different depths in a landfill for leachate recirculation, on the biodegradation of waste.

The central question to be answered is:

“Can any benefit be measured in using leachate for recirculation from shallower depths compared to the bottom of a deep landfill?”

10- year old waste from Kate Valley landfill in Canterbury, New Zealand was used in a laboratory experiment that simulated a landfill by using two large test cells with waste at different densities. It was expected that using waste of this age would ensure the waste was in a methanogenic phase of degradation, and methane composition and volumes would be sufficient to measure relevant properties.

By intercepting the leachate and using it for leachate recirculation and comparing this to leachate allowed to travel through the entire simulated waste pile, it was hoped that the various properties measured would demonstrate a difference in degradation efficacy.

The experiment failed to produce any gas volumes in any of the test cells. Post-mortem analysis to measure biomethanation potential showed there was little readily degradable organic material available for methanogenesis. The design parameters outlined in the research design section (section 3.3) had value and will help design future experiments. Measuring biomethanation potential via tube anaerobic respirometer is an effective means to determine likely successful gas production prior to committing to longer term landfill simulation experiments. Differentiating between slowly biodegradable organics from readily biodegradable organics during the experiment by measuring BOD, sbCOD and rbCOD may help to further clarify if readily biodegradable organics are available in the waste for methanogenesis.

This study also highlighted some interesting aspects of Kate Valley waste:

• There is significant evidence that much of the highly biodegradable organics expected in the 10-year old waste appear to have been degraded and can no longer support sufficient methanogenic activity for gas production. • Phosphate may be a limiting nutrient in older waste at Kate Valley despite theoretical requirements indicating otherwise. • Waste is considered to be a large particle sand, and as it degrades is more like a fine sand/silt. The particle sizing distribution shows that this assumption of waste is valid for the <20 mm distribution of this waste. • A decrease in hydraulic conductivity with density was experienced. Density approaching 1600kg/m³ experienced visible ponding although as waste is a highly heterogeneous material this is not a definitive density at which hydraulic issues can be said to occur within a landfill.