Quantification of factors affecting rate and magnitude of secondary settlement of landfills

Abstract

Laboratory scale experiments were conducted to estimate waste settlement rates under different operational-management practices including leachate recirculation and the addition of synthetic methanogenic mineral media and an anaerobic microbial seed culture.  The effect of these practices on settlement rates and magnitude was evaluated using four purpose-designed test cells-Consolidating Anaerobic Reactors (CARs) in which a waste sample was subjected to a constant applied load to simulate the waste overburden effect.

Prior to the establishment of the CARs, the degradability of the waste selected for the study was quantified by means of a series of Biochemical Methane Potential (BMP) tests.  Studies also allowed the determination of the most efficient proportion of anaerobically digested microbial seed to promote rapid methanogenic conditions in the CAR which was found to be 10% (v/v).  in both cases, the reactors were sacrificed at various states in the biodegradative process to allow the waste sample to be analysed for total carbon to facilitate the calculation of C mass balance for the biodegradative process.

The long-term secondary settlement was found to be dependent on waste depth.  The increased stress (150kPa) led to a 20% increase in the rates of long-term secondary compression in comparison to a stress level of 50 kPa.  For the fresh waste tested in this study (CARs 2, 3 and 4) secondary settlement due to biodegradable was found to be of compatible magnitude to the component of secondary settlement caused by mechanical creep.  The organic content of the waste was found to directly affect the volume of the gas produced which in turn directly affected the magnitude of secondary settlement observed in the CARs.  The enhanced biodegradative conditions were found to accelerate the waste biodegradation processes in the CARs and hence the establishment of methanogenic conditions.  The removal of all the sulphate in the leachate resulted in a simultaneous increase in the rate of gas production. The rate of biodegradation was found to vary with depth. This was established by obtaining core samples from the CARs at the end of the test. The BMP test results indicate that changes in values for NDF, ADF, cellulose, (C+H)/L ratio and methane potential for the BMP test samples to be interrelated. The good statistical correlation between all the chemical parameters mentioned above suggests that monitoring only one of these parameters may be sufficient to provide an accurate prediction of the biodegradability of a fresh MSW sample without extensive monitoring methods.

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