4. Investigating the Fate of a Synthetic Landfill Leachate Perfused Through Sequential
4.2 Results and Discussion
4.2.2 Redox potential (E h )
The redox potential of effluent leachate for both treatments over the course of the experiment is shown in Figure 4.4. At the first sampling interval (week 2) an average Eh of 672 mV was recorded for array Dh whereas for array Dl an Eh of 23 mV was recorded. The Eh of effluent leachate from array Dh decreased over the course of the study, reaching negative Eh conditions at week 26, stabilizing at approximately Eh -240mV from week 52 onward. In contrast, the Eh of the effluent leachate from array Dl increased for the first 24 weeks, reaching a maximum Eh of +220 mV before dropping down to negative Eh values.
From weeks 44 to 68 the redox states of both treatments were comparable. Thereafter, divergent paths were observed for arrays Dl and Dh with the former becoming less anaerobic while the latter stabilized between -210 mV and -240 mV, this occurs largely as a consequence of microbial metabolism which influences the redox cascade. This cascade generally begins with microorganisms utilizing aerobic respiration followed by those microorganisms that employ denitrification (nitrate reduction) as part of their metabolic pathways; this in turn leads to microbes using manganese and iron as terminal electron acceptors respectively. Depletion of these heavy metals facilitates the use of sulphate as the next terminal electron acceptor in the redox cascade. In other words, all microbes capable of using sulphate as a terminal electron acceptor during metabolism would thrive and out-compete non-sulphate reducing microorganisms (Lensing et al., 1994). The decrease in Eh occurs as a consequence of the availability of the different electron acceptors over time and space. Depletion of oxygen necessitates a switch to the next available terminal electron acceptor (nitrates), and this sequence of reactions continues until methanogenesis persists. Different electron acceptors yield different amounts of energy during microbial metabolism. The energy yield generally decreases as metabolically able microbes’ progress from oxygen through to sulphate as the terminal electron acceptor. The continuous acceptance of electrons by these terminal electron acceptors along the redox cascade ensures a continuous decrease in electrical charge along the redox cascade i.e. from positive Eh, associated with oxygen as the terminal electron acceptor, to negative Eh associated with sulphate acting as the terminal acceptor. Sulphate reduction activity was confirmed in each array by the deposition of metal sulphide in the gas traps connected to the soil arrays. The values never decreased below this and perhaps this explains
-400 -200 0 200 400 600 800
0 10 20 30 40 50 60 70 80 90
Time (weeks)
Redox Potential (mV)
Array Dh Array Dl
the failure to mimic complete methanogenic conditions, indicated by the recorded redox states of the emerging effluent and the lack of methane production. Among other factors such as pH and temperature, sulphate is a significant factor that influences redox states in subsurface environments (Beeman and Sulfita, 1990). Sulphate levels recorded for the soil arrays of both treatments remained elevated for the majority of the investigation (4.2.6), and only begin to recede after weeks 40 (array Dh) and 48 (array Dl), coinciding with the establishment of sulphate reducing potentials, particularly in array Dh (Figure 4.4). The composition of landfill leachate frequently includes sulphates (Christensen et al., 1994), and to this end Lovely and Klug (1983) concluded that this constituent contributes significantly to halting progression to methanogenesis. Ehlers (1999) achieved sulphate reducing conditions during his assessment of dual co-disposal of activated sewage sludge plus phenol with refuse but failed to mimic methanogenesis. He concluded that the continued presence of sulphate as an available electron acceptor, in the presence of a hypothetically common substrate, prevented effective competition from methanogens by virtue of metabolic energy yield-available substrate dynamics. Christensen et al. (2001) concluded that the limited time associated with laboratory experiments of this nature made it difficult for the development of an undisturebed and stable redox environment.
Figure 4.3 Transformation of the redox potential of effluent synthetic landfill leachate after leaching through soil microcosms at two different hydraulic loading rates (HLRs) over time. (▲) High HLR at 20 ml every 5 days, and (■) Low HLR at 10 ml every 5 days.
Table 4.2 Regression response functions of Redox Potential (y) on time (x) after grouping of treatments
yDh = 595.1 – 808.9 exp[-exp(-0.1190(x – 17.713))]
yDl = 192.0 – 348.6 exp[-exp(-0.1982(x – 34.40))]
Regression analysis accounted for 95.2 % of the variation when the redox data for both treatments were grouped and regressed against time. Table 4.3 contains two significantly different regression equations generated by the Gompertz model. The rate of change in Eh for array Dl (0.1982) was higher than that observed for array Dh (0.1190). However one has to consider that there was a steady drop in Eh recorded for array Dh throughout the investigation until stabilization from week 52 while the Eh state of array Dl was punctuated by periods of fluctuations (Figure 4.4). The Gompertz model further illustrates that the chief sequence of reduction was triggered at week 17 (17.713) and week 34 (34.40) for arrays Dh and Dl, respectively (Table 4.3). This indicated a lag of 17 weeks before reduction was effectively triggered in array Dl.
The actual redox conditions prevalent in each soil microcosm will play a significant role in determining the microbial populations present (Williams and Higgo, 1994). The survival and proliferation of these populations will in turn depend on their ability to utilize the organic and inorganic constituents introduced to the microenvironment. The governing redox processes play a pivotal role in determining the level of toxicity posed by the organic and heavy metal constituents of the landfill leachate on the microbial populations (Lensing et al., 1994). Hence, this “one-dimensional” approach attempts to define the prevalent Bacterial populations at various redox states achieved in the soil microcosms perfused with leachate at the two HLR investigated.