Permeability tests were conducted for a period of 13 months: 12 months under a hydraulic head of 2 m (representing a highly adverse, but realistic, scenario), followed by 1 month under a hydraulic head of 0.3 m (following the maximum allowable leachate head established by landfill design regulations). The materials through which landfill leachate were permeated to evaluate their potential to sustain biofilm formation and the creation of effective biobarriers were packaged into PVC columns. These materials consist of construction and demolition waste (CDW), mixtures of CDW and tire waste (CDW/TW), and CDW inoculated with anaerobic biomass (IN CDW). Hydraulic conductivity values and physicochemical parameters of inlet and outlet samples were monitored continuously.
Hydraulic conductivities reached values of 10-8 m/s for columns with CDW/TW and with IN CDW, operated with hydraulic heads of 2 m. The results were promising (considering that the initial permeability was as high as 10-4 m/s and that control columns reached 10-5 m/s), but were still above the legal limit (10-9 m/s). After day 360, columns were fed adopting a hydraulic head of 0.3 m (the maximum value recommended for landfills), which caused a reduction in the hydraulic conductivity values for all filling materials, enabling some CDW/TW columns to reach values as low as 10-9 m/s, after only 1 additional month of continuous operation (by day 420). Such results indicated that high hydraulic heads could impair the formation of thick biofilms and decelerate clogging. However, reducing the hydraulic head enhanced biobarrier formation and performance, as demonstrated when it was reduced nearly tenfold, complying the legal limits.
Regarding contaminants attenuation, most concentrations were below the WHO limits for agricultural reuse (AR), but not for potable water (PW), which would be desirable for avoiding groundwater contamination. Additionally, the concentrations fluctuated significantly, possibly due to the heterogeneity of the leachate stored in the inlet tank, which was refilled with newly collected leachate from the landfill each time it was depleted.
Temperature was assessed considering that the permeability test was conducted during one entire year, comprising a temperature variation of up to 30°C (5-35 °C). Regarding moisture, columns were disconnected and dried for 15 days by the end of the experiment and then reconnected for calculation of hydraulic conductivity. The temperature did not seem to impair the barrier development. Even though a more significant clogging occurred for higher temperatures, a continuous development was kept for lower temperatures, permitting an acceptable exponential fitting, which is expected for microbial processes. Regarding moisture, the dried columns presented an increase in conductivity values of up to 1 order of magnitude, for the CDW columns. This shows that the proposed biobarrier could have its performance reduced when used in dry climates or after the landfill closure, when leachate production decreases.
The processes involved with the retention of metals could be predicted considering the monitoring of other relevant physicochemical parameters, such as alkalinity (mainly HCO3-, as pH ranged from 7 – 9) and sulphates. Alkalinity values always decreased after passing through the barrier, showing a consumption of bicarbonates. It is known that many metals precipitate as carbonates/bicarbonates at the range of the pH studied (7-9), which was a geochemical process that was suggested to reduce the concentrations of metals in the permeates. Similarly, sulphate concentrations were also lower in outlet samples, when compared to inlet ones. In this regard, the BART ® tests for the identification of active sulphate reducing bacteria indicated a significant presence of them. These bacteria chemically reduce sulphates and form sulphides to obtain energy. It is known that metals precipitate as sulphide minerals in alkaline and reducing (low or negative Eh) environments, thus such a process was also found to be relevant to the decrease of pollutants concentrations in the permeates.
The best configuration for designing a biobarrier was using CDW/TW as the support material and adopting 0.3 m of hydraulic head. Such conditions permitted to obtain hydraulic conductivity values of 10-9 m/s. Lowering the pH till 5.0 did not cause an unwanted increase in hydraulic conductivities, whereas drying the columns for 15 days (intermittent flow) did impact such values, which reached 10-7 m/s.
Microscopical analysis validated the previous results, so that CDW/TW materials seemed to permit the formation of a more robust biofilm, which was thicker and more homogeneous than the other treatments; this was associated with the hydrophobic nature of TW.
