Periodic Reporting for period 1 - BioBar (BioBar: Biological barriers for a sustainable landfill design)
Berichtszeitraum: 2023-06-01 bis 2025-05-31
Waste final disposal can cause severe environmental impacts. Even in modern landfills, installed engineered barriers worsen their hydraulic performance after 8 years of landfill operation, which can lead to leachate leakage and environmental pollution in the medium to long term. In addition, the use of clay as landfill barriers relies on a non-renewable resource and entails high economic and environmental costs when such a resource is not locally available. Therefore, it is necessary to develop barriers made of renewable/recycled and locally available resources, which not only promote leachate containment but also enhance its in-situ treatment and attenuation.
The BioBar project proposed to develop an innovative barrier applied to landfill liners, taking advantage of the bacteria naturally occurring in leachate that grow and form biofilms in different support materials. We adopted both construction and demolition, and tire wastes as supports to permit the attachment of bacteria and promote the clogging of the media. Such natural clogging reduced the permeability of the materials to values compatible with landfill barrier layers, and, additionally, enhanced the occurrence of processes that attenuate pollution. In this case, as the leachate collection system would be above the bio-barrier, the clogging would not affect the drainage of leachate. Therefore, by adopting a bio-barrier between the liner and the drainage system, it would be possible to: a) reduce the flow of leachate that reaches the liner (since the natural clogging of the bio-barrier would reduce its permeability); b) reduce the contaminants concentrations reaching the liner, as physicochemical and biochemical processes would enhance their attenuation. Both processes would protect soils and groundwater from leachate leakages due to the liner’s failure and could enable the partial reduction of the clay liner thickness and/or the increase of its operational lifetime.
In this regard, the proposed project: 1) developed bio-barriers for landfill design by combining the rejected fraction from a construction and demolition waste treatment plant, tire waste and biofilm-forming bacteria; 2) verified the long-term performance of these new designs for representative environmental conditions.
We have observed that incorporating the proposed biobarrier within the landfill design could enable the reduction of clay liner thickness, which would contribute to a more renewable and sustainable perspective. Nevertheless, care should be taken in dry climates, and a multibarrier approach that combines clay layers is necessary, to avoid leachate leakages during the first stages of biofilm development.
The BioBar project proposed to develop an innovative barrier applied to landfill liners, taking advantage of the bacteria naturally occurring in leachate that grow and form biofilms in different support materials. We adopted both construction and demolition, and tire wastes as supports to permit the attachment of bacteria and promote the clogging of the media. Such natural clogging reduced the permeability of the materials to values compatible with landfill barrier layers, and, additionally, enhanced the occurrence of processes that attenuate pollution. In this case, as the leachate collection system would be above the bio-barrier, the clogging would not affect the drainage of leachate. Therefore, by adopting a bio-barrier between the liner and the drainage system, it would be possible to: a) reduce the flow of leachate that reaches the liner (since the natural clogging of the bio-barrier would reduce its permeability); b) reduce the contaminants concentrations reaching the liner, as physicochemical and biochemical processes would enhance their attenuation. Both processes would protect soils and groundwater from leachate leakages due to the liner’s failure and could enable the partial reduction of the clay liner thickness and/or the increase of its operational lifetime.
In this regard, the proposed project: 1) developed bio-barriers for landfill design by combining the rejected fraction from a construction and demolition waste treatment plant, tire waste and biofilm-forming bacteria; 2) verified the long-term performance of these new designs for representative environmental conditions.
We have observed that incorporating the proposed biobarrier within the landfill design could enable the reduction of clay liner thickness, which would contribute to a more renewable and sustainable perspective. Nevertheless, care should be taken in dry climates, and a multibarrier approach that combines clay layers is necessary, to avoid leachate leakages during the first stages of biofilm development.
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.
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.
BioBar aimed to design biobarriers applied to landfills, establishing requirements for their development and maintenance in such environments. Even though there are still restrictions to permit field applications, BioBar could contribute to several sectors:
- Circular economy: the research aligns with the increase in the number of publications on circular economy in the last 10 years. It has specifically contributed to the following sectors:
o Using wastes as support materials is possible and can lead to the establishment of media with hydraulic conductivity values compatible with barrier applications (10-9 m/s), after a clogging period of up to 12 months.
o Using hydrophobic wastes (such as tire wastes) can enhance biofilm attachment and clogging, being beneficial for barrier applications.
o Using wastes rich in sulphate and alkaline components can enhance sulphate reduction and act as pH buffers, contributing to metals precipitation in reducing and basic environments, such as the ones found in landfills.
o The use of wastes as support materials can be a cost-effective alternative for designing barriers, as the evaluated waste streams are currently disposed of in in landfills, without any form of recovery or added value.
- Reactive Barriers: such area of research has also an increasing number of publications in the recent years, with a broader adoption of wastes as support materials and biological processes. Biobar has specifically contributed to understanding that:
o Biological barriers cannot be adopted isolated in landfill environments, as it is necessary to count on a period of adaptation (clogging) and considering that drying can impair their effectiveness. Thus, it is necessary to adopt multibarrier concepts, also using clay layers, for example. Alternative applications, such as for the remediation of contaminated groundwater, could be less restrictive.
o Even chemical and biological attenuation occurring within the barrier, permitting reduction of some pollutant’s concentrations (up to 50% in salinity, 75% in Dissolved Organic Carbon), they were not considered to be enough for avoiding groundwater contamination from solid waste leachates. Thus, the physical containment (clogging) was the most relevant process to avoid contamination for the proposed barrier.
o Even not decreasing contaminants concentrations to levels compatible with WHO potable water standards, the proposed barrier did diminish pollution from initial values. Microbial communities developed in the proposed barriers get adapted to the support materials and perform important roles on attenuation of contaminants.
- Circular economy: the research aligns with the increase in the number of publications on circular economy in the last 10 years. It has specifically contributed to the following sectors:
o Using wastes as support materials is possible and can lead to the establishment of media with hydraulic conductivity values compatible with barrier applications (10-9 m/s), after a clogging period of up to 12 months.
o Using hydrophobic wastes (such as tire wastes) can enhance biofilm attachment and clogging, being beneficial for barrier applications.
o Using wastes rich in sulphate and alkaline components can enhance sulphate reduction and act as pH buffers, contributing to metals precipitation in reducing and basic environments, such as the ones found in landfills.
o The use of wastes as support materials can be a cost-effective alternative for designing barriers, as the evaluated waste streams are currently disposed of in in landfills, without any form of recovery or added value.
- Reactive Barriers: such area of research has also an increasing number of publications in the recent years, with a broader adoption of wastes as support materials and biological processes. Biobar has specifically contributed to understanding that:
o Biological barriers cannot be adopted isolated in landfill environments, as it is necessary to count on a period of adaptation (clogging) and considering that drying can impair their effectiveness. Thus, it is necessary to adopt multibarrier concepts, also using clay layers, for example. Alternative applications, such as for the remediation of contaminated groundwater, could be less restrictive.
o Even chemical and biological attenuation occurring within the barrier, permitting reduction of some pollutant’s concentrations (up to 50% in salinity, 75% in Dissolved Organic Carbon), they were not considered to be enough for avoiding groundwater contamination from solid waste leachates. Thus, the physical containment (clogging) was the most relevant process to avoid contamination for the proposed barrier.
o Even not decreasing contaminants concentrations to levels compatible with WHO potable water standards, the proposed barrier did diminish pollution from initial values. Microbial communities developed in the proposed barriers get adapted to the support materials and perform important roles on attenuation of contaminants.