Municipal Solid Waste Landfill Leachate Treatment and Electricity Production Using Microbial Fuel Cells
Microbial fuel cells were designed and operated to treat landfill leachate while simultaneously producing electricity. Two designs were tested in batch cycles using landfill leachate as a substrate without inoculation (908 to 3,200 mg/L chemical oxygen demand (COD)): Circle (934 mL) and large-scale microbial fuel cells (MFC) (18.3 L). A total of seven cycles were completed for the Circle MFC and two cycles for the larger-scale MFC. Maximum power densities of 24 to 31 mW/m2 (653 to 824 mW/m3) were achieved using the Circle MFC, and a maximum voltage of 635 mV was produced using the larger-scale MFC. In the Circle MFC, COD, biological oxygen demand (BOD), total organic carbon (TOC), and ammonia were removed at an average of 16%, 62%, 23%, and 20%, respectively. The larger-scale MFC achieved an average of 74% BOD removal, 27% TOC removal, and 25% ammonia reduction while operating over 52 days. Analysis of the microbial characteristics of the leachate indicates that there might be both supportive and inhibiting bacteria in landfill leachate for operation of an MFC. Issues related to scale-up and heterogeneity of a mixed substrate remain.
KeywordsLandfill leachate Leachate treatment Municipal solid waste MFC
Historically, landfills have been the primary form of waste management throughout the world, and in many locations, this is still the case. For example, in the United States of America (USA), of the 250 million tons of waste produced in 2010, 136 million tons (54.2%) went to landfills. While this number has been on the decline (from 89% in 1980), historical landfills still exist, and landfills will likely remain a disposal option into the future . Landfill leachate is liquid that emanates from the landfill system either produced by the waste within the system or occurring from a past infiltration, i.e., groundwater or rainfall. Leachate must be collected and managed per most regulations throughout the world. In the USA, Federal regulations require that municipal solid waste (MSW) landfills are lined and leachate be collected and managed to protect human health and the environment . Outside sources such as groundwater infiltration, precipitation, and/or surface drainage into the landfill can contribute significantly to the leachate volume. There are four primary categories of compounds in MSW landfill leachate: dissolved organic matter, inorganic macrocomponents, trace metals, and xenobiotic compounds . Leachate management may consist of recycling back into the landfill, evaporation, treatment followed by disposal, or direct discharge to a municipal wastewater collection system. Some wastewater treatment facilities have the capability and capacity to accept landfill leachate with no treatment; however, some require pre-treatment, and the large organic load of leachate can be difficult and expensive to manage. If organic loading and/or ammonia is to be reduced, the traditional leachate treatment options reviewed here require energy, time, and cost, often with no additional benefit.
Microbial fuel cells (MFCs) can be used to treat landfill leachate without additional energy input, while also producing a small amount of power. Bacteria that are capable of this exocellular electron transfer to an electrode can do this by artificial mediators, self-produced mediators, or through direct contact [25, 9]. The electrons flow through wiring and a resistor to produce electrical current and therefore direct electricity . Various other studies have examined both unaltered and amended landfill leachate as substrate in MFCs, evaluating various levels of treatment as well [38, 18, 10, 32, 7]. In a study related to this work, a voltage of 542 mV was produced from a landfill leachate MFC with a power density of 3 mW/m2 (94 mW/m3) with a coloumbic efficiency (CE) of 4 to 17%. Chemical oxygen demand (COD) removals were in the range of 2 to 43%; biological oxygen demand (BOD) removal was 50%; TOC removal was 26%, and removals of ammonia were from 18 to 70% . While these results were promising, this work includes a different design (horizontal instead of vertical), which allows for better contact of the leachate with the cathode. Additionally, an analysis of the microbial characteristics of the leachate was completed, indicating that there might be both supportive and inhibiting bacteria in landfill leachate for operation of an MFC.
Materials and Methods
Landfill leachate was collected from Cell III, Phases 1 and 2 of the Turnkey Recycling and Environmental Enterprises (TREE) facility in Rochester, NH. Phases 1 and 2 of Cell III were opened in 1995 and 1996, respectively, with a third phase opened in 1997. At TLR III (Phases 1 and 2), leachate was collected directly from the pumping station. The leachate was transported in either 2 or 19 L HDPE plastic containers and placed in the MFCs within an hour of arrival at the laboratory, with the exception of cycles 2b and 4b of the Circle MFC, which were stored at 8–9 ° C for 1 week prior to use. All leachates were characterized before use in the MFC to generate baseline data to compare with the output leachate characteristics.
Circle MFC Design
A larger MFC was created to determine electrical output and treatment capabilities of a scaled-up MFC (Fig. 1b). The anode chamber was made from a 5-gallon high-density polyethylene bucket with a diameter of 28 cm and 34 cm height. There was a total volume of 1.89 L, with a working volume of 1.83 L. The anode was constructed using a medium extruded graphite plate 30.5 × 30.5 × 0.635 cm and nine 1.27 cm diameter by 30.5 cm long, fine, extruded graphite rods. The plate was cut into an x-shape, and the rods were attached with silver epoxy (EE129-4, Epo-tek). The anode had a total surface area of 1,942 cm2 (1.942 m2). The cathode was constructed in the same manner as described for the Circle MFC. The cathode was allowed to float on the surface of the leachate to allow constant contact in this upright system and had a diameter of 30 cm (707 cm2). However, overlapping on the sides of the container was allowed to minimize air infiltration. The cathode sat 1 cm above the anode when installed. Silver epoxy (EE129-4, Epo-tek) was used to connect insulated copper wire to both the anode and carbon cloth.
An electrical breadboard was used for the wiring of both systems. A capacitor to compensate for electrical noise within the system was used, along with a 1 Ω resistor to compensate for the resistance of the data acquisition unit and a 470 Ω resistor to provide a load for the system . The internal resistance of this system was 400–500 Ω. For optimum operation of an MFC, the external resistance should be equal to that of the internal resistance , which, in this case, correlated well.
Leachate was used as both the substrate and innoculum in this research. No additional anaerobic bacteria or nutrients were added to the system. Leachate was added and removed between cycles with caution to limit disturbance of any biofilm formation on the anode. No cleaning was done between consecutive cycles of MFC operation, so that continual growth of an exoelectrogen community could be achieved. A cycle of operation for these MFCs began with the addition of recently sampled leachate into system. The cycle typically ended when voltage produced dropped below 50 mV. Three sets of data were obtained: electrical production, leachate treatment, and microbial characterization. The MFC was operated in batch mode (although because of evaporation and utilization, leachate was added to the MFC in a cycle when necessary), and data were collected for each cycle of operation; referred to as ‘cycle’ in the following sections and consecutively numbered, starting with 1. Leachate taken from the landfill closed cell, Cell III, Phase 1, is designated ‘b’ while leachate from the landfill Cell III, Phase 2, is designated ‘c’.
Electrical Measurements and Calculations
A data acquisition unit (National Instruments USB 6210) was used with a desktop computer and software to measure and record data from the microbial fuel cells using a LabView program. Current was calculated according to the Equation V = I × Rext, where V = voltage (millivolts); I = current (milliamperes); and Rext = external resistance . Current was normalized by the cathode area, since the anode materials had an abundance of surface area (geometric and internal) to calculate current density. Power density (for area and volume) is calculated by P (watts per square meter or cubic meter) = V2/(A or Vol × Rext), where V = voltage (volts); A = area of cathode or anode; Vol = volume of reactor; and Rext = external resistance . In this case, power density is reported with normalization to the cathode area and the reactor volume. Columbic efficiency is calculated by equation CE = (8 × I × t)/(F × VolA ×ΔCOD) where I = average current over time, t (amperes); t = time of cycle (seconds); F = Faraday’s constant (96,500C/mol); VolA = volume of anode compartment (liters), and ΔCOD = change in COD concentration over time, t (grams per liter) . Polarization curves illustrate how well the MFC can maintain voltage as a function of current production. To obtain the polarization curve and power density curve, the external resistance was varied from 40,000 Ω to 10 Ω. Voltage was recorded for each resistance once readings had stabilized.
For all measurements, data were recorded for the leachate prior to input into the MFC system and after treatment. Further description of residence times can be found in the results section of this paper. Temperature (Celsius, ±0.15° accuracy), pH (±0.2 units accuracy), oxidation reduction potential (ORP) (millivolts, ±20 mV accuracy), dissolved oxygen (milligrams per liter and percent, ±2% accuracy), conductivity (millisiemens per centimeter, ±0.5% accuracy), and specific conductivity (microsiemens per centimeter) were all measured using a YSI 556 MPS probe. Other analyses included: COD (accuracy, 778–822 mg/L COD, 95% confidence limits of distribution, run in triplicate) with Hach method 8000 ; BOD (detection limit 5 mg/L) with Standard Method 5210 B ; TOC (detection limit 20 mg/L) with US EPA SW-846 9060 ; ammonia (detection limit, 250 mg/L) with Standard Method 4500 ; alkalinity (detection limit, 5 mg/L) with Standard Method 2320 B ; nitrite (2 mg/L detection limit), nitrate (0.5 mg/L detection limit), sulfate (25 mg/L detection limit), and chloride (25 mg/L detection limit) with US EPA Method 300.0 A ; total phosphorus and phosphate (detection limit 0.5 mg/L) with US EPA Method 365.3 ; sulfide (±2% accuracy, run in triplicate) with US EPA Method 376.2 using Hach Method 813 . Replicates and blanks were included in discrete sample runs. Inductively coupled plasma-atomic emission spectroscopy was used to detect cations and inorganic (trace) metals in the leachate. Samples were analyzed for the presence and concentration of aluminum, antimony, arsenic, barium, calcium, cobalt, chromium, iron, magnesium, manganese, nickel, selenium, silver, vanadium, and zinc. NIST standards, calibration blanks, and calibration verifications were used for each analysis to ensure quality of the data. The calibration verifications and NIST standards were included at least every 20 samples to ensure the calibration remained consistent over the entire analysis, and that various labs, conducting the same trace metal analysis, were detecting similar concentrations of the same solution. Solution matrix spikes were performed to make sure elemental interferences were not affecting the detection capabilities of the analysis.
The microbial communities of four different leachate/biofilm samples were fingerprinted using terminal restriction fragment length polymorphisms. Details of methods used are provided in Damiano . A sample of leachate from TLR II, Phase 1, which was unable to produce electricity (non-producing), was tested to determine if the microbial community could be inhibiting electrical results. Leachate from TLR III, Phase 2, was also analyzed prior (pre-MFC) to entering the MFC system as well as after (post-MFC) running a complete cycle in the Circle MFC. A sample of biofilm scraped from the anode of the Circle MFC was also tested when the MFC had been consistently running for approximately 2 months with landfill leachate.
Results and Discussion
While the large-scale MFC increased the volume of substrate to 19 L, significant internal resistance is added to the system, with both protons and electrons having longer paths to travel to complete the circuit and reaction, so linear increases in voltage are not observed. Voltage was produced by the scaled-up MFC but quickly decreased, most likely due to the use of new materials for MFC construction and necessary acclimation of the bacteria (Fig. 2c). A second cycle was completed where voltage was maintained >52 days and had a peak of 635 mV (Fig. 2c).
Power Density and Coulombic Efficiency
Comparison of results of this study with values from the literature
Shape/architecture (all single chamber cells, except noted)
Power density (mW/m2, except noted)
COD removal (%)
501 ± 20
76 ± 4
16 ± 2
86 ± 6
72 ± 1
Landfill leachate (dilute)
Circular with activated carbon anode
49.2 ± 2.3 (699 ± 33 mW/m3)
74.7% ± 5.5%
0.58% ± 0.11%
Circular with biochar anode
40.4 ± 12 (575 ± 168 mW/m3)
28.6% ± 8.9%
1.27% ± 0.61%
31 (824 mW/m3)
27% ± 16%
17.4% ± 15.8%
Coulombic efficiency, CE, often used to describe the efficiency of MFC systems, was calculated for all cycles of each MFC. Values ranged from 7.9 to 41% for the Circle design and were 5.2% for the larger-scale design. When complex substrates are used in MFCs, CE is calculated based upon COD removals as a representation of the amount of organic degradation achieved by the system. The work of You et al. recorded a CE of 3.4%, with previous work by Ganesh and Jambeck , resulting in a CE of 1.27% ± 0.61% (Table 1).
MFC Leachate Characterization
Water Quality Parameter Treatment
Summary of influent, effluent, and percent difference values for leachate characterization of Circle and larger-scale MFC systems
17 ± 6
20 ± 0.6
7.8 ± 0.3
8.4 ± 0.2
14 ± 4.5
15 ± 3.2
0.55 ± 0.39
0.64 ± 0.38
−15 ± 63
−24 ± 60
2,130 ± 907
1,780 ± 811
238 ± 131
90 ± 28
1,028 ± 488
790 ± 472
45,00 ± 808
3,825 ± 359
970 ± 35
773 ± 87
1,500 ± 346
1,700 ± 408
5.5 ± 3.6
6.2 ± 3.5
38.5 ± 0.6
108 ± 95
28 ± 28
5.6 ± 3.0
0.24 ± 0.04
0.1 ± 0.07
There has been substantial research that illustrates conductivity is a key factor in the efficiency of an MFC system. Conductivity, through the increase in ionic strength, has been artificially increased in many wastewaters and artificial substrates that are used in MFCs [13, 20, 21, 24, 30]. One of the major benefits of using landfill leachate as a substrate in MFCs is that conductivity is relatively high. Influent conductivity was in the range of 11.2 to 17.3 mS/cm. Effluent readings were in the range of 10.4 to 19.1 mS/cm, creating a decrease in levels ranging from 1 to 17% (with one exception in cycle 2b where conductivity was increased).
Landfill leachate is generally an anaerobic substrate with low DO levels, which were variable throughout sampling and testing of the MFCs in this research. Increases in DO concentrations within the MFC systems during operation would be due to the system being open to the air. An aerobic zone could have formed near the cathode and resulted in increases of DO. For the cycles where a decrease in DO occurred, anaerobic conditions within the system were more efficient, and a larger anaerobic zone was allowed to control the system and reduce DO.
ORP can be a useful measure of the state of the system being tested, as it measures the tendency of a solution to gain or lose electrons. Although landfills are generally anaerobic systems, one exception to this anaerobic condition was the influent leachate from TLR II, Phase 2, during Cycles 3 and 4b for the Circle MFC. These influent values were +52.5 and +73 mV. This suggests that the leachate entering the MFC was actually aerobically active. Interestingly, these cycles also had significantly shorter total cycle time for both of the MFC designs. However, a connection cannot be definitively made between ORP and cycle time because of the dynamic values of other constituents. For all other incoming leachate samples, a range of negative ORP values were obtained; however, ORP values of effluent leachate did vary and had an overall increase in each MFC (Table 2). To produce electricity, MFCs must have anaerobic conditions for the appropriate bacteria to grow within the system; however, these results, along with the DO results, do suggest that the systems had aerobic zones.
COD, BOD, and TOC
The Circle COD removal range was 15 to 49% (mean = 16% reduction, Table 2); however, in Cycles 5 through 7c, COD increased during the cycle of the MFC. For the same leachate c, the larger-scale MFC had an increase in COD of 6.6% and a decrease of 3.1%. There are several inorganics that can exhibit COD in leachate, such as sulfides that can interfere in COD being a measure of just the organics of a system . BOD removals for these same cycles were 53 to 72% for the Circle MFC, and 47 to 86% for the larger-scale.
While the COD removals are lower than those recorded by other landfill leachate in MFC research (70 to 98%), BOD is in the range of 57 to 66% removal that has been previously reported [6, 1]. COD removals for other MFC systems utilizing different wastewaters are shown in Table 1; however, direct comparison is confounded because of highly variable operating conditions and architectures. Small volume systems (<50 ml) have COD removals of 42 to 87% while larger systems (300 to 400 ml) have 50 to 70% COD removals. Almost all MFC treatment efficiencies in the literature are only reported in COD removals; however, a BOD removal of 57 to 66% was found for a column MFC system using landfill leachate . TOC removals for the Circle and larger-scale MFC increased with each cycle of operation . TOC removals for the circle MFC ranged from 16.7% to 52.9% (one cycle had an anomaly increase of 8%) and removals of the larger-scale MFC were minimal for the first cycle of operation and 50.7% during the second cycle.
In every cycle of the Circle MFC, ammonia was removed for a mean of 20% reduction (Table 2). The larger-scale MFC had an increase in ammonia in the first cycle, yet a 60% reduction in the 52-day cycle. There are four removal mechanisms that could be occurring within this system to remove ammonia. If nitrifying bacteria were present, ammonia could be oxidized by nitrification in the aerobic region near the cathode and coupled with denitrification. Ammonia-oxidizing bacteria could also be oxidizing the ammonia in conjunction with ammonia oxidation and nitrite reduction by anaerobic ammonia oxidation bacteria. Ammonia-oxidizing bacteria could also be reducing nitrite and oxidizing ammonia. Unlikely, but possible, an unidentified bacteria could be oxidizing ammonia while reducing the anode of the MFC system. A fifth mechanism, which has not been studied at great length, is the chemical/physical removal of ammonia from the system .
In research with a single-chamber MFC treating animal wastewater, it was determined that ammonia loss in the system was due to ammonia volatilization at the elevated pHs near the cathode . A localized pH at this location could cause a shift in ammonium ions to ammonia, resulting in nitrogen losses through the cathode. It was found that this ammonia loss increased as power production of the system increased. While it is possible that there was still some ammonia loss due to nitrification at the cathode by ammonia oxidizing bacteria and oxygen that has diffused into the system, limited appropriate bacterial communities were found . While it is unclear what mechanisms were taking place within the MFC designs used in the research presented here; it is plausible that a mixture of the above mechanisms could account for the ammonia removal. There is a high probability that an aerobic zone occurred near the cathode of the MFCs due to the porous nature of the carbon cloth, which is supported by the DO and ORP data discussed previously. This could support an aerobic nitrifiying bacterial community along with an anaerobic ammonia oxidation bacteria in the rest of the MFC. Simultaneous nitrification, denitrification, and carbon removal were observed in microbial fuel cells with added aeration resulting in 94% ammonia removal . More recently, MFCs have been designed specifically for ammonia and nitrogen removal . Volatilization of ammonia was also possible with the increased surface area of the cathode in the designs of this research.
An increase in chloride concentration for each cycle was common for the Circle MFC design in the range of 8 to 17% (mean = 13%, Table 2). The large-scale MFC saw a large increase of 60% in Cycle 2c with only a small increase in Cycle 1c (mean = 30%, Table 2). This consistent increase could be due to both the evaporation and utilization of the leachate out of these systems that were open to the air. Continuous additions of leachate were needed as volume was lost over time in both the Circle and large-scale designs. As the leachate evaporated and was utilized, chloride likely remained in the system and accumulated, since these were not continuous flow designs. This explanation is supported by the fact that the previously researched Square MFC  and larger-scale MFC had higher accumulations of chloride and more evaporation/utilization. The Circle MFC, while still experiencing evaporation/utilization, experienced it at a much slower rate, and had less chloride accumulation. Cycle 2b of the Square MFC had a cycle time of 7 to 10 days longer than cycles 1, 3, and 4b while the larger-scale MFC had a 52-day cycle with a 60% increase of chloride . Furthermore, a mass balance was completed on each system, taking into account the additions of leachate and subsequent increase in chloride concentrations . Calculated final concentrations of chloride were close in value to the concentrations measured for each system, which suggests that evaporation and utilization of water from the system was the cause for the increasing levels of chloride.
In all influent leachate from this landfill, there were no detectable levels of nitrate or nitrite. The MFC anaerobic system is not designed to remove phosphorus; however, because aerobic and anaerobic zones do exist, it is reasonable to assume that phosphorus uptake and release were occurring within the MFCs as illustrated by the varying removals and increases observed (Table 2). Every cycle of the MFCs in this research produced an increase in sulfate levels in the effluent leachate (Table 2). At the same time as the sulfate increased, a reduction in sulfides was observed likely from sulfur oxidation occurring within the MFC. A more detailed discussion of potential oxidation mechanisms, as well as a sulfate mass balance, is located in . An analysis of cations in influent and effluent leachate in the Circle MFC to determine if any major changes in cation concentration occurred during MFC operation. While the amount of cations did vary, all concentrations remained in the typical range of values for landfill leachate on both the influent and effluent side . A table of values of all the cations in the influent and effluent is contained in .
A comparison of the four analyses of pre-MFC, post-MFC, biofilm, and non-producing leachate was conducted . The microbial communities that are present in MFC environments and facilitate voltage production are phylogenetically diverse. Initially, it was thought that only metal-reducing bacteria, such as Shewanella and Geobacter contributed to the exocellular electron transfer that is needed in MFCs . However, it has been determined that many different types of bacteria can take part in electricity production . Common phyla of bacteria that have been shown to be dominant in MFC microbial communities are alpha-, beta-, gamma-, and delta-proteobacteria along with firmicutes . A large difference was observed in the relative percentage of beta-proteobacteria populations between leachate that was conducive to voltage production and that which was not. This suggests that beta-proteobacteria were not major contributors to electron transfer in this system. Firmicutes and Bacteroidetes were microbial populations that were only present within the voltage-producing MFCs, with a small population of Firmicutes pre-MFC. This suggests that there are conditions within the MFC that facilitate growth of these types of bacteria. Firmicutes and Bacteroidetes are bacteria that have been found in MFC communities in previous literature [26, 22]. Alpha- and gamma-proteobacteria appeared to be limited in growth by the MFC environment.
While this microbial analysis is only a beginning step in understanding the microbial aspects of the MFC within landfill leachate systems, it is important to note that landfill leachate contains many of the bacteria that are necessary for voltage production in MFCs [26, 22]. This validates the finding of this research as well as others that inoculation of the MFC with anaerobic bacteria is not necessary when using landfill leachate as a substrate .
Leachate is a well-matched substrate for use in a microbial fuel cell because of its relatively high amount of organics, conductivity, and buffering capacity, yet minimal solids. All of these characteristics, with the exception of high organics, can limit the utilization of other wastewaters for use in MFCs. Even though substrates such as wastewater and leachate will not provide equal power production to MFCs utilizing pure substrates such as acetate; use of a substrate like leachate mimics more realistic conditions. MFCs utilizing landfill leachate also need no outside source of inoculation due to the presence of the appropriate mixed bacterial community. The removal of major constituents such as COD, BOD, TOC, total phosphorous, and ammonia suggest that this technology could be a viable option for leachate treatment or pre-treatment. While power production did not increase significantly with the volume increase to larger-scale, power production was higher than the small-scale cells and was maintained longer (over 52 days versus the typical 14 to 20 days for small-scale cells).
While material costs for scale-up are still a concern (e.g., rare earth catalysts), alternative materials are being investigated . There may be a role for MFCs as a pre-treatment for the leachate prior to other treatment or use. For example, ammonia can accumulate and resist treatment during leachate recirculation . If MFCs can reduce ammonia and some organic concentrations in leachate, then they could potentially be utilized as an intermediate step in landfill recirculation. However, before any integration, further research on landfill leachate treatment using microbial fuel cells is needed. Even though scale-up concerns with material costs and internal resistance remain, experiments should be conducted with cells wired in series, like batteries, in a field-size experiment. Different leachate from various landfills, and ages of landfills, should be utilized to evaluate variability in leachate treatment efficiencies, as well as MFC lifespans. In addition, the issue of recalcitrant COD (often corresponding to the age of landfill) is important when considering the amount of COD removal required for treatment. A more in-depth microbial analysis should also be completed to examine characteristics of the community within the leachate and MFC environment in detail. It is still unknown what specific bacterial species are providing the exocellular electron transfer in the landfill leachate MFC system.
The authors acknowledge the donation of Waste Management’s Turnkey Landfill leachate for this work and the Environmental Research and Education Foundation for funding this research.
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