Integration of MFC technology with lab-scale landfill bioreactors for organic matter removal: test configuration and preliminary results
Introduction and aim of the experiment
The present study was aimed to apply the MFC technology for the old landfill leachate treatment.
The MFC system was integrated with landfill bioreactors and coupled, for the first time with in situ-aeration technique (Figure 2).
The integration of MFC system was carried out looking for cheap and easily available materials for electrodes, in order to keep the costs as low as possible.
The main goal is to exploit the advantages of both technology:
• Increase the degradation rate of organic matter;
• Work at low COD concentration characterizing the old landfill leachate;
• Work at room temperature, saving heating costs;
• Produce electric currents.
Consequently, the employment of both technologies for in-situ treatment of landfill leachate can reduce the long-term emissions, obtain a steady condition in chemical, biological and geological terms in lower time (5 to 15 years from landfill closure) and guarantee the landfill conditions required for landfill area restoration.
The expected results of this new technology application are:
• Reduction of overall landfill costs (20% estimation);
• Possibility to invest saved money on on-site leachate treatment facilities (usually in Italy the leachate treatment is carried out in an ex-situ facility);
• Possibility to speed up the in-situ leachate treatment through lithotrophic metabolism, without further energetic costs;
• Possibility to recover fertilizers from leachate implementing circular economy concepts;
• High reduction of long-term emissions with the possibility to reduce aftercare time from 30 to 5-15 years;
• Landscape requalification of the landfill site with the possibility of creating a new use for the community.
The requalification of the landfill area gives it back to the community and allows new planned uses, such as photovoltainc plant, energy crops for biofuel production and so on.
Materials and methods
The waste material used in this experiment was the residual fraction coming from the separate collection of municipal solid waste disposed in the Legnago landfill (VR), Northern Italy. The waste material was characterized by high percentages of undersieve 20 mm, inerts, plastic, paper and small percentages of aggregates, textiles, wood and metals.
Reactors were filled just by the undersieve 20 mm fraction, in order to increase the homogeneity. Some plastic, (density of 23 kg/m3, size 10-30 mm) was added and mixed with the undersieve fraction in order to increase the final porosity and enhance air diffusion inside the waste mass.
The experiment was carried out using six Plexyglass® (polymethyl methacrylate) landfill bioreactors with a height of 106 cm and an internal diameter of 24 cm (Figure 3).
Each reactor was loaded with 32 kg of waste material, having a final density of 1129 kg/m3.
A 10-cm thick gravel layer (Ø 20–30 mm) was placed at the bottom and the top of each column as a drainage layer to facilitate the distribution of recirculated leachate. A leachate collection port was located at the bottom of each column for leachate extraction.
The inlet airflow into the waste body was provided by Newa Wind® 33 pump and then regulated through Sho-Rate GT1135 and Cole-Parmer® flow-meters.
Reactors were kept for the whole experiment at room temperature of 25 ± 2 °C.
Three landfill bioreactors were integrated with the MFC system, the others had a traditional configuration and were used as reference reactors.
The MFC reactors (MFC-1, MFC-2, MFC-R) were provided with electrodes: 40 cm pyrolyzed Arundo Donax L. hollow stick was used as cathode and granular graphite (Ø 3-8 mm) was used as anode; they were placed inside a plastic net which was in turn disposed within a HDPE slotted vertical pipe (Ø 75 mm), in the middle of the column. A felt layer separated the arundo stick from the granular graphite ensuring the electrical insulation of the two electrodes.
A carbon cloth rectangle (8×15 cm) was put inside the graphite mass and it was electrically connected to a plastic-insulated copper wire. The electrical connection was then insulated by four layers of a bi-component epoxy resin (UHU® Plus 5 minutes) and covered by pasta-fimo for ensuring rigidity to the connection. Electrical connection was tested for internal resistance and fluid contact/leakage by exposure to distilled water.
The Arundo Donax L. stick used as cathode is a biodegradable material, having a hollow cylindrical shape and porous texture; moreover, it is very cheap and easily available.
During the electrodes preparation, the arundo was cut into pieces of 20 cm -length for being put into the oven for pyrolysis. It was subjected to a stream of nitrogen of around 14 NL/h at 1.8 bar for one hour at room temperature (25° C); then, a temperature ramp provided for an increase of 10° C per minute, from 25 to 900° C. Once it reached 900° C, the temperature was kept constant for one hour. The oven was then turned off and, once cooled, nitrogen flow was stopped and the arundo was ready to be utilised.
In order to get a cathode of 40 cm-length, two pyrolyzed arundo were jointed together using a bi-component epoxy resin (UHU® Plus 5 minutes) and liquid black glue (Gomma Nera Semiliquida Sigill), protecting the junction from the entrance of leachate. The external surface was scratched and homogenized with abrasive paper (P400 grit). A 60 cm-long carbon cloth strip connected the upper part of arundo to a plastic-insulated copper wire.
The cathode was aerated from the bottom part by means a flexible silicon pipe. The air flowed along the cathode and reached the top, where it could diffuse through the waste mass.
In the reference bioreactors (B-1, B-2, B-R), the conductive materials used as cathode and anode were substituted by a vertical PVC pipe and gravel (Ø 3-8 mm) respectively.
Experimental set up and operations
Once the reactors were filled, they were kept under anaerobic conditions for one month. No additional bacteria, nutrients or mediators were added to the system to increase its performance.
At the beginning of the experiment 7 L of leachate were put inside each column. Two different phases characterized the experiment, as reported in Table 4.
Table 4. Experimental operating conditions during the first (I) and second (II) phase.
|I||MFC-1, MFC-2, B-1, B-2||17||2||Saturated zone|
|MFC-R, B-R||17||2||Vadose zone|
|II||MFC-1, MFC-2, B-1, B-2||15||29||Saturated zone|
During the first phase, a continuous airflow of 2 NL/h was applied to each reactor. This phase was interrupted because the cathode of MFC-1 was completely filled by leachate, air was prevented to flow within the hollow part of the arundo stick and anaerobic conditions established along its internal lateral surface.
During the second phase only MFC-1, MFC-2, B-1 and B-2 run. Additional leachate, up to 9.5 L, was added inside each reactor to increase the surface contact between the cathode and the substrate. The airflow increased from 2 NL/h to 29.13 NL/h to avoid the entrance of leachate inside the cathode.
During the whole experiment, leachate was recirculated once a week. In MFC-1, MFC-2, B-1 and B-2 leachate recirculation was provided directly in the saturated zone of the reactor in order to keep the substrate well mixed and increase the representativeness of the analysis on leachate samples. The leachate recirculation was accomplished by hand: leachate was pulled out by the collection port at the bottom of the column and collected in a tank. Then, it was recirculated at the top of the column using a funnel and a PVC pipe passing through the waste material and reaching the bottom.
In MFC-R and B-R the recirculated leachate passed through the unsaturated zone of the reactor. This enhanced the leaching of substances contained in the solid waste.
Results and discussion
In the first phase, MFC-1 showed negative cell potential values, whereas MFC-2 had only positive values, except during the leachate recirculation at day 7 and 14 when cell potential was negative. MFC-R had a positive cell potential peak value, but it was stopped by the leachate recirculation and after that the cell potential turned negative (Figure 4).
The leachate recirculation both in the saturated and in the vadose zone disturbed the system, not guaranteeing aerobic and anaerobic conditions at cathode and anode, respectively. In the first phase, MFC-R had the best performance due to the combination of MFC system and leachate recirculation into the vadose zone.
During the second phase, MFC-2 showed positive cell potential, which was negative in MFC-1. The zero-potential difference since day 4 to day 7 was caused by a sudden shut down of data logger, therefore data were not recorded.
During the first phase, MFC-R reached higher values of current and power densities, confirming the better functioning with respect MFC-1 and MFC-2. During the second phase, the highest values of current and power density were obtained. In fact, the range values of current density varied from -0.001 to 0.005 A/m2 during the first phase and from 0.01 to 0.015 A/m2 during the second phase. The maximum value of power density was 0.00035 W/m2.
Nevertheless, current and power density values were always about two orders of magnitude lower than values obtained in other studies (Marzorati et al., 2018; Santini et al., 2017). This was probably due to the different materials used as electrodes, and the absence of a catalyst.
During the whole experiment, all chemical parameters had similar trends in MFC-1 and MFC-2.
In MFC-R, the combination of aeration, MFC technology and leachate recirculation through the waste mass increased the removal efficiency of organic carbon and nitrogen compounds.
In particular, in MFC-R, COD decreased from 2184 to 1602 mgO2/L, with a final removal of about 26 %, whereas TOC decreased from 741 to 472 mg C/L, about 36 %. (Figure 5).
The influence of MFC system on the organic matter removal was evident also in MFC-1 and MFC-2 with respect the reference reactors (reactors B): TOC decreased on average of 13% in MFC-1 and MFC-2, while it reduced less than 5 % in reference reactors.
Considered that in the first phase leachate recirculation had great influence in leachate treatment, during the second phase, the goal was to study just the contribution of the MFC technology and aeration on the organic matter removal. Therefore, only the four reactors performing the leachate recirculation directly to the saturated zone were monitored.
During the second phase, MFC technology did not improve the organic matter removal, as can be seen from COD and TOC analysis (Figure 5). The average COD and TOC removal in MFC-1 and MFC-2 was about 20 % and 6 % respectively, as low as in B-1 and B-2.
A better removal of TKN and ammonia oxidation were obtained in MFC reactors but the differences were not so significant, as showed in Figure 6.
The main goal was to exploit the advantages of both technology: increase the degradation rate of organic matter, work at low COD concentration characterizing the old landfill leachate, work at room temperature saving heating costs and produce electric currents.
The preliminary results pointed out some critical aspects of the system, such as the weakness of the cathode material, the great influence of the leachate recirculation mode on the performance of the whole system and the optimal choice of the aeration flux required to keep the cathodic compartment aerated.
These considerations highlight the need of slightly modify the configuration of the MFC system within the landfill simulation reactors. First, more resistant and porous materials for the cathode should be used. Then, leachate recirculation should not increase the concentration of dissolved oxygen in the anodic compartment; therefore, it should be carried out directly in the saturated zone of the reactor. Finally, the choice of the optimum aeration flux have to be further investigated, since it strongly affects the biological removal processes of the organic matter.
Nevertheless, all the critical issues that appeared during the test allowed to acquire further theoretical and practical skills necessary for a future restart of the test and to undertake a research topic still not well developed in the framework of municipal solid waste landfill remediation.