The landfill studied is located in Piedmont (northern Italy) and consists of three lots. Two lots are full and are in the post-cultivation phase; one lot is currently in use. The cultivation was carried out in lot 1 from 1988 until 1994, and in lot 2, from 1994 until 2000. The landfill receives approximately 25,000 tonnes of MSW per year.

The basin has bottom proofing, which consists of an HDPE sheet and an underlying monitoring layer. Proofing of the various parts of the landfill was carried out using HDPE sheets, with a high adherence, joined with a double layer. Currently the leachate is collected, pumped and stored in tanks and treated out from the landfill plant, in a specific wastewater and liquid waste treatment plant (conventional biological treatment with the third chemical finishing step).

Lots 1 and 2 are currently exhausted.

Lot 3 is wider than the first two, is physically separate from lots 1 and 2, and is basically a separate landfill in terms of storage capacity, leachate, rainwater, and road network. It was dug in three stages (called the 1st, 2nd and 3rd hydraulic sectors), from east to west, and the last stage was completed in 2006. From a management point of view, the most western point (which corresponds to the first stage and part of the second stage) is almost full, and the western part, which began operating following completion of the third stage, is currently being used. The overall area of lot 3 is approximately 30,600 m².

The experimental apparatus used in the laboratory tests (Figure 1) consisted of:

A rectangular reactor (20 × 10 × 10 cm), in transparent PVC, with a thickness of 10 mm;

When carrying out the laboratory experiments using this apparatus, the electrodes were inserted into the grooves on the walls of the empty reactor. The empty space between the electrodes was then filled with the contaminated sample. The leachate samples were analyzed in the laboratory. Regarding the intermediary and final sampling, because the sample was liquid, only one mixed sample that characterized the entire sample was taken and then preserved at 4 to 5 °C or taken directly to the laboratory for analysis.

The methods used for the analyses were MI-127 rev1 2007 (digestion/titration) for COD, MI-128 rev 2 2007 (respirometric method) for BOD, MI-122 rev 1 2007 (UV/visible spectro-photometry) for ammonium and hexavalent chromium, UNI EN ISO 14911:2001 for nitrates and nitrites, MI-155 rev 1 2007 (gravimetry) for TSS, and EPA 3052 2006 + EPA 6010 B 1996 for As, Be, Cd, total Cr, Ni, Pb, Cu, Se, and Zn.

Table 1 shows the characterization of leachate in the experimental tests. Table 1 highlights that both the Italian regulatory limits for metals and the local sewage company limits were respected, except for copper and zinc. In any case, the main objective of this research did not include the treatment of metals, but rather the assessment of hexavalent chromium concentrations that can potentially change due to different oxidative conditions.

A comparison of data reported in Table 1 with literature data [37,38,39] demonstrates that the studied leachate is characteristic of an old landfill.

The parameters used in the treatments were adapted in accordance with the behavior of the matrix treated. All samples were characterized by high electrical conductivity values (between 14,500 μS·cm⁻¹ and 25,500 μS·cm⁻¹). It was therefore necessary to check the specific voltage values (V·cm⁻¹).

In each test, a contaminated sample was introduced into the test cell, and a constant voltage, between 10 and 30 V, was applied for a set period of time (seven days for the first three samples from the three lots of the landfill, and one day for the fourth, made up of perforation water). At the end of each test, the sample was removed from the reactor and analyzed, to determine COD, BOD₅, ammonium, total suspended solid and hexavalent chromium values.

For the first three samples, the specific voltage applied varied between 1.5 V·cm⁻¹ and 0.5 V·cm⁻¹. During the first hour of treatment, the specific voltage was set at 1.5 V·cm⁻¹, but the recorded current was too high (between 2 A and 3 A). In the next hour, the specific voltage was reduced to 1 V·cm⁻¹. In the end, it was decided that the specific voltage used should be 0.5 V·cm⁻¹.

The treatment period was seven days, with intermediate liquid samples and sludge taken after the first three days. In the literature, a maximum period of treatment is measured in hours (3–6 h) and not in days (as in this research), but the idea was to observe the behavior of the electrochemical treatment when no chemical substances were used.

We divided the treatment into two phases:

Phase 1: lasting 3 days, application of a specific voltage of 0.5 V·cm⁻¹;

The idea of two phases with two different specific voltages was due to the fact that at the beginning of the test, the current value was too high and the power supply was not able to sustain a high voltage that would have corresponded to a specific voltage of 1 V·cm⁻¹. After three days, the current value dropped and the specific value of 1 V·cm⁻¹ was established.

For the last sample, the treatment was applied only for one day, with a specific voltage of 1 V·cm⁻¹.

This test was carried out for a treatment period of seven days, with a specific voltage that varied from 0.5 to 1 V·cm⁻¹.

During the initial hours of treatment, it was decided that a voltage of 1.5 V·cm⁻¹ should be applied, however, given that, already after the first 30 minutes, the current value was greater than 2.8 A, the voltage was eventually reduced to 0.5 V·cm⁻¹. This voltage was kept constant for the first three days of treatment, after which, it was increased to 1 V·cm⁻¹ in the second part of the treatment, because the current tended to fall to a constant value, which, in the case in question, did not exceed 170 mA. The values recorded during monitoring are shown in Table 3.

Taking into account the current values measured, the energy consumed was also assessed, and was found to be 740 kWh·m⁻³ (0.74 kWh·L⁻¹).

By the end of the treatment, the pH increased slightly from its initial value of 7.5 to 7.7. The electrical conductivity fell from an initial value of 18.87 mS·cm⁻¹, to 13.04 mS·cm⁻¹, after seven days of treatment.

Table 4 shows the characterization of the sample, before, during and after the treatment.

Italian regulations for discharging into a sewage system establish require a maximum value for 271 COD of 500 mg·L⁻¹ for COD. For the sample to conform with this, it is was necessary to apply the treatment for a time significantly longer than the seven days already tested, given that it was demonstrated that the electrochemical treatment is was more effective with longer treatment times, but has not a linear kinetics [41].

After the first three days, sedimentation was observed, which had a volume of 0.3 L for lot 1.

From Table 4 it can be clearly observed that the efficiency of ammonium removal was low, the remaining ammonium concentration was about 1,000 mg·L⁻¹. The portion finally removed was 38.32%. The ammonium removed appeared to have been almost entirely transformed into nitrate.

An amount of 86.35% of total suspended solids had been removed from the liquid after the 7-day treatment.

This treatment lasted for seven days, with a specific voltage that varied between 0.5 and 1 V·cm⁻¹. The same treatment strategy described for the Lot 1 test was used. The trend of the current monitored during the test is shown in Table 5. The amount of energy consumed was 377 kWh·m⁻³ (0.38 kWh·L⁻¹).

The pH value increased slightly by the end of the treatment: from its initial value of 7.5 to a final value of 7.7. The electrical conductivity fell from the initial value of 25.4 mS·cm⁻¹, to 17.21 mS·cm⁻¹ (after seven days of treatment).

The characterization of the sample, before, during and after the treatment is shown in Table 6.

A modest removal of COD and BOD was observed after the first three days of treatment, which continued to improve by the end of the treatment (from 25% to 65% for COD, and from 30% to 64% for BOD).

In view of the national maximum value (COD = 500 mg·L⁻¹) for discharging into the sewage system, the treatment was continued at least another 14 days (in addition to the seven already tested).

After the first three days, sedimentation could be seen, which, for lot 2, had a volume of 0.35 L.

For ammonium there was a modest final removal, of only 15.09%. In addition, in this case, ammonium seemed to be transformed into nitrate, for which an increase was recorded.

In this case, the total suspended solids did not register any removal, but an increase by nearly 13% after the 7-day treatment. This was due, in part, to the corrosion of the steel plates, with a subsequent release of suspended matter, and, in part, to electro-flocculation following the release of iron ions into the solution.

Similar methods were used for the test for Lot 3 (duration 7 days, voltage 0.5–1 V·cm⁻¹) (Table 7). The energy consumed amounted to 689 kWh·m⁻³ (0.689 kWh·L⁻¹).

A comparison of the current values for all three samples (Figure 2), reveals that even though the maximum value for the Lot 3 experiment was the lowest, the steady state value was the highest. In addition, for Lot 3 the last three measured values were very similar compared with the same values as in the Lot 1 and Lot 2 tests, where the variation is more obvious.

By the end of the treatment, pH increased slightly from its initial value of 7.5, to 8.1. In contrast, the electrical conductivity decreased from 26.7 mS·cm⁻¹ to 19 mS·cm⁻¹ (after seven days of treatment) (Table 8).

Here, too, in view of the limits regarding discharging into the sewage system, the treatment was continued for at least another 14 days (in addition to the seven already tested) to allow the sample to conform.

After the first three days, sedimentation was seen, with a volume of 0.35 L.

The final amount of ammonium removed was 60.72%. Here, too, it appeared to have transformed, in part, into nitrate, which registered an increase.

In this case, the total suspended solids registered a removal of almost 39.4% after the 7-day treatment. This increase could be attributed to corrosion of the steel electrodes, which may also occur in laboratory electro-oxidation tests carried out with steel plates. In real-scale plants, probes made of titanium activated with noble metals, are used, which are entirely free of corrosion. The increase in the suspended solids was also due also to electro-flocculation, caused after iron ions were released into the solution.

Figure 3 compares the initial and final characterizations, for the three treated leachate samples.

Figure 4 and Figure 5 compare the discharge limits set by Italian regulations (Leg. Dec. No. 152/2006) [42] with the samples before and after the 7-day treatment, respectively.

In none of the cases were the limits established for COD, ammonium and nitrate complied with. Furthermore, for the samples from Lots 2 and 3, the limits established for total suspended solids, total chromium, nickel and copper were not complied with, because of the mechanisms described above, which are only important on a laboratory scale, when stainless steel electrodes are used.

Sedimentation in all three treatment (Lot 1, Lot 2 and Lot 3) samples was observed which may be due to the electrode corrosion.

Another important aspect is that the rate of ammonia converted into nitrate resulted variable: 35% for Lot 1, 10% for Lot 2 and 43% for Lot 3.

Moreover, data of Cr^VI in Table 4, Table 6 and Table 8 demonstrate that there is no detectable conversion of Cr^III into Cr^VI, with a clear advantage compared to other phenomena involving these forms of Cr (i.e., combustion [43], soils with specific composition [44]).

Very high values of power consumption were found, about ten times higher than previously reported data [41]. This is a demonstration that electro-oxidation is very sensible to the materials adopted at the electrodes.