Integrated Treatment at Laboratory Scale of a Mature Landﬁll Leachate via Active Filtration and Anaerobic Digestion: Preliminary Results

: The management of mature landﬁll leachate (MLL) represents an increasingly crucial issue to tackle. In this study, the feasibility of an integrated treatment was investigated at the laboratory scale using synthetic leachate with the objective of maximizing the recovery of potentially useful compounds present in leachate (especially ammonia nitrogen). First, in order to remove heavy metals, active ﬁltration of the MLL was carried out using zero-valent iron (ZVI) mixed with either lapillus or granular activated carbon (GAC). The average removal rates for the ZVI/lapillus and the ZVI/GAC ﬁlter were 33%, 85%, 66%, and 58% and 56%, 91%, 67%, and 75% for COD, Cu, Ni, and Zn, respectively. Then, pre-treated MLL was added during the anaerobic digestion (AD) of cellulose with the aim of providing bacteria with macro (i.e., ammonia nitrogen) and micro (e.g., residual heavy metals) nutrients. After 38 days, the best performance in terms of cumulative methane production (5.3 NL) and methane yield (0.26 NL/gVS added on average) was recorded in the reactor fed with the lowest dosage (17.9 mL/d) of MLL pre-treated by the ZVI/lapillus ﬁlter. The main issue that emerged during AD was the possible inhibition of the process linked to an excessive presence of humic substances; however, in future experiments, this problem can be solved through an optimization of the management of the whole process. The residual digestate from AD, rich in nitrogen and humic substances, may be safely used for agriculture purposes, closing the cycle of MLL management.


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The sustainable management of landfill leachate (LL) is particularly problematic. • Mature LL contains NH 3 -N and humic substances (HSs) that are potentially valuable. • Mature LL also contains toxic substances such as heavy metals (HMs). • Filtration on ZVI/GAC and ZVI/lapillus mixtures has been tested for removing HM. • Pre-treated LL is used for integration of NH 3 -N in anaerobic digestion of cellulose.

Introduction
Among the potential environmental impacts arising from landfilling of waste, leachate generation constitutes a critical issue and its management is a major challenge [1]. Leachate quality is largely affected by landfill age and specific composition of waste disposed of [2]. On account of landfill age, leachate can be defined as young (<5 years), medium age (5-10 years), and mature (>10 years) [3] according to the anaerobic waste degradation processes occurring over the landfill lifespan [4]. Leachate characteristics change over time and are usually represented by basic parameters such as pH, biochemical oxygen demand (BOD), chemical oxygen demand (COD), their ratio (BOD/COD), and ammonia nitrogen and heavy metal (HM) contents [3]. In particular, mature landfill leachates (MLLs) research, may limit the validity of the results and, therefore, they have to be considered to be preliminary and confirmation using a more complex matrix as substrate is needed.
The significance of this study lies in the possibility of closing the cycle of MLL treatment because methane produced in AD can generate energy, which can partially compensate for the impacts of uncontrolled landfill gas emissions [30], and digestate can replace fertilizers due to the presence of nitrogen and HSs deriving from the pre-treated MLL [31,32]. To the best of the authors' knowledge, neither the application of active filtration to MLLs nor the full integration of this physical-chemical process with the biological phase of leachate treatment have been previously investigated.

Synthetic MLL
The synthetic MLL was prepared following the typical composition described in the literature [3,33,34] by dissolving specific quantities of the reagents indicated in Table 1 into distilled water in order to reach the desired final concentration (C). The choice of the three heavy metals was dictated by their respective ability to activate the different removal mechanisms associated with the use of ZVI [35]. The pH of the synthetic solution was adjusted to the desired value of 8 with some drops of hydrochloric acid (Sigma-Aldrich, 37%).

Pre-Treatment Tests
The MLL pre-treatment was simulated in columns (polymethyl methacrylate (PMMA)-Plexiglas, internal diameter and height of 5 ± 0.1 and 50 cm, respectively) filled with two different granular active materials. Both columns were fed with the synthetic MLL by a peristaltic pump (Watson Marlow 205S) under a constant upward flow of 0.68 mL/min, which is equivalent to 0.5 m 3 /day of leachate per m 2 of granular filter. One column was filled with a granular mixture of ZVI/lapillus at a weight ratio of 20:80, whereas the other contained a granular mixture of ZVI/GAC at a weight ratio of 40:60 [15][16][17]. In both columns, the ZVI mass was fixed at 300 g, and the amount of the admixing agent (1200 g of lapillus and 450 g of GAC, respectively) was chosen on the basis of previous experiments [15][16][17] and in order to completely fill the columns. The final weight ratios were 20:80 and 40:60 for the ZVI/lapillus and ZVI/GAC columns, respectively. ZVI is a material that can be produced from scrap iron (e.g., from the automotive industry); lapillus is a volcanic material that is available worldwide and can be also produced as a by-product of the pumice extraction; and, finally, GAC is a material whose properties are well known and it is not particularly expensive.
A sampling port located at the top of the columns (50 cm) was used to sample the MLL filtered through the active medium during the test period through a 10 mL syringe. The pH of MLL samples was measured, and they were then analyzed for ammonium, chloride, and COD concentrations using specific pre-dosed cuvettes (Merck Millipore Ammonium Cell Test 114559, Chloride Cell Test 114730, and COD Cell Test 114555, respectively) using a photometer (WTW Photolab S12), and Cu, Ni, and Zn were evaluated by ICP-OES (Perkin Elmer OPTIMA 8000) in accordance with other reports [36]. The removal efficiency (R E ) of contaminants was calculated using Equation (1), where C in and C out stand for, respectively, the concentration (mg/L) of the contaminant in the input and output from the column.
Pre-treated MLLs, which came out from the columns, were separately collected and stored at 4 • C before their use in AD tests. The column tests were stopped after 38 days of continuous operation. The amount of leachate treated was equivalent to about 10.5 m 3 MLL /m 3 Filter_material .

Semicontinuous AD Tests
The purpose of this part of the experiment was to evaluate the use of the pre-treated MLLs, derived from the two column tests, as nutrient solutions for anaerobic digestion. For each column, a composite MLL sample was made by mixing weekly samples coming from the respective column outputs. The composition of the two composite samples used in the AD tests is summarized in Table 2. As already mentioned, metals and ammonium can be stimulatory to anaerobic microorganisms if present in adequate concentrations [23,37]. The presence of residual ammonium, Cu, Ni, and Zn makes pre-treated MLL analogous to the UNI/TS11703:2018 nutrient solutions (Italian norm for AD tests) prepared with CuCl 2 , NiCl 2 ·6H 2 O, ZnCl 2 , and other metal compounds. Similarly, ammonia can be toxic or beneficial to anaerobic microorganisms depending on its concentration [37].
The semicontinuous AD tests were carried out in four identical reactors ( Figure S1) designated as A, B, C, and D. In each reactor, microbial inoculum, distilled water as the diluting medium, pure cellulose as the organic substrate, and MLL as the nutrient solution were employed. In particular, reactors A and B were fed with MLL coming from the ZVI/lapillus column, whereas the MLL added to reactors C and D was the output from the ZVI/GAC pre-treatment column.
The inoculum used in the experiment was a liquid digestate from a full-scale AD plant that treats manure and various residues from the agro-industry. As soon as the digestate was collected, it was sieved in order to remove fibrous materials (e.g., straw) and then stored in anaerobic conditions at 35 • C to reduce the nonspecific biogas production as much as possible.
α-Cellulose (CAS 9004-34-6, Sigma-Aldrich, St. Louis, MO, USA) was used as the organic substrate due to its well-defined composition (only glucose as the monomer). Furthermore, it is an easily biodegradable material that stabilizes the AD process with a predictable methane production [21].
Inoculum and substrate were characterized according to standard methods [38] (see Supplementary Materials, Table S2).
Semicontinuous reactors consisted of three-neck bottles with 1.1 L volume (WTW-Germany), which were equipped with valves allowing biogas collection, substrate and MLL feeding, and sludge withdrawal. Biogas was collected from one of the valves connected to a tube placed in the headspace, while, with the biogas valve closed, a syringe was connected to the second one (feeding valve) connected to a tube immersed in the sludge, allowing excess sludge withdrawal and finally the feeding of a mixture containing dilution water, substrate and MLL. Because digestion was carried out in mesophilic conditions, the reactors were placed in a thermostatic cabinet at 35 ± 0.5 • C and mixed using a magnetic stirrer throughout the test period.
The total operational volume per reactor was 600 mL. At the beginning of the tests, the reactors were loaded with inoculum and distilled water in order to reach the desired total solids (TS) content of the mixture, which was set at 4%. Cellulose was fed into the reactors two times per week at an organic loading rate (OLR) of 1.0 gVS/L·day (1.25 gCOD/L·day) throughout the duration of the experiment; the feeding schedule was derived from previous experiments [39,40]. The hydraulic residence time (HRT) was set at 20 days for the first week (bacteria acclimatation period), and then at 10 days for the remaining part of the experiment, because cellulose degradation is quite fast.
As already mentioned, two times per week, generated biogas was slowly transferred into an alkaline trap, which was a second bottle containing 1 L of a 3 M NaOH solution, using a 100 mL syringe. In this manner, the carbon dioxide present in the biogas was absorbed in the alkaline solution, whereas methane caused a pressure increase in the trap and provoked the displacement of an equivalent volume of solution [39]. Due to the pH reduction, 4 g of NaHCO 3 was added to each reactor on days 20 and 27, whereas for reactor A only, feeding was suspended on day 27 because the biogas production was inhibited.
Because it was decided to test two different COD/NH 4 -N ratios per type of MLL (50 and 25, i.e., acceptable and low ratios, respectively, according to [41], different amounts of pre-treated MLLs were used to dilute cellulose along with distilled water. The semicontinuous reactor experimental program is summarized in Table 3. The AD tests were stopped on day 38 (3 HRT). During the semicontinuous experiments, the pH of the digestate extracted from reactors was measured twice per week, whereas TS, volatile solids (VS), COD, NH 4 -N, and volatile fatty acids (VFAs) were determined on average weekly digestate samples. The pH, TS, and VS of raw digestates were determined according to standard methods [38]. COD, NH 4 -N, and VFAs analyses were carried out on the liquid fractions resulting from digestate centrifugation (10,000 rpm for 10 min). The former two parameters were calculated using pre-dosed cuvettes (Merck Millipore COD Cell Test 114,555 and Ammonium Cell Test 114,559, respectively) and photometric determination (WTW Photolab S12), whereas VFAs were determined through a three-point titration of 20 mL of liquid digestate sample with 0.1 N sulfuric acid at pH 5.0, 4.3, and 4.0 [42,43]. An immersed electrode allowed for pH measurement, and the volumes of the added titrants were recorded. Total VFA concentration was calculated as follows: where V pH 4.0 is the volume in mL of added solution until pH = 4.0 was reached, V pH 4.3 is the volume in mL of added solution until pH = 4.3 was reached, V pH 5.0 is the volume in mL of added solution until pH = 5.0 was reached, V sample is the volume in mL of the sample (20 mL), and N H 2 SO 4 is the normality of the acid solution (0.1).

Pre-Treatment Tests
The pH values were measured in samples withdrawn from both columns at three sampling ports (5, 28, and 50 cm) on days 3 and 7 (see Supplementary Materials, Table S3). After the first week, pH stabilized at the influent value of about 8 for the whole duration of the tests.
Ammonium, chloride, and COD concentrations were measured in MLL samples withdrawn at the outlet of the columns. The results, which are the average of two replicates, are summarized in Table 4 for the column tests carried out with the ZVI/lapillus and ZVI/GAC granular reactive media, respectively. In general, the removal of ammonium and chloride was basically negligible for both granular media. Ammonium content decreased in the first days of operation in both pretreated MLLs, with the removal efficiency for the ZVI/GAC granular medium being better than that for the ZVI/lapillus mixture (83% and 57%, respectively). From day 8 onward, the amount of ammonium in the MLLs was close to the influent concentration. GAC was not expected to help with ammonium removal [15,44] due to its nonpolar surface, which results in poor interactions with polar adsorbates [45]. Nevertheless, it was noticeable that ZVI mixed with GAC showed a better performance compared to the ZVI/lapillus mixture (R E = 57%), even if only for a short period of time. Chloride was also basically unaffected by pre-treatment. COD concentration measured after 2 days witnessed a decrease in both tests (R E of 59% and 65% for ZVI/lapillus and ZVI/GAC column, respectively), whereas during the tests, it was slightly removed only by the ZVI/GAC granular mixture due to the key role played by the presence of GAC. Indeed, GAC is known as an effective adsorbent for organic matter [18], and its application is common in leachate treatment research [16,[46][47][48]. In general, as [49] asserted, the application of activated carbon adsorption is effective for the removal of organic compounds from MLL but not for ammonia nitrogen.
On the contrary, lapillus was found not to be a suitable material for ammonium, chloride, and COD removal. This proves that lapillus has no interactions with these tested contaminants, although they were not the specific targets of the pre-treatment.
Metal removal, at the outlet of the columns, is shown in Figure 1. After 2 days of operation, metal removal in the ZVI/lapillus filter was almost complete with an efficiency close to 100% for the three metals. Subsequently, the removal efficiency toward Cu was Water 2021, 13, 2845 7 of 15 slightly reduced assuming values between 84% and 93% until the end of the test. The reductions in Ni and Zn were more noticeable; for the former, it was in the range 55-67%, while for the latter, it gradually decreased from 61% on day 8 to 36% on day 38. close to 100% for the three metals. Subsequently, the removal efficiency toward Cu was slightly reduced assuming values between 84% and 93% until the end of the test. The reductions in Ni and Zn were more noticeable; for the former, it was in the range 55%-67%, while for the latter, it gradually decreased from 61% on day 8 to 36% on day 38.
Referring to the column filled with the ZVI/GAC mixture, Cu was completely removed at the beginning of the test (RE of about 100%). Its removal efficiency was reduced only in the last days of the test, assuming the lowest value of 75% on day 38. Ni removal was scarce until day 2 (RE = 27%) but significant on day 4 (RE = 93%). From this point, the removal efficiency toward Ni progressively decreased to 57% on day 38. Lastly, Zn removal efficiency increased to almost 100% until day 4, before gradually decreasing from 84% on day 8 to 66% on day 38.  Referring to the column filled with the ZVI/GAC mixture, Cu was completely removed at the beginning of the test (R E of about 100%). Its removal efficiency was reduced only in the last days of the test, assuming the lowest value of 75% on day 38. Ni removal was scarce until day 2 (R E = 27%) but significant on day 4 (R E = 93%). From this point, the removal efficiency toward Ni progressively decreased to 57% on day 38. Lastly, Zn removal efficiency increased to almost 100% until day 4, before gradually decreasing from 84% on day 8 to 66% on day 38.
In general, the results showed that both granular filters were efficient in removing the three contaminants according to the following sequences for ZVI/lapillus and ZVI/GAC granular mixtures, respectively: Cu > Ni > Zn and Cu > Zn > Ni.
ZVI was expected to remove HMs through a mechanism of reduction, adsorption onto its corrosion products, and precipitation and coprecipitation with its precipitating oxides [50,51]. Cu was removed via the well-described cementation process, whereas Ni and Zn removal relied on adsorption onto ZVI corrosion products and coprecipitation with ZVI oxides. Thus, the processes were facilitated by low pH values. Because, in these tests, an alkaline solution was used, it can be supposed that ZVI corrosion was slowed. However, this may imply the functionality of the filter for a longer period of time.
The role of GAC in HMs removal from leachate is well known [19]. For instance, its removal efficiency was tested by [52] for solutions with pH ranging from 6.0 to 7.7 with the removal of several HMs (Cd (II) , Cu (II) , Cr (III) , Mn (II) , Pb (II) , and Zn (II) ) in the range of 80-96% at an initial concentration of 184 mg/L. GAC can remove HMs through sorption on its porous surface, and through both surface and pore precipitation [53,54]. Moreover, sorption can also occur with organic matter because HMs in landfill leachate are thought to form complexes with it [55].
Similarly, lapillus is expected to remove HMs ions from aqueous solution via adsorption, as demonstrated in previous studies [17,51,56].
Cu was, as expected, the metal most easily removed by both filters (Figure 1). The ZVI/GAC filter performed slightly better than the ZVI/lapillus-filled column with R E values of 91% and 85% on average, respectively. Similar results in terms of removal efficiency were reported by [15] for a ZVI/GAC mixture (weight ratio 30:70 and 225 g of ZVI) but considering an acid leachate (pH = 5) and a longer residence time (flow rate of 0.1 mL/min).
Ni removal had an almost identical efficiency (66% and 67% on average) for both filters. However, the ZVI/lapillus-filled column removed Ni with a more regular trend compared to the ZVI/GAC filter (Figure 1). Ni removal was far more efficient in [15] where a ZVI/GAC granular mixture was tested toward an acid leachate (removal efficiency ranged from 93% to 80%). This behavior can be attributed to the aforementioned more favorable conditions for iron corrosion.
Lastly, the main difference between the two columns was exhibited on account of Zn removal. In fact, the ZVI/GAC granular mixture removed Zn better than the ZVI/lapillus filter with efficiencies of 75% and 58% on average, respectively. As in the case of Ni, Zn removal performance was considerably better for the ZVI/GAC filter studied by [15] (R E ranged from 98% to 95% using an acid leachate) confirming the better aptitude of the filter for the treatment of acidic solutions due to iron corrosion. Nevertheless, the metal removal sequence of this test (Cu > Ni > Zn) was in accordance with that of [15].
All results considered, the ZVI/GAC granular filter had a slightly better metal removal performance than the ZVI/lapillus granular mixture for MLL treatment. The aptitude of GAC for effective metal removal in alkaline conditions supported the ZVI, which performs better with acidic leachate. On the contrary, lapillus seemed less efficient in sufficiently supporting ZVI with metal removal; however, it is far cheaper because it is widely available and often produced as a by-product of pumice extraction, which is a significant advantage for its practical use.

Semicontinuous AD Tests
As shown in Table 2, the composite samples of MLL pre-treated by ZVI/lapillus and ZVI/GAC mixtures had very similar compositions in terms of pH, in addition to ammonium, chloride, Cu, and Ni concentrations. Conversely, COD and Zn concentrations were lower in the pre-treated MLL derived from the ZVI/GAC column.
Maximum concentrations of HMs and chloride were calculated for each reactor, according to the MLLs and water volumes added from the second week onward (Table 5), under the reasonable hypothesis that HMs and chloride were conserved (i.e., unaffected by the process). In the case of HMs and chloride, concentrations in the AD tests were far lower than the inhibitory thresholds, which were found to be 500, 100, and 50 mg/L for Cu, Ni, and Zn [25], respectively, and 4-9 g/L for chloride [57]. For reactors B and partially D the HMs maximum expected concentrations already complied with the Italian discharge limits for soil (Legislative Decree 152/2006; Cu < 0.1 mg/L; Ni < 0.2 mg/L; Zn < 0.5 mg/L). In a full-scale application, an appropriate dosage and/or and earlier replacement of the filter material used in the pretreatment can be considered in order to fully comply with the standard required for the intended use of digestate. The use of a cheap admixing Water 2021, 13, 2845 9 of 15 agent for the ZVI (i.e., the lapillus) may, in this case, reduce the cost of the replacement. Similarly, it can be stated that the low amounts added, in addition to the dilution with water and inoculum, would significantly reduce the concentration of persistent contaminants eventually present in AD reactors (e.g., pharmaceuticals and microplastics).
The pH trends of each semicontinuous reactor during the AD processes are depicted in Figure 2. During the semicontinuous AD tests, the pH of all reactors decreased from an initial value of around 7.8, showing similar trends to those during the AD tests. The lowest value (6.2) was reached in reactor A on day 20, probably due to an accumulation of VFAs. On days 20 and 27, 4 g of NaHCO 3 was added to all reactors in order to buffer the pH. As a result, a peak of 7.7 was measured on day 30 in reactor A. At the end of the tests, all reactors exhibited a pH of 6.9, except for reactor A (6.7). Despite the decreasing trends, the pH did not reach inhibitory values for methanogenic bacteria. However, the very irregular pH trend in reactor A could possibly hint at process imbalance.
Methane yields and cumulative methane productions for each reactor are depicted in Figure 2. Reactor A exhibited a gradually growing methane production up to 0.3 NL/gVS added (days [16][17][18][19][20] when methane production abruptly stopped. Reactor B reached the highest methane yield (0.37 L of methane per grams of VS added) after the first week of operation. From days 10 to 30, methane yield was fairly stable, remaining between 0.22 and 0.30 NL/gVS added (0.26 NL/gVS added on average). During the last week of operation, methane production sharply decreased to 0.06 NL/gVS added on the last day of the test. Reactor C showed a trend similar to reactor B but with lower methane yield values. The maximum (0.27 NL/gVS added ) was measured on day 13, before production gradually decreased. In reactor D, no considerable methane yield variations were measured. Indeed, methane yield remained close to 0.1 NL/gVS added for almost the entire test duration.
Cumulative methane productions of the four reactors were compared to that of the ideal AD of pure cellulose. The latter was determined by accounting for the cellulose biomethanation potential of 335 NmL/gVS (±25%) reported on the basis of the aforementioned UNI/TS 11703:2018 norm. After 38 days, the complete digestion of pure cellulose would have generated 7.2 NL of methane on average.
During the first 2 weeks of operation, the methane amount produced by reactor B process was consistent with the expected production from the complete conversion of cellulose, whereas at the end of the test, the final cumulative methane production was about 5.3 NL; in fact, this reactor was the best performing, with a final production equal to about 75% of the expected one. Reactor C was the second-best process in terms of the cumulative volume of methane produced, with a final value of about 4.1 NL and a production close to that of reactor B for the first 4 weeks. Reactor A reached a final methane production of 2.0 NL on day 16, as, from that point onward, methane generation was null. Conversely, reactor D generated methane throughout the experiment, despite maintaining low yields, achieving 1.6 NL of total methane production.
The analysis of average weekly digestates collected from the four reactors is summarized in Tables S4 and S5 (Supplementary Materials) and Figure 3.
Both TS and VS trends were similar among digestates and decreased during the tests in each reactor. Considering the initial TS value of each reactor mixture (4%), solid matter was scarcely consumed by the microbial community during the first week of operation, whereas the solid content decreased from the second week onward. At the end of the tests, TS reached values of 0.6-0.8%. VS remained almost constant for the first 3 weeks of tests in all reactors before their presence sharply decreased. The abrupt and abnormal reduction in VS/TS from week 4 onward hints at a reduction in microbial activity in the reactors as discussed below. During the first 2 weeks of operation, the methane amount produced by reactor B process was consistent with the expected production from the complete conversion of cellulose, whereas at the end of the test, the final cumulative methane production was about 5.3 NL; in fact, this reactor was the best performing, with a final production equal to about 75% of the expected one. Reactor C was the second-best process in terms of the cumulative 2.0 NL on day 16, as, from that point onward, methane generation was null. C reactor D generated methane throughout the experiment, despite maintaining achieving 1.6 NL of total methane production.
The analysis of average weekly digestates collected from the four reactors rized in Tables S4 and S5 (Supplementary Materials) and Figure 3. Both TS and VS trends were similar among digestates and decreased duri in each reactor. Considering the initial TS value of each reactor mixture (4%), s was scarcely consumed by the microbial community during the first week of During the first and second weeks of the tests, the highest contents of VFAs (852 and 975 mg/L, respectively) were recorded in reactor A, whereas those in reactors B, C, and D were similar (data for reactor C from the second week were not recorded because of a sampling problem). From week 3 onward, a continuous increase in VFAs was recorded in reactor A, again suggesting a probable imbalance of the process. The VFA concentration reached inhibitory levels from week 4 onward in this reactor. VFAs also increased to a lesser extent in reactors C and D in weeks 4 and 5. The drop in the level of VFAs in week 6 when methane production was already scarce, even in the best-performing reactor (i.e., B), may have been due to a possible inhibition of hydrolysis.
In the centrifuged composite samples from the first week, a COD of about 3200 mg/L was measured in each reactor. Despite the aforementioned sampling problem for reactor C, in the second week, slight decreases in COD content were recorded for all reactors. From the third week onward, COD concentration in reactor A started increasing again, reaching a peak of 4935 mg/L at the end of the experiment. The COD concentration in reactors B and C increased from the fourth week until the end of the tests, but with lower values compared to reactor A. Conversely, the COD concentration in reactor D smoothly decreased during the digestion, with the lowest value of 680 mg/L recorded in the fifth week, only increasing in the last week to 1755 mg/L.
The highest NH 4 -N concentrations were recorded in the first week of tests for reactors B, C, and D (519, 565, and 521 mg/L, respectively) and in the second week for reactor A (527 mg/L). In reactor A, ammonium content decreased to 306 in the fourth week, before increasing until the end of the experiment. In other reactors, analogous trends were recorded but with NH 4 -N concentration values, which were lower in reactors B and D and similar in reactor C compared to reactor A. As with HMs and chloride (Table 5), the NH 4 -N content in the AD reactors was lower than the inhibitory threshold (1.7 g/L) found in the literature [37,58].
Methane yields and cumulative methane productions depicted in Figure 2 clearly show that, with the exception of reactor D, methane production was not steady; moreover, after an initial production quite close to the "ideal" expected for cellulose, the process showed clear signs of inhibition. This was early and abrupt for reactor A, where after day 16, methane production totally stopped, whereas it was more delayed for reactor B, i.e., the best-performing system, where it was significant and quite steep from day 30 onward. By comparison, inhibition was more gradual in reactor C, with a steady and slow decline beginning on day 13. The behavior of reactor D was very peculiar, whereby the methane production was very slight but regular throughout the test, equal to approximately 25% of the "ideal" production expected for pure cellulose. The analysis of trends for pH, VFAs, and NH 4 -N ( Figure 3) did not justify this observation. This behavior could be classified as an "inhibited steady state", i.e., a condition where the process ran stably but with methane yield lower than expected due to limited bacterial activity [37]. The identified inhibition of methanogenesis is most probably not attributable to an excess of metals or chloride (Table 5) or NH 4 -N ( Figure 3) because, as already shown, the concentrations of those parameters were well below their respective inhibition thresholds. Ref. [29] reported the inhibition of anaerobic processes (particularly hydrolysis and methanogenesis) due to the presence, as in this case, of noticeable amounts of HSs; moreover, they also stated that the inhibition was irreversible and exacerbated by an increase in the HS/VS ratio. Because the biomass could not adapt to the presence of HSs, the abundance of microorganisms decreased, which is in agreement with the reduction in VS measured for all reactors. According to the same paper, HSs interfere with hydrolytic and methanogenic enzymes.
It is interesting to highlight that the two reactors where methane production was more noticeable (reactor B), or more regular albeit quite scarce (reactor D), were those where the addition of pre-treated synthetic MLL and, thus, of HSs was lower.
On the contrary, reactor A, supplemented with a higher amount of COD-rich pretreated leachate, had a more significant presence of HSs and showed a more abrupt inhibition of AD.

Conclusions
The proposed pre-treatment of MLL confirmed the excellent efficiency and selectivity for the removal of the HMs in the case of alkaline leachate.
Semicontinuous AD experiments revealed an early inhibition of the process, most probably linked to the presence of HSs in the MLL, whereas the accumulation of NH 4 -N up to a toxic level did not occur. Most probably, the inhibition was also exacerbated by the low HRT chosen, which favors wash-up of the active biomass, in addition to the relatively high dosage of MLL needed to reach the desired C/N values, because the tested co-substrate (cellulose) does not contain any nitrogen.
However, the value and the potential applications of the proposed treatment train (i.e., filtration using active materials and anaerobic co-digestion) remain intact. In fact, this process allows the use of the nitrogen present in the pre-treated MLL; moreover, several options exist for reducing or avoiding the inhibition due to HSs. The most promising of these are (i) a reduction in the pre-treated MLL dosage due to the fact that, for most substrates, it only results in limited nitrogen integration; (ii) an increase in the HRT to reduce the wash-off of the active biomass; and (iii) the addition of supplements of various type (salts or conductive materials) that may potentially reduce inhibition and/or enhance hydrolysis and methanization. Moreover, another interesting issue to explore would be to change the implementation of the AD process (e.g., batch or two phases).
For the possible pilot or full-scale implementation of this integrated process, it must be highlighted that the used synthetic MLL contains all the common leachate macroconstituents (e.g., organic matter including humic acids, ammonium, chlorides) and some of the micro-ones (e.g., HMs). Real MLLs can contain other micropollutants, normally present in limited concentration, so that the AD process would be most probably unaffected due to the limited amount of pretreated MLL to be added in the anaerobic reactor. However, with leachates of different composition, the duration of the filter material will be variable. In fact, its replacement will be guided by the desired characteristics of the pre-treated MLL. In addition, attention should be paid to the eventual presence of micropollutants strictly regulated by the norms for the agricultural use of the digestate derived from the process. As a consequence, further research in more realistic conditions (i.e., using a real MLL and a different AD substrate) is necessary to explore these options and to validate the results obtained herein.
Supplementary Materials: The following are available online at https://www.mdpi.com/article/10 .3390/w13202845/s1, Figure S1: experimental apparatus for leachate pre-treatment, Figure S2: Semicontinuous reactor, Table S1: Coefficient of uniformity, mean grain size, and particle density of materials, Table S2: Inoculum and substrate characterization, Table S3: pH values in MLL samples withdrawn from column tests, Table S4: TS content in weekly collected digestates, Table S5: VS content in weekly collected digestates.

Data Availability Statement:
The data presented in this study are available within the article and supplementary material. For the availability of detailed data sets the corresponding authors can be contacted.