Treatment of Low Biodegradability Leachates in a Serial System of Aged Refuse-Filled Bioreactors

: This paper presents a technology based on the use of aged refuse that has proven to be highly e ﬀ ective in the treatment of low biodegradability leachates. The tests were developed using two ﬁlled bioreactors arranged in series and operated at steady state. The aged refuse used as ﬁlling material was extracted from a city located in the southeast of Mexico and characterized by particle size, humidity, volatile solids, and volumetric weight. On the other hand, bacterial characterization made it possible to identify the presence of species related to the degradation and mineralization of organic compounds, as well as to processes of nitriﬁcation or reduction of phosphates and Cr (VI). The bioreactor system was operated under four hydraulic loads (10, 20, 35, and 50 L / m 3 · d). Maximum removal e ﬃ ciencies of 85, 86.1, 87.9, 98.6, 97.8, and 97.4% were achieved in COD, BOD 5 , Color, TP, TN, and N-NH 3 , respectively, complying with Mexican regulations (NOM-001-SEMARNAT-1996). The system also proved to be stable against shock loads, such as organic load ﬂuctuations in the inﬂuent or pH variations. The results of this study show that, in countries such as Mexico, aged refuse extracted from landﬁlls represents a promising option as a sustainable alternative for leachate treatment.


Introduction
The growth of commercial and industrial activity, as well as the general tendency of the society towards consumerism, has caused an undeniable increase in the rate of generation of municipal solid waste (MSW). Comparative studies on the various means of elimination of MSW have shown that the most economical way, in terms of operating costs and capital, are landfills [1]. In these sites, harmful liquids are generated as a result of the percolation of rainwater through solid waste, leaching a large amount of polluting materials, and producing very complex residual water, known as sanitary landfill leachates [2]. The contaminants present in the leachates are of both organic and inorganic nature; some

Extraction and Characterization of AR
The AR, aged 8-10 years, was obtained from the landfill in the city of Tuxtla Gutiérrez, located in the southeast of Mexico. The geographic location of the site (Latitude 16 • 40 7.57 N and Longitude 93 • 11 47.36 W) exposes it to a rainfall and annual temperature of 954.5 mm and 25.8 • C, respectively. To prevent contamination from the cover material, the AR sample was taken at a depth of 1.5 m. A fraction of the extracted material was characterized on a wet basis (WB) within 24 h after the excavation: volumetric weight and the composition (rigid plastics, bags, fine materials, and others) were determined. The rest of the extracted material was spread on tarpaulins and large-volume material (cloth, glass, cardboard, stones, iron, plastics, among others) was removed and discarded. The AR remained extended in the shade and at room temperature for 3 weeks, in order to remove enough moisture to facilitate handling and characterization. During this drying period, the AR was determined through pH, moisture content, and volatile solids. Once dry, the material was subjected to a granulometric analysis, separating it into different particle sizes: >40 mm, between 40 and 15 mm, and <15 mm. In the same way, the volumetric weight and its composition were determined.

Aged Refuse Filled Bioreactors (ARFBs)
The treatability tests were conducted in a system consisting of two ARFBs arranged in series ( Figure 1) and of identical dimensions (Ø = 20 cm, h = 230 cm). The 20 cm lower part of each bioreactor was packed with support material (crushed gravel, Ø = 2.5 cm) and 200 cm of packing material (AR of Ø ≤ 40 mm) was placed immediately on top. A 10 cm headspace was left in the upper part of each ARFB. In order to avoid the loss of fine AR, a "75% shade mesh" membrane was arranged between the support material and the packaging material. 93°11′47.36" W) exposes it to a rainfall and annual temperature of 954.5 mm and 25.8 °C, respectively. To prevent contamination from the cover material, the AR sample was taken at a depth of 1.5 m. A fraction of the extracted material was characterized on a wet basis (WB) within 24 h after the excavation: volumetric weight and the composition (rigid plastics, bags, fine materials, and others) were determined. The rest of the extracted material was spread on tarpaulins and large-volume material (cloth, glass, cardboard, stones, iron, plastics, among others) was removed and discarded. The AR remained extended in the shade and at room temperature for 3 weeks, in order to remove enough moisture to facilitate handling and characterization. During this drying period, the AR was determined through pH, moisture content, and volatile solids. Once dry, the material was subjected to a granulometric analysis, separating it into different particle sizes: >40 mm, between 40 and 15 mm, and <15 mm. In the same way, the volumetric weight and its composition were determined.

Aged Refuse Filled Bioreactors (ARFBs)
The treatability tests were conducted in a system consisting of two ARFBs arranged in series ( Figure 1) and of identical dimensions (Ø = 20 cm, h = 230 cm). The 20 cm lower part of each bioreactor was packed with support material (crushed gravel, Ø = 2.5 cm) and 200 cm of packing material (AR of Ø ≤ 40 mm) was placed immediately on top. A 10 cm headspace was left in the upper part of each ARFB. In order to avoid the loss of fine AR, a "75% shade mesh" membrane was arranged between the support material and the packaging material.

Sampling and Leachate Characterization
The leachate sample was collected from the young leachate evaporation pond of the sanitary landfill and kept refrigerated at 4 • C until use. The sample was characterized with the parameters Sustainability 2019, 11, 3193 4 of 16 of chemical oxygen demand (COD), biochemical oxygen demand (BOD), color, alkalinity, pH, total suspended solids (TSS), total nitrogen (TN), ammoniacal nitrogen (N-NH 3 ), total phosphorus (TP), and heavy metals (Cd, Ni, Cr, Pb, and Zn). The COD was quantified using the closed reflux micromethod, digesting the sample at 150 • C for 2 h, and subsequently read on a HACH DR-5000 spectrophotometer at 620 nm. The BOD was determined by quantifying the difference between the initial dissolved oxygen concentration and the concentration after five days of incubation at 20 ± 1 • C. Color determination was performed with a HACH DR/890 colorimeter, while alkalinity was determined through the volumetric method, titrating with 0.02 N sulfuric acid. All the analyses were conducted according to standardized methods [19] adapted to the particularities of leachates. Heavy metals were determined using atomic absorption spectrophotometry. In the case of Cd, Cr, Ni, and Pb, a graphite furnace (EPA, Method 7010a) was used, while Zn determination was carried out with flame atomic absorption spectrophotometry (EPA, Method 7000B).

AR Microbiological Characterization
Microbiological characterization of the aerobic fraction was conducted at the species level. For this purpose, samples were taken from the interior of the bioreactor at three different heights (0.5, 1.0, and 1.5 m). The extracted samples were seeded in nutritious agar; later, microscopic observation was carried out after applying Gram staining. DNA polymerase extraction was performed using the cell lysis-phenol-chloroform method, followed by polymerase chain reaction (PCR). The purification and sequencing was performed by MACROGEN Co, located in Seoul, Korea, on 16S rRNA with comparison to the GenBank database.

Operation and Monitoring of the ARFB System
The system was operated for eight months in the shade and at room temperature (25.8 ± 5.9). During this period, four hydraulic loads (HLs) (10,20,35, and 50 L/m 3 ·d) were evaluated, each with an operating time of two months. The bioreactors were spray-fed three times a day (9:00, 13:00 and 17:00 h), for 15 min each time, during which the volume corresponding to the HL evaluated was supplied. The parameters monitored during the period of operation of each HL were COD, color, BOD 5 , TSS, TN, N-NH 3 , and TP, estimating the removal percentage of each parameter through Equation (1). Additionally, pH, alkalinity, and five heavy metal concentrations (Cd, Ni, Cr, Pb, and Zn) were determined. All the analyses were carried out following standardized methods [19].
where C i is the initial concentration and C f is the final concentration

Analysis of Experimental Data
The results obtained were statistically analyzed by one-way analysis of variance (ANOVA), using a significance level of α = 0.05, with the Statistica 7.0 software. Before performing the ANOVA, it was verified that the data fulfilled the statistical assumptions. Table 1 shows the main byproducts found in the excavated materials, as well as the particle size distribution of the usable fraction (fine fraction). From Table 1, it is observed that in both WB and dry basis (DB), at least 10% are rigid plastics, higher than the 4.19% reported by Zhao et al. [20]. Similarly, nylon bags represent an important percentage (11.1%) of DB waste. Despite their high presence, both byproducts were easily separated. According to Zhao et al. [21], within the materials with 10 years of being disposed in a sanitary landfill, the usable fraction is one that has a particle size of up to 40 mm. In Table 1, it can be seen that, in our case, approximately 85% had a particle size ≤ 40 mm.

Excavated Material Characterization
AR characterization during the drying process allowed us to observe that the average vs. value remained at 19.2% because the materials had reached their highest degree of degradation. On the other hand, the pH presented an average value of 7.54, similar to that reported by Ziyang et al. [22]. These values above neutrality can be explained due to the type of leachate (mature) that percolates through the AR.

Characterization of Leachate in the Influent
The results of the characterization of the leachates used to feed the ARFB system are presented in Table 2, from which an BI of 0.07, pH values higher than 7 (8.58), and heavy metal concentrations are observed, which are mostly no more than 2 mg/L, characteristic of old leachates [1,23,24]. Considering that the samples were taken from a lagoon destined to accommodate fresh leachates, these results reveal that liquids of different ages are mixed in this lagoon. Thus, the influent used in the ARFB system had, in general, a low biodegradability and can be classified as a mature leachate.

Bacterial Characterization in the Packing Material
According to the bacterial analysis performed on the packaging material of the bioreactors, the identified bacterial genera were 34.5% Bacillus, 13.8% Pseudomonas, 10.3% Acinetobacter, 6.9% Lysinibacillus, and 3.4% Desulfovibrio and Enterobacter in total samples. Bacillus sp. and Bacillus cereus were the most abundant species, followed by Acinetobacter sp., Bacillus thuringiensis, Pseudomonas sp., aeruginosa, and mosselii, Lysinibacillus sp. and sphaericus, Desulfovibrio vulgaris and Enterobacter sp. Some of them specialized in the degradation and mineralization of organic compounds. Table 3 shows the main species related to the removal of macronutrients and other reduction processes, for example, Pseudomonas putida and Pseudomonas aeruginosa participate in nitrification processes, whereas Lysinibacillus sphaericus, Pseudomonas putida, and Desulfovibrio vulgaris are attributed the greatest activity in phosphorus removal. On the other hand, according to the literature [25][26][27], other identified species, such as Pseudomonas aeruginosa, Lysinibacillus fusiformis, Desulfovibrio vulgaris, and Pseudomonas putida, are capable of reducing Cr (VI) to Cr (III). These results reveal the bacterial varieties coexisting in the AR that can promote diverse processes of contaminant removal, such as metal and phosphate nitrification, denitrification, and reduction.

pH Behavior
The ARFB system was tested with four HLs: 10, 20, 35, and 50 L/m 3 ·d. Figure 2 shows that the pH values in the influent were around 8.6, highlighting the records close to 9.5 obtained during the feeding period at 20 L/m 3 ·d; for effluents 1 (E1) and 2 (E2) the pH value decreased to 8.1 on average. These slight reductions are related to biological degradation processes, such as the nitrification processes of organic pollutants that contribute to acidifying the environment. Another reason for the decrease in pH may be related to the hydrolysis of the humus present in the leachates and the AR [34]. Figure 3, shows that the color values in the influent were between 7150 and 13,950, and the final effluent between 655 and 2175 Pt-Co units. In general, the color of the influent was black and that of the effluent of the system was light yellow without odor. Regarding the removals achieved by the first bioreactor, similar behaviors are observed in the HL of 10, 35 and 50 L/m 3 ·d. According to the analysis of variance (p < 0.05), with the HL of 20 L/m 3 ·d, a significant difference is observed with respect to the other three HLs. This can be attributed to the high pH values of the influent (Figure 2), that moved away from the favorable interval (pH 6-8), hindering the biological growth before a greater presence of the ammonia at pH above 8 [35][36][37]. Considering the global removal, the ARFB system showed a removal between 84 and 88% for the different HLs tested, and according to the ANOVA (p < 0.05) there were no significant differences between them, despite the variations in organic load and pH in the influent, which shows that a second bioreactor has the ability to compensate for shock effects that may occur in the first bioreactor. Color removals greater than 80% were also reported by Li et al. [11] and Hassan and Xie [37], who worked with ARFB systems. Regarding the COD parameter, from Figure 4, the concentration in the influent ranged between 3874 and 6486 mg/L, and in the effluent (E2) between 575 and 1277 mg/L. The average removals achieved in E1, E2, and the total removal considering the higher HL (50 L/m 3 .d), were 79.7, 26.1, and 85%, respectively, which indicates that the greatest removal was achieved by the first bioreactor. However, as happened with the color parameter, low efficiencies in the first bioreactor were obtained in the HL of 20 L/m 3 .d, which can be attributed to the fact that the leachate of the influent was fed with pH values > 9. The works of Li et al. [10] and Li et al. [11] have documented similar behaviors in the removal of COD when using ARFB systems. Figure 5 shows the behavior profile of the biodegradable fraction, measured as BOD5, removed by the ARFB system. This parameter, like alkalinity, was monitored on a monthly basis. Figure 5 shows that there is an inverse relationship between the percentage of removal of BOD5 with respect to the HL fed, where the percentage of removal of the BOD5 ranged between 85.6 and 98.9%. It is also observed that the behavior profile of alkalinity, under the four HLs studied, is similar to the behavior of BOD5, which can be explained by the production of intermediate H + generated during the degradation processes of organic matter, which reacts with the species responsible for alkalinity. The relationship observed between the BOD5 and the alkalinity in the effluents of the ARFB systems makes it possible to quickly estimate the BOD5 value based on the determined alkalinity value. Regarding the COD parameter, from Figure 4, the concentration in the influent ranged between 3874 and 6486 mg/L, and in the effluent (E2) between 575 and 1277 mg/L. The average removals achieved in E1, E2, and the total removal considering the higher HL (50 L/m 3 ·d), were 79.7, 26.1, and 85%, respectively, which indicates that the greatest removal was achieved by the first bioreactor. However, as happened with the color parameter, low efficiencies in the first bioreactor were obtained in the HL of 20 L/m 3 ·d, which can be attributed to the fact that the leachate of the influent was fed with pH values > 9. The works of Li et al. [10] and Li et al. [11] have documented similar behaviors in the removal of COD when using ARFB systems. Figure 5 shows the behavior profile of the biodegradable fraction, measured as BOD 5 , removed by the ARFB system. This parameter, like alkalinity, was monitored on a monthly basis. Figure 5 shows that there is an inverse relationship between the percentage of removal of BOD 5 with respect to the HL fed, where the percentage of removal of the BOD 5 ranged between 85.6 and 98.9%. It is also observed that the behavior profile of alkalinity, under the four HLs studied, is similar to the behavior of BOD 5 , which can be explained by the production of intermediate H + generated during the degradation processes of organic matter, which reacts with the species responsible for alkalinity. The relationship observed between the BOD 5 and the alkalinity in the effluents of the ARFB systems makes it possible to quickly estimate the BOD 5 value based on the determined alkalinity value.

Behavior of the Biodegradability Index (BI)
An important relation in the monitoring of biological systems is the biodegradability index (BI). Figure 6 shows that the average initial BI (BI0) of the ARFB system evaluated was 0.07, which suggests that the leachate to be treated is low in biodegradable compounds and high in refractory organic compounds, such as humic substances [14]. According to Table 4, most of the studies in ARFB systems report BI0 between 0.2 and 0.7 (leachates from intermediate to young) and with final BI between 0.03 and 0.05. In the present study, the average BI0 was lower (0.07), even than the one reported by Xie et al. [17]. Despite this, removal efficiencies in biodegradable organic matter were high (> 85%), with an average final BI of 0.03. These results show an additional advantage of the ARFB systems, which can also be efficient in leachates with very low biodegradability, which is not the case with other biological systems.

Behavior of the Biodegradability Index (BI)
An important relation in the monitoring of biological systems is the biodegradability index (BI). Figure 6 shows that the average initial BI (BI 0 ) of the ARFB system evaluated was 0.07, which suggests that the leachate to be treated is low in biodegradable compounds and high in refractory organic compounds, such as humic substances [14]. According to Table 4, most of the studies in ARFB systems report BI 0 between 0.2 and 0.7 (leachates from intermediate to young) and with final BI between 0.03 and 0.05. In the present study, the average BI 0 was lower (0.07), even than the one reported by Xie et al. [17]. Despite this, removal efficiencies in biodegradable organic matter were high (>85%), with an average final BI of 0.03. These results show an additional advantage of the ARFB systems, which can also be efficient in leachates with very low biodegradability, which is not the case with other biological systems.   Figure 7 shows the behavior profile of the total phosphorus removal (TP) efficiency in the raw leachates reached by the system under study. As shown in this figure, the ARFB system achieved a total average removal of 99%, independent of the fluctuations of the TP concentration present in the influent (4.9-11.2 mg/L TP). It can also be seen that, despite HL feed increase, TP removal rates greater than 98% are reached in the first reactor alone. These rates are higher than the 96% reported in other studies [12,17]. The high TP removal rate can be explained by the presence of microbial consortiums, such as Pseudomonas putida, Desulfovibrio vulgaris, and Lysinibacillus sphaericus, found in the AR used in the present study.  Figure 7 shows the behavior profile of the total phosphorus removal (TP) efficiency in the raw leachates reached by the system under study. As shown in this figure, the ARFB system achieved a total average removal of 99%, independent of the fluctuations of the TP concentration present in the influent (4.9-11.2 mg/L TP). It can also be seen that, despite HL feed increase, TP removal rates greater than 98% are reached in the first reactor alone. These rates are higher than the 96% reported in other studies [12,17]. The high TP removal rate can be explained by the presence of microbial consortiums, such as Pseudomonas putida, Desulfovibrio vulgaris, and Lysinibacillus sphaericus, found in the AR used in the present study.  Figure 7 shows the behavior profile of the total phosphorus removal (TP) efficiency in the raw leachates reached by the system under study. As shown in this figure, the ARFB system achieved a total average removal of 99%, independent of the fluctuations of the TP concentration present in the influent (4.9-11.2 mg/L TP). It can also be seen that, despite HL feed increase, TP removal rates greater than 98% are reached in the first reactor alone. These rates are higher than the 96% reported in other studies [12,17]. The high TP removal rate can be explained by the presence of microbial consortiums, such as Pseudomonas putida, Desulfovibrio vulgaris, and Lysinibacillus sphaericus, found in the AR used in the present study.

Removal of TN and N-NH 3 .
Ammonia concentrations in the influent ranged between 297 and 1073 mg/L (Figure 8), while, in the case of effluents E1 and E2, it ranged between 9 and 31 mg/L and 8 and 13 mg/L, respectively, corresponding to an average global removal rate of 97%. Again, the greater E1 effluent, the greater contaminant removal rate (95%) is achieved in the E1 effluent. According to Li et al. [11], ARFB systems in series show strong nitrification capacity, where ammonium is first adsorbed before being removed by nitrifying microbial populations. This synergy allows the restoration of the adsorption capacity of the AR [38], which explains the stability of the ARFB system in series in the removal processes for long periods of operation. Ammonia concentrations in the influent ranged between 297 and 1073 mg/L (Figure 8), while, in the case of effluents E1 and E2, it ranged between 9 and 31 mg/L and 8 and 13 mg/L, respectively, corresponding to an average global removal rate of 97%. Again, the greater E1 effluent, the greater contaminant removal rate (95%) is achieved in the E1 effluent. According to Li et al. [11], ARFB systems in series show strong nitrification capacity, where ammonium is first adsorbed before being removed by nitrifying microbial populations. This synergy allows the restoration of the adsorption capacity of the AR [38], which explains the stability of the ARFB system in series in the removal processes for long periods of operation.  Figure 9 shows the elimination of total nitrogen (TN) in the raw leachates, where an average total removal of 97% was observed, independently of the fluctuations of the influent (377-1104 mg/L). According to Chen et al. [39], TN removal rate is favored when a source of high organic carbon is available. Regarding the present study, the high efficiencies achieved in TN removal rates can be attributed to the good COD/TN ratio, approximately 6.7, compared to other studies where this relationship was ≤2 and with lower TN removal rates (49-63%) [11,12]. Moreover, the presence of microbial consortiums, such as Pseudomonas putida and Pseudomonas aeruginosa, found in the AR of the present study can also explain the high removals achieved, as they relate to nitrification and denitrification processes [25,27,30].  Figure 9 shows the elimination of total nitrogen (TN) in the raw leachates, where an average total removal of 97% was observed, independently of the fluctuations of the influent (377-1104 mg/L). According to Chen et al. [39], TN removal rate is favored when a source of high organic carbon is available. Regarding the present study, the high efficiencies achieved in TN removal rates can be attributed to the good COD/TN ratio, approximately 6.7, compared to other studies where this relationship was ≤2 and with lower TN removal rates (49-63%) [11,12]. Moreover, the presence of microbial consortiums, such as Pseudomonas putida and Pseudomonas aeruginosa, found in the AR of the present study can also explain the high removals achieved, as they relate to nitrification and denitrification processes [25,27,30].

Physicochemical Quality of the Final Effluent
The physicochemical parameters contemplated in the Mexican regulations (NOM-001- SEMARNAT-1996) [40], such as BOD5, TSS, TP, and TN concentration, were all foundto be below the maximum permissible limits in the final effluent (Table 5). In order to test a higher HL, an additional experiment was performed with an HL of 100 L/m 3 .d and compared with the initial HLs (10,20,35, and 50 L/m 3 .d), monitoring the color and COD parameters. The results shown in Figure 10 show that for the initial HL, the removal efficiencies in both parameters were stable and higher than 80%, with no significant differences between them according to ANOVA. However, for the HL of 100 L/m 3 .d, the efficiency of the system decreased significantly, by up to 14 and 38% in COD and color removal, respectively.

Physicochemical Quality of the Final Effluent
The physicochemical parameters contemplated in the Mexican regulations (NOM-001-SEMARNAT-1996) [40], such as BOD 5 , TSS, TP, and TN concentration, were all foundto be below the maximum permissible limits in the final effluent (Table 5). In order to test a higher HL, an additional experiment was performed with an HL of 100 L/m 3 ·d and compared with the initial HLs (10,20,35, and 50 L/m 3 ·d), monitoring the color and COD parameters. The results shown in Figure 10 show that for the initial HL, the removal efficiencies in both parameters were stable and higher than 80%, with no significant differences between them according to ANOVA. However, for the HL of 100 L/m 3 ·d, the efficiency of the system decreased significantly, by up to 14 and 38% in COD and color removal, respectively. Sustainability 2019, 11, x FOR PEER REVIEW 13 of 16

Heavy Metal Removal
Elements such as Cd, Ni, Cr, Pb, and Zn were determined in the study and the values found are shown in Table 6. The concentration in the influent was low, typical of a mature leachate (<2 mg/L), and the values found in E1 and E2 varied very little with respect to the initial concentration, and elements such as Cd and Zn were below the maximum permissible limits according to Mexican regulations (NOM-001-SEMARNAT-1996) [40], except for Cr in the E2 stream and HL of 20 L/m 3 .d, (0.79 mg/L) which was slightly above the value of 0.5 mg/L set by the standard. According to Wang et al. [41], in ARFB systems for the treatment of wastewater, the Cr can be removed by processes of adsorption and reducing bacterial action from Cr (VI) to Cr (III); this observed slight increase could be due to a lower adsorption capacity of the system.

Heavy Metal Removal
Elements such as Cd, Ni, Cr, Pb, and Zn were determined in the study and the values found are shown in Table 6. The concentration in the influent was low, typical of a mature leachate (<2 mg/L), and the values found in E1 and E2 varied very little with respect to the initial concentration, and elements such as Cd and Zn were below the maximum permissible limits according to Mexican regulations (NOM-001-SEMARNAT-1996) [40], except for Cr in the E2 stream and HL of 20 L/m 3 ·d, (0.79 mg/L) which was slightly above the value of 0.5 mg/L set by the standard. According to Wang et al. [41], in ARFB systems for the treatment of wastewater, the Cr can be removed by processes of adsorption and reducing bacterial action from Cr (VI) to Cr (III); this observed slight increase could be due to a lower adsorption capacity of the system. On the other hand, although the oxidation states of Cr were not quantified, reduction processes in the system could take place, considering that species such as Bacillus thuringiensis, Desulfovibrio culgaris, Lysinibacillus fusiformis, Pseudomonas aeruginosa, and Streptomyces sp. were identified inside the reactor, which are able to reduce Cr [25][26][27]42].

Conclusions
The analysis of the obtained results demonstrates the ARFB system as a promising alternative for the treatment of leachates of very low biodegradability (BI = 0.07). This paper represents one of the first successful studies of a biological process used in the treatment of leachates that are difficult to degrade.
According to the ANOVA test performed on the obtained results, no significant differences were observed between the treatments. For example, for the highest HL tested (50 L/m 3 ·d), the ARFB system showed average COD, BOD 5 , and color removals of 85, 86.1, and 87.9%, respectively.
Other parameters, such as TP, TN, and N-NH 3 , were also evaluated, reaching average removals of 98.6, 97.8, and 97.4%, respectively. The high efficiencies in TP were related to the presence of microbial consortiums such as Pseudomonas putida, Desulfovibrio vulgaris, and Lysinibacillus sphaericus within the bioreactors; while for TN, species identified as Pseudomonas putida and Pseudomonas aeruginosa, in addition to the high COD/TN ratio (6.7), favored their removal.
The physicochemical quality (BOD 5 , TSS, TN, and TP) of the final effluent complies with the Mexican regulations (NOM-001- SEMARNAT-1996). This is also the case for the heavy metals evaluated (Cd, Ni, Cr, Pb, and Zn), but not for Cr in one of the HLs tested.
On the other hand, the ARFB system in series proved to be stable against possible shock loads, such as the fluctuations in organic load in the influent (COD from 3874 to 6485 mg/L and color from 7150 to 13,950 Pt-Co units), or variations in pH (6.25 to 9.55).
Thus, these results encourage further testing with compounds that are refractory to conventional biological processes, and, in general, with low biodegradability wastewater.
Finally, considering the little attention that has been given in Latin America to the treatment of leachates from sanitary landfills, the use of AR for the treatment of these liquids provides a window of opportunity for the revaluation of MSW disposed in landfills or any other place of final disposal whose age exceeds at least 8 years.