Waste Management xxx (2014) xxx–xxx
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The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process Gabriel Timm Müller a, Alexandre Giacobbo b, Edson Abel dos Santos Chiaramonte a, Marco Antônio Siqueira Rodrigues c, Alvaro Meneguzzi b, Andréa Moura Bernardes b,⇑ a b c
Universidade Estadual do Rio Grande do Sul (UERGS), R. Gal. João Manoel, 50, CEP 90010-030 Porto Alegre, RS, Brazil Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonçalves, 9500, Setor 4, Prédio 74, CEP 91501-970 Porto Alegre, RS, Brazil Universidade FEEVALE, ICET, RS 239, 2755, CEP 93352-000 Novo Hamburgo, RS, Brazil
a r t i c l e
i n f o
Article history: Received 14 July 2014 Accepted 28 October 2014 Available online xxxx Keywords: Sanitary landfill leachate Recalcitrant leachate treatment Effluent degradation Photoelectrooxidation
a b s t r a c t The sanitary landfill leachate is a dark liquid, of highly variable composition, with recalcitrant features that hamper conventional biological treatment. The physical–chemical characteristics of the leachate along the landfill aging, as well as their effects on the efficiency of the conventional treatment, were evaluated at this paper. The feasibility of photoelectrooxidation process as an alternative technique for treatment of landfill leachates was also determined. Photoelectrooxidation experiments were conducted in a bench-scale reactor. Analysis of the raw leachate revealed many critical parameters demonstrating that the recalcitrance of leachate tends to increase with time, directly influencing the decline in efficiency of the conventional treatment currently employed. The effects of current density and lamp power were investigated. Using a 400 W power lamp and a current density of 31.5 mA cm2, 53% and 61% efficiency for the removal of ammoniacal nitrogen and chemical oxygen demand were respectively achieved by applying photoelectrooxidation process. With the removal of these pollutants, downstream biological treatment should be improved. These results demonstrate that photoelectrooxidation is a feasible technique for the treatment of sanitary landfill leachate, even considering this effluent’s high resistance to treatment. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The sanitary landfill leachate is a dark liquid of highly variable composition with recalcitrant features, generated by urban waste degradation. The conditions for leachate formation and its composition are extremely complex and variable, and depend on a number of factors, such as environmental conditions, waste characteristics, the operational peculiarities of the landfill and, especially, the dynamics of the decomposition processes that take place within the landfill cells (Christensen et al., 2001). Decomposition processes vary depending on the age of the landfill and can be classified into four phases: aerobic, anaerobic, initial methanogenic and stabilization (Kjeldsen et al., 2002; Renou et al., 2008). These phases can be identified based on the characteristics of the leachate, such as concentration of organic matter, nitrogen and pH, and are important for evaluating and planning treatment systems.
⇑ Corresponding author. Tel.: +55 5133089428; fax: +55 5133089427. E-mail address: [email protected] (A.M. Bernardes).
A standardized, technically and economically viable methodology for the effective treatment of sanitary landfill leachate, which could be readily applied to all leachate types, has yet to be devised. The options that are available are generally similar to those used in the treatment of sewage, and involve physical, chemical and biological processes. Primarily due to their low costs, biological processes, in the form of aerobic, anaerobic, and facultative systems, remain the most widely implemented type of process. Experiments with activated sludge, biological filters, anaerobic reactors, and aerated, anaerobic and facultative ponds have provided satisfactory treatment results (Aziz et al., 2011; Bashir et al., 2013; Wang et al., 2011). However, they were all performed on leachate from young landfills, which is easily biodegradable. Studies described by Kjeldsen et al. (2002) demonstrate that the leachate from young landfills generally contains substances with low molar mass, primarily in the form of volatile fatty acids, which are biologically degradable. Nevertheless, it is known that the capacity of microorganisms to degrade certain organic compounds is limited (de Morais and Zamora, 2005; Renou et al., 2008). In fact, as leachate ages, its composition becomes less biodegradable, as it predominantly
http://dx.doi.org/10.1016/j.wasman.2014.10.024 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
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contains substances with low molar masses and very complex structures, such as humic substances (Kjeldsen et al., 2002). Furthermore, the increase in the concentration of ammoniacal nitrogen and the strong dark color inhibit proliferation of microorganisms, meaning that conventional biological treatment is not successful. Therefore, because of the problems associated with biological systems, there is a growing need for processes that offer better treatment efficiency or which are capable of integration with conventional processes, overcoming their disadvantages and improving treatment efficiency. In this context, Advanced Oxidation Processes (AOPs) can be an alternative, mainly because of their characteristics such as high decomposition capacity and rate. AOPs are defined as processes that involve the generation of transitory species with high oxidative power, the most notable of which is the hydroxyl radical (HO). This radical has a high oxidative power (E° HO/HO +2.8 V at 25 °C) and low selectivity, allowing the degradation of a large number of toxic contaminants in a relatively short time (da Silva et al., 2014). Some AOPs have been already studied for landfill leachate treatment (de Morais and Zamora, 2005; Tauchert et al., 2006; Turro et al., 2011; Zhao et al., 2010). All these researches evaluate the treatment for leachates containing COD from 560 to 5200 mg L1 and ammoniacal nitrogen from 14 to 1100 mg L1. One technique that has been widely studied for the treatment of complex effluents is photoelectrooxidation (Bertoldi et al., 2012; Catanho et al., 2006; da Silva et al., 2014; Molina et al., 2013; Rodrigues et al., 2008; Siqueira et al., 2011; Xavier et al., 2011), which is a combination of electrochemical process and heterogeneous photocatalysis. Photoelectrooxidation (PEO) consists of percolating the effluent to be treated through an electrolytic reactor in which the anode, plated with metallic oxides, is irradiated with ultraviolet (UV) light. This technology has demonstrated an impressive synergic effect leading to increases on the rate and efficiency of decomposition, when compared with the contribution of individual processes (Moraes and Bertazzoli, 2005; Zhao et al., 2010). Three basic mechanisms drive the oxidation of organic compounds by the HO: proton abstraction, electron transfer and radical addition. These mechanisms are represented by RH, RX e PhX as described by Eqs. (1)–(3), respectively.
HO þ RH ! R þ H2 O
ð1Þ
HO þ RX ! RX þ OH
ð2Þ
HO þ PhX ! OHPhX
monthly activity reports provided by the company responsible for managing the landfill were evaluated in order to assess the situation of the landfill, as well as to determine the quality of the treatment used. Subsequently, a homogeneous aliquot of 40 L of raw leachate was collected directly from the equalization tank and kept refrigerated at 4 °C until the experiments were performed. Cyclic voltammetry was run in a Potentiostat (EG&G Princeton Applied Research, model 273A), using an acrylic cell, containing 300 mL of raw leachate, in order to determine the range of current density to be applied to oxidize the organic matter. The working electrode, with an area of 1.5 cm2, was titanium coated with 70TiO2/30RuO2; the counter electrode was platinum and the reference electrode was calomel. Potential was ranged from 2 to 2 V, returning to 2 V, with a scan rate of 5 mV s1 and without addition of supporting electrolytes. Subsequently, experiments to evaluate the effectiveness of the PEO process as a leachate treatment method were conducted. They were performed on 1.3 L batches in a photoelectrochemical benchscale reactor. A complete mixing was assured by a magnetic stirrer. The reactor consisted of a 3 L jacket glass cylinder, with cooling solution that was circulated by an ultra-thermostatic bath. The reactor also included a pair of electrodes and mercury vapor lamps (AvantÒ) and was powered by an electrical current. The schematic in Fig. 1 illustrates the photoreactor used in the experiments. Since the lamp would be immersed in the solution, it had to be placed inside a protective tube. However, since glass blocks UV radiation, a tube made of quartz was used. This material allows radiation to go through it and prevents the solution from coming into direct contact with the interior of the lamp. For optimizing the use of the radiation emitted, the electrodes were arranged concentrically around the lamp (da Silva et al., 2014). A pair of commercial electrodes, dimensionally stable (DSAÒ), manufactured by De Nora do BrasilÒ were used; the anode had a surface area of 475.2 cm2 and was made of titanium coated with a mixture of titanium and ruthenium oxides in the proportion of 70TiO2/ 30RuO2, while the cathode had a surface area of 118 cm2 and was made of titanium oxide. The gap between quartz tube and anode, as well as the one between electrodes was 0.5 cm.
ð3Þ
Basically, carbon dioxide, organic acids and inorganic ions are the final products generated by these reactions (da Silva et al., 2014). The main objectives of this research were to study the evolution of the physical and chemical characteristics of the leachate with respect to landfill age, to evaluate the efficiency of the conventional treatment employed, and to evaluate the potential of the photoelectrooxidation process as a treatment system for sanitary landfill leachate. A leachate with recalcitrant features, containing more than 1700 mg L1 ammoniacal nitrogen and 6500 mg L1 COD, was studied. Although the use of advanced oxidation processes has been recently studied to the treatment of leachates, the behavior of a leachate with the characteristics presented here has not been sufficiently studied before. 2. Experiments This work was performed using samples of leachate from a sanitary landfill located in Southern Brazil. This sanitary landfill receives around 2000 tons of waste per day. Initially, data from
Fig. 1. Batch photoreactor used in the photoelectrooxidation processes: (1) glass reservoir with an entrance and exit for the refrigeration fluid, (2) anode, (3) cathode, (4) quartz tube and (5) mercury vapor lamp.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx
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Table 1 Characteristics of the raw leachate sample collected from the sanitary landfill. Parameter
Raw leachate
COD (mg L1) BOD5 (mg L1) TKN (mg L1) NH3AN (mg L1) Total solids (mg L1) Conductivity (mS cm1) pH Temperature (°C)
6560 ± 226 3326 ± 565 2021 ± 97 1740 ± 83 14013 ± 258 30.1 ± 0.1 8.2 ± 0.1 22 ± 1
Three hour experiments were conducted for treatment process optimization, and the following variables were evaluated: current density and lamp power. Temperature was kept below 50 °C in order to avoid polymerization of intermediate compounds (Arslan et al., 2005). Initially the lamp power was examined (125, 250 and 400 W), using a current density of 31.5 mA cm2. After that, using a 250 W lamp, three different current densities, 10.5, 21 and 31.5 mA cm2, selected by means of cyclic voltammetry assay, were tested. Duo to the high conductivity of the raw leachate, 30 mS cm1 (see Table 1), no supporting electrolytes were added to PEO experiments. To characterize the leachate and to evaluate the efficiency of the treatment systems, the followed parameters were analyzed: total solids (TS), chemical oxygen demand (COD), five-day biological oxygen demand (BOD5), ammoniacal nitrogen (NH3AN), total Kjeldahl nitrogen (TKN), color, alkalinity, hardness, conductivity and pH. All the analyzes were performed according to the Standard Methods (Eaton and Franson, 2005). According to IUPAC guidelines, the removal of organic contaminants through advanced oxidation processes can be often described phenomenologically by simple rate expressions that are either zero-order or first-order (Bolton et al., 2001), being represented by Eqs. (4) and (5), respectively.
C ¼ C0 k t ln
C C0
¼ k t
ð4Þ ð5Þ
The reaction kinetics were calculated based on the COD, where t is the exposure time; k is the kinetic constant; C0 is the COD of initial effluent and C is the COD of final effluent, in mg L1. Electric energy per mass, EEM (kW h kg1), was calculated as described by Bolton et al. (2001). EEM is defined as the electric energy in kilowatt–hour (kW h) required to degrade a kilogram of a specific pollutant in contaminated water or air, as described by Eq. (6):
EEM ¼
P t 106 VðC 0 CÞ
Fig. 2. Characteristics of the landfill leachate over the years: (a) TS, alkalinity, COD and BOD5; (b) color, hardness, TKN and NH3AN. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
large increase, however, the biodegradability, i.e., the BOD/COD ratio was declining over the years, as shown in Fig. 3. Nitrogen is also an important parameter, especially considering the different oxidation states that it can reach. Furthermore, the combination of high nitrogen, COD and alkalinity values presented by the effluent can be toxic to biological treatment processes (de Morais, 2005). Additionally, the increase in color is another strong indication of the tendency towards landfill stabilization, and consequently, the accumulation of humic and fulvic substances (Tauchert et al., 2006).
3.1. Treatment efficiencies achieved by the leachate treatment plant at the sanitary landfill The biological treatment plant currently operating at the sanitary landfill comprises two anaerobic filters that are operating
ð6Þ
where P is the rated power (kW), V is the volume (L) of treated water in the time t (h) and the pollutant concentration is given in mg L1, while the factor 106 converts mg to kg. 3. Results The data collected from the landfill managing company are presented in Fig. 2. Over the years, there has been a significant increase for all monitored parameters. Some of these deserve greater emphasis: COD, BOD5, TKN, NH3AN, color and alkalinity. The parameters related to the concentration of organic matter, COD and BOD5, showed a
Fig. 3. BOD5/COD ratio of the leachate over the years.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
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in parallel, and a series of three facultative ponds. In the first year of operation, this plant presented high removal rates for all monitored parameters. Nevertheless, over the years, it has experienced a significant reduction in the removal efficiency of these parameters (Fig. 4). This can be attributed to the low effectiveness of biological processes in treating this particular effluent, especially after the fourth year of operation of the sanitary landfill. The efficiency of the biological treatment (anaerobic filters and facultative ponds) used by the effluent treatment plant can be seen to have declined from 88% to 33% with respect to BOD5; from 87% to 35% with respect to COD removal; from 94% to 25% for TKN removal; from 95% to 13% for ammoniacal nitrogen; and from 84% to 14% efficiency with respect to total solids. Regarding to color, as can be seen in Fig. 5, the biological treatment practically do not remove it, and in some cases even worse the leachate color. This significant loss in the efficiency of the conventional treatment sequence is directly related to the evolution of the characteristics of the leachate produced by the sanitary landfill (Fig. 2). As was described earlier, with the aging of the landfill, the characteristics of the leachate produced change drastically, and these changed properties reduce biodegradability, making biological treatment more difficult. There is a great deal of information in the literature (Abu Amr et al., 2013; Cai et al., 2014; Cortez et al., 2011; Erkan and Apaydin, 2014; Turro et al., 2011; Venu et al., 2014) that can explain the low efficiency of biological treatment for leachates with high recalcitrant characteristics, which can be applied to this study. The recalcitrance of the leachate is also associated with the presence of humic substances, indicated by the strong dark
coloration (Fig. 5). According to Kang et al. (2002), humic substances make up more than 60% of the organic matter present in mature leachates. These substances are macromolecules with very complex structures and intermediates (1000 g mol1, fulvic acids) or high (10000 g mol1, humic acids) molecular mass and, as a result of these characteristics, are not biodegradable and accumulate in landfills over time (El-Fadel et al., 2002). Another factor that interferes with biological treatment is the high concentration of ammoniacal nitrogen, which is toxic to the microorganisms. In the sixth year of operation of the sanitary landfill, the leachate reached high concentrations of COD and ammoniacal nitrogen, as huge as 8000 and 2000 mg L1, respectively. According to de Morais (2005), an effluent with an ammoniacal nitrogen concentration above 1000 mg L1, along with a COD greater than 2000 mg L1 and high alkalinity, is considered difficult to treat with biological processes, primarily due to the high toxicity of ammonia. In addition to the recalcitrant characteristics of the leachate, the low efficiency of the treatment may also be related to the low phosphorus concentrations in this effluent, since this element is an essential nutrient for the microorganisms and, therefore, should be available in the medium in sufficient concentrations to guarantee biomass growth. According to the literature (Metcalf and Eddy, 2003), the relative quantities of nitrogen and phosphorus required for biological processes should be at a N:P ratio of 5:1. Leachate characterization studies demonstrate that the nitrogen concentration is on average 30 times that of the phosphorus, and this may be the factor limiting the growth of microorganisms. 3.2. Photoelectrooxidation process
Fig. 4. Mean annual efficiency in reducing major physico-chemical parameters achieved by the biological treatment plant of the sanitary landfill leachate.
Fig. 5. Color evolution of the raw and biologically treated leachate over sanitary landfill operation time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 lists the results of the analyzes carried out on the raw leachate sample collected at the sanitary landfill. The characteristics of this leachate sample are consistent with the data from the characterization study described previously (Kang et al., 2002; Moraes and Bertazzoli, 2005), where high concentrations of organic matter, ammoniacal nitrogen and total solids were observed, in addition to the mildly basic pH, all of which further reinforce the recalcitrance prevision of this effluent for the conventional treatment. A cyclic voltammetry assay was carried out to determine the current density range that would oxidize the compounds present in the leachate. It was found that the most intense reactions occurred in the range from 8 to 35 mA cm2, as illustrated in Fig. 6. Therefore, intermediate densities were applied throughout all treatment tests.
Fig. 6. Cyclic voltammogram of raw leachate. Scan rate: 5 mV s1; working electrode: 70TiO2/30RuO2; counter electrode: platinum and; reference electrode: calomel.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
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G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx Table 2 COD removal and specific energy consumption in photoelectrooxidation experiments in the final treatment time (3 h). Experiment
Current density (mA cm2)
Potential (V)
Lamp power (W)
COD removal (%)
EEM (kw h kg1 COD)
PEO PEO PEO PEO PEO
31.5 31.5 31.5 21.0 10.5
1.37 1.37 1.37 0.96 0.8
125 250 400 250 250
37.7 46.1 57.3 31.2 19.8
136 206 258 293 451
1 2 3 4 5
Table 2 shows the effects of lamp power and current density on COD removal and specific energy consumption in PEO process treating a landfill leachate. One can verify that, at constant current density of 31.5 mA cm2, PEO 1–3, COD removal was directly dependent on lamp power and the greatest efficiency was obtained with the 400 W lamp, while the lowest one was achieved with the 125 W lamp. These results are in agreement with several studies that have evaluated this variable in treatment processes that use UV radiation, where usually an increase in degradation occurs when higher lamp powers are employed (Bauer et al., 1999; Li et al., 2002; Pera-Titus et al., 2004). Li et al.’s (2002) studies of humic acid treatment with PEO process demonstrated that the increase in UV radiation intensity produces greater and faster removal rates. According to Pera-Titus et al. (2004), as the intensity of the light increases, the quantity of photons emitted by the system into the solution also increases, meaning that more TiO2 particles are activated. Furthermore, direct photolysis also has an effect that arises from the interaction between photons and pollutant molecules. However, once the electrodes are already sufficiently illuminated, there are no more TiO2 molecules left to interact and, hence, no further effects occur. In PEO 2, 4 and 5, using a 250 W lamp and current densities of 31.5, 21 and 10.5 mA cm2, COD removal increases directly with the current density applied, which has a greater influence than lamp power; besides, the efficiency achieved applying a current density of 31.5 mA cm2 is significantly greater than the one with 10.5 mA cm2. These results are in line with the literature, which describes studies that have assessed the influence of current density on process efficiency, involving the application of electric power. In all of these studies, which used photoelectrooxidation for the treatment of humic acid (El-Fadel et al., 2002), pesticides (Quan et al., 2004), sanitary landfill leachate (Moraes and Bertazzoli, 2005), and nonylphenol ethoxylate (da Silva et al., 2014), the greatest degradation efficiencies were obtained with the highest current densities applied.
Fig. 7. Progression of COD removal efficiency over time with 400 W lamp and current density of 31.5 mA cm2.
Another important factor to be considered is the EEM. It provides the energy consumption associated with the performed process, enabling comparisons between the different investigated parameters, lamp power and current density, regarding to the COD removal. In PEO 1–3, at constant current density and different lamp powers, although the COD removal is higher with the increase in lamp power, the energy consumption tends also to linearly rise. On the other hand, in PEO 2, 4 and 5, using a 250 W lamp power and different current densities, the highest current density
Fig. 8. Reaction kinetics calculated based on the COD removal from the landfill leachate by PEO (a) pseudo first-order kinetic model; (b) first-order kinetic model and; (c) zero-order kinetic model. Conditions: C0 = 6560 mg L1, 400 W lamp power and current density of 31.5 mA cm2.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
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G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx Table 3 Leachate treated by photoelectrooxidation for 5 h, with a 400 W lamp power and current density of 31.5 mA cm2. Parameter 1
COD (mg L ) pH TKN (mg L1) NH3AN (mg L1) Temperature (°C)
Raw leachate
Treated leachate
Removal efficiency (%)
EEM (kW h kg1)
6560 ± 226 8.2 ± 0.1 2021 ± 97 1740 ± 83 22 ± 1
2558 ± 123 8.1 ± 0.1 909 ± 88 817 ± 64 43 ± 1
61 – 55 53 –
404 – 1454 1752 –
(31.5 mA cm2) promoted higher COD removal, 46.1%, and lower EEM, 206 kw h kg1 COD, i.e., the best energetic efficiency. These findings are in line with those from the literature, where is stated that photoelectrochemical processes present substantially lower specific energy consumption at higher current densities (Catanho et al., 2006). Once the experimental conditions yielding the maximum response for COD removal from the photoelectrooxidation process had been determined (specifically, a 400 W lamp with a current density of 31.5 mA cm2), a 5 h degradation experiment was performed, using the same reactor described earlier. 20 mL samples were collected every 1 h until a final time of 5 h for COD and apparent color analyzes. As can be seen in Fig. 7, after 5 h of treatment, the photoelectrooxidation process achieved a COD removal efficiency of 61%. However, it was noted that there is a tendency for the removal rates to stabilize as time of treatment increases, since the increase in treatment duration from 3 to 5 h resulted in a further increase of only 10% in removal efficiency, which is marginal when compared with the rate of 51% at 3 h. Fig. 8a–c, presents the reaction kinetics calculated based on COD removal, where ‘‘a’’ is the pseudo first-order model, ‘‘b’’ is the firstorder model and ‘‘c’’ is the zero-order model. Bolton et al. (2001) assert that, for high concentrations of contaminants, oxidation reactions in AOPs follow zero-order kinetic. Nonetheless, one can verify that the first-order kinetic fits better to the data than the zero ones. On the other hand, in the current work, the best fitting achieved to represent the COD removal in photoelectrooxidation reactions were the pseudo first-order kinetic model, with regression coefficients (R2) 0.99. The pseudo first-order kinetic model presents two distinct linear regions: Phase I, until 3 h, has a kinetic constant (k1) of 0.23 h1 and Phase II, from 3 to 5 h, has a kinetic constant (k2) of 0.1141 h1. Thus, Phase I presents reaction kinetic twice faster than the one from Phase II, what can justify the fact that 51% of COD removal was achieved at the first 3 h of PEO process, while an increase in efficiency of only 10% more was obtained in the following 2 h of treatment (Fig. 7). Other researchers also stated that photoelectrooxidation reactions follow the pseudo first-order kinetic (Nageswara Rao et al., 2009; Xiao et al., 2013). The final analysis of the characteristics of the treated leachate included, in addition to COD, the parameters pH, TKN, ammoniacal nitrogen (NH3AN) and temperature (see Table 3). Under these conditions, TKN and NH3AN removal were 55% and 53%, respectively. Regarding to energy consumption, after 5 h of treatment using a current density of 31.5 mA cm2 and a 400 W lamp power, the PEO of nitrogen compounds (TKN and NH3AN) consumes threefourfold more energy than the one for COD removal (Table 3). This behavior may be due to more complex heterocyclic molecules having CAN and C@N bonds which account for nitrogen compounds (Nageswara Rao et al., 2009). Fig. 9 shows the evolution of color of the leachate recorded at sampling intervals of 1 h, in the experiment with 5 h of treatment. In Fig. 9, ‘‘a’’ represents raw leachate and ‘‘f’’ is the sample after 5 h of treatment, one can noted a sharp reduction in apparent color of the leachate over treatment time by the PEO process. This data demonstrates that photoelectrochemical treatment resulted in significant changes in the leachate composition,
Fig. 9. Evolution of apparent discoloration of leachate with the treatment of 5 h. (a) Raw leachate, (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, (f) 5 h treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
through the removal of color, organic matter, and nitrogen. Although the efficiency of the process is not sufficient to make photoelectrooxidation process viable as a single-stage treatment, it can be considered an improvement, if one takes into consideration the complexity and recalcitrance of the effluent studied, and also when compared to the efficiency of the conventional treatment sequence employed at the sanitary landfill discussed above. Therefore, it can be concluded that photoelectrooxidation process is an efficient treatment method for complex wastes such as leachates with recalcitrant pollutants, and can be used as a supplementary treatment to complement other systems. It is also observed that the treatment studied here brought about a high removal of ammoniacal nitrogen and color, what should allow a downstream biological treatment with higher efficiency. 4. Conclusion The characteristics of the leachate studied here allow its classification as an intermediate leachate, and, during the period of study, major physical and chemical changes took place, demonstrating the implications of the age of sanitary landfills on leachate composition, including the reduction in the biodegradability index, the increase in concentration of ammoniacal nitrogen and color, in addition to a mildly basic pH, alkalinity and the presence of solids. The leachate tendency towards stabilization and a consequent reduction in biodegradability is observed in the reduction of the efficiency of conventional biological treatment as measured by the parameters COD, TKN, NH3AN and TS. This effect may primarily be related to the increased proportion of non-biodegradable organic matter, represented by humic substances, and increased concentration of ammonia, which are toxic to the microorganisms. By the photoelectrooxidation of this leachate, COD removal increase directly with the lamp power and the current density. The highest removal rates were obtained using the highest lamp power and the greatest current density, with the latter producing
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx
a more significant effect. Nevertheless, higher current densities yield lower energy consumption per mass of COD removed. PEO reactions for COD removal followed the pseudo first-order kinetic and the specific energy consumption for nitrogen compounds was 3-4fold higher than the one for COD removal. Summing up, photoelectrooxidation produced significant changes in the leachate matrix, removing ammoniacal nitrogen, which should ease the biological process. The photoelectrooxidation process is then feasible as an option for leachate pretreatment before biological processes.
Acknowledgements The authors acknowledge FINEP, CAPES, CNPq, SEBRAE/RS and FAPERGS for their financial support.
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The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process Gabriel Timm Müller a, Alexandre Giacobbo b, Edson Abel dos Santos Chiaramonte a, Marco Antônio Siqueira Rodrigues c, Alvaro Meneguzzi b, Andréa Moura Bernardes b,⇑ a b c
Universidade Estadual do Rio Grande do Sul (UERGS), R. Gal. João Manoel, 50, CEP 90010-030 Porto Alegre, RS, Brazil Universidade Federal do Rio Grande do Sul (UFRGS), Av. Bento Gonçalves, 9500, Setor 4, Prédio 74, CEP 91501-970 Porto Alegre, RS, Brazil Universidade FEEVALE, ICET, RS 239, 2755, CEP 93352-000 Novo Hamburgo, RS, Brazil
a r t i c l e
i n f o
Article history: Received 14 July 2014 Accepted 28 October 2014 Available online xxxx Keywords: Sanitary landfill leachate Recalcitrant leachate treatment Effluent degradation Photoelectrooxidation
a b s t r a c t The sanitary landfill leachate is a dark liquid, of highly variable composition, with recalcitrant features that hamper conventional biological treatment. The physical–chemical characteristics of the leachate along the landfill aging, as well as their effects on the efficiency of the conventional treatment, were evaluated at this paper. The feasibility of photoelectrooxidation process as an alternative technique for treatment of landfill leachates was also determined. Photoelectrooxidation experiments were conducted in a bench-scale reactor. Analysis of the raw leachate revealed many critical parameters demonstrating that the recalcitrance of leachate tends to increase with time, directly influencing the decline in efficiency of the conventional treatment currently employed. The effects of current density and lamp power were investigated. Using a 400 W power lamp and a current density of 31.5 mA cm2, 53% and 61% efficiency for the removal of ammoniacal nitrogen and chemical oxygen demand were respectively achieved by applying photoelectrooxidation process. With the removal of these pollutants, downstream biological treatment should be improved. These results demonstrate that photoelectrooxidation is a feasible technique for the treatment of sanitary landfill leachate, even considering this effluent’s high resistance to treatment. Ó 2014 Elsevier Ltd. All rights reserved.
1. Introduction The sanitary landfill leachate is a dark liquid of highly variable composition with recalcitrant features, generated by urban waste degradation. The conditions for leachate formation and its composition are extremely complex and variable, and depend on a number of factors, such as environmental conditions, waste characteristics, the operational peculiarities of the landfill and, especially, the dynamics of the decomposition processes that take place within the landfill cells (Christensen et al., 2001). Decomposition processes vary depending on the age of the landfill and can be classified into four phases: aerobic, anaerobic, initial methanogenic and stabilization (Kjeldsen et al., 2002; Renou et al., 2008). These phases can be identified based on the characteristics of the leachate, such as concentration of organic matter, nitrogen and pH, and are important for evaluating and planning treatment systems.
⇑ Corresponding author. Tel.: +55 5133089428; fax: +55 5133089427. E-mail address: [email protected] (A.M. Bernardes).
A standardized, technically and economically viable methodology for the effective treatment of sanitary landfill leachate, which could be readily applied to all leachate types, has yet to be devised. The options that are available are generally similar to those used in the treatment of sewage, and involve physical, chemical and biological processes. Primarily due to their low costs, biological processes, in the form of aerobic, anaerobic, and facultative systems, remain the most widely implemented type of process. Experiments with activated sludge, biological filters, anaerobic reactors, and aerated, anaerobic and facultative ponds have provided satisfactory treatment results (Aziz et al., 2011; Bashir et al., 2013; Wang et al., 2011). However, they were all performed on leachate from young landfills, which is easily biodegradable. Studies described by Kjeldsen et al. (2002) demonstrate that the leachate from young landfills generally contains substances with low molar mass, primarily in the form of volatile fatty acids, which are biologically degradable. Nevertheless, it is known that the capacity of microorganisms to degrade certain organic compounds is limited (de Morais and Zamora, 2005; Renou et al., 2008). In fact, as leachate ages, its composition becomes less biodegradable, as it predominantly
http://dx.doi.org/10.1016/j.wasman.2014.10.024 0956-053X/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
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G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx
contains substances with low molar masses and very complex structures, such as humic substances (Kjeldsen et al., 2002). Furthermore, the increase in the concentration of ammoniacal nitrogen and the strong dark color inhibit proliferation of microorganisms, meaning that conventional biological treatment is not successful. Therefore, because of the problems associated with biological systems, there is a growing need for processes that offer better treatment efficiency or which are capable of integration with conventional processes, overcoming their disadvantages and improving treatment efficiency. In this context, Advanced Oxidation Processes (AOPs) can be an alternative, mainly because of their characteristics such as high decomposition capacity and rate. AOPs are defined as processes that involve the generation of transitory species with high oxidative power, the most notable of which is the hydroxyl radical (HO). This radical has a high oxidative power (E° HO/HO +2.8 V at 25 °C) and low selectivity, allowing the degradation of a large number of toxic contaminants in a relatively short time (da Silva et al., 2014). Some AOPs have been already studied for landfill leachate treatment (de Morais and Zamora, 2005; Tauchert et al., 2006; Turro et al., 2011; Zhao et al., 2010). All these researches evaluate the treatment for leachates containing COD from 560 to 5200 mg L1 and ammoniacal nitrogen from 14 to 1100 mg L1. One technique that has been widely studied for the treatment of complex effluents is photoelectrooxidation (Bertoldi et al., 2012; Catanho et al., 2006; da Silva et al., 2014; Molina et al., 2013; Rodrigues et al., 2008; Siqueira et al., 2011; Xavier et al., 2011), which is a combination of electrochemical process and heterogeneous photocatalysis. Photoelectrooxidation (PEO) consists of percolating the effluent to be treated through an electrolytic reactor in which the anode, plated with metallic oxides, is irradiated with ultraviolet (UV) light. This technology has demonstrated an impressive synergic effect leading to increases on the rate and efficiency of decomposition, when compared with the contribution of individual processes (Moraes and Bertazzoli, 2005; Zhao et al., 2010). Three basic mechanisms drive the oxidation of organic compounds by the HO: proton abstraction, electron transfer and radical addition. These mechanisms are represented by RH, RX e PhX as described by Eqs. (1)–(3), respectively.
HO þ RH ! R þ H2 O
ð1Þ
HO þ RX ! RX þ OH
ð2Þ
HO þ PhX ! OHPhX
monthly activity reports provided by the company responsible for managing the landfill were evaluated in order to assess the situation of the landfill, as well as to determine the quality of the treatment used. Subsequently, a homogeneous aliquot of 40 L of raw leachate was collected directly from the equalization tank and kept refrigerated at 4 °C until the experiments were performed. Cyclic voltammetry was run in a Potentiostat (EG&G Princeton Applied Research, model 273A), using an acrylic cell, containing 300 mL of raw leachate, in order to determine the range of current density to be applied to oxidize the organic matter. The working electrode, with an area of 1.5 cm2, was titanium coated with 70TiO2/30RuO2; the counter electrode was platinum and the reference electrode was calomel. Potential was ranged from 2 to 2 V, returning to 2 V, with a scan rate of 5 mV s1 and without addition of supporting electrolytes. Subsequently, experiments to evaluate the effectiveness of the PEO process as a leachate treatment method were conducted. They were performed on 1.3 L batches in a photoelectrochemical benchscale reactor. A complete mixing was assured by a magnetic stirrer. The reactor consisted of a 3 L jacket glass cylinder, with cooling solution that was circulated by an ultra-thermostatic bath. The reactor also included a pair of electrodes and mercury vapor lamps (AvantÒ) and was powered by an electrical current. The schematic in Fig. 1 illustrates the photoreactor used in the experiments. Since the lamp would be immersed in the solution, it had to be placed inside a protective tube. However, since glass blocks UV radiation, a tube made of quartz was used. This material allows radiation to go through it and prevents the solution from coming into direct contact with the interior of the lamp. For optimizing the use of the radiation emitted, the electrodes were arranged concentrically around the lamp (da Silva et al., 2014). A pair of commercial electrodes, dimensionally stable (DSAÒ), manufactured by De Nora do BrasilÒ were used; the anode had a surface area of 475.2 cm2 and was made of titanium coated with a mixture of titanium and ruthenium oxides in the proportion of 70TiO2/ 30RuO2, while the cathode had a surface area of 118 cm2 and was made of titanium oxide. The gap between quartz tube and anode, as well as the one between electrodes was 0.5 cm.
ð3Þ
Basically, carbon dioxide, organic acids and inorganic ions are the final products generated by these reactions (da Silva et al., 2014). The main objectives of this research were to study the evolution of the physical and chemical characteristics of the leachate with respect to landfill age, to evaluate the efficiency of the conventional treatment employed, and to evaluate the potential of the photoelectrooxidation process as a treatment system for sanitary landfill leachate. A leachate with recalcitrant features, containing more than 1700 mg L1 ammoniacal nitrogen and 6500 mg L1 COD, was studied. Although the use of advanced oxidation processes has been recently studied to the treatment of leachates, the behavior of a leachate with the characteristics presented here has not been sufficiently studied before. 2. Experiments This work was performed using samples of leachate from a sanitary landfill located in Southern Brazil. This sanitary landfill receives around 2000 tons of waste per day. Initially, data from
Fig. 1. Batch photoreactor used in the photoelectrooxidation processes: (1) glass reservoir with an entrance and exit for the refrigeration fluid, (2) anode, (3) cathode, (4) quartz tube and (5) mercury vapor lamp.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx
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Table 1 Characteristics of the raw leachate sample collected from the sanitary landfill. Parameter
Raw leachate
COD (mg L1) BOD5 (mg L1) TKN (mg L1) NH3AN (mg L1) Total solids (mg L1) Conductivity (mS cm1) pH Temperature (°C)
6560 ± 226 3326 ± 565 2021 ± 97 1740 ± 83 14013 ± 258 30.1 ± 0.1 8.2 ± 0.1 22 ± 1
Three hour experiments were conducted for treatment process optimization, and the following variables were evaluated: current density and lamp power. Temperature was kept below 50 °C in order to avoid polymerization of intermediate compounds (Arslan et al., 2005). Initially the lamp power was examined (125, 250 and 400 W), using a current density of 31.5 mA cm2. After that, using a 250 W lamp, three different current densities, 10.5, 21 and 31.5 mA cm2, selected by means of cyclic voltammetry assay, were tested. Duo to the high conductivity of the raw leachate, 30 mS cm1 (see Table 1), no supporting electrolytes were added to PEO experiments. To characterize the leachate and to evaluate the efficiency of the treatment systems, the followed parameters were analyzed: total solids (TS), chemical oxygen demand (COD), five-day biological oxygen demand (BOD5), ammoniacal nitrogen (NH3AN), total Kjeldahl nitrogen (TKN), color, alkalinity, hardness, conductivity and pH. All the analyzes were performed according to the Standard Methods (Eaton and Franson, 2005). According to IUPAC guidelines, the removal of organic contaminants through advanced oxidation processes can be often described phenomenologically by simple rate expressions that are either zero-order or first-order (Bolton et al., 2001), being represented by Eqs. (4) and (5), respectively.
C ¼ C0 k t ln
C C0
¼ k t
ð4Þ ð5Þ
The reaction kinetics were calculated based on the COD, where t is the exposure time; k is the kinetic constant; C0 is the COD of initial effluent and C is the COD of final effluent, in mg L1. Electric energy per mass, EEM (kW h kg1), was calculated as described by Bolton et al. (2001). EEM is defined as the electric energy in kilowatt–hour (kW h) required to degrade a kilogram of a specific pollutant in contaminated water or air, as described by Eq. (6):
EEM ¼
P t 106 VðC 0 CÞ
Fig. 2. Characteristics of the landfill leachate over the years: (a) TS, alkalinity, COD and BOD5; (b) color, hardness, TKN and NH3AN. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
large increase, however, the biodegradability, i.e., the BOD/COD ratio was declining over the years, as shown in Fig. 3. Nitrogen is also an important parameter, especially considering the different oxidation states that it can reach. Furthermore, the combination of high nitrogen, COD and alkalinity values presented by the effluent can be toxic to biological treatment processes (de Morais, 2005). Additionally, the increase in color is another strong indication of the tendency towards landfill stabilization, and consequently, the accumulation of humic and fulvic substances (Tauchert et al., 2006).
3.1. Treatment efficiencies achieved by the leachate treatment plant at the sanitary landfill The biological treatment plant currently operating at the sanitary landfill comprises two anaerobic filters that are operating
ð6Þ
where P is the rated power (kW), V is the volume (L) of treated water in the time t (h) and the pollutant concentration is given in mg L1, while the factor 106 converts mg to kg. 3. Results The data collected from the landfill managing company are presented in Fig. 2. Over the years, there has been a significant increase for all monitored parameters. Some of these deserve greater emphasis: COD, BOD5, TKN, NH3AN, color and alkalinity. The parameters related to the concentration of organic matter, COD and BOD5, showed a
Fig. 3. BOD5/COD ratio of the leachate over the years.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
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in parallel, and a series of three facultative ponds. In the first year of operation, this plant presented high removal rates for all monitored parameters. Nevertheless, over the years, it has experienced a significant reduction in the removal efficiency of these parameters (Fig. 4). This can be attributed to the low effectiveness of biological processes in treating this particular effluent, especially after the fourth year of operation of the sanitary landfill. The efficiency of the biological treatment (anaerobic filters and facultative ponds) used by the effluent treatment plant can be seen to have declined from 88% to 33% with respect to BOD5; from 87% to 35% with respect to COD removal; from 94% to 25% for TKN removal; from 95% to 13% for ammoniacal nitrogen; and from 84% to 14% efficiency with respect to total solids. Regarding to color, as can be seen in Fig. 5, the biological treatment practically do not remove it, and in some cases even worse the leachate color. This significant loss in the efficiency of the conventional treatment sequence is directly related to the evolution of the characteristics of the leachate produced by the sanitary landfill (Fig. 2). As was described earlier, with the aging of the landfill, the characteristics of the leachate produced change drastically, and these changed properties reduce biodegradability, making biological treatment more difficult. There is a great deal of information in the literature (Abu Amr et al., 2013; Cai et al., 2014; Cortez et al., 2011; Erkan and Apaydin, 2014; Turro et al., 2011; Venu et al., 2014) that can explain the low efficiency of biological treatment for leachates with high recalcitrant characteristics, which can be applied to this study. The recalcitrance of the leachate is also associated with the presence of humic substances, indicated by the strong dark
coloration (Fig. 5). According to Kang et al. (2002), humic substances make up more than 60% of the organic matter present in mature leachates. These substances are macromolecules with very complex structures and intermediates (1000 g mol1, fulvic acids) or high (10000 g mol1, humic acids) molecular mass and, as a result of these characteristics, are not biodegradable and accumulate in landfills over time (El-Fadel et al., 2002). Another factor that interferes with biological treatment is the high concentration of ammoniacal nitrogen, which is toxic to the microorganisms. In the sixth year of operation of the sanitary landfill, the leachate reached high concentrations of COD and ammoniacal nitrogen, as huge as 8000 and 2000 mg L1, respectively. According to de Morais (2005), an effluent with an ammoniacal nitrogen concentration above 1000 mg L1, along with a COD greater than 2000 mg L1 and high alkalinity, is considered difficult to treat with biological processes, primarily due to the high toxicity of ammonia. In addition to the recalcitrant characteristics of the leachate, the low efficiency of the treatment may also be related to the low phosphorus concentrations in this effluent, since this element is an essential nutrient for the microorganisms and, therefore, should be available in the medium in sufficient concentrations to guarantee biomass growth. According to the literature (Metcalf and Eddy, 2003), the relative quantities of nitrogen and phosphorus required for biological processes should be at a N:P ratio of 5:1. Leachate characterization studies demonstrate that the nitrogen concentration is on average 30 times that of the phosphorus, and this may be the factor limiting the growth of microorganisms. 3.2. Photoelectrooxidation process
Fig. 4. Mean annual efficiency in reducing major physico-chemical parameters achieved by the biological treatment plant of the sanitary landfill leachate.
Fig. 5. Color evolution of the raw and biologically treated leachate over sanitary landfill operation time. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Table 1 lists the results of the analyzes carried out on the raw leachate sample collected at the sanitary landfill. The characteristics of this leachate sample are consistent with the data from the characterization study described previously (Kang et al., 2002; Moraes and Bertazzoli, 2005), where high concentrations of organic matter, ammoniacal nitrogen and total solids were observed, in addition to the mildly basic pH, all of which further reinforce the recalcitrance prevision of this effluent for the conventional treatment. A cyclic voltammetry assay was carried out to determine the current density range that would oxidize the compounds present in the leachate. It was found that the most intense reactions occurred in the range from 8 to 35 mA cm2, as illustrated in Fig. 6. Therefore, intermediate densities were applied throughout all treatment tests.
Fig. 6. Cyclic voltammogram of raw leachate. Scan rate: 5 mV s1; working electrode: 70TiO2/30RuO2; counter electrode: platinum and; reference electrode: calomel.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
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G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx Table 2 COD removal and specific energy consumption in photoelectrooxidation experiments in the final treatment time (3 h). Experiment
Current density (mA cm2)
Potential (V)
Lamp power (W)
COD removal (%)
EEM (kw h kg1 COD)
PEO PEO PEO PEO PEO
31.5 31.5 31.5 21.0 10.5
1.37 1.37 1.37 0.96 0.8
125 250 400 250 250
37.7 46.1 57.3 31.2 19.8
136 206 258 293 451
1 2 3 4 5
Table 2 shows the effects of lamp power and current density on COD removal and specific energy consumption in PEO process treating a landfill leachate. One can verify that, at constant current density of 31.5 mA cm2, PEO 1–3, COD removal was directly dependent on lamp power and the greatest efficiency was obtained with the 400 W lamp, while the lowest one was achieved with the 125 W lamp. These results are in agreement with several studies that have evaluated this variable in treatment processes that use UV radiation, where usually an increase in degradation occurs when higher lamp powers are employed (Bauer et al., 1999; Li et al., 2002; Pera-Titus et al., 2004). Li et al.’s (2002) studies of humic acid treatment with PEO process demonstrated that the increase in UV radiation intensity produces greater and faster removal rates. According to Pera-Titus et al. (2004), as the intensity of the light increases, the quantity of photons emitted by the system into the solution also increases, meaning that more TiO2 particles are activated. Furthermore, direct photolysis also has an effect that arises from the interaction between photons and pollutant molecules. However, once the electrodes are already sufficiently illuminated, there are no more TiO2 molecules left to interact and, hence, no further effects occur. In PEO 2, 4 and 5, using a 250 W lamp and current densities of 31.5, 21 and 10.5 mA cm2, COD removal increases directly with the current density applied, which has a greater influence than lamp power; besides, the efficiency achieved applying a current density of 31.5 mA cm2 is significantly greater than the one with 10.5 mA cm2. These results are in line with the literature, which describes studies that have assessed the influence of current density on process efficiency, involving the application of electric power. In all of these studies, which used photoelectrooxidation for the treatment of humic acid (El-Fadel et al., 2002), pesticides (Quan et al., 2004), sanitary landfill leachate (Moraes and Bertazzoli, 2005), and nonylphenol ethoxylate (da Silva et al., 2014), the greatest degradation efficiencies were obtained with the highest current densities applied.
Fig. 7. Progression of COD removal efficiency over time with 400 W lamp and current density of 31.5 mA cm2.
Another important factor to be considered is the EEM. It provides the energy consumption associated with the performed process, enabling comparisons between the different investigated parameters, lamp power and current density, regarding to the COD removal. In PEO 1–3, at constant current density and different lamp powers, although the COD removal is higher with the increase in lamp power, the energy consumption tends also to linearly rise. On the other hand, in PEO 2, 4 and 5, using a 250 W lamp power and different current densities, the highest current density
Fig. 8. Reaction kinetics calculated based on the COD removal from the landfill leachate by PEO (a) pseudo first-order kinetic model; (b) first-order kinetic model and; (c) zero-order kinetic model. Conditions: C0 = 6560 mg L1, 400 W lamp power and current density of 31.5 mA cm2.
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
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G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx Table 3 Leachate treated by photoelectrooxidation for 5 h, with a 400 W lamp power and current density of 31.5 mA cm2. Parameter 1
COD (mg L ) pH TKN (mg L1) NH3AN (mg L1) Temperature (°C)
Raw leachate
Treated leachate
Removal efficiency (%)
EEM (kW h kg1)
6560 ± 226 8.2 ± 0.1 2021 ± 97 1740 ± 83 22 ± 1
2558 ± 123 8.1 ± 0.1 909 ± 88 817 ± 64 43 ± 1
61 – 55 53 –
404 – 1454 1752 –
(31.5 mA cm2) promoted higher COD removal, 46.1%, and lower EEM, 206 kw h kg1 COD, i.e., the best energetic efficiency. These findings are in line with those from the literature, where is stated that photoelectrochemical processes present substantially lower specific energy consumption at higher current densities (Catanho et al., 2006). Once the experimental conditions yielding the maximum response for COD removal from the photoelectrooxidation process had been determined (specifically, a 400 W lamp with a current density of 31.5 mA cm2), a 5 h degradation experiment was performed, using the same reactor described earlier. 20 mL samples were collected every 1 h until a final time of 5 h for COD and apparent color analyzes. As can be seen in Fig. 7, after 5 h of treatment, the photoelectrooxidation process achieved a COD removal efficiency of 61%. However, it was noted that there is a tendency for the removal rates to stabilize as time of treatment increases, since the increase in treatment duration from 3 to 5 h resulted in a further increase of only 10% in removal efficiency, which is marginal when compared with the rate of 51% at 3 h. Fig. 8a–c, presents the reaction kinetics calculated based on COD removal, where ‘‘a’’ is the pseudo first-order model, ‘‘b’’ is the firstorder model and ‘‘c’’ is the zero-order model. Bolton et al. (2001) assert that, for high concentrations of contaminants, oxidation reactions in AOPs follow zero-order kinetic. Nonetheless, one can verify that the first-order kinetic fits better to the data than the zero ones. On the other hand, in the current work, the best fitting achieved to represent the COD removal in photoelectrooxidation reactions were the pseudo first-order kinetic model, with regression coefficients (R2) 0.99. The pseudo first-order kinetic model presents two distinct linear regions: Phase I, until 3 h, has a kinetic constant (k1) of 0.23 h1 and Phase II, from 3 to 5 h, has a kinetic constant (k2) of 0.1141 h1. Thus, Phase I presents reaction kinetic twice faster than the one from Phase II, what can justify the fact that 51% of COD removal was achieved at the first 3 h of PEO process, while an increase in efficiency of only 10% more was obtained in the following 2 h of treatment (Fig. 7). Other researchers also stated that photoelectrooxidation reactions follow the pseudo first-order kinetic (Nageswara Rao et al., 2009; Xiao et al., 2013). The final analysis of the characteristics of the treated leachate included, in addition to COD, the parameters pH, TKN, ammoniacal nitrogen (NH3AN) and temperature (see Table 3). Under these conditions, TKN and NH3AN removal were 55% and 53%, respectively. Regarding to energy consumption, after 5 h of treatment using a current density of 31.5 mA cm2 and a 400 W lamp power, the PEO of nitrogen compounds (TKN and NH3AN) consumes threefourfold more energy than the one for COD removal (Table 3). This behavior may be due to more complex heterocyclic molecules having CAN and C@N bonds which account for nitrogen compounds (Nageswara Rao et al., 2009). Fig. 9 shows the evolution of color of the leachate recorded at sampling intervals of 1 h, in the experiment with 5 h of treatment. In Fig. 9, ‘‘a’’ represents raw leachate and ‘‘f’’ is the sample after 5 h of treatment, one can noted a sharp reduction in apparent color of the leachate over treatment time by the PEO process. This data demonstrates that photoelectrochemical treatment resulted in significant changes in the leachate composition,
Fig. 9. Evolution of apparent discoloration of leachate with the treatment of 5 h. (a) Raw leachate, (b) 1 h, (c) 2 h, (d) 3 h, (e) 4 h, (f) 5 h treatment. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
through the removal of color, organic matter, and nitrogen. Although the efficiency of the process is not sufficient to make photoelectrooxidation process viable as a single-stage treatment, it can be considered an improvement, if one takes into consideration the complexity and recalcitrance of the effluent studied, and also when compared to the efficiency of the conventional treatment sequence employed at the sanitary landfill discussed above. Therefore, it can be concluded that photoelectrooxidation process is an efficient treatment method for complex wastes such as leachates with recalcitrant pollutants, and can be used as a supplementary treatment to complement other systems. It is also observed that the treatment studied here brought about a high removal of ammoniacal nitrogen and color, what should allow a downstream biological treatment with higher efficiency. 4. Conclusion The characteristics of the leachate studied here allow its classification as an intermediate leachate, and, during the period of study, major physical and chemical changes took place, demonstrating the implications of the age of sanitary landfills on leachate composition, including the reduction in the biodegradability index, the increase in concentration of ammoniacal nitrogen and color, in addition to a mildly basic pH, alkalinity and the presence of solids. The leachate tendency towards stabilization and a consequent reduction in biodegradability is observed in the reduction of the efficiency of conventional biological treatment as measured by the parameters COD, TKN, NH3AN and TS. This effect may primarily be related to the increased proportion of non-biodegradable organic matter, represented by humic substances, and increased concentration of ammonia, which are toxic to the microorganisms. By the photoelectrooxidation of this leachate, COD removal increase directly with the lamp power and the current density. The highest removal rates were obtained using the highest lamp power and the greatest current density, with the latter producing
Please cite this article in press as: Müller, G.T., et al. The effect of sanitary landfill leachate aging on the biological treatment and assessment of photoelectrooxidation as a pre-treatment process. Waste Management (2014), http://dx.doi.org/10.1016/j.wasman.2014.10.024
G.T. Müller et al. / Waste Management xxx (2014) xxx–xxx
a more significant effect. Nevertheless, higher current densities yield lower energy consumption per mass of COD removed. PEO reactions for COD removal followed the pseudo first-order kinetic and the specific energy consumption for nitrogen compounds was 3-4fold higher than the one for COD removal. Summing up, photoelectrooxidation produced significant changes in the leachate matrix, removing ammoniacal nitrogen, which should ease the biological process. The photoelectrooxidation process is then feasible as an option for leachate pretreatment before biological processes.
Acknowledgements The authors acknowledge FINEP, CAPES, CNPq, SEBRAE/RS and FAPERGS for their financial support.
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