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R.M. Ram´ırez Zamora*, A. Durán Moreno**, M.T. Orta de Velásquez* and I. Monje Ram ´ırez* *Instituto de Ingenier ı´a, UNAM. Apdo. Postal 70-472, Coyoacán 04510, México, D.F. (E-mail:
[email protected])
**Fac. De Qu´ımica, UNAM, Edif. E planta baja, Paseo de la Investigación Cientifica, Coyoacán 04510, México, D.F. Abstract This work compares two pre-treatments (coagulation-flocculation process (CF) and the Fenton oxidation Method (FE)) of the activated carbon adsorption process (AC) to optimize the removal of the organic compounds in landfill leachates. The content of organic compounds was measured in terms of three global parameters: colour, chemical oxygen demand (COD) and dissolved organic carbon (DOC). The result obtained in discontinuous reactor conditions showed an increase in colour removal from 1.5 to 2.0 times and a decrease of COD between 0.3 to 0.5 times for the FE-AC treatment, in relation to the CF-AC treatment. On the other hand, the data obtained in continuous reactor conditions (packed columns) showed that the column fed with leachate CF exhibited operation times 1.3 times longer and a better physiochemical quality in the filtrate (COD and colour) than the one fed with the FE leachate. Nevertheless, the adsorption capacities in the colour removal column of COD and DOC were higher for the FE leachate. Keywords Activated carbon; adsorption; coagulation-flocculation; Fenton oxidation; leachate
Introduction
Landfills constitute the most common method for the confinement of municipal solid wastes due to their low operation and maintenance costs (Diamondapoulos, 1994). Nevertheless, this type of waste disposal shows the problem of leaching, as a result of liquid infiltration through the solid wastes. The leachates produced in landfills may contaminate aquifers. This contamination is avoided by collecting and treating the leachates by different procedures. Leachates are classified by their residence time in a landfill, as young and old leachates. Young leachates are characterised by COD contents higher than 5 g L–1 (represented by volatile fatty acids (VFAs), which are intermediate products in the anaerobic degradation that takes place in the landfill), and by a low nitrogen concentration (400 mg L–1 of N), a high content of recalcitrant compounds and a low biodegradable organic fraction (BOD5/COD=0.1). It is important to note that the organic carbon in old leachates is mainly due to substances with high molecular weight and recalcitrant characteristics (Diamondapoulos, 1994). The recalcitrant compounds in old leachates are not amenable to biological processes. The effective removal of these substances can only be achieved through the application of more sophisticated methods such as advanced oxidation and specially adsorption with activated carbon, or a combination of these with conventional physiochemical methods. In this context, we report a study based on two conventional physiochemical pre-treatments (coagulation-flocculation (CF) and an advanced oxidation or Fenton method (FE)), both coupled to an adsorption process to achieve the efficient removal of organic compounds in an old landfill leachate.
Water Science and Technology Vol 41 No 1 pp 231–235 © IWA Publishing 2000
Treatment of landfill leachates by comparing advanced oxidation and coagulation-flocculation processes coupled with activated carbon adsorption
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Materials and methods
R.M. Ram ı´rez Zamora et al.
Activated carbon pre-treatments (coagulation-flocculation (CF) and Fenton oxidation (FE) were carried out in laboratory batch reactors using a mechanical stirring equipment of the “jar test’’ type, at 20ºC. The CF optimal conditions were determined and reported in a previous work for this leachate (Orta et al., 1997). The pH of the leachate was 5.75 with coagulating doses of 738 mg L–1 of aluminium sulphate and 1136 mg L–1 of ferric chloride. In the case of the FE pre-treatment, the conditions were determined using a mathematical model that describes the colour removal in terms of the pH, reactants dose and reaction time. The model was obtained based on an experimental design with a fractional factorial plan (Adler et al., 1975). According to the model, the best conditions for colour removal using FE method are pH=4, with 30 minutes of reaction time combined with 1000 mg L–1 of FeSO4 and 1000 mg L–1 of H2O2. The isotherms and the adsorption kinetics were determined with activated carbon “LQ 1000’’ in closed batch reactors to avoid liquid phase evaporation. The reactors were equipped with magnetic stirrers and a constant temperature regulation device. The adsorption process in continuous reactor conditions was carried out using glass columns of 3 cm of internal diameter and an activated carbon layer of 25 cm high. The characteristics of the different activated carbons used are presented in Table 1. Table 1 Activated carbons characteristics TYPE OF CARBON
232
CHARACTERISTIC
F200
LQ100
CG700
MD (NB)
1) Origin
Mineral
Mineral
Coconut
Wood
2) Manufacturer
CALGON
CARBOCHEM
NOBRAC
NOBRAC
3) Activation
Physical
Physical
Physical
Chemical
4) Apparent density (g mL–1)
0.51
0.47
0.55
0.26
5) Hardness
80
75
98
—
6) Specific area (m2 h–1)
875
1,100
800
1,000
7) Iodine index (mg g–1)
900
1,000
700
900
8) Porous volume (mL g–1)
0.7
0.90
—
—
The experimental technique proposed by Noll et al., (1993) was used to obtain the kinetics parameters and adsorption isotherms. The experimental conditions for the isotherm determinations were pH=5.75, an adsorbent material dose ranging from 1 to 7 g L–1 for the different powder activated carbons (PAC), a temperature of 20ºC and according to the adsorption kinetics results a pseudo equilibrium time of 24 hours. The experimental method to study the adsorption process in continuous reactor was performed by feeding the pretreated leachates to the packed column with 70 g of the best grain activated carbon (GAC) as determinate in a batch reactor (LQ 1000). The leachate was fed to the column in upflow mode with a flow rate of 7 m L min –1 using a peristaltic pump. The resulting empty column contact time (EBCT) was 15 minutes. The parameters analysed were colour, chemical oxygen demand (COD) and dissolved organic carbon (DOC), which are the best indicators of the content of organic compounds. The analytical techniques applied for the determination of COD, DOC and colour are those recommended in Standard Methods for the Examination of Water and Wastewater (1995). The COD and colour parameters were determined using a spectrophotometer HACH model 2010 and, the DOC with a SHIMADZU TOC equipment, model 5050, with a UV detector.
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R.M. Ram ı´rez Zamora et al.
Figure 1 Adsorption kinetics in AC LQ 1000 (dose 2 g L–1) for (a) colour and (b) COD removal of the leachates treated by CF and FE processes
Results Adsorption kinetics
The four different activated carbons that were studied presented similar adsorption kinetics. Figure 1 illustrates the adsorption kinetics results for colour and COD of the AC type LQ1000. The data are presented in terms of the “q’’ adsorption capacity (mg of solute/g AC) for the CF and FE leachates. Figures 1a and 1b show that the necessary time for reaching the “pseudo equilibrium’’ is approximately 24 hours. Therefore this time was applied to the isotherms. For this contact time between solid and liquid phase, we observe that the AC LQ 1000 presents greater adsorption capacities for the FE leachate than for the CF leachate. Adsorption isotherms
The treatment of the leachates by CF-AC and FE-AC in a batch reactor allowed to remove colour with efficiencies ranging from 55 to 70% when using similar doses of adsorbent than those employed in other works (Diamadopoulos, 1994). The residual values of this parameter were acceptable for both series. As far as pH is concerned, it is worth mentioning that before the adsorption process, the leachates of both series exhibited an initial pH equivalent to 5 and, afterwards, between 6 and 7, which leads to compliance with regulations for treated wastewaters. The numerical results for the COD and colour removal were determined using the lineal form of the (Equation 1) Freundlich and the (Equation 2) Langmuir models, given by: ln qe=ln KF +1/n ln Ce 1 qe
=
1 qm
+
1 1 qmb Ce
(1) (2)
In these equations, KF , 1/n are Freundlich model constants, Ce is the equilibrium concentration of the solute in liquid phase (mg L–1), qm, qe are the adsorption capacities at maximum and at equilibrium conditions (mg g–1 AC) and b is Langmuir model constant. The values of the constants colour removal based on these models are presented in Table 2. It is important to note that the constants obtained for COD removal show a similar trend to those for colour removal. Considering the values of the coefficient for the linear fit (r 2), we observed that the experimental data are better adjusted to the Langmuir model (0.996 arithmetical mean) than to the Freundlich model (0.852 arithmetical mean). The Langmuir model allows calculating the maximum capacity of colour and COD (qm). Based on this parameter, the results obtained in discontinuous reactors, for three of the four activated car-
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Table 2 Values of the constants for the Freundlich and Langmuir models of different AC for colour removal FREUNDLICH MODEL (1906)
LANGMUIR MODEL (1915)
TYPE OF CARBON
KF
l/n
r2
qm (mg L–1)
b (L mg–1)
r2
F200
5.4 E-2
1.23
0.832
134.92
0.0025
0.960
adsorption
MD (NB)
1.6 E-4
2.53
0.990
190.24
0.0031
0.996
(CF-AC)
LQ 1000
5.1 E-1
0.88
0.937
193.45
0.0019
0.906
CG 700
2.9 E-1
0.97
0.897
111.1
0.0021
0.965
LEACHATE
Leachate clarify+
R.M. Ram ı´rez Zamora et al.
Fenton Method
F200
2.4 E-2
2.03
0.989
322.6
0.0045
0.938
leachate+
MD (NB)
2.9 E-2
0.80
0.997
375.4
0.0026
0.997
adsorption
LQ 1000
1.7 E-2
1.89
0.986
442.8
0.0003
0.982
(FE-AC)
CG 700
1.08 E-2
4.67
0.880
256.4
0.0008
0.987
bons of different origin using FE-AC, showed a 5 to 10 times increase in colour removal and a 0.3 to 0.5 decrease for COD in relation to the leachates from the CF-AC treatment. The COD removal was apparently carried out in a less effective way than the colour removal in the pre-oxidised leachates (FE-AC) when compared to the clarified (CF-AC), due to the fact that the Fenton method degrades the structure of certain organic compounds. This degradation allows the measurement of the oxidation by-products after the Fenton method, increasing the values of this parameter in this way. With regard to the effectiveness of activated carbon, we observed that the LQ 1000 allows to obtain the best colour and COD adsorption capacities for both leachates. This activated carbon possesses the largest specific area of all the ACs studied. Adsorption columns
Figures 2 and 3 present the results for colour residual COD and DOC with regard to their initial values obtained in adsorption columns for the CF and FE leachates. As it can be seen, the obtained values in adsorption columns with AC type LQ 1000 agree with the ones
234
Figure 2 Adsorption results in packed columns with AC (LQ 1000) for (a) colour, (b) COD and (c) DOC removal of the CF and FE leachates
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observed for the batch reactor adsorption isotherms. This means that, in the first 48 hours of operation time, better colour, COD and DOC adsorption capacities are observed for the FEAC leachate than for the CF-AC. Conclusions R.M. Ram ı´rez Zamora et al.
For old leachates, an advanced oxidation pre-treatment or Fenton method allows to obtain greater adsorption capacities of organic compounds by activated carbon as compared to a conventional pre-treatment or coagulation-flocculation process. This phenomenon is attributed to the transformation of organic compounds into oxidation by-products, which have a smaller size than the molecules of the starting compound. Because of their size, byproducts are able to pass through micropores of the activated carbon. So, the specific area for the adsorption of these oxidation by-products is greater than that of the starting compounds because of their size. In adsorption columns with activated carbon, the results obtained agree with those obtained in a batch reactor. Nevertheless, the difference between CF-AC and FE-AC leachates for the adsorption capacities of organic compounds is less significant in packed columns than in batch reactors. With regard to the physiochemical quality of the leachate, it must be said that the one that comes from CF pre-treatment is better than the one from a FE pre-treatment. As a consequence, the operation times for the adsorption column and the filtrate quality are better for FC-AC than for FE-AC. Acknowledgements
The authors are grateful to DGAPA, UNAM, for the facilities made available for the realisation of this work and to Alicia Margarita Romero, who did most of the experiments and analysis. References Adler, Y.C., Markova, E.V. and Granousky, Y.V. (1975). The design of experiments to find optimal conditions, MIR Publisher. Diamandopoulos, E. (1994). Characterization and treatment of recirculation-stabilized leachate, Wat. Res., 28(12), 2439–2445. Noll, K.E., Gounaris, V. and Hou, W. (1992). Adsorption Technology for air and water pollution control. Lewis Publishers Inc., USA. Orta de Velásquez, M.T., Ram´ırez Zamora, R.M., Monje Ram´ırez, I. (1997). Sustitución y disminución del consumo de reactivos quimicos en la planta de tratamiento de lixiviados de Bordo Poniente, Informe elaborado para la Dirección General de Servicios Urbanos por el Instituto de Ingenier ´ıa, UNAM. Standard Methods for the Examination of Water and Wastewater (1995). 19th edn., American Public Health Association/American Water Works Assocation/Water Environment Federation, Washington DC, USA.
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