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Physico-chemical removal of iron from semi-aerobic landfill leachate by limestone filter
Waste Management 24 (2004) 353–358 www.elsevier.com/locate/wasman
Physico-chemical removal of iron from semi-aerobic landfill leachate by limestone filter Hamidi Abdul Aziz*, Mohd Suffian Yusoff, Mohd Nordin Adlan, Nurul Hidayah Adnan, Salina Alias School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Received 8 October 2003
Abstract Limestone has been proven effective in removing metals from water and wastewater. A literature review indicated that limestone is capable of removing heavy metals such as Cu, Zn, Cd, Pb, Ni, Cr, Fe and Mn are through a batch process or by filtration technique. The removal capability is reported at up to 90%. However, to date most of the studies have been focused on synthetic wastewater. The present study attempts to investigate the suitability of limestone to attenuate total iron (Fe) from semi aerobic leachate at Pulau Burung Landfill Site in Penang, Malaysia. Iron was found in significant quantities at the landfill site. The study also aims to establish the Fe isotherm and breakthrough time of the proposed limestone filter for post-treatment to the migrating landfill leachate before its release to the environment. The Fe isotherms were established using a batch equilibrium test, while the breakthrough characteristics were determined using continuous flow permeating through a limestone column. The latter was used in order to simulate the continuous flow of leachate that would occur in the proposed limestone filter. The limestone media used in the experiment contain more than 90% CaCO3 with particle sizes ranging from 2 to 4 mm. Four filter columns (each 150 mm in diameter and 1000 mm depth) were installed at the landfill site. Metal loadings were kept below 0.5 kg /m3 day and the experiment was run continuously for 30 days. Initial results indicated that 90% of Fe can be removed from the leachate based on retention time of 57.8 min and surface loading of 12.2 m3/m2 day. For the batch study on the Fe isotherm, the results indicated that limestone is potentially useful as an alternative leachate treatment system at a relatively low cost. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction Leachate is produced when moisture enters the refuse in a landfill, extracts contaminants into the liquid phase, and produces moisture content sufficiently high to initiate liquid flow. Leachate composition depends on many factors such as the waste composition, site hydrology, the availability of moisture and oxygen, design and operation of the landfill and its age. There is a considerable number of studies reported in the literature for metals concentrations from full-scale landfills, test cells, and laboratory works (Christensen et al., 2001, 1994; Kjeldsen and Christophersen, 2001; Revans et al., 1999; Reinhart and Grosh, 1998; Robinson, 1995). Heavy
* Corresponding author. Tel.: +60-45937788x6215; fax: +6045941009. E-mail address: [email protected] (H.A. Aziz). 0956-053X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2003.10.006
metals, which are commonly found in high concentrations in landfill leachate include iron, manganese, zinc, chromium, lead, copper and cadmium. They are a potential source of pollution for ground water, surface water and reservoirs. Iron concentration in leachate is normally contributed to by iron-base material waste, such as construction materials, paints, pigments, colour compounds, polishing agents and electrical materials. There are three mechanisms by which metals concentration could be increased in an aerobic landfill. These include the oxidation of metals sulfides to metal sulfates that are more soluble, the complexation capacity of oxidized humic acids relative to reduced humic acids (Martensson et al., 1999) and the oxidation of sulfides which results in the production of sulfuric acid that reduces the pH and increases the solubility of metal. Theoretically, the solubility of metal in the acidogenic phase is higher because of the production of acid in an
354
H.A. Aziz et al. / Waste Management 24 (2004) 353–358
anaerobic condition. Once the landfill has reached the methanogenic phase, heavy metal concentrations are generally low (Revans et al., 1999). Various studies have been carried out to remove undesirable constituents from landfill leachate. Removal of ammonium by chemical precipitation has been reported by Li et al. (1999). In 2002, Palma et al. reported on the treatment of industrial landfill leachate using evaporation and reverse osmosis. However, removal of metals from leachate is not well documented, especially in Malaysia. There are numerous techniques used for iron removal. These include aeration, softening, chlorination, ozonation and filtration. Aeration is the most common. For the filtration technique, the common media used are activated carbon and clay. The use of limestone for removing metals from water and industrial wastewater was found to be effective (Aziz, 1992, 1997; Aziz and Smith, 1992, 1996; Aziz and Mohd, 1998; Othman and Aziz, 1999; Othman et al., 1999a, 1999b; Aziz et al., 2000, 2001). More than 80% of heavy metals such as copper, iron, manganese, cadmium and others can be removed using a batch or continuous flow filtration process. Some of the removal mechanisms have been established. For example, removal of copper was due to the adsorption and absorption of this metal towards limestone particles (Aziz et al., 2001). However, to date, no studies have been reported on the use of limestone for leachate treatment in Malaysia. This study focuses on the leachate generated from Pulau Burung Landfill Site (PBLS); which is situated in Penang, Malaysia. PBLS has a semi-aerobic system and it is one of the only three sites of its kind found in Malaysia. A semi-aerobic system can be achieved through a convection process. The latter involves the decomposition of organic matter inside the landfill and will cause an increase in temperature. The difference in temperature between inside and outside of the landfill will generate a heat convection current into the landfill through the leachate pipe. PBSL has been developed semi-aerobically into a sanitary landfill Level II by establishing a controlled tipping technique in 1991. It was further upgraded to a sanitary landfill Level III employing controlled tipping with leachate recirculation in 2001. It was found that the leachate from a semi-aerobic system has slightly lower organic contaminants compared with an anaerobic landfill in terms of BOD and COD (Basri et al., 2000; Aziz et al., 2001). This site receives 1500 tons of solid waste daily. At PBLS, leachate is collected through collection pipes that feed into a detention pond. Table 1 shows details of the main characteristics of leachate used in the experiment. The table indicates that Fe is present in significant quantities with a maximum value of approximately 20 mg/l. The main source of Fe is from the ore waste industry.
Table 1 The characteristics of raw leachate from old detention pond at Pulau Burung Landfill Site (PBLS) Parameter
Unit
Average Value
BOD5 COD Suspended Solid Turbidity pH Zinc Manganese Iron Copper Chromium (VI) Colour
mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l Platinum unit, Pto
48–105 1533–2580 159–233 61–198 7.5–9.4 0.1 15.5 4.1–19.5 4.6 0.6 1000–4000
Base on 30 days reading.
The main objective of the present study is to investigate the suitability of limestone particles to be used as a filter, capable of attenuation of heavy metals, especially total iron (Fe). Fe was emphasized in this study due to its high concentration at PBLS, which is between 4 and 20 mg/l. This study also focused on the establishment of necessary parameters for the design of the limestone filter; which offers post-treatment to migrating leachate before its release to the environment. These include the determination of Fe isotherms and breakthrough characteristics of Fe. The study on adsorption isotherms was carried out through a batch process while the breakthrough characteristics were determined by a continuous process where leachate permeated through limestone columns. The latter was carried out in order to simulate the continuous flow of leachate through the proposed limestone filter.
2. Materials and methods The composition of limestone (in powder form with particle size of less than 75 mm) was determined by Xray fluorescence using a Rigaku RIX3000 and the X-ray using a Camscan Editor EDX. Limestone grains were sieved to obtain a uniform size between 2 and 4 mm. The density of limestone used in the experiment was 2573 kg/m3. Four perspex filter columns, each with a 150 mm diameter, were placed beside the old leachate pond (Fig. 1). Each column has several outlets set at different depths for sampling purposes. The filter columns were covered with waterproofing material to prevent the entrance of rainwater. A storage tank for raw leachate with a capacity of 100 l was placed at about 1 m above the top of the filter column to achieve a constant head for gravity feeding. The thickness of the limestone filter bed was 1000 mm. The bed was conditioned with three rinses of distilled water before the experiment was conducted. Then raw leachate contain-
H.A. Aziz et al. / Waste Management 24 (2004) 353–358
ing 4–20 mg/l of total iron was pumped from the detention pond into the storage tank. Leachate from the tank was released by gravity into the filter column with an up-flow rate of 150 ml/min. As the Fe concentration was less than 20 mg/l, and in order to extend the breakthrough time in the course of experiment, the metals loading was maintained below 0.5 kg metals/m3 day, at a surface loading of 12.2 m3/m2 day and a retention time of 57.8 min. Details of the design of the Fe filter are given in Table 2. The effluents from raw and filtered leachate were collected through sampling points after each day to study filtration efficiency. The effluent was analyzed for pH and heavy metals (total Fe) based on the Standard Method for the Examination of Water and Wastewater (APPA, AWWA, WPCF, 1992). The batch equilibrium test was conducted by shaking the leachate in the presence of different volumes of limestone media in a 250 ml conical flask at 350 rpm. A
Fig. 1. Set up of the filtration column. Table 2 Design data for Fe removal in filtration experiment Parameters
Data
Unit
Particle size Density Void Column’s diameter Filter depth Vneta Retention time Maximum Fe concentration Surface Loading Rate Fe loading
2–4 2573 49 150 1000 0.00866 57.8 20 12.2 0.5
mm Kg/m3 % mm mm m3 min mg/l m3/m2 day kg Fe/m3 day
a
Net volume of filter column (void’s volume).
355
shaking time of 60 min followed by a settling time of 90 min (Aziz, 1992) was used in the experiment before the samples were analysed. pH and iron concentrations were analyzed before and after the experiment. A Hanna portable pH meter was used for pH determination; the Atomic Absorption Spectrophotometer (model Shimadzu AA 660) and Hach’s DR2010 spectrophotometer were used for the determination of iron concentration.
3. Results and discussion The limestone chips used in the experiment were composed of 95.5% CaCO3, 3.0% MgCO3, and 1.5% impurities. Fig. 2 summarizes the results of the filtration experiment after 30 days of continuous operation. Fig. 3 shows the results of the adsorption experiment (shaking experiment) at different pH values. Table 3 gives predicted results of limestone filter bed capacity for iron removal for a proposed plant, with assumed values of flow rates and influent Fe. Fig. 4 shows a plot of an equilibrium test aimed at determining the Fe isotherm. It can be observed from Fig. 2 that the average pH of the effluent increased from approximately 7.6–9.1. This is due to the presence of CO3 in limestone as stated elsewhere (Aziz, 1992; Aziz and Smith, 1996; Aziz et al., 2001). It can also be observed that the increase in metals removal is related to the increase in pH. For example, on the first day of filtration, up to 83% of Fe was removed (Fig. 2). The overall removal was between 83 and 95% with an average value of 89%. Breakthrough did not occur during the 30 days of the experiment. Similar patterns of removal were observed in the shaking experiment (Fig. 3). The latter indicated that the removal of iron was between 80 and 95%. Previous research suggested that there are four possible mechanisms that control heavy metal removal in a solute environment. These mechanisms include complexation, oxidation-reduction reactions, sorption, and precipitation (Christensen et al., 2001, 1994; Revans et al., 1999; Flyhammar and Hakansson, 1999). Aziz et al. (2001) concluded that heavy metals precipitated in the form of metal’s carbonate (Aziz, 1992). The surface charge of limestone is also predicted to be a contributing factor for the removal of heavy metals. Divalent metal cation (i.e., Fe2+) which is positively charged will be attracted to the negatively charged calcite surface at pH levels higher than 8.3 (Stumm and Morgan, 1996). The same mechanisms are expected to take place in the present experiment. Figs. 3 and 4 show plots of Freundlich’s isotherm in the present experiment. The coefficient of determination, R-squared, observed from Fig. 4 is 0.8119. The straight line proves the occurrence of adsorption that contributes to the removal of Fe from the solution.
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Fig. 2. Results of filtration experiment for the first 30 days of experiment at surface loading rate 12.2 m3/m2 day, metals loading 0.5 kg metals/m3 day, filter depth 1000 mm, and particle size 2–4 mm.
Fig. 3. Plot of volume of media verses leachate pH and% of Fe removal.
Fig. 4. Freundlich Isotherm of Fe removal from leachate.
The 1/n value represents the slope and Kf value is the antilog of the interception of log Ce. Hence, the Freundlich isotherm can be rewritten as: x ¼ 0:000498Ce1:4668 m
ð1Þ
The constant Kf is 4.98104 and primarily related to the capacity (effectiveness) of limestone to adsorb Fe. Larger values of Kf mean greater capacities of adsorption. The constant 1/n is 1.4668 and it is a function of the strength of adsorbent. Higher values of 1/n suggest that the adsorption bond is weak. This means the value of (x/m) experiences large charges for a small change in Ce. The smaller value of 1/n, (approaching 1), means the adsorption bond is strong and when the isotherm plot is horizontal, it is termed as irreversible. When 1/n > 1, the sorption constant increases with the increasing solution concentration, perhaps reflecting an increase in hydrophobic character of the surface after a monolayer. When 1/n < 1, Kf decreases with solution concentration as low energy sites are occupied. The breakthrough time, tb can be computed using the following equation: tb ¼
ðx=mÞb Mc Q½Ci ðCb =2Þ
ð2Þ
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357
Table 3 Predicted results of limestone filter bed capacity for proposed Fe removal plant Parameter
Value
Unit
1/n Kf Density CaCO3 (r) Influent conc., Ci Breakthrough conc., Cb Field breakthrough adsorption capacity, (x/m)=Kf (Ci)1/n 50% (x/m)b=0.5 (x/m) Mass of CaCO3, Mc=(V) Flow rate, Q Empty bed contact time, EBCT=Vv/Q Breakthrough time, tb=Eq. (2) Filter bed area (1.51.5), A Filter Depth (d) Void Bed volume (growth), Vg Bed volume (void), Vv=(Void)(Vg) Volume treated, Vb=(Q)(tb) Limestone usage rate=Mc/Vb Limestone usage rate=Mc/tb
1.4668 0.000498 2573 20 1 0.04033 0.02016 11578.5 100 31.75 239.5 2.25 2 49 4.5 2.205 23,950 483 48,340
kg/m3 mg/l mg/l mg/mg mg/mg Kg m3/day min day m2 M % m3 m3 m3 g/m3 g/day
where tb=time to breakthrough (day); (x/m)b=field breakthrough adsorption capacity (g/g); Mc=mass of adsorbent (limestone) (g); Q=flow rate (m3/day); Ci=influent leachate concentration (mg/l); and Cb=breakthrough leachate concentration (mg/l). By knowing tb, limestone adsorption capacity can be calculated and simplified as given in Table 3. For example, for a filter bed of 2.25 m2 and 2 m depth, as much as 23,950 m3 of leachate can be treated in about 8 months at a leachate flow rate of 100 m3/day. The usage rate for limestone media is about 48,340 g/ day.
Acknowledgements The author acknowledges the Ministry of Science, Technology and Environment Malaysia for the National Scientific Fellowship and IRPA research grant provided by Ministry of Science, Technology and Environment that has resulted in this article. The author also wishes to acknowledge cooperation given by the Majlis Perbandaran Seberang Perai, Penang and the contractor Idaman Bersih Sdn. Bhd., Penang during the study.
References 4. Conclusion Based on the preliminary results, it can be concluded that, at a retention time of 57.8 min, surface loading rate 12.2 m3/m2 day and metals loading below 0.5 kg metals/m3 day, the removal of Fe was more than 90% for the first 30 days of experiment. Thus limestone has a potential to be used as an alternative media for leachate treatment. It is also cheaper than activated carbon. The breakthrough time from the Fe isotherm of iron removal showed that reversible reaction occurred due to 1/n> 1. It has been proven that adsorption phenomenon contributes to the removal of Fe from its solution. The result of the experiment implies that the estimated breakthrough time for a filter bed of 4.5 m3 will be 8 months with a leachate production of 100 m3/day. This result suggests that limestone is potentially useful as an alternative leachate treatment system at a relatively low cost.
APHA, AWWA, WPCF, 1992. Standard Methods for the Examination of Water and Waste Water, nineteenth ed. Aziz, H.A., 1992. A study on the Removal of Manganese from Water in the Presence of Coarse Media. PhD thesis, University of Strathclyde, Glasgow (unpublished). Aziz, H.A., 1997. Heavy metals removal from water and wastewater using limestone filtration technique—a potential use for small and medium industries. Journal Institution of Engineers Malaysia (IEM) December 58 (3), 27–35. Aziz, H.A., Mohd, M., 1998. Potential use of low cost filtration technique for removing selected heavy metals from wastewater at pH less than 5. Indian Journal of Engineering and Material Sciences 5 (Aug), 194–201. Aziz, H.A., Smith, P.G., 1996. Removal of manganese from water using crushed dolomite filtration technique. Water Research 30 (2), 489–492. Aziz, H.A., Smith, P.G., 1992. The influence of pH and coarse media on manganese precipitation from water. Water Research 26 (6), 853–855. Aziz, H.A., Adlan, M.N., Perwira, M., Asaari, F., Basri, D.R., Yusuff, M.S., 2000. Removal of Cu, Pb and Zn from synthetic industrial wastewater using adsorption media-comparative study between activated carbon and limestone particle. Special Issue, Al-Azhar
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University Engineering Journal, Faculty of Engineering, Al-Azhar University 4, 33–40. Aziz, H.A., Othman, N., Yusuff, M.S., Basri, D.R.H., Asaari, F.A.H., Adlan, M.N., Othman, F., Johari, M., Perwira, M., 2001. Removal of copper from water using limestone filtration technique-determination of mechanism of removal. Environment International 26, 395–399. Basri, D.R.H., Mohamed, A.A.K., Aziz, H.A., Asaari, F.A.H., 2000. The Effectiveness of Cockle Shells in Treating Leachate Organic Pollutants’ in ISWA International Symposium and Exhibition On Waste Management in Asian Cities, 23–25 October 2000, Hong Kong Convention and Exhibition Centre. Christensen, T.H., Kjeldsen, P., Albrechtsen, H.J., Heron, G., Neilsen, P.H., Bjerg, P.L., Holm, P.E., 1994. Attenuation of landfill leachate pollutants in aquifers. Critical Reviews in Environmental Science and Technology 24 (2), 119–202. Christensen, T.H., Kjeldsen, P., Bjerg, P.L., Jensen, D.L., Christensen, J.B., Baun, A., Albrechtsen, H.J., Heron, G., 2001. Biogeochemistry of landfill leachate plumes. Applied Geochemistry 16, 659–718. Flyhammar, P., Hakansson, K., 1999. The Mobilization of Heavy Metals in Partially Stabilized MSW During Oxidation. Sardinia, 99, Seventh International Waste Management and Landfill Symposium, 4–7 October, Volume I, pp. 213–217. Kjeldsen, P., Christophersen, M., 2001. Composition of leachate from old landfills in Denmark. Waste Management & Research 19 (3), 249–256. Li, X.Z., Zhao, Q.L., Hao, X.D., 1999. Ammonium removal from landfill leachate by chemical precipitation. Waste Management 19, 409–415. Martensson, A.M., Aulin, C., Wahlberg, O., Agren, S., 1999. Effect of humic substances on the mobility of toxic metals in a mature landfill. Waste Management & Research 17, 296–304.
Othman, N., Aziz, H.A., Basri, D.R.H., Assari, F.A.H., 1999a. Removal Of Lead From Water Using Limestone Filtration Technique-Laboratory Scale. Malaysian Technologist, 2/ 99, 36–39. Othman N., Aziz, H.A., Basri, D.R.H., Ashaari, F.A.H., 1999b. The Influence Of Retention Time And Surface Loading Rate In The Removal Of Copper, Zinc And Chromium From Water Using Limestone Filtration Technique-Laboratory Scale. Proceedings, World Filtration Congress 8, Brighton, UK, 4–7 April 2000, Vol. 1, pp. 359–362, Elsevier Science. Othman, N., Aziz, H.A., 1999. The Influence of Flowrates and Particle Size in the Removal of Copper from Water Using Limestone Filtration Technique-Laboratory Scale. Proceedings International Conference of Recent Advances in Materials and Mineral Resources, RAMM’ 2, Penang Park Royal Penang, 3–5 May. Palma, L.D., Ferrantelli, P., Merli, C., Petrucci, E., 2002. Treatment of industrial landfill leachate by means of evaporation and reverse osmosis. Waste Management 22, 951–955. Reinhart, D.R., Grosh, C.J., 1998. Analysis of Florida MSW landfill leachate quality. Florida Center for Solid and Hazardous Waste Management Report #97-3. Revans, A., Ross, D., Gregory, B., Meadows, M., Harries, C., Gronow, J., 1999. Long Term Fate of Metals in Landfill. Sardinia, 99, Seventh International Waste Management and Landfill Symposium, 4–7 October, Volume I, pp. 199–206. Robinson, H.D. (1995). A Review of the Composition of Leachates from Domestic Wastes in Landfill Sites. A Report for the UK Department of the Environment, Reference: DE0918A/FR1. Stumm, W., Morgan, J.J., 1996. Aquatic Chemistry—Chemical Equilibria and Rates in Natural Waters, third ed. Wiley-Interscience, New York.
Physico-chemical removal of iron from semi-aerobic landfill leachate by limestone filter Hamidi Abdul Aziz*, Mohd Suffian Yusoff, Mohd Nordin Adlan, Nurul Hidayah Adnan, Salina Alias School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Received 8 October 2003
Abstract Limestone has been proven effective in removing metals from water and wastewater. A literature review indicated that limestone is capable of removing heavy metals such as Cu, Zn, Cd, Pb, Ni, Cr, Fe and Mn are through a batch process or by filtration technique. The removal capability is reported at up to 90%. However, to date most of the studies have been focused on synthetic wastewater. The present study attempts to investigate the suitability of limestone to attenuate total iron (Fe) from semi aerobic leachate at Pulau Burung Landfill Site in Penang, Malaysia. Iron was found in significant quantities at the landfill site. The study also aims to establish the Fe isotherm and breakthrough time of the proposed limestone filter for post-treatment to the migrating landfill leachate before its release to the environment. The Fe isotherms were established using a batch equilibrium test, while the breakthrough characteristics were determined using continuous flow permeating through a limestone column. The latter was used in order to simulate the continuous flow of leachate that would occur in the proposed limestone filter. The limestone media used in the experiment contain more than 90% CaCO3 with particle sizes ranging from 2 to 4 mm. Four filter columns (each 150 mm in diameter and 1000 mm depth) were installed at the landfill site. Metal loadings were kept below 0.5 kg /m3 day and the experiment was run continuously for 30 days. Initial results indicated that 90% of Fe can be removed from the leachate based on retention time of 57.8 min and surface loading of 12.2 m3/m2 day. For the batch study on the Fe isotherm, the results indicated that limestone is potentially useful as an alternative leachate treatment system at a relatively low cost. # 2003 Elsevier Ltd. All rights reserved.
1. Introduction Leachate is produced when moisture enters the refuse in a landfill, extracts contaminants into the liquid phase, and produces moisture content sufficiently high to initiate liquid flow. Leachate composition depends on many factors such as the waste composition, site hydrology, the availability of moisture and oxygen, design and operation of the landfill and its age. There is a considerable number of studies reported in the literature for metals concentrations from full-scale landfills, test cells, and laboratory works (Christensen et al., 2001, 1994; Kjeldsen and Christophersen, 2001; Revans et al., 1999; Reinhart and Grosh, 1998; Robinson, 1995). Heavy
* Corresponding author. Tel.: +60-45937788x6215; fax: +6045941009. E-mail address: [email protected] (H.A. Aziz). 0956-053X/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.wasman.2003.10.006
metals, which are commonly found in high concentrations in landfill leachate include iron, manganese, zinc, chromium, lead, copper and cadmium. They are a potential source of pollution for ground water, surface water and reservoirs. Iron concentration in leachate is normally contributed to by iron-base material waste, such as construction materials, paints, pigments, colour compounds, polishing agents and electrical materials. There are three mechanisms by which metals concentration could be increased in an aerobic landfill. These include the oxidation of metals sulfides to metal sulfates that are more soluble, the complexation capacity of oxidized humic acids relative to reduced humic acids (Martensson et al., 1999) and the oxidation of sulfides which results in the production of sulfuric acid that reduces the pH and increases the solubility of metal. Theoretically, the solubility of metal in the acidogenic phase is higher because of the production of acid in an
354
H.A. Aziz et al. / Waste Management 24 (2004) 353–358
anaerobic condition. Once the landfill has reached the methanogenic phase, heavy metal concentrations are generally low (Revans et al., 1999). Various studies have been carried out to remove undesirable constituents from landfill leachate. Removal of ammonium by chemical precipitation has been reported by Li et al. (1999). In 2002, Palma et al. reported on the treatment of industrial landfill leachate using evaporation and reverse osmosis. However, removal of metals from leachate is not well documented, especially in Malaysia. There are numerous techniques used for iron removal. These include aeration, softening, chlorination, ozonation and filtration. Aeration is the most common. For the filtration technique, the common media used are activated carbon and clay. The use of limestone for removing metals from water and industrial wastewater was found to be effective (Aziz, 1992, 1997; Aziz and Smith, 1992, 1996; Aziz and Mohd, 1998; Othman and Aziz, 1999; Othman et al., 1999a, 1999b; Aziz et al., 2000, 2001). More than 80% of heavy metals such as copper, iron, manganese, cadmium and others can be removed using a batch or continuous flow filtration process. Some of the removal mechanisms have been established. For example, removal of copper was due to the adsorption and absorption of this metal towards limestone particles (Aziz et al., 2001). However, to date, no studies have been reported on the use of limestone for leachate treatment in Malaysia. This study focuses on the leachate generated from Pulau Burung Landfill Site (PBLS); which is situated in Penang, Malaysia. PBLS has a semi-aerobic system and it is one of the only three sites of its kind found in Malaysia. A semi-aerobic system can be achieved through a convection process. The latter involves the decomposition of organic matter inside the landfill and will cause an increase in temperature. The difference in temperature between inside and outside of the landfill will generate a heat convection current into the landfill through the leachate pipe. PBSL has been developed semi-aerobically into a sanitary landfill Level II by establishing a controlled tipping technique in 1991. It was further upgraded to a sanitary landfill Level III employing controlled tipping with leachate recirculation in 2001. It was found that the leachate from a semi-aerobic system has slightly lower organic contaminants compared with an anaerobic landfill in terms of BOD and COD (Basri et al., 2000; Aziz et al., 2001). This site receives 1500 tons of solid waste daily. At PBLS, leachate is collected through collection pipes that feed into a detention pond. Table 1 shows details of the main characteristics of leachate used in the experiment. The table indicates that Fe is present in significant quantities with a maximum value of approximately 20 mg/l. The main source of Fe is from the ore waste industry.
Table 1 The characteristics of raw leachate from old detention pond at Pulau Burung Landfill Site (PBLS) Parameter
Unit
Average Value
BOD5 COD Suspended Solid Turbidity pH Zinc Manganese Iron Copper Chromium (VI) Colour
mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l mg/l Platinum unit, Pto
48–105 1533–2580 159–233 61–198 7.5–9.4 0.1 15.5 4.1–19.5 4.6 0.6 1000–4000
Base on 30 days reading.
The main objective of the present study is to investigate the suitability of limestone particles to be used as a filter, capable of attenuation of heavy metals, especially total iron (Fe). Fe was emphasized in this study due to its high concentration at PBLS, which is between 4 and 20 mg/l. This study also focused on the establishment of necessary parameters for the design of the limestone filter; which offers post-treatment to migrating leachate before its release to the environment. These include the determination of Fe isotherms and breakthrough characteristics of Fe. The study on adsorption isotherms was carried out through a batch process while the breakthrough characteristics were determined by a continuous process where leachate permeated through limestone columns. The latter was carried out in order to simulate the continuous flow of leachate through the proposed limestone filter.
2. Materials and methods The composition of limestone (in powder form with particle size of less than 75 mm) was determined by Xray fluorescence using a Rigaku RIX3000 and the X-ray using a Camscan Editor EDX. Limestone grains were sieved to obtain a uniform size between 2 and 4 mm. The density of limestone used in the experiment was 2573 kg/m3. Four perspex filter columns, each with a 150 mm diameter, were placed beside the old leachate pond (Fig. 1). Each column has several outlets set at different depths for sampling purposes. The filter columns were covered with waterproofing material to prevent the entrance of rainwater. A storage tank for raw leachate with a capacity of 100 l was placed at about 1 m above the top of the filter column to achieve a constant head for gravity feeding. The thickness of the limestone filter bed was 1000 mm. The bed was conditioned with three rinses of distilled water before the experiment was conducted. Then raw leachate contain-
H.A. Aziz et al. / Waste Management 24 (2004) 353–358
ing 4–20 mg/l of total iron was pumped from the detention pond into the storage tank. Leachate from the tank was released by gravity into the filter column with an up-flow rate of 150 ml/min. As the Fe concentration was less than 20 mg/l, and in order to extend the breakthrough time in the course of experiment, the metals loading was maintained below 0.5 kg metals/m3 day, at a surface loading of 12.2 m3/m2 day and a retention time of 57.8 min. Details of the design of the Fe filter are given in Table 2. The effluents from raw and filtered leachate were collected through sampling points after each day to study filtration efficiency. The effluent was analyzed for pH and heavy metals (total Fe) based on the Standard Method for the Examination of Water and Wastewater (APPA, AWWA, WPCF, 1992). The batch equilibrium test was conducted by shaking the leachate in the presence of different volumes of limestone media in a 250 ml conical flask at 350 rpm. A
Fig. 1. Set up of the filtration column. Table 2 Design data for Fe removal in filtration experiment Parameters
Data
Unit
Particle size Density Void Column’s diameter Filter depth Vneta Retention time Maximum Fe concentration Surface Loading Rate Fe loading
2–4 2573 49 150 1000 0.00866 57.8 20 12.2 0.5
mm Kg/m3 % mm mm m3 min mg/l m3/m2 day kg Fe/m3 day
a
Net volume of filter column (void’s volume).
355
shaking time of 60 min followed by a settling time of 90 min (Aziz, 1992) was used in the experiment before the samples were analysed. pH and iron concentrations were analyzed before and after the experiment. A Hanna portable pH meter was used for pH determination; the Atomic Absorption Spectrophotometer (model Shimadzu AA 660) and Hach’s DR2010 spectrophotometer were used for the determination of iron concentration.
3. Results and discussion The limestone chips used in the experiment were composed of 95.5% CaCO3, 3.0% MgCO3, and 1.5% impurities. Fig. 2 summarizes the results of the filtration experiment after 30 days of continuous operation. Fig. 3 shows the results of the adsorption experiment (shaking experiment) at different pH values. Table 3 gives predicted results of limestone filter bed capacity for iron removal for a proposed plant, with assumed values of flow rates and influent Fe. Fig. 4 shows a plot of an equilibrium test aimed at determining the Fe isotherm. It can be observed from Fig. 2 that the average pH of the effluent increased from approximately 7.6–9.1. This is due to the presence of CO3 in limestone as stated elsewhere (Aziz, 1992; Aziz and Smith, 1996; Aziz et al., 2001). It can also be observed that the increase in metals removal is related to the increase in pH. For example, on the first day of filtration, up to 83% of Fe was removed (Fig. 2). The overall removal was between 83 and 95% with an average value of 89%. Breakthrough did not occur during the 30 days of the experiment. Similar patterns of removal were observed in the shaking experiment (Fig. 3). The latter indicated that the removal of iron was between 80 and 95%. Previous research suggested that there are four possible mechanisms that control heavy metal removal in a solute environment. These mechanisms include complexation, oxidation-reduction reactions, sorption, and precipitation (Christensen et al., 2001, 1994; Revans et al., 1999; Flyhammar and Hakansson, 1999). Aziz et al. (2001) concluded that heavy metals precipitated in the form of metal’s carbonate (Aziz, 1992). The surface charge of limestone is also predicted to be a contributing factor for the removal of heavy metals. Divalent metal cation (i.e., Fe2+) which is positively charged will be attracted to the negatively charged calcite surface at pH levels higher than 8.3 (Stumm and Morgan, 1996). The same mechanisms are expected to take place in the present experiment. Figs. 3 and 4 show plots of Freundlich’s isotherm in the present experiment. The coefficient of determination, R-squared, observed from Fig. 4 is 0.8119. The straight line proves the occurrence of adsorption that contributes to the removal of Fe from the solution.
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Fig. 2. Results of filtration experiment for the first 30 days of experiment at surface loading rate 12.2 m3/m2 day, metals loading 0.5 kg metals/m3 day, filter depth 1000 mm, and particle size 2–4 mm.
Fig. 3. Plot of volume of media verses leachate pH and% of Fe removal.
Fig. 4. Freundlich Isotherm of Fe removal from leachate.
The 1/n value represents the slope and Kf value is the antilog of the interception of log Ce. Hence, the Freundlich isotherm can be rewritten as: x ¼ 0:000498Ce1:4668 m
ð1Þ
The constant Kf is 4.98104 and primarily related to the capacity (effectiveness) of limestone to adsorb Fe. Larger values of Kf mean greater capacities of adsorption. The constant 1/n is 1.4668 and it is a function of the strength of adsorbent. Higher values of 1/n suggest that the adsorption bond is weak. This means the value of (x/m) experiences large charges for a small change in Ce. The smaller value of 1/n, (approaching 1), means the adsorption bond is strong and when the isotherm plot is horizontal, it is termed as irreversible. When 1/n > 1, the sorption constant increases with the increasing solution concentration, perhaps reflecting an increase in hydrophobic character of the surface after a monolayer. When 1/n < 1, Kf decreases with solution concentration as low energy sites are occupied. The breakthrough time, tb can be computed using the following equation: tb ¼
ðx=mÞb Mc Q½Ci ðCb =2Þ
ð2Þ
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Table 3 Predicted results of limestone filter bed capacity for proposed Fe removal plant Parameter
Value
Unit
1/n Kf Density CaCO3 (r) Influent conc., Ci Breakthrough conc., Cb Field breakthrough adsorption capacity, (x/m)=Kf (Ci)1/n 50% (x/m)b=0.5 (x/m) Mass of CaCO3, Mc=(V) Flow rate, Q Empty bed contact time, EBCT=Vv/Q Breakthrough time, tb=Eq. (2) Filter bed area (1.51.5), A Filter Depth (d) Void Bed volume (growth), Vg Bed volume (void), Vv=(Void)(Vg) Volume treated, Vb=(Q)(tb) Limestone usage rate=Mc/Vb Limestone usage rate=Mc/tb
1.4668 0.000498 2573 20 1 0.04033 0.02016 11578.5 100 31.75 239.5 2.25 2 49 4.5 2.205 23,950 483 48,340
kg/m3 mg/l mg/l mg/mg mg/mg Kg m3/day min day m2 M % m3 m3 m3 g/m3 g/day
where tb=time to breakthrough (day); (x/m)b=field breakthrough adsorption capacity (g/g); Mc=mass of adsorbent (limestone) (g); Q=flow rate (m3/day); Ci=influent leachate concentration (mg/l); and Cb=breakthrough leachate concentration (mg/l). By knowing tb, limestone adsorption capacity can be calculated and simplified as given in Table 3. For example, for a filter bed of 2.25 m2 and 2 m depth, as much as 23,950 m3 of leachate can be treated in about 8 months at a leachate flow rate of 100 m3/day. The usage rate for limestone media is about 48,340 g/ day.
Acknowledgements The author acknowledges the Ministry of Science, Technology and Environment Malaysia for the National Scientific Fellowship and IRPA research grant provided by Ministry of Science, Technology and Environment that has resulted in this article. The author also wishes to acknowledge cooperation given by the Majlis Perbandaran Seberang Perai, Penang and the contractor Idaman Bersih Sdn. Bhd., Penang during the study.
References 4. Conclusion Based on the preliminary results, it can be concluded that, at a retention time of 57.8 min, surface loading rate 12.2 m3/m2 day and metals loading below 0.5 kg metals/m3 day, the removal of Fe was more than 90% for the first 30 days of experiment. Thus limestone has a potential to be used as an alternative media for leachate treatment. It is also cheaper than activated carbon. The breakthrough time from the Fe isotherm of iron removal showed that reversible reaction occurred due to 1/n> 1. It has been proven that adsorption phenomenon contributes to the removal of Fe from its solution. The result of the experiment implies that the estimated breakthrough time for a filter bed of 4.5 m3 will be 8 months with a leachate production of 100 m3/day. This result suggests that limestone is potentially useful as an alternative leachate treatment system at a relatively low cost.
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