the use of alum, ferric chloride and ferrous sulphate as coagulants

The use of Alum, Ferric chloride and Ferrous sulphate as coagulants in removing suspended solids, colour and COD from semi-aerobic landfill leachate at controlled pH

*Hamidi Abdul Aziz1, Salina Alias2, Faridah Assari3, Mohd Nordin Adlan4,1,3,4Associate Prof., Ph.D., 2Research Student

School of Civil Engineering, Engineering Campus, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, MALAYSIA.

Tel: 04-5995999 Fax: 04-5941009Email: [email protected]

Abstract

Suspended solids, colour and COD are amongst the main form of pollutants in leachate. Application of physical or biological process alone is normally not effective to remove these constituents, especially for leachate with lower BOD/COD ratio. The main objective of this research is to investigate the efficiency of coagulation and flocculation processes for removing suspended solids, colour and COD which present in significant quantity at a semi-aerobic Pulau Burung Landfill Site (PBLS) in Penang, Malaysia. Twelve months characterization of leachate indicated that it was stable with yearly average BOD/COD ratio of 0.15 and biologically difficult for further degradation. Particle size analysis of raw leachate indicated that its d50 was 11.676 µm. Three types of coagulants were examined using standard jar test apparatus, i.e., aluminum sulphate (alum), ferric chloride (FeCl3) and ferrous sulphate (FeSO4). The effects of agitation speed, settling time, pH, coagulant dosages and temperature were exermined. At 300 rpm of rapid mixing and 50 rpm of slow mixing and 60 minutes settling time, higher removals of suspended solids (over 95%), colour (90%) and COD (43%) were achieved at pH 4 and 12. FeCl3 was found to be superior compared with other coagulants. At pH 4 and 12, fair removal of suspended solids was observed at reasonably lower amount of coagulant, i.e., 600 mg/L. However, about 2500 mg/L of coagulant was required to achieve good removals at pH 6. Better removals were achieved at higher temperature. The d50 of sludge after coagulation at pH 4 and 2500 mg/L FeCl3 was 60.163 µm which indicated that the particles had been removed effectively from the leachate. The results indicated that coagulation and flocculation processes had contributed bigger roles in the integrated treatment system.

Keywords: Leachate, semi-aerobic, coagulation and flocculation, alum, ferric chloride, ferrous sulphate

1.0 Introduction

One of the most important aspects related to siting, planning, design, operation and long-term management of Municipal Solid Waste (MSW) landfill is the management of leachate. Leachate from MSW landfill sites are often characterized as heavily polluted wastewater. Leachate is a complex organic liquid formed primarily by the percolation of precipitation water through open landfill or through the cap of the

completed site (Senior, 1995). Leachate may contain large amount of organic contaminants which can be measured as chemical oxygen demand (COD) and biological oxygen demand (BOD), ammonia, halogenated hydrocarbons, suspended solid, significant concentration of heavy metals and inorganic salts (Trebouet et al., 2000; Bagchi, 1990). If it was not treated and safely disposed, landfill leachate could be a potential source of surface and groundwater contamination, as it may percolate through

soils and subsoils, causing pollution to receiving waters (Tatsi et al., 2003).

The specific composition of leachate influences its relative treatability and depends to a large extent on the contamination to be removed (Kreith, 1994). In general, the treatment of landfill leachate often involves a combination of various techniques. Several processes derived from water and wastewater treatment have been applied for the treatment of leachate. These include aerobic and anaerobic biological treatments, photo-oxidation and membrane processes, chemical oxidation and precipitation, activated carbon and adsorption and coagulation-flocculation (Trebouet et al., 2001; Marttinen et al., 2002; Wang et al., 2002; Enzminger et al., 1987; Lisk, 1991). Biological processes are quite effective for younger leachates (containing mainly volatile fatty acids), but less efficient for the treatment of stabilized leachate (Amokrane et al., 1997; Wang et al., 2002). However, leachate with low organic content is best treated with physical/chemical process (Baghi, 1990). Chemical precipitation using lime indicated that between 70 and 90% removal of color, turbidity, suspended matter and dispersed oil could be achieved (Amokrane et al., 1997). Therefore, a combination of physical-chemical and biological methods is often required for the efficient treatment of leachate (Rautenbach & Millis, 1994).

Coagulation and flocculation is widely used in water and wastewater treatment and these techniques form an important step in the treatment process (Muhammad et al., 1998; Amokrane et al., 1997; AWWA, 1971). Coagulation process is effective for removing high concentration organic pollutants (Wang et al., 2002), heavy metals and some anions (Kreith, 1994). Aluminum and iron coagulants have been widely used as coagulants for the removal of humic substances from water (Lu et al., 1999; Wang et al., 2002; Amokrane et al., 1997;

Koether et al., 1997; Ching et al., 1994). Chemical coagulants can destabilize colloidal particles by four distinct mechanisms: double layer compression; charge neutralization; enmeshment in a precipitate and inter-particle bridging (Al-Malack et al., 1999). Different coagulants provide different degrees of destabilization. The higher the valence of the counter-ion, the more will the destabilizing effect and the less amount of dose required for coagulation.

Studies on the coagulation-flocculation processes for the treatment of landfill leachate have been reported by a few researchers (Sletten et al., 1995; Ehrig, 1984; Amokrane et al., 1997; Diamadopoulos, 1994). Alum, ferrous sulphate, ferric chloro-sulphate and ferric chloride were commonly used (Ehrig, 1984; Amokrane et al., 1997). Amokrane et al. (1997) reported that iron salts seem to be more efficient than aluminum. They reported that the addition of flocculants together with coagulants may enhance the flocs-settling rate. Additionally they reported that nonionic, cationic or anionic polyelectrolyte could be used as coagulant aids to increase flocs settling rate, without really improving the turbidity removal efficiency.

However, limited information exists on the efficiency of the coagulation-flocculation process, when applied for the removal of suspended solids, colour and COD from semi-aerobic landfill leachate. A semi-aerobic landfill is a system where semi-aerobic condition is maintained in the waste body through a convection process. It involves the decomposition of organic matter inside the landfill and causes an increase in temperature. The difference in temperature between the inside and outside of the landfill generates a heat convection current in the landfill through the leachate pipe (Aziz et al., 2004). It was found that the leachate from semi-aerobic system has slightly lower organic contaminants compared with an anaerobic landfill in terms of BOD, COD, and other

constituents (Basri et al., 2000; Aziz et al., 2004).

This study focuses on the leachate generated from Pulau Burung Landfill Site (PBLS); which is situated within Byram Forest Reserve at 5° 24’ N, 100° 24’ E in Penang, Malaysia. The total area of the landfill is 23.7 ha and equipped with a leachate collection pond but without other treatments. This site is contained by a natural marine clay liner. PBLS has a semi-aerobic system and it is one of the only three sites of its kind found in Malaysia. PBLS has been developed as a sanitary landfill Level II by establishing a controlled tipping technique in 1991. It was further upgraded to a sanitary landfill level III by employing controlled tipping with leachate recirculation in 2001. This site receives 1,500 tons of solid waste daily (Aziz et al., 2004).

The objective of this study was to examine the efficiency of coagulation-flocculation processes for the removal of suspended solids, colour and COD from a semi-aerobic landfill leachate using three types of coagulants, i.e., aluminum sulphate (alum), ferric chloride and ferrous sulphate. The experiments involved with the determination of the most appropriate coagulant types and dosages, the examination of the effect of pH on the removal capacity and the identification of the optimum experimental conditions for the efficient application of these processes.

2.0 Materials and method

Leachate samples were collected from PBLS between January and December 2003. If tests could not be carried out on the same day, the samples were stored in a refrigerator and maintained at 4°C Coagulation and flocculation studies were performed in a standard jar-test apparatus (Jar Tester Model CZ150) comprises of six paddle rotors (24.5mm x 63.5mm) and equipped with 6 beakers of 1 L volume. Leachate samples were removed from the refrigerator and conditioned for about 3

hour under ambient temperature. Samples were thoroughly agitated for re-suspension of settled solids before any tests were conducted. Chemical reagents used as coagulants are commercially available from R&M Marketing, UK which include aluminum sulphate [Al2(SO4)3.18H2O], ferric chloride (FeCl3) and ferrous sulphate [FeSO4]. Stock solutions were prepared by dissolving 50g of these salts in 1 L of deionized water. Each beaker used for testing was filled with 500mL of sample. The influences of agitation speed (rapid and slow mixing) and settling time were examined in the pre-experiments at pH 6 and at a coagulant dosage of 500 mg/L. The influence of agitation speed was determined in the subsequent experiment by varying the rapid mixing from 250 to 350 rpm and slow mixing from 20 to 70 rpm. Variations of settling times were made between 10 and 210 seconds. In the pre-experiments suspended solids were used as controlled parameter. In the main experiments, constant dosage of coagulants (1000 mg/L) was used to determine the optimum pH. In order to determine the optimum dosage of coagulant, initial pH of 4, 6 and 12 were used. The initial rapid mixing for all experiments was taken as 60 seconds (range between 30-300 seconds, Ramirez & Velasquez, 2004; Aquilar et al., 2005) and for slow mixing at 19 minutes (range between 5-55 seconds, Tatsi, et.al, 2003; Aquilar et al., 2005).

After settling (duration, pre-determined in the pre-experiment), about 50 mL of supernatant was withdrawn using plastic syringe from the point located about 2 cm below liquid level for the determination of pH, suspended solids, colour and COD. Analyses were made in triplicates. Experiments were conducted at 25°C and 10°C.

Selected flocs formed after settling were sampled for particle size distribution using Malvern 2000-S Mastersizer. In this study, the analysis was conducted for raw leachate and coagulated leachate (sludge)

at pH 4, using 2500 mg/L FeCl3 as coagulant.

The pH was measured by pH meter (CyberScan 20). Colour measurements were reported as true colour (filtered using 0.45µm filter paper) assayed at 455 nm using DR 2000 HACH spectrophotometer that was adapted from Standard Methods for the Examination of Water and Wastewater (APHA, 1995) (Method No 2120C). The result is reported in Platinum-cobalt (PtCo), the unit of colour being produced by 1 mg platinum/L in the form of the chloroplatinate ion. The effect of filtration on colour removal was corrected by means of a control sample. COD were determined in accordance with Method 5220D (closed reflux, colourimetric method). Removal efficiency of suspended solids, colour and COD was obtained using the following equation:

(Eqn. 1)

Where Ci and Cf are the initial and final concentrations of leachate, respectively.

2.0 Results and discussion

The characteristics of raw leachate are presented in Table 1. In terms of pH, the leachate could be categorized as alkaline. In terms of COD, Table 1 suggests that the leachate has moderate concentration of organic matter. The annual average BOD/COD ratio was 0.15 which reflects the presence of high concentrations of recalcitrant organic matter which is stable and biologically difficult for further degradation. The Table also indicates that the values of suspended solids and turbidity had exceeded the standard discharge limit of the Malaysian Environmental Quality Act 1974 (MDC, 1997). The permissible limits for Standard A and B are 50 mg/L and 100 mg/L respectively. The presence of high concentrations of colour was contributed

by the presence of dissolved organics. These organic compounds may present in the form of recalcitrant material of humic acids (Artiola and Fuller, 1982). Humic substances are natural organic matters made up of complex structures of polymerized organic acids, carboxylic acids, and carbohydrates (Langlais et .al., 1991). The level of NH3-N was very high (average value 970 mg/L) which was the result of slow leaching and the release of soluble nitrogen from solid waste in landfills, which may last for several decades (Jokela et al., 2002; Tyrrel et al., 2002; Hoilijoki et al., 2000). This implies that there is a need to treat the leachate before it can be discharged. Integrated treatment systems consisting of biological, physical and chemical treatments are often necessary to treat this type of leachate. However, the focus of this paper is on the coagulation and flocculation processes.

Table 1: Lechate characterisation of PBLS in 2003PARAMETERS YEARLY

AVERAGE

JANUARY FEBRUARY MARCH APRIL MAY JUNE6/1/200

3 29/1/03 5/2/2003 27/2/03 6/3/2003

28/3/03

7/4/2003 27/4/03 6/5/20

03 26/5/03 6/6/2003

pH 8.31 8.35 7.43 8.16 8.19 8.36 8.46 8.62 8.78 8.42 8.78 8.44BOD (mg/L) 377 75 184 105 80 104 65 59 68 168 140 181COD (mg/L) 2098 1749 1912 1641 1763 1806 1650 1684 1164 1625 1887 2338BOD/COD 0.15 0.04 0.10 0.06 0.05 0.06 0.04 0.04 0.06 0.10 0.07 0.04Turbidity (NTU) 151 150 105 104 109 150 68 120 58 124 100 110Suspended Solids

(mg/L) 297 216 233 220 233 164 181 196 86 234 210 251

Colour (ptCo) 3423 2730 2130 2090 2970 2950 2940 3160 1535 3540 3960 3360Ammoniacal Nitrogen

(mg/L) 970 685 1224 367 491 1263 772 982 678 524 729 506

Chromium+6 (mg/L) 0.04 0.02 0.07 0.07 0.05 0.04 0.04 0.03 0.01 0.07 0.05 0.04

Chromium+3 (mg/L) 0.12 0.08 0.08 0.09 0.08 0.1 0.06 0.12 0.32 0.31 0.08 0.14Iron (mg/L) 4.84 5 5.9 6.1 4.8 5.6 5 3.4 3.5 4.4 3.2 7.3Temperature (°C) 27.8 27.8 27.6 27.9 28.0 28.1 28.1 28.4 28.6 28.8 28.6 28.1Rainfall (mm/day) 21.1 0.7 1.3 1.1 - 1.4 - 1.5 0.2 10.2 0.9 -

PARAMETERSYEARLY AVERA

GE

JUNE18/6/03

JULY AUGUST SEPTEMBER OCTOBER NOVEMBER DECEM

BER3/7/200

3 21/7/03 6/8/2003 25/8/03 9/9/20

03 17/9/03 27/10/03 5/11/2003

20/11/03

15/11/2003

pH 8.31 8.4 8.32 8.56 8.05 8.3 8.84 8.05 7.91 8.39 7.8 8.24

BOD (mg/L) 377 708 2526 182 548 176 603 419 1120 345 149 285

COD (mg/L) 2098 2122 4779 2338 2118 1850 1535 2134 3450 2197 1795 2608

BOD/COD 0.15 0.33 0.53 0.08 0.26 0.10 0.39 0.20 0.32 0.16 0.08 0.11Turbidity (NTU) 151 420 240 110 100 160 116 90 450 170 172 100Suspended Solids

(mg/L) 297 550 320 251 303 303 329 285 936 380 310 342

Colour (ptCo) 3423 5060 4550 3380 3560 3160 2760 3000 6540 3630 3500 4800Ammoniacal Nitrogen

(mg/L) 970 1292 754 888 1071 1332 2233 1214 1325 1104 810 1104

Chromium+6 (mg/L) 0.04 0.03 0.04 0.02 0.02 0.05 0.04 0.03 0.02 0.02 0.05 0.05Chromium+3 (mg/L) 0.12 0.09 0.06 0.12 0.09 0.1 0.12 0.12 0.09 0.1 0.08 0.1

Iron (mg/L) 4.84 4.5 4.7 5.2 2.8 4 4.6 3.1 5.9 5.1 7.2 5.1Temperature (°C) 27.8 28 27.8 27.8 27.7 27.8 27.6 27.4 26.4 26.6 27 26.9Rainfall (mm/day) 21.1 17 30.9 5 - - 70.5 34.2 123.7 24.1 14.6 -

Figure 1: Influence of agitation speed on suspended solids removals at pH 4, 1000

mg/L FeCl3 as coagulant.

The influence of agitation speed on the removal of suspended solids is presented in Figure 1. It can be observed that at 300 rpm of rapid mixing and 50 rpm of slow mixing had exhibited the highest removal of suspended solids (over 98% removal) as

compared to the other speeds. The slow mixing speed was in agreement with the range of 20-60 rpm recommended by Aguilar et al., (2005) and Tatsi et al., (2003).

Figure 2: Influence of settling time on

suspended solids removals at pH 4, 1000 mg/L FeCl3 as coagulant

0

20

40

60

80

100

4 6 8 10 12

pH values

% C

olou

r Rem

oval

Alum

FeCl3

FeSO4

The influence of agitation speed on the removal of suspended solids is presented in Figure 1. It can be observed that at 300 rpm of rapid mixing and 50 rpm of slow mixing had exhibited the highest removal of suspended solids (over 98% removal) as compared to the other speeds. The slow mixing speed was in agreement with the range of 20-60 rpm recommended by Aguilar et al., (2005) and Tatsi et al., (2003). However, the rapid mixing was slightly higher than the range of 80-250 rpm as suggested by Tan et al., (2000) and Connolly et al., (2004). The influence of settling time shown in Figure 2 indicated that almost complete removal of suspended solids was achieved after 60 minutes of settling. The retention time was in agreement with the established range of between 30 and 120 minutes (Amokrane et al., 1997; Connolly et al., 2004). As the concentration of suspended solids in the raw leachate was considerably high, the destabilization of colloidal particles occurred due to the adsorption of strongly charged partiallly hydrolysed metallic ions. Continued adsorption will result in charge reversal and restabilization of the suspension which does occur at higher coagulant dosages (Tebbutt, 1998).

The influence of pH on the removal of suspended solids, colour and COD is important in coagulation process. According to AWWA (1971), pH is the most important variable in the coagulation process for water treatment.

Figure 3: Removal of suspended solids at different pH values with addition of

1000 mg/L coagulants

The extent of pH range is affected by the types of coagulant used and by the chemical composition of water as well as by the concentration of coagulant. Figures 3 to 5 show the effects of pH on the overall removal performance of suspended solids, colour and COD at a random dosage of 1000 mg/L. It can be noted from these figures that, relatively higher removals of suspended solids, colour and COD were observed at lower and higher pH values (pH 4 and 12).

Figure 4: Removal of colour at different pH values with addition of 1000 mg/L

coagulants

Figure 5: Removal of COD at different pH values with addition of 1000 mg/L

coagulants

For example, at pH 4 and 12, FeCl3

had removed 95% of suspended solids at pH 4 compared with less than 38% at pH 6 (Figure 3). The same pattern of removal was observed for colour (Figure 4) and COD (Figure 5).

Figure 6: Effect of different dose of coagulant on suspended solids at pH 4.

Figure 7: Effect of different dose of coagulant on suspended solids at pH 6.

Figure 8: Effect of different dose of

coagulant on suspended solids at pH 12


coagulant on colour at pH 4.

Figure 10: Effect of different dose of coagulant on colour at pH 6.


coagulant on colour at pH 12.

Figure 12: Effect of different dose of coagulant on COD at pH 4.


coagulant on COD at pH 6.

Figure 14: Effect of different dose of coagulant on COD at pH 12.

Figures 6 to 14 present the results of the effect of coagulation dosages at pH 4, pH 6 and pH 12. The effects of coagulation on suspended solids, colour and COD are shown on Figures 6 to 8, 9 to 11 and 12 to 14, respectively. From these figures, at pH 4 and 12, good removals of suspended solids, colour and COD were achieved with reasonably lower amount of

coagulant (600 mg/L). However, about 2500 mg/L of coagulants were required to give good removal at pH 6.FeCl3 was found to be generally superior to the other two coagulants in removing all the parameters. The same pattern of removals was noted for color and COD. For example, at pH 4, over 90% of suspended solids were removed using FeCl3 compared with only 75% and 46% removals using alum and FeSO4, respectively Final effluent was less than 60 mg/L using FeCl3 as coagulant and this has complied with the Standard B of the Malaysian Environmental Quality Act 1974 (less than 100 mg/L in terms of suspended solids).

Table 2:Removal of suspended solids, colour and COD at different pH values and dosages of coagulants.

pH Results Suspended Solids Colour CODAlum FeCl3 FeSO4 Alum FeCl3 FeSO4 Alum FeCl3 FeSO4

4 Optimum/economical dosage (mg/L)

600 600 600 600 600 600 600 600 600

Initial* concentration

1106 1106 1068 6450 6460 7275 2660 2565 3320

Final** concentration

282 59 582 2554 626 5485 1862 1472 1291

% Removal 74.5 94.7 45.5 60.4 90.3 24.6 30 42.6 61.16 Optimum/

economical dosage (mg/L)

2500 2500 2500 2500 2500 2500 2500 2500 2500


983 786 878 7005 7100 7003 3015 2980 3066


291 8 506 3159 249 5953 2204 1648 2422

% Removal 70.4 99 42.4 54.9 96.5 15 26.9 44.7 2112 Optimum/

economical dosage (mg/L)

600 600 600 600 600 600 600 600 600


1106 932 1068 6460 6658 7270 3210 3565 3320


90 52 197 2558 1738 2690 2793 2777 2825

% Removal 91.9 94.4 81.6 60.4 73.9 63 13 22.1 14.9

This is in agreement with the findings of Amokrane et al., (1997) who indicated that ferric chloride was more effective than aluminum sulphate (94% and 87%, respectively) for turbidity removal for the pretreatment of landfill leachate by coagulation-flocculation processes.

Alum has performed better than FeSO4, especially in removing suspended

solids. For example, at pH 12, the removals of suspended solids by alum and FeSO4 were 92% and 82%, respectively (Table 2).

Significant reductions of colour using FeCl3 and alum were observed at pH 4 (at a coagulant dosage of 600 mg/L) and pH 6 (at a coagulant dosage of 2500 mg/L). At these dosages, alum had

removed 60% of colour at pH 4 whereas 55% removal was achieved at pH 6. In comparison, FeCl3 exhibited better performance with 90% and 97% removals at pH 4 and 6, respectively. Selcuk (2005) reported that 79% colour removal was achieved using 1500 mg/L ferrous sulphate and only 50% absorbances were achieved at a dosage of 1000 mg/L.

Fair removals of COD were observed at 40% and 60% using FeCl3 and FeSO4 respectively. These were achieved at pH 4 and coagulant dose of 600 mg/L. The results are in agreement with the findings of other workers. Lin & Peng (1996) had reduced 33% of COD from 900 mg/L through continuous treatment of an influent of textile wastewater. Nicolaou & Hadjivassilis (1992) had reduced 44% of COD from textile influent of 694 mg/L. Kim, et.al., (2003) had reduced 63% COD for his biologically pretreated textile influent using FeCl3.6H2O of 3.25x10-3

mol/L as coagulant. Selcuk (2005) reported that 59% of COD were removed from raw wastewater using 1000 mg/L ferrous sulphate as coagulant compared with 56% removal by alum.

The removals of colour and COD were mainly attributed by the formation of precipitates from the combination of the soluble organics and the coagulant (Tebbutt, 1998). Basic reactions occur during coagulation process are shown by the following equations:FeSO4 + 2HCO3- ↔ Fe(OH)2 (↓)+ (SO4)2-

+ 2CO2 (2)2FeCl3 + 6HCO3- ↔ 2Fe(OH)3 (↓)+ 6Cl- + 6CO2 (3)Al2(SO4)3 + 6HCO3- ↔ 2Al(OH)3 (↓)+ 3(SO4)2- + 6CO2 (4)

Figure 15: Influence of temperature on suspended solids removals

In terms of temperature, it has been demonstrated that better removal had been achieved at higher temperature for all parameters (Figure 15). Ozbelge et al., (2002) concluded that flocs settlement was reduced with a decrease in temperature from 23°C to 15°C using FeCl3 and carbonate. Hammer & Hammer (2004) reported that the chemical reactions had increased at higher temperature. Reduction of turbidity at higher temperature was attributed by higher Brownian and Van Der Waals movements in water which had enhanced the chemical reactions (Rios et al., 1998).The results of the present study are in agreement with the findings of various other workers, even though there was no single pH region had been agreed by them. Wang et al., (2002) indicated that, the lower the pH value, the higher the efficiency of the treatment in the pH range of 3 to 8. Other researchers suggested that, organic colloids are favourable in the flocculation process between pH 4 and 4.5 using ferric salts and between 5 and 6 with aluminum salts (Babcock and Singer, 1979; Semmens and Field, 1980; Blanc and Navia, 1991). Amokrane et al., (1997) also observed that turbidity removal was decreased with an increase in pH. They reported that ferric chloride coagulant is generally effective at pH 4.9 while aluminum sulphate at pH 5.5. They found that 94% of turbidity had been removed when ferric chloride was used as coagulant. About the same results were obtained by several authors in their

leachate coagulation-flocculation experiments (Stegmann and Ehrig, 1980; Ehrig, 1984). The differences in results may be due to different experimental conditions and types of leachate used as most of their studies involved an anaerobic leachates. Nevertheless, the decrease in pH can be explained by the acidic character of Fe3+ or Al3+. When reacting with OH- ions of leachate, aluminum or iron will precipitate in the form of Fe(OH)3 or Al(OH)3 (O’melia, 1969). According to Stephenson and Duff (1996), the influence of pH on chemical coagulation/floccculation may be considered as a balance of two competitive forces: (1) between H+ and metal hydrolysis products for interaction with organic ligands and (2) between hydroxide ions and organic anions for interaction with metal hydrolysis products.

Table 3: An example of particle size distribution of raw and coagulated

leachate samplesSample D10

(µm)D50

(µm)D90

(µm)Specific surface

area (m2/g)

Uniformity

Raw leachate

3.045 11.676 60.039 1.16 1.33

Settled leachate after coagulation by 2,500 mg/L FeCl3 at pH 4

27.358 96.319 282.843 0.104 0.829

Results of particle size distribution of flocs presented in Table 3. Figures 16 and 17 indicated that the d50 of raw leachate was 11.676 µm. The d50 after coagulation became 96.319 µm which indicated that the particles had been settled and removed effectively (as detailed in Figures 6 to 8).

4.0 Conclusion

Leachate at Pulau Burung Landfill Site in Penang, Malaysia was characterized for 12 months in this study. It was slightly alkaline with moderate

concentration of organic matter. The yearly average of BOD/COD ratio was 0.15 which reflects the presence of high concentrations of recalcitrant organic matter which is stable and biologically difficult for further degradation. Coagulation and flocculation studies conducted on the leachate indicated that FeCl3 was capable to remove suspended solids, colour and COD at lower dosages as compared with alum and FeSO4. Better removal was observed at pH 4 and pH 12. For an effective removal of the above parameters, the optimum rapid and slow mixing was 300 rpm and 50 rpm respectively. At pH 4 and 12, a fair removal at reasonably lower amount of coagulant can be achieved using FeCl3

with a dosage of 600 mg/L. However, about 2500 mg/L of coagulants were required to give good removals at pH 6. Better removal was achieved at higher temperature. The d50 of sludge after coagulation at pH 4 with 2500 mg/L FeCl3

was 60.163 µm compared with 11.676 µm before coagulation. The study indicated that coagulation and flocculation processes are significantly important in the overall integrated treatment system for semi-aerobic leachate. Integrating the process into the existing biological and physical treatment may enhance the treatment performance, particularly for removal of suspended solids, colour and COD.

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 Prai, Penang and the contractor Idaman Bersih Sdn. Bhd., Penang during the study.

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the use of alum, ferric chloride and ferrous sulphate as coagulants

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