Chemical Engineering Journal 221 (2013) 492–499

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Chemical Engineering Journal

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Pretreatment of stabilized leachate using ozone/persulfate oxidationprocess

1385-8947/$ - see front matter � 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.cej.2013.02.038

⇑ Corresponding author. Tel.: +60 45996215; fax: +60 45941009.E-mail addresses: cehamidi@eng.usm.my, cehamidi@yahoo.com (H.A. Aziz).

Salem S. Abu Amr a, Hamidi Abdul Aziz a,⇑, Mohd Nordin Adlan a, Mohammed J.K. Bashir b

a School of Civil Engineering, Engineering Campus, UniversitiSains Malaysia, 14300 NibongTebal, Penang, Malaysiab Department of Environmental Engineering, Faculty of Engineering and Green Technology, Universiti Tunku Abdul Rahman, 31900 Kampar, Perak, Malaysia

h i g h l i g h t s

" Ozone and persulfate reagent ðO3=S2O2�8 Þ was used to treat stabilized leachate .

" Removal efficiency of COD, Color and NH3–N was 72%, 93% and 55%, respectively." Biodegradability (BOD5/COD) was improved from 0.043 to 0.29." A good value of ozone consumption (OC) obtained with 0.76 (kg O3/kg COD)." Performance of ozone/persulfate is higher than Ozone/Fenton in leachate treatment.

a r t i c l e i n f o

Article history:Received 19 December 2012Received in revised form 9 February 2013Accepted 11 February 2013Available online 18 February 2013

Keywords:Stabilized leachateOzonePersulfateAdvanced oxidationBiodegradability

a b s t r a c t

A combination of ozonation and persulfate ðO3=S2O2�8 Þ was used to treat stabilized landfill leachate. COD,

color, and NH3–N, ozone and persulfate doses, pH and reaction time were evaluated to define the opti-mum operational conditions. The results indicated that under optimum operation conditions (i.e.210 min of ozonation, a COD=S2O2�

8 ratio (1 g/7 g), and pH 10), the best removal values of COD, color,and NH3–N were 72%, 93%, and 55%, respectively. The biodegradability (BOD5/COD ratio) has improvedfrom 0.05 to 0.29. The ozone consumption for COD removal was 0.76 kg O3/kg COD. The results obtainedby employing the combined use of persulfate and ozone were compared with those by employing ozoneonly and persulfate only. The results achieved with the combined use of persulfate and ozone were com-pared with those obtained with ozone only and persulfate only. The combined method (i.e., O3=S2O2�

8 )achieved higher removal efficiencies for COD, color, and NH3–N compared with other studied applica-tions. Additionally, the combined of ozone/persulfate method proved to be more efficient than the com-bined use of ozone/Fenton in advanced oxidation processes in the treatment of the same studied leachate.

� 2013 Elsevier B.V. All rights reserved.

1. Introduction

Continuous population growth and industrial developmenthave led to an increase in waste generation. To date, land fillingis the preferred option for the disposal and management of solidurban wastes [1]. Despite the advantages of this disposal method,the highly polluted leachate that is produced elicits significant con-cern [2]. Landfill leachate is defined as liquid that seeps throughsolid waste in a landfill, producing extracted, dissolved, or sus-pended materials [3]. The age of the landfill site is one of the mostimportant factors that affect leachate characteristics. Old landfills(>10 years) produce mature leachate with stable characteristics,also called stabilized leachate, which is relatively less biodegrad-able (BOD5/COD ratio <0.1) and contains low COD concentration

compared with the young leachate [4,5]. Leachate, which is recog-nized as a potential source of groundwater and surface water con-tamination, contains large quantities of organic compounds,ammonia, heavy metals, a complex variety of materials, and manyother hazardous chemicals [6,7]. Regardless of changes in the con-centration of landfill leachate, its complexity can be categorizedbased on the four major groups of pollutants depending on a com-plex set of interrelated factors: dissolved organic matter, inorganicmacro-components, heavy metals, and xenobiotic organic com-pounds [8].

One of the main problems encountered in landfill managementis the establishment of efficient treatment methods for large quan-tities of polluted leachate. The environmental effects of leachateare influenced by its strength, proper collection, and efficiency oftreatment. Leachate requires treatment to reduce the quantity ofpollutants to an acceptable level prior to discharge into watersources [9]. A number of leachate treatment techniques, including

Fig. 1. Schematic diagram of semi-aerobic landfill[39].

S.S. Abu Amr et al. / Chemical Engineering Journal 221 (2013) 492–499 493

biological, physical, and chemical processes, have been applied[10,11]. Ozone is one of the chemical processes that has recentlyreceived considerable attention in landfill leachate treatment be-cause of its oxidation potential and ability to reduce leachatestrength and the quantity of non-biodegradable organics [12–14].Advanced oxidation processes (AOPs) have received attention inwastewater and leachate treatment because of their good perfor-mance in removing color and organic load from leachate [15,16].AOPs, such as O3, H2O2, UV, persulfate, and a combination of O3,H2O2, and UV, are involved with producing powerful oxidizingagents. Hydroxyl radical (OH_) is a primary oxidizing agent pro-duced from single oxidants such as ozone or from a combinationof strong oxidants (e.g., O3 and OH�, H2O2, ultraviolet, ultrasound,or electron beam, and catalysts such as transition metal ions orphoto catalysts) [17–23,14]. The effectiveness of ozone alone inleachate treatment is low; however, its effectiveness can be im-proved using advanced oxidation materials and techniques (i.e.,O3/H2O2, O3/UV, O3/Fe2+/H2O2). Furthermore, significant improve-ments in treatment efficiency of ozone can be obtained from pHvariance.

Using ozone alone; Tizaoui et al. [17] obtained 27% and 87% re-moval rates for COD and color, respectively, during 60 min ozona-tion at 80 g/m3 O3 dosage and natural pH (7). Hagman et al. [24]obtained a 22% COD reduction, using 4 g/L O3 dosage at pH 8–9,whereas Rivas et al. [25] achieved a 30% reduction in COD during1 h ozonation using 1.3–1.5 g O3 per g COD. Furthermore, AbuAmr and Hamidi [26] obtained 15% and 27% removal of COD andcolor, respectively, used 80 g/m3 O3 dosage, pH (8.5) at 60 minozonation.

Using hydrogen peroxide in AOPs, Tizaoui et al. [17] improvedthe removal efficiency from 27% to 50% and from 87% to 95% forCOD and color, respectively, using 2 g/L of H2O2 dosage. Hagmanet al. [24] obtained an improvement of COD removal rate from22% (ozone alone) to 50%, using 1 g/L of H2O2 at pH 8. Moreover,COD removal improved from 49%, using ozone alone, to 78%,using O3/UV process at 120 min of ozonation [18] and theO3/Fe2+/H2O2 process. Improvements in COD and color removalfrom 15% and 27% to 65% and 98% were obtained, respectively,using 0.05 mol L�1 of H2O2 and 0.05 mol L�1 of Fe2+ dosage atpH 7 and 90 min of ozonation [26]. Using pH variation, Goiet al. [9] achieved a COD removal rate ranging from 24% to 41%at pH varying from 4.5 to 11.

However, the use of ozone and ozone in AOPs has a low ammo-nia removal efficiency, which exists in high levels in stabilizedleachate and is extremely toxic to the environment and aquaticorganisms [27]. Persulfate ðS2O2�

8 Þ is the newest oxidant used inthe chemical oxidation of groundwater and soil cleanup [28]. Per-sulfate, which has standard oxidation potential (Eo = 2.01 V) com-pared with ozone (Eo = 2.07 V) [29], has recently received anattention in landfill leachate treatment because of its effectivenessin removing organics and ammonia [30]. Persulfate activation canbe investigated by examining naturally occurring iron oxides, man-ganese oxides, clay minerals, trace minerals, base, iron chelates,and organic compounds [31]. Although persulfate anion can actas a direct oxidant, its reaction rates are limited in more recalci-trant contaminants [32]. Furthermore, S2O2�

8 can initiate sulfiteradical based-AOPs that are strong oxidants (E� = 2.7 V) [33]. Shab-iimam and Dikshit [19] obtained 35% and 79% removal of TOC andcolor from stabilized leachate using sodium persulfate oxidationalone in acidic media (pH 2–4.5). Xu et al. [34] employed potas-sium persulfate (K2S2O8) combined with activated carbon in stabi-lized leachate treatment and obtained 77.8% COD removal andenhanced biodegradability. Ghauch et al. [35,36] utilized heatedsodium persulfate(Na2S2O8) for ibuprofen and methylene blue deg-radation in aqueous solution. Yang et al. [37] used activated carbonas catalyzed sodium persulfate for azo dye acid orange 7 removal.

Thus, persulfate oxidation in combination with other treatmentprocesses is more efficient for stabilized leachate treatment.

Consequently, the current study investigates the effectivenessof cooperatively employing S2O2�

8 and O3 in stabilized landfillleachate treatment. The scope and benefit of this new method isan increase in the oxidation potential using S2O2�

8 in the advancedoxidation processes during the ozonation of stabilized leachate atone treatment reactor stage. The effectiveness of the S2O2�

8 dosage,pH variation, and reaction time in an ozone reactor are investi-gated in the current work. Furthermore, the treatment efficiencyof the combined ozone/persulfate method as one oxidation stepin an ozone reactor was assessed and compared with those pro-cesses conducted with ozone alone and persulfate alone.

2. Materials and methods

2.1. Leachate sampling and characteristics

Leachate samples were collected from an aeration pond atPulauBurung Landfill Site (PBLS), at the Byram Forest Reserve inPenang, Malaysia. The leachate samples were collected from thearea of the landfill that continues to receive waste. The PBLS is clas-sified as a semi-aerobic stabilized landfill and has an area of62.4 ha, of which 33 ha are currently operational, receiving approx-imately 2200 t of municipal solid waste daily. The site is equippedwith a natural marine clay liner and three leachate collectionponds [27]. In the first 10 years of operation, from the 1980s until1990, waste disposal suffered from lack of proper management andfrom inappropriate leachate control. In 1990, PBLS started operat-ing as a semi-aerobic system, and the leachate was collectedthrough a leachate collection pipe that flows into a detention pond[7,27]. A semi-aerobic landfill is an effective system for early stabil-ization of landfill sites and improvement of waste decomposition.Leachate from a semi-aerobic system is characterized by slightlylower organics compared with that in an anaerobic landfill,although still not subjected to biological treatment [38]. The sche-matic diagram of semi-aerobic (Fukuoka method) landfills is dem-onstrated in Fig. 1 [39]. In this study, the leachate samples werecollected 9 times manually from March 2011 to July 2012 andplaced in 20 L plastic containers. The samples were immediately

Table 1Characteristics of semi-aerobic landfill leachate from PBLS.

Parameters Min. Max. Value (average)

COD (mg/L) 1780 2530 2025BOD5 (mg/L) 90 107 93NH3–N (mg/L) 780 1090 810Color (PT Co.) 3390 4100 3550TOC 650 1010 860pH 8.3 9.2 8.5Suspended solids (mg/L) 197 220 212Conductivity, (lS/cm) 17,480 18,920 18,670Iron as Fe2+ (mg/L) 6.5 12 9Sulfate (mg/L) 154 207 187

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transported to the laboratory, characterized, and cooled to 4 �C tominimize the biological and chemical reactions. The average char-acteristics of the leachate used in the experiments are summarizedin Table 1. Sample collection and preservation were performed inaccordance with the Standard Methods for the Examination ofWater and Wastewater [40].

2.2. Experimental procedures

All experiments were conducted with a 2 L leachate sample vol-ume using an ozone reactor with a height of 65 cm and an innerdiameter of 16.5 cm, as presented in Fig. 2. The reactor was sup-ported by a cross column ozone chamber to enhance the diffusionof ozone gas. The water bath and cooling system helped the ozonereactor to maintain internal reaction temperature below 15 �C,which is optimal for the half-life of the dissolved ozone (30 min)in aqueous solutions [41]. Ozone was produced by a BMT 803 gen-erator (BMT Messtechnik, Germany) fed with pure dry oxygen, fol-lowing the recommended gas flow rate of 200 mL/min ± 10%. Theinput ozone concentration was 80 g/m3 non-thermal plasma(NTP) ± 0.5% under 1 bar of pressure. The gas ozone concentration(in g/m3 NTP) was measured using an ultraviolet gas ozone ana-lyzer (BMT 964). S2O2�

8 as sodium persulfate (Na2S2O8) M = 238 g/mol) was employed for advanced oxidation during the ozonationof stabilized leachate, as shown in Fig. 2. A three-stage experimentwas conducted to determine the optimum conditions.

Ore

O2

tank

O3 Generator

O3 analyzer

Gas flow meter

CoolingSystem(< 150C)

O2

O3

Fig. 2. Schematic diagram of ozone equi

Stage 1: In this stage, persulfate dosage (Na2S2O8) was deter-mined as COD=S2O2�

8 ratio (g/g); different ratios ranging from1/1 to 1/10 were added gradually to the 2 L of leachate samplein the ozone reactor for determining the optimum persulfatedosage for the efficiencies of COD, color, and NH3–N removal.In this stage, pH was not adjusted and all runs were performedwith natural leachate pH (8.5) during 60 min of ozonation. ThepH was measured after adding persulfate reagent, and nochanges were observed in pH value after adding the reagentor after the ozonation process. The best COD=S2O2�

8 ratio (g/g)was determined from Stage 1.Stage 2: Based on the optimum persulfate ratio obtained fromstage 1 (i.e., 1 g/7 g COD=S2O2�

8 , the effect of the initial pH var-iation of the leachate ranging from 3 to 11 was examined andadjusted using a 5 M sulfuric acid solution and a 5 M sodiumhydroxide solution. All runs were performed at 60 min ofozonation.Stage 3: The reaction time was examined from 30 min to240 min at the optimal pH (10) and persulfate dosage obtainedfrom stages 1 and 2.

Persulfate oxidation was conducted in a 2 L sample volumereactor, maximum rotation: 350 rpm, natural leachate pH (8.5),room temperature (28 �C) [30], and maximum reaction time 4 h[34]. Three experiments of pesulfate oxidation were performedusing 2 L of leachate with different persulfate reagent dosages (5,35, and 50 g).

2.3. Analytical methods

COD, color, NH3–N, and pH were immediately tested before andafter each run of the experiments in accordance with the StandardMethods for the Examination of Water and Wastewater [40].Leachate was stirred uniformly before analyzed. NH3–N concentra-tion and pH were measured by the Nessler Method using a HACHDR 2800 spectrophotometer and a portable digital pH/Mv meter,respectively. COD concentration was determined by the closed re-flux colorimetric method using a DR 2800 HACH spectrophotome-ter. Color concentration was measured using a DR 2800 HACHspectrophotometer. The test values are presented as the averageof the three measurements, and the difference between the

zoneactor

Water bath

Sample out

Sample in + Persulfate

ActivatedCarbon

O3 Catalyzingcartridge

pment and experiments procedures.

S.S. Abu Amr et al. / Chemical Engineering Journal 221 (2013) 492–499 495

measurements of each value was less than 3%. The removal effi-ciencies of COD, color, and ammonia were obtained using the fol-lowing equation:

Removelð%Þ ¼ ½ðCi � Cf Þ=Ci� � 100 ð1Þ

where Ci and Cf refer to the initial and final COD, color, and NH3–Nconcentrations respectively.

3. Results and discussion

3.1. Effect of S2O2�8 dosage

The Na2S2O8 dosage was fixed as a COD=S2O2�8 ratio (g/g),

namely, 1/1 to 1/10 during 60 min ozonation of leachate (Fig. 3),to evaluate the role of S2O2�

8 in ozonation improvement. Persulfateoxidation can be enhanced by the release of sulfate radicals, whichhave powerful effects on the oxidation of organics [30,32]. The gen-eration of sulfate radicals during oxidation can be significantly en-hanced by catalysts, such as heat and UV radiation (Eqs. (2)–(4)),which were found to improve the persulfate oxidation potential[42,43].

S2O2�8 þ 2Hþ þ 2e� ! 2HSO��4 ð2Þ

SO2�4 þHO� ! SO��4 þ OH� ð3Þ

S2O2�8 þ Thermal activation! SO��4 þ SO��4 ð30 �C < T

< 90 �CÞ ð4Þ

Shiying et al. [44] initiated sulfate radical by activating persul-fate, using Microwave (MW)–activated persulfate oxidation. Theeffectiveness of the process was evaluated by the degradation oforganics in wastewater; 83–95% of COD removal was obtained.

Consequently, to investigate the performance of ozone in initi-ating sulfate radical formation from persulfate during the ozona-tion process, the influence of Na2S2O8 concentration on COD,color and NH3–N was investigated with the optimal ozone condi-tions fixed at 15 �C, max O3 dosage 80 g/m3, reaction time 60(min), and high pH (8.5) [17,26,43]. The initial pH (8.5) of theleachate samples was maintained. In the O3=S2O2�

8 system, themaximum removal efficiencies of COD, color, and NH3–N were44%, 65%, and 32%, respectively, at 1/7 COD=S2O2�

8 ratio (g/g)

Fig. 3. Effect of persulfate dosage in removing COD, color and NH3–N during

(35 g Na2S2O8) (Fig. 3). Reacting with several organic compounds,sulfate radical is more effective oxidant than OH� because it is moreselective to oxidation, whereas OH� may react rapidly by hydrogenabstraction [45]. Although the auto decomposition of persulfate isslow at temperatures 620 �C, iron ions as ferrous salts in leachatecan initiate and activate the decomposition of persulfate through aone-electron transfer reaction analogous to the Fenton initiationreaction (Eq. (5)). Such initiation reactions result in the formationof sulfate radicals [31].

SO2�4 þ Fe2þ ! SO��4 þ SO2�

4 þ Fe3þ ð5Þ

However, using the Fenton reaction cycle as an analogy, thereduction of ferric ions and regeneration of ferrous ions in the pres-ence of persulfate had been proposed as in Eq. (6) [46]. However,Liang et al. [47] reported the inability of ferric ions to oxidizepersulfate.

Fe3þ þ S2O2�8 ! 2SO��4 þ Fe2þ ð6Þ

Rostagy et al. [48] used iron (Fe2+ and Fe3+) ions for generatingsulfate radical from persulfate. Iron (Fe2+) acts as a sulfate radicalscavenger at higher concentrations (Eq. (7)), indicating that anoptimum concentration of ferrous ions leads to the most effectivedegradation of the target contaminants, such as polychlorinatedbiphenyl, 2-chlorobiphenyl, and total organic carbon (TOC) re-moval [48].

Fe2þ þ SO��4 þ ! Fe3þ þ SO2�4 ð7Þ

In an ozone/persulfate system, ozone has a significant role byincreasing hydroxyl radicals in the solution, which directly acti-vates persulfate and initiate sulfate radical. Sulfate ions cannot re-act with ozone, but radical reactions are possible, as shown in thereaction in Eq. (8) [17].

SO��4 þHO� ! SO2�4 þ 1=2 O2 ð8Þ

The removal efficiencies of the target parameters were im-proved by increasing the S2O2�

8 dosage (Fig. 3). Both sulfate and hy-droxyl radicals may have played a part in the degradation of COD,color, and ammonia. However, the excess quantity of S2O2�

8 con-sumes hydroxyl radicals and inhibits the reaction [17]. Neverthe-less, the sulfate radical has the ability to oxidize the targetpollutants despite the increase in S2O2�

8 dosage to higher than

60 min ozonation of stabilized leachate (O3 dosage: 80 g/m3, pH: 8.5).

496 S.S. Abu Amr et al. / Chemical Engineering Journal 221 (2013) 492–499

35 g, which explains the balance and stability of the reaction(Fig. 3). Xu et al. [34] obtained 77.8% COD removal using K2S2O8

and an activated carbon process at high temperature (150 �C).

3.2. Effect of pH

The pH of the initial leachate sample was gradually adjustedfrom 3 to 11, based on the optimal S2O2�

8 dosage (1 g/7 gCOD=S2O2�

8 ) obtained from the previous reaction step, to deter-mine the optimal pH for the O3=S2O2�

8 system. Fig. 4 shows thatby increasing the pH, the removal efficiencies for COD, color, andammonia were also increased. The maximum removal efficiencieswere 51%, 78%, and 41% for COD, color, and ammonia, respectively.These phenomena are attributed to the ability of O3 to initiate hy-droxyl radical formation at high pH. These hydroxyl radicals havean oxidation potential (E� = 2.80) higher than O3 (E� = 2.07) in thedirect reaction under acidic conditions [49]. The effect of basicityon persulfate reactivity was discussed by Furman et al. [50], andthe persulfate can be activated to initiate sulfate radical at highpH, under the effect of hydroxyl radical (Eq. (9)) [51].

S2O2�8 þ OH� ! HSO��4 þ SO��4 þ 1=2 O2 ð9Þ

Although S2O2�8 is more active at high pH, Deng and Ezyske [30]

reported higher removal of COD and NH3–N at low pH (4), hightemperature (40 �C), and higher persulfate dosage. Meanwhile,the optimal removal efficiency in the present study was obtainedat the optimal ozone temperature of 15 �C and high pH (10). Shab-iimam and Dikshit [19] obtained lower organic removal in stabi-lized leachate using sodium persulfate reaction in an optimallyacidic medium (pH 2.5). Furthermore, Lin et al. [52] reported thata high pH (11) is preferred in the removal of Phenol from wastewa-ter using UV activation persulfate oxidation.

3.3. Effect of reaction time

The effect of reaction time on the O3=S2O2�8 process was inves-

tigated to determine a suitable experimental condition. In the pres-ent study, the efficiency of O3/S2O8 was evaluated in terms of COD,color, and NH3–N removal (Fig. 5). The reaction time was variedbetween 30 min and 240 min, and the initial pH and persulfatedosage were kept constant at 10 and a COD=S2O2�

8 ratio of 1/7,respectively. The degradation of organics and ammonia and thedecolorization of leachate were improved as the reaction time in-

Fig. 4. Effect of pH on COD, color and NH3–N removal during 60 min ozonation of stabiltime: 60 (min).

creased (Fig. 5). The maximum removal efficiencies for COD, color,and NH3–N reached 72%, 93%, and 55%, respectively, but did notchange considerably after 210 min. Thus, the optimum reactiontime was considered to be 210 min. Xu et al. [34] reported an opti-mal reaction time of 4 h using a K2S2O8 and activated carbon pro-cess in stabilized leachate.

3.4. Ozone consumption

Ozone consumption (OC) was calculated during the treatmentof stabilized leachate using O3=S2O2�

8 in the advanced oxidationprocess and is given by the following equation:

OC ¼ Q G

V�

R t0 1� CAG

CAG0

� �dt

ðCOD0 � CODÞ ð10Þ

where QG is the gas flow rate, V is the sample volume, CAG is the off-gas ozone concentration, CAG0 is the input ozone concentration, t isthe time, and COD0 and COD correspond to the initial and final COD,respectively.

Table 2 illustrates the OC at different reaction times for stabi-lized leachate compared with the COD removal efficiency. Theminimum value of OC (0.76) was reported at the optimal reactiontime (210 min) and with maintenance of the optimum conditions(Fig. 5). Previous studies reported OC values of 16 [53], between2 and 3 [54,17], and approximately less than 1 kg O3/kg COD[55]. The differences in OC in literature are mainly due to the widevariations in experimental conditions and leachate characteristics(e.g., O3 dosage, sample volume, pH variation, and oxidants suchas H2O2 and UV).

3.5. Treatment efficiency and effect on biodegradability

The treatment efficiency of the O3=S2O2�8 system was compared

with other treatment applications (i.e., ozone alone, persulfatealone, and persulfate + ozone) under the experimental conditions(Table 3). The efficiency of O3 alone is insufficient for the removalof COD, color, and ammonia (15%, 27%, and 0.25%, respectively).Compared with that of the other processes, these lower valueswere attributed to the high concentration of organics in leachatecontaining a considerable amount of dissolved ozone in the aque-ous phase from the beginning of the reaction [25]. Thus, persulfateoxidation is more efficient for leachate treatment than O3 alone.

ized leachate at optimum COD=S2O2�8 ratio (1 g/7 g), O3 dosage: 80 (g/m3), reaction

Fig. 5. Effect of reaction time on COD, color and NH3–N removal during ozonation of stabilized leachate at optimum COD=S2O2�8 ratio (1 g/7 g) and pH (10).

Table 2Ozone consumption at different reaction time (ozone/persulfate).

RTa (min) 30 60 90 120 150 180 210 140

COD removal (%) 33% 51% 53% 55% 59% 64% 72% 72%OC (kgO3/kg COD) 1 1.01 1.39 1.74 1.73 1.64 0.76 0.87

a Reaction time.

S.S. Abu Amr et al. / Chemical Engineering Journal 221 (2013) 492–499 497

However, increasing the persulfate dosage higher than 35 g did notimprove removal efficiency. Although the performance of O3 afterS2O2�

8 is improved, the removal efficiency is also improved by theadvanced oxidation system ðO3=S2O2�

8 Þ to 72%, 93%, and 55% forCOD, color, and ammonia, respectively, under optimal conditions.

In comparison with ozone/Fenton in AOPs, our previous work[26] obtained 65%, 98%, and 22% removal efficiencies for COD, col-or, and NH3–N, respectively. In the O3/Fenton oxidation, the coop-erative reaction of ozone and Fe2+ ions with H2O2 was performed atnatural pH (7) to form hydroxyl radical (Eqs. (10) and (11)). More-over, H2O2 was dissolved in water and dissociated into hydro per-oxide ion (HO2�), which rapidly reacted with the ozone to initiate aradical chain mechanism that generates hydroxyl radicals, as pre-sented in Eq. (12) [56–58].

Fe2þ þH2O2 ! Fe3þ þ OH� þ OH� ð10Þ

H2O2 þ 2O32! OH� þ 3O2 ð11Þ

HO�2 þ O3 ! HO�2 þ O3 ð12Þ

Table 3Comparing the performance of persulfate in advanced oxidation of ozone with other appl

Experimental conditions COD

Initial(mg/L)

F(mL

Ozone only (RTa:60 min, O3: 80 g/m3 pH = 8.5) 2480 2

Persulfate ðS2O2�8 Þ only (RT: 240 min, pH = 8.5, COD=S2O2�

8 ratio (1 g/7 g), rotation: 350 rpm, temp. 28 �C)

2500 1

Persulfate (RT: 240 min, pH = 8.5, CODÞ0=S2O2�8 ratio (1 g/7 g), rotation:

350 rpm, temp. 28 �C) + ozone (RT:60 min, O3: 80 g/m3 pH = 8.5)

2500 1

Ozone/persulfate in the AOPs (RT: 210 min, pH = 10, COD0=S2O2�8 ratio

(1 g/7 g), O3 = 80 g/m3)

2500

a Reaction time.

Although the optimal reaction time in the ozone/Fenton process(90 min) was shorter than that in the current process, the perfor-mance of cooperative hydroxyl and sulfate radicals in removingCOD and NH3–N was much higher at 210 min of ozonation. More-over, one of the main environmental problems in the final effluentdischarge for the ozone/Fenton process is the release of a signifi-cant amount of iron sludge, which requires additional appropriatemanagement prior to discharge. O3=S2O2�

8 does not produce anyundesirable sludge after the treatment process. The residual of sul-fate ions caused by ozone/persulfate oxidation was higher(520 mg/L) than that in raw leachate (187 mg/L). Nevertheless, sul-fate ions can be included in bacterial uptake during further aerobicand anaerobic treatment [59–61].

The effect of ozone/persulfate oxidation on the biodegradabilityof stabilized leachate was investigated. The COD, BOD5, and BOD5/COD ratio of the stabilized leachate were tested before and afterthe ozonation process. Fig. 6 presents the effect of O3=S2O2�

8 onthe COD, BOD5, and BOD5/COD ratio. The O3=S2O2�

8 system wascompared with other treatment techniques to evaluate its perfor-mance. Biodegradability was examined after both persulfate oxida-tion only and S2O2�

8 oxidation followed by ozone. Ozone/persulfatemethod was more efficient in enhancing the biodegradability ofstabilized leachate than other treatment techniques. Fig. 6 showsthat the COD decreased from 2500 mg/L to 700 mg/L, and BOD5

was increased from 107 to 202 mg/L. The suitable biological reduc-tion of sulfate required long-term anaerobic conditions [60,61].Thabet et al. [61] obtained 91% sulfate removal from leachate usinganaerobic reactor for 194 days. Accordingly, the contribution of

ications for the treatment of stabilized leachate.

Color NH3–N

inalg/

)

Removal(%)

Initial(Pt.Co.)

Final(Pt.Co.)

Removal(%)

Initial(mg/L)

Final(mg/L)

Removal(%)

108 15 3460 2526 27 780 778 0.25525 39 3650 1642 55 790 616 22

125 55 3650 511 86 790 545 31

700 72 3650 255.5 93 790 355.5 55

Fig. 6. Effect of ozone and persulfate processes on biodegradability of stabilizedleachate.

498 S.S. Abu Amr et al. / Chemical Engineering Journal 221 (2013) 492–499

residual sulfate in BOD (5 days) test was negligible. Consequently,the biodegradability described in terms of the BOD5/COD ratio in-creased from 0.043 to 0.29, whereas the ratio only improved to0.12 and 0.16 after S2O2�

8 oxidation and S2O2�8 + ozone, respec-

tively. Cortez et al. [62] obtained an improvement in biodegrad-ability from 0.01 to 0.17 using O3/H2O2 in the advancedoxidation process.

4. Conclusion

In the current study, the performance of an advanced oxidationprocess, including the combined use of persulfate and ozone for thetreatment of a Malaysian stabilized solid waste leachate, wasinvestigated. The maximum removal efficiency for COD, color,and NH3N were 72%, 93%, and 55%, respectively, at optimaloperational conditions of 80 g/m3 O3 dosage, 35 g Na2S2O8 (1 g/7 g COD=S2O2�

8 ) persulfate dosage, pH 10, and a reaction time of210 min. Moreover, the biodegradability of the stabilized leachatewas enhanced, suggesting a biological treatment process afterO3/persulfate. The combination of persulfate and ozone performedmore efficiently in stabilizing leachate treatment compared withusing ozone only and persulfate only. Accordingly, the new treat-ment increased the oxidation potential of organics during ozona-tion. This study yielded a greater value of OC for COD removal(0.76 kg/kg COD) compared with other experiments conductedon leachate. The study also introduced new limitations on differenttreatment operational variables that could be useful in futureexperimental designs.

Acknowledgements

This study was made possible through the support of the Schoolof Civil Engineering, Engineering Campus, UniversitiSainsMalaysia.

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