cod removal from anaerobically treated palm oil mill effluent (at - pome) via aerated heterogeneous...

1

2 COD removal from anaerobically treated palm oil mill effluent3 (AT-POME) via aerated heterogeneous Fenton process:4 Optimization study

5 M.R. TahaQ1a, A.H. Ibrahim b,*

6 aDepartment of Civil and Structural Engineering, Universiti Kebangsaan Malaysia,Bangi 43600, Malaysia

7 b School of Environmental Engineering, Universiti Malaysia Perlis, Perlis 01000, Malaysia

89 1. Introduction

10 Preservation of fresh water is a great concern of people

11 worldwide. With the increasing population in every part of the

12 world, the demands for clean and fresh water becomes a crucial

13 issue. On the other hand, industries also require a large amount of

14 fresh water to sustain their activities and processes. In 2012,

15 Malaysia was recorded as the world’s second largest producer of

16 palm oil with the production of 18.7 million tons of crude palm

17 oil (CPO) [1]. This crude palm oil was produced from 429 palm oil

18 mills located all over Malaysia. Fig. 1 shows the number of palm

19 oil mills in Malaysia by year, and the trend shows that the number

20 of mills is increasing over the years.

21 As all oil palm mills in Malaysia utilize a wet milling method, a

22 lot of water is used during the milling process. This results in a

23 significantly large production of wastewater called palm oil mill

24 effluent (POME). It is estimated that for every ton of crude palm oil

25 yield, 2.5 tons of palm oil mill effluent (POME) is produced [2]. This

26effluent contains extremely high quantities of organic content, for

27which the chemical oxygen demand (COD) and biological oxygen

28demand (BOD) are between 45,000 and 65,000 mg/l and 21,500

29and 28,500 mg/l respectively [3]. The characteristics of raw POME

30are summarized in Table 1.

31Due to the extreme organic pollutant loading, palm oil mills

32employ three stages of treatment to ensure that the POME

33complies with the discharge standard enforced by the local

34authority. Fig. 2 shows the typical POME treatment process at palm

35oil mill. As an anaerobic treatment process properly treats

36wastewater containing high organic loading, most mill employ

37this type of treatment at their primary stage. In addition, studies by

38Poh [4], Choi [5] and Zinatizadeh [6] also demonstrated the success

39of anaerobic process to treat POME.

40While for the tertiary or polishing stage, the mills as well as

41technology providers are still looking for the best treatment for

42POME. Some of the approaches that have been studied for tertiary

43treatment of POME are ultra-filtration [7], biological aerated filter

44(BAF) [8], adsorption [9] and membrane bioreactor process (MBR)

45[10]. Although all methods show a positive result for POME

46treatment, they still have drawbacks which need to be addressed.

47Membrane fouling/scaling, frequent back washing, and high

48capital cost are main problems with membrane filtration process

Journal of Water Process Engineering xxx (2014) xxx–xxx

A R T I C L E I N F O

Article history:

Received 18 December 2013

Received in revised form 6 February 2014

Accepted 11 February 2014

Available online xxx

Keywords:

Palm oil mill effluent (POME)

Anaerobic digestion

Fenton process

Nano zero-valent iron (nZVI)

A B S T R A C T

Due to the extremely high content of organic pollutants, palm oil mill effluent (POME) requires multiple

stages of treatment. At the primary stage, because of the high loading of organics, anaerobic treatment

method was found to be the best practice nowadays. However, at the later stages, the treatment methods

vary. As an option to available secondary and tertiary treatment method, we used an aerated

heterogeneous Fenton process to remove chemical oxygen demand (COD) from anaerobically treated

palm oil mill effluent (AT-POME). The Box–Behnken design (BBD) and response surface method (RSM)

were used to design and optimize the performance of the process. Furthermore, the regression quadratic

model representing the COD removal efficiency of aerated heterogeneous Fenton was developed and

validated by the analysis of variances (ANOVA). The optimum parameters were determined as 3.91 g/l of

nZVI dosage, 1.84 g/l of H2O2 dosage, and 23.84 l/h of aeration and 240 min of reaction time. As 75% of

COD was predicted to be removed at the optimum condition, the aerated heterogeneous Fenton process

is a promising treatment method for AT-POME.

� 2014 Published by Elsevier Ltd.

* Corresponding author. Tel.: +60 125353897; fax: +60 389216147Q2 .

E-mail addresses: [email protected] (M.R. Taha), [email protected]

(A.H. Ibrahim).

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Please cite this article in press as: M.R. Taha, A.H. Ibrahim, COD removal from anaerobically treated palm oil mill effluent (AT-POME) via

aerated heterogeneous Fenton process: Optimization study, J. Water Process Eng. (2014), http://dx.doi.org/10.1016/j.jwpe.2014.02.002

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49 [11]. While for BAF and MBR, the long hydraulic retention time

50 (HRT) becomes the major problem for the treatment process.

51 On the other hand, advance oxidation processes (AOPs),

52 particularly the Fenton process, seem to be a potential option

53 for POME treatment especially at the tertiary level. Kellel [12] and

54 Nieto [13] used the Fenton process to treat olive oil mill effluent,

55 which has a similar characteristic to POME. At the same time, many

56 works been done to improve the efficiency of the process, including

57 the application of ultrasound or nanomaterials (i.e. nZVI) in the

58 Fenton process. In addition, nanomaterial such as nZVI also has a

59 great potential for recycling and could be used again in the

60 treatment process [14,15].

61 In this research, we study the effect of nZVI and H2O2 dosage,

62 aeration flow rate, and reaction time on the COD removal from AT-

63 POME. The main purpose of vigorous aeration was to disperse the

64 nZVI in the entire AT-POME samples. In addition, we also

65 investigate the optimum conditions for the aerated heterogeneous

66 Fenton process by conducting a series of factorial designs.

67 2. Materials and methods

68 2.1. Materials/chemicals

69 Sulfuric acid (H2SO4) (Merck, 95–97%), sodium hydroxide

70 (NaOH) (Merck, M = 40 g/mol), hydrogen peroxide (H2O2) (R&M

71 Chemicals, 30%), nZVI particles by (Nanofer Star by NANO IRON.)

72 and AT-POME from KKS East Mill, Sime Darby Plantation Sdn.

73 Bhd.

742.2. Characterization of AT-POME

75The COD content of AT-POME was determined through USEPA

76reactor digestion method [16]. While the amount of oil and grease

77in AT-POME were determined using method 2450 D by standard

78method for the examination for water and wastewater [17]. The

79content of soluble tannin/lignin and total suspended solid (TSS)

80were determined by tyrosine method [18] and photometric

81method [19] respectively. The concentration of COD, TSS and

82tannin/lignin were then determined by using Hach DR 6000

83spectrophotometer. In addition the pH and turbidity of AT-POME

84were also measured using Eutech Cyberscan pH meter and LaMotte

852020 Turbidimeter respectively.

862.3. Experimental design and statistical analysis

87Experimental design and statistical analysis were performed

88using Design Expert software (version 7.1). The Box–Behnken

89Design (BBD) and response surface methods (RSM) were used to

90set up the experimental protocols and optimize the independent

91process variables for aerated heterogeneous Fenton. The variables

92and their levels are summarized in Table 2. The COD removal

93efficiency was selected as the dependent response variable for

94aerated heterogeneous Fenton process.

95The analysis of variance (ANOVA) was conducted using Design

96Expert software (version 7.1) to analyze the result and to check the

97significance of the fitted models. On the other hand, the interaction

98between the variables was illustrated by the contour plots. The

99optimum process parameters for aerated heterogeneous Fenton

100process was calculated using the fitted model.

1012.4. Aerated heterogeneous Fenton process

102AT-POME was diluted 4 times before being treated through

103aerated heterogeneous Fenton process. The aerated heterogeneous

104Fenton process was performed in A 250 ml reactor where the

105aeration was supplied from the bottom of the reactor, as illustrated

Year

1975 198 0 198 5 199 0 199 5 200 0 200 5 201 0 201 5

Num

ber

of m

ills

0

100

200

300

400

500

Fig. 1. Number of mills in Malaysia by year.

Table 1

Characteristics of palm oil mill effluent (POME) [3].

Characteristic Concentrationa

COD 45000–65000

BOD 21500–28500

Total suspended solid (TSS) 15660–23560

Oil and grease 1077–7582

pH 4.15–4.45

a All concentrations are in units of mg/l with the exception of

pH.

Fig. 2. An anaerobic treatment of POME for land application.

M.R. Taha, A.H. Ibrahim / Journal of Water Process Engineering xxx (2014) xxx–xxx2

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106 in Fig. 3. 100 ml of diluted AT-POME was placed inside the reactor

107 and the pH was adjusted to pH 2 using 0.5 M H2SO4. The pH

108 adjustment was done to suit the condition for Fe2+ production by

109 nZVI [14]. Then, certain dosage of nZVI particles and H2O2 was put

110 inside the diluted AT-POME solution.

111 The dosage of H2O2 used for the experiment was based on the

112 stoichiometric weight ratio between H2O2 and COD as in Eq. (1).

1 gCOD ¼ 1 gO2 ¼ 0:03125 molO2 ¼0:0625 molH2O2 ¼2:125 gH2O2

(1)

113114115 The upper and lower level of H2O2 dosage in BBD was based on

116 1.0 and 0.25 times the stoichiometric amount of complete

117 oxidation of COD, respectively. Details of the parameters for all

118 experiments are summarized in Table 3. At the end of every

119 experiment, the pH of the treated AT-POME was adjusted to 8.5.

120 This pH adjustment eliminates any available ferrous ion (Fe2+) and

121 H2O2 inside the solution, which could cause interference with COD

122 test. At this pH, H2O2 was not stable and decomposed into water

123 and oxygen while Fe2+ reacted with hydroxide ion (OH�) to form

124 insoluble ferrous hydroxide (Fe(OH)2). Next, the treated AT-POME

125 was centrifuged at 4000 rpm for 10 min to separate the insoluble

126 Fe(OH)2 from the solution.

127 The aliquot was then collected for COD determination. In

128 addition, the size of nZVI particles in AT-POME were measured

129 using Malvern Zetasizer (Nano ZS/ZEN 3600).

130 3. Results and discussion

131 3.1. Characteristics of AT-POME

132 AT-POME contains approximately 4568 mg/l of COD which

133 indirectly indicates that AT-POME need to undergo some other

134 treatments before it can be discharge into any water bodies. Due to

135 the addition of sodium hydroxide (NaOH) or potassium hydroxide

136 (KOH) at initial stage of anaerobic digestion process, the pH of AT-

137 POME was affected and resulted in high pH. pH of AT-POME was

138 recorded as 8.5. In addition, the concentration of oil and grease, TSS

139 and turbidity of AT-POME recorded as 1100 mg/l, 1070 mg/l and

140 400 NTU respectively. The characteristics of AT-POME were

141 summarized in Table 4.

142AT-POME also exhibited darker color compared to raw POME

143due to the presence of soluble tannin and lignin. Tannin and lignin

144in AT-POME was resulted from the digestion of lignocellulosic

145biomass presence in raw POME by microorganism during the

146anaerobic digestion process. Concentration of tannin and lignin

147was measured based on tannic acid content in AT-POME and the

148concentration was 600 mg/l.

149However, after the pH was adjusted to pH 2, the concentration

150of tannic acid was reduced significantly. The removal of tannin acid

151in AT-POME by pH change is shown in Fig. 4. As tannin acid affected

152the color of the solution, reduction of tannin acid in AT-POME can

153also be observed through the color change of AT-POME. As shown

154in Fig. 5, AT-POME immediately turned into lighter color once the

155pH was reduced.

156As lignin can also impart the color of the AT-POME, the color

157change of the solution can be explained through the precipitation

158of soluble lignin by H2SO4. The soluble lignin started to

159precipitate at pH 5 and achieved its peak at pH 4 where all

160the lignin was precipitated. At this stage, the turbidity was

161increased due to the floc formation inside the AT-POME and

162highest NTU was recorded at pH 3. Further down the pH of AT-

163POME to pH 2 had made the colloid to be in a very unstable

164condition as indicated by the zeta potential (z) magnitude as

Table 2

Coded levels and independent variables for the experimental design for aerated

heterogeneous Fenton.

Factor Variables Level

�1 0 1

A nZVI dosage (g/l) 1.20 2.45 3.70

B Hydrogen peroxide (g/l) 0.60 1.45 2.30

C Reaction time (min) 30 135 240

D Aeration rate (l/h) 0 25 50

Fig. 3. An aerated heterogeneous Fenton reactor.

Table 3

BBD experimental design and results of COD removal efficiency.

Experiment nZVI dosage

(g/l)

H2O2

dosage

(g/l)

Reaction time

(min)

Aeration

flow rate

(l/h)

1 1.20 0.60 135.00 25.00

2 3.70 0.60 135.00 25.00

3 1.20 2.30 135.00 25.00

4 3.70 2.30 135.00 25.00

5 2.45 1.45 30.00 0.00

6 2.45 1.45 240.00 0.00

7 2.45 1.45 30.00 50.00

8 2.45 1.45 240.00 50.00

9 1.20 1.45 135.00 0.00

10 3.70 1.45 135.00 0.00

11 1.20 1.45 135.00 50.00

12 3.70 1.45 135.00 50.00

13 2.45 0.60 30.00 25.00

14 2.45 2.30 30.00 25.00

15 2.45 0.60 240.00 25.00

16 2.45 2.30 240.00 25.00

17 1.20 1.45 30.00 25.00

18 3.70 1.45 30.00 25.00

19 1.20 1.45 240.00 25.00

20 3.70 1.45 240.00 25.00

21 2.45 0.60 135.00 0.00

22 2.45 2.30 135.00 0.00

23 2.45 0.60 135.00 50.00

24 2.45 2.30 135.00 50.00

25 2.45 1.45 135.00 25.00

26 2.45 1.45 135.00 25.00

27 2.45 1.45 135.00 25.00

28 2.45 1.45 135.00 25.00

29 2.45 1.45 135.00 25.00

30 2.45 1.45 135.00 25.00

Table 4

Characteristics of AT-POME.

Parameters Concentration (mg/l)

COD 4568

Oil and grease 1100

Tannin and lignin 600

TSS 1070

pH 8.5

Turbidity 400

All parameters in mg/l unit except for pH and turbidity.

M.R. Taha, A.H. Ibrahim / Journal of Water Process Engineering xxx (2014) xxx–xxx 3

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165 shown in Fig. 6. Hence, at pH 2, the lignin particles easily

166 agglomerated to each other and formed bigger and denser flocs.

167 As a result, these flocs settled at the bottom and produced clearer

168 AT -POME. Due to this, AT-POME exhibited lowest NTU reading as

169 well as tannic acid content at pH 2. Fig. 7 shows the trend of NTU

170 reading due to pH adjustment of AT- POME.

171 3.2. COD removal by aerated heterogamous Fenton process

172 In the Fenton process, organic pollutants present in AT-POME

173 are oxidized by hydroxyl radical (OH�

) according to the reaction of

174 Fe2+ and H2O2 as in Eq. (2). In this study, Fe2+ was produced

175 through the oxidation (corrosion) of nZVI particles. The oxidation

176 of nZVI particles could happen in two ways (anaerobic and aerobic)

177 as mentioned in Eqs. (3) and (4) respectively [20]. However, under

178 very acidic condition, the oxidation process of nZVI particles was

179 accelerated and Eq. (4) became a dominant reaction compared to

180 Eq. (3).

Fe2þ þ H2O2 ! Fe3þ þ OH�

þ OH� (2)

181182 Fe0 þ 2H2O ! Fe2þ þ H2 þ 2OH� (3)

183184 Fe0 þ 4Hþþ O2 ! 2Fe2þ þ 2H2O (4)

185186187As OH�was produced as shown in Eq. (2), a slight increase in pH

188can be observed in all experiments, as shown in Fig. 8. OH�

189produced according to Eq. (2) attacked organic pollutants in AT-

190POME and initiated the oxidative destruction of the organic

191substrate through radical addition and/or hydrogen abstraction

192[21] and directly reduced the COD content. However, as the Fenton

193process is dependent on Fe2+ and H2O2 concentration [22] as well

194as the reaction time, the COD removal efficiency for each

195experiment varies, as shown in Fig. 9. The highest and the lowest

196COD removal were recorded in experiment 25 (70.3%) and

197experiment 14 (1.45%), respectively. The effect of all factors on

198COD removal from AT-POME can be compared through a

199perturbation plot, as shown in Fig. 10. The COD removal was

200increased by increasing the nZVI particles content and extending

201the reaction time. By increasing the H2O2 concentration and the

202aeration flow, the COD removal also increased up to a certain

203extent. However beyond a certain limit, excessive H2O2 negatively

204affected the process.

2053.3. ANOVA analysis of experimental results

206Results for the aerated heterogeneous Fenton process was fitted

207with a quadratic model, as in Eq. (5), with F-test of the regression

208model producing a very low p-value (<0.0001), which indicates the

209model was of high significance. where A is nZVI dosage (g/l), B is

pH of AT - POME solut ion

1 2 3 4 5 6 7 8 9

Tannic

acid

rem

oval (%

)

0

10

20

30

40

50

Fig. 4. Tannic acid removal from AT-POME due to pH adjustment.

Fig. 5. Color of AT-POME at different pH.

pH of AT - POME

2468

Zeta

pote

ntial (m

V)

-13

-12

-11

-10

-9

-8

-7

-6

-5

Fig. 6. The magnitude of zeta potential of AT-POME at different pH.

pH

2345678

Turb

idity (

NT

U)

0

100

200

300

400

500

600

700

Fig. 7. Turbidity of AT-POME at different pH.


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210 H2O2 dosage (g/l), C is reaction time (min) and D is aeration flow

211 rate (l/h).

212COD removal ¼ þ65:10 þ 12:85A � 11:93B þ 14:79C þ 9:54D

þ 10:83AB � 12:25AC � 8:87AD þ 18:178BC

� 6:40CD � 4:98A2� 14:48B2

� 6:04C2� 9:44D2

(5)

213214215 The determination coefficient (R2) of the model indicated that

216 97% of the variability can be explained by the model. On the other

217 hand, the value of adjusted determination coefficient (adjusted

218 R2 = 93%) also proved the high significance of the model. Fig. 11

219 shows the graphical presentation of the effect magnitude of each

220 parameter on the COD removal by heterogeneous aerated sono-

221 Fenton process. nZVI dosage (A), reaction time (C) and aeration

222 flow (D) yield a positive effect, while H2O2 has a negative effect on

223 the COD removal from AT-POME. In addition, excess nZVI dosage

224 (AA), H2O2 concentration (BB), reaction time (CC) and aeration flow

225 (DD) yield a negative effect on the COD removal.

226 With the code levels in Table 2 and Eq. (5), the COD removal

227 efficiency at each experiment could be predicted by the model. The

228 ‘‘Lack of Fit F-value’’ of 1.47 implies the lack of fit is not significant

229 relative to the pure error.In addition, the insignificant of lack of fit

230 (p-value of 0.3502) also indicated that the model fit the data well.

231 Furthermore, as the normal probability plot of residual formed

232 almost a straight line as shown in Fig. 12 and the residual

233vs. predicted plot in Fig. 13 also shows that vertical spread of the

234studentized residuals were approximately the same across all

235levels of the predicted values, these indicated no abnormalities of

236the model. Additionally, a lambda of 1 as shown by Box–Cox plot in

237Fig. 14 also indicated that no transformation is needed for the

238model.

239Considering all the statistics and diagnostics done, the

240regression model could be used to predict the COD removal from

241the experimental conditions. The actual and predicted data of COD

242removal efficiency were in good agreement (Pearson’s coefficient

243was 0.98) as shown in Fig. 15.

2443.4. Response surface plotting and optimization of the experimental

245conditions

246As shown in Eq. (5), the presence of quadratic terms indicates

247that the COD removal is represented by a curved surface.

248Fig. 16(a)–(c) shows the graphical representation of the response

249curve at different reaction time. For reaction time of 30 min, the

250increase of nZVI loading clearly improved the COD removal from

251the AT-POME. This can be clearly seen in Fig. 16(a) where the

252response surface exhibited a rising ridge pattern. Higher loading of

253nZVI particles produce more active sites which are available to be

254oxidized to produce Fe2+, and this is essential to initiate the OH�

255production, as in Eq. (3). This OH�

which later acts as an oxidant to

Experiment

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30

pH

0

1

2

3

4

5

Initial pH

Final pH

Fig. 8. pH of AT-POME after the aerated heterogeneous Fenton process.

Experiment

0 5 10 15 20 25 30

CO

D r

em

oval (%

)

0

20

40

60

80

Fig. 9. COD removal from AT-POME in each experiment.

Fig. 10. Comparison the effect of all the factors at a particular point in the design

space.

Main, in tera ctions an d se condorder

A B C D AB AC AD BC CD AA BB CC DD

Coeffic

ients

-20

-15

-10

-5

0

5

10

15

20

Fig. 11. Significant main, interaction and second order terms for aerated

heterogeneous Fenton process.


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256 oxidize organic pollutants and reduce the COD content in AT-

257 POME.

258 However, an opposite trend was observed for the effect of H2O2

259 concentration on COD removal. After this reaction time, low H2O2

260 concentration is preferred by the process. Hence, after 30 min of

261 reaction time, the maximum COD removal occurred with a high

262 loading of nZVI particles and low concentration of H2O2. In

263 addition, negative COD removal (increment from initial COD

264 content) was observed at high concentration of H2O2 and low nZVI

265 dosage. As less Fe2+ was available to react with H2O2 (to produce

266 OH�

) during the treatment process, the remaining H2O2 in AT-

267 POME had interfered the COD test and contributed to high COD

268 reading [14].

269 A change in pattern was noticeable at 135 min of reaction time.

270 At this point, the maximum COD removal occurred at a medium

271 level, instead of low level, of H2O2 concentration, as shown in

272 Fig. 16(b). The respond curve for aerated heterogeneous Fenton

273 process shows the simple maximum response pattern after

274 240 min of reaction time. After this reaction time, the maximum

275 COD removal was obtained at a point between the medium and

276 high level for nZVI loading and H2O2 concentration, as shown in

277 Fig. 16(c). After 240 min of reaction time, the maximum COD

278 removal from AT-POME was 75.49%, obtained approximately at

279 3.13 g/l of nZVI and 1.845 g/l of H2O2.

280 As Fe2+ determines the rate of the conversion of H2O2 to OH�

, a

281 higher concentration of Fe2+ will increase the efficiency of the

282 process [23]. Compared to iron salt (i.e. FeSO4) which instantly

283 dissociates into Fe2+ and SO42� in water, production of Fe2+ from

284nZVI particles is slower. Production of Fe2+ by nZVI particles is

285dependent on the oxidation (corrosion) rate of nZVI particles

286present in AT-POME. Hence by lengthening the reaction time, more

287Fe2+ can be produced to improve the COD removal efficiency.

288This trend can be clearly seen in the contour plots shown in

289Fig. 17(a)–(c). In Fig. 17(a), 70% of the COD could be removed after

290240 min of reaction at a dosage of 1.43 g/l of nZVI particles.

291However, the same COD removal efficiency can also be obtained in

292much shorter time if a higher dosage of nZVI particle was used. This

293is due to more nZVI particles being available to be oxidized and to

294produce more Fe2+. Furthermore, available nZVI particles can

295accelerate the recycling of ferric ion (Fe3+) at the nZVI particle

296surface for further use in the process [24]. Eq. (6) shows the

297reaction of ferric ion (Fe3+) recycling on nZVI surface.

2Fe3þ þ Fe0 ! 3Fe2þ (6)

298299300Fig. 17(b) and (c) shows that, by increasing the nZVI dosage to

3012.87 g/l and 3.52 g/l, respectively, 70% of the COD could be

302removed within 135 min and 30 min, respectively, instead of

303240 min of reaction time.

304On the other hand, the effect of H2O2 concentration on COD

305removal efficiency is dependent on nZVI dosage and reaction time.

306This again is related to the concentration of Fe2+ inside the AT-

307POME. As the concentration of Fe2+ increased, more H2O2 is

308required to react with Fe2+ to produce OH�

which later attacks the

309organic pollutants and reduces the COD content inside the AT-

310POME. Hence in this case, the maximum removal of COD happened

311at different dosages of H2O2 depending on the Fe2+ concentration

Fig. 12. Normal plot of residuals.

Fig. 13. Residuals vs. predicted response values.

Fig. 14. Box–Cox plot for power transformation.

Experimental COD removal efficiency (%)

0 20 40 60 80

Pre

dic

ted C

OD

rem

oval effic

iency (

%)

0

20

40

60

80

Fig. 15. Predicted vs. experimental results of COD removal efficiency.


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312 inside the AT-POME. After 30 min of reaction time, the maximum

313 COD removal was obtain slightly above the low level of H2O2

314 (0.86 g/l). However, after 135 min and 240 min of reaction time,

315 the optimum dosage of H2O2 to obtain the maximum COD removal

316 increased to 1.422 g/l and 1.837 g/l, respectively. The changes of

317 optimum dosage of H2O2 can be seen clearly in every contour plot

318 of reaction time, as shown in Fig. 17(a)–(c).

319 Although higher dosage of H2O2 works well at a higher

320 concentration of Fe2+ to remove COD from AT-POME, an excessive

321 amount of H2O2 could negatively affect the process. This can be

322 explained by the scavenger effect of OH�

by excessive H2O2

323 remaining in the solution [25]. Instead of oxidizing the organic

324 pollutants in AT-POME, the available OH�

reacted with H2O2 to

325 produce hydroperoxy radicals (HO2�

) which is less reactive

326 compared to OH�

as shown in Eq. (7).

OH�

þ H2O2 ! H2O þ HO2 (7)

327328329 Due to its size, nZVI particles have different surface structures

330 and surface interactions especially the hydrophobic interaction

331between them [26]. As a result, they tend to agglomerate or attach

332to one another and thus form bigger nZVI particles. In addition, as

333the particles were denser compared to AT-POME, they settled at

334the bottom of the reactor immediately after being poured into the

335solution. Consequently, this contributed to the poor dispersion of

336nZVI in AT-POME and jeopardized the efficiency of COD removal

337process.

338One way to disperse the nZVI particles effectively in AT-POME is

339by sonication. However, as ultrasound uses a very high energy

340density, utilization of sonication method to disperse nZVI particles

Fig. 16. (a) Response surface for the COD removal at 30 min. (b) Response surface for

the COD removal at 135 min. (c) Response surface for the COD removal at 240 min.

Fig. 17. (a) Contour lines which represent the COD removal at different dosages of

nZVI and H2O2 after 240 min of reaction time. (b) Contour lines which represent the

COD removal at different dosages of nZVI and H2O2 after 135 min of reaction time.

(c) Contour lines which represent the COD removal at different dosages of nZVI and

H2O2 after 30 min of reaction time.


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341 in large volume of wastewater would consume much energy and

342 lead to cost issues to the industries [27]. To overcome this problem,

343 in this study, the dispersion of nZVI was done by creating a

344 turbulence flow inside the reactor through a vigorous aeration.

345 Without aeration, due to its density, all nZVI particles accumulated

346 at the bottom of the reactor after being poured into the solution.

347 The effect of aeration on COD removal at 30 min of reaction time

348 can be seen clearly in Fig. 18(a)–(c).

349 As shown in Fig. 18(a), without aeration, most of the plotting

350 area is covered by blue and green color which indicate a low (less

351than 50%) COD removal from AT-POME. However, the COD removal

352efficiency was improved after the nZVI particles were dispersed in

353the solution by the air supplied from the bottom of the reactor.

354Fig. 18(b) shows that the blue area is decrease and red area started

355to appear. By supplying air flow (25 l/h) to the system, the

356maximum COD removal was increased from approximately 56% to

35771%. However, increasing the aeration flow to higher level (50 l/h)

358did not give any significant change to the COD removal efficiency.

359This can be seen in Fig. 18(c) where the color tone of the contour

360(represent the percentage of COD removal) is similar to Fig. 18(b).

361In addition Q3, a perturbation plot as in Fig. 10 also shows that the

362effect of aeration is almost negligible (flat line) when the air flow

363changed from 25 l/h to 50 l/h. (For interpretation of the references

364to color in the text, the reader is referred to the web version of the

365article.)

366Furthermore, beside the air flow, the presence of natural

367surfactant inside the AT-POME (at pH 2) also helped to disperse

368nZVI particles in the solution. The formation of surfactant can be

369noticed through the foam formation in AT-POME when air was

370supplied to the solution. Fig. 19 shows that more foam is formed in

371AT-POME at pH 2 compared to its original pH (pH8.5), and the

372foams also exhibited a better stability. The ease of foam formation

373in the solution indicated that the surface tension of AT-POME was

374reduced and thus contributed to better dispersion of nZVI particles

Fig. 18. (a) Contour lines which represent the COD removal at different dosages of

nZVI and H2O2 (reaction time: 30 min, aeration: 0 l/h). (b) Contour lines which

represent the COD removal at different dosages of nZVI and H2O2 (reaction time:

30 min, aeration: 25 l/h). (c) Contour lines which represent the COD removal at

different dosages of nZVI and H2O2 (reaction time: 30 min, aeration: 50 l/h).

Fig. 19. Formation of foams in AT-POME at two different pH levels.

pH

1 2 3 4 5 6 7 8 9

Siz

e (

nm

)

650

700

750

800

850

900

950

1000

1050

Fig. 20. Measured nZVI particle size at different pH values of AT-POME.


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375 in AT-POME. The particles size analysis shows a decreasing trend

376 when AT-POME was reduced to lower pH as shown in Fig. 20.

377 Smaller size of nZVI as recorded at pH 2 indicated that the particles

378 were dispersed better in AT-POME at this pH condition compared

379 to its initial pH (pH 8.5).

380 By considering all parameters (dosage of nZVI and H2O2, air flow

381 and reaction time) and their ranges, the predicted maximum COD

382 removal from AT-POME at optimum condition was 75.5% as shown

383 in Fig. 21. The optimum process parameters predicted by the RSM

384 are shown in Table 5. With these optimum conditions, the

385 suggested H2O2 dosage was 1.845 g/l which also represents 0.81 of

386 full stoichiometric weight ratio between H2O2 and COD, as

387 mentioned in Eq. (1). Also, to obtain this optimum conditions,

388 the weight ratio of nZVI and H2O2 was suggested to be 2.125.

389 4. Conclusion

390 AT-POME was treated using an aerated heterogeneous Fenton

391 process. Application of Box–Behnken design (BBD) and response

392 surface methods (RSM) in the operative condition successfully

393 built a quadratic model for the process. The quadratic model yields

394 high significance and high R2 coefficient, which indicates that the

395 surface response model was accurate to predict the experimental

396 result. By prediction, at optimum conditions of the process, 75% of

397 the COD content could be removed from AT-POME. In addition, we

398 also found that vigorous aeration of AT-POME was sufficient to

399 disperse the nZVI particles in the solution, and hence provide a

400 simple and cheaper way of dispersion for the process. Therefore,

401 aerated heterogeneous Fenton process can be a promising option

402 to other treatment methods for AT-POME.

403References

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Fig. 21. Predicted maximum COD removal from AT-POME at optimum process

parameters.

Table 5

The optimum process parameters predicted by the RSM.

Parameters Optimum value

nZVI (g/l) 3.91

H2O2 (g/l) 1.84

Aeration (l/min) 23.84

Reaction time (min) 240


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cod removal from anaerobically treated palm oil mill effluent (at - pome) via aerated heterogeneous...

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