Disintegration of Waste Activated Sludge by Thermally-Activated

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Disintegration of Waste-Activated Sludge by ThermallyActivated Persulfates for Enhanced Dewaterability Min Sik Kim, Ki-Myeong Lee, Hyung-Eun Kim, Hye-Jin Lee, Changsoo Lee, and Changha Lee Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00019 • Publication Date (Web): 07 Jun 2016 Downloaded from http://pubs.acs.org on June 11, 2016

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Environmental Science & Technology

Disintegration of Waste Activated Sludge by Thermally-Activated Persulfates for Enhanced Dewaterability

Min Sik Kim, Ki-Myeong Lee, Hyung-Eun Kim, Hye-Jin Lee, Changsoo Lee, Changha Lee*

School of Urban and Environmental Engineering, and KIST-UNIST-Ulsan Center for Convergent Materials (KUUC), Ulsan National Institute of Science and Technology (UNIST), 50 UNIST-gil, Ulju-gun, Ulsan 689-798, Republic of Korea

*Corresponding author Phone: +82-52-217-2812 Fax: +82-52-217-2809 E-mail: [email protected]

ACS Paragon Plus Environment


TOC/Abstract art


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1

ABSTRACT

2 3

Oxidation by persulfates at elevated temperatures (thermally activated persulfates)

4

disintegrates bacterial cells and extracellular polymeric substances (EPS) composing waste-

5

activated sludge (WAS), facilitating the subsequent sludge dewatering. The WAS

6

disintegration process by thermally activated persulfates exhibited different behaviors

7

depending on the types of persulfates employed, i.e., peroxymonosulfate (PMS) versus

8

peroxydisulfate (PDS). The decomposition of PMS in WAS proceeded via a two-phase

9

reaction, an instantaneous decomposition by the direct reaction with the WAS components

10

followed by a gradual thermal decay. During the PMS treatment, the WAS filterability

11

(measured by capillary suction time) increased in the initial stage but rapidly stagnated and

12

even decreased as the reaction proceeded. In contrast, the decomposition of PDS exhibited

13

pseudo first-order decay during the entire reaction, resulting in the greater and steadier

14

increase in the WAS filterability compared to the case of PMS. The treatment by PMS

15

produced a high portion of true colloidal solids (< 1 µm) and eluted soluble and bound EPS,

16

which is detrimental to the WAS filterability. However, the observations regarding the

17

dissolved organic carbon, ammonium ions, and volatile suspended solids collectively

18

indicated that the treatment by PMS more effectively disintegrated WAS compared to PDS,

19

leading to higher weight (or volume) reduction by post-centrifugation.

20

1



21

INTRODUCTION

22 23

The management of waste activated sludge (WAS) has become one of the main

24

challenges in wastewater treatment plants as the production of WAS continues to increase

25

worldwide along with the increasing demand for sanitation and water treatment facilities.

26

WAS management accounts for up to approximately 60% of the operating cost of the

27

wastewater treatment plant.1 To reduce the costs for the storage, transport, and disposal of

28

produced WAS, the volume of sludge must be reduced by a dewatering process. WAS

29

disintegration by proper treatments can facilitate the dewatering process. These sludge

30

disintegration treatments can also enhance the efficacy of subsequent processes for WAS

31

recycling such as biogas production by anaerobic digestion,2 nutrient and energy recovery,3

32

and composting.4 However, the disintegration of the cells composing WAS is not easy due to

33

the rigid structures of cell walls and membranes and extracellular polymeric substances

34

(EPS).5

35

As options for WAS disintegration, oxidative treatment methods including chlorination,6

36

ozonation,7 and advanced oxidation processes (AOPs)8-10 have been suggested. Among them,

37

AOPs, which utilize a highly reactive and nonselective hydroxyl radical (•OH) (Eo(•OH,

38

H+/H2O) = 2.813 VNHE11), are effective tools for disrupting rigid cell membranes and

39

simultaneously destructing EPS and other polymeric substances in WAS. Several AOPs such

40

as the Fenton process,8 microwave-assisted peroxidation,9 and TiO2 photocatalysis10 have

41

been demonstrated to improve the dewaterability (often expressed as filterability measured by

42

capillary suction time), settleability, and bioavailability (for anaerobic digestion) of WAS as

43

well as to remove refractory chemicals and pathogens in the sludge. 2


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44

Recently, oxidation technologies using sulfate radical anion (SO4•−) have emerged as

45

alternatives to AOPs because SO4•− is a highly reactive oxidant (E0(SO4•−/SO42−) = 2.43

46

VNHE12) that can be generated by manifold pathways. SO4•− is usually produced from

47

persulfates (i.e., peroxymonosulfate (PMS) and peroxydisulfate (PDS)) by thermal

48

activation,13,14 UV photolysis,15,16 alkali activation,17,18 and transition metal catalysis.19-22

49

These persulfate-based technologies have been extensively studied for the degradation of

50

organic contaminants in wastewater or groundwater.21 Several studies have also demonstrated

51

that iron-catalyzed and thermally-activated persulfate oxidation can be applied to WAS

52

treatment for the enhancement of filterability and settleability.24-27 However, there is still a

53

lack of available information on the behavior of WAS disintegration by persulfate oxidation.

54

In particular, the effect of persulfate oxidation on EPS and the micro-floc structures in WAS

55

and in turn on WAS dewaterability is barely known.

56

The objectives of the present study are i) to assess the potential of thermally activated

57

persulfate oxidation to disintegrate and enhance the dewaterability of WAS and ii) to compare

58

the characteristic behaviors of the sludge disintegration by the two persulfates (i.e., PMS vs.

59

PDS) in terms of EPS, micro-flocs, solubilization, and the reduction in VSS, weight, and

60

volume.

61 62

EXPERIMENTAL SECTION

63 64

Materials. Concentrated WAS was collected from a full-scale wastewater treatment plant

65

(WWTP) located in Ulsan, South Korea. The WWTP employs the combined sequential batch

66

reactor (SBR)-anaerobic/anoxic/oxic (A2O) process, treating 47,000 m3 of domestic sewage 3



67

per day, and the sampling point was the end of the secondary clarifier. The collected WAS

68

sample was thickened to the required solid concentration (approximately 15 g VSS/L) and

69

stored at 4˚C for less than 2 days prior to the experiments. The representative characteristics

70

of the WAS are shown in the supporting information (Table S1). All chemicals were of

71

reagent grade and used without further purification. Potassium peroxymonosulfate (PMS,

72

KHSO5 as Oxone®, 2KHSO5·KHSO4·K2SO4, DuPont Co., USA) and sodium peroxydisulfate

73

(PDS, Na2S2O8) were obtained from Sigma-Aldrich Co. (USA). Stock solutions of PMS (300

74

mM) and PDS (300 mM) were prepared prior to the experiments.

75 76

Procedures for WAS Treatment. The WAS treatment experiments were conducted in a

77

batch reactor using a 500-mL Erlenmeyer flask with 300 mL of WAS. The reaction was

78

initiated by adding an aliquot of the persulfate stock solution (PMS or PDS) into the reactor

79

containing WAS. The concentrations of persulfates for the experiments were 0.5, 1, and 2

80

mmol/g VSS of WAS. Upon the addition of persulfate, the reactor was immediately heated

81

with stirring at 700 rpm by a ceramic hot plate stirrer. The reactor was continuously heated

82

until the temperature reached 80°C (for 5 min), and the temperature was controlled constant

83

(± 1.0 ˚C) by intermittent heating during the entire reaction (60 min). The solution pH

84

decreased from 6.8 to 2.0−4.2 depending on the condition as persulfates were decomposed

85

(refer to the supporting information, Figure S1). The WAS samples were withdrawn at

86

predetermined time intervals and immediately immersed into an ice bath to quench further

87

reaction. The samples were cooled for 3 min in the ice bath prior to analysis.14 For some

88

experiments, microwave (MW) heating was used for comparison. The MW irradiation

89

(2.45 GHz) was performed by placing the reactor into a MW chamber equipped with a probe 4


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digital thermometer and a magnetic stirrer. Further details on the MW experiments are

91

described in the supporting information (Figures S2−S5).

92 93

All experiments were conducted in duplicate at least, and the average values with the standard deviations are presented.

94 95

Filterability. The filterability of WAS was evaluated by the capillary suction time (CST) as

96

commonly used in the literature.28,29 A commercial capillary suction timer (Type 304B, Triton

97

Ltd., UK) equipped with a stainless steel funnel (1.0-cm inner diameter and 5.0-cm height)

98

and standard CST filter papers was used for the CST measurement. The variations in

99

filterability were represented by the reciprocal ratio of CST to its initial value (CST0/CST). A

100

CST0/CST value higher than unity indicates an increase in filterability.

101 102

EPS Stratification and Analysis. EPS in the WAS was stratified into four fractions (i.e.,

103

soluble EPS, loosely bound EPS (LB-EPS), the first and second tightly bound EPS (TB-EPS-

104

1 and TB-EPS-2)) by the EPS extraction protocol slightly modified from the method

105

described in the literature.30 The WAS samples were centrifuged (2000 g for 15 min), and the

106

supernatants were collected as soluble EPS. The bottom pellets were fully re-suspended to the

107

original volume using a phosphate buffer saline (PBS, pH 7). The suspensions were

108

centrifuged (5000 g for 15 min), and the supernatants were collected as LB-EPS. The bottom

109

pellets were re-suspended using PBS and ultrasonicated at 20 kHz and 480 W for 10 min

110

while maintaining a temperature of 4˚C in an ice bath. The suspensions were centrifuged

111

(20,000 g for 20 min), and the supernatants were collected as TB-EPS-1. The residues were

112

again subject to re-suspension, ultrasonication, and centrifugation under the same conditions 5



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as the last run, and the supernatants were collected as TB-EPS-2. Protein (PN) and

114

polysaccharide (PS) (assumed to be dominant components of EPS5,31) were quantified in each

115

fraction of the extracted EPS.

116

For the analyses of PN and PS, the EPS samples were filtered by a 0.45 µm PTFE

117

membrane syringe filter, and the filtrates were used. PN was measured by the Lowry method

118

using bovine serum albumin as the standard.32 PS was measured by the phenol-sulfuric acid

119

method using glucose as the standard.33

120 121

Centrifuged Weight Reduction. Centrifuged weight reduction (CWR) was analyzed by

122

measuring the weights of the centrifuged WAS before and after the treatment. To measure the

123

centrifuged weight (CW) of WAS, 30 ml of the WAS sample was centrifuged at 3,000 g for

124

30 min; then, the supernatant was gently discarded, and the remaining sludge sediment was

125

weighed. CWR was calculated using the CW values of the raw and treated WAS samples

126

(equation 1).

127

CWR (%) = [1 − CWtreated /CWraw] × 100

(1)

128 129

Other Analyses. To analyze the dissolved organic carbon (DOC), total dissolved nitrogen

130

(TDN), ammonium ion (NH4+), and persulfates in the WAS samples, the liquid was separated

131

by centrifugation (1308 g, 15 min) and subsequently filtered through a 0.45-µm-pore-size

132

polytetrafluoroethylene (PTFE) membrane syringe filter (HP045AN, Advantec MFS Inc.

133

Japan). DOC and TDN were analyzed using a TOC/TN analyzer (Shimadzu Co., Japan). The

134

concentration of NH4+ was quantified by ion chromatography (ICS-1600, Dionex Co., USA)

135

with a conductivity detector. The separation was performed on a cationic column (4 mm × 6


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136

250 mm, IonPac CS12A, Dionex Co., USA) using 18 mM methanesulfonic acid as the eluent

137

at a flow rate of 1.0 mL/min. Persulfates were analyzed by iodometry.34

138

The sub-micron particle size distribution and the zeta potential of the WAS samples were

139

analyzed by a Zetasizer Nano ZS (Malvern Instruments Ltd., UK). A rotational viscometer

140

(LVDV-I Prime, Brookfield Eng. Labs. Inc., USA) was used to measure the apparent

141

viscosity of the WAS samples and the measurement was performed using spindle (No. 63) at

142

20 rpm. The samples were centrifuged at 300 g for 10 min, and the supernatants were used

143

for the analysis. The total suspended solids (TSS), volatile suspended solids (VSS), water

144

content, and alkalinity were analyzed in accordance with the Standard Methods.35

145 146

RESULTS

147 148

WAS Filterability. The time-dependent variation of the WAS filterability (denoted as

149

CST0/CSTt) was monitored during the thermally activated PMS and PDS treatments (80˚C)

150

with different persulfate doses (Figures 1a and 1b). The control conditions generally

151

decreased the WAS filterability; the persulfate treatment at room temperature (RT, 24±0.5˚C)

152

slightly decreased the CST0/CSTt value over the entire reaction time, and the thermal

153

treatment at 80˚C without persulfate resulted in a rapid decrease in the CST0/CSTt value

154

within 5 min.

155

The PMS treatment activated at 80˚C enhanced the WAS filterability during the initial

156

stage of the reaction (10 min); the CST0/CSTt value increased 2.17, 2.08, and 1.58 times at

157

PMS doses of 0.5, 1.0 and 2.0 mmol/g VSS, respectively (Figure 1a), and then after 10 min,

158

the WAS filterability stagnated or began to decrease. The thermally-activated PDS treatment 7



159

exhibited different trends from the PMS treatment (Figure 1b). At the beginning of the

160

reaction (within 5 min), the PDS treatment at 80˚C decreased the WAS filterability similar to

161

the thermal treatment without persulfate. However, after 5 min, the WAS filterability began to

162

increase enormously; in 60 min, the CST0/CSTt value increased by 2.39 and 4.31 times at

163

PDS doses of 1.0 and 2.0 mmol/g VSS, respectively. It appeared that 0.5 mmol PDS/g VSS

164

was not sufficient to recover the WAS filterability.

165 166

Decomposition of Persulfates. The decomposition of persulfates was monitored during the

167

WAS treatment (Figures 2a and 2b). The decomposition of PMS at 80˚C proceeded in two

168

stages (Figure 2a). In the first stage of the reaction, a certain amount of PMS was

169

instantaneously decomposed possibly due to the rapid direct reactions of PMS with the WAS

170

constituents. The second stage reaction exhibited a gradual decomposition of PMS following

171

pseudo-first-order kinetics. At RT, the instantaneous decomposition of PMS occurred initially,

172

but no further decomposition followed in the second stage. In contrast with PMS, PDS

173

continuously decomposed during the entire reaction time at 80˚C, exhibiting pseudo-first-

174

order decay. The PDS decomposition at RT was negligible.

175 176

Particle Size Distribution. The particle size distribution of WAS has been examined as an

177

important factor affecting the sludge filterability.8,26,36 Mikkelsen noted that particles in WAS

178

generally exhibits bimodal size distributions in ranges of 0.5–5 µm (primary particles) and

179

25–1,000 µm (macro-flocs), and primary particles have a dominant effect on the WAS

180

filterability characterized by CST.29 Shao et al. also reported that solids smaller than 10 µm

181

have the most critical effect on the WAS filterability.37 In this study, therefore, the size

8


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182

distribution of (supra- and true-38) colloidal particles in the range of 0.01–10 µm was

183

examined during thermal and thermally activated persulfate treatments (Figures 3a−3c).

184

The particle size distribution of raw WAS exhibited a major peak centered at 5.25 µm.

185

The thermal treatment without persulfate did not significantly change the distribution profile

186

(Figure 3a); the major peak at 5.25 µm slightly increased, and a very minor peak evolved at

187

approximately 1.36 µm. However, the treatments by thermally activated PMS and PDS

188

appeared to fragment the major group of particles in WAS into smaller groups (Figures 3b

189

and 3c). In the PMS treatment at 80˚C (Figure 3b), the peak at 5.25 µm was rapidly reduced

190

with evolving new peaks in the lower particle size range. Those peaks were downshifted as

191

the reaction proceeded; 0.25 and 1.27 µm at 15 min, 0.06, 0.71 µm at 30 min, and 0.06,

192

0.29−0.54 µm at 60 min. The PDS treatment at 80˚C downshifted the major peak at 5.25 µm

193

to those at 5.06 µm (15 min), 3.57 µm (30 min), and 1.86 µm (60 min) (Figure 3c). A very

194

minor peak was also evolved at 0.25 µm (refer to the inset). Compared to the PMS treatment,

195

the fragmentation rate of the major colloidal group (at approximately 5.25 µm) was slower in

196

the PDS treatment. In addition, the PDS treatment did not significantly produce true colloidal

197

particles less than 1 µm.

198 199

Behaviors of EPS. The concentrations of PN and PS in four different fractions of EPS (i.e.,

200

soluble-, LB-, TB-EPS-1 and TB-EPS-2) were monitored over time during the thermal and

201

thermally activated persulfate treatments (Figures 4a−4c for PN and Figures 4d−4f for PS).

202

The thermal treatment at 80˚C without persulfate increased the soluble- and LB-EPS (both

203

PN and PS) with slightly decreasing fractions of TB-EPS-1 and TB-EPS-2 (Figures 4a and

204

4d). 9



205

In the PMS treatment activated at 80˚C, the PN content in all fractions of EPS rapidly

206

decreased in the initial 5 min and then gradually increased over the rest of the reaction time;

207

in particular, an increase in soluble- and TB-EPS-2 was evident (Figure 4b). Similarly, the

208

overall PS content exhibited a rapid initial decrease and a subsequent gradual increase

209

(Figure 4e). However, unlike the PN content, the PS content in the soluble EPS continued to

210

increase during the entire reaction time.

211

On the other hand, the treatment by thermally activated PDS continuously decreased both

212

the PN and PS contents in the TB-EPS-1 and TB-EPS-2 while increasing those in the soluble-

213

and LB-EPS (Figure 4c and 4f).

214 215

Dissolved Organics and Ammonium Ion. To examine the solubilization of the WAS

216

constituents, variations in DOC, TDN, and NH4+ were monitored during the thermally

217

activated persulfate treatments (Figures 5a−5d, and Figures S6a and S6b in the supporting

218

information).

219

The thermal treatment without persulfate was effective in the WAS solubilization; DOC

220

increased by 19-fold in 1 h (refer to “No oxidant” in Figures 5a and 5b). The thermally-

221

activated persulfate treatments did not increase DOC as much as the thermal treatment

222

(Figures 5a and 5b), due to the oxidative mineralization of eluted organic substances.39 An

223

increase in persulfate dose accelerates both elution and mineralization of DOC. For the PDS

224

oxidation (Figure 5a), DOC increased with increasing the oxidant dose, indicating that the

225

elution of organics is more pronounced than its mineralization. However, the PMS oxidation

226

exhibited less DOC at higher oxidant doses (Figure 5b), indicating that the mineralization of

10


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DOC dominates. TDN exhibited very similar trends to DOC in both the PMS and PDS

228

treatments (Figure S6a and S6b).

229

The concentration of NH4+ did not change during the thermal treatment (refer to “No

230

oxidant” in Figures 5c and 5d). However, the treatments by thermally activated persulfates

231

increased the level of NH4+ in solution (Figures 5c and 5d). A greater amount of NH4+

232

accumulated at higher doses of persulfates. The PMS treatment generally eluted more NH4+

233

than the PDS treatment.

234 235

Reduction of VSS and Centrifuged Weight. The VSS reduction by thermal and thermally

236

activated persulfate treatments was examined (Figure 6a). The thermal treatment without

237

persulfate reduced VSS by 2% in 1 h. The PMS treatment at RT reduced VSS by

238

approximately 2.5% in 1 h at all PMS doses. The VSS reduction by the PDS treatment at RT

239

was negligible. The treatments by thermally activated persulfates reduced VSS by 4.9–15.4%

240

for PMS and by 4.1–7.7% for PDS depending on the persulfate dose.

241

The centrifuged weight reduction (CWR) exhibited similar trends to the VSS reduction,

242

with higher overall values and less difference between the PMS and PDS treatments (Figure

243

6b). The CWR by thermal treatment without persulfate was relatively low. The changes in

244

color and volume of the WAS samples during the persulfate treatments were also presented

245

(Figure S7 in the supporting information).

246 247

MW Effects. MW heating was compared with conventional heating to examine any

248

synergistic effects of MW irradiation with persulfate oxidation (i.e., non-thermal effects other

249

than the temperature elevation by MW40). However, notable differences between the two

11



250

heating methods were not found in any of the results regarding WAS filterability, persulfate

251

decomposition, and dissolution of DOC and NH4+. Further details are described in the

252

supporting information (Figures S2−S5).

253 254

DISCUSSION

255 256

Factors Affecting WAS Filterability. In WAS flocs, water is usually entrapped in the

257

interstitial space of the EPS matrix and partially in the intracellular region of microbial cells,

258

or adsorbed on the EPS surfaces. The literature has reported that WAS filterability is affected

259

by multiple factors such as the concentration and properties of EPS, floc size, the surface

260

charge of flocs, and viscosity.24,28-30,37,38,41-43 A high concentration of EPS generally leads to

261

low WAS filterability with different sensitivities depending on the classifications of EPS

262

(soluble, LB-, TB-EPS, and PS, PN, etc.).30,37,41,42 In terms of the floc size, the abundance in

263

supra- and true colloidal particles (smaller than 10 µm) has been reported to decrease WAS

264

filterability.29,37,38 An increased negative surface charge also decreases the filterability by

265

enhancing the dispersivity of WAS suspension (i.e., the increased stability of colloidal

266

particles results in less aggregation and subsequently increases the packing density of

267

flocs).28,29,41 An increase in WAS viscosity generally decreases WAS filterability.24,42,43

268

However, the deterioration of WAS filterability with decreased viscosity has been also

269

reported in some non-oxidative treatments (e.g., thermal, ultrasonic and alkali

270

treatments).44,45 In this study, no clear correlation between viscosity and WAS filterability

271

was found (compare Figure 1 and Figure S9 in the supporting information).

272 12


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Effects of Thermal Treatment on WAS Filterability. The thermal treatment without

274

persulfate deteriorated the WAS filterability (Figure 1), which is consistent with the

275

observations in earlier studies.26,36,46 The release of soluble EPS (Figures 4a and 4d, also

276

evidenced by the increase in DOC and TDN, Figures 5 and S6) appears to be the main cause

277

for the decreasing filterability. The thermal treatment at 80˚C did not significantly cause the

278

fragmentation of major flocs (Figure 3a)26,36,46 or the lysis of cellular proteins in WAS (no

279

production of NH4+, Figures 5c and 5d). Tian et al. reported that the peptide bonds of proteins

280

in WAS start to be cleaved at above 300˚C, eventually leading to the release of NH4+.3 The

281

increase in surface negative charge (Figure S8 in the supporting information) may partially

282

increase the stability of micro-flocs (e.g., a small group of micro-flocs at 1.36 µm was formed,

283

Figure 3a), contributing to the decrease in filterability.

284 285

Reactions of Persulfates with WAS Constituents. Persulfates are strong oxidants with high

286

redox potentials for two-electron transfer reactions (E0(S2O82−/SO42−) = 1.96 VSHE47;

287

E0(HSO5−/SO42−) = 1.75 VSHE48). They can also be converted into reactive radical species

288

such as SO4•− and •OH at elevated temperatures. Both direct (by persulfates) and indirect (by

289

radical species) reactions will oxidatively disintegrate WAS components.

290

As shown in Figures 2a and 2b, the reactions of persulfates with WAS proceeded in

291

different patterns for PMS versus PDS. The instantaneous decomposition of PMS in the

292

initial stage of the reaction (both at RT and 80˚C) implies that specific substrates in WAS

293

rapidly consume PMS via selective direct reactions. PMS has been reported to oxidize

294

various organic compounds (e.g., indole, vanillin and aromatic anils) via non-radical

295

mechanisms such as oxygen transfer and nucleophilic addition.49-51 The decomposition of 13



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296

residual PMS in the second stage (i.e., the gradual pseudo-first-order decay) proceeded via

297

the thermal decomposition of PMS itself which produces •OH and SO4•− (reaction 2).52 The

298

resultant radical species non-selectively oxidize substrates in WAS.

299

HSO5−

∆

•

OH + SO4•−

(2)

300

In the second stage, the possibility for PMS to directly react with WAS substrates is excluded

301

because the PMS decomposition rate in WAS is identical to that in deionized water (Figure

302

S10a in the supporting information).

303

On the other hand, PDS did not undergo the initial fast decomposition by direct reactions

304

with WAS constituents (Figure 2b). It is well known that PDS is thermally decomposed to

305

produce two equivalents of SO4•− by the symmetrical fission of the peroxide bond (reaction

306

3),53 and it appears that SO4•− is primarily responsible for the oxidative disintegration of WAS

307

components.

308

∆

S2O82−

2SO4•−

(3)

309

A notable observation is that the PDS decomposition is significantly accelerated in the WAS

310

medium compared to in deionized water (Figure S10b in the supporting information),

311

suggesting the possibility of direct reactions of PDS with WAS components (relative slow

312

and steady compared to the reactions of PMS). This behavior can also be explained by the

313

radical chain reactions initiated by the reaction of SO4•−. It has been suggested that the one-

314

electron oxidation of organic substrates by SO4•− produces an organo radical as an

315

intermediate, which converts another PDS molecule into SO4•−, propagating the chain

316

reactions (reactions 4 and 5).54,55

317

RH + SO4•−

→

R• + SO42− + H+

(4)

14


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318


S2O82− + R• →

SO4•− + SO42− + R+

(5)

319 320

Effects of Thermally-Activated Persulfate Treatments on WAS Filterability. The

321

variations in WAS filterability during thermally-activated persulfate treatments (Figure 1) are

322

interpreted by the behaviors of colloidal particles (Figure 3) and EPS (Figure 4).

323

The initial enhancement in WAS filterability by the thermally-activated PMS treatment

324

(Figure 1a) primarily resulted from the fast decrease of EPS in the first stage of the reaction

325

(< 5 min, Figures 4b and 4e). The destruction of EPS leads to the release of interstitial and

326

surface-bound water molecules. In particular, the decrease in TB-EPS (rather than soluble and

327

LB-EPS) and in PN (rather than PS) was pronounced, which was consistent with the

328

observations in previous studies; TB-EPS has been suggested to be a substantial factor

329

reciprocally correlated with the WAS filterability in the Fe(II)-catalyzed PDS treatment25

330

(however, other studies have reported the importance of soluble- and LB-EPS in raw and

331

thermally treated WAS30,37,42,46), and PN is known as a more critical constituent of EPS for

332

holding water molecules than PS.30,37,41 In the second stage (after 5 min), the WAS

333

filterability began to deteriorate as the overall EPS increased (Figures 4b and 4e); the increase

334

in soluble- and TB-EPS-2 was pronounced. The decreased filterability in this stage can also

335

be associated with the fragmentation of micro-flocs into fine colloidal particles (Figure 3b).

336

Previous studies have demonstrated that the water filterability through the particle cake

337

decreases mainly by fine particles smaller than 1 µm by experiments using inorganic particles

338

(silicon carbide and alumina) of different sizes.56,57 The reactions of •OH and SO4•− appear to

339

be responsible for the intensive fragmentation of micro-flocs.

15



340

On the other hand, in the thermally-activated PDS treatment, the WAS filterability

341

slightly decreased in the initial stage of the reaction, but then (after 5 min) it significantly

342

increased throughout the entire reaction (Figure 1b). The initial decrease in filterability may

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have resulted from the instant increase in soluble EPS (Figures 4c and 4f) as was in the

344

thermal treatment; in the initial stage, the thermal effect appears dominant over the chemical

345

oxidation. In the subsequent process, the filterability was substantially improved due to the

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overall decrease in EPS (Figures 4c and 4f); particularly, the decrease in TB-EPS was critical.

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In addition, contrary to the PMS treatment, the PDS treatment did not produce a significant

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faction of true colloidal particles (

Disintegration of Waste Activated Sludge by Thermally-Activated

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