Influence of thermal hydrolysis pretreatment on organic transformation characteristics of high solid anaerobic digestion

Accepted Manuscript Influence of Thermal Hydrolysis Pretreatment on Organic Transformation Characteristics of High Solid Anaerobic Digestion Yun Han, Yang Zhuo, Dangcong Peng, Qian Yao, Huijuan Li, Qiliang Qu PII: DOI: Reference:

S0960-8524(17)31282-8 http://dx.doi.org/10.1016/j.biortech.2017.07.166 BITE 18586

To appear in:

Bioresource Technology

Received Date: Revised Date: Accepted Date:

2 June 2017 25 July 2017 27 July 2017

Please cite this article as: Han, Y., Zhuo, Y., Peng, D., Yao, Q., Li, H., Qu, Q., Influence of Thermal Hydrolysis Pretreatment on Organic Transformation Characteristics of High Solid Anaerobic Digestion, Bioresource Technology (2017), doi: http://dx.doi.org/10.1016/j.biortech.2017.07.166

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

1

Influence of Thermal Hydrolysis Pretreatment on Organic Transformation

2

Characteristics of High Solid Anaerobic Digestion Yun HANa,b,* , Yang ZHUOa,b, Dangcong PENGa,b, Qian YAOa,b, Huijuan LIa,b, Qiliang QUa,b a

School of Municipal and Environmental Engineering, Xi’an University of Architecture and Technology, Xi’an, 710055,

China b

Key Laboratory of Membrane Separation of Shaanxi Province, Xi’an University of Architecture and Technology, Xi’an,

710055, China

3 4 5 6 7 8 9 10 11 12 13 14 15 16

Keywords: high solid sludge; thermal hydrolysis pretreatment; anaerobic digestion; substance transformation. Abstract: The study evaluated the influence of thermal hydrolysis pretreatment (THP) on anaerobic digestion (AD) ability of high solid sludge. The transformation characteristics of organics during the THP+AD process of dewatering sludge from wastewater treatment plant was investigated using a lab-scale THP reactor and four anaerobic digesters. The reduction efficiency of volatile suspended solids using THP+AD exceeded 49%. The acceleration of biogas production during AD was due to the enhancement of protein hydrolysis and acidogenesis by THP. THP had only minimal influence on the improvement of carbohydrate acidogenesis. The hydrolysis of poly phosphates was likely the main reaction of phosphorus transformation. Biochemical generation of sulfide and ammonia nitrogen occurred during the acidogenesis.

17 18

1

Introduction

19

Anaerobic digestion (AD) of waste activated sludge (WAS) in municipal wastewater

20

treatment plants (WWTPs) could reduce environmental pollution and recover energy (Ennouri

21

et al., 2016). Typically, the treatment of WAS aims at sludge reduction and stabilization,

22

which requires concentration, anaerobic digestion, dewatering, and disposal. AD is the core

23

treatment in the sludge stabilization process, and has been extensively studied. However, in

24

China WAS is still not sufficiently stabilized and dewatered, as only 20% of the anaerobic

25

digesters in WWTPs are well operated (Dai et al., 2013). The current operating conditions of

26

digesters in China reflect economic viability of the AD unit, which is influenced by the

27

energy recovery ratio and digester volume. These limitations are associated with the first step

28

of the AD process, the hydrolysis of organics.

29

Strengthening the rate-limiting hydrolysis step of AD could increase the biodegradation

30

of biosolids. A recent quantitative, comprehensive, multi-subject study of sludge pretreatment

31

technologies (thermal, ultrasonic, alkaline, enzymatic and mechanical) alone and in various

32

combinations was carried out with the goal of enhancing the hydrolysis of organics and the

33

release of intracellular material (Gonzalez-Fernandez et al., 2015). Of the variety of

34

pretreatments, thermal hydrolysis pretreatment (THP) seems most likely to increase the

35

efficiency of removal of organics (Li and Noike, 1992). THP intensifies the liquefaction and

36

release of macromolecular organic particulates at high temperatures (Li et al., 2016).

37

Temperature, retention time and solid content are influential factors of THP. Optimal

38

conditions for THP are a temperature of 160~180 ℃ for 30~60 min (Carrère et al., 2010).

39

WAS treated by THP reportedly displays better digestion capabilities in the subsequent

40

reactions of AD. However, this remarkable improvement of biogas production yield comes at

41

the cost of high energy consumption, which cannot be supplemented by that of the biogas

42

produced.

43

THP demands an input of high grade heat. The key parameter to assure energy

44

self-sufficiency in THP+AD was the total solids (TS) concentration, which has been

45

suggested as 5% of minimum concentration (Cano et al., 2015). High solid treatment (solid

46

content >10%) has been suggested to reduce energy cost. Accordingly, THP and high solid

47

anaerobic digestion (HSAD) were integrated. The THP+HSAD process might achieve energy

48

self-sufficiency and stabilize sludge effectively and adequately. The process is an alternative

49

upgraded sludge management practice in WWTPs.

50

Most research on THP+HSAD has focused on operating performance, inhibition and

51

optimize. Liu et al. (2016A) observed that the hydrolysis remains the rate-limiting step of

52

HSAD. Xue et al. (2015) reported that higher temperature and longer reaction time of THP are

53

beneficial for the hydrolysis of proteins and carbohydrates. The mass balances and

54

distribution of chemical oxygen demand (COD), nitrogen and phosphorus contributed by

55

straws, manures and foodstuff during AD were elucidated by Li et al. (2016). The authors

56

described that methane production can be predicted by derived linear models. The diffusion

57

pattern is the key to determine the inhibitory level of HSAD system (Aymerich et al., 2013).

58

The AD ability of formate, acetate, propionate, pyruvate, lactate, butyrate and valerate was

59

investigated by Yang et al. (2015). However, the detailed understanding of variation of sludge

60

during THP+AD is lacking, and few papers have addressed the performance of the

61

THP+HSAD process.

62

In this paper, we briefly introduce related concepts and discuss the details of the

63

performance of mesophilic and thermophilic AD of high solid sludge (solid content: 10%)

64

using THP device and completely mixed reactors. The anaerobic acidogenesis phase reactor

65

was used to investigate the biodegradation ability of pretreated sludge. The study investigates

66

the transformation characteristics of COD, nitrogen, phosphorus and sulfur during

67

THP+HSAD process, evaluated the influence of THP on organic removal efficiency during

68

HSAD, and clarified the transformation sequence during organic acidogenesis.

69

2

70

2.1 Sludge Sampling and Characterization

Materials and Methods

71

The high solid sludge was sampled from the dewatering sludge of the fourth WWTP in

72

Xi’an, China, which uses anaerobic-anoxic-aerobic (A2/O) process to remove nitrogen and

73

phosphorus. The solid content of sludge was adjusted to 10% by adding tap water and the

74

sludge was stored at 4 ℃. The average characteristics of the raw sludge are as follows: TS

75

101.97 g·L-1, suspended solids (SS) 101.05 g·L-1, volatile solids (VS) 71.05 g·L-1, volatile

76

suspended solids (VSS) 66.00 g·L-1, total chemical oxygen demand (TCOD) 93.56 g·L-1, pH

77

7.13 and volatile fatty acids (VFAs) 0.25g·L-1. The detail of the raw sludge is listed in Table

78

1.

79

2.2 THP Device and AD System

80

Experiments were performed in a thermal hydrolysis laboratory device comprising which

81

a 200 mL stainless pressure vessel, heating device (oil bath) and temperature sensor. The heat

82

conducting oil was dimethicone. The sensor was put inside the pressure vessel to monitor the

83

sludge temperature. The pretreatment of high solid sludge was conducted as follows. The

84

vessel was filled with the sludge (about 100 mL per vessel) and sealed. The conducting oil

85

was heated to the desired temperature. The vessel was put into the THP laboratory device and

86

heated from conducting oil immersion. The time when the value of temperature sensor

87

reached the set value was recorded. The vessel was removed from the THP device at that

88

time. Finally, the vessel was allowed to naturally cool and the sludge was stored at 4 ℃.

89

To investigate the transformation characteristics of sludge during THP+HSAD process,

90

four different processes were used. These included mesophilic anaerobic digestion (MAD),

91

thermophilic anaerobic digestion (TAD), THP+MAD and THP+TAD. Borosilicate glass

92

vessels (5L) that were continuously stirred served as the anaerobic digester. The schematic

93

diagram of THP device and AD system is presented in Fig. 1. The AD reactors had a head

94

space of 2 L. A water seal was used to avoid the risk of seal failure during the AD. Specific

95

temperature of reactors in all processes was maintained by heating of the device to ensure the

96

stability of AD. The AD reactors were equipped with stirrers to ensure the completely mixing

97

of the sludge that was loaded daily using a peristaltic pump. Biogas production was recorded

98

every day at 9:00 a.m. Pressure and temperature of the biogas were recorded at the same time.

99

Biogas production of all reactors was monitored daily using wet gas meters, and normalized

100

to the STP conditions.

101

Four reactors were operated at mesophilic (35±1 ℃) and thermophilic (55±1 ℃)

102

conditions, with feeding by pretreated and unpretreated sludge, respectively. All reactors were

103

set at a hydraulic retention time (HRT):20 d. Seed sludge of all the reactors was used from

104

anaerobically digested sludge of the first WWTP in Xi’an, China. The SS concentration of

105

inoculum sludge was 13.12 g·L-1.

106

To investigate the acidogenesis characteristics of pretreated sludge, acidogenesis phase

107

(AP) reactors were used. The AP reactors consisted of a shaking table at 35±1 ℃ and 100

108

mL detest oxygen bottles. The inoculum sludge was obtained from a lab scale mesophilic AD

109

reactor (high solid, HRT:20 d, stable for over 200 days). To separate the methanogens

110

microorganisms

from

the

AP

reactors,

the

inoculum

sludge

was

added

with

111

2-bromoethanesulfonic acid sodium salt (BES), eluted with oxygen-free water and

112

centrifuged. Thus, the reactors would remain in the acidogenesis phase. Four mesophilic AP

113

reactors were fed with thermal pretreated high solid sludge. The completely mixed sludge in

114

AP reactors was discharged and thermal pretreated sludge was added daily with HRT ranged

115

from 2d~5d.

116

2.3 Analytical Methods

117

Analyses to characterize both sludge and biogas included pH, alkalinity, TS, VS, SS,

118

VSS, COD, soluted COD (SCOD), VFAs, total proteins (Pro), dissolved proteins (DPro), total

119

carbohydrates (Cab), dissolved carbohydrates (DCab), total ammonia nitrogen (TAN), total

120

nitrogen (TN), total dissolved nitrogen (DTN), phosphate (PO43-), total phosphorus (TP), total

121

dissolved phosphorus (DTP), hydrogen sulfide (H2S), total sulfur (TSul), dissolved sulfide and

122

particulate sulfide (PS2-). Calculations of the transformation ratio are presented in Table 2 and

123

described in subsection 2.4.

124

TS, VS, SS, VSS, TN, TDN, TAN, PO43-, TP, TDP, COD and alkalinity were analyzed

125

according to standard methods for the examination of water and wastewater (APHA, 2005).

126

pH was determined using a model PHS-3c pH meter (Shanghai Jingke Corp. China).

127

Carbohydrates and proteins were measured using previously described Pheno-H2SO4 (Dubois

128

et al., 1956) and the Lowry (Frølund et al., 1995) methods. Methane in biogas and VFAs were

129

monitored by gas chromatograph (TCD and FID, 3420A, BEIFEN Corp. China). VFAs were

130

defined including fatty acids from C2 to C6 (acetic, propionic, butyric, etc.). The dissolved

131

and particulate sulfide were measured by p-aminodimethylaniline photometry with

132

acidification air striping absorption pretreatment (APHA, 2005). H2S was detected by gas

133

chromatograph (FPD, Clarus 600; PerkinElmer Inc. US). Total sulfur was detected by barium

134

chromate spectrophotometry with HNO3-HClO4 digestion pretreatment (APHA, 2005).

135

2.4 Calculation of Transformation Ratio

136

To describe the transformation characteristics of organics during THP+HSAD process, it

137

is essential to convert the concentration of parameters to the transformation ratio. A brief

138

summary of some of the relevant parameters is presented in Table 2, including COD,

139

nitrogen, phosphorus and sulfur. All were measured and the average concentrations were

140

converted to transformation ratios.

141

3

Results and Discussion

142

3.1 Performance of THP

143

This research focused on the effect of THP on organic AD. Optimization of THP

144

condition was necessary. Based on the results of orthogonal test on VSS reduction ratio, 165

145

℃ for 50 min was selected as the optimized THP condition.

146

A set of batch tests were performed to assess the performance of THP on high solid

147

sludge. Characteristics of raw sludge and pretreated sludge are shown in Table 1. SCOD/COD

148

reached as high as 45.21% under THP at the optimized conditions. This result agreed with

149

prior findings (Xue et al., 2015). Organics, which could be transformed into biogas during

150

HSAD, were transferred mainly to the hydrolysate. Proteins and carbohydrates, which were

151

the main organic components of hydrolysate, were solubilized due to the THP process. VFAs

152

accounted for only 9.35% of the hydrolysate. Over half (54.23%) of the particulate organic

153

nitrogen present in the form of dissolved nitrogen, and 22.13% of dissolved nitrogen, were

154

converted to ammonia nitrogen. The latter is a significant inhibitor of methanogenesis in high

155

TS content sludge digestion systems, and can also be hydrolyzed from proteinaceous material

156

in the following AD. The conversion percent of particulate phosphorus was 30.52%.

157

Dissolved phosphorus was mostly transformed to phosphate (79.84%) with the lysis of

158

phosphorus-accumulating bacteria. Little change was observed for sulfide (0.38%).

159

3.2 Transformation Characteristics of COD during THP and AD

160

The variation of daily biogas production, pH, alkalinity and ammonia nitrogen

161

concentration were measured during long term operation, allowing the estimating of the

162

relative stability in each reactor (Fig. 2.). After 50 days of operation, the curve appeared

163

stable. As shown in Table 3, methane concentrations of unpretreated AD ranged from

164

63~65%, which was consistent with the average biogas methane concentration of 63% for

165

conventional AD (Appels et al., 2008). The methane concentration of THP+MAD exceeded

166

70%, probably due to the transformation of CO2 to HCO3- (pH >8.1). Characteristics of biogas

167

production rate in stable stage was measured. Large differences were encountered in terms of

168

the biogas production rate (Lbiogas/Lfeed) between the reactors. Biogas production (Lbiogas/Lfeed)

169

in the conventional AD was approximately 6~10 Lbiogas/Lfeed, but was raised to 24.69

170

Lbiogas/Lfeed for THP+MAD process. The result highlights that THP accelerates the biogas

171

production rate of MAD compared to unpretreated HSAD. However, the biogas production

172

rate of unpretreated HSAD was similar to that of conventional AD. These results indicate that

173

the pretreatment of sludge is effective in enhancing anaerobic biodegradability.

174

To compare the AD performance of pretreated and unpretreated sludge, the

175

characteristics of sludge in the stable stage were determined (Table 1). The VSS reduction

176

efficiency of high solid sludge in the MAD and TAD reactors were 31.05% and 33.65%,

177

respectively, which were lower than the average reduction efficiency (40%~50%). The

178

hindered efficiency could reflect the fluidity and mass transfer performance. The VSS

179

hydrolysis ratio by THP significantly differed from the VSS reduction ratio after anaerobic

180

digestion (P<0.05). VSS and VS reduction efficiency of pretreated MAD both exceeded

181

49.00%, which demonstrated the extreme operating stability and high efficiency of organic

182

removal. Furthermore, the HRT of AD in high solid THP+MAD processing could be further

183

decreased. Low VS reduction efficiency of high solid THP+TAD process was observed, since

184

most of the hydrolyzed organics existed in the form of dissolved proteins, carbohydrates and

185

VFAs. With the increasing organic load of the digester, pH in MAD, TAD and THP+MAD

186

processes under stable operating condition were increased up to 8.03~8.11, which excluded

187

the THP+TAD reactor, in that the alkalinity, which consisted of ammonia nitrogen and

188

HCO3-, was also increased with solid content.

189

AD is a microbial degradation process where organics are broken down to produce CO2

190

and CH4. The organics in sludge, as the biomass energy resource, is converted to biogas by the

191

sequence of hydrolysis, acidogenesis, acetogenesis and methanogenesis (Metcalf, 2002).

192

Presently, COD, methane, proteins, carbohydrates and VFAs were measured to investigate the

193

conversion characteristics of organic. Referring to the content of lipid in WAS and its THP

194

efficiencies (Li and Noike, 1992; Liu et al., 2016B), proteins and carbohydrates were

195

considered as the analytical parameters of COD.

196

The transformation ratios of organic compounds during HSAD in all processes are

197

shown in Fig. 3. As expected, the reaction rate of each step showed that THP accelerated AD

198

The bio-energy recovery ratio of MAD fed by pretreated sludge (165 ℃, 50 min) was

199

obviously higher than those corresponding to the raw sludge. The particulate proteins

200

hydrolysis ratio of THP+MAD process (59%) was higher than the simple MAD and TAD

201

(35~41%), the transformation ratio of dissolved proteins to VFAs was up to 99%, which was

202

directly caused the doubling of biogas production. The results of the statistical analysis

203

between VSS hydrolysis ratio of THP and hydrolysis/acidogenesis ratio of proteins revealed

204

an obvious difference of hydrolysis and the acidogenesis ratio of proteins before and after

205

THP (P<0.05).

206

The hydrolysis ratio of carbohydrates was increased from 60%~62% (unpretreated) to

207

77%~81% (pretreated). However, with the enhancement of carbohydrates hydrolysis, the

208

acidogenesis ratio of carbohydrates indicated only a marginal difference between the

209

pretreated and unpretreated sludge (Fig. 3). This phenomenon could be explained by the

210

component of carbohydrates in waste activated sludge, which is mainly composed of cell

211

walls (Li and Noike, 1992). Even cell lysis was achieved after THP, and the transformation

212

ratio of carbohydrates was limited by its biodegradation ability.

213

THP accelerates the rate of biogas production during AD due to the solubilization of

214

organics (Matthew et al., 2017). Once the hydrolysis of organics is completed by THP, the

215

following steps can proceed rapidly. That is the reason why the evaluation of THP often

216

selects VSS hydrolysis ratio as the indicator to optimize the pretreatment condition. After

217

comparing the transformation ratio of proteins and carbohydrates with or without THP, the

218

effect of THP on anaerobic biodegradation characteristics of high solid sludge was

219

investigated. The improved rate of biogas production contributed to the acceleration of

220

hydrolysis and acidogenesis of proteins by THP. Since proteins are the main organic

221

component of WAS, optimized THP conditions will have greater influence on the hydrolysis

222

ratio of proteins than VSS.

223

3.3 Transformation Characteristics of Nitrogen, Phosphorus and Sulfur During AD

224

during THP

225

With the biodegradation of organics in WAS, nitrogen, phosphorus and sulfur

226

compounds were released to the liquid phase. The inhibitor and by-production needed to be

227

investigated to analyze their influence on the stability of WAS AD.

228

Nitrogen was measured in all processes. The transformation ratios are shown in Fig. 4.

229

Depending on the pH and temperature of the solution, ammonia nitrogen in aqueous solutions

230

is present either as free ammonia or as ammonium (Nie et al., 2015). Presently, the

231

concentration of free ammonia nitrogen (FAN) in each AD reactor was calculated according

232

to the equation (1) (Emerson et al., 1975). After AD, over 50% of the particulate nitrogen was

233

hydrolyzed and TAN concentrations of THP+MAD and THP+TAD reached 3.02~3.57 g·L-1.

234

The biogas production yield is known to increase with organic contents. But, it should be

235

noted that when dissociated from TAN, FAN becomes the main inhibitor of AD. The pH and

236

concentration of TAN of THP+TAD at start stage were 8.00 and 3.33 g·L-1, respectively.

237

FAN concentration in the THP+TAD process was 1.13 g·L-1. Duan et al. (2012) reported that

238

FAN that exceeds 0.60 g·L-1 inhibits the AD process. Therefore, to prevent systemic inhibited

239

by ammonia, the FAN concentration should be lower than the threshold value of 0.6 g·L-1.

240 241

As the equation (1) shows, FAN is determined by pH, temperature and TAN. FAN (mgFAN·L-1)=17/14×TAN×1/(100.09018+2729.82/(272.16+T)-pH+1)

(1)

242

To avoid the inhibition of FAN during HSAD, the solid content of sludge could be

243

calculated by equation (1) at the FAN concentration of 600 mg·L-1. Considering the

244

concentration of nitrogen compounds (about 0.06 TN·gTS-1) and the ammonia transformation

245

ratio (about 50% of TKN for HRT of 20 d) of sludge and pH (8.10), the suggested TS

246

concentration for the MAD and TAD process should be lower than 136 gTS·L-1 and 55

247

gTS·L-1, respectively. TAN concentration of the anaerobic digester is affected by the protein

248

concentration and its efficiency of hydrolysis. However, the hydrolysis process is closely

249

related to the biodegradability and energy recovery ratio. This means that controlling pH

250

value is the only measure to limit FAN concentration under certain temperature conditions.

251

The digester pH is determined by the feed content and HRT, and affects alkalinity, acidity,

252

VFA concentration, organic reduction rate and microbial activity. Mesophilic AD can reduce

253

about two-thirds of the FAN concentration compared to thermophilic AD at a certain pH.

254

The transformation ratios of phosphorus in sludge of each step are presented in Fig. 4.

255

Phosphorus compounds in sludge mainly reflected the presence of organic phosphorus and

256

poly phosphate. The present results show that THP had little influence on the transformation

257

characteristics of phosphorus from subsequent AD. The results determined that 32%~35% of

258

total phosphorus in the sludge was converted to phosphate.

259

To understand the source of phosphate generated after AD, an anaerobic phosphate

260

release test of WAS was carried out. WAS was sampled and sealed in 100 mL bottles for 8 h

261

to attain the complete release of poly phosphate. The ratio of poly phosphate to total

262

phosphorus in the phosphorus accumulating organisms (PAO) could be measured through this

263

test. The ratio of phosphate to total phosphorus of WAS was 28%~30%, which was identical

264

to the ratio of phosphate to total phosphorus after HSAD. These results indicated that the

265

phosphate produced during HSAD process was hydrolyzed from poly phosphate. Given the

266

ratio of PAO/WAS in the A2/O system of 5~17% (Wong et al., 2005), it can be suggested that

267

the structure of organic phosphorus, exists in WAS, was difficult to break down by

268

biochemical process or thermal process.

269

H2S is a toxic by-product in biogas during anaerobic treatment of WAS. It can be

270

generated by sulfate-reducing bacteria (SRB) or by the decomposition of the sulfur amino

271

acids methionine (C5H11NO2S) and cysteine (C3H7NO2S) by other anaerobic bacteria (Du and

272

Parker, 2012). SRB are important in H2S production during the anaerobic treatment of

273

sulfate-rich wastewater (Mizuno et al., 1988; Chen et al., 2008). Moreover, in WWTP sludge

274

treatment, especially for WAS, proteins are the main organic compounds of sludge and the

275

H2S in biogas is generally associated with the destruction of organic material (Dewil et al.,

276

2009). Therefore, the concentration and transformation ratio of sulfur-containing proteins are

277

the key factors influencing the concentration of H2S production in biogas.

278

The transformation ratios of sulfur in each step are presented in Fig. 4. The TSul in all

279

processes ranged from 1.42~1.52 g·L-1. H2S concentration in biogas in this study was

280

146.6±18.7 ppm, 112.0±12.8 ppm, 74.1±24.1 ppm and 29.5±20.9 ppm in MAD, TAD,

281

THP+MAD and THP+TAD process, respectively, while it is approximately 1000 ppm in

282

conventional AD (Appels et al., 2008). Three possible approaches can be used to analyze this

283

phenomenon: pH, heavy metal concentration and hydrolysis efficiency of sulfur proteins.

284

Sulfide of the sludge exists as H2S, dissolved and particulate forms (Peu et al., 2012). The

285

ratio of H2S/TSfd (total sulfide) decreases with increasing pH, and the pH value of in all

286

HSAD processes (about 8.0) is higher than conventional AD (7.0~7.5). Consequently, H2S

287

concentration in biogas decreased with raised alkalinity during AD, which was increased with

288

the solid content of feeds. On the other hand, the dissolved sulfide tends to form precipitates

289

in the presence of combinations of heavy metals. Inadequate hydrolysis of sulfur proteins may

290

occur during HSAD. As mentioned above, H2S concentration in biogas is reduce with

291

increased solid content of sludge.

292

3.4 Transformation Characteristics of VFAs, Nitrogen, Phosphorus and Sulfur during

293

Acidogenesis Phase

294

The transformation characteristics of COD, nitrogen, phosphorus and sulfur during

295

mesophilic AD showed that THP influences hydrolysis in addition to acidogenesis. To

296

investigate the influence of THP on acidogenesis of organics in sludge, four AP reactors were

297

operated with the aim of understanding the mechanism of biodegradation of thermal

298

pretreated sludge.

299

With degradation of organics and the production of bicarbonate and ammonia nitrogen in

300

AP reactor, the concentration of alkalinity increased with the concentration increase of HRT.

301

There was no significant methane detected (<0.1%) in biogas. From the methane

302

concentration in biogas, pH and alkalinity of sludge and AP reactor, the phase separation in

303

AP reactor was achieved. The pH of the four AP reactors was 6.27 of HRT:2d, 6.30 of

304

HRT:3d, 6.34 of HRT:4d and 6.52 of HRT:5d, respectively. The variation of HRT had an

305

obvious difference in pH with the raw sludge (P<0.05).

306

Ratios of acetic acid to VFAs in AP reactors were 3.1% of HRT:2d and 62.35% of

307

HRT:5d, respectively. Dissolved proteins (3.27 g·L-1) and carbohydrates (4.83 g·L-1)

308

remained in the liquid phase of the HRT:5d AP reactor. Acidogenesis of the unconverted

309

dissolved organic, which were not transformed to VFAs in the HRT:2d AP reactor, was

310

difficult. The dissolved organic nitrogen was almost completely (about 90%) transformed to

311

ammonia nitrogen in the HRT≥3d AP reactor. The composition of phosphorus in sludge was

312

not due to the change after pretreatment and biological hydrolysis. The main form of sulfide

313

was particulate sulfide in the AP reactor. With the hydrolysis of organic sulfur in sludge, the

314

H2S concentration in biogas gradually increased from 155 mg·m-3 in the HRT:2d reactor to

315

1744 mg·m-3 in the HRT:5d reactor.

316

To evaluate the acidogenesis of organics in the AP process, VFAs, acetic acid, ammonia,

317

phosphate and total sulfide were analyzed. Referring to the concentrations in lab scale

318

THP+MAD (high solid, HRT:20d), the concentrations were converted to the transformation

319

ratio as follows: VFAs=VFAsAP/(MethaneTHP+MAD+VFAsTHP+MAD); acetic acid=acetic

320

acidAP/(Acetic acidTHP+MAD+MethaneTHP+MAD); TAN=TANAP/TANTHP+MAD; PO43-= PO43-AP/

321

PO43-THP+MAD; TS2-=TS2-AP/TS2-THP+MAD.

322

The calculation results of transformation ratio in the AP process are shown in Fig. 5. Of

323

note, VFAs could be rapidly transformed from SCOD (HRT:2d) after pretreatment, and

324

significantly increased acetic acid concentration was detected in the AP reactor of HRT:5d. In

325

the AP reactors, the rate limiting step was acetogenesis. The calculation results of phosphate

326

indicated that the hydrolysis of phosphate was completed by THP in THP+HSAD process.

327

The variation of ammonia nitrogen showed that the deamination of amino acids, as the only

328

source of ammonia nitrogen during AD, was completed at HRT≥3d. For the rapid generation

329

of ammonia nitrogen, the thermal pretreated high solid WAS could be considered as the ideal

330

provider of alkalinity in co-digestion with wastes of high carbon to nitrogen ratio.

331

Considering to the H2S concentrations in biogas produced by AP reactors, pre-separation of

332

H2S is feasible by adding an AP reactor between THP and the AD unit. The transformation

333

sequence of organics in AP reactor was P> N> S> VFAs> acetic acid. It can be inferred that

334

the biochemical generation of sulfide and ammonia nitrogen occur prior to the acetogenesis

335

during anaerobic biodegradation of thermal pretreated sludge.

336

4

Conclusion

337

The effect of THP on organics anaerobic biodegrading was assessed in addition via the

338

transformation ratio analysis. In summarize, THP is suitable for WAS pretreatment to

339

accelerate the anaerobic digestion process. Also, THP has the potential on applying waste

340

treatment of low carbon to nitrogen ratio. Results showed that THP has the potential to

341

enhance the anaerobic biodegradation ability of wastes of low carbon to nitrogen ratio. For

342

the thermal pretreated sludge, the generating of by-products (like ammonia, sulfide and

343

phosphate) is prior to the acetic acid during HSAD. In future work, this characteristic could

344

be used to separate H2S, recover phosphate, provide alkalinity or remove ammonia nitrogen.

345

Acknowledgment

346

This work was supported by the National Natural Science Foundation of China [grant

347

number 51408458], the key projects of National Water Pollution Control and Management of

348

China [grant number 2013ZX07315-001-04], the Natural Science Foundation of Shaanxi

349

Province [grant number 2017JM5090], and the Innovative Research Team of Xi’an

350

University of Architecture and Technology.

351

References

352 353 354 355 356 357 358 359 360

1. 2. 3.

4.

APHA, 2005. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC. Appels, ., ae ens, ., egr ve, J., Dewil, R., 2008. Principles and potential of the anaerobic digestion of waste-activated sludge. Prog. Energy Combust. Sci. 34, 755–781. Aymerich, E., Esteban-Gutiérrez, M., Sancho, L., 2013. Analysis of the stability of high-solids anaerobic digestion of agro-industrial waste and sewage sludge. Bioresour. Technol. 144, 107–114. Cano, R., Pérez-Elvira, S.I., Fdz-Polanco, F., 2015. Energy feasibility study of sludge pretreatments: A review. Appl. Energy. 149, 176–185.

361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411

5.

6. 7.

8. 9. 10.

11.

12.

13. 14.

15.

16. 17. 18. 19.

20.

21. 22. 23.

Carr re, H., umas, C., attimelli, A., atstone, . ., elgen s .P., Steyer, J.P., Ferrer, I., 2010. Pretreatment methods to improve sludge anaerobic degradability: A review. J. Hazard. Mater. 183(1-3), 1-15. Chen, Y., Cheng, J., Creamer, K.S., 2008. Inhibition of anaerobic digestion process: A review. Bioresour. Technol. 99(10), 4044–4064. Dai, X., Duan, N., Dong, B., Dai, L., 2013. High-solids anaerobic co-digestion of sewage sludge and food waste in comparison with mono digestions: Stability and performance. Waste Manage. 33(2), 308–316. Dewil, R., Baeyens, J., Roels, J., Steene, B.V.D., 2009. Evolution of the Total Sulphur Content in Full-Scale Wastewater Sludge Treatment. Environ. Eng. Sci. 26(4), 867–872. Du, W., Parker, W., 2012. Modeling volatile organic sulfur compounds in mesophilic and thermophilic anaerobic digestion of methionine. Water Res. 46(2), 539–546. Duan, N., Dong, B., Wu, B., Dai, X., 2012. High-solid anaerobic digestion of sewage sludge under mesophilic conditions: Feasibility study. Bioresour. Technol. 104, 150–156. Emerson, K., Russo, R.C., Lund, R.E., Thurston, R.V., 1975. Aqueous ammonia equilibrium calculations: effect of pH and temperature. J. Fish. Res. Board Can., 32(12), 2379–2383. Ennouri, H., Miladi, B., Diaz, S.Z., Güelfo, L.A.F., Solera, R., Hamdi, M., Bouallagui, H., 2016. Effect of thermal pretreatment on the biogas production and microbial communities balance during anaerobic digestion of urban and industrial waste activated sludge. Bioresour. Technol. 214, 184–191. Frølund, B., Griebe, T., Nielsen, P.H., 1995. Enzymatic activity in the activated-sludge floc matrix. Appl. Microbiol. Biotechnol. 43(4), 755–761. Gonzalez-Fernandez, C., Sialve, B., Molinuevo-Salces, B., 2015. Anaerobic digestion of microalgal biomass: Challenges, opportunities and research needs. Bioresour. Technol. 198, 896–906. Li, H., Tan, F., Ke, L., Xia, D., Wang, Y., He, N., Yanmei, Z., Qingbiao, L., 2016. Mass balances and distributions of C, N, and P in the anaerobic digestion of different substrates and relationships between products and substrates. Chem. Eng. 287, 329–336. Li, Y., Noike T., 1992. Upgrading of anaerobic digestion of waste activated sludge by thermal treatment. Water Sci. Technol. 26(3-4), 857-866. Li, Y., Jin, Y., Li, J.H., Nie, Y., 2016, Enhanced nitrogen distribution and biomethanation of kitchen waste by thermal pre-treatment. Renew Energy. 89, 380–388. Liu, C., Li, H., Zhang, Y., Chen, Q, 2016A, Characterization of methanogenic activity during high-solids anaerobic digestion of sewage sludge. Biochem. Eng. 109, 96–100. Liu, C., Li, H., Zhang, Y., Liu, C., 2016B. Improve biogas production from low-organic-content sludge through high-solids anaerobic co-digestion with food waste. Bioresour. Technol. 219, 252-260. Higgins, M.J., Beightol, S., Mandahar, U., Suzuki, R., Xiao, S., Lu, H.-W., Le, T., Mah, J., Pathak, B., DeClippeleir, H., Novak, J.T., Al-Omari, A., Murthy, S.N., 2017. Pretreatment of a primary and secondary sludge blend at different thermal hydrolysis temperatures: Impacts on anaerobic digestion, dewatering and filtrate characteristics. Water Res. 122, 557–569. Mizuno, O., Li, Y., Noike, T., 1998. The behavior of sulfate-reducing bacteria in acidogenic phase of anaerobic digestion. Water Res. 32(5), 1626–1634. Metcalf, E., 2002. Wastewater Engineering: Treatment and Reuse, 4th ed. New York: McGraw-Hill. Nie, H., Jacobi, H.F., Strach, K., Xu, C., Zhou, H., Liebetrau, J., 2015. Mono-fermentation of chicken manure: ammonia inhibition and recirculation of the digestate. Bioresour. Technol. 178, 238–246.

412 413 414 415 416 417 418 419 420 421 422 423

24. Peu, P., Picard, S., Diara, A., Girault, R., Béline, F., Bridoux, G., Dabert, P.,2012. Prediction of hydrogen sulphide production during anaerobic digestion of organic substrates. Bioresour. Technol. 121, 419–424. 25. Wong, M.T., Mino, T., Seviour, R.J., Onuki, M., Liu, W.T., 2005. In situ identification and characterization of the microbial community structure of full-scale enhanced biological phosphorous removal plants in japan. Water Res. 39(13), 2901-2914. 26. Xue, Y., Liu, H., Chen, S., Dichtl, N., Dai, X., Li, N., 2015. Effects of thermal hydrolysis on organic matter solubilization and anaerobic digestion of high solid sludge. Chem. Eng. 264, 174–180. 27. Yang, Y., Chen, Q., Guo, J., Hu, Z., 2015. Kinetics and methane gas yields of selected C1 to C5 organic acids in anaerobic digestion. Water Res. 87, 112–118.

424 425 426 427

Fig. 1. Schematic diagram of THP device and anaerobic digestion system

Stable

Start up

Ammonia (g·L-1 )

Alk(gCaCO3·L-1 )

pH

Daily Biogas Produuction (L·d -1 )

8.0 TAD

THP+MAD

THP+TAD

6.0 5.0 4.0 3.0 2.0 1.0 0.0 9.0 8.5 8.0 7.5 7.0 6.5 6.0 20.0

(b)

(c)

18.0 16.0 14.0 12.0 10.0 5.0

(d)

4.0 3.0 2.0 1.0 0.0 0

428 429 430 431

MAD

(a)

7.0

20

40

60

80

100 Time (d)

120

140

160

180

200

Fig. 2. Variation of daily biogas production, pH, alkalinity and ammonia nitrogen concentrations over long term operation

432 Hydrolysis

Acidogenesis

PCOD

SCOD

PPro

37%

PCab

VFAs

DPro, AA

68%

34%

PCab

75%

74%

99%

DCab

69%

54% PPro

99%

75% DPro, AA

62%

THP

88%

62%

41% PPro

Methane

Acetic Acid

DCab

60%

TAD

Methanogenesis

65%

35%

MAD

Acetogenesis

46%

PCab

22% DPro, AA

9%

3%

DCab

COD Protein Carbohydrate

65% 99%

59%

THP+MAD

PPro

56%

PCab

THP+TAD

PPro

86%

100%

100%

55%

64%

DCab

77%

73%

59%

71%

57%

DPro, AA

31%

DCab

PCab

433 434 435 436

DPro, AA

84%

81%

Fig. 3. Transformation ratio of COD in all processes

Nitrogen Particulate Nitrogen

MAD

Dissolved Nitrogen

51%

Phosphorus

Free Ammonia Nitrogen Ammonia Nitrogen

51%

13%

Particulate Dissolved Phosphate Phosphorus Phosphorus

39%

83%

Sulfur Total Sulfur

Dissolved Particulate Hydrogen Sulfide Sulfide Sulfide

83%

55%

TAD

62%

29%

30%

94%

85%

54%

30%

24%

94%

0.3% 7.7%

73%

THP+MAD

439

71%

11%

33%

102%

6.9%

1.9% 85%

78%

437 438

3.2%

1.9%

THP

THP+TAD

6.0%

2.0%

81%

1.2%

31%

89%

0.4%

1.7% 97%

Fig. 4. Transformation ratio of nitrogen, phosphorus and sulfur in all processes

440

160% Phosphate Total Sulphide Acetic Acid Produced

Transformation Ratio

140%

Ammonia Nitrogen VFAs Produced

120% 100% 80% 60% 40% 20% 0%

441 442 443

Raw

THP

HRT:2d

HRT:3d

HRT:4d

Fig. 5. Transformation ratio of organics during THP+AP process

HRT:5d

444 445

Table 1 Characteristics sludge before and after THP. Raw

THP

VSS (g·L )

66.00±5.08

37.39±4.66

pH

7.13±0.45

5.40±0.13

Alkalinity (gCaCO3·L-1)

0.67±0.35

4.23±1.26

-1

2.11±0.78

44.41±3.39

47.52±2.39

21.69±4.36

1.87±2.36

22.07±6.00

13.00±2.08

4.54±1.29

0.60±0.03

9.26±1.60

0.25±0.28

4.21±1.93

5.98±1.87

2.74±0.57

0.06±0.02

2.51±0.39

Ammonia Nitrogen (g·L )

0.27±0.02

1.06±0.11

Particulate Phosphorus (g·L-1)

1.94±0.18

1.35±0.15

0.07±0.06

0.11±0.08

0.05±0.01

0.70±0.08

1.50±0.19

1.52±0.13

Dissolved Sulfide (mg·L )

1.88±0.45

5.46±0.81

-1

0.00±0.00

0.30±0.07

-1

SCOD (g·L ) -1

Particulate Protein (g·L ) -1

Dissolved Protein (g·L ) -1

Particulate Carbohydrate (g·L ) -1

Dissolved Carbohydrate (g·L ) -1

VFAs (g·L ) -1

Particulate Organic Nitrogen (g·L ) -1

Dissolved Organic Nitrogen (g·L ) -1

-1

Dissolved Organic Phosphorus (g·L ) -1

Phosphate (g·L ) -1

Total Sulfur (g·L ) -1

Particulate Sulfide (mg·L )

446 447

448 449

Table 2 Calculation methods of transformation ratio Formula (PCODinf-PCODeff)/PCODinf

Hydrolysis

(PProinf-PProeff)/PProinf

Hydrolysis of Protein

(PCabinf-PCabeff)/PCabinf

Hydrolysis of Carbohydrate Acidogenesis

(Methane+VFAseff-VFAsinf)/(Methane+SCODeff-SCODinf) (PProinf-PProeff-DProeff+DProinf)/(PProinf-PProeff)

Acidogenesis of Protein Acidogenesis of Carbohydrate

(PCabinf-PCabeff-DCabeff+DCabinf)/(PCabinf-PCabeff)

Acetogenesis

(Methane+Aceff-Acinf)/(Methane+VFAseff-VFAsinf)

Methanogenesis Particulate Nitrogen

(PTNinf-PTNeff)/PTNinf

Dissolved Nitrogen

(TANeff-TANinf)/(DTNeff-DTNinf)

Ammonia Nitrogen

(FANeff-FANinf)/(TANeff-TANinf)

Particulate Phosphorus

(PTPinf-PTPeff)/PTPeff

Dissolved Phosphorus

(Phosphateeff- Phosphateinf)/(DTPeff-DTPinf)

Total Sulfur

450 451 452 453 454

Methane /(Methane +Aceff-Acinf)

(TS2-eff-TS2-inf)/TSulinf

Dissolved Sulfide (to Hydrogen Sulfide)

H2S/(TS2-eff-TS2-inf)

Dissolved Sulfide (to Particulate Sulfide)

(PS2-eff-PS2-inf)/(TS2-eff-TS2-inf)

inf: influent; eff: effluent; Ac: acetic acid; PCOD: particulate COD; PCab: particulate carbohydrate; PPro: particulate protein; PTN: particulate total nitrogen; PTP: particulate total phosphorus; TS2-: total sulfide.

455 456

Table 3 Characteristics of digested sludge in stable stage. MAD

TAD

THP+MAD

THP+TAD

45.51±6.46

43.79±7.08

31.20±6.25

28.10±9.74

VS (g·L )

55.91±5.59

57.52±5.75

35.94±4.01

50.72±5.21

pH

8.11±0.15

8.03±0.14

8.03±0.17

6.50±0.09

12.76±0.43

16.00±0.46

17.82±0.36

15.96±0.49

72.49±2.82

75.23±2.74

47.89±2.55

87.52±4.36

14.47±3.42

15.08±2.43

7.38±3.11

48.49±8.10

3.24±0.51

7.04±0.61

0.28±0.18

11.26±3.56

1.68±0.18

1.78±1.00

3.02±0.15

3.57±0.40

62.95±4.37

63.34±5.36

70.82±3.62

14.79±11.18

0.07

0.05

0.18

0.01

0.33

0.25

0.38

0.06

10.97

7.17

24.69

2.78

-1

VSS (g·L ) -1

-1

Alkalinity (gCaCO3·L ) -1

COD (g·L ) -1

SCOD (g·L ) -1

VFAs (gCOD·L ) +

-1

NH4 -N (g·L ) Methane (%) -1

Methane Production (L·(gCODadd·d) ) -1

Methane Production (L·(gCODremove·d) ) -1

Biogas Production (L·(Lsludge·d) )

457 458 459

Particulate Dissolved Degraded

Proteins

Carbohydrates

Poly PO43- to PO43-

High Solid Sludge

Thermal Hydrolysis Pretreatment

Anaerobic Digestion

460 461 462 463 464 465

Highlights： • Major influence of THP on AD is to accelerate proteins hydrolysis and acidogenesis. • THP has insignificant effects on carbohydrates anaerobic biodegradation. • Phosphorus transformation was mainly contributed by poly phosphates hydrolysis. • Transformation sequence of organic during acidogenesis was P>N>S>VFAs>acetic acid.