Free Nitrous Acid (FNA)-Based Pretreatment ... - ACS Publications

Subscriber access provided by Otterbein University

Article

Free Nitrous Acid (FNA)-Based Pre-treatment Enhances Methane Production from Waste Activated Sludge Qilin Wang, Liu Ye, Guangming Jiang, Paul Jensen, Damien Batstone, and Zhiguo Yuan Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es402933b • Publication Date (Web): 17 Sep 2013 Downloaded from http://pubs.acs.org on September 20, 2013

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

Environmental Science & Technology is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 29

Environmental Science & Technology

1

Free Nitrous Acid (FNA)-Based Pre-treatment Enhances Methane Production from

2

Waste Activated Sludge

3 4

Qilin Wang,† Liu Ye,†,‡ Guangming Jiang,† Paul D. Jensen,† Damien J. Batstone,†

5

and Zhiguo Yuan†,*

6 7

†

Australia

8 9

Advanced Water Management Centre (AWMC), The University of Queensland, QLD 4072,

‡

School of Chemical Engineering, The University of Queensland, QLD 4072, Australia

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25

1

ACS Paragon Plus Environment


TOC Art

26

27 Bioreactor Wastewater

Settler

Effluent

Waste activated sludge Thickener

FNA production

FNA treatment of sludge

0.8-1.5 g NH4+-N/L

CH4

Anaerobic digester

To head of the WWTP or side-stream treatment

Anaerobic digestion liquor

28

Dewatered sludge Transport+disposal Dewatering

2


Page 2 of 29

Page 3 of 29


29

ABSTRACT: Anaerobic digestion of waste activated sludge (WAS) is currently enjoying

30

renewed interest due to the potential for methane production. However, methane production is

31

often limited by the slow hydrolysis rate and/or poor methane potential of WAS. This study

32

presents a novel pre-treatment strategy based on free nitrous acid (FNA or HNO2) to enhance

33

methane production from WAS. Pre-treatment of WAS for 24 h at FNA concentrations up to

34

2.13 mg N/L substantially enhanced WAS solubilization, with the highest solubilization (0.16

35

mg chemical oxygen demand (COD)/mg volatile solids (VS), at 2.13 mg HNO2-N/L) being

36

six times that without FNA pre-treatment (0.025 mg COD/mg VS, at 0 mg HNO2-N/L).

37

Biochemical methane potential tests demonstrated methane production increased with

38

increased FNA concentration used in the pre-treatment step. Model-based analysis indicated

39

FNA pre-treatment improved both hydrolysis rate and methane potential, with the highest

40

improvement being approximately 50% (from 0.16 to 0.25 d-1) and 27% (from 201 to 255 L

41

CH4/kg VS added), respectively, achieved at 1.78-2.13 mg HNO2-N/L. Further analysis

42

indicated that increased hydrolysis rate and methane potential were related to an increase in

43

rapidly biodegradable substrates, which increased with increased FNA dose, while the slowly

44

biodegradable substrates remained relatively static.

45 46 47 48 49 50 51 52 53

3



54

INTRODUCTION

55

Large amounts of organics in wastewater are converted to waste activated sludge (WAS)

56

during biological wastewater treatment. Most WAS disposal methods (e.g. agricultural use,

57

incineration) either require, or benefit from biological stabilisation, which destroys organics

58

and controls pathogens. In particular, anaerobic digestion is attracting extensive attention

59

since it converts organics in WAS to a renewable bioenergy resource in the form of

60

methane.1-4 However, methane production through anaerobic digestion is often limited by the

61

slow hydrolysis rates and poor biochemical methane potential of the WAS.1,5 Thus, effective

62

improvement of methane production in anaerobic digestion, particularly through thermal,

63

chemical, and mechanical pre-treatments has become an important research topic and an

64

industrial application area.1,6-12 These technologies destroy cells and/or extracellular

65

polymeric substances (EPS) with the release of intracellular and/or extracellular constituents

66

to the aqueous phase.1,6,7 The released constituents are more easily biodegraded during

67

anaerobic digestion, thereby enhancing methane production. For example, it was observed

68

that methane production in anaerobic digestion increased by 42% after pre-treating WAS at

69

175 °C for 60 min.9 However, most of the above mentioned approaches are cost intensive due

70

to high energy and/or chemical requirements and have negative environmental consequences

71

(e.g. higher net CO2 emissions compared with the case without pre-treatment).7,11 Thus,

72

alternative methods to enhance methane production in anaerobic digestion are needed.

73 74

Our recent studies showed that free nitrous acid (FNA or HNO2), at parts per million (ppm)

75

levels, had a strong biocidal effect on anaerobic sewer biofilms13 and microorganisms in

76

WAS.14 It was reported that WAS treatment using FNA at 1-2 mg N/L for 24-48 h reduced

77

WAS metabolic activity to zero and killed 50%-80% of the cells (i.e. damaged cell

78

membrane) in WAS. 14 Aerobic digestion tests showed that the degradation of WAS with

4


Page 4 of 29

Page 5 of 29


79

FNA pre-treatment was more than two times higher in comparison to the WAS without FNA

80

pre-treatment.14 More recently, WAS production in a reactor operated under alternating

81

anoxic-aerobic conditions was reduced by 28%, by treating part of the returned activated

82

sludge with FNA for 24 h at an FNA level of 2.0 mg N/L.15 However, the FNA induced

83

improvement in WAS degradation, as revealed by the above research, all occurred under

84

aerobic/anoxic conditions. No information has been reported so far on the effect of WAS pre-

85

treatment using FNA on methane production in anaerobic digestion of WAS.

86 87

The aim of this study is to assess a novel method based on FNA pre-treatment to enhance

88

methane production in anaerobic digestion of WAS. FNA is a renewable and low cost

89

chemical that can be produced on site by nitritation of the anaerobic digestion liquor,16,17 thus

90

achieving sustainable use of the anaerobic digestion liquor otherwise requiring extra

91

treatment. The effect of FNA pre-treatment at 0-2.13 mg N/L for 24 h on WAS solubilization

92

and biochemical methane production was evaluated. The FNA derived enhancement to both

93

the hydrolysis rate and the methane production potential was revealed through model-based

94

analysis.

95 96

MATERIALS AND METHODS

97

Sludge sources

98

WAS was collected from the dissolved air flotation thickener of a local biological nutrient

99

removal wastewater treatment plant (WWTP) with a sludge retention time (SRT) of 15 d in

100

Queensland, Australia. Its main characteristics (with standard errors obtained from triplicate

101

measurements) were: total solids (TS) 42.6 ± 0.2 g/L, volatile solids (VS) 33.7 ± 0.2 g/L, total

102

chemical oxygen demand (TCOD) 54.1± 0.3 g/L, soluble chemical oxygen demand (SCOD)

103

0.65 ± 0.03 g/L, pH=6.4 ± 0.0.

5



104 105

For the biochemical methane potential (BMP) tests to be further described below, the

106

inoculum was harvested from a mesophilic anaerobic digester treating mixed primary sludge

107

and WAS in the WWTP from which WAS was collected. Its main characteristics (with

108

standard errors obtained through triplicate measurements) were: TS 31.2 ± 0.2 g/L, VS 22.5 ±

109

0.1 g/L, TCOD 31.5 ± 0.2 g/L, SCOD 0.76 ± 0.03 g/L, pH=7.5 ± 0.0.

110 111

FNA pre-treatment on waste activated sludge

112

Contact batch tests were performed to assess the effect of FNA pre-treatment on the

113

characteristics of WAS. 2.1 L of WAS was evenly distributed into seven batch reactors. A

114

nitrite stock solution was then added to the batch reactors in different volumes to achieve the

115

designated nitrite concentrations of 0, 50, 100, 150, 200, 250 and 300 mg N/L, respectively.

116

Each test lasted for 24 h, during which pH was controlled at 5.5 ± 0.1 using 1.0 M HCl

117

solution via a programmable logic controller (PLC) except for the case of 0 mg N/L, where

118

pH was not controlled and was observed to be around 6.4 (i.e. original pH of the WAS). This

119

served as a control. The nitrite concentrations and pH levels applied gave rise to FNA

120

concentrations of 0, 0.36, 0.71, 1.07, 1.42, 1.78 and 2.13 mg N/L, respectively, which were

121

calculated using the formula

122

temperature T (°C) (25°C in this study) with the formula

S NO − − N /(K a × 10 pH ) 2

with the Ka value determined as a function of

Ka = e−2,300/(273+T) .18

123 124

In each test, the VS, SCOD, soluble Kjeldahl nitrogen (SKN), ammonium nitrogen (NH4+-N),

125

soluble proteins, soluble polysaccharides and volatile fatty acid (VFA) were measured in

126

triplicate both before and after FNA pre-treatment. The measured changes were then

127

expressed as a biomass specific value by dividing by the corresponding VS of WAS measured

128

before the pre-treatment. 6


Page 6 of 29

Page 7 of 29


129

Anaerobic batch biochemical methane potential tests

130

Methane production from WAS with and without FNA pre-treatment (0-2.13 mg HNO2-N/L,

131

for 24 h) was assessed using BMP tests, as described in Angelidaki et al.19 The BMP tests

132

were carried out in 160 mL serum vials (100 mL working volume). Each BMP test contained

133

75 mL inoculum and 25 mL WAS with an inoculum to WAS ratio of 2.0 on a dry VS basis.

134

The vials were flushed with helium gas for 1 min (1 L/min), sealed with a butyl rubber

135

stopper retained with an aluminium crimp-cap and stored in a temperature controlled

136

incubator at 37 ± 1°C. Tests were mixed by inversion prior to each sampling event. Three sets

137

of blanks (blanks I, II and III) were also set up. Blank I contained inoculum and MilliQ water

138

without WAS. Blanks II and III were identical to blank I except with the addition of nitrite

139

stock solution, which resulted in an initial nitrite level of around 12.5 and 75 mg N/L,

140

respectively, in blanks II and III. This was to evaluate the effect of nitrite on the performance

141

of the inoculum. The initial nitrite levels of 12.5 and 75 mg N/L in blanks II and III were

142

similar to the lowest and highest initial nitrite levels in the serum vials with the addition of

143

FNA-treated WAS. All tests were carried out in triplicates. The BMP tests lasted for 44 days,

144

when biogas production dropped to insignificant levels. The biogas (CH4, CO2, H2, N2, N2O)

145

production was monitored on a daily basis over the first week and every 2-4 days afterwards.

146

The biogas production from WAS was obtained by subtracting biogas production from blank

147

I. The methane production was reported as the volume of methane produced per kilogram of

148

VS added (L CH4/kg VS added).

149 150

Biochemical methane potential tests modelling

151

The hydrolysis rate (k) and biochemical methane potential (B0), two key parameters

152

associated with methane production from WAS, were used to evaluate and compare methane

153

production kinetics and potential of the WAS with and without FNA pre-treatment. They

7



Page 8 of 29

154

were estimated by fitting the methane production data from BMP tests to a first-order kinetic

155

model using a modified version of Aquasim 2.1d with sum of squared errors (Jopt) as an

156

objective function.20 The uncertainty surfaces of k and B0, based on a model-validity F-test

157

with 95% confidence limits, were estimated using the modified version of Aquasim 2.1d.120

158

The degradation extent (Y) of WAS was determined using equation (1):

159

Y= B0/380×0.62+LN2/880×1.71/(54.1×0.025)

160

where B0 =biochemical methane potential (L CH4/kg VS added); 380=theoretical biochemical

161

methane potential under standard conditions (25 °C, 1 atm) (L CH4/kg TCOD);21 0.62=ratio

162

of VS/TCOD in the studied WAS; LN2=volume of N2 produced under standard conditions (25

163

°C, 1 atm) (L); 880=N2 yield (L N2/kg NO2--N);21 1.71=oxygen equivalent of nitrite (kg O2/kg

164

NO2--N);21 54.1×0.025=WAS fed to the BMP vials (kg COD).

(1)

165 166

Two models were used. The first considered a single substrate type (i.e. one-substrate model)

167

in the first-order kinetic model,20, 22 as shown in equation (2):

168

B (t) = B0 1 − e − kt

169

where B(t)=cumulative methane production at time t (L CH4/kg VS added); t=time (d).

(

)

(2)

170 171

In the second model, the WAS samples were considered to consist of a rapidly biodegradable

172

substrate type and a slowly biodegradable substrate type (i.e. two-substrate model).22 The aim

173

of introducing the two-substrate model was to explore the effect of FNA pre-treatment on the

174

rapidly biodegradable substrates and slowly biodegradable substrates, respectively.

175

The equation of the two-substrate model is shown below:

176

B (t) = B0, rapid 1 − e

177

where B0,rapid=biochemical methane potential of the rapidly biodegradable substrates (L

178

CH4/kg VS added); krapid=hydrolysis rate of the rapidly biodegradable substrates (d-1);

(

− krapid t

)+ B (1 − e ) − kslowt

(3)

0 ,slow

8


Page 9 of 29


179

B0,slow=biochemical methane potential of the slowly biodegradable substrates (L CH4/kg VS

180

added); kslow=hydrolysis rate of the slowly biodegradable substrates (d-1).

181 182

Analysis

183

WAS samples were filtered through disposable Millipore ﬁlter units (0.45 µm pore size) for

184

the analyses of NH4+-N, NO2--N, SCOD, SKN, soluble proteins, soluble polysaccharides and

185

VFA. The NH4+-N and NO2--N concentrations were analyzed using a Lachat QuikChem8000

186

Flow Injection Analyzer (Lachat Instrument, Milwaukee, Wisconsin). The TS, VS, TCOD,

187

SCOD, SKN and VFA concentrations were determined according to the standard methods.23

188

The soluble protein concentration was measured by the BCA method with BSA as standard.24

189

The soluble polysaccharide concentration was determined by the Anthrone method with

190

glucose as standard.25

191 192

The biogas volume was measured by a manometer at the start of each sampling event.

193

Cumulative volumetric gas production was calculated from the pressure increase in the

194

headspace volume (60 mL) and expressed under standard conditions (25 °C, 1 atm). At each

195

sampling event, the biogas quality (CH4, CO2, H2, N2, N2O) was determined using a Perkin

196

Elmer autosystem gas chromatograph equipped with a thermal conductivity detector (GC-

197

TCD), a gas chromatograph GC8-AIT equipped with a Porapak Q column and a thermal

198

conductivity detector, and an Agilent 7890A Gas chromatograph equipped with an electron

199

capture detector.23, 26

200 201

RESULTS

202

Effect of FNA pre-treatment on characteristics of waste activated sludge

203

Figure 1 shows the changes in WAS characteristics after 24 h FNA pre-treatment at different

204

FNA concentrations (0-2.13 mg N/L). Figure 1A shows increased FNA level resulted in 9



205

increased release of SCOD. For the untreated WAS (exposing to 0 mg HNO2-N/L for 24 h),

206

SCOD only increased by around 0.025 mg COD/mg VS. In contrast, SCOD increased by

207

approximately 0.16 mg COD/mg VS at the highest FNA level tested (2.13 mg N/L) over the

208

same period, which indicates WAS treated by FNA at 2.13 mg N/L was solubilized six times

209

(0.16 versus 0.025 mg COD/mg VS) that without FNA pre-treatment (0 mg N/L). This

210

implies more cells and/or EPS became soluble substrates from particulate substrates in the

211

case of FNA-treated WAS. A similar trend was also observed in the cases of SKN, soluble

212

proteins and soluble polysaccharides (see Figures 1B and 1C). The increases in the SKN,

213

soluble protein and soluble polysaccharide concentrations went from 0.002 mg N/mg VS,

214

0.003 mg/mg VS and 0.002 mg/ mg VS, respectively, for the untreated WAS (FNA at 0 mg

215

N/L) to approximately 0.015 mg N/mg VS, 0.034 mg/mg VS and 0.021 mg/mg VS,

216

respectively, for the WAS treated at an FNA level of 2.13 mg N/L. This implies that more

217

intracellular and/or extracellular constituents were released from the cells and/or EPS, which

218

corroborated the SCOD results. However, the NH4+-N and VFA results display a different

219

trend, with the amounts of NH4+-N and VFA produced decreased with the increase of FNA

220

concentration (Figures 1B and 1D). This could be attributed to the inhibitory/toxic effect of

221

FNA on sludge hydrolytic enzymes (e.g. protease) and/or enzymes responsible for

222

acidogenesis.14

223 224

Effect of FNA pre-treatment on biochemical methane production

225

The effect of nitrite on the performance of inoculum is shown in Figure S1. Similar methane

226

production was observed in blanks I, II and III throughout the BMP tests period. This

227

indicates nitrite concentrations used in this study did not have a significant effect on the

228

performance of inoculum. Therefore, blank I (i.e. 0 mg NO2--N/L) is a valid blank for

229

correcting methane production for all tests.

10


Page 10 of 29

Page 11 of 29


230 231

The measured methane production in all tests over the whole BMP test period is shown in

232

Figure 2. In general, pre-treatment of WAS with increased FNA levels resulted in increased

233

methane production throughout the 44 days’ BMP test period except for the case of 2.13 mg

234

N/L, where cumulative methane production ranked the first among all the studied cases only

235

after around fifteen days. This suggests that FNA pre-treatment is effective in enhancing

236

methane production during anaerobic digestion of WAS. This also indicates although FNA

237

pre-treatment at 2.13 mg N/L can further improve biochemical methane potential in

238

comparison to pre-treatment with a lower FNA level (e.g. 1.78 mg N/L), the corresponding

239

hydrolysis rate of WAS decreased slightly. The production of N2 in the BMP tests with FNA-

240

treated WAS was only observed on the first day and N2O was not detected at any time (data

241

not shown), indicating all the nitrite/FNA could be quickly removed and did not contribute to

242

WAS degradation from the second day onwards.

243 244

Determination of hydrolysis rate and biochemical methane potential

245

The hydrolysis rate (k) and biochemical methane potential (B0) were estimated using both

246

one-substrate and two-substrate models.

247 248

One-substrate model

249

The simulated methane production curves using one-substrate model are shown in Figure 3A,

250

which indicates the fit of methane production to the one substrate model was satisfactory

251

(R2>0.97 in all studied cases). Table 1 and Figure 4 show the estimated k and B0 and their

252

95% confidence regions at different FNA concentrations. In general, k and B0 increased with

253

increased FNA level except for the case of 2.13 mg N/L, where a decreased k was determined.

254

This is in accordance with the slow methane production observed during the first fifteen days

11



255

of the BMP tests at 2.13 mg N/L (see Figure 2). The reason for this slight, undesirable

256

decrease of k is not apparent and requires further clarification in future studies. The highest

257

improvement in k and B0 was achieved at FNA concentrations of 1.78 - 2.13 mg N/L, and was

258

determined as approximately 50% (from 0.16 to 0.25 d-1) and 27% (from 201 to 255 L

259

CH4/kg VS added), compared to the WAS without FNA pre-treatment (0 mg N/L).

260

Correspondingly, Y also increased (from 0.33 to 0.43 d-1) with increased FNA levels (see

261

Table 1). This indicates FNA pre-treatment improved k, B0 and Y.

262 263

Two-substrate model

264

The simulated methane production curves using two-substrate model are shown in Figure 3B,

265

which indicates methane production was well predicted (R2>0.99 in all studied cases). This

266

reflects the WAS composition was heterogeneous. The estimated values of krapid, B0,rapid,

267

Yrapid, and kslow, B0,slow, Yslow are shown in Table 2. Although poor parameter identification

268

existed in some cases (see shading area in Table 2), increased FNA levels resulted in

269

increased krapid, B0,rapid and Yrapid in general. They changed from 0.25 d-1, 136 L CH4/kg VS

270

added and 0.22, respectively, in the case of untreated WAS, to 0.42 d-1, 175 L CH4/kg VS

271

added and 0.29, respectively, at an FNA level of 1.78 mg N/L. This indicates FNA pre-

272

treatment had a significant effect on the hydrolysis rate, biochemical methane potential and

273

degradation extent of the rapidly biodegradable substrates. However, no significant difference

274

was observed in kslow, B0,slow and Yslow with the changes in FNA concentration, suggesting that

275

the slowly biodegradable substrates were not significantly affected.

276 277

DISCUSSION

278

Mechanistic analysis of performance improvements

279

This study demonstrated that WAS pre-treatment using FNA can improve k and B0, which

12


Page 12 of 29

Page 13 of 29


280

translated into the improved methane production performance. The relationship between FNA

281

levels and the two parameters (k and B0) is shown in Figure S2. The statistical analysis

282

indicates that FNA pre-treatment had a strong impact on both B0 and k (P

Free Nitrous Acid (FNA)-Based Pretreatment ... - ACS Publications

Recommend Documents