Net-Zero-Energy Model for Sustainable ... - ACS Publications

Dec 12, 2016 - treatment, and a large value of Yobs indicates that material and energy flow are directed toward anabolism, yielding sludge. 3. RESULTS...
0 downloads 0 Views 7MB Size
Subscriber access provided by Washington University | Libraries

Article

A net-zero energy model for sustainable wastewater treatment Peng Yan, Rongcong Qin, Jinsong Guo, Qiang Yu, Zhe Li, You-Peng Chen, Yu Shen, and Fang Fang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04735 • Publication Date (Web): 12 Dec 2016 Downloaded from http://pubs.acs.org on December 13, 2016

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

2

A net-zero energy model for sustainable wastewater treatment

3

Peng Yan1* Rong-cong Qin1*, Jin-song Guo1**, Qiang Yu1, Zhe Li1, You-peng Chen1, Yu Shen1, Fang

4

Fang2

1

5

1

Key Laboratory of Reservoir Aquatic Environment of CAS, Chongqing Institute of Green and

6

Intelligent Technology, Chinese Academy of Sciences, Chongqing 400714, China

7

2

8

400045, China

College of Urban Construction and Environmental Engineering, Chongqing University, Chongqing

9 10

* The two authors contributed equally to this work

11 12

**Corresponding Author

13

Prof. Jin-song Guo

14

Fax: +86-23-65935901, Tel: +86-23-65935901

15

Email: [email protected]

16 17 18 19 20 21

1

ACS Paragon Plus Environment

Environmental Science & Technology

22 23

Abstract: The large external energy input prevents the wastewater treatment from being

24

environmentally sustainable. A net-zero energy (NZE) wastewater treatment concept based on

25

biomass energy recycling was proposed to avoid wasting resources and promote energy recycling in

26

wastewater treatment plants (WWTPs). Simultaneously, a theoretical model and boundary condition

27

based on energy balance were established to evaluate the feasibility of achieving NZE in WWTPs;

28

the model and condition were employed to analyze data from 20 conventional WWTPs in China.

29

Six WWTPs can currently export excess energy, eight WWTPs can achieve 100% energy

30

self-sufficiency by adjusting the metabolic material allocation, and six municipal WWTPs cannot

31

achieve net-zero energy consumption based on the evaluation of the theoretical model. The NZE

32

model offset 79.5% of the electricity and sludge disposal cost compared with conventional

33

wastewater treatment. The NZE model provides a theoretical basis for optimization of material

34

regulation for effective utilization of organic energy from wastewater, and promotes engineering

35

applications of the NZE concept in WWTPs.

36 37 38

39

Keywords: net-zero energy; wastewater treatment; biomass energy recycling; theoretical model; anaerobic digestion

1. INTRODUCTION

40

Wastewater purification consumes a large amount of electricity.1 Treatment of wastewater

41

currently consumes approximately 4% of all electrical power produced in the United States,

42

whereas the electricity consumption of wastewater treatment plants (WWTPs) in China is 1 × 1011

43

kWh.2,3 The electricity required for wastewater treatment will increase by 20% in the next 15 years 2

ACS Paragon Plus Environment

Page 2 of 29

Page 3 of 29

Environmental Science & Technology

44

in developed countries, leading to significantly increased CO2 emissions and resource consumption,

45

because WWTPs have a large carbon footprint.4,5,6 The consequences of the massive energy

46

consumption of WWTPs include damage to the environment, depletion of natural resources, and

47

significant economic burden.7,8,9 Therefore, sustainable wastewater treatment processes must be

48

developed to decrease electricity consumption and the greenhouse gas (GHG) footprint of

49

WWTPs.4,10,11

50

Wastewater contains a significant amount of potential energy.12,13 The organic energy in

51

wastewater is approximately 9–10 times greater than that used to treat it.14 Wastewater treatment is

52

a complex microbial metabolic process, in which microbes utilize pollutants to achieve sustained

53

growth and propagation.15 Some organic energy is converted into biomass energy, while remaining

54

organic energy dissipates as heat.16 In conventional WWTPs, biomass energy is wasted with sludge

55

discard.5 Because the substantial energy in organic matter currently lost in conventional treatment

56

processes,1 intensive attention has been focused on recovering energy from wastewater to power

57

wastewater treatment and evaluating the energy efficiency of WWTP. Several energy recovery

58

methods developed with the aim of achieving energy self-sufficiency in WWTPs have been

59

discussed in previous reports.17 However, the most feasible approach to recovering internal energy

60

in existing treatment plants is using CH4 biogas produced during sludge anaerobic digestion as

61

biofuel to generate power and heat by cogeneration.12,18 If more organic energy is captured from

62

wastewater than is used for wastewater treatment, external energy input is not required; thus, an

63

independent and self-sufficient energy recycling system, with the characteristic of “net-zero energy”

64

(NZE), may be established. NZE has been partially or fully achieved in some WWTPs.19,20

65

Although these practical cases of energy self-sufficiency in WWTPs have been discussed in depth 3

ACS Paragon Plus Environment

Environmental Science & Technology

66

in previous studies, there have been no reports including statistical analysis of such cases and

67

mathematical derivation of energy utilization patterns; additionally, a theoretical model is urgently

68

needed to be developed to evaluate the feasibility of achieving NZE in WWTPs.

69

As earlier studies,21-22 the attention has been devoted to direct in-plant energy consumption and

70

generation in this paper (indirect energy use, energetic value of N and P as fertilizers, chemicals

71

input, etc. are not considered), and it seems to be more pertinent with respect of the direct energy

72

exploitation in WWTPs and the practical interest of stakeholders. The objective of the present study

73

is to establish a NZE theoretical model based on substance transformation and energy utilization to

74

systematically evaluate the feasibility and level of energy self-sufficiency of WWTPs, as well as to

75

obtain optimal organic matter allocation between catabolism and anabolism. The theoretical model

76

was employed to analyze data from 20 WWTPs in China to evaluate energy self-sufficiency based

77

on anaerobic digestion of excess sludge. Finally, a statistical analysis of NZE cases was conducted.

78

The results of this study provide a foundation for engineering applications of the NZE concept.

79

2. METHODS AND MATERIALS

80

2.1. Material-energy cycle of conventional and NZE WWTPs

81

Metabolism is typically divided into two categories: catabolism, the decomposition of organic

82

matter by cellular respiration with energy release; and anabolism, the synthesis of components of

83

microbial cells with energy utilization. Microorganisms decompose organic matter to sustain life

84

and reproduction via material-energy transformation in municipal WWTPs. Chemical energy from

85

organic matter is converted into biomass energy during biochemical treatment. 23

86

Organic matter, the main exploitable energy source in wastewater [represented as chemical 4

ACS Paragon Plus Environment

Page 4 of 29

Page 5 of 29

Environmental Science & Technology

87

oxygen demand (COD)], is completely biodegraded into simple molecules, such as CO2 and H2O,

88

during wastewater treatment. Two pathways transform COD into CO2 and H2O: (1) a conventional

89

pathway, in which COD is oxidized directly into CO2 by aerobic processes that require significant

90

input of external electrical energy; and (2) a sustainable pathway, in which COD is maximally

91

transformed into biomass (sludge) by bio-synthesis processes, after which the biomass is

92

biodegraded into CH4. CH4 is ultimately utilized to produce electricity, heat, and simple molecules

93

(CO2 and H2O). The conventional pathway is widely applied in existing WWTPs, in which most of

94

chemical energy from organic matter is expended by consuming electric energy in the pathway,

95

with CO2 escaping and energy depletion. The chemical energy in sludge is squandered with sludge

96

waste in conventional pathway (Figure. 1a); In contrast, in the sustainable pathway, most of the

97

chemical energy from organic matter is effectively utilized by sludge anaerobic digestion; therefore,

98

external energy consumption is offset by the electricity and heat generated by cogeneration,

99

reducing the cost and energy consumption associated with sludge treatment and disposal. Biomass

100

recycling and energy-saving are achieved by the sustainable pathway; thus, the NZE mode of

101

WWTPs is postulated based on the matter-energy cycle (Figure. 1b). A part of organic pollutants as

102

substrate were completely oxidized (O2) into simple, stable molecules (CO2 and H2O) by

103

microorganisms via catabolism with energy release. Remaining organic matter was directly used to

104

synthesize structural molecules and reproduce cells via anabolism; the energy required for

105

anabolism was provided by the energy released by catabolism. In order to ensure smooth

106

metabolism of microorganism, the organic matter allocated to anabolism cannot exceed 2/3 of the

107

total organic matter in bio-treatment unit.15 In the NZE WWTP model, an independent

108

energy-recycling system (no need for external energy input) for wastewater treatment was 5

ACS Paragon Plus Environment

Environmental Science & Technology

Page 6 of 29

109

developed based on the biomass energy utilization. In addition, a mathematical formulation of the

110

energy supply-demand balance of wastewater treatment in the NZE wastewater treatment system

111

was established.

112

2.2 Establishment of the NZE WWTP theoretical model

113

2.2.1 Specific energy consumption and COD flow of anabolism

114

COD is more directly related to energy and carbon substrate is usually used in energy balance

115

calculation, therefore specific energy consumption µ was defined as the electricity consumption per

116

unit of COD removed (kWh/kgCOD), and it can be calculated with Eq. [1]:

117

µ =

118

where Ew represents the electricity consumption of wastewater treatment [kWh]. Mo (ton) represents

119

the total mass of organic matter removed as measured by COD (Equation [3]).

Ew 1000M o

(1)

120

In the NZE mode, sludge handling is the process of energy production based on self-energy

121

balance, and the Ew is only contributed by the energy for aeration (Eaera); therefore, Eaera can be

122

obtained by the function of the removed COD Mo (ton) with the specific energy consumption as

123

follows:

124

Eaera =1000µ M o

(2)

125

As presented in section 2.1, there are two material flows of COD, catabolism and anabolism,

126

which are represented by CODcata and CODana, respectively. Thus, the total mass of organic matter

127

was divided into two parts, which were consumed by catabolism Mcata and anabolism Mana, and

128

their quantitative relation was derived as Equation [3]. In addition, the total mass of organic matter

129

Mo was related to influent CODinf, effluent CODeff, and wastewater flow Qw (m3/d), which was 6

ACS Paragon Plus Environment

Page 7 of 29

Environmental Science & Technology

130

expressed as Equation [4]:

131

M o = M cata + M ana

132

Mo =

133

where the units of COD and M were mg/L and ton/d, respectively.

(3)

(

Qw CODinf − CODeff 10

)

(4)

6

134

In this paper, the MLVSS/MLSS ratio of sludge in practical municipal WWTPs is considered

135

to be 0.75.24 The relationship between sewage sludge amount Mts (ton/d), organic matter Mvs (ton/d),

136

and inorganic matter Mis (ton/d) was derived as Equation [5] and Equation [6]:

137

M ts = M is + M vs

(5)

138

M vs = 0.75M ts

(6)

139

where Mvss is the sludge reproduced by anabolism of microorganisms, and it is assumed that

140

CODana contributes completely to MLVSS.

141

Microbial cells in sludge are typically expressed as C5H7O2N;16 the amount of oxygen required

142

to oxidize one microbial cell to carbon dioxide, water, and ammonia is given by:

143

C5 H 7 O2 N + 5O2 → 5CO2 + 2 H 2O + NH 3

144

(7)

According to Formula [7], one gram of C5H7O2N is equivalent to 1.42 grams of COD; thus,

145

Mana in sewage sludge was expressed as Equation [8]:

146

M ana = 1.42 M vs

147

2.2.2 Electricity and heat requirements of the anaerobic digestion system

(8)

148

Thermophilic digestion is more beneficial for energy utilization than mesophilic digestion. 25,26

149

Therefore, thermophilic digestion was adopted to generate energy, and the digestion temperature

150

was assumed to be 50 °C. The energy requirement of digestion stirring is related to the excess

7

ACS Paragon Plus Environment

Environmental Science & Technology

Page 8 of 29

151

sludge volume Qs (m3/d), which is a function of Mts (ton/d) (Equation [9]). In the calculation, the

152

moisture content of the sludge was assumed to be 96% (a common value), whereas the density of

153

the digested sludge ρs was assumed to be 1000 kg/m3 (approximately equal to that of water).

154

Equations [5] and [8] were substituted into Equation [9] to simplify the Equation. A digestion time

155

of 25 days and typical power per unit volume of stirring of 0.008 kW/m3 were chosen;27 after which

156

the energy for stirring Ediges (kWh) per day was obtained by Equation [10]:

157

Qs =

1000(kg/t) ⋅ M ts (t ) ρ s (kg / m3 ) ⋅ (1 − 96%)

(9)

3

= 25(0.7 M ana + M is ) (m / d ) 158

Ediges = 25( d ) ⋅ Qs ( m 3 / d ) ⋅ 0.008( kW / m 3 ) ⋅ 24( h / d ) = 4.8Qs ( kWh / d )

159

(10) The heat requirement includes heat loss of digesters and the heat necessary for raising the

160

incoming sludge temperature. To obtain the total heat demand of sludge digestion, an inlet ambient

161

sewage sludge temperature of 15 °C was assumed. Before feeding, the sludge was pre-heated to

162

30 °C through heat convection with high-temperature sludge discharged from digestion tanks.28 The

163

heat requirement Hs (MJ) for raising the sludge temperature to 50 °C was calculated using Equation

164

[11]. The heat loss of digesters is usually 2–8% of the heat requirement of the sludge.29 Thus, the

165

total heat requirement Ht (MJ) of complete digestion of 1 m3 of sludge was obtained by Equation

166

[12] (with 8% heat loss assumed):

167

H s = ρ s Qs cs ⋅ ( t s − to )

(11)

168

H t =( 1 + 8%) H s = 90.4Qs ( MJ )

(12)

169

where cs is the specific heat capacity of sludge (96% moisture), Qs is the volume of sludge, and ts

170

and to represent the final and initial temperature, respectively, of the input sludge.

171

2.2.3 Energy recovery of CHP system 8

ACS Paragon Plus Environment

Page 9 of 29

Environmental Science & Technology

172

Energy production by anaerobic processes can be expressed in a simplified model as a function

173

of the mass of biodegradable organic compounds (Mana). Generally, the biogas yield of sludge and

174

its calorific value are assumed to be 0.35 m3/kg COD and 11 kWh/m3, respectively; thus, total

175

energy recovery Pt (kWh) is calculated as Equation [13].18,29-31 Fifty percent heat efficiency ηh and

176

40% electricity efficiency ηe of CHP are assumed, achieving an overall efficiency of 90%.32 All

177

design parameters of the NZE WWTP model are provided in Table S1.

178 179

180

181

182

183

Electricity recovery Pe (kWh) and heat recovery Ph (MJ) are calculated as Equation [14] and Equation [15], respectively:

Pt = 1000(kg / t )M ana × 0.35(m3 / kg ) ×11(kWh / m3 ) = 3850M ana (kWh)

Pe = ηe Pt

(13)

(14)

= 1540M ana (kWh) Ph = η h Pt × 3.6 ( MJ / kWh)

(15)

= 6930 M ana ( MJ )

2.2.4 NZE WWTP theoretical model

184

In the NZE WWTP model, energy self-sufficiency of 100% is assumed; thus, there exists an

185

energy balance between electricity production Pe and electricity consumption Eaera and Ediges

186

(Equation [16]). At the same time, the heat requirement should be provided by heat recovery, as

187

expressed by Inequality [17]. However, only some WWTPs can achieve 100% energy

188

self-sufficiency in reality. Anaerobic digestion was applied to recover energy based on the premise

189

that electricity production Pe and heat recovery Ph should meet the digestion energy consumption

190

(Ediges and Ht), presented as Inequality [17] and Inequality [18]. Inequality [18] always holds based

191

on Equations [16] (Eaera ≥0), therefore, Inequality [18] is not considered in this model. Equation [19]

192

and [20] can be obtained by substituting Equations [2], [9], [10], [12], [14], and [15] into Equations 9

ACS Paragon Plus Environment

Environmental Science & Technology

Page 10 of 29

193

[16] and [17].

194

Pe = Eaera + Ediges

(16)

195

Ph ≥ Ht

(17)

196

Pe ≥ Ediges

(18)

197

1456M ana = 120 M is + 1000 µ M o

(19)

198

2.37 M ana ≥ M is

(20)

199

Inequality [20] is an energy boundary condition in the NZE WWTP model. The minimum of

200

Mvs/Mts (organic matter content of sludge) was obtained by solving Inequality [20] with Equation [5]

201

and Equation [8], yielding minimum of 0.23 which was selected as a comprehensive energy

202

boundary condition of energy self-sufficiency in the NZE WWTP model. The Mvs/Mts is generally

203

less than 0.9;33 therefore, an inequality regarding Mvs/Mts can be obtained (Inequality [21]).

204

Ultimately, Equation [19] and Inequality [21] constituted the NZE theoretical model.

205

0.23 ≤ M vs / M ts ≤ 0.9

206

Furthermore, the observed yield of excess sewage sludge was defined as follows:

207

208

Yobs =

(21)

M vs CODt

(22)

Where CODt is the total of removed COD in the wastewater treatment, and a large value of

209

Yobs indicates that material and energy flow are directed towards anabolism, yielding sludge.

210

3. RESULTS AND DISCUSSION

211

3.1. Case studies

212

Twenty municipal WWTPs in China (Table S2) were investigated to validate the theoretical

213

model. Characteristic parameters of the WWTPs are shown in Table S3. In the matter-energy cycle, 10

ACS Paragon Plus Environment

Page 11 of 29

Environmental Science & Technology

214

metabolic energy allocation corresponds to COD flow (material flow), and the amount of removed

215

COD is a relatively constant value during wastewater treatment. It is clear that increasing the

216

amount of COD allocated to anabolism benefits energy recovery; however, the maximum benefit is

217

achieved when anabolism of COD utilizes 2/3 of the total COD in the bio-treatment process.15

218

Figure 2 shows the COD flow of anabolism and catabolism in twenty WWTPs. The CODana/CODt

219

ratios of municipal WWTPs were in the range of 0.34–0.66 (average = 0.53). Only 6 WWTPs had

220

CODana/CODt ratios greater than 0.6 (close to the extremum). These results indicate that there is

221

significant potential to reduce energy dissipation by improving the CODana/CODt ratios in the

222

WWTPs. To avoid energy waste, WWTPs need optimized strategies to regulate the COD flow

223

between catabolism and anabolism; the NZE model provided a theoretical basis to optimize such

224

allocation.

225

According to the NZE theoretical model, when given a specific value of Mvs/Mts, the

226

relationship between specific electricity consumption µ and the material allocation ratio Mana/Mo

227

(represented as the CODana/CODt ratio) may be obtained by substituting Eqs. [5] and [8] into Eq.

228

[19]. Consequently, in the range of 0.23 to 0.9, each specific Mvs/Mts corresponds to an NZE

229

equation for µ and the CODana/CODt ratio, which can be shown as a line in a graph. As the Mvs/Mts

230

ratio changed, a series of lines was drawn to show the relationship between µ and CODana/CODt

231

(Figure 3A). The frontiers of these curves are the lines obtained when Mvs/Mts was its most extreme

232

values of 0.23 and 0.9. Changing the value of CODana/CODt can alter the amount of recovered

233

energy so that it meets the energy needs of water treatment.

234

The Mvs/Mts ratio (organic matter content of the sludge) in the wastewater treatment process is

235

usually determined as 0.75, and the corresponding zero energy curve was drawn to analyze the 11

ACS Paragon Plus Environment

Environmental Science & Technology

236

sample points (Figure. 3B). Because the maximum of Mana/Mo is 2/3 (0.67), the maximum specific

237

energy consumption was calculated to be 0.95. The sample points were categorized as type A, type

238

B, or type C according to the probability of realizing NZE status (Table 1).

239

Type A cases were those in which specific energy consumption was greater than 0.95

240

irrespective of CODana regulation [region (a) in Figure 3B]; 100% energy self-sufficiency cannot be

241

achieved in these WWTPs, which require external energy. The low influent CODinf and high Ew led

242

to the phenomenon in these WWTPs (Table S3). However, biomass energy can be recovered by

243

anaerobic digestion with cogeneration to compensate for some external power consumption by

244

these WWPTs. The energy self-sufficiency rates of the municipal WWTPs of Degan, Xiyong,

245

Jijiang, Changshou, Jiuquhe, and Jiangdong are 94.6%, 79.4%, 73.9%, 93.8%, 87.4%, and 88.0%,

246

respectively.

247

Type B cases can currently achieve NZE status and export excess energy [region (b) in Figure

248

3B]. The energy self-sufficiency rates of the municipal WWTPs in Zhouping, Xiaojiahe, Wushan,

249

Chengebei, Dajiu, and Chayuan are 158.2%, 127.9%, 103.4%, 122.4%, 101.7%, and 112.9%,

250

respectively.

251

Type C cases are those in which municipal WWTPs can achieve 100% energy self-sufficiency

252

by adjusting CODana; these WWTPs must increase the value of CODana to increase sludge yield.

253

Ultimately, the energy produced by increasing sludge yield will offset the energy requirement of

254

wastewater treatment.

255

There are eight municipal WWTPs can achieve 100% energy self-sufficiency and six

256

municipal WWTPs can export excess energy in the investigated twenty municipal WWTPs in this

257

study. The theoretical model of NZE wastewater treatment was successfully employed to evaluate 12

ACS Paragon Plus Environment

Page 12 of 29

Page 13 of 29

Environmental Science & Technology

258

the feasibility and level of energy self-sufficiency in practical wastewater treatment cases, and the

259

mode can be utilized to conserve resources by biomass energy recycling during wastewater

260

treatment, allowing WWTPs to achieve environmental sustainability.

261

3.2. Statistical analysis of specific energy consumption and CODana/CODt in the NZE model

262

In order to assess the predicted values of µ and CODana/CODt calculated using the NZE

263

WWTP model using real-life examples, the µ (kWh/kgCOD) values of 53 municipal WWTPs

264

(Table S4) were subjected to normality tests and statistical description. The Shapiro-Wilk test

265

yielded the following results: w = 0.97118, p-value = 0.2268 (>α, 0.05). Therefore, the null

266

hypothesis can be accepted, indicating that specific energy consumption is distributed normally

267

(Figure 4a; mean = 0.69, standard deviation = 0.28).

268

According to the µ distribution, the sample size was enlarged to 1000 by generating random

269

numbers, which were substituted into the NZE WWTP model to obtain corresponding

270

CODana/CODt ratios. The obtained CODana/CODt ratios were filtered under the condition of 0