The Economics of Wastewater Treatment Decentralization: A Techno

Jul 2, 2018 - *Phone: +1-949-233-6446; e-mail: [email protected]. ... This paper explores the economic viability of adopting more sustainable ... The Sup...
1 downloads 0 Views 2MB Size
Subscriber access provided by - Access paid by the | UCSB Libraries

Energy and the Environment

The economics of wastewater treatment decentralization: A techno-economic evaluation Manel Garrido-Baserba, Sergi Vinardell, Maria Molinos-Senante, Diego Rosso, and M. Poch Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b01623 • Publication Date (Web): 02 Jul 2018 Downloaded from http://pubs.acs.org on July 3, 2018

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 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 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.

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

The economics of wastewater treatment decentralization: A techno-

2

economic evaluation

3

Manel Garrido-Baserba1,2*, Sergi Vinardell3, María Molinos-Senante4,5, Diego Rosso1,2, Manel

4

Poch3

5 6

1 Department of Civil & Environmental Engineering, University of California, Irvine, CA 92697-2175, U.S.A. (Email:[email protected])

7

2

Water-Energy Nexus Center, University of California, Irvine, CA 92697-2175, U.S.A.

8

3

LEQUiA, Institute of the Environment, University of Girona, E-17071, Girona, Spain

9 10

4 Department of Hydraulic and Environmental Engineering, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile.

11 12

5 Center for Sustainable Urban Development, CONICYT/FONDAP/15110020, Av. Vicuña Mackenna 4860, Santiago, Chile.

13

*Corresponding author (T: +1-949-233-6446), E-mail: [email protected])

14 15

ABSTRACT

16

The existing wastewater treatment infrastructure has not adequately established an efficient and

17

sustainable use of energy, water, and nutrients. A proposed scheme based on source separation

18

and water-efficient use is compared to the current wastewater management paradigm (one

19

largely based on activated sludge) using techno-economic terms. This paper explores the

20

economic viability of adopting more sustainable management alternatives and expands the

21

understanding of the economics of decentralization and source-separation. The feasibility of

22

three different potential types of source-separation (with different levels of decentralization) are

23

compared to the conventional centralized activated sludge process by using recognized

24

economic assessment methodologies together with widely accepted modeling tools. The

25

alternatives were evaluated for two common scenarios: new developments and retrofit due to

26

the aging of existing infrastructures. The results prove that source-separated alternatives can be

27

competitive options despite existing drawbacks (only when countable incomes are included),

28

while the hybrid approach resulted in the least cost-effective solution. A detailed techno-

29

economic evaluation of the costs of decentralization provides insight into the current constraints

30

concerning the paradigm shift and the cost of existing technologic inertia.

31 32

KEY WORDS: Activated sludge; Decentralization; Water management; Source-separation;

33

Nitrogen removal

34 1 ACS Paragon Plus Environment

Environmental Science & Technology

Page 2 of 29

35

1.

36

Economic and environmental sustainability are shifting the current paradigm in urban

37

wastewater management

38

worldwide water sanitation significantly after increased urbanization and industrialization by

39

providing safe effluent from wastewater. But this process is now being recognized as lacking

40

economic and environmental sustainability, especially with respect to the inefficient use of

41

energy, (recycled) water, and nutrients

42

and constructed on the basis of outdated views, requirements, conditions, and technologies of

43

decades ago 8. Greenhouse gas emissions from the AS process itself (e.g., N2O) and from

44

sources of energy production, lack of recovery of finite nutrients such as phosphorus (even

45

given current depletion rates), continually rising energy costs, resilience limitations, and the

46

need for cost-efficient technologies are among the forces that will drive cities to start building

47

the next generation of urban wastewater solutions 3. Furthermore, the construction of new

48

developments in cities will be a worldwide phenomenon in the coming decades 9. Similarly,

49

existing and aging infrastructures in wastewater treatment facilities will soon need to be

50

replaced.

51

In the pursuit of sustainable wastewater management options, source separation and

52

decentralization are slowly becoming realistic alternatives for these new and expected

53

developments10,11. Both wastewater treatment alternatives lead to several advantages:

54

Introduction

-

1–4

. The century-old activated sludge (AS) process improved

5–7

. Current wastewater infrastructures were designed

The organic carbon in a typical combined municipal wastewater represents a chemical

55

energy content of approximately 1.9 kWh/m3

56

potential to harvest this energy within the order of 0.6 to 0.9 kWh/m3 for the most

57

concentrated BW stream in households

. While anaerobic digestion has the

, the current AS process on the other hand

3

consumes about 0.3-0.7 kWh/ m of wastewater 15–17.

58 59

13

12–14

-

The current practice is to merge, dilute, and treat both gray wastewater (GW) and black

60

wastewater (BW) streams, which hinders the feasibility of nutrient recovery. Highly

61

concentrated BW streams can be treated separately to facilitate nutrient recovery

62

Similarly, using a minimal amount of water yields concentrated wastewater flows which

63

are more cost-effective for removing harmful micropollutants

64

valuable constituents 22.

65

-

20,21

18,19

.

or recovering

Recovering nitrogen could reduce the production of artificial fertilizers via the Haber-

66

Bosch process, which fixes nitrogen from the air but uses up to 2% of the world’s

67

energy 23 and represents 50% of the energy in European agriculture8.

2 ACS Paragon Plus Environment

Page 3 of 29

Environmental Science & Technology

68

-

The energy demand to run aeration blowers in the aeration-based AS process accounts

69

for more than 50-75% of the net power demand in wastewater treatment plants

70

(WWTPs) needed to meet the mandated amount of dissolved oxygen 24–26.

71

-

Source separation and decentralization could reduce the current increase in energy

72

demand (and concurrent carbon footprint) caused by the implementation of new

73

technologies that achieve higher effluent quality at the expense of higher energy

74

demand

75

avoiding energy-demanding AS processes and transport 28–30

76

-

27

by producing renewable energy in useful forms (heat, methane) and by

The current trend in clean decentralized energy (i.e., biogas, solar, wind) offers new

77

possibilities of decentralized wastewater treatment, making new water reuse systems

78

scalable, off-grid, and without the need for the transport of fossil fuels 31–36.

79

-

Vacuum toilets, as a way of source separation, can reduce BW water consumption by 90% to 35 liters per person/day 10,37–40., and the overall consumption by about 25%41.

80 81

-

Increase the ability of urban wastewater systems to adapt as a response to change 42 and enhance climate-resilient infrastructures 6,43–46.

82 83

Despite new available knowledge, expertise, and technologies to develop more economically

84

and

environmentally

sustainable 47–50

water

resource

management

alternatives,

practical

85

implementation remains slow

86

and hardly any resources were allocated to their development because they are still considered

87

immature and risky by most wastewater professionals

88

that developing alternative cost-efficient wastewater management systems is an issue of

89

governance rather than technology

90

consumers (>64%) express highly favorable views of new systems combining elements of

91

source separation, local treatment, and reduced water use 54,55. A lack of evidence pertaining to

92

the economic viability of these alternatives hinders their consideration as feasible and credible

93

options. Therefore, we aimed to present a clear and simple approach to the economics of source

94

separation and decentralization to provide sound information that can support the decision-

95

making of (waste) water authorities.

96

One of the first comparison studies to date stated that depending on the scenario, source

97

separated systems are more cost effective than conventional systems

98

instead estimated that the overall costs of source-separated approaches could be about twice the

99

conventional system

57

. Source separating technologies are considered “low-tech,”

50,52,53

10,50,51

. Similarly, some authors suggest

. Furthermore, recent studies have highlighted that

56

. Other authors have

. However, none of these studies used standardized and recognized

100

methods of cost assessment (i.e., licensed software), and their calculations were mainly based

101

on their own experience and references. Including the potential incomes from source separation

102

is also essential to evaluate decentralized alternatives 58.

3 ACS Paragon Plus Environment

Environmental Science & Technology

103

The aim of this paper is to expand the understanding of the economics of decentralization and

104

source-separation by using standardized approaches for economic projections and evaluations in

105

wastewater systems. The feasibility of three different potential types of source-separated

106

systems were compared with the AS process using commercially available modeling software

107

(i.e., CapdetWorks). The main novelty of this study is to provide a comprehensive comparison

108

and assessment of wastewater treatment alternatives, including the following: 1) A methodology

109

based on reliable cost-estimation software (i.e., CapdetWorks; Hydromantis, Inc.) and state-of-

110

the-art literature for estimating construction and operation costs; 2) Consideration of only

111

existing technologies (not requiring further innovation before their deployment) that have been

112

accepted as feasible alternatives among wastewater experts; 3) Inclusion of the potential income

113

produced by source-separated alternatives; 4) Implementation of the aforementioned analytical

114

analysis in two of the most common scenarios in developing and developed societies: New

115

wastewater treatment developments and the aging of existing infrastructures (retrofit),

116

respectively. Therefore, a detailed and integrated economic analysis including the sewer system,

117

existing and realistic alternative wastewater treatments, and resource efficiency is presented.

118

2.

119

2.1 Wastewater treatment alternatives

120

Two main scenarios were considered for the economic assessment of the selected wastewater

121

management alternatives: new developments, and the retrofit of an existing WWTP. Both

122

scenarios are evaluated for a medium-sized population of 30,000 population equivalent (PE).

123

The population size was selected to represent an average, intermediate city scenario in which

124

activated sludge-related configurations would typically be the preferred option to implement.

125

Moreover, the installation of anaerobic digesters (AD) is often discouraged in centralized

126

communities smaller than 40,000 PE due to economic and technical reasons. Phosphorus re-

127

solubilization during the hydrolysis step in AD drives highly concentrated phosphorus flows

128

back to the main stream, leading to recirculation instead of actual P-removal, plus increased

129

piping blockage by spontaneous struvite precipitation

130

without anaerobic digestion may facilitate the comparison with source-separated alternatives

131

(Anaerobic-based). Note that larger treatment plants would benefit from the use of side-stream

132

AD (specifically by using co-digestion strategies) and operational savings should be included in

133

the case of a techno-economic analysis.

Methodology

5959–62

4 ACS Paragon Plus Environment

. The consideration of an AS

Page 4 of 29

Page 5 of 29

Environmental Science & Technology

134 135 136 137

Figure 1. Flow diagrams for three wastewater treatment alternatives. a) Fully centralized alternative using activated sludge; b) Hybrid alternative following a centralized approach for gray water and a decentralized approach for black water; c) Fully decentralized approach for both black and gray water streams. Alternatives B and C can have two differentiated treatments for the liquid effluent from the anaerobic processes unit (Figure 2).

5

ACS Paragon Plus Environment

Environmental Science & Technology

138 139 140

Figure 2. Flow diagram alternatives for the liquid effluent from the anaerobic processes unit. Option 1) Nitrogen recovery by a stripping-absorption system; Option 2) Nitrogen removal by the Oland process.

141

6

ACS Paragon Plus Environment

Page 6 of 29

Page 7 of 29

Environmental Science & Technology

142

For each scenario, three wastewater management alternatives or flow diagrams are applied: i)

143

centralized (alternative A); ii) hybrid (alternative B1 and B2) and; iii) decentralized (alternative

144

C) (Figures 1 and 2; See S12 for a detailed description in supporting information). The

145

centralized alternative consisted of a typical AS process. Alternative B (hybrid) is characterized

146

by a partially centralized scheme, while alternative C represents a fully decentralized scheme.

147

For both source-separated alternatives (B and C), the treatment of nitrogen was evaluated

148

considering two different technologies. One approach was based on the physical-chemical

149

recovery of the nitrogen (i.e., stripping-absorption process, alternative B1), while the other was

150

based on the biological removal of nitrogen (i.e. ,Oland/anammox process, alternative B2 and

151

C). For the sake of simplicity, the results of the two nitrogen treatment alternatives were shown

152

only for alternative B. For the decentralized alternative (C), only the output of the most cost-

153

effective alternative, i.e., nitrogen removal by the Oland process, is presented in this paper. A

154

detailed description of each alternative is provided as supplemental material.

155

2.2 Influent composition

156

Table 1 shows the typical values for an urban influent in a centralized WWTP, which

157

traditionally combines BW and GW10. The wastewater composition shown in table 1 was

158

selected as the influent for all the wastewater alternatives evaluated. In the source-separation

159

cases, the two different streams were split accordingly, as shown in Table 1. Following current

160

practices, BW is expected to be collected with vacuum toilets, which means a consumption of

161

water per people equivalent (PE) of 5 L/PE/day 63. As for GW, its water consumption is about

162

108 L/PE/day 10,64.

163

Table 1. Typical pollutant concentrations for the three main influents (i.e., combined, gray and

164

black water) 64–69. Combined

Source-separated streams

mg/L

mg/L (GW)

mg/L (BW)

TSS - Total Suspended Solids

410.8

175.9

8,360

COD - Chemical Oxygen Demand

701.3

472.2

10,560

BOD - Biological Oxygen Demand

248.6

175.9

3,560

TKN - Total Kjeldahl Nitrogen

86.1

2.3

2,500

NH4+_N - Ammonium-N

6.1

2.3

132.4

TP - Total Phosphorus

13.7

4.6

306.0

4.3

5.9

-

-

N-NO3 - Nitrate

165 166

Conventional toilets were used in the sewer combined alternative (alternative A, combining and

167

not discerning between BW and GW), which means a consumption of about 40 L/p/day10.

7 ACS Paragon Plus Environment

Environmental Science & Technology

Page 8 of 29

168 169

2.3 Sewer infrastructure

170

For comparative and standardization purposes the sewer distribution of the new development

171

(Scenario 1) and retrofit (Scenario 2) scenarios were adapted from Roefs et al. (2016). Each

172

development consisted of a series of districts (See figure S4, supporting information) servicing

173

1200 PE. Each district was distributed in neighborhoods of 50 households representing a total of

174

120 PE. Each neighborhood had a surface area of 2.5 ha.

175

In both the conventional and hybrid alternatives, districts were connected to a collection system

176

that connected district to district and to the central WWTP. A backbone pipe was used as

177

connection between neighborhoods within the district (Fig. S4). At the neighborhood level, both

178

private and public sewers were taken into account. Private sewers were defined as the sewers

179

from the house to the first Y-joint that makes connection with the water main

180

description of sewer infrastructure is shown in the supplemental information.

56,70

. A detailed

181 182

2.4 Model domain.

183

Influent variability. This study assumes low variability in the influent concentrations for the

184

source-separated options as the main uncertainty contributors are avoided: Industrial effluents

185

(traditionally representing 15-30% of flow composition), storm water episodes, sewage

186

characteristics (i.e., combined and separated), uncontrolled infiltrations or additions, public

187

spaces effluents (i.e., swimming pools, malls), etc. It is assumed that each person would produce

188

a similar waste water composition (Table1) from the daily theoretical 1.2-1.5l of urine and feces

189

71,72

190

further studies should be carried out to determine the exact impact of non-average

191

concentrations.

192

On the other hand, hybrid and centralized alternatives will inevitably be sensitive to load

193

fluctuations that cannot be predicted with the present methodology. The reduction of uncertainty

194

could be reduced further by studies incorporating wastewater dynamics and site-specific

195

fluctuation.

196

Sewer System. This study was based on a density of about 20 household/hectare, which is a

197

common value for European residential areas with free standing or double houses

198

studies that explore the variability depending on other common population densities would be

199

required, as there can be lower densities (i.e., US and Canada) or higher densities (i.e., highly

200

populated cities). The model used in this study was developed by Maurer et al. (2013), and may

. Nevertheless, the authors acknowledge the potential influence of influent variability and

8 ACS Paragon Plus Environment

73

. Further

Page 9 of 29

Environmental Science & Technology

201

yield significantly different results, especially if some pipe layouts needed to be arranged

202

differently. Furthermore, the impact of assuming different distances to the central plant

203

(currently 3km) may result in close to negligible results for distances shorter than 30-40km

204

(following the aforementioned methodology). The methodology considers straight long-distance

205

pipes (Main column or artery, HDPE) with significantly lower costs (