Lifecycle comparison of mainstream anaerobic baffled reactor and

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Environmental Modeling

Lifecycle comparison of mainstream anaerobic baffled reactor and conventional activated sludge systems for domestic wastewater treatment Andrew Ross Pfluger, Jennie Callahan, Jennifer R. Stokes-Draut, Dotti Ramey, Sonja Gagen, Linda A. Figueroa, and Junko Munakata Marr Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b06684 • Publication Date (Web): 21 Aug 2018 Downloaded from http://pubs.acs.org on August 24, 2018

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Environmental Science & Technology

Title: Lifecycle comparison of mainstream anaerobic baffled reactor and conventional activated sludge systems for domestic wastewater treatment

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Authors: Andrew R. Pfluger1,2*, Jennie L. Callahan1,2, Jennifer Stokes-Draut1,3, Dotti F. Ramey1,2, Sonja Gagen1,4, Linda A. Figueroa1,2, Junko Munakata-Marr1,2

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NSF Engineering Research Center ReNUWIt, Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO 80401 2 Department of Civil and Environmental Engineering, Colorado School of Mines, Golden, CO 80401 3 Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720 4 Department of Civil and Environmental Engineering, Clarkson University, Potsdam, NY 13699

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*Corresponding Author ([email protected])

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Keywords: Lifecycle assessment, domestic wastewater treatment, anaerobic wastewater treatment

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TOC Art

Wastewater Treatment System Design Wastewater Treatment

LCA Methodology Goal & Scope Definition

Biosolids Treatment

Inventory Analysis

Impact Assessment

Anaerobic Primary Treatment Anaerobic Digestion

Interpretation

Impacts Conventional Activated Sludge

Aerobic Digestion

Combined Heat & Power Environmental

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Abstract

ACS Paragon Plus Environment

Net Energy Balance


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The objective of this study was to evaluate the lifecycle impacts of anaerobic primary treatment

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of domestic wastewater using anaerobic baffled reactors (ABRs) coupled with aerobic

28

secondary treatment relative to conventional wastewater and sludge/biosolids treatment

29

systems through the application of wastewater treatment modeling and three lifecycle-based

30

analyses: environmental lifecycle assessment, net energy balance, and lifecycle costing. Data

31

from two pilot-scale ABRs operated under ambient wastewater temperatures were used to

32

model the anaerobic primary treatment process. To address uncertain parameters in the scale-

33

up of pilot-scale anaerobic reactor data, uncertainty analysis and Monte Carlo simulation were

34

employed. This study demonstrates that anaerobic primary treatment of domestic wastewater

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using ABRs can be incorporated with existing aerobic treatment strategies to reduce aeration

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demand, reduce sludge production, and increase energy generation. The net result of coupling

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anaerobic primary treatment with aerobic secondary treatment is a more favorable net energy

38

balance, reduced environmental impacts in most examined categories, and lower lifecycle costs

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relative to conventional treatment configurations; however, the removal and/or capture of

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dissolved methane is required to reduce global warming impacts and increase on-site energy

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generation. With further study, anaerobic primary treatment can be a path forward for energy-

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positive wastewater treatment.

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1. Introduction

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Current

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technologies, e.g., conventional activated sludge.1,2 Municipal wastewater treatment accounts

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for approximately 3% of U.S. electricity consumption;3,4 the percentage of electricity used for

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wastewater treatment in other countries can be up to 5%.5,6 Electricity costs can range from 25

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to 40% of total wastewater treatment facility (WWTF) costs with aeration equipment typically

49

accounting for approximately half of all electricity use.7–9 WWTFs, however, have the potential to

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be energy-positive, i.e., generate more energy than they use, by recovering methane-rich

wastewater

treatment

practices

are dominated


by

energy-intensive

aerobic

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biogas

using

technologies

that

take

advantage

of

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microorganisms.3,10 Multiple-compartment anaerobic reactor configurations, such as the

53

anaerobic baffled reactor (ABR), are promising energy-generating wastewater treatment

54

technologies.11,12 Shoener et al. (2014) showed that the ABR was capable of recovering more

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energy (kJ per g chemical oxygen demand (COD) removed) than other available anaerobic

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technologies, such as the anaerobic membrane bioreactor (AnMBR) or the single-compartment

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upflow anaerobic sludge blanket (UASB).13

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In addition to energy-generation from methane production, other potential advantages of the

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ABR for domestic wastewater treatment include minimal sludge production, reduced biosolids

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treatment requirements, lower operations and maintenance costs, decreased energy

61

requirements, and reduced WWTF physical footprint requirements.14 Despite the potential of

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multiple-compartment anaerobic reactors, ABRs operated at pilot- or full-scale under colder

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water temperatures (< 25°C) have not been shown to consistently achieve effluent discharge

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standards in developed nations, such as the U.S. Environmental Protection Agency’s

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Secondary Standards for biochemical oxygen demand (BOD) and total suspended solids (TSS)

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(30-day average of 30 mg L-1), a result achieved by several more energy-intensive mainstream

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anaerobic technologies, such as the AnMBR or the staged anaerobic fluidized membrane

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bioreactor (SAF-MBR).12,13,15,16 Dissolved methane (dCH4) in the anaerobic reactor effluent is an

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additional consideration, as uncontrolled methane volatilization to the atmosphere represents

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both lost energy and potent greenhouse gas emissions.17 Further, the performance of ABRs

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varies substantially with temperature, as lower BOD and TSS removal efficiencies and

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decreased methane generation are observed at lower water temperatures (i.e., < 15°C).12,16

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Therefore, ABRs, at present, are considered enhanced primary treatment for domestic

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wastewater and need to be coupled with a secondary treatment technology, such as activated

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sludge, to meet discharge standards.18,19


the

metabolism

of

anaerobic


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To date, few studies have examined ABRs treating domestic wastewater from a systems

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perspective. Sillis et al. (2016) compared the lifecycle impacts of an ABR treating synthetic

78

wastewater to that of an aerobic trickling filter; however, data used for the analysis were derived

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from one bench-scale ABR operated under controlled temperatures for < 100 days,20 whereas

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this study uses data from two pilot-scale systems treating raw wastewater operated for two

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years. As anaerobic wastewater treatment gains increasing attention, promising energy-

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generating technologies, such as the ABR for mainstream domestic wastewater treatment,

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require lifecycle-based analyses to evaluate their efficacy, benefits, and drawbacks compared to

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more conventional wastewater treatment systems. The objective of this study was to evaluate

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the efficacy of mainstream anaerobic primary treatment of domestic wastewater relative to

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conventional treatment configurations that include processes such as primary clarification,

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conventional activated sludge, anaerobic digestion, aerobic digestion, and combined heat and

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power using three lifecycle-based analyses: environmental lifecycle assessment (LCA), net

89

energy balance, and lifecycle costing (LCC).

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2. Materials and Methods

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2.1. System Boundaries and Functional Unit

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To examine the efficacy of mainstream anaerobic treatment of domestic wastewater, this study

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compares a theoretical wastewater treatment scenario including anaerobic primary treatment

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(AnP) coupled with aerobic secondary treatment (AeS) to a typical wastewater treatment

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scenario consisting of primary clarification coupled with conventional activated sludge (CAS).

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For each scenario, two biosolids treatment approaches were examined: anaerobic digestion

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(AnD) and aerobic digestion (AeD). In total, four complete treatment configurations were

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examined using wastewater and biosolids treatment process modeling: (1) CAS with AnD of

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primary sludge and waste activated sludge (WAS) (CAS/AnD), (2) CAS with AnD of primary

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settled solids and AeD of WAS (CAS/AnD+AeD), (3) AnP with AeS and AnD of WAS


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(AnP/AeS+AnD), and (4) AnP with AeS and AeD of WAS (AnP/AeS+AeD). Figure 1 depicts the

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processes examined and the system boundaries for the CAS/AnD and AnP/AeS+AnD

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configurations. Supporting Information Figure 1 (S1) depicts the processes examined and

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system boundaries for the CAS/AnD+AeD and AnP/AeS+AeD configurations.

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A

Aeration

DWW

Primary Clarifier

CAS/AnD

Electricity

Secondary Clarifier

Activated Sludge

Downstream Impacts

Effluent

RAS (50%) WAS (50%)

Electricity

Legend CHP

Biogas

Heat to Buildings

Heat

Process Input

Filtrate

Sludge Thickening

Anaerobic Digestion

Biosolids Dewatering

Electricity

Electricity

Output

N, P Recovery Credit

Biosolids Storage

Hauling

Electricity

Fuel

Land Application

Downstream Impacts

Polymer Electricity

Polymer

Centrate

106 B

AnP/AeS+AnD Electricity Biogas

CHP

Heat to Buildings

Heat

Anaerobic Primary

DWW

Aerobic Secondary Electricity

Legend Process Input

Sludge Thickening

Secondary Clarifier WAS (50%)

Aeration

Anaerobic Digestion

Downstream Impacts

Effluent

N, P Recovery Credit

Biogas

Filtrate Electricity

Output

RAS

(50%)

Electricity

Polymer

Biosolids Dewatering Electricity

Polymer

Biosolids Storage

Hauling

Electricity

Fuel

Land Application

Downstream Impacts

Centrate

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Figure 1. (A) processes and system boundary for CAS/AnD; (B) processes and system boundary for AnP/AeS+AnD.

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CAS/AnD was selected as a baseline for comparison because it is among the “most common”

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process configurations for wastewater treatment,2 and allows comparison of biogas generating

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technologies, i.e., AnD and AnP. AeD was included as a comparison WAS treatment technology

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as some WWTFs treating low wastewater flows (e.g., 5 million gallons per day (MGD), 18950

115

m3d-1, or less), opt for aerobic sludge digestion.2 While AeD precludes energy recovery, its use

116

can be advantageous. As 20-35% of WAS is not biodegradable, diverting WAS to AeD and

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employing dedicated AnD for primary sludge treatment can result in a smaller overall footprint

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and more stable AnD operation.21

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To provide reasonably equivalent wastewater treatment performance for comparison, CAS

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reactors were designed to remove BOD only. The following processes were also modeled in

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each treatment configuration: secondary clarification, gravity belt thickening, centrifuge

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dewatering, biogas treatment (moisture, siloxane, and H2S removal), CHP using a microturbine,

123

on-site biosolids storage, and biosolids hauling and land application. As neither activated sludge

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systems designed for BOD removal only nor pilot-scale ABRs remove nitrogen or

125

phosphorous,21,22 each configuration was assumed to have the same follow-on nutrient removal

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requirements: advanced nutrient removal was not considered. Preliminary treatment prior to

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primary clarification and disinfection of effluent wastewater were also considered common to

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each configuration and were excluded from analysis. The following processes were considered

129

outside of the system boundary and not analyzed: environmental impacts of effluent wastewater

130

of the WWTF beyond immediate discharge to the environment, environmental impacts of

131

biosolids beyond initial land application, and excess heat produced by combined heat and

132

power (CHP) beyond AnD heating requirements (i.e., to maintain a 35°C digester temperature).

133

Excess heat was assumed routed outside of the system boundary to on-site buildings or

134

wasted. Finding uses for excess heat can be challenging for facilities intentionally sited away

135

from population centers. SI Section 1 provides a complete list of general assumptions and a


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more complete explanation of system boundary considerations. The functional unit is treatment

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of one m3 of domestic wastewater to a minimum of U.S. EPA secondary effluent standards. The

138

reference flowrate for WWTF design is 5 MGD (18950 m3d-1). WWTFs treating flowrates 5 MGD

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or greater have been suggested as the appropriate size for economically feasible CHP.23

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2.2. System Design for Life Cycle Inventory and Modeling

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Influent domestic wastewater was modeled on wastewater characteristics at the Mines Park

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Wastewater Test Bed at the Colorado School of Mines in Golden, CO (Table S1). The observed

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wastewater strength was considered medium-high strength relative to previously described

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wastewater characteristics.21 SI Section 1 provides a description of each process, including

145

assumptions used for modeling. Of note, a synthetic fertilizer offset credit for land application of

146

biosolids (i.e., nitrogen and phosphorus addition) was included in the environmental impact

147

analysis.24,25 Modeled polymer and chemical additions included: (1) acrylonitrile polymer

148

addition to sludge thickening and dewatering processes (5 g polymer per kg of sludge),25 and

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(2) NaOH addition to AeD processes (325-gal d-1).2 Two additional parameters expected to

150

affect the treatment model were varied, creating 12 total treatment scenarios (Figure S2). First,

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as biological activity and bioreactor performance are affected by wastewater temperature, two

152

conditions were examined: 15 and 25°C. Second, for AnP/AeS+AnD and AnP/AeS+AeD

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scenarios, dCH4 produced in AnP was modeled as either (1) uncontrolled volatilization to the

154

atmosphere, or (2) increased COD loading and managed removal via microbial activity in the

155

AeS bioreactor with no volatilization. Estimates of methane losses in anaerobic bioreactor

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effluent vary widely, from 11% to 100%.26,27

157

The general steps in ISO 14040 (Environmental management – Life cycle assessment –

158

Principles and framework) were used to compare environmental attributes of each

159

configuration.28,29 BioWin Version 5.2 supplemented by spreadsheet analysis was used to

160

model performance of each treatment configuration as described in SI Section 1. AnP was



161

modeled using real-world performance data from two pilot-scale multiple-compartment reactors

162

operating under ambient conditions in Colorado treating 800 and 1,728 liters d-1. Uncertainty in

163

the scale-up of performance from pilot-scale to full-scale was addressed using uncertainty and

164

sensitivity analyses described in Section 2.3 and SI Section 2. A solids balance was used to

165

validate model accuracy. Table S3 provides representative solids balances for each

166

configuration. SimaPro Version 8.0.3.14 was used to identify environmental impacts using

167

TRACI 2.1 V1.00 as described in SI Section 3 for the following impact categories (reference unit

168

shown in parentheses): acidification (kg SO2-eq), carcinogens (CTUh), ecotoxicity (CTUe),

169

eutrophication (kg N-eq), fossil fuel depletion (MJ surplus), global warming (kg CO2-eq), non-

170

carcinogens (CTUh), ozone depletion (kg CFC-11-eq), respiratory effects (kg PM2.5-eq), and

171

smog (kg O3-eq). SimaPro’s U.S. electricity mix was used for analysis. Gaseous emissions

172

(e.g., CH4 or N2O) appended to SimaPro for each relevant process are described in SI Section

173

1.

174

The net energy balance (NEB) was calculated as the sum of WWTF electricity demands minus

175

electricity produced (electricity generated by CHP and the heat generated by CHP used to offset

176

electricity required to heat anaerobic digesters to 35°C). Heat produced above and beyond

177

digester heating requirements was not accounted for in the NEB. Values used to calculate the

178

NEB were derived from BioWin modeling results or other sources as described in Table S5. For

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CHP, a microturbine (29% efficiency) for electricity and heat production was used for all

180

scenarios. NEB modeling is described further in SI Section 4.

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CAPDETWorks Version 2.5 (Hydromantis, Inc.) was used to model both capital and annual

182

costs. Costs appended to CAPDETWorks are provided in Table S7. As multiple compartment

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reactor systems are not available in CAPDETWorks, AnP was modeled as three UASBs in

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series. A treatment facility life of 40 years was assumed for all configurations.25 Costs derived

185

from CAPDETWorks were adjusted from 2007 dollars to 2016 dollars using annual inflation


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rates (Table S8).30 Net present value was calculated using discount rates of 5%, 8%, and

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10%.25 LCC modeling is further described in SI Section 5.

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2.3. Uncertainty and Sensitivity Analyses

189

Due to uncertainty in the performance of AnP at full-scale, two years of pilot-scale data for

190

organic material (i.e., COD) removal, TSS removal, and methane generation (gaseous and

191

dissolved) from two ABRs were subjected to predictive analysis using Oracle’s Crystal Ball

192

Predictor. Performance data from both reactor systems were analyzed over 60 forecast periods

193

to determine probable minimum (i.e., low-end), mean, and maximum (i.e., high-end) values for

194

each temperature condition (15 and 25°C) (Table S2.A). Identified values were then linearly

195

scaled to full-scale, i.e., to performance of a WWTF treating 5 MGD, and propagated through

196

the entire analysis, to include performance modeling in Biowin, LCA in SimaPro, and LCC in

197

CAPDETWorks (Figure S4). AnP/AeS+AnD and AnP/AeS+AeD scenarios, therefore, were

198

defined as a triangular distribution using low-end, mean, and high-end estimates in the

199

uncertainty analysis. When only two data points were available, i.e., a low-end and high-end

200

value, a uniform distribution was assumed. For example, energy use for AnP was assumed to

201

vary from 0 kWh m-3 as a low-end value (i.e., no energy was used because AnP treatment was

202

located within the hydraulic gradient of the facility and no pumping was required) to 0.011 kWh

203

m-3 as a high-end value (i.e., the same energy required for pumping in traditional primary

204

clarification). When one value was available, a normal distribution (i.e., mean value with

205

standard deviation ± 20%) was assumed. Table S4 lists uncertainty parameters used in this

206

analysis with example values from the AnP/AeS+AnD (25°C, dCH4 volatilization) scenario. The

207

impacts of data uncertainty were evaluated using Monte Carlo analysis (50,000 simulations) in

208

Oracle Crystal Ball Release 11.1.2.4.850.

209

A sensitivity analysis was performed on uncertain parameters in Oracle Crystal Ball, which

210

produced ranked correlation coefficients between each assumed parameter and every forecast



211

in the Monte Carlo analysis. The value of the correlation coefficient was then used to determine

212

sensitivity by ranking parameters from the most important, or most sensitive (i.e., maximum

213

absolute value of the correlation coefficients), to the least important, or least sensitive (i.e., the

214

minimum absolute value of the correlation coefficients). All values presented are median values;

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10th and 90th percentile results are also noted in parentheses where appropriate. Error bars

216

depicted in each figure represent the 10th and 90th percentile results.

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3. Results and Discussion

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3.1. NEB for configurations with AnP are lower than conventional systems

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Figure 2 depicts energy use, energy generation, and the NEB for each treatment scenario.

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AnP/AeS+AnD and AnP/AeS+AeD scenarios had a lower NEB relative to their CAS/AnD

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baseline scenarios. Specifically, AnP/AeS+AnD scenarios used only 34 to 53% of the energy

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required to operate CAS/AnD, while AnP/AeS+AeD scenarios used 46 to 66% of the energy

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required for CAS/AnD. AnP/AeS+AnD scenarios, which included biogas generation from both

225

AnP and AnD, had the lowest NEBs relative to all other comparative scenarios within different

226

configurations. For example, at 25˚C, AnP/AeS+AnD (dCH4 volatilization) had a median NEB of

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0.10 kWh m-3 (10th percentile = 0.09; 90th percentile = 0.13), while the median NEB for

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AnP/AeS+AeD (dCH4 volatilization), CAS/AnD, and CAS/AnD+AeD were 0.12 kWh m-3 (0.10;

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0.14), 0.24 kWh m-3 (0.19; 0.31), and 0.30 kWh m-3 (0.24; 0.35), respectively. The lower NEB for

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scenarios with AnP can be primarily attributed to two factors. First, CAS/AnD and

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CAS/AnD+AeD scenarios produced more sludge in the CAS process than AnP/AeS+AnD and

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AnP/AeS+AeD scenarios produced in the AeS process. While the increased volume of WAS

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from CAS produced more biogas in AnD, the resulting increase in energy production from CHP

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was negated by the requirement for additional energy to process sludge and biosolids.


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Second, AnP/AeS+AnD and AnP/AeS+AeD scenarios produced more biogas than CAS/AnD

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and CAS/AnD+AeD scenarios. For example, CAS/AnD (25°C) offset 35% of electricity

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requirements via CHP, while the scenario with the lowest NEB (AnP/AeS+AnD, 15°C, dCH4

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volatilization) offset 71% of electricity requirements. For AnP/AeS+AnD scenarios, between

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63% and 85% of biogas captured and used for electricity and heat generation was produced by

240

AnP, not AnD of primary sludge and WAS. Modeled AnP removed 45% (15°C, low-end value

241

from Crystal Ball Predictor) to 81% (25°C, high-end value) of influent organic carbon (as COD),

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which was directly converted to methane gas via anaerobic microbial activity (i.e. hydrolysis,

243

acetogenesis, and methanogenesis).31 Traditional primary clarification typically removes less

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COD, approximately 40%, in the influent wastewater as primary sludge.21 In conventional

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aerobic treatment (i.e., CAS/AnD), the remaining COD, approximately 60%, is converted to

246

biomass in energy-intensive activated sludge aeration basins before eventually being removed

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as WAS. WAS is then usually combined with primary sludge and subjected to AnD to create

248

biogas. AnP, therefore, represents a redirection of carbon compared to conventional aerobic

249

treatment systems. This redirection decreases energy use, while enhancing methane production

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and energy-generating potential.

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Two temperature-related factors substantially affected the NEB and caused modeled energy

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use to be greater in 25°C scenarios than in 15°C scenarios. First, increased energy use in the

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activated sludge process was observed at 25°C due to decreased oxygen transfer efficiency at

254

the higher wastewater temperature. As indicated by sensitivity analysis (Tables S6.A-C), energy

255

use within the NEB was most sensitive, i.e., had the highest magnitude of correlation, to energy

256

demand for the aeration basin (i.e., CAS or AeS). Increases in energy use in the aeration basin

257

at 25°C, therefore, had the greatest negative impact on the NEB. Second, at lower wastewater

258

temperatures, anaerobic microbial metabolic activity decreases, thereby producing less

259

methane in the AnP process. Methane solubility also increases at lower wastewater



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temperatures, reducing biogas capture and energy/heat generating potential.26 Energy

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production within the NEB was most sensitive to electricity produced by CHP from methane

262

generation in either AnP (for configurations with anaerobic primary) or AnD (for configurations

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without anaerobic primary). For example, less biogas was produced in AnP at the lower

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wastewater temperature and more heat


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0.5

CAS/AnD

CAS/AnD +AeD

25◦C

25◦C

AnP/AeS+AnD

AnP/AeS+AeD

Energy Use & Generation (kWh m-3)

0.4

0.3

0.2

0.1

0

-0.1

-0.2

-0.3

15◦C

15◦C

25◦C

15◦C

dCH4 Volatilization

25◦C

15◦C

dCH4 Removal

25◦C

15◦C

25◦C

dCH4 Volatilization

15◦C

dCH4 Removal

Net Energy Balance (kWh m-3 )

0.4

0.3

0.2

0.1

0

Energy Use

Energy Generation - AnD

Energy Generation - AnP

Energy Offset from Heat Production

Potential Energy Recovery – dCH4

265



266 267 268 269 270 271 272 273

Figure 2. Energy use, energy generation, energy offset from heat generation, and NEB for each configuration in terms of electricity (kWh m-3). Electricity generated by CHP is differentiated by biogas source, i.e., electricity generated from AnP and AnD are depicted separately. NEB does not include potential energy generation from dissolved methane capture; however, potential energy recovery is depicted as a separate item on the energy use & generation portion of the figure. In each scenario, excess heat beyond digester heating requirements was generated, but was routed outside of the system boundary and not considered in the NEB. Median values are depicted. Error bars represent 10th and 90th percentiles from the uncertainty analysis (50,000 Monte Carlo simulations).

274 275

was required to maintain AnD at 35°C; therefore, more electricity generated off-site was

276

required, increasing the NEB.

277

3.2. Recovery of dCH4 is required to reduce global warming impacts and enhance energy

278

generation

279

Each AnP/AeS+AnD and AnP/AeS+AeD scenario was modeled with two variations regarding

280

dCH4 produced by anaerobic primary treatment: complete uncontrolled volatilization to the

281

atmosphere or complete removal in AeS treatment. Figure 3 shows the contribution of each

282

modeled process to global warming for each treatment scenario. AnP/AeS+AnD scenarios had

283

reduced global warming impacts relative to their comparative baseline CAS/AnD scenario when

284

dCH4 emissions were controlled. In this case, the use of electricity generated off-site was the

285

biggest contributor to global warming impacts for all scenarios, accounting for between 43% and

286

65% of emissions. Uncontrolled volatilization of dCH4 to the atmosphere significantly increased

287

global warming impacts for scenarios with AnP, accounting for between 87 and 93% of the total

288

global warming impacts for these scenarios. For example, AnP/AeS+AnD (25°C, dCH4

289

volatilization) scenario emitted > 690% more CO2-eq than did its comparative baseline scenario

290

(CAS/AnD, 25°C) and > 720% more CO2-eq than did the AnP/AeS+AnD (25°C, dCH4 removal)

291

scenario. Global warming impacts were greater for 15°C scenarios with AnP. Here, the

292

increased solubility of methane led to decreased gaseous methane capture in the AnP process

293

but increased uncontrolled volatilization to the atmosphere in the AeS process. For example, the


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AnP/AeS+AnD (15°C, dCH4 volatilization) scenario emitted > 1130% more CO2-eq than did its

295

comparative baseline scenario (CAS/AnD, 15°C) and approximately 1200% more CO2-eq than

296

the AnP/AeS+AnD (15°C, dCH4 removal) scenario.

297

The aforementioned values represent a “worst case” emissions scenario where all dCH4 is

298

assumed volatilized; however, direct emissions from AnP are likely to be lower due to some

299

level of dCH4 removal in follow-on aerobic treatment by methanotrophic bacteria.32 dCH4 can

300

also be used as a carbon source for heterotrophic denitrification in an anoxic-aerobic secondary

301

process.14,33 In this case, dCH4 would reduce the need for an exogenous electron donor and

302

carbon source that would otherwise be required for denitrification. Other means to biologically

303

remove dCH4 have been suggested, e.g., the downflow hanging sponge, which was observed to

304

recover between 57 and 88% of dCH4 from UASB effluents.34–37

305

Perhaps more advantageous than biogenic dCH4 removal, which limits emissions and reduces

306

global warming impacts but can increase sludge production, is dCH4 capture for energy

307

generation. In addition to the NEB, Figure 2 also depicts modeled dCH4 capture after conversion

308

to electricity via CHP. When complete dCH4 capture was modeled, no scenario in this study

309

achieved an energy-positive state, i.e., no scenario generated more energy and heat than were

310

required for facility operation; however, several approach energy neutrality. For example, in the

311

scenario with the lowest NEB, AnP/AeS+AnD (15°C, dCH4 volatilization), modeled electricity

312

production from dCH4 (assuming a median value of 0.02 kWh m-3) would decrease the NEB

313

from 0.06 kWh m-3 (71% energy offset) to 0.04 kWh m-3 (81% energy offset). In a hypothetical

314

scenario where complete capture of dCH4 was achieved and routed to CHP for additional

315

energy generation, a total of 0.04 kWh m-3 of electrical energy from dCH4 is possible for 15°C

316

scenarios. With complete dCH4 capture, the NEB for the AnP/AeS+AnD (15°C, dCH4



Page 16 of 30

3.00

2.50

Global warming (CO2-eq) m-3 wastewater treated

2.00

1.50

0.35

0.25

0.15

0.05

AnP/Aes+AeD (15◦C, dCH4 removal)

AnP/Aes+AeD (15◦C, dCH4 volat.)

AnP/Aes+AeD (25◦C, dCH4 removal)

AnP/Aes+AeD (25◦C, dCH4 volat.)

AnP/AeS+AnD (15◦C, dCH4 removal)

AnP/AeS+AnD (15◦C, dCH4 volat.)

AnP/AeS+AnD (25◦C, dCH4 removal)

AnP/AeS+AnD (25◦C, dCH4 volat.)

CAS/AnD+AeD (15◦C)

CAS/AnD+AeD (25◦C)

CAS/AnD (15◦C)

CAS/AnD (25◦C)

-0.05

Anaerobic Digester

Biosolids Storage

Fossil Fuel Emissions

Conventional Activated Sludge

Electricity Use

Combined Heat & Power

Land Application Nitrogen Credit

Sludge Dewatering

Sludge Thickening

Transportation

Aerobic Digestion

Dissolved Methane Volatilization

317


Page 17 of 30

318 319 320 321 322 323


Figure 3. Global warming impacts by process within each configuration (median values) in CO2-eq per m3 of wastewater treated. AnP/AeS+AnD and AnP/AeS+AeD scenarios that modeled uncontrolled volatilization of dCH4 rather than managed dCH4 removal in aerobic secondary units each have substantially increased global warming impacts; therefore, a broken y-axis is depicted so that all global warming impacts can be visualized. Assumptions concerning emissions that influenced modeled global warming impacts are found in SI Section 1.

324 325

volatilization) scenario would decrease to 0.02 kWh m-3 (91% energy offset). To date, however,

326

no economically or energetically viable dCH4 capture solution has been identified.25 For

327

example, several studies examining dCH4 capture using membrane degasification use more

328

energy than can theoretically be recovered.26,38–40

329 330

3.3. AnP/AeS+AnD configurations have lower impacts relative to conventional treatment

331

configurations in most impact categories

332

Environmental impacts for each scenario are depicted in Figure 4. AnP/AeS+AnD scenarios had

333

reduced environmental impacts relative to baseline CAS/AnD scenarios regardless of

334

temperature for most impact categories, to include acidification, fossil fuel depletion, ozone

335

depletion, respiratory effects, ecotoxicity, and smog. AnP/AeS+AeD scenarios had reduced

336

environmental impacts relative to CAS/AnD scenarios for fewer impact categories: acidification,

337

ozone depletion, respiratory effects, and ecotoxicity. CAS/AnD+AeD scenarios had increased

338

impacts relative to CAS/AnD scenarios in all impact categories, regardless of temperature.

339

Using acidification as an example, AnP/AeS+AnD scenarios produced between 17% and 37%

340

less kg SO2-eq than comparative CAS/AnD scenarios, whereas AnP/AeS+AeD scenarios

341

produced approximately 6% less kg SO2-eq, and CAS/AnD+AeD scenarios produced between

342

32% and 42% more kg SO2-eq than comparative CAS/AnD scenarios.

343

CAS/AnD scenarios had lower emissions of carcinogens and non-carcinogens relative to

344

AnP/AeS+AnD scenarios; however, no scenario had a substantial relative impact on human



345

health, i.e., the difference between emissions of each scenario was negligible. As CAS and AeS

346

processes were modeled for BOD removal only, no N and P was removed and substantial

347

eutrophication impacts from the effluent wastewater were observed for each scenario. When the

348

impacts of N and P in the effluent wastewater were removed and the impact of eutrophication

349

from other processes isolated, a negative value was observed for both CAS/AnD and

350

AnP/AeS+AnD scenarios due to the land application of biosolids and associated nitrogen (N)

351

and phosphorus (P) offset credits, which avoided impacts from synthetic fertilizer production.

352

For further comparison, Figures S5.A and S5.B show impacts for like each scenario normalized

353

to the baseline CAS/AnD scenario.

354

Emissions in each impact category were primarily influenced by off-site fossil fuel use for

355

electricity (e.g. coal, natural gas, crude oil, etc.). AnP/AeS+AnD scenarios relied less on off-site

356

electricity relative to CAS/AnD scenarios, which reduced environmental impacts in the

357

aforementioned impact categories. AnP/AeS+AeD scenarios also generated more on-site

358

electricity relative to baseline CAS/AnD scenarios; however, the use of NaOH for AeD increased

359

acidification, fossil fuel depletion, respiratory effects, non-carcinogens, and smog. The quantity

360

of sludge produced in each scenario also impacted the quantity of emissions. The increased

361

removal of COD in AnP relative to primary clarification reduced sludge generation in the AeS

362

process, thereby reducing downstream sludge/biosolids treatment requirements and reducing

363

environmental impacts relative to CAS/AnD scenarios. The land application of biosolids as

364

fertilizer provided a N and P recovery credit, reducing modeled emissions in several impact

365

categories, including carcinogens, non-carcinogens, ecotoxicity, and eutrophication. The

366

avoided use of synthetic fertilizer most benefited CAS/AnD scenarios, which produced the

367

greatest volume of sludge/biosolids. N and P recovery credit resulted in negative environmental

368

impacts for carcinogens in all scenarios, and for non-carcinogens in CAS/AnD scenarios. The

369

ability to land-


Page 18 of 30

Ozone Depletion (kg CFC-11-eq)

370 0.0E+00 5.0E-04

-2.0E-03

-4.0E-03

-6.0E-03

-8.0E-03

4.0E-10

0.0E+00

-4.0E-10

-8.0E-10

-1.2E-09 2.2E-01

1.6E-01

1.0E-01

Non-carcinogens (CTUh)

-7.0E-10

-1.0E-09

-1.3E-09

Ecotoxicity (CTUe)

2.5E-03

1.0E-04

7.5E-05

5.0E-05

Smog (kg O3-eq)

1.0E-03

Carcinogens (CTUh)

1.5E-03

Fossil Fuel Depletion (MJ Surplus)

Acidification (kg SO2-eq) 2.0E-03

Respiratory Effects (kg PM 2.5-eq)

Eutrophication (kg N-eq)

dCH4 volat.

4.0E-02 1.3E-04

2.5E-05


-4.0E-10 8.0E-02

-1.6E-09 2.8E-01

6.5E-02

5.0E-02

3.5E-02

1.2E-08

2.0E-02

8.0E-09

4.0E-09

0.0E+00

-4.0E-09 1.7E-02

1.4E-02

1.1E-02

8.0E-03

5.0E-03

dCH4 removal

25◦ C,

15◦C, dCH4 removal

15◦ C, dCH4 volat.


25◦C, dCH4 volat.


15◦C, dCH4 volat.

dCH4 volat.

25◦C,

15◦C

25◦C

15◦C

25◦C


15◦C, dCH4 volat.

25◦ C, dCH4 removal

25◦C, dCH4 volat.


15◦C, dCH4 volat.


25◦C,

15◦C

25◦C

15◦C

25◦ C

15◦ C, dCH4 removal

15◦ C, dCH4 volat.


25◦C, dCH4 volat.


15◦C, dCH4 volat.


25◦C, dCH4 volat.

15◦C

25◦C

15◦C

25◦C

S/ A nD

eD

nD +A nP ED +A eS /A nD A nP +A eS /A eD

A

nD S/ A

A S/ A C A

C

+A nP ED +A eS /A nD A nP +A eS /A eD

A

S/ A nD

nP +A eS /A

A

A

C

C

A

nD A nD +A nP ED +A eS /A nD

S/

S/ A A A

C

C A

Page 19 of 30 Environmental Science & Technology


371 372 373 374 375 376 377

Figure 4. Environmental impacts by configuration for each impact category examined minus global warming impacts, which are depicted in Figure 3. Units presented are normalized to m3 wastewater treated. Error bars represent 10th and 90th percentiles from uncertainty analysis (50,000 Monte Carlo simulations).

378

regulations.41

379

Regarding temperature, lower environmental impacts were observed in almost all impact

380

categories for 15°C scenarios relative to 25°C scenarios principally due to reduced fossil fuel

381

use. Exceptions include eutrophication and ozone depletion, which had greater impacts at 15°C.

382

Both impact categories were strongly impacted by nitrogen offset credits and synthetic fertilizer

383

use. As more sludge is produced at the higher wastewater temperature (25°C), more biosolids

384

were land applied; therefore, an increased nitrogen offset credit was modeled and lower relative

385

eutrophication and ozone depletion impacts were observed. An additional exception was

386

observed for global warming impacts in scenarios where complete dCH4 volatilization was

387

assumed. Here, increased solubility of CH4 at lower temperatures decreased gaseous CH4

388

available for energy generation, while simultaneously increasing dCH4 volatilization (see Section

389

3.2). Figures S5.C and S5.D compare impacts of temperature for scenarios of the CAS/AnD

390

configuration and the AnP/AeS+AnD configuration, respectively.

apply wastewater treatment biosolids, however, may be restricted in some areas based on local

391 392

3.4. Lifecycle costs of configurations with anaerobic primary treatment are lower than

393

conventional systems

394

Figure 5 depicts the net present value of each treatment configuration at an 8% discount rate;

395

net present value at 5% and 10% discount rates are also shown for comparison. Table S9.A

396

further lists values for net present value (5%, 8%, 10% discount rates), project capital costs, and

397

annual costs for each treatment scenario. In general, all AnP/AeS+AnD and AnP/AeS+AeD


Page 20 of 30

Page 21 of 30


398

scenarios had lower net present values than their comparative baseline CAS/AnD scenario,

399

while CAS/AnD+AeD scenarios were found to be costlier than their comparative CAS/AnD

400

scenarios. Variation between configurations was observed in both capital costs and annual

401 0.25 Operations & Maintenance Costs 5% Discount Rate

Net present value ($) m- 3 wastewater treated

Capital Construction Costs 10% Discount Rate

0.20

0.15

0.10

0.05

25◦C

15◦C

25◦C

15◦C

25◦C

15◦C

dCH4 Volatilization

402 403 404 405 406 407 408

CAS/AnD

CAS/AnD+AeD

25◦C

15◦C

dCH4 Removal

AnP/AeS+AnD

25◦C

15◦C

dCH4 Volatilization

25◦C

15◦C

dCH4 Removal

AnP/AeS+AeD

Figure 5. Lifecycle costing for each treatment configuration per m3 of wastewater treated. Net present value of each configuration at an 8% discount rate (2016 U.S. dollars) is depicted. Capital costs and annual operations and maintenance costs are displayed separately. Error bars represent 10th and 90th percentile from uncertainty analysis. For comparison, the diamond represents net present value at a 10% discount rate and the triangle represents net present value at a 5% discount rate.

409 410

costs. As each configuration treated the same flow of wastewater (5 MGD) and required

411

similarly sized physical structures, the variation in capital costs was primarily observed in the

412

sludge/biosolids treatment processes. As less sludge was produced in the AeS treatment



413

process than in the CAS process, lower sludge/biosolids equipment capital costs and a smaller

414

required treatment footprint was observed for AnP/AeS scenarios. For example, the modeled

415

primary and WAS sludge production for CAS/AnD (25°C) was 0.32 kg sludge m-3 wastewater

416

treated, while the modeled WAS production from the AeS unit in the AnP/AeS+AnD (25°C,

417

dCH4 volatilization) scenario was only 0.10 kg sludge m-3 wastewater treated (Table S3).

418

Correspondingly, the total capital costs for the AnP/AeS+AnD (25°C, dCH4 volatilization)

419

scenario were 17% less than the capital costs for the CAS/AnD (25°C) scenario with a 50%

420

reduction in capital costs for sludge/biosolids treatment processes. Table S10 shows median

421

capital cost of each process adjusted for inflation (25°C scenarios). Annual costs, which

422

includes operations, maintenance, material, chemicals, and energy, were lower for

423

AnP/AeS+AnD scenarios relative to baseline CAS/AnD scenarios by 27% to 55%. Reduced

424

annual costs were observed in each cost category with the greatest reductions in energy costs

425

and material costs. Table S9.B shows median annual costs for each treatment scenario.

426 427

3.5. AnP coupled with other anaerobic technologies may be a path forward for energy-

428

positive domestic wastewater treatment with further study

429

In the future, AnP using ABRs may be coupled with other anaerobic reactor technologies;

430

however, more study of reactor performance is required before full-scale implementation of a

431

potentially anaerobic energy-positive wastewater treatment scheme is achievable.1,13,25,42,43

432

Beyond wastewater treatment, creating resource recovery facilities that incorporate anaerobic

433

wastewater treatment with other anaerobic approaches for the digestion of organics, such as

434

the co-digestion of wastewater biosolids with food waste, can be a means for creating energy-

435

generating facilities.44 Some activated sludge WWTFs, such as the East Bay Municipal Utility

436

District in Oakland, CA, currently supplement AnD of wastewater biosolids with high-strength


Page 22 of 30

Page 23 of 30


437

organic wastes to achieve energy self-sufficiency,45,46 and can provide a starting point for the

438

design of a full-scale completely anaerobic resource recovery facility.

439

Despite potential advantages in energy generation, environmental impacts, and cost reduction,

440

undesirable anaerobic effluent constituents must be addressed before mainstream anaerobic

441

primary treatment schemes can achieve widespread implementation. In addition to dCH4,

442

anaerobic effluents also contain residual organic carbon, nutrients (i.e., ammonia and

443

phosphorus) that exceed discharge standards, hydrogen sulfide, and volatile fatty acids.47

444

Energy-efficient follow-on treatment technologies that remove one or more of these

445

contaminants need to be explored. Nitrogen is typically removed in energy-intensive,

446

conventional aerobic treatment by adding an anoxic denitrification step, often achieved through

447

supplemental carbon addition or through an alternate configuration, such as the Modified

448

Ludzack-Ettinger process.21,48 Several nitrogen removal strategies requiring little aeration and no

449

organic carbon addition are promising, but full-scale, mainstream wastewater treatment has not

450

been demonstrated. Nitritation coupled with anammox is one such technology for anaerobic

451

effluents with low carbon-to-nitrogen ratios as observed with anaerobic primary treatment.49,50

452

Bioreactors coupling methane-oxidizing microbial communities and microalgae may also be a

453

means of removing dCH4, ammonia, and excess carbon.51 Other phototrophic technologies,

454

such as high-rate algal ponds or photobioreactors, may also be coupled with anaerobic

455

wastewater treatment for nutrient recovery or biofuel production.13,52

456

Figure 6 compares the net present value (8% discount rate) to the NEB for each treatment

457

scenario. The intersection of these two lifecycle-based analyses depicts the advantage in

458

energy and costs of modeled AnP scenarios compared to conventional wastewater treatment

459

systems. This study shows that anaerobic primary treatment coupled with aerobic secondary

460

treatment strategies can enhance methane production from wastewater organics while

461

minimizing biosolids production, thereby creating a more favorable net energy balance, reducing



462

environmental impacts in most examined categories, and lowering lifecycle costs relative to

463

conventional treatment configurations. With further research and process improvements, AnP

464

with multiple compartment reactors, such as the ABR, may be able to reduce effluent COD and

465

TSS to meet secondary effluent standards and eliminate the need for aerobic secondary

466

treatment altogether, or may be coupled with other anaerobic technologies to enhance energy

467

generation and further reduce environmental impacts beyond those modeled in this analysis.

468

[Figure 6 uploaded separately]

469 470

Figure 6. Comparison of Net Present Value (8% discount rate, normalized to m3 wastewater treated) and Net Energy Balance (kWh m-3 wastewater treated) for each treatment scenario.

471 472

Acknowledgements

473

The study was supported by the U.S. National Science Foundation (NSF) under CBET-151278

474

and by the U.S. NSF Engineering Research Center (ReNUWIt) (EEC-1028968).

475

Supporting Information

476

The supporting information includes additional methodology, data, figures, and tables. This

477

information is available free of charge via the Internet at http://pubs.acs.org.

478 479

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Figure 6. Comparison of Net Present Value (8% discount rate, normalized to m3 wastewater treated) and Net Energy Balance (kWh m-3 wastewater treated) for each treatment scenario. 859x464mm (72 x 72 DPI)


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Lifecycle comparison of mainstream anaerobic baffled reactor and

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