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Sustainability Engineering and Green Chemistry
Quantitative Evaluation of an Integrated System for Valorization of Wastewater Algae as Bio-oil, Fuel Gas, and Fertilizer Products Yalin Li, William A. Tarpeh, Kara L. Nelson, and Timothy J. Strathmann Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b04035 • Publication Date (Web): 26 Sep 2018 Downloaded from http://pubs.acs.org on October 1, 2018
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Environmental Science & Technology
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Quantitative Evaluation of an Integrated System for
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Valorization of Wastewater Algae as Bio-oil, Fuel
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Gas, and Fertilizer Products
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Yalin Li†,‡, William A. Tarpeh‡,§,⊥, Kara L. Nelson ‡,§ Timothy J. Strathmann*,†,‡,∥
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†
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‡
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§
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∥National
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*Corresponding Author: +1.303.384.2226.
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⊥Present
Colorado School of Mines, Department of Civil and Environmental Engineering, Golden, CO 80401, USA. Engineering Research Center for Re-inventing the Nation’s Urban Water Infrastructure (ReNUWIt). Department of Civil and Environmental Engineering, University of California, Berkeley, CA 94720, USA. Bioenergy Center, National Renewable Energy Laboratory, Golden, CO 80401, USA. T.J.
Strathmann,
E-mail:
[email protected],
Phone:
Address: W.A. Tarpeh, Department of Chemical Engineering, Stanford University, Stanford, CA 94305, USA.
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ABSTRACT
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Algal systems have emerged as a promising strategy for simultaneous treatment and valorization
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of wastewater. However, further advancement and real-world implementation are hindered by
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the limited knowledge on the full energetic and nutrient product potentials of such systems and
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the corresponding value of these products. In this work, an aqueous-based system for the
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conversion of wastewater-derived algae and upgrading of crude products was designed and
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demonstrated. Bio-oil, fuel gas, and fertilizer products were generated from algal biomass
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harvested from a municipal wastewater treatment facility. Experiments showed that 68% of
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chemical energy contained in the algal biomass could be recovered with 44% in upgraded bio-oil
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and 23% in fuel gas (calculated as higher heating values), and 44% and 91% of nitrogen and
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phosphorus element contents in the original feedstock could be recovered as fertilizer products
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(ammonium sulfate and struvite), respectively. For 1,000 kg of such dry algal biomass, these
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products had an estimated total value of $427 (in 2014 US dollars). For the first time,
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experiment-based energy and nutrient recovery potentials of wastewater-derived algae were
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presented in an integrated manner. Findings also revealed critical research needs and suggested
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strategies to further improve resource recovery and waste valorization in these systems.
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Table of Contents (TOC)/Abstract Art
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1. INTRODUCTION
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Growing population and urbanization are putting increasing pressure on wastewater treatment
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plants (WWTPs) to meet discharge standards at lower cost. However, these facilities largely
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continue to exercise energy-intensive operations and disregard the valorization potential held by
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organic matter and nutrients present in wastewater.1,2 Wastewater algae (WW-algae) treatment
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systems leverage the versatile metabolism of microalgae to convert the carbon and nutrient
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constituents into cellular products (e.g., medium chain length fatty acids, proteins).3,4 Multiple
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existing and ongoing studies have supported the use of WW-algae systems for contaminant
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removal and algal biomass production, with the potential to meet discharge regulations and
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replace secondary treatment processes.5–8 In addition, WW-algae systems can also reduce the
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costs and environmental impacts of algae-based products by utilizing the excess nutrients in
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wastewater (as opposed to supplying external nutrients), potentially turning WWTPs into net
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energy producers.9–11 The algal biomass can be harvested and used for the generation of valuable
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products (e.g., biofuels, chemicals, nutrient products),7,12 and aqueous conversion technologies
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are increasingly being employed to minimize the energy needed for reducing moisture content of
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the harvested algae. Typical aqueous technologies include, but are not limited to, anaerobic
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digestion (AD), fermentation, lipid extraction, and hydrothermal liquefaction (HTL).13–17 Though
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many of these processes have been shown to be promising for liquid biofuel production, more
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affordable sources of algal biomass are needed to make biofuels cost-competitive with
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conventional fossil fuels, and the algal product portfolio should be expanded to include co-
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products like fuel gases (mainly composed of CH4, H2, etc.) and fertilizers to further reduce the
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costs and environmental impacts of biofuels.18,19 It follows that integration of aqueous
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conversion technologies with low-cost WW-algae can lead to paradigm-shifting scenarios for 3 ACS Paragon Plus Environment
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generation of biofuels and bio-derived co-products that are economically viable and
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environmentally sustainable.
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To date, aqueous-phase biomass conversion processes have examined mostly freshwater15- or
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saltwater20- cultivated algal biomass; however, WW-algae have distinct properties (e.g., higher
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protein and ash contents, lower lipid and carbohydrate contents2,7,21–23) due to the wastewater
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matrix and target growth conditions (e.g., nutrient-rich influent, shorter residence times).
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Moreover, studies examining aqueous conversion of WW-algae have largely limited their scope
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to crude products that require further upgrading. Even for studies where valuable end products
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have been produced, only a portion of the biomass is typically recovered (e.g., nutrient co-
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products are ignored18). Therefore, the total resource recovery and valorization potential of
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harvested algal biomass, especially wastewater-originated algae, has not been evaluated. The
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absence of comprehensive feed-to-product mass balance studies also limits techno-economic and
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life cycle analyses (TEA/LCA) that are needed to identify critical barriers to the widespread
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deployment of WW-algae systems.
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To overcome these critical challenges, this research aims to design and demonstrate an
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integrated aqueous system for the complete conversion of WW-algae to valuable end products.
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Mass, element, and energy flows along the integrated process chain were quantified for algal
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biomass harvested from a pilot-scale WW-algae treatment system, and a preliminary economic
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analysis was conducted to estimate the maximum revenue that can be derived collectively from
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WW-algae fuel and fertilizer products, setting the stage for TEA/LCA studies that will guide
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future developments in the field.
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2. MATERIALS AND METHODS
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2.1 Wastewater Algae. Algal biomass was supplied by Clearas Water Recovery from a pilot
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photobioreactor system at the Missoula Wastewater Treatment Plant (Missoula, MT). The
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harvested algae consisted primarily of Chlorella, Scenedesmus, and other minor protozoa. The
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algae were received as a frozen slurry, and were lyophilized, ground, and stored at 4°C prior to
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use for experimental expediency. Detailed methods for biochemical and elemental analyses of
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the algal biomass were provided in Section S1 in the Supporting Information (SI).
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2.2 Algae Conversion and Product Upgrading. The integrated scheme for WW-algae
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valorization includes five thermo-, physico-, and electrochemical processes (Figure 1). First, the
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biomass was prepared into a 20% aqueous slurry (dry weight basis, dw%), which was subjected
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to hydrothermal liquefaction (HTL, Step 1) at 350°C for 1 h in a continuously-stirred batch
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reactor under N2 headspace (atmospheric pressure at room temperature). This generated a self-
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separating biocrude oil product along with aqueous, biochar, and gas co-products. The HTL
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biocrude product was then upgraded to liquid bio-oil by hydrotreating (HT, Step 2) with a
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sulfided cobalt oxide/molybdenum oxide on alumina (CoMo) catalyst. HT was conducted under
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H2 headspace (960 psi at room temperature) in a continuously-stirred batch reactor, and the
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temperature was ramped to 405°C at about 5°C·min-1 and held for 2 h. HT upgrading also
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produced an energetic fuel gas co-product.
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During the initial HTL process, most of the nutrient content in the algal biomass was routed to
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aqueous and biochar co-products, with aqueous product rich in N and biochar rich in P.10,24,25
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The HTL aqueous co-product was treated by catalytic hydrothermal gasification (CHG, Step 3)
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using a ruthenium catalyst on carbon support (Ru/C) at 350°C for 4 h. After CHG, organic
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carbon in water was converted to energetic fuel gas product (mixture of H2, CH4, CO2 and minor 5 ACS Paragon Plus Environment
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volatile hydrocarbons) and aqueous carbonate species.26,27 This process also reduced the organic
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loading (i.e., chemical oxygen demand COD) and toxicity of the aqueous phase (inhibiting algal
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growth when used for cultivation28), yielding a nutrient-rich aqueous co-product stream.18,26
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Phosphorus associated with the biochar co-product was extracted (Step 4A, 2 g per batch for 3
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batches) by 0.5 M sulfuric acid (80°C, 1:10 g:mL), and the extract was amended with MgCl2 (1:1
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Mg:P molar ratio) and mixed with the ammonium-rich HTL aqueous co-product streams to
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precipitate magnesium ammonium phosphate (MAP or struvite, MgNH4PO4·6H2O, Step 4B).
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Two scenarios were evaluated for recovering P as struvite: (1) mixing the Mg-amended P extract
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with the HTL aqueous co-product before applying CHG (solid line to Step 4B), and (2) mixing
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with the aqueous product stream after applying CHG (dotted line to Step 4B).
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Figure 1. Scheme, material inputs, and products of the proposed integrated system for WWalgae valorization. Five conversion processes were applied: HTL (Step 1); HT (Step 2), CHG (Step 3), acid extraction of P (Step 4A) and magnesium ammonium phosphate (struvite) precipitation (Step 4B), and ECS (Step 5). In Step 4A, the biochar acid extract was alternatively combined with either the HTL aqueous co-product (solid line) or CHG aqueous co-product 6 ACS Paragon Plus Environment
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(dotted line) in separate experiments to compare P recovery and the quality of struvite solids generated in step 4B (denoted as HTL/CHG struvite, respectively).
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Finally, residual NH4+ present in the CHG aqueous co-product stream (after struvite
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precipitation) was recovered as ammonium sulfate, a commercial fertilizer product, by
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electrochemical stripping into a sulfuric acid trap (ECS, Step 5). NH4+ was transported from the
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feedwater (anodic chamber) to the cathodic chamber through a cation exchange membrane
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(CMI-7000, Membranes International Inc., Ringwood, NJ).29 In the cathode chamber, oxygen
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and water reduction increased pH, converting NH4+ to NH3, allowing for passage through a gas
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permeable microporous polypropylene membrane (3M, St. Paul, MN) into a sulfuric acid trap to
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produce ammonium sulfate. Additional details of the conversion steps and product analyses were
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provided in Section S1 in the SI.
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2.3 Estimation of product values. All prices were converted to 2014 US dollars using GDP
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chain-type price index from different years in the U.S. Energy Information Administration’s
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Annual Energy Outlook.30 The market value for bio-oil was converted from the price of gasoline
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($3.38 gal-1) on equivalent higher heating value (HHV) basis.31,32 Market values for HT/CHG
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gas products were converted from the price of natural gas ($7.08 mcf-1) on equivalent HHV
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basis.31,32 Market value for ammonium sulfate was $533 US ton-1 (average US farm prices).33
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Currently, struvite is not commercially available in the US, so the market value was estimated
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from a previous study in Japan,34 and the price (€245 US ton-1) was converted to US dollars
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using an exchange rate of 0.833 (to $1).35
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3. RESULTS AND DISCUSSION
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3.1 Algal Biomass. Analyses of the WW-algal biomass (Table S1 in the SI) revealed low lipid
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(14.4±0.7 dw%) and carbohydrate contents (13.3±0 dw%, an uncertainty of 0 indicates duplicate 7 ACS Paragon Plus Environment
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experiments yielded same results), and a correspondingly high protein content (53.1±0.4 dw%),
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which was consistent with previous reports for WW-algae.2,7,21–23 Hence the selected HTL-
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initiated conversion scheme was ideal due to its higher protein conversion potential compared to
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more conventional processes like AD.15,36 Additionally, the ash content (16.5±0.5 dw%) was
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higher than that typically reported for biomass cultivated specifically for biofuel production
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(typically
