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Sustainability analysis of microalgae production systems A review on resource with unexploited high-value reserves Arun K Vuppaladadiyam, Pepijn Prinsen, Abdul Raheem, Rafael Luque, and Ming Zhao Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b02876 • Publication Date (Web): 12 Nov 2018 Downloaded from http://pubs.acs.org on November 14, 2018

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Environmental Science & Technology

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Sustainability analysis of microalgae production

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systems - A review on resource with

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unexploited high-value reserves

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Arun K. Vuppaladadiyam,‡a,b Pepijn Prinsen,‡c Abdul Raheem,‡a,b Rafael Luquec and

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Ming Zhao,a,b,*

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a

b

Key Laboratory for Solid Waste Management and Environment Safely, Ministry of

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School of Environment, Tsinghua University, Beijing 100084 China.

Education, Beijing, 100084, China c

Departamento de Química Orgánica, Universidad de Córdoba, Campus de

Rabanales, Edificio Marie Curie (C-3), Ctra. Nnal. IV, Km 396, Córdoba, Spain.

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KEYWORDS. Microalgae, biorefinery, sustainability, techno-economic assessment, life-

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cycle analysis, socio-economic impact

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ABSTRACT. Sustainability, at present, is a prominent aspect in the development of

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production systems that aim to provide the future´s energy and material resources.

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Microalgae are a promising feedstock, however, the sustainability of algae-based

ACS Paragon Plus Environment

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production systems is still on debate. Commercial market volumes of algae derived

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products are still narrow. The extraction and conversion of primary metabolites to

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biofuels requires cultivation at large scales; cost-effective methods are therefore highly

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desirable. This work presents a complete and up to date review on sustainability

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analysis of various microalgae production scenarios, including techno-economic,

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environmental and social impacts, both in large-scale plants for bioenergy production as

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in medium-scale cultivars intended for the production of high added-value chemicals.

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The results show that further efforts in algal based research should be directed to

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improving the productivity, the development of multi product scenarios, a better

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valorization of co-products, the integration with current industrial facilities to provide

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sustainable nutrient resources from waste streams and the integration of renewable

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technologies such as wind energy in algae cultivars.

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Table of Contents

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1. INTRODUCTION

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The global population could reach and even exceed 9 billions by 2050.1 The

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development of efficient conversion methods that use sustainable feedstocks to meet


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the growing global energy demand and to reduce the use of fossil resources, is now an

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imminent challenge for the research community. Around 87 % of the global CO2 emitted

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by anthropogenic activities result from fossil resources, with coal, oil and natural gas

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contributing 43, 36 and 20 %, respectively.2 In the EU, ca. 30 % of the total energy use

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is spent in transport fuels.3 One way to convert CO2 and light energy into renewable

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fuels, chemicals and energy is to store them in microalgae (biosequestration).

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Microalgae can fix CO2 more efficiently than terrestrial plants (with biomass yields ca.

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55 kg ha-1 year-1, twice as high) and do not directly compete with food crops for arable

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land.4,5 Still, significant bottlenecks exist in the road towards sustainable commercial

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microalgae derived production systems, due to techno-economic, environmental and

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social constraints and challenges in their cultivation, harvesting and associated

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downstream processes.

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Recently, various reviews have been published in the field of sustainable microalgae

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based production systems, including conversion to biofuels6-9 and high added-value

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chemicals.10-12 Some of them focused on specific sustainability aspects such as the use

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of industrial flue gas as CO2 source13 or wastewater (WW) as nutrient source,14-16 and

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environmental applications.17,18 Some reviews deal with techno-economic assessment

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(TEA) studies,19-21 others with life cycle analysis (LCA)22-24 and some with socio-

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economic indicators.25 Rather few works evaluated the sustainability in all its

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aspects.26,27 As progress is continuously being developed in various fields, including

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metabolic engineering, cultivation, harvesting, extraction and conversion, these review

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studies may help to clear out favorable routes towards sustainable industrial algal

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production systems. The data available in the literature is hardly assesed in a


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quantitative manner because they are rather dificult to compare due to differences in

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model definitions, assumptions and boundaries and the high amount of biorefinery

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scenarios and input parameters used.

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The present work aims to evaluate the sustainability of different microalgae

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biorefinery scenarios in a quantitative and up to date study based on techo-economic

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assessment and life-cycle analysis. Key papers from the last seven years on TEA and

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LCA studies were selected. Production costs of algal biomass, algal oil/biocrude and

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algal biofuel were contrasted by comparing MAFBSP (minimum ash-free biomass

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selling price) and MFSP (minimum fuel selling price) in various biorefinery scenarios

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together with high impact variables (according to the sensitivity analysis if any).

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MAFBSP and MFSP refer to prices to obtain a zero net present value (NPV) for a

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specified internal rate of return after taxes, typically set at 10 % target). MAFBSP and

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MFSP are accounting concepts, not real selling prices. Higher MAFBSP/MFSP means

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that the (final) product needs to be sold at higher value for revenues and inflow cash

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reaching breakeven level of the original capital investments. To correct these cost data

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for the currency and inflation between different publication years, MAFBSP and MFSP

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values are expressed in USD (2018), recalculated with the mean annual currency and

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inflation rates (with respect to publication year). Next, important findings from the

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literature are highlighted involving sensitivity, cost breakdown and market analysis,

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which are important tools in TEA studies. In the section on LCA, net energy ratios

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(NER), greenhouse gas (GHG) emissions and water footprints (WF) were compared for

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the most prominent scenarios. All energy balances were expressed as NER values.

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Data were also collected on algal biomass productivities using different wastewater


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(WW) sources to evaluate their potential as nutrient source. In the last section, various

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indicators of socio-economic impacts are highlighted. Finally, based on the main

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conclusions, future prospects for algae-based research and commercial production

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systems are summarized.

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2. TECHNO-ECONOMIC ASSESSMENT (TEA)

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Production systems must be techno-economically viable to be sustainable. This aspect

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is currently still on debate in algae based biorefinery, due to the large uncertaincy in the

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extrapolation of lab-scale data to large scale scenarios and depending on the data

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available for every biorefinery scenario and the model boundary limits, giving raise to a

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high degree of heterogeneity among the data.26-29

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2.1.

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An important part in the cost distribution is attributed to the algae biomass production

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itself. This includes cultivation and harvesting. Figure 1 shows the biomass production

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cost of various algal production systems including open raceway ponds (ORP) and

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photobioreactors (PBR). The results show considerable variations in production costs

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(MAFBSP), depending on the input parameters used and assumptions made in the TEA

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scenarios. In some cases the production cost is competitive with the USA soybean

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market price as the benchmark feedstock for competitiveness with first generation

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biodiesel, whereas in other cases it largely exceeds it. The USA 2022 target price (entry

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3) of 2.25 USD kg-1 may be a better reference to compare with.52 The large differences

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in production costs are mainly the effect of varying biomass productivity, scale,

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cultivation method and nutrient costs. The highest impact variable was the scale of

Algal biomass production


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Figure 1. Minimum biomass selling price in TEA scenarios 1-7.

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biomass production (1, 10, 100 and 400 ha in scenarios 1-2). But, the scale of economy

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effect may be subjected to uncertainity, as real life data are rather scarce (instead they


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are extrapolated from lab-scale or pilot scale data). The economy of scale effect in

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algae production systems is limited by the modular character of the cultivation

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systems.27,30

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The cultivation method affects the cost considerably, as illustrated for ORP, tubular

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(TPBR) and flat-plate (FPPBR) photobioreactors in scenario 1. Hoffmann compared

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algal turf scrubbers (ATS, scenario 4a) with classic ORP cultivars (scenario 4b).33 ATS

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systems are based on native cultures which dynamically adapt to changing conditions to

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improve the culture stability and avoid crash events. The lipid content of ATS algal

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biomass was however low (10 %) and they presented high ash content. Nevertheless,

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the ATS cultivars produced algal biomass at 2620 USD ton-1, while ORP cultivars at

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3460 USD ton-1, mainly due to the difference in cell density at harvesting (200 vs. 0.5 g

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L-1, respectively). Rotating algal biofilm reactors (RABR, scenarios 5a-5e) are another

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innovative cultivation method.34

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The effect of zero cost nutrients is also noticeable (scenarios 2a and 2c vs. 2b and

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2d). Some studies reported the use of WW as nutrient feed source and the supply of

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CO2 from flue gas sources, but not all of them included the carbon credits associated

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with the estimated storage and pump costs. Rezvani et al. studied the integration of

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cultivars with different CO2 biosequestration methods (scenarios 6a-6j).35 The cultivation

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method (ORP/PBR) and photosynthetic efficiency (PE) were also included as variables.

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The effects were rather small and the production costs were low in all cases compared

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with other studies, possibly due to the effect of other variables such as productivity, lipid

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content and scale. The substantial lower cost also may have resulted from MAFBSP´s


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definition; in this case it was calculated at an electricity price similar to that from a

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conventional PP without CO2 capture and storage.

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Important parameters in the selection of a suitable cultivar location include not only

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the estimated PE and seasonal variations, but also governmental regulations and labour

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costs as illustrated in scenarios 7a-7d. Ruiz et al. (2016) reported a tidous and realistic

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analysis of ORP and PBR cultivars.27 They compared the capital and operational costs

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on a 100 ha scale from current production facilities in several countries. They concluded

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that FPPBR are the most cost-effective production system, with the best projections for

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southern Spain (3.4 EUR kg-1).

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2.2.

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Algae harvesting (often followed by dewatering) is a bottleneck step, as it contributes

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considerably in the overall biomass production cost.36,37 The energy consumed during

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harvesting and dewatering can account up to 90 % of the total energy required for algal

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biodiesel production.38 Typically, algae slurries ca. 1 wt% dry solids must be dewatered

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to ca. 20 wt%. Centrifugation technologies work efficient but face large initial capital

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investments. Figure S1 (see Supporting Information) shows the effect of the cell density

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and lipid content on the energy use and the cultures size required to produce 1 L of

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algal oil, showing that centrifugation should be considered more appropriate as a final

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step in dewatering methods, especially at large scales.

Algae harvesting

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Aggregation by flocculation and coagulation is one of the most cost-effective

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harvesting technologies.39,40 Bioflocculation (BF) occurs at pH > 9, whereas chemical

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flocculation techniques cover a broader pH range, employing cationic iron, aluminium

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salts, lime, cellulose, polyacrylamide polymers, cationic starch or surfactants to alter the


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physiochemical interaction between algae cell walls (negatively charged) and induce the

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formation of aggregates. Around 95 % cell flocculation efficiency was achieved when

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the pH of Nannochloropsis sp. cultures (107 cells mL-1) was adjusted to 10 by adding

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Ca(OH)2.41 The corresponding harvesting cost was estimated as low as 7.5 USD ton-1

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biomass and was further reduced to 3.5 USD ton-1 when the cell density reached 108

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cells mL-1. When using chitosan flocculation (CHF) in a pre-concentration step, Xu et al.

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found that up to 95 % of the energy required for harvesting via centrifugation can be

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saved.42 Despite the promising results, chemical flocculation is not the best option from

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an environmental point of view, especially when using aluminium salts.38 Chemical

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flocculation technologies also face lower biomass recovery, typically 1-20 % lower

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compared to other harvesting methods.43 In some cases, negative effects can be

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observed in product quality.40 Harvesting microalgae via aqueous ammonia hardly

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affected the metabolites content distribution. The liquid fraction may be re-used as

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nutrient feed for algae cultivation.44 Aggregation induced by micro-organisms avoids the

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use of chemical flocculants.45,46 However, relatively large inoculant sizes (30:1) were

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needed and flocculation was rather slow. Powell and Hill accelerated the BF process

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drastically and reduced the bacteria cell:algae cell ratio to 1:1.47

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Direct filtration, cross-flow filtration or combinations with inverse osmosis were also

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demonstrated to recover algae aggregates efficiently.48-50 NAABB researchers

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assembled a thin porous nickel-alloy metal-sheet membrane in a cross-flow module for

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dewatering microalgae cultures up to 24 % solids.51 Filter pore sizes need to be

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carefully designed in function of the aggregation rate, as smaller cells (< 10 µm) are not

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recovered and small pores lead to filter blocking. Recently, Global Algae Innovations


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(Hawaii) developed an advanced membrane filtration system for combined harvesting

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and dewatering without using flocculants, demonstrated at large scales (20 m3 h-1) with

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an energy use ca. 0.04 kWh m-3 and 100 % harvest efficiency.28 Another elegant way to

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facilitate membrane filtration is to ´pelletize´ microalgae in fungi-algae complexes (2-5

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mm), though the process depended on glucose addition.52 Electrolytic flocculation

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through release of metal ions from a sacrificial anode is another alternative.53 In 2009,

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Originoil (USA) announced Live ExtractionTM, a technology which extracts oil from algae

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on a frequent basis based on the balance of living and dead algae cells using

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electromagnetic pulses.54

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Despite the development of novel harvesting and dewatering technologies, only few

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works evaluated their cost-effectiveness and in particular combinations of them. This is

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important as probably not just one harvesting or dewatering technology may be the

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most suitable candidate but rather a combination of them, depending on biomass

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composition and downstream processes. Recently, Fasaei et al. (2018) described the

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effect on the energy use and costs associated with sequential combinations of various

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harvesting and dewatering operations,19 as illustrated in Figure S2. Low-energy

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combinations were chemical flocculation, either cationic (CAF) or chitosan (CHF)

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mediated, followed by centrifugation or pressure filtration. Different effects were

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observed from the cell culture density and feed rate in ORP, TPBR and FPFBR. CAF

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showed higher costs as higher flocculant dosage is required compared to CHF.

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2.3.

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For the production of lipid derived biofuels and chemicals, first oil or biocrude needs to

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be extracted or produced from algal biomass. Figure 2 compares the outcome of recent

Algal oil/biocrude production


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Figure 2. MFSP of algal lipid oil/biocrude in TEA scenarios 1-7.


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TEA studies on the production cost of algal oil or biocrude, obtained either via dry or wet

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solid extraction (DSE or WSE) or via hydrothermal liquefaction (HTL), respectively. The

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MFSP varied typically between 2 and 7 USD L-1 oil, still considerably higher than fossil

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derived crude oil and soya oil. Given the high amount of variables in algae strains,

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cultivation and harvesting methods, conversion techniques and co-product valorizations,

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it is no surprise that one finds considerable discrepancies in the results reported. Some

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uncertainity contributors in the models are even not related to cultivation and refining

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(e.g. market price, risk tolerance, capital finance, etc.). Beal et al. analysed an

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interesting set of scenarios for hybrid PBR-ORP cultivars (Figure 2, scenarios 4a-4h)

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with various combinations of harvesting, nutrient feeding, conversion pathways and co-

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product valorizations based on 100 ha data obtained during 1 year in Texas and Hawaii

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facilities.58 In the OpenAlgae process, lipid oil is produced via WSE and the solid is

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valorized as animal feed. The Valicor process is similar to the OpenAlgae process, but

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the residual aqueous stream is also converted to combined heat and power (CHP) via

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catalytic hydrothermal gasification (CHG). In the HTL process, biocrude is produced and

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both residual solids and aqueous streams are used for CHP. The advantage of

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including solids is that their residues after CHG are recycled to decrease the chemical

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nutrient requierements (at least for nitrogen). HTL is also the most favorable conversion

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process from the energetic point of view, as the dewatering step is minimized. The U.S.

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Department of Energy set the 2018 HTL productivity goal for algal biofuels at 3.8 L m-2

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year-1 biocrude (33 g m-2 day-1 productiviy and 35 % conversion). Interestingly, the

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lowest MFSP values were reached when considering wind energy to reduce the share

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of grid electricity. In fact, the use of wind and solar energy has been considered in only


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few TEA scensaios. Pearce et al. considered solar powered thermal processing (CSP,

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scenario 5),59 using a 100 m parabolic trough integrated succesfully with a HTL reactor,

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which works more efficiently compared to the classic set-up for parabolic troughs in grid

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electricity generation, as the use of heat transfer fluids, counter current heat

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exchangers, fluid transfer interconnectivity and electrical power control systems is

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minimal. They reported a competitive biocrude production cost of 1.37 USD L-1.

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2.4.

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Once algal oil or biocrude is obtained, it can be converted to biofuels. Whereas

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conversion to fatty acid methyl esters (FAME) is an already established route, more

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recent research focused on the conversion to renewable diesel (RD) via hydrotreating

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(HT). One motivation for this shift is that the distribution pattern of the fatty acid chain

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length and insaturation degree in microalgae is not optimal compared to oleo-rich

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crops,61 affecting the physico-chemical properties of FAME mixtures intented for

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combustion use in vehicles. Other motivations for this shift include the high costs

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associated with algal biomass drying:32 1) FAME production from wet algae slurries is

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only possible via wet conversion technologies whereas RD is produced from wet algae

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slurries via WSE or HTL, 2) promotion of lipid productivity is of primordial importance in

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FAME production whereas in RD production proteins and carbohydrates also contribute

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in the total fuel yield. Figure 3 shows the outcome of recent TEA studies on the

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production of algal derived RD, produced either via WSE (scenarios 1 and 6) or via HTL

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(scenarios 2-5 and 7-9). The MFSP results are in the range of competitiveness

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compared to those of RD obtained via pyrolysis (scenario 10) and those of advanced

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FAME production (integration with bioethanol production, either via fermentation of the

Algal biofuel production


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Figure 3. MFSP of algal RD (scenarios 1-10) and FAME (scenario 11).


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whole algae slurry (scenario 11a) or after solid-liquid separation (scenario 11b)). The

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production costs of wood pyrolysis and Fischer-Tropsch corn derived RD are also

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displayed for comparison,62 showing that micro-algae are currently less cost-

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competitive. Lipid extracted algal biomass (LEA) in contrast, was valorized at much

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lower cost by Ou et al. (30-140 USD ton-1),63 resulting in HTL derived RD at competitive

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prices (scenarios 4a-4c). The oil was hydrotreated in a two-stage process at 200 and

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400 °C resulting in RD containing 51 % cycloalkanes, as the LEA was enriched in

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proteins and carbohydrates and depleted in fatty acids. The RD product composition,

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important in view of application in combustion engines, is an aspect which is not

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accounted for in most TEA studies. Zhu et al. (2015) compared fresh - and seawater

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microalgae as feedstock for HTL biocrude production (scenarios 5a-5d), containing 4

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and 16 wt% lipids (ash-free basis), and 8 and 22 wt% ashes, respectively.64 This

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resulted in higher biocrude and RD yield for seawater algae (whereas more naphtha as

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co-product was produced from freshwater algae). Seawater microalgae are appealing

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for large scale biofuel production, as freshwater consumption can be reduced drastically

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and fuel yields are generally higher, but their high ash content affects the overall

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process economy. Hoffman compared RD produced from ATS and ORP cultivars

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(scenarios 7a-7b).33 Although the algal biomass production cost was lower in the ATS

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model (Figure 1, scenarios 4a-4b), this was not the case for the RD production cost,

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mainly due to the high ash content and low lipid content. Juneja and Murthy presented a

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TEA analysis of RD production via HTL using CO2 supply from a PP at 4.5 km distance

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and WW from a facility located at 3 km distance (scenarios 9a-9g).14 The nitrogen

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utilization degree (0.08-0.12 g L-1) from the WW had high impact on the MFSP of RD, in


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the range of the effect of the lipid content (15-35 wt%), followed by the scale of the WW

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facility, diesel yield and algae concentration at harvest.

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The results in Figures 1-3 show, at least in equicomparable conditions, the

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importance of reaching higher biomass productivities (g m2 day or g L-1 day-1). The

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productivity as key parameter to attain feasible biorefinery scenarios was previously

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highlighted as one of the main conclusions in various studies.56-58,61 Productivity

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improvements require efforts in microalgae based biotechnology, e.g. via the

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development of genetically modified species.28 Another strategy to increase the

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productivity is the cultivation of microalgae in co-cultures, either with other algae

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species or with fungi, yeasts or bacteria. Gomez et al. used genome-based metabolic

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network modelling to demonstrate this in large scale open pond cultivars.67 First, they

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modelled single algae and single oleaginous yeast cultivation in flue gas enriched

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medium. Next, an algal-fungal ORP with a feed of cellulosic glucose and oleaginous

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yeast was considered, which consumed glucose/xylose mixtures resulting from

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lignocellulose hydrolysis waste. Co-cultures of algae and yeast produced FAME at

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competitive prices, ca. 2.0 USD L-1 for pure cellulosic glucose and 1.4 USD L-1 for

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lignocellulosic glucose/xylose, whereas the algae monoculture gave similar results only

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at very short distances from the flue gas source. Oleaginous yeasts such as

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Cryptococcus albidus, Rhodotorula glutinis and Yarrowia lipolytica are attractive for

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biofuel production as they can convert lignocellulosic sugars into lipids, accumulating up

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to 36-72 %.68 These cultivation systems could be applied in the vicinity of PP, but

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optimization is required as most oleaginous yeasts are not extremophiles. The

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importance of these findings is the fact that the introduction of yeast or fungi enables


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lignocellulosic (waste) materials to be digested and to become metabolized by

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microalgae, which are otherwise not available for assimilation in single algae cultures.

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This cultivation strategy enables the partial transformation of these carbon sources into

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algae fixed CO2 . At the same time, the yeast or fungi can benefit from the O2 produced

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by algae and increase their lipid production. In addition, microbial communities are

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better protected against microbial invasion.68

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2.5

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Cost breakdown analysis is often divided in fixed capital costs and operational costs, as

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illustrated in Figure S3 for various algal biofuel production scenarios. Doshi et al. (2017)

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analysed the financial investment for a 250 ha FAME production plant consisting of 175

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ha ORP cultivars, fed with WW and CO2 refined from flue gas, supplemented by urea

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and diammonium phosphate as fertilizers (based on nutrient availability in WW

306

medium).69 An important detail in the study was that the cost of flue gas refining was

307

assumed to be fully covered by the waste producers. Pond construction and installation

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costs represented the highest capital investment (34 %), whereas the sum of

309

maintenance, supplies and insurance costs dominated (91 %) the total operational cost.

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The present value depended on the revenues from FAME (1.50 USD L-1) as the main

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product and glycerol (1.52 USD L-1), fertilizer (12.0 USD kg-1) and animal feed (12.0

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USD kg-1) as the co-products. The results demonstrated that valorization of the LEA

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residue was essential for the financial feasibility (NPV > 0). Maximizing FAME yields in

314

compromise with lower LEA yields for animal feed lead to a negative NPV. Batan et al.

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reported the capital and operational costs for a PBR cultivar intended for RD production

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via WSE followed by HT.60 Surprisingly, the harvesting costs were estimated much

Cost Breakdown Analysis


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higher than those reported by Doshi et al (ORP),69 despite no use of flocculant was

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reported and despite exhibiting higher biomass concentrations at harvest. Although

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higher capital investment was required to purchase PBR, the installation cost and

320

working capital was considerably lower compared to ORP. Hoffman contrasted the cost

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analysis of ATS against ORP cultivars intented for RD production via HTL and HT.33

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The total capital investment was similar whereas the operational cost was drastically

323

lower for ATS cultivars, as it was assumed that all nutrients were provided from WW

324

streams (without credits for removing N and P) and that CO2 was provided by a nearby

325

source of flue gas. Flocculants were not required for dewatering of ATS cultures (algae

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can be harvested at concentrations up to 200 g L-1) in contrast with ORP cultures

327

(harvested at 0.5 g L-1). Recently some hybrid PBR-ORP cultivars were analysed, in

328

which algal strains are inoculated in PBR followed by large scale growth in ORP.70 The

329

hybrid system showed attractive capital investment requirements. Capital costs of 269,

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83 and 101 million USD year-1 for PBR, ORP and hybrid PBR-ORP, respectively (0.7-

331

108 dry ton year-1). The effects on the operational costs were similar.

332

2.6 Sensitivity Analysis

333

Sensitivity analysis is an important tool in TEA, as it indicates more clearly the fields that

334

must be improved in the road to commercial algae production systems. It shows how a

335

certain output value (MFSP, NPV, t-values, etc.) varies with a changing input parameter

336

in the calculation model (Excel, AspenPlus, FARM, etc.), including a baseline case, a

337

lower case and an upper limit case, in the form of probability curves, histograms, etc.

338

Bravo-Fritz et al. (2016) considered an interesting set of biorefinery scenarios and

339

compared them between Isochrysis sp. and Tetraselmis sp. cultivars at medium and


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340

large scale sizes.70 All the scenarios (Figure S4) resulted in negative NPV, except for

341

protein extraction (albeit it had the worst energy balance evaluation). The most

342

promising scenarios included: a) drying + ball-milling + lipid/debris separation, b) WSE

343

and c) WSE + anaerobic digestion (AD). The effect on MFSP of the assessment context

344

(moderate, intermediate and optimistic) and the scale were actually more significant

345

than the biorefinery scenario itself. Only with Isochrysis sp. in the optimistic scenario

346

and at large scale competitive production costs (MFSP) were achieved ca. 1 USD L-1.

347

Here is the point where TEA results start to get speculative; data on larger scale are

348

required to confirm the data used from small scale. One succesful example of this was

349

reported by Wen et al. (2016) on the up-scaling of Chlorophyta cultivars from pilot scale

350

(0.01 m3 reactor) to outdoor (40 m3 ponds), in which both the biomass productivity and

351

lipid content remained stable.72

352

Whereas sensitivity analysis of single product scenarios (e.g. biodiesel) indicates that

353

higher lipid contents will lead to lower MFSP, the behavior is different in multi product

354

scenarios.69 Therefore, sensibility studies which evaluate on a NPV basis are more

355

appropiate, rather than evaluation of the production cost only (MFSP), because a better

356

valorization of residual process streams can improve the total revenue value.

357

Importantly, in a multi product scenario of biodiesel, glycerin, animal feed and fertilizer,

358

it was found that a lower biodiesel price was off set by sales of high-value feed and

359

fertilizer and hence the feasibility range based on realistic potential prices for

360

commercial diesel fuel was hardly affected. In other words, the key for biofuels to

361

become price competitive (not cost competitive) is a better valorization of the co-


19


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362

products. This was also one of the main conclusions of a very recent review on

363

microalgae based biorefinery concepts.72

364

In sensitivity analysis, the input parameters are based on results from the literature

365

which in turn are based on past events or past interpretations of future outcomes. In this

366

sense, switch-value (SV) analysis is more appropiate to compare the financial

367

feasability of biorefinery scenarios in function of a certain input parameter. The

368

parameter values are calculated at which NPV values turn to zero. In other words, SV

369

values describe how close the parameter set for the baseline scenario corresponds with

370

NPV = 0 situations. Doshi et al. (2017) calculated for a multiproduct scenario (biodiesel,

371

glycerol, animal feed and fertilizer) SV values of 19.6 g m-2 day-1 (biomass growth rate),

372

40 % lipids (dry content), 11.8 USD kg-1 (both fertilizer and animal feed price), 41 % use

373

of biomass allocated for biodiesel production, 19.5 years (operation period) and 97 %

374

use of the LEA residue.69 In the sensitivity analysis, lipid contents higher than the

375

baseline case (40 %) resulted in negative NPV values, showing that further

376

improvements in lipid extraction and transesterification are still highly desired before

377

increasing the biomass proportion allocated for biodiesel. By improving the cost-

378

efficiency of these proceses with 20 % (via reduced capital investment and maintenance

379

costs), the pay-back period was reduced from 20 to 12.3 years. TEA clusters that

380

explore fast and efficient new scenarios based on novel product and technological

381

developments are highly desired, as recently addressed by an expanded biorefinery

382

superstructure proposed by Rizwan et al., including the processing of microalgae

383

residues and solvent recycling.73 The model (Figure S5) was developed for C. vulgaris,

384

but it can easily be extended to other species.


20

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385

2.7.

386

Market analysis is an essential part of TEA analysis and should be thoroughly

387

conducted, as it defines the potential market niches, values and volumes. Microalgae

388

derived products find their market majorly in 4 sectors: (i) bio-energy and renewable

389

bulk chemicals, (ii) agricultural products (biopesticides and biofertilizers), (iii) animal

390

feed (supplements) and (iv) human use (food, nutraceuticals and cosmetics). Algal

391

biomass is considered as a suitable feedstock, but the reality is that industrial

392

applications are related to almost exclusively human consumption and animal feed.74 In

393

the bioenergy sector, the production of biofuels requires large scale algal biomass

394

production (ca. 107 tons year-1), which is way too far from the actual global production

395

volume (104 tons year-1). This production scenario also falls short compared to the

396

production required for agricultural (105 tons year-1) and animal feed (106 tons year-1)

397

and only meets the requirements for human applications (104 tons year-1). A market

398

analysis for different microalgae derived products is presented in Figure S6. To be

399

economically feasible NPV must be positive, but only products for human consumption

400

and animal feed have market values higher than the production cost. High added-value

401

products include animal feed products which are often protein enriched, while products

402

for human consumption are usually obtained from polyunsaturated fatty acids (PUFAs)

403

in lipid fractions and secondary metabolites (i.e. extractives). It is estimated that the

404

market value of carotenoids would reach 1000 million US$ by 2020. Commercially

405

produced astanxanthin from microlalgae has a market value of 15,000 USD kg-1.

406

2.8

Market analysis

High Added-Value Chemicals


21


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407

Large scale cultivation plants for the production of biofuels (as described in sections

408

2.1-2.5) do not have the same cost distribution, revenue and profitability compared to

409

small-scale specilalized cultivars for the production of high added-value chemicals, as

410

illustrated in a comparison between algal FAME and β-carotene production plants

411

(Table S1). Although the market volumes of secondary metabolites are very low

412

compared to bulk chemicals and biofuels, their value is much higher, as illustrated in

413

Table S2 for astaxanthin, lutein, β-carotene and phycocyanins. Ruiz et al. designed

414

specific biorefinery process chains for the production of pigments in function of different

415

market scenarios (cosmetics, healthcare, food and natural/synthetic pigments), e.g. for

416

the production of omega-3 fatty acid and astaxanthin.27 Based on a realistic cost and

417

market analysis, the authors demonstrated a higher profitability for cosmetic and food

418

related products with better projections for the near future, compared to the production

419

of bulk chemicals and in particular biofuels. Other examples of commercial speciality

420

products from microalgae are PUFAs,75 natural (fluorescent) dyes76 and stable isotope

421

chemicals for research and pharmaceuticals.77 Biofuels in contrast have a relatively low

422

commercial value and need to be produced at large scale or need a better co-product

423

valorization to become cost and price competitive. These findings has partially moved

424

the

425

products.5,21,27,74,75 This shift has also been stimulated by the fall of oil prices.5

interest

in

algae

based

research

from

biofuels

to

high

added-value

426

Despite various improvements, the selective extraction of valuable compounds

427

remains a key challenge.27 The highest costs are attrituted to biomass drying and cell

428

disruption (e.g. bead-milling, 1 kWh kg-1 ~ 0.17 USD kg-1 for 95 % disruption). The use

429

of pulsed electric fields may drop the energy use to 0.06 kWh kg-1 for 70 % cell


22

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430

disruption.78 A lot of energy is also put in solvent extraction and recovery. It was

431

estimated that the use of heat for biomass drying, lipid extraction and solvent recovery

432

reaches 0.21 USD kg-1. Supercritical fluids and switcheable solvent systems are

433

attractive alternatives to traditional solvents. Aqueous ammonia extraction, in similiarity

434

with the AFEX process,79 or anhydrous liquid ammonia,79,80 is also an option as

435

ammonia can be recycled efficiently. Ammonia residue streams could be recycled to the

436

algae nutrient feeding.44

437

3. ENVIRONMENTAL IMPACT

438

Most of the environmental impact studies related to microalgae biorefinery scenarios

439

are conducted and evaluated via Life Cycle Analysis (LCA) of carbon, energy, water

440

and nutrients. The most frequently used environmental sustainability indicators are net

441

energy ratio (NER), greenhouse gas (GHG) emissions and water footprint (WF). Similar

442

to the data from TEA studies, progress is highly desired in further collecting data from

443

pilot plants to estimate better productivity data for large scale plants, and to contrast

444

them with the current available data from large plants.81

445

3.1 Net Energy Ratio and Greenhouse Gas Emissions

446

GHG emissions produced during the life cycle of algal biofuels are reported as CO2

447

equivalents (g CO2eq) by combining CO2, CH4 and N2O emissions scaled by their global

448

warming potentials. NER indicates the ratio of energy demand (from cultivation to final

449

production stage) to energy content of the biofuel. Some works consider only the well to

450

pump cycle (WTP: feedstock terminal and retail station), while others also consider the

451

pump to wheels cycle (PTW: CO2, CH4 and N2O emissions associated with biofuel


23


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452

combustion). Well to wheels (WTW, also ´cradle to grave´) results consider the entire

453

biofuel life cycle. Emission data on the combustion of biofuels in engines are still lacking

454

and often the value for low-sulfur diesel is used instead. A substantial amount of the

455

reports in literature used the GREET model (Greenhouse Gases Regulated Emissions

456

and Energy Use in Transportation). This model is updated regularly and can be

457

downloaded as an excel file.82 While most LCA studies on microalgae derived biofuel

458

production reported promising results, others did not.83-87 Clarens et al. (2010)

459

illustrated that life cycle impacts of algae cultivation are sensitive to several input

460

parameters, some of which are still overlooked.88 Collet et al. (2014) reported

461

recommendations for LCA studies on algal biofuels to harmonize results in order to

462

improve their validity.89 Improvements could be made in the life cycle inventory (LCI)

463

and the functional unit itself. At the LCI level, special attention should be paid to the

464

perimeter of the study (e.g. inclusion of infrastructures) and to the valorization of co-

465

products.

466

3.1.1 Algal Oil/Biocrude and FAME

467

Figure 4 shows the high impact scenario variables and the outcome of recent LCA

468

studies on the production of FAME (via transesterification of lipid oil), algal oil (via

469

DSE/WSE) and biocrude (via HTL). NER and GHG emissions for fossil derived low-

470

sulfur diesel are shown for comparison, as well as the GHG reduction thresholds for

471

2018 set by the European Directive on Renewable Energy in 2009.97 The general trend

472

for FAME showed a less favorable energy balance (higher NER) and higher GHG

473

emissions, with exception of some scenarios. Note that equal biomass productivities

474

and FAME yields can result in different NER and GHG emissions, depending on the


24

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475 476

Figure 4. Net energy ratios (NER) and greenhouse gas (GHG) emissions associated with

477

the production of algal derived lipid oil (green), biocrude (blue) and FAME (red).


25


Page 26 of 62

478

carbohydrate and protein contents (co-product valorization). An important part of the

479

energy and emission balance is related to drying requirements of algal biomass.

480

Whereas HTL requires minimal drying, DSE requires important amounts of energy. DSE

481

scenarios can be improved by integrating heat recovery with the drying process, as

482

illustrated by Zaimes and Khanna (2013) in scenarios 2a-2b, still the WSE method

483

showed better results (scenario 2c).91 Quinn et al.

484

(scenarios 3a-3d), despite having superior extraction performance compared to hexane,

485

was not as favorable as expected.92 This was mainly because it was supposed that CO2

486

extraction required dry conditions. Soh et al. (2014) conducted LCA studies based on

487

lab-scale (0.5 L) data from 2 freshwater (N. oleoabundans and C. sorokiniana) and 2

488

marine (N. oculata and T. suecica) microalgal species, both with nitrogen deprivation

489

and repletion (scenarios 4a-4h).93 Higher lipid productivity did not lead to lower NER

490

and lower GHG emissions in all cases, because AD also has favorable impacts on

491

these indicators (as drying is not required for AD). Still, considerable uncertainty exists

492

on this effect as the CH4 yields from LEA are poorly described in the literature.83

493

Ponnusamy et al. (2014) compared hexane extraction in near dry conditions with

494

subcritical water extraction, as a variant to HTL.94 The total energy requirements for

495

subcritical water extraction were estimated similar to those for hexane extraction and

496

recovery (33 MJ kg-1 FAME). For the base case (scenario 5a) they assumed 50 % heat

497

exchanger efficiency and 60 % in the optimized case (scenario 5b), whereas 85-90 %

498

was used in previous literature.

(2014)

showed that supercritical CO2

499

The use of external fossil energy is mainly governed by the electricity demand in the

500

cultivation stage for mixing, pumping and injecting gas and can vary widely with


26

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501

considerable effects on both NER and GHG emissions.98,99 It was determined that in the

502

case of Nannochloropsis sp. ORP cultivars, a possible reduction of 70 % of the

503

electricity consumption at the cultivation stage would reduce the GHG emissions with

504

ca. 42 % (resulting in an emission of 0.85 kg CO2eq) and decrease NER to values below

505

1.99 In classic ORP cultivars the energy consumption can vary between 0.24-1.12 W m-2

506

or more specifically between 3.7-5.7 kWh per kg algal oil.5,99 The reason for this

507

variation is that the main parameter to be optimized during cultivation is the productivity,

508

more than the energy consumption. Microalgae need proper mixing to avoid

509

photoinhibition and photolimitation and to attain high photosynthetic efficiency, for

510

instance by keeping high flow velocities and turbulence levels. Chiaramonti et al. (2013)

511

showed how redesign of raceway ponds can optimize the energy consumption without

512

compromising productivity.100 Another strategy to reduce the input of fossil energy is to

513

increase the share with renewable energy. LCA studies which included renewable

514

electricity as an alternative to grid electricity are rather scarce. Note that in real life

515

situations a photovoltaic energy panel has larger PE compared to most microalgae; the

516

advantage of microalgae is that excess energy can be stored efficiently. Another

517

advantage is that energy consumed on site only has minor transport and distribution

518

losses. Collet et al. (2014) demonstrated how wind turbines and photovoltaic panels

519

could be integrated on site to provide the electricity demand of an 80 ha ORP cultivar.89

520

At 20 g m-2 day-1 algal productivity, the NER and GHG emissions were reduced with 18

521

and 21 %, respectively. They demonstrated that the impact of the electricity source on

522

GHG emissions corresponds with the same effect as increasing the algal productivity

523

from 10 to 30 g m-2 day-1. This three-fold increase in productivity would require


27


Page 28 of 62

524

biochemical and technical developments on the long-term, while installing wind turbines

525

and solar panels is more straightforward at short-term. Positive effects of wind energy

526

on energy balances were also demonstrated by Beal et al. (2015) in scenarios 10e, 10g

527

and 10i.58

528

Effects from the nutrient source were also studied. Woertz and co-workers conducted

529

an LCA study on the production of algal biodiesel, in which CO2 and WW were

530

considered as inputs for cultivation (scenario 7).83 The energy demand for CO2 supply

531

and distribution was ca. 17 % of the total demand (1.05×107 MJ year-1). Based on

532

detailed mass and energy balances, calculated GHG emissions were 70 % lower than

533

those of conventional diesel fuel, meeting the minimum 50 % reduction requirements

534

set by EPARFS2 and even below the GHG reduction threshold for 2018 set by the

535

European Directive on Renewable Energy.97 GHG emissions from algal biodiesel were

536

estimated at 29 g CO2eq MJ-1, beneath the level for low-sulfur diesel and biodiesel from

537

soya bean (83 g CO2eq MJ-1), at least when taking into consideration also the indirect

538

land use changes. The lower oil content (10 %) implies low biodiesel but high LEA yield

539

(LEA was used for the generation of electricity via AD). This case resulted in GHG

540

emissions as low as to 3 g CO2eq MJ-1, showing again the prominent role of the

541

electricity demand in the GHG emission indicator. The energy balance (NER) in turn

542

was high (2.2 MJ per MJ-1 FAME). The authors also quantified the increase in emissions

543

associated with the use of chemical fertilizers. Having a manufacturing GHG emission

544

factor of 3 g CO2eq g-1 nitrogen, emissions increased with 6 % with respect to the 89 %

545

N recycle case represented in scenario 7, requiring a fertilizer input of 116,250 kg

546

nitrogen year-1.


28

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547

3.1.2 Algal Renewable Diesel (RD)

548

Figure 5 shows the NER and GHG emissions associated with the production of algal

549

derived biofuels. The general trend is that the results do not vary a lot whether

550

producing FAME or RD. This was confirmed by Zaimes and Khanna (2010) (scenarios

551

2a-2c in Figure 4 compared to scenarios 4a-4c in Figure 5).91 The impact of the

552

scenario prior to the final conversion step was higher. The results showed that HTL

553

would be the preferred pathway rather than lipid extraction or pyrolysis, at least from an

554

LCA point of view. Frank et al. (2011; 2013) compared RD production obtained either

555

via lipid extraction (scenario 3a) or via HTL (scenario 3b), both followed by HT.90,103 Key

556

variables were the biocrude yield and nitrogen content, along with the hydrogen

557

demand for HT. They concluded that too high HTL yields impedes the valorization of the

558

solid LEA residue via AD (too low C:N ratios). Instead, catalytic hydrothermal

559

gasification (CHG) of LEA to biogas and ammonia was used for the production of CHP

560

in the HTL route. Despite the fact that HTL requires high pressure and temperature, the

561

direct energy use was higher for the WSE route as pressure-homogenization was

562

required in the latter (high electricity demand). The HTL route required ca. 2 times less

563

algal biomass to reach similar RD yields compared to the WSE route. Still, WSE

564

resulted in considerably lower CHG emissions, because after nutrient recycling (NR)

565

from the residual aqueous phase up to 5 times less ammonia and 1.5 times less

566

phosphorus were required additionally, whereas in the HTL process an important

567

amount of nitrogen ends up in the biocrude (5.7 wt% N compared to 0.2 wt% N in WSE

568

lipid oil). The lower GHG emissions in the WSE route were also the result from AD (low

569

heat demand + electricity generation). The scenarios 2a-2b (WSE route) and 12a-12b


29


Page 30 of 62

570 571

Figure 5. NER and GHG emissions associated with the production of algal derived RD

572

production, obtained after hydrotreating of extracted lipid oil (green), HTL biocrude (blue)

573

and pyrolysis oil (red). Gaseous fuels (black) are shown for comparison.


30

Page 31 of 62


574

(pyrolysis route) showed high NER and GHG emissions, as in both belt drying was

575

used.102 The effect of drying is also observed by comparing the results in scenarios 1a-

576

1b (for WSE only dewatering required) compared to scenarios 10a-10d (dewatering +

577

thermal drying required for intake in pyrolysis unit). Another example of the impact from

578

drying activities is shown in the scenarios 7a-7b (HTL, minimal dewatering) and

579

scenarios 11a-11b (pyrolysis, dewatering + thermal drying).105

580

By using flue gas as carbon source significant reductions in GHG emissions can be

581

achieved.55,106,107 Rickman et al. (2013) conducted an LCA study on utility-connected

582

systems to evaluate the feasibility of integrating algae cultivars in PP for CO2

583

biosequestration.108 As considerable energy requirements were associated with

584

pumping of large gas and fluid volumes, the authors pointed out the need of integrated

585

systems which effectively can reduce CO2 emissions. The costs and credits associated

586

with the processing of flue gas is not fully clear in the current literature, some do take

587

these into account and others not. Recently, Laurens (2017) also claimed the urgent

588

need for more detailed studies on how microalgae cultivars could be integrated within or

589

close to existing industrial facilities,5 including PP, natural gas plants, bioethanol and

590

ammonia plants, each of them having different CO2 purities and supply costs.109

591

3.2. Water Footprint

592

The water footprint (WF) is the total freshwater quantity embedded in a production

593

scenario, including ground and surface water (blue WF) and rain water (green WF). This

594

indicator is important, particularly in regions that experience water shortage and

595

aridification risks. The WF depends on the local climate and the actual process

596

design.110 Estimation of the WF is a complex task as it is highly sensitive to evaporation

597

rates and hydraulic retention times. Yang et al. (2011) reported 3727 kg water per dry


31


Page 32 of 62

598

kg algal biomass.111 A better comparison is based on the water quantity embedded

599

against the energy content of the biofuel produced. WF between 1-62 L water per MJ−1

600

of energy produced were reported.87,112,113 By recirculating harvest water the WF was

601

reduced by 84 % and by using sea water it was further reduced to 90 %. However,

602

using seawater has indirect effects on MFSP, NER and GHG, as the presence of salt is

603

considered as ´dead´ mass to be processed. In comparison, the WF of lignocellulosic

604

bioethanol, corn bioethanol and soya biodiesel were estimated at 11-171, 1-18, 2-91 L

605

MJ-1.113,114 Data from pilot-scale reactors (ORP and PBR) operated in 3 different

606

seasons (summer, fall and winter) were considered by Pérez-López et al. to evaluate

607

the environmental burdens.115 The energy use for temperature regulation contributed

608

significantly. The production of the high added-value phycocyanin was reported by

609

Papadeki et al., including associated environmental impacts to evaluate the

610

sustainability of the extraction process.116 The recovery of this bioactive compound was

611

highly dependent on the amount of biomass, consumables and energy supplied.

612

Advanced extraction processes such as ultrasound assisted extraction were

613

recommended to decrease the environmental impact.

614

The impact on the water usage from large-scale cultivation of microalgae is still on

615

debate. Introduction of large water volumes at high temperature may have effects on

616

the evaporation rate, especially in arid regions. Increasing the reutilization of harvest

617

water and the adaption towards seawater input are research areas which deserve

618

further attention.117 The recovery of nutrients from harvest water, which otherwise also

619

would be an environmental burden, also reduces the WF.115 The high impact of

620

freshwater availability in the USA on the algae cultivar location was recently


32

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621

demonstrated by Venteris et al.119 Important in the decision taking is the actual salinity

622

of the freshwater source, which should vary close to the salinity of the cultivars to

623

reduce the amount of make-up water. Prospects are therefore strain dependent. The

624

cultivation of microalgae in the vicinity of thermal PP is attractive, not only for CO2

625

biosequestration but also because PP consume large amounts of cooling water, as

626

recently quantified for China.120 The cooling water could be re-utilized for microalgae

627

cultivation and the WW could be treated to use again as cooling water, closing the water

628

cycle.

629

3.3 Toxicity and Biodiversity in Aquatic Ecosystems

630

The water quality and consumption are important sustainability indicators of aquatic

631

cultivation systems. They depend on the algal strains used and on the microbial

632

ecology. Many algae species can be grown in low-grade WW to levy pressure on

633

natural freshwater resources.121-123 By doing so, alongside WW remediation credits, it is

634

also possible to procure water and nutrients at lower cost for cultivation at large scale. A

635

wide range of pollutants can be assimilated by microalgae including carbon, NOx, SOx

636

and heavy metals.4 Microalgae can use both organic and inorganic C, N (in the form of

637

ammonium, nitrate or nitrite) and P. Elevated levels may trigger negative impacts such

638

as algal blooms and oxygen depletion during nights (due to decomposition of dead

639

algae).124 Eutrophication due to accidental release of culture media into the environment

640

is a potential risk for the ecologic biodiversity.125,126 The bioremediation of polluted water

641

streams suffering from algal blooms could generate additional biomass which can be

642

used to increase the biofuel production capacity, provided residual N, P and S can be

643

controlled properly. Large-scale cultivation of microalgae can be considered as a


33


Page 34 of 62

644

“controlled eutrophication” process and needs to be well managed by an adequate

645

nutrient supply and by harvesting at regular intervals.

646

3.4 Wastewater Treatment and Nutrient Recycle

647

Microalgae cultivation can fit in as a secondary treatment unit in traditional WW

648

treatment facilities, with possibilities to obtain effluents within standards set for surface

649

discharge.51 This approach alleviates negative impacts on the aquatic biodiversity and

650

allows to recover valuable nutrients, which favors the overall energy balance and GHG

651

emissions.87,127 The use of WW not only can reduce the chemical fertilizer demand but it

652

can also minimize the resources needed for chemical WW treatment. Several types of

653

wastewater (WW), produced by municipal (MWW), agricultural (AWW) or industrial

654

(IWW) sources, may be used for microalgae cultivation.127-131 Microalgae based

655

research has demonstrated the potential and the challenges in combining WW nutrient

656

removal and biofuel production.123,132-142 The algae growth is strongly affected by the

657

WW composition and even for the same WW source population dynamics exist.140

658

Table S3 in the Supporting Information shows the potential of various microalgae

659

strains in different WW treatments. The results show that especially AWW sources

660

provide higher biomass and lipid productivities, which plays in favor of algal farming in

661

rural areas, though MWW sources in non-rural areas may provide nutrients on a larger

662

and more continuous basis. In animal manure effluents, the N:P ratio is so high that it

663

cannot be remediated by crops only, but too high nutrient concentrations in AWW may

664

require dilution first, otherwise it would reduce light penetration considerably.144 Dilution

665

however has a great impact on nutrient removal efficiency, biomass accumulation and

666

lipid productivity.142,145-148 Research has been carried on primary and secondary treated

667

MWW, essentially in activated sludge plants, as well in municipal centrates obtained


34

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668

from the sludge centrifuge. Municipal centrate has been found as an encouraging

669

growth medium, especially for Chlorella which provided the highest lipid productivity

670

reported.132 Industrial WW contains much lower levels of phosphorous and ammonia,

671

and in some occasions it is enriched with heavy metals, which can affect growth rates.

672

Ruiz-Martinez and co-workers studied the removal of N and P from the effluent of a

673

submerged anaerobic membrane bioreactor.145 They used a lab-scale PBR in which

674

algae were cultured in semi-continuous mode for 42 days, assuring stable pH in the

675

growth medium by adding CO2. Despite the variations in N and P concentrations, the

676

anaerobic effluent resulted to be suitable for growing microalgae, with biomass

677

productivities reaching 0.23 g L-1 day-1 and nutrient removal efficiencies of 67 and 98 %

678

for NH4+ and PO43- at optimized conditions, respectively. Similarly, submerged

679

membrane photobioreactors (MPBR, see Figure S7) were recently reviewed by Luo et

680

al. for microalgae cultivation applied to WW treatment.148 MPBR technology combines

681

conventional PBR with a membrane to allow higher flexibility for WW feed composition

682

and operational conditions. MPBR play an important role in optimization, but the

683

challenge is to avoid fouling which can lead to operational problems. Applying

684

immobilized microalgal technology in MPBRs has the potential to mitigate fouling risks.

685

4. SOCIO-ECONOMIC IMPACT

686

Data available till-date focus on benefits and hurdles related to the economy of the

687

production process itself, rather than on socio-economic concerns.149 Only few reviews

688

on the sustainability of microalgae production systems included socio-economic

689

impacts.25,26,150 Qualitative or semi-quantitative indicators include social well-being and

690

acceptability. Social well-being refers to fulfilment of basic human needs such as food


35


Page 36 of 62

691

security and employment. Social acceptability includes factors such as public opinion,

692

transparency, effective stakeholders’ participation and waste management risks.

693

Recently a set of more specific socio-economic indicators were proposed by the US

694

Department of Energy:109 food security, employment, ROI, NPV, energy security

695

premium, depletion of non-renewable energy, fuel price volatility, trade volume and

696

terms, effective stakeholder participation, transparency, public opinion, income and

697

works days lost due to injury. The public confidence in the microalgae based industry is

698

hindered by the lack of reliable information, production transparency and by the data

699

heterogeneity on health and environmental issues.151 Aspects of the public opinion

700

include potential for generating new jobs, odours, esthetical aspects, water usage,

701

recent media reports, perception towards potential use of genetically modified algae and

702

already established perceptions such as rise in food prices and deforestation associated

703

with first generation biofuels.152,153 One important benefit of setting up an algae based

704

industry is the projection and creation of new jobs in the farming, refining and supply

705

sector.150-153 Established algae companies have reported considerable numbers of jobs

706

for pilot plants.5,28 Algae based industry and rural development can support each other

707

mutually, as land costs in rural areas are lower and biomass transport costs strongly

708

motivates biofuel processing near the algal cultivars.75,150 Depending on whether the

709

project is local or global, the public acceptability may vary.128 Technologies that are

710

accepted in one region may be rejected in other regions. As the large scale algal

711

biobased industry is not established yet, the public acceptability may vary over time.

712

The competition for the use of arable land is an important aspect, often widely

713

discussed in different social communities. Laurens (2017) proposed to perform


36

Page 37 of 62


714

Resource Assessment in addition to LCA, quantifying the total amount of product

715

manufactured using a specific process given the amount of input resources (land, water,

716

CO2 and nutrients) available within a specific area.5 These data should indicate how

717

much extra resources should be transported from more distant areas. A study published

718

by Wigmosta et al. (2011) considered the land, water and resource availability in the

719

USA, and concluded that ca. 4.3×107 ha of available land was suitable for algal

720

cultivation open ponds, which corresponded with a potential production of 2.20×1011 L

721

of algal oil per year, equivalent 48 % of the annual petroleum imports in the USA

722

(2011).154 It was estimated that 5.5 % of USA land area would be required in addition to

723

reach these levels of production. The water consumption, however, would exceed 2-3

724

times the current agricultural water needs. The impact of land use can be minimized to

725

a great extent as algae can be cultivated on marginal lands. However, with regard to

726

temperature and light intensity, many areas identified as suitable for algae cultivation

727

are tropical, where the availability of water is limited and evaporation losses are

728

considerable (arid zones). Concerns still exist in the public opinion regarding the use of

729

land for large scale biofuel production.150 Impacts resulting from direct changes (gas flux

730

due to construction of ponds on arable land) and indirect changes (purpose of land used

731

and associated emissions) and the pressure on freshwater availability can be minimized

732

when off-shore cultivation of (macro)algae is implemented. For instance, the

733

“Submariner” research group studied the prospects of associating algae cultivation with

734

an off-shore wind farm in the Baltic sea, with annual algal biomass yields of 1.2 kg per

735

m2 sea surface.155

736

5. CONCLUSIONS AND FUTURE PROSPECTS


37


Page 38 of 62

737

Microalgae can play an important role in the development of sustainable production

738

systems. Sustainability is the capacity of a process or system to continue while being

739

able to meet the needs of future generations. In practice, the sustainability of

740

microalgae production systems is evaluated based on techno-economic assessment

741

(TEA), life cycle analysis (LCA) and socio-economic impact. Recent TEA studies

742

pointed out that current projections for large scale production of microalgal biofuels are

743

not for the near future, due to their low cost competitiveness as compared to fossil fuels

744

and biofuels from other biomass sources. Although the data are highly heterogeneous

745

in nature (depending on the model assumptions and boundaries), the studies agreed in

746

the fact that biomass productivity was the parameter with the highest impact. The high

747

cost is mainly associated with the high energy demand for algae cultivation, harvesting

748

and drying. Flocculation combined with centrifugation or filtration technology is actually

749

the most cost-effective harvesting method. Conversion methods that directly act on

750

diluted wet algae biomass slurry are highly desired to reduce the effect of drying, such

751

as hydrothermal liquefaction (HTL) and anaerobic digestion (AD). In large scale biofuel

752

production, the financial feasibility of multi product scenarios is improved signifcantly

753

compared to single product scenarios. The unit production costs of high added-value

754

chemicals are much higher, as they are typically produced in non-optimal growth

755

conditions and at smaller scales. But, these costs are countered by a high revenue and

756

therefore their commercial production has better projections for the near future. Still,

757

these compounds have limited market niches and volumes at present. LCA of various

758

algal biofuel production scenarios have shown considerable variations, not only

759

depending on the scenario input parameters but also depending on model assumptions


38

Page 39 of 62


760

and boundaries. Recent studies have demonstrated the positive effect of the integration

761

of renewable energy technologies within algal cultivars to reduce the greenhouse gas

762

emissions emitted during the life cycle of algal biofuels. Whereas on the long-term algal

763

biotechnology will play an important role to increase the biomass productivity,

764

renewable energy technologies can offer innovative solutions on the short term. Other

765

imminent algae based research fields include the integration of cultivation with industrial

766

CO2 point source facilities and the use of wastewaters (WW) and/or seawater to reduce

767

the nutrient requirements and the water footprint. Agricultural WW sources can provide

768

a sustainable nutrient source for cultivation in rural areas, whereas municipal WW may

769

be used for cultivars in urban areas. Finally, as part of the overall sustainable analysis,

770

the socio-economic benefits and burdens require a more uniform and quantified study in

771

the final evaluation.

772

AUTHOR INFORMATION.

773

Corresponding Author

774

*Phone: +86 10 6278 4701; e-mail: [email protected].

775

Notes

776

The authors declare no competing financial interest.

777

Author contributions

778

The manuscript was written through contributions of all authors. All authors have given

779

approval to the final version of the manuscript. ‡These authors contributed equally.

780

ACKNOWLEDGEMENTS


39


Page 40 of 62

781

This work was supported by the Tsinghua University Initiative Scientific Research

782

Program (grant number: 20161080094) and National Recruitment Program of Global

783

Youth Experts (The National Youth 1000 – Talent Program) of China (grant number:

784

20151710227).

785

LIST OF ACRONYMS

786

AD

anaerobic digestion

787

ASPFT

advanced supercritical pulverized fuel technology

788

ATS

algal turf scrubbers

789

AWW

agricultural wastewater

790

BF

bioflocculation

791

C

Centrifugation

792

CHF

chitosan flocculation

793

CHG

catalytic hydrothermal gasification

794

CHP

combined heat and power

795

CAF

cationic flocculation

796

DAF

dissolved air flotation

797

DSE

dry solid extraction

798

FAME

fatty acid methyl esters

799

FARM

Farm-level Algae Risk Model

800

FPPBR

flat panel photobioreactors

801

FT

Fischer-Tropsch synthesis

802

GHG

greenhouse gas

803

HT

hydrotreating


40

Page 41 of 62


804

HTL

hydrothermal liquefaction

805

IGCC

integrated gasification combined cycle

806

IWW

industrial wastewater

807

LCA

Life cycle analysis

808

LCI

Life cycle inventory

809

LEA

lipid extracted algal biomass

810

MPBR

membrane photobioreactors

811

MWW

municipal wastewater

812

NER

net energy ratio

813

NGCC

natural gas combined cycle

814

NPV

net present value

815

NR

nutrient recycling

816

ORP

open raceway ponds

817

PBR

photobioreactors

818

PF

Pressure filtration

819

PTW

pump to wheels

820

PE

photosynthetic efficiency

821

PP

power plant

822

PPC

paddle wheel pond circulation

823

RABR

rotating algal biofilm reactor

824

SV

Switch-value

825

TEA

techno-economic assessment

826

TPBR

tubular photobioreactors


41


827

WSE

wet solid extraction

828

WTP

well to pump

829

WTW

well to wheels

830

WW

wastewater

831

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