Life cycle assessment of microalgae systems for wastewater treatment and bioproducts recovery: natural pigments, biofertilizer and biogas

1 Life Cycle Assessment of microalgae systems for wastewater treatment and 1 bioproducts recovery: natural pigments, biofertilizer and biogas 2 3 Larissa T. Arashiroa,b, Irene Josaa,c, Ivet Ferrera, Stijn W.H. Van Hulleb, Diederik P.L. 4 Rousseaub, Marianna Garfía1 5 6 aGEMMA - Group of Environmental Engineering and Microbiology, Department of Civil and 7 Environmental Engineering, Universitat Politècnica de Catalunya · BarcelonaTech, c/ Jordi 8 Girona 1-3, Building D1, 08034 Barcelona, Spain 9 bLIWET – Laboratory of Industrial Water and Ecotechnology, Department of Green Chemistry 10 and Technology, Ghent University Campus Kortrijk, Sint-Martens Latemlaan 2B, 8500 11 Kortrijk, Belgium 12 cC3S - Concrete Sustainability and Smart Structures, Department of Civil and Environmental 13 Engineering, Universitat Politècnica de Catalunya · BarcelonaTech, c/ Jordi Girona 1-3, 14 Building D1, 08034 Barcelona, Spain 15 16 Arashiro, L.T., Josa, I., Ferrer, I., Van Hulle, S.W.H., Rousseau, D.P.L., Garfí, M.* (2022) 17 Life Cycle Assessment of microalgae systems for wastewater treatment and bioproducts 18 recovery: natural pigments, biofertilizer and biogas. Science of the Total Environment, 847, 19 157615 20 21 22 1Corresponding author: Tel: +34 9340 16412 E-mail address: marianna.gar[email protected] (M. Garfí). 2 Abstract 23 The aim of this study was to assess the potential environmental impacts associated with 24 microalgae systems for wastewater treatment and bioproducts recovery. In this sense, a Life 25 Cycle Assessment was carried out evaluating two systems treating i) urban wastewater and ii) 26 industrial wastewater (from a food industry), with the recovery of bioproducts (i.e. natural 27 pigments and biofertilizer) and bioenergy (i.e. biogas). Additionally, both alternatives were 28 compared to iii) a conventional system using a standard growth medium for microalgae 29 cultivation in order to show the potential benefits of using wastewater compared to typical 30 cultivation approaches. The results indicated that the system treating industrial wastewater with 31 unialgal culture had lower environmental impacts than the system treating urban wastewater 32 with mixed cultures. Bioproducts recovery from microalgae wastewater treatment systems can 33 reduce the environmental impacts up to 5 times compared to a conventional system using a 34 standard growth medium. This was mainly due to the lower chemicals consumption for 35 microalgae cultivation. Food-industry effluent showed to be the most promising scenario for 36 bioproducts recovery from microalgae treating wastewater, because of its better quality 37 compared to urban wastewater which also allows the cultivation of a single microalgae species. 38 In conclusion, microalgae wastewater treatment systems are a promising solution not only for 39 wastewater treatment but also to boost the circular bioeconomy in the water sector through 40 microalgae-based product recovery. 41 42 Keywords: Bioproducts; Environmental impact assessment; Life Cycle Assessment; 43 Microalgae; Wastewater 44 45 3 1. Introduction 46 Microalgae have shown a great potential for the production of several bioproducts with a wide 47 variety of applications such as biofuels and chemicals as well as food and feed (Christaki et 48 al., 2015; Michalak and Chojnacka, 2015; Nagappan et al., 2021; Spolaore et al., 2006). The 49 advantages of using microalgae are their high productivity, the possibility to grow on marginal 50 land in fresh or saltwater, which can avoid competition with food crops, and the option of 51 combining biomass growth with the treatment of waste streams (Clarens et al., 2011; Ahmad 52 et al., 2022). 53 Natural pigments from microalgae, which are particularly strong dyes even at very low 54 concentrations (parts per million), are now strongly demanded by the market as renewable 55 natural colour enhancers for foods and feeds, which simultaneously provide certain health 56 benefits (Christaki et al., 2015; Villaró et al., 2021). Among the pigments present in microalgae 57 cells, the phycobiliproteins have important applications in the pharmaceutical, food and 58 cosmetic industry due to their fluorescence properties (Cuellar-Bermudez et al., 2015; Qiang 59 et al, 2021; Dagnino-Leone et al., 2022). Phycobiliproteins have been extracted and purified 60 from several microalgae species, but commercial production is mainly from Arthrospira spp. 61 (Spirulina) for phycocyanin, and Porphyridium spp. for phycoerythrin (Borowitzka, 2013; 62 Christaki et al., 2015; Bayu et al., 2022). In particular, A. platensis (Spirulina) is widely chosen 63 as a host for phycocyanin production merely because of its availability and favourable growing 64 conditions rather than due to the particular qualities of its pigments (Eriksen, 2008; Chaiklahan 65 et al., 2022). A. platensis (Spirulina) tolerates alkaline conditions and is grown at pH values up 66 to 10.5, being among the few photoautotrophic microorganisms able to grow in open ponds 67 without high risks of being out-competed by contaminating organisms (Richmond and 68 Grobbelaar, 1986; Mona et al., 2021). 69 4 Both microalgae biomass and residual biomass generated from the production of natural 70 pigments can be used for biogas and biofertilizer production (Solé-Bundó et al., 2017; Ramos-71 Suárez et al., 2020; Ammar et al., 2022). On the one hand, the high potential of microalgae to 72 produce biofuels, such as biogas, has been intensively researched in the last decades (Iyovo et 73 al., 2010; Arias et al., 2018; Solé-Bundó et al., 2019). In comparison to other biofuels like 74 bioethanol or biodiesel, the process used to obtain biogas does not require complex extraction 75 methods (Solé-Bundó et al., 2019). On the other hand, biofertilizers are obtained from the solid 76 phase of the digestate and represent a more environmentally friendly alternative to synthetic 77 fertilizers (Albuquerque et al., 2012). 78 Although the demand for natural pigments is increasing and microalgae are considered a 79 potential candidate for natural pigments (e.g. phycobiliproteins) production, the requirement 80 of huge quantities of water and chemicals (i.e. nutrients) in large scale systems leads to high 81 costs and further hinders the production and commercialisation of these bioproducts. 82 Microalgae cultivation using wastewater and/or recycled water has been recently explored as 83 a potential solution to produce natural pigments (Acién et al., 2016; Delrue et al., 2016; Ho et 84 al., 2018; K. Li et al., 2019; Jiang et al., 2022). In addition to this, the potential of applying 85 anaerobic digestion at the end of the process to recover bioenergy from the residual biomass is 86 interesting in environmental and economic terms (Ramos-Suárez et al., 2020). 87 Even though previous studies have shown the technical feasibility of such processes, 88 little is still known regarding the environmental implications of using recycled water for natural 89 pigment, biogas and biofertilizer recovery from microalgae. Evaluating the environmental 90 performance of these processes is particularly important to support informed decision-making 91 processes, as well as for identifying the main bottlenecks to be addressed during the scale-up 92 towards sustainable industrial facilities (Pérez-López et al., 2017). It needs to be mentioned 93 that other authors have carried out environmental performance analyses of microalgae 94 5 cultivation using wastewater (Arashiro et al., 2018) and natural pigments recovered from 95 microalgae systems with standard growth media (Papadaki et al., 2017). However, to the best 96 of the authors’ knowledge, there is still no study that analyses the environmental impacts of 97 microalgae-based systems and bioproducts recovery using wastewater and including natural 98 pigment recovery. 99 In light of the above, it is essential to better understand which are the environmental 100 impacts of bioproducts recovery from microalgae-based systems treating wastewater and using 101 standard grown medium. To this aim, this paper provides, for the first time, a comparative Life 102 Cycle Assessment (LCA) of two microalgae-based systems for wastewater treatment and 103 bioproducts recovery (i.e. natural pigments, biofertilizer and biogas): i) a high rate algal ponds 104 (HRAPs) system treating urban wastewater followed by closed photobioreactors (PBRs) 105 cultivating a mixed culture dominated by cyanobacteria; and ii) an up-flow anaerobic sludge 106 blanket (UASB) reactor treating food-industry wastewater followed by HRAPs cultivating A. 107 platensis (Spirulina). Moreover, both scenarios were compared to a conventional system 108 (HRAPs) for bioproducts recovery using a standard growth medium. The main environmental 109 burdens of each option were evaluated to compare their performances and to identify 110 bottlenecks for up-scaling. 111 112 2. Materials and methods 113 2.1 Wastewater treatment systems description 114 The studied systems were hypothetical full-scale wastewater treatment plants based on 115 extrapolation from pilot-scale studies (from 5 up to 600 m2). The systems were designed to 116 serve a population equivalent of 10,000 p.e. and treat a flow rate of 1,500 m3/d. For the 117 6 microalgae-based system treating urban wastewater (hereafter referred to as scenario UWW), 118 the design parameters were based on experimental results obtained in lab-scale and pilot 119 systems (5 m2) located at the Universitat Politècnica de Catalunya-BarcelonaTech (UPC) 120 (Barcelona, Spain) (García et al., 2006, 2000; Gutiérrez et al., 2016; Passos and Ferrer, 2014; 121 Solé-Bundó et al., 2019, 2017). This scenario is a combination of HRAPs for urban wastewater 122 treatment and PBRs for cyanobacteria biomass cultivation based on the system previously 123 described in Arashiro et al. (2020b). The flow diagram of this case study is shown in Figure 1 124 (1) and the characteristics and design parameters are listed in Table 1. Firstly, it comprises a 125 primary settler (hydraulic retention time (HRT)=2.5 h) followed by four HRAPs working in 126 parallel, cultivating a mixed culture of green microalgae. From these units, wastewater goes 127 through a secondary settler (HRT=3 h) where microalgal biomass is harvested and separated 128 from wastewater. Part of the harvested microalgal biomass (2 and 10% on a dry weight basis 129 in summer and winter, respectively) is recycled in order to enhance spontaneous flocculation 130 (bioflocculation) and increase microalgae harvesting efficiency (Gutiérrez et al., 2016). The 131 remaining harvested biomass is thickened (HRT=24 h) and co-digested with primary sludge 132 (35 °C, 20 days). In this context, the HRT of each HRAP has to be modified over the year (8, 133 6 and 4 days) according to weather conditions (i.e. solar radiation and temperature) in order to 134 accomplish wastewater treatment and meet effluent quality requirements for discharge 135 (Arashiro et al., 2018; García et al., 2000; Gutiérrez et al., 2016). For this reason, it was 136 considered that during the summer months (from May to July) only two HRAPs work in 137 parallel (HRT=4 days), whereas all of them are operated during winter months (from 138 November to April) (HRT=8 days). During the rest of the year (from August to October), the 139 HRT is 6 days (3 HRAPs working in parallel). Secondly, the cultivation of cyanobacteria-140 dominated biomass is done in hybrid tubular PBRs, which are tubular horizontal semi-closed 141 reactors, each one consisting of 2 lateral open tanks made from polypropylene connected 142 7 through 16 low-density polyethylene tubes (García et al.. 2018). In this study, the design of the 143 PBRs was based on a demo scale plant, which is described elsewhere (García et al., 2018; 144 Uggetti et al., 2018). For that, most of the HRAP effluent is discharged into a surface water 145 body, but part of it (6.5%) is used to support the cyanobacteria-dominated biomass growth. 146 The secondary effluent is filtered (to avoid any possible grazer contamination) and used to 147 dilute the centrate (the liquid part of digestate) from the microalgae anaerobic digestion unit. 148 The effluent of the PBRs goes through a tertiary settler (HRT=3 h) where microalgal biomass 149 is harvested and separated from wastewater that is discharged into a surface water body. The 150 microalgae biomass is then centrifuged and the biomass paste is used for phycobiliproteins 151 recovery, which is done through ultrasound extraction with phosphate buffer. The residual 152 biomass (after extraction) is also used as a substrate for the anaerobic digester. The biogas 153 produced is then converted into electricity and heat in a combined heat and power (CHP) unit, 154 while the centrate is recirculated to the PBR and the solid part of the digestate is transported 155 and reused in agriculture as biofertilizer. 156 For the microalgae-based system treating industrial wastewater (hereafter referred to as 157 scenario IWW), the design parameters were based on data obtained by a company that produces 158 plant-based food (located in Wevelgem, Belgium) and experimental results obtained in lab-159 scale systems at Ghent University (Kortrijk, Belgium) (Arashiro et al., 2020a). This scenario 160 is a combination of a UASB reactor (2 m3), to reduce the organic matter concentration of the 161 wastewater, and HRAPs cultivating A. platensis (Spirulina). The flow diagram of this case 162 study is shown in Figure 1(2) and the characteristics and design parameters are listed in Table 163 2. Firstly, the industrial wastewater goes through a drum sieve (0.5 mm) to remove the large 164 particles. The wastewater is then treated in a UASB (HRT=30 h), from which the biogas 165 produced is converted into electricity and heat through a CHP unit. The UASB effluent is 166 filtered to remove suspended solids and the solids from both the UASB (digestate) and the 167 8 filtration process (retained solids) are hypothetically transported and reused in agriculture as 168 biofertilizer. After filtration, the wastewater is mixed with seawater to ensure enough salinity 169 to cultivate Spirulina biomass in the HRAPs. The portion of the seawater was estimated in 170 order to reach a similar concentration as the study described in (Arashiro et al., 2020a) (75% 171 wastewater and 25% seawater, v/v). In this scenario, the HRT of each HRAP was also modified 172 over the year (8, 6 and 4 days) assuming similar weather conditions to the first scenario 173 (scenario UWW). The effluent from HRAPs goes through a secondary settler (HRT=3 h) where 174 microalgal biomass is harvested and separated from the treated water. The microalgae biomass 175 is then centrifuged and the biomass paste is used for natural pigments recovery, which is done 176 through ultrasound extraction with phosphate buffer (Arashiro et al., 2020). The residual 177 biomass (after extraction) is also used as a substrate for the UASB. 178 For reference purposes, the potential environmental impacts of the microalgae-based 179 wastewater treatment systems were compared to those generated by a conventional microalgae 180 cultivation system. For that purpose, the design of a typical facility for natural pigments 181 production from A. platensis (Spirulina) using a standard growth medium (SGM) was 182 considered, as described by Papadaki et al. (2017). The flow diagram of this study case 183 (hereafter referred to as scenario SGM) is shown in Figure 1(3) and the characteristics and 184 design parameters are listed in Table 3. It comprises HRAP systems to cultivate microalgae, 185 followed by a centrifuge to recover the biomass paste, which is further used for the natural 186 pigments recovery. As in the previous scenarios, an anaerobic digester is also considered to 187 generate biogas (later converted into electricity and heat in a CHP unit) and the digestate is 188 transported and reused in agriculture. 189 9 2.2 Life cycle assessment 190 The LCA was conducted following the ISO standards (ISO, 2006, 2000) in order to 191 assess and quantify the potential environmental impacts of each scenario under study. The 192 technical framework for LCA methodology consisted of four phases: 1) goal and scope 193 definition; 2) inventory analysis; 3) impacts assessment; and 4) interpretation of the results 194 (ISO, 2006). The following sub-sections describe the specific content of each phase. 195 2.3 Goal and scope definition 196 The goal of this study was to analyse and compare the potential environmental impacts 197 associated with different microalgae-based systems for wastewater treatment and bioproducts 198 (i.e. natural pigments, biogas and biofertilizer) recovery and to identify the vulnerable aspects 199 in which the technologies studied can potentially improve in terms of environmental 200 performance. To this aim, two configurations were compared: 201 a) an urban wastewater treatment system based on HRAPs followed by PBRs cultivating 202 cyanobacteria-dominated biomass (scenario UWW); 203 b) an industrial wastewater (from a food company) treatment system based on a UASB 204 reactor followed by HRAPs cultivating A. platensis (Spirulina) (scenario IWW). 205 The functional unit (FU) for this comparison was set as 1 m3 of treated water since the 206 main function of the technologies proposed is to treat wastewater (Arashiro et al., 2018). 207 Additionally, both scenarios were compared to c) a conventional microalgae cultivation system 208 using a standard growth medium (scenario SGM) in order to show their benefits compared to 209 conventional cultivation systems. For the comparison of the three scenarios, the FU of 1 kg of 210 microalgal biomass produced (i.e. kilogram of Total Suspended Solids (kgTSS)) was used 211 (Pérez-López, 2017). 212 16 industrial wastewater (scenario IWW), mostly as a result of emissions to water. This is 351 explained by the higher total phosphorous concentrations in the effluent of HRAPs treating 352 urban wastewater (scenario UWW), compared to industrial wastewater (scenario IWW) (Table 353 4 and Table 5). The difference between the effluent quality of the two systems is related not 354 only to its source (one being urban and the other food wastewater) but also to the initial 355 nutrients concentrations when they enter the systems (industrial wastewater with 356 concentrations about 2-fold higher than urban wastewater, Table 1 and Table 2). Nevertheless, 357 in any case, the effluent concentrations of phosphorus fulfill the discharge requirements set by 358 the regulation. 359 The scenario treating urban wastewater (scenario UWW) shows better environmental 360 performance than the scenario treating industrial wastewater (scenario IWW) in only 2 impact 361 categories: Marine Eutrophication and Mineral Resource Scarcity. Regarding Marine 362 Eutrophication, the latter (scenario IWW) showed significantly higher (by 3.74-fold) 363 environmental impacts than the former (scenario UWW). This can be explained by the treated 364 final effluent quality, in which the industrial wastewater had a higher total nitrogen 365 concentration in the effluent (scenario IWW) (around 10 mg N/L) compared to the urban 366 wastewater scenario (scenario UWW) (around 0.9) (Table 4 and Table 5). Concerning the 367 Mineral Resource Scarcity impact category, the scenario treating industrial wastewater 368 (scenario IWW) showed a slightly higher impact (only by 1.2-fold) than the scenario treating 369 urban wastewater (scenario UWW), mostly due to the amount and type of construction 370 materials required for the HRAPs. Indeed, the amount of steel needed in the first case (scenario 371 IWW) is around 29% higher than the last (scenario UWW) (Table 4 and Table 5). This is 372 related to the higher surface area of HRAPs estimated for this scenario (scenario IWW) (Table 373 1 and Table 2) since the industrial wastewater is mixed with seawater at a 75/25% (v/v%) ratio 374 to ensure enough salinity level. 375 17 Electricity consumption was by far the most impacting aspect, in 5 impact categories (i.e. 376 Global Warming, Stratospheric Ozone Depletion, Human carcinogenic Toxicity, Fine 377 Particulate Matter Formation and Fossil Resource Scarcity), accounting for 43 to 81% of the 378 overall impacts. Next, construction materials were the major contributor to the highest impacts 379 (83 and 85%) in the Mineral Resource Scarcity impact category, but also a secondary 380 contributor in 4 impact categories (i.e. Global Warming, Stratospheric Ozone Depletion, 381 Human carcinogenic Toxicity, and Fossil Resource Scarcity), representing from 13 to 35% of 382 the overall impacts. Subsequently, emissions to water through nutrients were the major 383 contributor to the highest impacts in Freshwater Eutrophication potential (91% in scenario 384 UWW and 84% in scenario IWW) and in Marine Eutrophication potential (72% in scenario 385 UWW and 96% in scenario IWW). Emissions to the air of the scenario treating urban 386 wastewater (scenario UWW) through NH3 volatilisation from HRAPs were the main 387 contributor to Terrestrial Acidification potential (accounting for 64% of the overall impacts) 388 and secondary contributor to Fine Particulate Matter Formation and Marine Eutrophication 389 potentials (39 and 18% of the overall impacts, respectively). Finally, digestate reuse in 390 agriculture was the main contributor in the scenario treating urban wastewater (scenario UWW) 391 for Terrestrial Ecotoxicity potential (accounting for 66% of the overall impacts, due to heavy 392 metals concentrations) and a secondary contributor in the scenario treating industrial 393 wastewater (scenario IWW) for Terrestrial Acidification (accounting for 24% of the overall 394 impacts, due to nitrogen volatilisation). 395 Based on this, in order to improve the environmental performance of the microalgae-396 based systems studied, the following issues should still be studied: 1) increasing energy 397 efficiency by optimising processes (e.g. natural pigments extraction, harvesting), maximising 398 biogas production or integrating renewable sources to reduce impacts related to electricity 399 consumption; 2) improving HRAP design to reduce construction materials consumption (e.g. 400 18 excavation instead of concrete structure); 3) improving nutrients removal efficiencies (e.g. 401 installations in warmer regions); and 4) recovering heavy metals from digestate before 402 application in agriculture. 403 The results shown in this study suggested the use of food-industry effluent (scenario 404 IWW) as a more promising scenario for bioproducts recovery from microalgae treating 405 wastewater mainly for the following reasons: 1) Cultivation system: several researchers have 406 reported that HRAPs are more energetically self-sufficient and more environmentally 407 sustainable than PBRs, especially in cases in which the heat and power requirement of the 408 process can be provided by combusting the methane generated from the anaerobic digestion of 409 the residual microalgae biomass (Moon, 2022; Stephenson et al., 2010; Shormeh Darko, 2022); 410 2) Microalgae biomass: to be deemed suitable for producing natural pigments commercially, 411 microalgae strains have to meet various criteria, such as ease of culture, lack of toxicity, high 412 nutritional value, and presence of digestible cell walls to make the pigments available 413 (Christaki et al., 2015; Siddiki et al., 2022). Based on that, the most frequently used species are 414 Dunaliella salina, Haematococcus pluvialis, Chlorella spp., Muriellopsis spp., Scenedesmus 415 spp., Arthrospira spp. (Spirulina), and Porphyridium spp. (Borowitzka, 2013; Christaki et al., 416 2015; Eriksen, 2008; Ho et al., 2018; Patel et al., 2022; Spolaore et al., 2006). For this reason, 417 cultivating a single species might be a better strategy than mixed cultures. This way, the 418 cultivation parameters can be adjusted accordingly in order to maximise natural pigments 419 recovery; 3) Risks of contamination and social acceptance: the application of the natural 420 pigments recovered in the scenario treating urban wastewater (scenario UWW) is much more 421 limited than in the scenario treating food-industry wastewater (scenario IWW). Indeed, urban 422 wastewater usually contains a wider variety of contaminants (e.g. pathogens, heavy metals, 423 micropollutants) than food-industry wastewater. Although the purity of the final product could 424 be proved to be suitable according to the application of the natural pigments, the cultivation in 425 19 urban wastewater could raise more concerns in terms of social acceptance and regulatory 426 issues, which could hinder industrial-scale production. For this reason, the use of food-427 processing waste streams could be a more appropriate alternative for providing nutrients for 428 microalgae biomass growth while ensuring no risks of contamination. 429 Comparing both scenarios with the conventional system for microalgae-based products 430 production using a standard growth medium (scenario SGM), both systems investigated 431 (scenarios UWW and IWW) showed better environmental performance. The scenario using 432 standard growth medium (Scenario SGM) showed higher environmental impacts than the 433 scenario treating urban wastewater (scenario UWW) in 6 out of 10 impact categories (from 434 1.7-fold to 3-fold higher) (i.e. Global Warming, Stratospheric Ozone Depletion, Terrestrial 435 Ecotoxicity, Human carcinogenic Toxicity, Mineral Resource Scarcity and Fossil Resource 436 Scarcity) (Figure 3). This was mainly due to the lower chemicals consumption for microalgae 437 cultivation. On the other hand, it showed higher environmental impacts than the scenario 438 treating industrial wastewater (scenario IWW) in 8 out of 10 impact categories (from 1.3-fold 439 to 5.3-fold higher) (i.e. all impact categories except for Freshwater and Marine Eutrophication) 440 (Figure 3). As expected, the main contributors to the higher impacts in the scenario using 441 standard growth medium (scenario SGM) were electricity consumption and chemicals input, 442 which represented from 82 to 99% of the overall impacts in all the impact categories evaluated. 443 The only impact categories in which both wastewater systems (scenarios UWW and IWW) 444 showed way higher environmental impacts than the scenario using standard growth medium 445 (scenario SGM) were Marine Eutrophication (from 3.7 to 12-fold higher environmental 446 impacts) and Freshwater Eutrophication (from 2 to 4-fold higher environmental impacts). Yet, 447 it is important to note that these impacts were associated with the discharge of nutrients in the 448 treated effluent, as previously explained. In the case of the scenario using a standard growth 449 medium (scenario SGM), there were no discharges to water bodies, since the inventory was 450 20 based on systems in which all nutrients are taken up by the microalgae by recycling the medium 451 (Papadaki et al., 2017). However, the impacts related to nutrients discharge could be minimised 452 in a full-scale plant, by optimising operational conditions, which could favour even more the 453 use of wastewater for recovering bioproducts and bioenergy. 454 The results in this work are in accordance with previous research on microalgae and 455 valuable compounds production. Ye et al. (2018) carried out a comparative LCA of industrial-456 scale production of Spirulina tablets and found out that the most impacting stage along the 457 entire process was the cultivation, responsible for approximately 60% of the total impacts, 458 followed by harvesting (1-20%) and tablets production (<10%). From the cultivation stage, the 459 growth medium was the major contributor, accounting for 80% of the impacts due to the high 460 nutrients needed for cultivation. In this context, extensive research has been done to identify 461 the advantages and potential risks of either recycling growth medium or using waste streams 462 in order to reduce costs and impacts of cultivation. However, the effects of recycling the 463 medium reported in the literature are contradictory, with some studies revealing positive 464 aspects of recycling (Ho et al., 2018; Y. Li et al., 2019; Wang et al., 2018) while others are 465 highlighting inhibitory effects on biomass growth (Hadj-Romdhane et al., 2013; Loftus and 466 Johnson, 2019). Therefore, the use of wastewater is a considerable option as it provides the 467 necessary nutrients and environmental conditions required for the enhanced metabolite content 468 of microalgae, while being a low-cost media and, thus, a better approach compared to the 469 processing involved by using standard growth media (Alam and Wang, 2019; Cinq-Mars, 470 2022). 471 Finally, microalgae wastewater treatment systems are a promising solution not only for 472 wastewater treatment but also for bioproducts recovery. Indeed, the use of wastewater can 473 reduce the environmental impacts associated with the production of microalgae-based products 474 21 (e.g. natural pigments and biofertilizer) and bioenergy (e.g. biogas), boosting the circular 475 bioeconomy. 476 3.2 Normalisation 477 The normalised results showed that Freshwater Eutrophication, Marine Eutrophication, 478 Terrestrial Acidification and Human carcinogenic Toxicity were the most significant impact 479 categories for all the scenarios considered (Figure 4), which was in accordance with previous 480 LCAs on wastewater treatment systems (Fang et al., 2016; Gallego et al., 2008; Hospido et al., 481 2004). The scenario treating industrial wastewater (scenario IWW) showed to be the solution 482 with the lowest environmental impacts in 3 out of 4 of these impact categories (i.e. Freshwater 483 Eutrophication, Terrestrial Acidification and Human carcinogenic Toxicity potentials). 484 Regarding Marine Eutrophication, operational conditions could be addressed in order to 485 optimise the nitrogen removal efficiency in such a system. 486 The results of the normalization confirmed that the scenario treating industrial 487 wastewater (scenario IWW) is the solution for wastewater treatment and bioproducts recovery 488 from microalgae with the lowest environmental impacts among the compared options. 489 490 4. Conclusions 491 The aim of this study was to compare the environmental impacts of two microalgae-492 based systems for wastewater treatment and bioproducts recovery (i.e. natural pigments, 493 biofertilizer and biogas): i) a high rate algal ponds (HRAPs) system treating urban wastewater 494 followed by closed photobioreactors (PBRs) cultivating a mixed culture dominated by 495 cyanobacteria, and ii) an up-flow anaerobic sludge blanket (UASB) reactor treating food-496 industry wastewater followed by HRAPs cultivating A. platensis (Spirulina). For reference 497 22 purposes, both scenarios were compared to a conventional system for microalgae cultivation 498 using a standard growth medium. 499 Results indicated that the scenario treating industrial wastewater had lower 500 environmental impacts than the scenario treating urban wastewater in 8 out of 10 impact 501 categories (from 1.2-fold to 2.4-fold). This was mainly due to i) the higher amount of biogas 502 generated from industrial wastewater which was converted into bioenergy (electricity and heat) 503 compared to the urban wastewater; ii) the lower emissions to air (i.e. NH3 volatilisation) 504 associated with a lower ammonia concentration in industrial wastewater compared to urban 505 wastewater; iii) the lower concentration of heavy metals in food-derived digestate compared to 506 the urban one. 507 Comparing both scenarios with the conventional system for microalgae cultivation using 508 a standard growth medium, both microalgae wastewater treatment systems investigated 509 showed better environmental performance in most of the impact categories analysed 510 (environmental impacts up to 5-fold lower). This was mainly due to the lower chemicals 511 consumption for microalgae cultivation. 512 On the whole, food-industry effluent showed to be the most promising scenario for 513 bioproducts recovery from microalgae treating wastewater mainly for the following reasons: i) 514 microalgae cultivation in HRAP is more sustainable than PBRs (less energy and construction 515 materials consumption); ii) cultivating a single species might be a better strategy than mixed 516 cultures, since the cultivation parameters can be adjusted accordingly in order to maximise 517 bioproducts recovery; iii) urban wastewater contains a wider variety of contaminants (e.g. 518 pathogens, heavy metals, micropollutants) than food-industry wastewater which raise more 519 concerns in terms of social acceptance and regulatory issues. 520 23 Finally, microalgae wastewater treatment systems are a promising solution not only for 521 wastewater treatment but also for microalgae-based products recovery. Indeed, the use of 522 wastewater can reduce the environmental impacts associated with the production of 523 microalgae-based products (e.g. natural pigments and biofertilizer) and bioenergy (e.g. biogas) 524 boosting the circular bioeconomy. 525 526 Acknowledgments 527 528 This research was funded by the European Union’s Horizon 2020 research and innovation 529 programme under the Marie Skłodowska-Curie Grant Agreement No 676070 (SuPER-W). 530 This communication reflects only the author’s view and the Research Executive Agency of the 531 EU is not responsible for any use that may be made of the information it contains. 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Flow diagram and system boundaries of the three scenarios: 1) scenario UWW: urban 786 wastewater treatment in high rate algal ponds (HRAPs) followed by photobioreactors (PBRs) 787 35 cultivating cyanobacteria-dominated biomass; 2) scenario IWW: industrial wastewater from a 788 food company treated in an up-flow anaerobic sludge blanket (UASB) reactor followed by high 789 rate algal ponds (HRAPs) cultivating A. platensis (Spirulina); 3) scenario SGM: high rate algal 790 ponds (HRAPs) cultivating A. platensis (Spirulina) with a standard growth medium (SGM). 791 792 36 793 794 Figure 2. Potential environmental impacts for the two scenarios: 1) urban wastewater treatment 795 in high-rate algal ponds (HRAPs) followed by photobioreactors (PBRs) cultivating 796 cyanobacteria-dominated biomass (scenario UWW); 2) industrial wastewater from a food 797 company treated in an up-flow anaerobic sludge blanket (UASB) reactor followed by high rate 798 algal ponds (HRAPs) cultivating A. platensis (Spirulina) (Scenario IWW). Values are referred 799 to the functional unit (1 m3 of treated water). 800 37 801 Figure 2. (Continued). 802 803 38 804 Figure 3. Potential environmental impacts for the three scenarios: 1) urban wastewater 805 treatment in high rate algal ponds (HRAPs) followed by photobioreactors (PBRs) cultivating 806 cyanobacteria-dominated biomass (scenario UWW); 2) industrial wastewater from a food 807 company treated in an up-flow anaerobic sludge blanket (UASB) reactor followed by high rate 808 algal ponds (HRAPs) cultivating A. platensis (Spirulina) (Scenario IWW); 3) high rate algal 809 ponds (HRAPs) cultivating A. platensis (Spirulina) with a standard growth medium (SGM) 810 (scenario SGM). Values are referred to the functional unit (1 kgTSS of microalgal biomass 811 produced). 812 813 814 39 815 Figure 3. (Continued). 816 817 40 818 Figure 4. Normalised potential environmental impacts for the three scenarios: 1) urban 819 wastewater treatment in high rate algal ponds (HRAPs) followed by photobioreactors (PBRs) 820 cultivating cyanobacteria-dominated biomass (scenario UWW); 2) industrial wastewater from 821 a food company treated in an up-flow anaerobic sludge blanket (UASB) reactor followed by 822 high rate algal ponds (HRAPs) cultivating A. platensis (Spirulina) (scenario IWW); 3) high 823 rate algal ponds (HRAPs) cultivating A. platensis (Spirulina) with a standard growth medium 824 (SGM) (scenario SGM). 825 826 41 Table 1. Characteristics and design parameters of scenario UWW: urban wastewater treatment 827 in high rate algal ponds (HRAPs) followed by photobioreactors (PBRs) cultivating 828 cyanobacteria-dominated biomass. 829 System characteristics Unit HRAPs PBRs Flow rate m³/d 1500 96.20 Total surface area m² 30000 13000 Channel width m 12 5 Channel length m 625 50 Water depth m 0.4 - Influent concentrations BOD mg O₂/L 300 171 TSS mg/L 150 17 Total Nitrogen mg/L 39 30.6 Total Phosphorus mg/L 5 0.6 Effluent concentrations BOD mg O₂/L <25 79 TSS mg/L <35 35 Total Nitrogen mg/L 0.94 17.7 Total Phosphorus mg/L 3.69 0.6 Design parameters Summer Winter Rest of the year Hydraulic retention time d 4 8 6 5 Number of HRAP/PBR - 2 4 3 52 Average microalgae biomass production g TSS/m²d (HRAP) g TSS/m³d (PBR) 25.8 6.4 10.5 313 Acronyms: Biochemical oxygen demand (BOD); High rate algal ponds (HRAPs); Photobioreactors (PBRs); Total suspended solids (TSS). Summer: May to July; Winter: November to April. 830 831 48 Table 6. Summary of the inventory for scenario SGM: high rate algal ponds (HRAPs) 855 cultivating A. platensis (Spirulina) with a standard growth medium (SGM). Values are referred 856 to the functional unit (FU) (1 kg of Total Suspended Solids (TSS) of microalgal biomass 857 produced). 858 . 859 Scenario SGM Functional unit: 1 kgTSS of biomass Inputs Unit Construction materials HRAPs Concrete 1.035E-03 m³/kgTSS Steel 8.279E-02 kg/kgTSS Centrifuge Steel 2.159E-04 kg/kgTSS Anaerobic digester Concrete 2.714E-06 m³/kgTSS Steel 2.171E-04 kg/kgTSS Operation Energy consumptiona HRAPs 5.904E-02 kWh/kgTSS Centrifuge 1.585E+00 kWh/kgTSS Pigment extraction 5.282E+00 kWh/kgTSS Anaerobic digester 1.818E-03 kWh/kgTSS Total energy consumption 6.927E+00 kWh/kgTSS Chemicalsa Medium Water, salt, ocean 1.58E+00 kg/kgTSS Carbon dioxide 1.13E+01 kg/kgTSS Nitrogen fertilizer 5.47E-02 kg/kgTSS Phosphorus fertilizer 3.72E-02 kg/kgTSS Iron sulphate 7.92E-04 kg/kgTSS Pigments extraction Sodium phosphate 5.437E-03 kg/kgTSS Outputs Emissions to watera Water 3.151E+03 kg/kgTSS Salts 5.182E+00 kg/kgTSS Emissions to aira Cultivation in HRAPs Carbon dioxide 1.529E-01 kg/kgTSS Nitrogen 7.923E-04 kg/kgTSS Digestate for agricultural reuse NH3 3.963E-01 g/kgTSS 49 N 2 O 1.585E-02 g/kgTSS Avoided productsa Biogas cogeneration Electricity production 1.811E-04 kWh/kgTSS Heat production 2.845E-04 kWh/kgTSS Digestate for agricultural reuse N as fertilizer 1.585E+00 g/kgTSS P as fertilizer 1.761E-01 g/kgTSS Pigments as chemical 1.081E-01 kg/kgTSS aAnnual averages 860 861 862 863 864 865