UNIVERSIDADE FEDERAL DO ESPÍRITO SANTO CENTRO TECNOLÓGICO PROGRAMA DE PÓS-GRADUAÇÃO EM ENGENHARIA AMBIENTAL RENAN BARROSO SOARES TOWARD AN EFFECTIVE USE OF MICROALGAE BIOMASS FROM UASB REACTORS: FROM HARVESTING TO SMALL GASIFICATION VITÓRIA 2020 RENAN BARROSO SOARES TOWARD AN EFFECTIVE USE OF MICROALGAE BIOMASS FROM UASB REACTORS: FROM HARVESTING TO SMALL GASIFICATION The thesis presented to the Graduate Program in Environmental Engineering of the Federal University of Espírito Santo, as part of the requirements to obtain the title of Doctor of Science in Environmental Engineering. Advisor: Prof. Ricardo Franci Gonçalves, Ph.D. Advisor: Prof. Márcio Ferreira Martins, Ph.D. VITÓRIA 2020 Ficha catalográfica disponibilizada pelo Sistema Integrado de Bibliotecas - SIBI/UFES e elaborada pelo autor S676t Soares, Renan Barroso, 1986- SoaTOWARD AN EFFECTIVE USE OF MICROALGAE BIOMASS FROM UASB REACTORS : FROM HARVESTING TO SMALL GASIFICATION / Renan Barroso Soares. - 2020. Soa119 f. : il. SoaOrientador: Ricardo Franci Gonçalves. SoaCoorientador: Márcio Ferreira Martins. SoaTese (Doutorado em Engenharia Ambiental) - Universidade Federal do Espírito Santo, Centro Tecnológico. Soa1. Esgoto. 2. Biomassa. 3. Algas. 4. Carvão - gaseificação. I. Gonçalves, Ricardo Franci. II. Martins, Márcio Ferreira. III. Universidade Federal do Espírito Santo. Centro Tecnológico. IV. Título. CDU: 628 UNIVERSIDADE FEDERAL DO ESPÍRITO SANTO CENTRO TECNOLÓGICO PROGRAMA DE PÓS-GRADUAÇÃO EM ENGENHARIA AMBIENTAL Av. Fernando Ferrari, 514 Campus Universitário, Goiabeiras - Vitória - ES - CEP 29075-910 - Tel. (27) 3335 2324 - Ramal *9510. TOWARD AN EFFECTIVE USE OF MICROALGAE BIOMASS FROM UASB REACTORS: FROM HARVESTING TO SMALL GASIFICATION Renan Barroso Soares Banca Examinadora: ________________________________________ Prof. Dr. Ricardo Franci Gonçalves Orientador - PPGEA/CT/UFES ______________________________________ Prof. Dr. Márcio Ferreira Martins Coorientador – DEM/CT/UFES _______________________________________ Prof.ª Dr.ª Edumar Ramos Cabral Coelho Examinadora Interna – PPGEA/UFES ________________________________________ Prof. Dr. Sérvio Túlio Alves Cassini Examinador Interno – PPGEA/UFES ___________________________________ Prof. Dr. André Bezerra Examinador Externo – UFC ____________________________________ Prof. Dr. Antônio Geraldo de Paula Oliveira Examinador Externo – IET/Norway Diogo Costa Buarque Coordenador do Programa de Pós-Graduação em Engenharia Ambiental UNIVERSIDADE FEDERAL DO ESPÍRITO SANTO Vitória/ES, 03 de abril de 2020 Este documento foi assinado digitalmente por RICARDO FRANCI GONCALVES Para verificar o original visite: https://api.lepisma.ufes.br/arquivos-assinados/17390?tipoArquivo=O PROTOCOLO DE ASSINATURA UNIVERSIDADE FEDERAL DO ESPÍRITO SANTO O documento acima foi assinado digitalmente com senha eletrônica através do Protocolo Web, conforme Portaria UFES nº 1.269 de 30/08/2018, por RICARDO FRANCI GONCALVES - SIAPE 1176053 Departamento de Engenharia Ambiental - DEA/CT Em 17/04/2020 às 11:56 Para verificar as assinaturas e visualizar o documento original acesse o link: https://api.lepisma.ufes.br/arquivos-assinados/17390?tipoArquivo=O Este documento foi assinado digitalmente por RICARDO FRANCI GONCALVES Para verificar o original visite: https://api.lepisma.ufes.br/arquivos-assinados/17390?tipoArquivo=O vi Dedication I dedicate this work to my wife Priscila for patience and understanding on this long journey. I also dedicate to my parents Uilians and Luciana, for the construction of worth without which nothing would be possible. vii Acknowledgments I thank God and life who endowed me with health and tenacity. To my wife Priscila, who abdicated family moments and understood my dreams. To my family, Uilians, Luciana and Vinícius, for the initial encouragement, at a time when I was not entirely convinced. To my advisor, Ricardo Franci, who believed in my work and guided me on this path. To my advisor, Márcio Martins, for friendship and dedication in the articles and experiments. To the teachers Elias and Oswaldo for their initial attention and guidance. To the several laboratories and researchers who participated in my experimental results, such as Zudivan (NEST), Rodrigo Oss and Paulão (LABSAN), Guto (LABPETRO) and Elson (Air Pollution Laboratory). To the various teachers who shared knowledge. To the friends, who brought joy and distanced discouragement. Finally, to everyone who directly or indirectly contributed to this work. viii “Work and trust”. Jerônimo Monteiro ix Abstract Sanitary sewage, traditionally seen as a source of expenses and problems, has come to be seen as an opportunity and source of funds. This is because of the three major current demands of modern society, two can be extracted directly from sewage (water and energy), and one (food) can benefit from the recovery of nutrients to agriculture. Therefore, Wastewater Treatment Plants (WWTP) which go beyond the treatment itself and reuse by-products to improve their energy and economic performance is being increasingly studied. This thesis discussed the reuse of microalgae biomass produced in WWTP as a source of energy. Based on the data from the literature survey, a conceptual scenario for the use of microalgae biomass for microgeneration in WWTP was built. Thermochemical gasification was the chosen conversion process since it is one of the most promising for the microgeneration of electricity. The results showed a production potential of 0.167 kWh/m3 of treated sewage, and investments financially returned after five years. After this theoretical approach, an experimental investigation was carried out using the microalgae produced in a WWTP pilot, constructed within the area of Companhia Espírito Santense de Saneamento (CESAN), with resources from Financiadora de Estudos e Projetos (FINEP), and in partnership with the company Fluir Engenharia Ambiental. The microalgae were cultivated in two high-rate algal ponds (HRAP), fed with the effluent obtained after wastewater treatment in the UASB reactor (Up-flow Anaerobic Sludge Blanket). The biomass was then harvested in a coagulation-flocculation system, dried, characterized in terms of its calorific value, ultimate, proximate, ash, thermogravimetric, and differential thermal analysis. The effects of seven commercial coagulants on the thermochemical conversion of microalgae were evaluated and the results revealed that coagulants could affect the energy recovery. Some coagulants showed catalytic effects and were beneficial to the gasification process, while others impaired the energy recovery of biomass. Lastly, experimental microalgae gasification was evaluated in a pilot-scale downdraft gasifier. Unlike other studies reported in the literature, which simulate microalgae gasification in laboratory equipment, the gasifier used in this study is a commercial technology, already widespread in the market and present in over 40 countries. Another important difference of this work in relation to the published ones concerns the microalgae type. While the literature generally reports the gasification of pure microalgae species (monoculture), obtained in a controlled manner and free of chemical coagulants used in the harvest stage, this study presents the gasification of biomass composed of different species of microalgae, bacteria and other organisms present in the HRAP, besides chemical coagulants. The effects of air-fuel equivalence ratio (ER) on the produced gas composition, higher heating value (HHV), cold gas efficiency (CGE), and production rate were presented. An increasing and then decreasing trend with ER with a peak was seen, indicating that there is an optimum ER of 0.23 for the best performance of the process. The cold gas efficiency, syngas composition, HHV, and production rate were 87%, 11.86% H2, 19.45% CO, 8.5% CH4, 9.82% CO2, 6.23 MJ/Nm3, and 2.79 Nm3/kg biomass dry, respectively. The tests demonstrated the possibility to use wastewater microalgae as fuel in downdraft gasifier. The energy recovery could help drive the WWTP to a more economical and sustainable process. Keywords: Wastewater; microalgae; biomass; harvesting; gasification; downdraft gasifier; pilot-scale; equivalence ratio; energy; microgeneration; coagulant; catalytic effect; thermochemical behavior; high-rate-ponds. x Resumo O esgoto sanitário, tradicionalmente visto como fonte de despesas e problemas, passou a ser visto como uma oportunidade e fonte de recursos. Isso porque das três maiores demandas da sociedade, duas podem ser extraídas diretamente do esgoto (água e energia) e uma (alimento) pode se beneficiar da recuperação de nutrientes para a agricultura. Por isso, Estações de Tratamento de Esgoto (ETE), que vão além do tratamento em si e reutilizam subprodutos para melhorar seu desempenho energético e econômico, estão sendo cada vez mais estudadas. Esta tese discutiu a reutilização da biomassa de microalgas produzida na ETE como fonte de energia. Com base na literatura, foi construído um cenário conceitual para o uso de microalgas para produção de eletricidade dentro da ETE. O processo termoquímico de gaseificação foi escolhido, já que é um dos mais promissores para a microgeração. Os resultados mostraram um potencial de produção de 0,167 kWh / m3 de esgoto tratado e retorno dos investimentos em cinco anos. Após essa abordagem teórica, foi realizada uma investigação experimental utilizando microalgas produzidas em uma ETE piloto, construída dentro da Companhia Espírito Santense de Saneamento (CESAN), com recursos da Financiadora de Estudos e Projetos (FINEP), e em parceria com a empresa Fluir Engenharia Ambiental. As microalgas foram cultivadas em duas lagoas de alta taxa (LAT), alimentadas com o efluente do reator UASB (Up- flow Anaerobic Sludge Blanket). A biomassa foi colhida em um sistema de coagulação- floculação, seca, caracterizada em termos de seu valor calorífico, análise elementar, imediata, cinzas, termogravimétrica e fluxo térmico. Os efeitos de sete coagulantes comerciais sobre a conversão termoquímica de microalgas foram avaliados e os resultados revelaram que os coagulantes podem afetar a recuperação de energia. Alguns coagulantes apresentaram efeitos catalíticos e foram benéficos ao processo de gaseificação, enquanto outros prejudicaram a recuperação de energia da biomassa. Por fim, a gaseificação experimental de microalgas foi avaliada em um gaseificador downdraft em escala piloto. Diferentemente de outros estudos relatados na literatura, que simulam a gaseificação de microalgas em equipamentos de laboratório, o gaseificador utilizado neste estudo é uma tecnologia comercial, já difundida no mercado e presente em mais de 40 países. Outra diferença importante deste trabalho diz respeito ao tipo de microalgas. Enquanto a literatura geralmente relata a gaseificação de espécies de microalgas puras (monocultivo), obtidas de maneira controlada e livre de coagulantes químico, este estudo apresenta a gaseificação de biomassa composta por diferentes espécies de microalgas, bactérias e outros organismos presentes na LAT, além do coagulante. Os efeitos da razão de equivalência ar-combustível (ER) na composição do gás produzido, poder calorífico (PC), eficiência do gás frio (EGF) e taxa de produção do gás foram avaliados. Uma tendência crescente e decrescente com a variação do ER foi observada, com um pico, indicando um ER ideal de 0,23 para um melhor desempenho do processo. A eficiência do gás frio, a composição de gás, o PC e a taxa de produção foram 87%, 11,86% H2, 19,45% CO, 8,5% CH4, 9,82% CO2, 6,23 MJ / Nm3 e 2,79 Nm3 / kg de biomassa seca, respectivamente. Os testes demonstraram a possibilidade de usar microalgas de águas residuais como combustível. A recuperação de energia pode ajudar a conduzir a ETE a um processo mais econômico e sustentável. Palavras-chaves: Esgoto; microalga; biomassa; colheita; gaseificação; gaseificador downdraft; escala piloto; razão de equivalência; energia; micro geração; coagulante; efeito catalítico; comportamento termoquímico; lagoas de alta taxa. xi List of Figures Fig. 2.1. Graphical abstract chapter 2. _________________________________________ 22 Fig. 2.2. Types of microalgae and cyanobacteria studied in gasification. ______________ 37 Fig. 2.3. WWTP model with microgeneration. ___________________________________ 43 Fig. 3.1. Graphical abstract chapter 3. _________________________________________ 66 Fig. 3.2. Experimental unit: (a) Pilot wastewater treatment plant; (b) High-rate ponds; (c) First microalgae harvesting method; (d) Second microalgae harvesting method; (e) Third microalgae harvesting method. ______________________________________________ 70 Fig. 3.3. Efficiency achieved at the harvesting process for different coagulants. Data are shown as mean (n = 3) ± standard deviation. ____________________________________ 72 Fig. 3.4. Aspects of microalgae biomass and ash obtained: (a) Polyquaternium polymer; (b) Hydrated lime; (c) Ferric chloride; (d) Ferrous aluminum sulfate; (e) Aliphatic amines polymer; (f) Aluminum polychloride; (g) Tannin-based polymer; (h) No coagulant. _____ 73 Fig. 3.5. Thermal analysis of microalgae biomass using TGA/DTG/DTA. ______________ 82 Fig. 3.6. DTG characteristics of microalgae at different coagulants.__________________ 84 Fig. 4.1. Graphical abstract chapter 4. _________________________________________ 92 Fig. 4.2. Wastewater microalgae biomass aspect before (a) and after (b) drying. ______ 95 Fig. 4.3. Thermochemical profile of microalgae. _________________________________ 96 Fig. 4.4. Experimental downdraft gasification unit and main chemical reactions involved. 98 Fig. 4.5. Variation of cold gasifier efficiency and syngas production rate with equivalence ratio. Error bars represent standard deviation. _________________________________ 101 Fig. 4.6. Equivalence ratio influence on syngas composition and calorific value. Error bars represent standard deviation. _______________________________________________ 102 xii List of Tables Table 2.1. Comparison between High Rate Algal Ponds and Photobioreactor. _________ 28 Table 2.2. Microalgal productivity in open ponds using wastewater._________________ 29 Table 2.3. Comparison of microalgae harvesting techniques. _______________________ 31 Table 2.4. Comparison between the main routes of biomass to energy conversion. _____ 34 Table 2.5. Methods of harvesting microalgae reported. ___________________________ 37 Table 2.6. Syngas contaminant limits in equipment. ______________________________ 39 Table 2.7. Energy comparison between different fuels. ____________________________ 40 Table 2.8. Electric energy requirements and production for the proposed wastewater treatment process. _________________________________________________________ 47 Table 3.1. Coagulants evaluation in the harvest process. Data are shown as mean (n = 3) ± standard deviation. ________________________________________________________ 72 Table 3.2. Biomass characterization. Data are shown as mean (n = 3) ± standard deviation. ________________________________________________________________________ 75 Table 3.3. Ash composition in microalgae. ______________________________________ 78 Table 4.1. Microalgae’s characterization. Data are shown as mean (n = 3) ± Standard deviation. ________________________________________________________________ 95 Table 4.2. GEK Gasifier technical data. _________________________________________ 96 Table 4.3. Experimental gasification results and data from the literature. ___________ 100 Table 1 (Appendix A). Compilation of the microalgae characterization. _____________ 110 Table 2 (Appendix A). Compilation of hydrothermal gasification of microalgae._______ 112 Table 3 (Appendix A). Compilation of conventional gasification on bench scale. ______ 115 xiii List of Contents Dedication ________________________________________________________________ vi Acknowledgments _________________________________________________________ vii Abstract __________________________________________________________________ ix Resumo ___________________________________________________________________ x List of Figures ______________________________________________________________ xi List of Tables ______________________________________________________________ xii Chapter 1 ________________________________________________________________ 16 Introduction ___________________________________________________________________ 16 1.1 Motivation ____________________________________________________________________ 17 1.2 Thesis outline __________________________________________________________________ 18 1.3 Research objectives _____________________________________________________________ 19 References _______________________________________________________________________ 19 Chapter 2 ________________________________________________________________ 21 State of the art and microgeneration _______________________________________________ 21 A conceptual scenario for the use of microalgae biomass for microgeneration in wastewater treatment plants _____________________________________________________________________________ 22 Abstract _________________________________________________________________________ 22 Graphical abstract _________________________________________________________________ 22 Highlights ________________________________________________________________________ 23 2.1 Introduction ___________________________________________________________________ 23 2.2 Microalgae ____________________________________________________________________ 24 2.2.1 Media cultivation______________________________________________________________ 25 2.2.1.1 Wastewater as a cultivation medium _____________________________________________ 26 2.2.2 Microalgae production _________________________________________________________ 27 2.2.2.1 Systems____________________________________________________________________ 27 2.2.2.2 Productivity ________________________________________________________________ 28 2.2.3 Harvesting methods ___________________________________________________________ 29 2.2.4 Drying ______________________________________________________________________ 31 2.2.5 Conversion technologies vs. fraction converted ______________________________________ 32 2.2.5.1 Mechanical extraction ________________________________________________________ 32 2.2.5.2 Biochemical processes ________________________________________________________ 32 2.2.5.3 Thermochemical processes ____________________________________________________ 33 2.3 Thermal gasification process ______________________________________________________ 34 2.4 Microalgae gasification: a systematic review __________________________________________ 35 2.4.1 Species gasified _______________________________________________________________ 36 2.4.2 Characteristics of biomass _______________________________________________________ 38 2.4.3 Gasifiers for microalgae gasification _______________________________________________ 40 2.5 Results and discussion ___________________________________________________________ 42 2.5.1 Modeling the conceptual scenario ________________________________________________ 44 2.5.1.1 Microalgae production estimation _______________________________________________ 44 2.5.1.2 Power generation evaluation ___________________________________________________ 45 2.5.1.3 Energy balance of the conceptual scenario ________________________________________ 46 2.5.1.4 Cost estimation______________________________________________________________ 48 xiv 2.6 Trends and knowledge gaps _______________________________________________________ 49 2.7 Conclusion ____________________________________________________________________ 50 References _______________________________________________________________________ 51 Chapter 3 ________________________________________________________________ 65 Microalgae production and characterization _________________________________________ 65 Thermochemical Conversion of Wastewater Microalgae: The Effects of Coagulants Used in the Harvest Process ____________________________________________________________________________ 66 Abstract _________________________________________________________________________ 66 Graphical Abstract _________________________________________________________________ 66 Highlights ________________________________________________________________________ 67 3.1 Introduction ___________________________________________________________________ 67 3.2 Materials and Methods __________________________________________________________ 68 3.2.1 The wastewater treatment plant _________________________________________________ 68 3.2.2 Jar Test simulations ____________________________________________________________ 68 3.2.3 Microalgae biomass harvesting and preparation _____________________________________ 69 3.2.4 Thermochemical methods _______________________________________________________ 70 3.2.5 Statistical analyses _____________________________________________________________ 71 3.3 Results and discussion ___________________________________________________________ 71 3.3.1 Coagulants evaluation in the harvest process ________________________________________ 71 3.3.2 Microalgae biomass characterization ______________________________________________ 73 3.3.2.1 Qualitative analysis___________________________________________________________ 73 3.3.2.2 Quantitative analysis _________________________________________________________ 74 3.3.3 Ash content evaluation _________________________________________________________ 77 3.3.4 Thermogravimetric analysis _____________________________________________________ 80 3.4 Conclusion ____________________________________________________________________ 85 Acknowledgments _________________________________________________________________ 85 References _______________________________________________________________________ 85 Chapter 4 ________________________________________________________________ 91 Wastewater microalgae gasification _______________________________________________ 91 Experimental investigation of wastewater microalgae in a pilot-scale downdraft gasifier ____________ 92 Abstract _________________________________________________________________________ 92 Graphical Abstract _________________________________________________________________ 92 Highlights ________________________________________________________________________ 93 4.1 Introduction ___________________________________________________________________ 93 4.2 Experimental details _____________________________________________________________ 94 4.2.1 Wastewater microalgae production _______________________________________________ 94 4.2.2 Wastewater microalgae characterization ___________________________________________ 95 4.2.3 Gasifier _____________________________________________________________________ 96 4.2.4 Gasification setup _____________________________________________________________ 96 4.2.5 Process evaluation _____________________________________________________________ 98 4.2.6 Statistical analyses _____________________________________________________________ 99 4.3 Results and discussion ___________________________________________________________ 99 4.4 Conclusions ___________________________________________________________________ 103 Acknowledgments ________________________________________________________________ 103 References ______________________________________________________________________ 103 Chapter 5 _______________________________________________________________ 107 Conclusions and suggestions _____________________________________________________ 107 5.1 Conclusions ___________________________________________________________________ 108 5.2 Recommendations for future work ________________________________________________ 109 xv Appendix A. Data compilation of microalgae from characterization to products of gasification ______________________________________________________________ 110 Appendix B. Publications ___________________________________________________ 118 Chapters in books _________________________________________________________________ 118 Articles in journals ________________________________________________________________ 118 Articles in congress ________________________________________________________________ 119 Chapter 1 Introduction 17 1.1 Motivation Basic sanitation prevents human contact with waste, being important for the public health, the environment, and the economy of the country [1]. Even so, according to the National Sanitation Information System (SNIS) [2], the sewage treatment rate in Brazil is only 46.3%. The economic conditions of the population limit the full transfer of the cost of services to the tariff, especially in the poorest cities, making the necessary investments for the sector unfeasible [3]. Sanitation problems can be overcome if the technology adopted presents an attractive economic return. For this, one of the trends is energy recovery from wastewaters [4]. Build Wastewater Treatment Plant (WWTP) more energy-efficient can improve project profitability and attract investment to the sector [5]. One possibility of energy recovery in WWTP involves biogas produced in anaerobic treatment systems [6]. For the state of Espírito Santo alone, it is estimated potential production of 18.5 MW of energy if 60% of the generated sewage will be treated by anaerobic systems [7]. Among the anaerobic treatment systems, UASB reactors (Up-flow Anaerobic Sludge Blanket) are the technology that has experienced the greatest acceptance in the last 20 years. However, to meet the most stringent environmental standards, these processes require an additional treatment step to reduce the remaining organic matter and nutrients [8]. High-rate algal ponds (HRAP) can be used as a complementary treatment of UASB reactors, and it is an interesting alternative from technical, economic and environmental points of view [9]. Such treatment ponds have gained extra motivation in recent years since their large production of microalgae is no longer seen as a problem, but as a raw material for biofuels [10]. Within WWTP, the use of microalgae to produce energy is an alternative that can increase efficiency and reduce process costs [11,12]. On the other hand, when left unused, this amount of energy is not only lost but can also add costs to conventional sludge disposal systems. In this context, WWTP composed by the association of UASB reactors and HRAP represents an interesting alternative for the development of a more economical WWTP. Azeredo [13] evaluated this new integrated WWTP model and demonstrated an energy surplus performance. In addition, the author reported operational simplicity, satisfactory performance at the tertiary level of sewage treatment, and the possibility of phosphorus recovery and sequestration of CO2. Among the various processes to convert biomass into energy, the gasification process presents important advantages, such as higher efficiency, lower CO2 emissions, rapid conversion, and hydrogen production. It is worth mentioning that hydrogen is appointed as the substitute fuel for gasoline and diesel in the future [14]. In addition, the gasification process can overcome typical problems observed in incineration processes such as the need for additional fuel, and emissions of sulfur and nitrogen oxide, heavy metals, ashes, chlorinated dibenzofurans and dioxins [15]. The conversion of biomass to fuel gas still allows the generation of electricity in systems more efficient than steam boilers, such as turbines and gas engines [16]. This is relevant since most of the energy consumed in WWTP is electrical [17]. 18 Thus motivated, this work discusses the energy recovery from microalgae produced in WWTP through the thermochemical gasification process. Studies involving wastewater microalgae gasification are scarce in the literature, such as the works of Zhu et al. [18], Sharara and Sadaka [19] and Zhu et al. [20]. The most gasification studies have been carried out for pure species of microalgae (monoculture) obtained commercially instead of microalgae grown in wastewater. Moreover, the gasifying agent used in these works was not air, the most economical and traditional gasifying agent. To the best of our knowledge, it is the first work involving the wastewater microalgae gasification using a commercial downdraft gasifier and air as the gasifying agent, which gives the work a unprecedentedness. 1.2 Thesis outline This thesis is written in “Integrated Article Format”. The following chapters are based on two articles already published and one submitted to Fuel Journal, which will be presented in full versions. Chapter 2 presents a review article on a conceptual scenario for the use of microalgae biomass for microgeneration in WWTP and the scale in which it is possible. A systematic mapping of literature work was done and a scenario was constructed. All process steps, from microalgae cultivation to energy production, were discussed in this chapter. The reasons that led to the choice of gasification as the energy conversion route for the produced biomass over other processes such as fermentation, digestion and lipid extraction are also described in this chapter. Finally, a cost estimate was made and the suggested WWTP model was displayed in a flowchart. This chapter was published on October 03rd, 2019 in the Journal of Environmental Management. https://doi.org/10.1016/j.jenvman.2019.109639. Chapter 3 shows a published article on the thermochemical conversion of wastewater microalgae and the effects of coagulants used in the harvest process. This chapter analyzed the influence of seven commercial coagulants on the characterization of microalgae biomass. The main thermochemical characteristics of microalgae biomass evaluated were higher heating value (HHV), proximate, ultimate, ash, thermogravimetric and differential thermal analysis. This chapter described all the methodology of biomass production and characterization. This chapter was published on March 03rd, 2020 in the Algal Research. https://doi.org/10.1016/j.algal.2020.101864. Chapter 4 introduces the article submitted with the results obtained from the experimental investigation of wastewater microalgae in a pilot-scale downdraft biomass gasifier. The whole experimental gasification apparatus is described, and the effects of equivalence ratio (ER) on the produced gas composition, HHV, cold gas efficiency (CGE), and production rate were illustrated. This chapter was submitted on February 21st, 2020 in the Fuel. https://www.sciencedirect.com/science/journal/03014797 https://doi.org/10.1016/j.jenvman.2019.109639 19 Finally, Chapter 5 brings the general conclusions of the thesis and suggestions for future works. 1.3 Research objectives The main objective of this thesis was to evaluate the gasification of microalgae grown in wastewater using a commercial downdraft biomass gasifier. The effect of the ER, the most important parameter on the gas calorific value, was evaluated on the performance of the process in order to find the best experimental condition. It is expected that this experimental investigation contributes to the development of more efficient WWTP. The specific objectives of this research are: ✓ To describe the state of the art of microalgae production, its potential to generate electricity in WWTP and the scale in which it is possible (Chapter 2). ✓ To evaluate the effects of coagulants used in the harvest process on the thermochemical conversion of microalgae (Chapter 3). ✓ To study the experimental investigation on wastewater microalgae gasification in a pilot-scale downdraft biomass gasifier (Chapter 4). References [1] F.N. de S. (Brasil), Manual de saneamento, Funasa, 2006. [2] SNIS, Diagnósticos dos serviços de água e esgoto 2018, (2020). [3] S.T. Cassini, Digestão de Resíduos Sólidos Orgânicos e Aproveitamento do Biogás, ABES, RiMa, Rio de Janeiro, 2003. [4] S. TRIPATHI, Sanitation and Energy, NEW DELHI, INDIA, 2016. [5] E. Metcalf, M. Eddy, Wastewater engineering: treatment and Resource recovery, Mic Graw-Hill, USA. 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Ashman, Fluidized Bed Co-gasification of Algae and Wood Pellets: Gas Yields and Bed Agglomeration Analysis, Energy and Fuels. 30 (2016) 1800–1809. doi:10.1021/acs.energyfuels.5b02291. Chapter 2 State of the art and microgeneration A version of this chapter has been published: Soares, R.B., Martins, M.F., Ricardo Franci Gonçalves, 2019. A conceptual scenario for the use of microalgae biomass for microgeneration in wastewater treatment plants. Journal of Environmental Management (Elsevier - Impact Factor: 4.865) 252, 109639. https://doi.org/10.1016/j.jenvman.2019.109639. 22 A conceptual scenario for the use of microalgae biomass for microgeneration in wastewater treatment plants Abstract Microalgae are a potential source of biomass for the production of energy, which is why the amount of research on this topic has increased in recent years. This work describes the state of the art of microalgae production from wastewater treatment plants (WWTP), its potential to generate electricity and the scale in which it is possible. The methodology used was a systematic review of the gasification of microalgae from 49 articles selected. Based on the review, a conceptual scenario for microgeneration in WWTP using as feedstock microalgae for thermal gasification was developed. The most consistent assumptions for a real scale microgeneration are microalgae production in open ponds using domestic sewage as a nutritional medium; the use of the flocculation process in process of harvesting; microalgae to energy through thermal gasification process using a downdraft gasifier. Considering a WWTP with a 3,000 m3/d flux capacity, 860 kg / d of dry microalgae biomass might be produced. For which, gasification has a production potential of 0.167 kWh / m3 of treated sewage, but the energy balance is compromised by the drying process. However, when the biogas produced in anaerobic treatment enter in the model, it is possible to add a surplus of electricity of 0.14 kWh / m3 of treated sewage. Finally, a cost estimate is made for the acquisition of drying and gasification-electricity generation systems. For this scenario, the results suggest that the investments may be financially returned after five years, with additional potential for further optimization. Graphical abstract Fig. 2.1. Graphical abstract chapter 2. 23 Keywords: Wastewater; microalgae production; HRAP; gasification; microgeneration. Highlights • Microalgae production from wastewater treatment plants. • Wastewater as a nutritional medium is a tendency and requires more studies. • A conceptual scenario for microgeneration is developed. • The microalgae drying process makes microgeneration unfeasible. • Hybrid scenario: energy input from biogas makes the process economically viable. 2.1 Introduction The great biodiversity of microalgae offers numerous applications, including biofuel production for power generation. Microalgae can produce energy-rich substances such as lipids for biodiesel production, calorific gases (hydrogen, methane, carbon monoxide) by water photolysis, biological or thermochemical gasification. Production processes involve major constraints, however, for instance, in minimizing energy consumption for sustainability or in maximizing fuel production. Microalgae is biomass, since it absorbs the solar radiant energy and CO2 from the atmosphere to growth [1], and maybe a biosolid as a by-product of the wastewater treatment process [2]. According to Patel et al. [3], these types of biomasses can partially meet the need for liquid and gaseous fuels for integration with existing power generation infrastructure. Another advantage is that microalgae are the most promising alternative for biofuel production, particularly because it does not compete with food production [1,4]. Manara and Zabaniotou [5] suggest the combination of microalgae production and sewage treatment for biofuel production as the most plausible scenario for the commercial application of microalgae cultivation in the short term. The coexistence of aerobic bacteria and microalgae - which are indigenous to the type of wastewater - presents a synergy that favors biomass growth. While bacteria use oxygen produced by microalgae, microalgae use carbon dioxide produced by the bacteria, increasing the microalgae- bacteria biomass productivity [6]. There are thus two potential scenarios in the above context. One is to grow microalgae using water and nutrients as culture media; the other is to recover microalgae from the high rate open ponds in a wastewater treatment plant (WWTP). In the latter scenario, the biomass is composed of bacteria and different indigenous microalgae strains; for example in a case from southeastern Brazil, such strains are Chlorococcum sp., Chlorella sp., 24 Scenedesmus sp. and Tetradesmus sp. [6]. In both scenarios, harvesting and drying are necessary sub-processes in order to make biomass production feasible. The use of microalgae to produce energy is an alternative that increases efficiency and reduces WWTP costs. Around 7% of the total energy produced in the world is consumed in WWTPs [7,8]. Moreover, most of the consumed energy is electrical, due to the use of pumps, valves, compressors and other equipment [9]. This energy represents 5 to 30% of the operation costs [10] and is globally the greatest proportion of WWTP costs [10–12]. Microgeneration is often discussed in the context of energy costs reduction. It generally refers to the size and configuration of small to medium energy systems, which make use of renewable energy sources, and can operate independently of grid-supplied power [13]. The type of biomass sources can be incorporated into microgeneration by means of a thermochemical conversion process such as gasification [14]. It offers some important advantages, such as higher energy efficiency; lower CO2 emission; the rapid conversion of all biomass fractions (lipid, protein, and carbohydrate); and hydrogen production, a calorific gas seen by some researchers as a good future substitute for gasoline and diesel [15]. The objective of this review is to consider the discussion of WWTP based on microalgae recovery, energy demand, and microgeneration, in order to develop a scenario involving the potential production of microalgae biomasses from high rate open ponds to generate electricity. The state of the art in microalgae cultivation, harvesting, drying, and gasification are discussed, and the scale of heat/power generation and economic feasibility are estimated. The review is concluded by presenting a conceptual model and exploring the research gaps and opportunities for microgeneration in WWTP. The paper is organized as follows. Section 2.2 presents an overview of microalgae, from their characteristics to the advantages of gasification as conversion technology. The gasification process is explained in Section 2.3, as well as the factors leading to the downdraft gasifier as a promising commercial technology for microgeneration. Using the arguments built in this section, a literature review methodology is presented in Section 2.4, to map the publications on microalgae biomass gasification. Extensive data regarding microalgae/gasification is compiled. The compiled data serves to map opportunities for the use of microalgae resource in conversion technologies. A conceptual scenario of using microalgae biomass for microgeneration in a surplus energy system is developed in Section 2.5, based on the data from the literature survey. Economic viability is discussed. Finally, knowledge gaps are highlighted in Section 2.6 in order to guide new studies, and optimization trends to enable the process. A flowchart presents the proposed final process, an optimized hybrid system incorporating gasification into the sewage treatment process. 2.2 Microalgae In general, microalgae are unicellular organisms that use sunlight, water, and atmospheric CO2 to grow. Their structure does not involve roots, a stem, and leaves as in plants. Their 25 general composition is lipid (9.5 to 42%), carbohydrate (17 to 57%) and protein (20 to 50%), in proportions that vary according to the species and growth conditions. Around 100,000 species have been identified, however, only 35,000 have been characterized [16,17]. Currently, only a few species are produced on a scale of hundreds of thousands of tons, such as Chlorella, and about ten additional species are commercially explored on a smaller scale. Although there has been commercial production of microalgae for many decades, current production is mainly focused on products of high commercial value, such as nutritional supplements and natural pigments [18]. The use of microalgae for the production of biofuel has only begun to attract interest in recent years, with the need for new sources of clean energy [17,19]. The production of microalgal biomass has several advantages compared to the traditional biomass of terrestrial cultures. Growth rates can be up to 100 times greater than those of terrestrial plants, doubling their biomass in less than a day [1,20]. This means less demand for land and the sustainable use of land. The cultivation of microalgae also does not compete with food production, which promotes more sustainable energy development [1,20]. Other advantages include the potential to grow microalgae in infertile areas, such as deserts and coastal regions, in saline waters, and brackish, and wastewater. Cultivation allows the incorporation of CO2 generated in industrial processes, adding an extra benefit [16,17,21]. As a result, microalgae has become consolidated as an essential matrix for third-generation biofuels, opening a new dimension in the renewable energy industry. More than 150 companies worldwide, including large oil companies, are interested in producing biofuel from microalgae [22]. 2.2.1 Media cultivation According to Chen et al. [23], there are four main types of microalgae cultivation: phototrophic, heterotrophic, mixotrophic and photoheterotrophic. Phototrophic cultivation refers to processes where the microalgae use light as the energy source and CO2 as the carbon source. In the heterotrophic cultivation, microalgae can grow not only under phototrophic conditions, but also in the absence of light, using organic carbon as the energy source. When the microalgae can use organic and inorganic carbon and it is able to live in both phototrophic and heterotrophic conditions, mixotrophic cultivation is under control. Photoheterotrophic involves the microalgae using inorganic and organic carbon, plus a light source. Heterotrophic conditions are most easily contaminated and the cost of an organic carbon source is another challenge for commercial production, due to the extra cost of the input [23]. Microalgae are thus normally cultivated under phototrophic conditions, using photosynthesis to capture light energy and fixing inorganic carbon in the form of CO2 [24]. The growth and composition characteristics of microalgae under phototrophic conditions depend on cultivation variables such as carbon/nutrient sources and amount, irradiance intensity, temperature level, and pH values. Other operating variables are shear stress, inoculum conditions, and the type of reactor [1,25]. 26 Microalgae can assimilate up to 150,000 ppmv CO2, and as air contains only 360 ppmv, extra CO2 is supplied to growth media either from external sources such as power plants or in the form of soluble carbonates such as Na2CO3 and NaHCO3. Other inorganic nutrients required for microalgae include nitrogen and phosphorus [26]. The solar radiant energy captured by microalgae is used in the Calvin cycle to produce carbohydrates by utilizing CO2 and H2O molecules. A minimum of eight light photons are required to generate one mole of base carbohydrate (CH2O) [26]. Solar energy is equally important to temperature control. Optimal growth takes place between 15 and 30 °C associated with pH, between 7.0 and 8.0 [1]. Simple changes in the environment might decrease microalgae productivity, and increase undesired competitive or predatory organisms [27]. Other factors such as the shear stress, inoculum conditions, type of reactor, and operating modes are thus important to maintain a homogeneous media cultivation and to minimize undesired organisms. 2.2.1.1 Wastewater as a cultivation medium One of the main reasons to combine WWTP and microalgae cultivation is the high costs of the input. In traditional algal farms, it is common to use nutrients from chemical fertilizers and water from distribution systems. This decreases the system viability [6]. Indeed, microalgae usually require more nutrients for their metabolism than terrestrial plants. Nutrients such as phosphorus need to be added in large amounts to improve cultivation since a significant quantity is captured in the formation of complexes with ions [28,29]. For these reasons, the use of traditional nutrients is not considered sustainable, and a reduction in their use is imperative [30]. Some authors estimate that nutrients will become scarce in the future if agriculture does not change. Phosphorus deposits will be limited and the high nitrogen consumption will cause environmental pollution, such as an increase of NO2 in the atmosphere and acidification of the environment [30]. Several authors have performed a Life Cycle Assessment (LCA) in processes involving microalgae and pointed out the use of nutrients in the crop as an important contributor to their environmental impact [31,32]. The valorization of sewage and by-products as a nutritional source has been considered as a possible alternative in order to reduce these impacts and make the process more sustainable [33–35]. If transporting nutrients and water is not necessary, a significant amount of money can be saved [36]. Microalgae grown in sewage has been reported as having high productivity [5]. Production in wastewater can be increased with the addition of CO2 to increase the carbon and nitrogen ratio. The typical wastewater C:N ratio is to 7:1, which is low when compared to the microalgae ratio of 15:1 (C:N) [37]. Microalgae cultivation in wastewater can offer benefits beyond saving water and nutrients, such as nutrient removal from the effluent, the adsorption of heavy metals, generation of oxygen to bacteria and disinfection of the tank [4]. The effluent from anaerobic treatment, for example, have characteristics 27 that are promising for cultivation, such as low turbidity, which allows the passage of light, organic load and nitrogen predominantly in the ammonium form, best assimilated by microalgae [6]. 2.2.2 Microalgae production 2.2.2.1 Systems Microalgae production can be carried out in closed and opened systems (which have contact with the atmospheric air) [38]. Open systems are usually commercialized at a large scale since they are simpler and cheaper to construct and operate, however, they have low biomass productivity, higher sensibility to biological contaminants and, consequently, limited microalgae species cultivation [39]. Open systems require highly selective environments to avoid contamination from other microalgae species and protozoa. Monoculture cultivation is possible through the maintenance of extreme conditions, although few strains are suitable [26]. pH control can have plays an important role in the maintenance of a high standard in monoculture for certain species, such as Spirulina [1] since this species can grow under extreme alkalinity conditions where other organisms cannot live [26]. In commercial terms, high rate algal ponds (HRAP) are the most commonly used open system for microalgae production [18,40] and are designed to maximize production in shallow reservoirs [2]. In order to do so, depths between 0.15 and 0.45 m, typically 0.30 m, are adopted to guarantee light penetration [1], and a paddlewheel is used to improve circulation during the process. Closed systems have been developed to overcome the aforementioned difficulties. Transparent systems known as photobioreactors (PBR), designed to optimize the growth of photosynthetic organisms, have been improved according to the economic and operational conditions of each project. For example, tubular photobioreactors are usually made of glass or transparent plastic, with a diameter of less than 0.1 m to allow light penetration even in high microalgae concentrations [29]. In comparison to HRAP, PBR allows the production of pure microalgae species [29], however, when the sewage is used as a cultivation medium, the production of pure species is difficult due to the presence of indigenous microalgae in the wastewater, which could grow [6]. Photobioreactors allow higher production than open systems due to the higher operational control (temperature control, low contamination levels, and high CO2 trapping) [18,29,38], however the use of PBR on a large scale is limited due to the difficulty in exchanging O2 and CO2, and the high operation and installation costs that lead to a decrease in their economic viability [24]. Closed PBR is also not well developed for microalgae cultivation using sewage, and requires more study to achieve commercial production [38]. This is not true of HRAP, whose application in WWTP is nowadays relatively common in countries with hot climates [2]. The advantages and disadvantages 28 of each type of system are discussed by Adeniyi et al. [41] and this natural process presents low Capital Expenditures (CAPEX) and Operating Expenses (OPEX) in a large scale scenario. A comparison of the two systems is presented in Table 2.1, adapted from Rösch and Posten [42]. Table 2.1. Comparison between High Rate Algal Ponds and Photobioreactor. Parameter HRAP PBR Land footprint High Low Energy requirement Low High Temperature control No needed Required Reactor cleaning No needed Required Risk of contamination High Low Product quality Variable Reproducible Microbiology safety No Yes Biomass productivity Low High Capital and operational cost Low High 2.2.2.2 Productivity According to Farag and Price [43], microalgae growth in batch cultures involves five different phases: the Lag Phase - initial period of slow growth; Exponential Phase - rapid growth and often cell division; Declining Relative Growth Phase - when a growth requirement for cell division is limiting; Stationary Phase - cell division slows due to the lack of resources necessary for growth; Death/Lysis Phase - cells begin to die due to lack of resources. Initially, the indigenous strain in wastewater is grown in batch mode but once the culture reaches the stationary phase, fresh wastewater is supplied continuously, resulting in a steady-state condition, and the beginning of harvest and productivity. Different values of microalgae productivity in municipal wastewater cultivation are reported by Park et al. [44], with values between 12.7 and 35 g / m2 / d for cultivation in many countries of the world. Posadas et al. [45] also present productivity within this range, with values of 17 g / m2 / d for the cultivation in the summer and lower in the winter. Table 2.2 presents microalgae productivity using wastewater and open ponds. The productivity of pure species with other conditional parameters can be found in the review paper by Enamala et al. [46]. 29 Table 2.2. Microalgal productivity in open ponds using wastewater. Species Areal productivity (g / m2 / d) Ref. Actinastrum sp. 35 [44] Micractinium sp. 33 [44] Pediastrum sp. 25 [44] Unknown 18.4 [44] Chlorella sp. Ankistrodesmus sp. 18 [44] Scenedesmus sp. 17 [45] Coelostrum sp. 15.3 [44] Unknown 12.7 [44] 2.2.3 Harvesting methods The main difficulty in the harvesting stage is the large amount of water associated with the microalgae crop. It is precisely in the microalgal biomass recovery stage that the highest energy demand of the entire production system is concentrated [47]. This is one reason that microalgae cultivation for biofuel production is not yet economically viable. The high cost of harvesting, estimated at 30% of the total cost of biomass production [46,48], may reach 60% of the total biofuel production cost [49]. Developing a cost-effective harvesting method is the industry's biggest challenge. To become competitive, operating costs must be significantly reduced. Finding an alternative that allows the processing of large volumes of microalgae suspensions with minimal cost is essential in order to scale-up the process [18]. The large amount of water results in suspensions that may range from 0.1 to 5 g of total suspended solids (TSS) / L, and are often less than 1 g TSS / L in the effluents from open ponds. Normally, the production of large-scale algal biomass does not exceed 0.5 g / L, which means processing a huge volume of suspension to obtain a significant amount of biomass [47,50,51]. Other microalgae properties that make the harvesting process difficult are density - similar to water; particle size (between 2 and 50 μm); and the electrical surface charge, which is negative (between -7.5 and -40 mV) making the suspension dispersed and electrostatically stable, which prevents the self-aggregation of particles [47,49,50]. This surface charge arises predominantly from the presence of carboxyl groups (-COOH) on the cell surface. At pH values above 4.5, these groups dissociate and become negatively charged [18]. Currently developed technologies for microalgae separation are based on filtration, centrifugation, flotation, and flocculation, or some combination of those methods [52]. The centrifugation process is the fastest and most reliable process, and is indicated to harvest microalgae from pure cultures to recover high-value products such as natural pigments [47], however this method is the most energy intensive and is economically 30 unfeasible at large scales for biofuels [18,49]. The filtration process is cheaper and presents low efficiency to obtain microalgae in large quantities. The flotation process using microbubbles is notably more unstable and inefficient for microalgae harvesting [47]. A variety of physical, chemical and biological strategies have been developed to harvest microalgae via flocculation processes. These options can be applied upstream of other processes, aiming to concentrate the biomass and reduce the costs of the following stages [47]. The flocculation process allows concentration of the diluted solutions of 0.5 g / L up to 100 times, forming a liquid and viscous sludge with 50 g TSS / L [18]. The TSS concentration achieved after harvesting is between 2 and 7% of the microalgae in sludge [17]. A downstream dewatering process, such as centrifugation, is required to obtain a cake with 25% of dry material. The energy demand, in this case, is acceptable since the agglutinated microalgae particles are bigger and the volume processed is smaller [18]. For example, more than 95% of the energy required in microalgae centrifugation can be saved if there is a pre-concentration stage with specific coagulants. Due to the potential to treat high inflows, the cost of the process is also cost-effective, and many types of flocculants have been applied [47]. Flocculation usually allows dewatering processes to produce cakes with a total solid concentration with 10 to 30% [51]. Since flocculation allows the fast treatment of large volumes [48], and based on its operational costs, efficiency and technological possibilities, it is currently the most economical and efficient method to harvest of microalgae [47,50,53]. Most microalgae flocculation studies have involved a single species under particular conditions, however flocculation depends on the surface properties of cells and these properties differ between species and vary within the same species depending on culture conditions [18]. Ideal coagulant concentrations may therefore vary significantly from design to design. Values such as 200 mg / L ferric chloride are reported to harvest Chlorella sp. [54] and 5 mg / L cationic polymer to harvest Chlorella vulgaris [48]. Jar test assays will determine the optimal dosage of coagulant. Despite its advantages, chemical flocculation incorporates chemical compounds into the biomass. Changes in harvested biomass staining and in the culture medium, have already been pointed out [50]. Aluminum salts can cause cell damage, while iron salts affect the quality of pigments, especially chlorophyll. Aluminum chlorides can inhibit transesterification reactions and impair biodiesel production with biomass [47]. Aluminum sulfate and ferric chloride can also negatively affect anaerobic digestion and biogas production [53]. It is worth noting that the physicochemical characteristics of the biomass harvested, in addition to being dependent on the microalgae species present, are also affected by the culture medium and the harvesting process [22]. The harvesting process should thus be chosen to integrate the downstream treatment stages and biomass conversion, and not only in an isolated form. Table 2.3 presents dry biomass concentrations obtained in the major microalgae harvest processes described by Shah et al. [55]. Each process received a score for their efficiency 31 and economy according to Al Hattab et al. [56]. Eight criteria were used for evaluation: (a) dewatering efficiency (b) cost (c) toxicity (d) suitability for industrial-scale (e) time (f) species specificity (g) reusability of media and (h) maintenance. Each criterion was assigned a score between 7 and 15 based on its degree of importance. Higher values were given to the criteria that were deemed most important for the development of an efficient and economic largescale dewatering method for microalgae. Coagulation with organic chemical compounds and centrifugation were the only processes with a total score over 80, suggesting that the methods could be a good combination in order to obtain a more efficient and cost-effective harvesting solution. Table 2.3. Comparison of microalgae harvesting techniques. Harvesting Techniques Dry solids output concentration (%)[55] Process Score [56] Sedimentation 0,5-3 Sedimentation 61 Flotation 7 Dispersed air flotation Dissolved air flotation 77 70 Chemical coagulation 3-8 Inorganic coagulation Organic coagulation 65 80 Centrifugation 10-22 Disc stack centrifugation Decanter centrifugation 87 80 Filtration 2-27 Pressure filtration Vacuum filtration 74 75 2.2.4 Drying The last stage in algal biomass production is drying the wet paste obtained in the harvest process. In general, the moisture is reduced to a 12-15 wt.% content in which the biomass can be stored [57]. The drying stage is normally carried out to extend the life use of the material and needs to be done quickly after harvest, so that the biomass does not spoil [58]. There are many drying systems, which differ in cost and energy demand. The selection of method will depend on the operation and scale of the biomass [57]. For instance, solar drying is the cheapest method of microalgae drying, however it requires a long time and large areas. In this case, part of the energy content of the biomass and some specific compounds can be lost [59]. More efficient and expensive methods to dry microalgae have been studied, such as drum drying, spray drying, fluidized bed drying, freeze-drying and refraction window dehydration technology [59]. Freeze-drying, or lyophilization, has been largely used to dry microalgae in labs, however, the method is very expensive for use at a large scale. Spray drying is the method chosen for products that have high value [58], and rotary kilns 32 are currently commonly used to dry sludge and biosolids [60], and, as pointed by Bennion et al. [61], to dry microalgae. 2.2.5 Conversion technologies vs. fraction converted Microalgae can be converted into products using a mechanical process for oil extraction, biochemical processes for biogas and alcohol production, and thermochemical processes producing oil, gas, and heat. The selection of the right conversion technology is a key step in ensuring a viable and environmentally sustainable production process [16]. 2.2.5.1 Mechanical extraction There has been a recent resurgence of interest in microalgae as an oil producer for biofuel applications. The extraction of oil is conditioned by the lipid content of the microalgae and not all species have satisfactory amounts, with as low as 2% in M. aeruginosa species [62]. Lipids content in microalgae may vary from 1 – 85% of the dry weight, and factors such as nutrient availability have been shown to affect lipid content in many microalgae. When nitrogen deprivation is imposed, for example, photosynthesis continues more slowly, and the flow of fixed carbon is diverted from protein to either lipid or carbohydrate synthesis. Lipid accumulation can be initiated in microalgae by imposing nutrient deficiency like N, P, and K but also reduces growth rates [63]. In general, species with higher lipid content have a slower growth rate [62]. High nutrient (N and P) wastewater crops, such as sanitary sewage, can also inhibit lipid accumulation [64]. The cost of microalgae biodiesel production in the conventional process is still very high, around U$8 per gallon, double the of soy biodiesel [4]. Chisti [29] suggests that, just like a petroleum refinery, a biorefinery could be used to take advantage of each component of microalgae, not just lipids, as a way to cut costs. 2.2.5.2 Biochemical processes Another route to the recovery of energy from microalgae is reached by biochemical processes, however, they require a long reaction time and involve less conversion than thermal processes [65], so the industry generally does not choose these processes [1]. The production of bioethanol by fermentation involves long processing stages, as they depend on the enzymatic and cellular activities that make up the biochemical processes. The need for pre-treatment in the feedstock also increases the cost of production and only one final product is obtained [1]. The carbohydrate content of alcohol production is also relatively low in microalgae [4]. The degradation of microalgae is incomplete, and involves low biogas production, for anaerobic digestion at 35 ºC. Improvement is only found at a higher temperature, where 33 the cell wall is broken, exposing its intracellular content to the bacteria. Another difficulty is that the C/N ratio of this biomass is low and does not favor anaerobic digestion [65]. The process provides only partial degradation of the biomass and the use of partially degraded microalgae as a substrate is not viable because it interferes in the metabolism of organisms, and should be avoided owing to its potential to generate sulfides [66]. 2.2.5.3 Thermochemical processes Thermochemical conversion is the most direct path to transforming biomass into different forms of energy [67]. The process involves the thermal decomposition of the organic compounds present in the biomass to produce biofuels [1]. The main advantages are the conversion of whole biomass regardless of the type of macromolecule present [68], high efficiency and low conversion time [69]. This type of conversion can be done by pyrolysis, liquefaction, and gasification, and is applied more commonly than biochemical conversion [1]. The major challenge of pyrolysis is the need to purify the oil produced and the salt content involved in liquefaction is problematic [36]. These techniques are also immature and require considerable development before large-scale application. In contrast, biomass gasification is a well-developed technology and has been applied for decades [65]. Gasification seems to be the most advantageous process. The production of fuel gas makes the subsequent stage of energy generation more direct, simple and compact. Gasifiers coupled with internal combustion engines and generators are already commercialized with high conversion efficiencies [70]. Other processes require intermediate stages, increasing system complexity to an undesirable level, such as with alcohol, biodiesel and steam production, which demand distillation columns, transesterification reactors, and boilers, respectively. Anaerobic digestion does not involve an intermediate conversion process since microorganisms convert biomass directly into biogas, however, only the biodegradable organic fraction of biomass is converted into biogas, and the non-biodegradable organic fraction would need to be removed periodically and sent to final disposition, implying in extra costs. Gikas [71] compared anaerobic digestion and gasification processes, such as energy recovery systems with sewage sludge produced in a pilot WWTP, with a 380 m3/d flux capacity. To reduce energy demand in the WWTP, the author developed a different WWTP model, substituting the aerobic process, high in energy consumption, with microwaving and filtration. According to the flowchart developed, the sludge obtained was dewatered in an auger press until 55% humidity. A rotary kiln then reduced the humidity of the solids to 20%, in order to condition the feed to the gasification process. The gas produced fed an internal combustion engine, generating electricity. In the anaerobic digestion system, the dewatered sludge generated biogas in an anaerobic reactor, with 60% of converted organic fraction. When the author compared both systems, 34 gasification had a more favorable energy balance, even considering the drying energy requirement. Table 2.4 compares the differences between the main routes of biomass to energy conversion discussed in the text. Table 2.4. Comparison between the main routes of biomass to energy conversion. Process Conversion Time Conversion Fraction Biomass Requirements Final Product Technology Extraction Fast Partial High lipid Oil Mature Fermentation Slow Partial High carbohydrate Alcohol Mature Anaerobic digestion Slow Partial High biodegradable Biogas Mature Liquefaction Medium Partial Low salt Oil Immature Pyrolysis Medium Total - Oil/Gas/Char Immature Gasification Fast Total Low moisture Gas Mature 2.3 Thermal gasification process Gasification is a partial oxidation thermochemical process in which carbonaceous substances are converted to gas in the presence of a gasifying agent - usually air - oxygen, water steam, carbon dioxide or mixtures thereof [72]. The flow of the gasifying agent is controlled and partial oxidation takes place [73]. The objective of the process is to produce a highly efficient clean gas synthesis (syngas) [5]. Syngas production can vary from 1 to 3 m3 / kg of dry biomass [74]. Syngas consists mainly of CO and H2, mixed with other components, such as CH4 and CO2. It may include some light hydrocarbons, such as ethane and propane, and also, heavy hydrocarbons such as tar, condensable between 250-300 °C. Small particles of solid coal waste, alkali metal species, ash and other gases, such as H2S, HCl, NH3, H2O, and N2, may be present in small quantities. The presence of these gases depends on the characteristics of the biomass, the gasifying agent, the process conditions and the type of gasifier [72,74–76]. The gasifier is the main component of a gasification plant and the biomass and gasifying agent reactants are mixed therein so that the reactions can occur. In some cases, catalysts, additives and inert materials are also fed into the process to improve their performance. The manner in which the reactants come into contact in the gasifier affects the performance of the process and forms the basis for classifying the gasifiers [75]. Two typical gasifier configurations with a fixed bed are updraft and downdraft [72]. In the first, the solids move down relative to the gasifying agent, and the syngas produced move upwards. In the second configuration, both the solid and the syngas are moved downwards [74]. In this reactor, the pyrolytic gases pass through the oxidation zone at high temperatures as the syngas is removed from the bottom of the equipment, and 35 therefore almost all tar is converted to gas and the syngas is much cleaner than that generated in updraft gasification [76]. The updraft reactor is the oldest model and involves more simplicity and lower costs [75]. This equipment can gasify most of the biomass, however, the syngas produced in these systems contains high quantities of tar, between 10 and 20 wt.%, and therefore, these gasifiers are not suitable when one desires to use syngas in the internal combustion engines [76]. On the other hand, in downdraft gasifiers, tar production can be inferior to 1 wt.% [74,77], making this process preferable to electricity generation in engines and turbines [76]. Fluidized bed gasifiers and other gasifiers variations are also reported in the literature. Gikas [71] shows a horizontally disposed rotary cylindrical gasifier developed by Greene Waste to Energy S.L. (Spain), which allows each stage of the process of gasification to be controlled, increasing the performance of the equipment. In each internal area of the equipment, stirrers homogenize the flux during the process and a thermal jacket covers the equipment controlling all thermal exchanges. Recent publications involving the gasification process have been addressed in literature review studies. Ruiz et al. [72] reviewed the main factors to be considered in the gasification process, and pointed out the barriers to the generation of electricity. Asadullah [76] reviewed the barriers of each stage of the gasification process, from the collection of biomass to the generation of electricity, and noted syngas cleaning as an important step. Heidenreich and Foscolo [73] provided a detailed review of new concepts of gasification, such as the UNIQUE gasifier, which integrates gasification with syngas cleaning in a single reactor. Abdoulmoumine et al. [78] presented a review of the methods of purification of syngas, discussing the removal of the main contaminants. Molino et al. [74] detailed the gasification technologies, assessing advantages and disadvantages and the potential for use of syngas. Ahmad et al. [75] highlighted the characteristics and performance of different types of gasifiers over different operating parameters, and discussed the economic evaluation of the gasification process. Despite the difficulties pointed out and the challenges still to be overcome, the potential advantages of the biomass gasification process continue to motivate research. 2.4 Microalgae gasification: a systematic review We used the method developed by Ensslin et al. [79], known as the Proknow-C method, Knowledge Development Process - Constructivist to map the research on microalgae gasification. It is a systematic approach used to organize the information collected in the literature and includes the construction of knowledge in three main stages: preparation of a bibliographic portfolio, bibliometric analysis, and systematic analysis. Articles are selected from defined databases using keywords, then filtered based on specific criteria, such as alignment with the topic of interest and scientific relevance. Finally, redundant and unavailable papers are eliminated [80]. After the establishment of the bibliographic 36 portfolio, a systematic analysis is conducted in order to elucidate points of interest and gaps to be filled in current research. The keywords adopted in this review were "microalgae" and "gasification". The SCOPUS database was chosen because it is the largest database [81] of abstracts and citations of literature reviewed, including scientific journals, books and conference work. According to Ferenhof et al. [81], the SCOPUS database had 15,000 indexed newspapers, almost 265 million websites and 18 million patents and other documents in 2014. The database is thus able to provide a comprehensive view of the outcome of the worldwide survey. 169 documents were found in the SCOPUS database on September 2018, the first of which was published in 1991 [82], however the topic has only gained more relevance in the literature in the last decade, and notably in the last five years. The number of documents was reduced to 121 when only journal articles with an impact factor greater than 1.0 were selected. The titles and summaries of these papers were read in order to select only the studies involving the experimental thermal gasification of microalgae. Macroalgae, residual microalgae after extraction of compounds, such as lipids, and non- experimental studies were not considered. Forty-two articles were thus selected and, after careful reading, seven additional references, not included in the SCOPUS but fitting with the criteria of paper selection. were added to the bibliographic portfolio, which was finally composed of 49 documents [28,30,62,64,65,68,83–125]. 2.4.1 Species gasified Despite the variety of known microalgae species (thousands), only 19 different species of microalgae (and Spirulina cyanobacterium) have been studied for gasification (Fig. 2.2). Considering the variety of existing and known species, this number is very low and clearly illustrates current ignorance on the subject. It is believed to be related to the ease of culturing and the rate of growth of certain strains. Among the 19 species of microalgae and cyanobacteria reported, there is a predominance of the Chlorella Vulgaris and Spirulina commercialized species, used in 50% of the experimental gasification studies. In fact, most of the authors (72%) acquired the microalgae in powder or paste form, and the biomass harvesting process was not clear. Because these species are mostly used in the food industry, it is believed that the harvesting processes employed were not based on the incorporation of products for coagulation-flocculation, to keep the biomass free of chemical contaminants. In today's commercial microalgae systems, despite being expensive and energy-intensive, centrifuges and filters are the most commonly used equipment [126]. In order to obtain bulk products of lower value, such as biofuels, however, the investment and operating costs of these processes compromise the economic viability of the projects [127]. At large scales, the most efficient method of microalgae separation is coagulation-flocculation- sedimentation [53]. Even so, no author evaluated the biomass obtained by coagulation- flocculation and the effect of the chemicals mixed with the biomass on the gasification process. The few authors who reported on microalgae culture and harvesting themselves 37 adopted incompatible techniques in the context of a large-scale industrial process, such as the scaling of the decanted bed at the bottom of the pond, centrifugation, vacuum and membrane filtration and electro flocculation (Table 2.5). The combination of the coagulation and centrifugation processes, described in Table 2.3 as the highest score, is not reported. Fig. 2.2. Types of microalgae and cyanobacteria studied in gasification. Table 2.5. Methods of harvesting microalgae reported. Microalgae Harvest Methods Ref. Scenedesmus sp. cultivated in sewage Spontaneous decanting [65] Scenedesmus sp. cultivated in sewage Spontaneous decanting [4] Chlorella vulgaris Vacuum Filtration [128] Algae biomass cultivated in sewage Scraping the bottom of the pond [64] Chlorella vulgaris Centrifugation [22] Chlorella vulgaris Centrifugation [22] Tetraselmis sp. Electro flocculation and centrifugation [129] Tetraselmis sp., Schroederiella apiculata and Scenedesmus dimorphus Centrifugation [119] Microcystis aeruginosa Filtration with membrane [64] Chlorella sp. Centrifugation [118] Only three marine species were reported, Nannochloropsis gaditana, Nannochloropsis sp. and Tetraselmis sp., which together contributed only 21% of the total papers. One 38 explanation for this may be the higher salt and ash content of marine microalgae, which may hinder the gasification process or require additional pretreatment steps such as washing to remove excess salt. Marine microalgae also typically exhibit lower conversion than freshwater species in some gasification process, although the comparison is made difficult by the different process conditions [106]. The gasification of microalgae grown in wastewater was studied by a few researchers, such as Zhu et al. [65,130]. In these two publications, they used Scenedesmus sp. obtained in stabilization ponds fed with industrial and municipal sewage. Sharara and Sadaka [64] recovered a microalgae biomass comprised of native and Diatomaceous species in a stabilization pond applied to sewage treatment. The treatment systems were not detailed in terms of configuration and performance, or the characterization of the sewage introduced into the ponds. To the best of our knowledge, there is an important gap in terms of research in this area. Most gasification studies refer to pure microalgae species, even with sewage cultivation being suggested as the most promising way to convert microalgae into biofuels. The cultivation of pure species in photobioreactors using water and fertilizers does not appear to be the most viable path for biofuel production, for the reasons discussed in Section 2.2. This paper considers biomass as a mixture of microalgae, bacteria, and chemical coagulants, and the gasification process for this type of biomass is still little reported in the literature. 2.4.2 Characteristics of biomass Different types of microalgae have different compositions (Table 1 in Appendix A), which are normally reported in terms of their proximate analysis, elemental analysis, macromolecules, and higher heating value (HHV). Proximate analysis can be defined as a technique to measure the chemical properties of a compound based on four particular elements: moisture content, fixed carbon, volatile matter and ash content [131]. These characteristics affect the syngas yield and quality [106]. The analysis of ash and its components were predominant in the discussion of the experimental results, as used to justify the kinetic performance, the conversion and the composition of the syngas. This is because the different chemical elements present in the ash can affect the process in various ways, such as acting as a catalyst, catalyst promoter, catalyst support or sorbents in the gas cleanup. Abdoulmoumine et al. [78] compiled these effects into a Periodic Table, indicating the effect caused by different chemical elements in the process. The average ash content was 14.3%, while the fractions of volatile material and fixed carbons were 66.5% and 14.7%, respectively. Disregarding seawater species, the average ash content is reduced to less than 12%. The species of saline microalgae Tetraselmis sp. presented the highest ash content, with 64.4%. An elemental analysis defines the mean chemical elements present in biomass, which are carbon, hydrogen, oxygen, nitrogen, and sulfur. The higher proportion of oxygen and 39 hydrogen, compared with carbon, reduces the energy value of a fuel, due to the lower energy contained in carbon-oxygen and carbon-hydrogen bonds, than in carbon-carbon bonds [132]. The presence of oxygen in the biomass reduces its calorific value [77,133], and the presence of sulfur and nitrogen can lead to process contaminants such as HCN, NH3, CS2, and H2S [78]. Of the 34 articles that reported microalgae sulfur content, only six reported a higher percentage than 1% [94,117,119,129,134,135]. The low sulfur content of the algal biomass is an advantage since the thermochemical conversion of biomass can release sulfur in the form of H2S, a corrosive gas for equipment. According to Molino [74], the maximum sulfur levels in the syngas allowed for use in internal combustion engines and turbine are 20 ppmv. This means that the use of such equipment in electricity generation systems could be limited to high H2S content. Table 2.6 shows typical engine and turbine requirements for the major syngas contaminants. The elemental analysis results in 46% carbon, 7% hydrogen, 30% oxygen, 8% nitrogen and 1% sulfur. The species of saline microalgae Tetraselmis sp. presented the highest sulfur content reported, 6.9%. Table 2.6. Syngas contaminant limits in equipment. Equipment Tar (mg/m3) Sulfur (ppmv) Nitrogen (ppmv) Alkali (ppmv) Halides (ppmv) Particulate (mg/m3) Ref. Turbine - < 20 < 50 < 0,02 < 1 - [78] Turbine < 10 < 20 - < 0,025- 0,1 - < 2,4 [74] Engine < 100 < 20 - < 0,025- 0,1 - < 50 [74] The composition of the microalgal biomass in terms of macromolecules, as reported by Onwudili et al. [106], means that carbohydrates form H2 more easily than lipids and proteins. The latter are responsible for inhibiting some reactions of hydrogen formation. Few studies have quantified the levels of protein, carbohydrate and lipid in the biomass to be gasified. Without stratified chemical species present in the biomass studied, the average values of the protein, carbohydrate and lipid levels of microalgae were 46%, 19%, and 19%, respectively. Shumbulo Shuba and Klife [4] compiled information about carbohydrate, protein and lipid microalgae composition in different species and found a wide variation. Table 1 in Appendix A also presents the HHV content for different microalgae. The mean HHV established was 18.93 MJ / kg. This characterization corroborates the interest of converting microalgae to energy, considering the energy content and high carbon content. Table 2.7 shows a comparison with the calorific value of other fuels, demonstrating the significant amount of energy from microalgae biomass, and its superiority to wood. 40 Table 2.7. Energy comparison between different fuels. Fuel MJ/Kg Ref. Gas-oil 45.5 [5] Natural Gas (NTP) 38.1 [74] House coal 27 - 31 [74] Microalgae 18.93 Table 1. in Appendix A (average) Plastics, wood, paper, rags, garbage 17.6 - 20 [5] Wood 12 - 19 [74] Dry sewage sludge 12 - 20 [5] Black liquor 12.5 - 15.5 [5] 2.4.3 Gasifiers for microalgae gasification Of the 49 papers evaluated, 29% are simulations of gasification in a thermogravimetric analyzer (TGA). These studies were not considered as gasification experimental tests since the conditions of this equipment do not reflect a realistic scenario of the process. Only 16 papers were found for conventional gasification, less than one-third of the total, [64,65,88,92,94,101,105,107,110,112,113,115,117,119,121,122]. Of those, only six used fixed bed gasifiers, a very low number, considering the greater simplicity, and operational and economic advantages of this equipment [105,110,112,117,119,136]. This may be related to the lower syngas quality obtained in these reactors when compared to other technologies, however it is possible to adjust operational parameters to reduce the contaminants in the product. A small predominance of studies used fluidized bed gasifiers. Manara and Zabaniotou [5] pointed out that the fluidized bed gasifiers allow a better mass and energy exchange, as well as a homogeneous heat transfer at a higher velocity, facilitating the reaction and reducing the residence time of the load [76]. The process has the advantage of allowing consistent temperatures, kept well below the problematic levels that could lead to the synthesis or accumulation of ash, however these reactors are more operationally sensitive to the type of biomass [72]. The high ash content of some microalgae may limit the use of this technology, due to problems such as defluidization and bed agglomeration, as observed in [137]. According to Alghurabie et al. [137], there are several operational difficulties when gasifying a marine species of microalgae with high ash content, due to the high salt content in these species of microalgae. Freshwater species may also have high ash content in their composition, depending on the culture medium and harvesting processes. This was demonstrated by Zhu et al. [65], for example, during the gasification of the species Scenedesmus sp. The authors associated the high ash content after gasification with the cultivation of the species in industrial and municipal wastewater and the harvesting conditions. The high silicon content observed could be related to contamination from sand and other minerals during the cultivation and harvesting processes in the ponds. Elliot et 41 al. [125] reported a high ash content in the microalgae gasification residue and associated it with mud contamination during the harvest stage. Hydrothermal processes correspond to 55% of the studied processes in the publications analyzed. The high interest in hydrothermal processes is due to the use of humid biomass, without the need for drying. Conventional gasification requires low moisture biomass, which consumes a lot of energy and reduces efficiency to uneconomic levels [134]. Hydrothermal gasifiers, which adopt fluids under supercritical conditions of pressure and temperature, therefore have a higher thermal efficiency than conventional gasifiers. The process with water also produces less tar [3], which is considered a critical point of biomass gasification [74]. Hydrothermal gasification occurs under pressure and the formation of hydrogen or methane can be prioritized depending on the temperature and the catalyst used. At moderate temperatures (below 500 °C), catalysts are required in order to increase the conversion. Ruthenium is singled out as the most active and selective catalyst for generating methane, however the deactivation of ruthenium by sulfur has been pointed out as a problem, and attempts at regeneration [22] and reuse [138] have been unsuccessful. Hydrothermal gasification is also still under development [16] and is considered a very expensive process for implantation and operation [83]. For this reason, Elliot et al. [125] believed that more research is needed before scaling the technology to a commercial demonstration level. One of the challenges is the presence of salt in these reactors, which can cause corrosion and clogging [36]. The water itself is corrosive under supercritical conditions and high levels of nickel have been observed in the residual liquid of the process, indicating corrosion of the reactor wall [83]. Due to the severe conditions of the process, metals can also be washed from the catalyst. The incorporation of heavy metals and organic compounds makes it difficult to manage large volumes of residual liquid. Attempts to re-use this liquid as a nutritional medium in the cultivation of microalgae have encountered many difficulties. Haiduc et al. [134] found that a nickel concentration of 28 ppm completely inhibited the growth of the microalgae species studied. According to Bagnoud-Velásquez et al. [135], aluminum may form relatively inert complexes, which interfere with cell metabolism. Phosphorus becomes unavailable for enzymatic transport when complexed with aluminum, so that the growth rates of microalgae are inhibited. Patzelt et al. [30] identified at least 28 toxic organic compounds that could cause inhibition in the growth of microalgae, requiring dilution of 1 to 355 to allow the cultivation. Onwudili et al. [106] indicated an even greater dilution, from 1 to 400, due to the presence of phenol. The amount of water for dilution would exceed the capacity of the culture medium of an integrated gasification process and would not be a satisfactory option [30]. A treatment step would thus be necessary to reduce the harmful compounds to tolerable levels. Elsayed et al. [139] used activated carbon and ultra-violet radiation treatment systems to remove most of the pollutants from the liquid, but they observed a delayed growth of microalgae. Patzelt et al. [30] used the same treatment processes and noted that desirable compounds are also removed, especially nitrogen in the ultraviolet treatment by ammonia volatilization. A 79.6% decrease in nitrogen content and an important limitation of 42 phosphorus in the feed liquid of the process were observed. Minowa and Sawayama [28] observed a growth of only one-eighth of that expected, for cultivation in recovered and treated solutions of the gasification process, assigning the problem to the lack of nutrients in the diet. The authors suggested the integration of preliminary stages of salt separation, common in hydrothermal processes, with the culture stage, aiming to reuse the salts and supply these nutrients, however such integration would mean even greater plant complexity and higher costs. Catalysts commonly used to increase the conversion of these processes also increase the complexity of these systems, since compounds present in the biomass rapidly deactivate the catalyst, especially sulfur [22]. The process is also not fully developed, due to the extreme pressure and temperature conditions required [119]. The critical point of the water is 374 ºC and 22 MPa [140]. In contrast, the common gasification of biomass has been well developed and applied for decades. Nevertheless, the technological bottleneck represented by the need to reduce the moisture content of the biomass previously fed in the process remains. The microalgae drying method, based on the circulation and reuse of heat, could considerably reduce energy consumption [121]. In this sense, several drying technologies have been developed [141]. The equipment used in the experiments was practically all on a laboratory scale. Only the experiments of Yang et al. [92] were performed on a larger pilot scale, adopting a bubbling fluidized bed gasifier of 30 kW. Other models of gasifiers, including the more traditional ones, such as the fixed bed downdraft and updraft, have not yet been experimented with on a pilot scale. These reactors have simpler operations and designs, which makes them the preferred and most feasible option for small power generation units [73,76]. Tables 2 and 3 (in Appendix A) compile the information found for hydrothermal and conventional gasification, respectively. The tables include operational parameters, equipment dimensions and some results in order to introduce a summary view of the literature. The gaps in the tables are due to non-available information. In general, it may be noted that the carbon conversion of the process is greater in conventional gasification than in hydrothermal gasification, and tar levels are often not reported. 2.5 Results and discussion A microgeneration scenario in a WWTP is built in this section, considering the premises that are more similar to a real scale scenario, as described in the text. Premises: • Microalgae production in HRAP using domestic sewage as a nutritional medium; • Use of the flocculation process in the biomass harvesting process; 43 • Production of mixed biomass composed of different species of microalgae, bacteria and chemical coagulant; • Use of rotary kilns to dry the biomass to reach the maximum limit of moisture allowed in the gasifier; • Conversion of biomass to energy through a thermal gasification process using a conventional downdraft gasifier. Scale: • The construction of ponds to wastewater treatment is more common in less urbanized places with more area available and smaller demographic density [2]. In Brazil, 70% of cities have less than 20,000 inhabitants [142], therefore, it was estimated the design of a WWTP for 20,000 people; • Using a per capita sewage flow rate of 150 L / day, as suggested by Sperling [2], it is equivalent to a sewage flow of 3,000 m3 / day. Figure 2.3 presents the WWTP model with microgeneration proposed in this study with high rate algae pond cultivation, using the Upflow Anaerobic Sludge Blanket (UASB) effluent. Fig. 2.3. WWTP model with microgeneration. The UASB reactor is commonly applied in Brazil and the association of both treatment systems results in better effluent quality when compared to a single system. Since the anaerobic process does not remove nutrients to any great extent, there is no harm to the 44 microalgae production [2]. On the contrary, the effluent nitrogen is predominantly in the ammonium form, which facilitates microalgae assimilation and low turbidity facilitates light penetration in the medium [6]. Another advantage of this association is that the anaerobic reactor does not need a supply of energy for its function, as it also produces biogas, an important additional source of energy, due to the high energy demand in the biomass algae drying process. 2.5.1 Modeling the conceptual scenario 2.5.1.1 Microalgae production estimation The microalgae-bacteria biomass production in high rate algal ponds can be estimated, according to Park et al. [44], from the maximum photosynthetic conversion rate of sunlight (Equation 2.1). Pba = Io . nmáx / H (2.1) Where: Pba = Production of microalgae-bacteria biomass (g / m².d); I0 = Average solar radiation (MJ / m².d); ηmáx = Maximum photosynthetic conversion efficiency of sunlight (%); H = Energy value of the biomass as heat, calorific value (kJ / g). The maximum conversion efficiency of light through photosynthetic processes adopted was 2.4%, according to Park et al. [44]. The smallest annual solar radiation index for Brazil is 16 MJ / m2.d, according to the Solarimetric Atlas of Brazil [143], and the highest energy value of microalgae indicated in this work, is 23.2 MJ/kg. Both values were adopted in order to obtain more conservative biomass prod