Periodic optimal control for biomass productivity maximization in a photobioreactor using natural light

We address the question of optimization of the microalgal biomass long term productivity in the framework of production in photobioreactors under the influence of day/night cycles. For that, we propose a simple bioreactor model accounting for light attenuation in the reactor due to biomass density and obtain the control law that optimizes productivity over a single day through the application of Pontryagin's maximum principle, with the dilution rate being the main control. An important constraint on the obtained solution is that the biomass in the reactor should be at the same level at the beginning and at the end of the day so that the same control can be applied everyday and optimizes some form of long term productivity. Several scenarios are possible depending on the microalgae's strain parameters and the maximal admissible value of the dilution rate: bang-bang or bang-singular-bang control or, if the growth rate of the algae is very strong in the presence of light, constant maximal dilution. A bifurcation diagram is presented to illustrate for which values of the parameters these different behaviors occur. Finally, a simple sub-optimal bang-bang strategy is proposed that numerically achieves productivity levels that almost match those of the optimal strategy

Hydrogen photo-production by mixotrophic cultivation of chlamydomonas reinhardtii: Interaction between organic carbon and nitrogen

Hydrogen photo-production by a wild type and two engineered strains of Chlamydomonas reinhardtii was investigated. Growth rate values and hydrogen yields attained as the concentration of acetate and nitrogen vary were compared. In the analysis of microalgal growth, the interaction between organic carbon (acetate) and nitrogen (nitrate) was investigated by recourse to an experimental factorial design. This analysis evidenced the existence of a statistically significant interaction between organic carbon and nitrate. Hydrogen production was attained by cultivating microalgae previously grown in mixotrophic regime with sulphur deprived medium. The influence of varying the photobioreactor headspace on hydrogen production was investigated. This analysis revealed an increase in the hydrogen produced per unit volume of culture of about one order of magnitude when the headspace volume is modified from 100 to 350 mL. This result provides valuable indications on how to design and operate photobioreactors for hydrogen production optimization and was thoroughly discussed in terms of the metabolic pathways activated by sulphur depletion. ©2014, AIDIC Servizi S.r.l

Knowledge of regulation of photosynthesis in outdoor microalgae cultures is essential for the optimization of biomass productivity

Microalgae represent a sustainable source of biomass that can be exploited for pharmaceutical, nutraceutical, cosmetic applications, as well as for food, feed, chemicals, and energy. To make microalgae applications economically competitive and maximize their positive environmental impact, it is however necessary to optimize productivity when cultivated at a large scale. Independently from the final product, this objective requires the optimization of biomass productivity and thus of microalgae ability to exploit light for CO2 fixation. Light is a highly variable environmental parameter, continuously changing depending on seasons, time of the day, and weather conditions. In microalgae large scale cultures, cell self-shading causes inhomogeneity in light distribution and, because of mixing, cells move between different parts of the culture, experiencing abrupt changes in light exposure. Microalgae evolved multiple regulatory mechanisms to deal with dynamic light conditions that, however, are not adapted to respond to the complex mixture of natural and artificial fluctuations found in large-scale cultures, which can thus drive to oversaturation of the photosynthetic machinery, leading to consequent oxidative stress. In this work, the present knowledge on the regulation of photosynthesis and its implications for the maximization of microalgae biomass productivity are discussed. Fast mechanisms of regulations, such as Non-Photochemical-Quenching and cyclic electron flow, are seminal to respond to sudden fluctuations of light intensity. However, they are less effective especially in the 1–100 s time range, where light fluctuations were shown to have the strongest negative impact on biomass productivity. On the longer term, microalgae modulate the composition and activity of the photosynthetic apparatus to environmental conditions, an acclimation response activated also in cultures outdoors. While regulation of photosynthesis has been investigated mainly in controlled lab-scale conditions so far, these mechanisms are highly impactful also in cultures outdoors, suggesting that the integration of detailed knowledge from microalgae large-scale cultivation is essential to drive more effective efforts to optimize biomass productivity

Nutrient removal from an anaerobic membrane bioreactor effluent using microalgae. Study and modeling of the process

Tesis por compendio[EN] Anaerobic membrane bioreactors for urban wastewater treatment present interesting advantages when compared with aerobic treatments, such as less sludge production, lower energy demand and biogas generation. However, the generated effluent cannot generally be discharged without further ammonium and phosphate elimination. This thesis studies the removal of these inorganic nutrients by means of microalgae cultivation. The main objective of this work is therefore to obtain an autochthonous microalgal culture and to investigate its ability to grow on an already existing anaerobic effluent, as well as to research the extent to which ammonium and phosphate can be removed. Moreover, this thesis aims at providing the kinetic expressions which reproduce the main processes involved, in order to provide the basis for process simulation and design. Microalgae were isolated from a local wastewater treatment plant and their ability to grow on the anaerobic effluent -while successfully removing ammonium and phosphate- was demonstrated. An excellent water quality was obtained with a semicontinuous cultivation mode under constant illumination. The Scenedesmus and Chlorococcum genus proliferated more efficiently and thus became predominant in the culture. Results also showed that phosphorus was the limiting nutrient in the anaerobic effluent to be treated. The influence of phosphorus limitation on ammonium and phosphate removal, as well as the influence of temperature in ammonium removal, were then studied under laboratory conditions. Kinetic expressions which reproduce the observed effects were proposed and validated, taking also into account the effect of light intensity. Additionally, a Scenedesmus-dominated culture was grown under varying light and temperature in an outdoor flat-plate photobioreactor, with constant monitoring of light intensity, temperature and ammonium concentration. Acceptable results were obtained in the reproduction of the experimental data, albeit with less accuracy than under laboratory conditions. The work here presented demonstrates the feasibility of coupling a microalgal cultivation system to an anaerobic membrane bioreactor for urban wastewater treatment. The basic factors affecting microalgal nutrient removal are researched, and mathematical models are provided which reproduce these effects. This Ph.D. thesis is enclosed in a national research project funded by the Spanish Ministry of Economy and Competitiveness entitled "Estudio experimental de la recuperación como biogás de la energía de la materia orgánica y nutrientes del agua residual, acoplando un AnBRM y un cultivo de microalgas" (MINECO project CTM2011-28595-C02-01/02). This research was also supported by the Spanish Ministry of Education, Culture and Sport via a pre doctoral FPU fellowship to the author (AP2009-4903)[ES] En el tratamiento de aguas residuales urbanas, los bioreactores anaerobios de membranas presentan ventajas interesantes frente a los tratamientos aerobios. Algunas de estas ventajas son la menor producción de fangos, un menor consumo energético y la producción de biogás. Sin embargo, y generalmente, el efluente obtenido no puede ser vertido al medio sin una etapa previa de eliminación de amonio y fosfato. La presente tesis estudia la eliminación de dichos nutrientes inorgánicos empleando para ello un cultivo de microalgas. El objetivo principal de este trabajo es, por tanto, la obtención de un cultivo autóctono de microalgas y la evaluación de la capacidad que éstas tienen tanto de crecer en un efluente anaerobio como de eliminar el amonio y el fosfato presentes. Asimismo, se pretenden proporcionar las bases para la simulación y el diseño del sistema de depuración propuesto, mediante la obtención de las expresiones cinéticas que reproducen los principales procesos involucrados. En primer lugar se ha demostrado la capacidad de las microalgas, aisladas en una estación depuradora de aguas residuales, de crecer en el efluente anaerobio y de eliminar con éxito el amonio y fosfato en éste presente. El agua tratada, obtenida a mediante un proceso semicontinuo y con iluminación constante, presenta una excelente calidad. Los géneros Scenedesmus y Chlorococcum han proliferado más eficientemente y han llegado a ser los predominantes en el cultivo. Los resultados obtenidos indican que el nutriente limitante en el efluente a tratar es el fósforo, y por tanto la influencia de la limitación de fósforo en la eliminación de nutrientes ha sido estudiada en condiciones de laboratorio, junto con la influencia de la temperatura en la velocidad de eliminación de amonio. Han sido propuestas y validadas las correspondientes expresiones cinéticas que reproducen los efectos observados, teniendo en cuenta en todo momento la influencia de la intensidad de la luz. Por otro lado, un cultivo de Scenedesmus ha sido cultivado en el exterior, bajo condiciones cambiantes de luz y temperatura, que a su vez han sido monitorizadas constantemente, junto con la concentración de amonio. Los datos obtenidos han sido reproducidos mediante modelación matemática con resultados aceptables, aunque la precisión obtenida es menor que en condiciones de laboratorio. La presente tesis demuestra la viabilidad de combinar un cultivo de microalgas con un bioreactor de membranas para el tratamiento de agua residual urbana. Se exponen asimismo los factores básicos que influyen en la velocidad de eliminación de nutrientes, y se presentan los modelos matemáticos necesarios para reproducir los efectos observados. La presente tesis doctoral se incluye en el marco de un proyecto nacional de investigación financiado por el Ministerio de Economía y Competitividad de título "Estudio experimental de la recuperación como biogás de la energía de la materia orgánica y nutrientes del agua residual, acoplando un AnBRM y un cultivo de microalgas" (CTM2011-28595-C02-01/02). La presente tesis doctoral ha sido también financiada por el Ministerio de Educación, Cultura y Deporte a través de una ayuda para contratos predoctorales de Formación del Profesorado Universitario (AP2009-4903).[CA] En el tractament d'aigües residuals urbanes, els bioreactors anaerobis de membrana tenen avantatges interessants respecte als tractaments aerobis. Alguns d'aquests avantatges són: menys producció de fangs, menys consum energètic i la producció de biogàs. No obstant això, i en general, l'efluent obtingut no es pot tornar al medi sense una etapa prèvia d'eliminació d'amoni i fosfat. Aquesta tesi estudia l'eliminació d'aquests nutrients inorgànics emprant per a fer-ho un cultiu de microalgues. L'objectiu principal d'aquest treball és, per tant, l'obtenció d'un cultiu autòcton de microalgues i l'avaluació de la capacitat que aquestes tenen tant de créixer en un efluent anaerobi com d'eliminar l'amoni i el fosfat presents. Així mateix, volem proporcionar les bases per a la simulació i el disseny del sistema de depuració proposat, mitjançant l'obtenció de les expressions cinètiques que reprodueixen els principals processos involucrats. En primer lloc, s'ha demostrat la capacitat de les microalgues, aïllades en una estació depuradora d'aigües residuals, de créixer en l'efluent anaerobi i d'eliminar amb èxit l'amoni i el fosfat presents. L'aigua tractada, obtinguda mitjançant un procés semicontinu i amb il·luminació constant, presenta una qualitat excel·lent. Els gèneres Scenedesmus i Chlorococcum han proliferat més eficientment i han arribat a ser els predominants en el cultiu. Els resultats obtinguts indiquen que el nutrient limitant en l'efluent per tractar és el fòsfor, i per tant la influència de la limitació de fòsfor en l'eliminació tant d'amoni com de fosfat ha sigut estudiada en condicions de laboratori, juntament amb la influència de la temperatura en la velocitat d'eliminació d'amoni. S'han proposat i validat les expressions cinètiques corresponents que reprodueixen els efectes observats, tenint en compte en tot moment la influència de la intensitat de la llum. D'altra banda, s'ha cultivat a l'exterior un cultiu predominat per Scenedesmus, sota condicions canviants de llum i temperatura, que al seu torn s'han monitorat constantment, juntament amb la concentració d'amoni. Les dades obtingudes s'han reproduït mitjançant simulació matemàtica amb resultats acceptables, encara que la precisió obtinguda és més baixa que en condicions de laboratori. La nostra tesi demostra la viabilitat de combinar un cultiu de microalgues amb un bioreactor de membrana per al tractament d'aigua residual urbana. La tesi exposa així mateix els factors bàsics que influeixen en la velocitat d'eliminació de nutrients, i presenta els models matemàtics necessaris per a reproduir els efectes observats. Aquesta tesi doctoral s'inclou en el marc d'un projecte nacional de recerca finançat pel Ministeri d'Economia i Competitivitat amb el títol "Estudio experimental de la recuperación como biogás de la energía de la materia orgánica y nutrientes del agua residual, acoplando un AnBRM y un cultivo de microalgas" (CTM2011-28595-C02-01/02). La tesi doctoral ha sigut també finançada pel Ministeri d'Educació, Cultura i Esport a través d'una ajuda per a contractes predoctorals de formació del professorat universitari (AP2009-4903).Ruiz Martínez, A. (2015). Nutrient removal from an anaerobic membrane bioreactor effluent using microalgae. Study and modeling of the process [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/59409TESISCompendi

Development and optimization of microalgae cultivation systems: microalgal composition, photobioreactor design and characterization

Tese de doutoramento em Biological and Chemical EngineeringThis work aimed at the development and optimization of systems and techniques for microalgae cultivation, in order to make the process economically and environmentally sustainable. Three different strategies were adopted: i) maximize productivity through the optimization of culture conditions, ii) maximize productivity and decrease costs by the use of agro-industrial waste as cultivation medium; iii) development of a new, low cost and highly productive microalgae cultivation system. Carbon dioxide (CO2) is the most widely used carbon source for photoautotrophic growth of microalgae. The rate of CO2 fixation (RCO2) by Chlorella vulgaris was maximized by defining the values of CO2 concentration in air feed and aeration rate. The results revealed that the maximum RCO2 (2.22 g L-1 d-1) was obtained using 6.5% (v/v) CO2 and 0.5 vvm. Although biomass concentration and mass productivity were affected by growth conditions, no differences were obtained in the biochemical composition of cells. The optimization of specific productivity (starch and lipids for the production of bioethanol and biodiesel, respectively) was performed using strategies of nutrient limitation. Starch accumulation in C. vulgaris cells was evaluated under different initial concentrations of urea (nitrogen source) and FeNa-EDTA (iron source) in the medium. Based on the results, a two-stage process for obtaining culture cells with high concentrations of starch (> 40%) was proposed: a first stage of cultivation with initial urea and FeNa-EDTA concentrations of 1 and 0.08 g L-1, respectively, which aims at maximizing biomass productivity, followed by a second stage of cultivation in the absence of these nutrients to induce starch accumulation. The increase of lipid content in Parachlorella kessleri cells was induced using a culture medium dilution strategy. Photosynthetic carbon partitioning into starch and neutral lipid, as well as the influence of nutrient depletion and repletion on growth and pigment content in the green microalga P. kessleri were studied. The study revealed that P. kessleri used starch as a primary carbon and energy storage, but the stress caused by the decrease of nutrients concentration made the microalgae to shift the fixed carbon into reserve lipids as a secondary storage product. These findings indicate that nutritional limitation can be used in P. kessleri cultivation as a very effective and cheap strategy to increase lipid productivity, for biofuel production. Growth parameters and biochemical composition of the green microalga C. vulgaris cultivated under different mixotrophic conditions were determined and compared to those obtained from a photoautotrophic control culture. Supplementation of the inorganic culture medium with hydrolysed cheese whey powder solution, when compared photoautotrophic growth, led to a significant improvement in microalgal biomass production (from 0.10 ± 0.01 to 0.75 ± 0.01 g L d-1) and an increase in carbohydrate utilization when compared with the culture enriched with a mixture of pure glucose and galactose (from 80.5 and 49.5% of glucose and galactose utilization, respectively, to an utilization of 100% of these carbohydrates), possibly due to the presence of growth promoting nutrients in cheese whey. Mixotrophic cultivation of C. vulgaris using the main dairy industry by-product could be considered a feasible alternative to reduce the costs of microalgal biomass production, since it does not require the addition of expensive carbohydrates to the culture medium. A characterization of liquid and gas phases was performed, the mass transfer coefficient was determined, together with the light distribution profiles and flow patterns of three different photobioreactors (PBRs), namely bubble column (BC), split cylinder airlift photobioreactor (SCAPBR) 75 and SCAPBR 50. The effect of these parameters on biomass productivity was discussed. The developed SCAPBRs proved to be extremely suitable for microalgae cultivation. The design of photobioreactors (PBR), particularly the designed gas sparger, allowed meeting the needs of microalgae in terms of efficient mixing and good mass transfer coefficients (efficient supply and removal of CO2 and O2, respectively). SCAPBR 50 (at UGr = 0.0044 m s-1) showed, among the tested PBRs, the highest value of biomass productivity (0.75 g L-1 d-1). This result has been attributed to a higher efficiency of light distribution inside the PBR and to a regular and defined flow pattern, which allows exposing cells to regular light-dark periods, as demonstrated in the present workA realização deste trabalho visou o desenvolvimento e optimização de sistemas e técnicas de cultivo de microalgas de forma a tornar o processo económica e ambientalmente sustentável. Três estratégias distintas foram adoptadas: i) maximização da produtividade recorrendo à optimização das condições de cultivo; ii) maximização da produtividade e diminuição de custos recorrendo à utilização resíduos agroindustriais como meio de cultivo; iii) desenvolvimento de um novo sistema de cultivo de baixo custo e elevada produtividade. O dióxido de carbono (CO2) é a fonte de carbono mais utilizada no crescimento fotoautotrófico de microalgas. A taxa de fixação de CO2 (RCO2) por parte da Chlorella vulgaris, foi optimizada através da definição dos valores de concentração de CO2 e taxa de arejamento. Os resultados obtidos revelaram que a RCO2 máxima (2,22 g L-1 d-1) foi observada utilizando 6,5 % CO2 e 0,5 vvm. Apesar da concentração de biomassa e produtividade mássica terem sido afectadas pelas condições de cultivo, não foram obtidas diferenças na composição bioquímica das células. A optimização da produtividade específica (amido e lípidos destinados à produção de bioetanol e biodiesel, respectivamente) foi efectuada recorrendo a estratégias de limitação nutricional. A acumulação de amido em células de C. vulgaris foi avaliada sob diferentes concentrações iniciais de ureia (fonte de azoto) e FeNa-EDTA (fonte de ferro) no meio de cultivo. Com base nos resultados obtidos, foi proposto um processo de cultivo para a obtenção de células com elevadas concentrações de amido (> 40%), composto por duas fases: uma primeira fase de cultivo com concentrações iniciais de ureia e FeNa-EDTA de 1,1 e 0,08 g L-1, respectivamente, que tem como objectivo maximizar a produtividade em biomassa; seguida por uma segunda etapa de cultivo sem a presença destes nutrientes, induzindo a acumulação de amido nas células. O aumento do teor de lípidos em células Parachlorella kessleri foi induzida utilizando como estratégia a diluição do meio de cultura. A partição do carbono fotossintético em amido e lípidos neutros, bem como a influência da depleção e repleção de nutrientes no crescimento e teor de pigmentos na microalga P. kessleri foi estudada. O estudo revelou que a P. kessleri utiliza amido como fonte primária de armazenamento de carbono e energia, mas o stress causado pela diminuição da concentração de nutrientes faz a microalga direcionar o seu metabolismo para a acumulação de lípidos, sendo estes reserva energética secundária. Estes resultados indicam que a limitação nutricional pode ser usada na P. kessleri cultivo como uma estratégia muito eficaz e barata para aumentar a produtividade de lípidos. Foram determinados os parâmetros de crescimento e composição bioquímica da microalga C. vulgaris, cultivada em diferentes condições de mixotrofia, e comparados com os obtidos no cultivo padrão, efectuado em condições fotoautotróficas. A suplementação do meio de crescimento com soro de queijo hidrolisado levou a um aumento muito significativo da produtividade em termos de biomassa quando comparado com o crescimento fotoautotrófico (de 0,10 ± 0,01 para 0,75 ± 0,01 g L d-1) e a um aumento da utilização dos hidratos de carbono presentes no meio quando comparado com uma cultura enriquecida apenas com glucose e galactose (de 80,5 e 49,5% de consumo de glucose e galactose, respectivamente, para 100% de utilização destes hidratos de carbono), possivelmente devido à presença de nutrientes do soro de queijo que promovem o crescimento. O cultivo mixotrófico de C. vulgaris recorrendo ao principal subproduto da indústria dos lacticínios, pode ser considerada como uma alternativa bastante promissora para a redução de custos da produção de microalgas. A caracterização das fases líquida e gasosa, bem como a determinação do coeficiente de transferência de massa, a determinação do perfil de distribuição da luz e do padrão de fluxo foi efectuada em três fotobioreactores diferentes (BC, SCAPBR 75 e 50). Os SCAPBRs desenvolvidos revelaram-se extremamente adequados para o cultivo de microalgas. O design do SCAPBR, particularmente o sistema de arejamento desenvolvido, permitiu colmatar na totalidade as necessidades da microalga em termos de coeficientes de massa de mistura eficientes (fornecimento eficiente e remoção de CO2 e O2, respectivamente). SCAPBR 50 (com UGr = 0,0044 m s-1) apresentou o valor mais elevado de produtividade (0,75 g L-1 d-1). Este resultado deveu-se a uma maior eficiência da distribuição de luz no interior da PBR e um padrão de fluxo regular e definido, o que permite expor as células a ciclos regulares de luz e sombra

Sustainable conversion of light to algal biomass and electricity: A net energy return analysis

A substantial interest is growing in the cultivation of microalgae as a source of biofuel production, considering their relatively high lipid content, fast growth rates, use of alternative water sources, and growth on non-arable land. This paper conducts an energy life cycle analysis for a novel hypothetical hybrid energy system where the electricity required for microalgae cultivation is generated from semi-transparent PV panels to energise paddle wheels and light emitting diodes installed on raceway ponds. The combined system configuration allows for a full utilisation of the solar spectrum, while enhancing the photosynthetic productivity of microalgae cultivation and reducing the evaporation from raceway ponds. The findings of study for a hypothetical system installed in Western Australia show that the amount of land use substantially decreases by 43%, the productivity of microalgae cultivation increases by 75%, while the net energy return of the system remains significantly higher than one, in comparison with a microalgae cultivation system energised by grid electricity. Among a range of variables affecting the energy performance of the proposed system, the primary energy demand for PV panels and conversion efficiency of LEDs exert the highest impact on energy life cycle of the syste

Microalgae Cultures

Microalgae have been intensively studied for CO2 capture, nutrient removal from wastewater, and biofuels production. These photosynthetic microorganisms use solar energy with efficiency ten times greater than terrestrial plants and are responsible for about 50% of the world’s oxygen production. Therefore, microalgae have been considered a sustainable solution for CO2 capture. Besides carbon, their growth also requires other macronutrients: nitrogen and phosphorus. To avoid the addition of fertilizers (increasing the production costs), these nutrients can be supplied if wastewater is used as the culture medium. The integration of biomass production with wastewater treatment enables a reduction in operational costs and the environmental impact. Microalgae are also known for their high lipid contents and high growth rates and are a promising oil source for biodiesel production. This Special Issue Book presents the recent research activities concerning the environmental applications of microalgae and their potential for biofuels production, focusing on the main challenges for their large-scale application. Since microalgal culturing can address different environmental and non-environmental issues, the achievements from the integration of multiple microalgal applications are also considered in this Special Issue Book

Artificial Leaf for Biofuel Production and Harvesting: Transport Phenomena and Energy Conversion

textMicroalgae cultivation has received much research attention in recent decades due to its high photosynthetic productivity and ability to produce biofuel feedstocks as well as high value compounds for the health food, cosmetics, and agriculture markets. Microalgae are conventionally grown in open pond raceways or closed photobioreactors. Due to the high water contents of these cultivation systems, they require large energy inputs for pumping and mixing the dilute culture, as well as concentrating and dewatering the resultant biomass. The energy required to operate these systems is generally greater than the energy contained in the resultant biomass, which precludes their use in sustainable biofuel production. To address this challenge, we designed a novel photobioreactor inspired by higher plants. In this synthetic leaf system, a modified transpiration mechanism is used which delivers water and nutrients to photosynthetic cells that grow as a biofilm on a porous, wicking substrate. Nutrient medium flow through the reactor is driven by evaporation, thereby eliminating the need for a pump. This dissertation outlines the design, construction, operation, and modeling of such a synthetic leaf system for energy positive biofuel production. First, a scaled down synthetic leaf reactor was operated alongside a conventional stirred tank photobioreactor. It was demonstrated that the synthetic leaf system required only 4% the working water volume as the conventional reactor, and showed growth rates as high as four times that of the conventional reactor. However, inefficiencies in the synthetic leaf system were identified and attributed to light and nutrient limitation of growth in the biofilm. To address these issues, a modeling study was performed with the aim of balancing the fluxes of photons and nutrients in the synthetic leaf environment. The vascular nutrient medium transport system was also modeled, enabling calculation of nutrient delivery rates as a function of environmental parameters and material properties of the porous membrane. These models were validated using an experimental setup in which the nutrient delivery rate, growth rate, and photosynthetic yield were measured for single synthetic leaves. The synthetic leaf system was shown to be competitive with existing technologies in terms of biomass productivity, while requiring zero energy for nutrient and gas delivery to the microorganisms. Future studies should focus on utilizing the synthetic leaf system for passive harvesting of secreted products in addition to passive nutrient delivery.Mechanical Engineerin

Feasibility study of biogas upgrading coupled with nutrient removal from anaerobic effluents using microalgae-based processes

Producción CientíficaThe present research was conducted to simultaneously optimize biogas upgrading and carbon and nutrient removal from centrates in a 180-L high-rate algal pond interconnected to an external CO2 absorption unit. Different biogas and centrate supply strategies were assessed to increase biomass lipid content. Results showed 99 % CO2 removal efficiencies from simulated biogas at liquid recirculation rates in the absorption column of 9.9 m3 m−2 h−1, concomitant with nitrogen and phosphorus removal efficiencies of 100 and 82 %, respectively, using a 1:70 diluted centrate at a hydraulic retention time of 7 days. The lipid content of the harvested algal–bacterial biomass remained low (2.9–11.2 %) regardless of the operational conditions, with no particular trend over time. The good settling characteristics of the algal–bacterial flocs resulted in harvesting efficiencies over 95 %, which represents a cost-effective alternative for algal biomass reutilization compared to conventional physical–chemical techniques. Finally, high microalgae biodiversity was found regardless of the operational conditions.Junta de Castilla y León (programa de apoyo a proyectos de investigación – Ref. (Project GR76, VA024U14, and RTA2013-00056-C03-02