Aim of the project
Nowadays, the problems associated to pollution are a major concern of society. Strick environmental laws have appeared to account for that, since, in terms of health, environment and economy, the fight against pollution has become a major issue. The EU advices that, since the world population will reach 9000 million of people in 2050, it is mandatory to look for new methodologies and solutions to produce, consume, transform, store, recycle and remove residuals; at the same time that the environmental impact is reduced. This objective can be reached by an equilibrated use of clean resources and by transforming the residual in valuable bioproductors, bioenergy, food, and clean water. Thus, in this context, wastewater treatment and mitigation of industrial flue gases to reduce the greenhlouse gases emissions are two challenge problems to contribute in this concern of the society (Abdel-raouf, 2012; Union Europea, 2014).
Today, although the strategic importance of fresh water is universally recognized more than ever before, and although issues concerning sustainable water management can be found almost in every scientific, social, or political agenda all over the world, water resources seem to face severe quantitative and qualitative threats. The pollution increase, industrialization and rapid economic development, impose severe risks to availability and quality of water resources, in many areas worldwide. The main problem of the existing wastewater treatment systems is that they are economically unefficient because of land and energy costs requirements (Abdel-raouf, 2012). Research is needed to improve the understanding of the dynamics of emerging pollutants and improve methods to remove these pollutants from wastewater (Acién, et. al., 2016). Thus, efficient wastewater treatment solutions are required.
On the other hand, regarding the reduction of industrial flue gases problem, microalgae are becoming an important solution that help to fix CO2 and contribute to their mitigation, as well as, to produce biomass to be use for food, cosmetis or energy purposes. Neverthenless, despite the large potential of products derived from algae, its implementation is still limited mainly due to unfavourable economics. Thus, microalgae are being applied only in niche markets as food supplements and cosmetics. However, they have a large potential as source of bioactive compounds due to their high productivity compared to other biomass sources, and the possibility to be produced by using only wastewaters and flue gases as nutrient source. The microalgae production cost is currently high, but it can be reduced by using wastewaters as nutrients source, improving the productivity and robustness of production systems, and maximizing the revenues from the final microalgae (Acién, et. al., 2016).
Wastewater is a source of high-value hydrocarbons through microalgae. Biofuel/biomass can be produced from wastewater through conversion of its nutrients in into microalgae biomass. Moreover, microalgae can be used for cleaning wastewater, capturing carbon dioxide and producing alternative sustainable energy without competing with agriculture for water, fertilizer and land. Also, wastewater combined with microalgae can be used to produce biodegradable bioplastics replacing traditional petroleum based plastics at lower costs, and to produce cosmetics and medical products (WWAP, 2017). Microalgae culture offers an interesting step for wastewater treatments, because they provide a tertiary biotreatment coupled with the production of potentially valuable biomass, which can be used for several purposes. Microalgae offer an elegant solution to tertiary treatment due to the ability of microalgae to use inorganic nitrogen and phosphorus for their growth. Furthermore, due to their capacity to remove heavy metals, as well as some toxic organic compounds, it does not lead to secondary pollution (Abdel-Raouf et al. 2012). It is important to note that the main raw materials required for the production of microalgae are CO2 along with nitrogen and phosphorus (Gómez et al. 2013). So, if the microalgae production plants are placed closed to CO2-emitting industries, several environmental advantages are achieved with low cost: reduction of greenhouse gases, wastewater treatment and production of clean energy. The microalgae production costs using wastewater treatment can be covered by the wastewater treatment plant capital and operation cost themselves (Park et al. 2011).
The combination of microalgae production and wastewater treatment has been proposed as an option since the 1960s, but however, this technology has not been expanded to an industrial scale (Acién et. al. 2016). The reason this technology has not been applied at the large scale is because it is not as efficient as conventional processes. Recently, it has been analyzed that combining microalage production with wastewater treatment in an adequate way, it can provide imoprtant advantages for both processes. The utilization of microalgae for wastewater treatment allows a reduction of the energy associated to conventional wastewater treatment processes up to achieve an energy positive process, consuming less than half of energy required in conventional treatments also reducing the wastewater treatment cost to less than half of conventional systems (by refunding costs thanks to the derived products). On the other hand, such as comented above, wastewater may be used as cheap nutrient sources for algal biomass production, since it contains carbon, nitrogen, phosphorus and other minor components required for microalgal growth. Thus, an adequate sinergy of these two processes regarding to energy requirements, nutrient demands, and CO2 injections, can contribute to obtain clean water and biomass-based products, and reducing at the same time the pollution coming from wastewater and CO2.
It has been demostrated that the performance of microalgae-based processes for wastewater treatment is principally centred on adequate microalgae growth conditions. Bacteria digest the organic matter very rapidly and produce CO2, NH4+ and PO4−3, which then have to be assimilated by the microalgae. If the culture conditions are inadequate for the microalgae, the CO2 is stripped to the air instead of being consumed or stored in the water as bicarbonate buffer. The NH4+ can be oxidised to nitrate and then denitrified to N2 as a function of the bacterial metabolism, or directly stripped to the air as NH3 if the pH is higher than 9. The PO4−3 can be precipitated under alkaline conditions as calcium salt. To maximise the assimilation of inorganic compounds, the microalgal productivity has to be optimised—the higher the microalgal biomass productivity, the higher the system’s nutrient removal capacity (Morales-Amaral et al. 2015). In order to achieve these objectives, both the biological and the engineering aspects of the process must be considered. Optimal strategies to maximise the system’s efficiency must be implemented, and the developed processes’ performance must be validated under real conditions over a long period. Thus, methodologies based on modelling and control approaches are becoming a solution to reach these objectives.
Hence, the focus of this project will address issues that are related to improve the efficiency, productivity, design and optimization of large-scale raceway combined wastewater treatment and microalgae production processes cultivated in outdoor conditions by means of using adequate modelling and control strategies. The main objective is to obtain adequate modelling and control approaches to contribute in better reproducible conditions with competitive market costs by analysing/simulating new photobioreactor designs, compensating for the permanent non-stationary behaviour of the processs, the presence of disturbances, taking advantage of nutrients provided by wastewater to the culture (mainly carbon, nitrogen, oxygen and phosphorous), removing any toxic metabolic products (e.g. CO2 mitigation), and controlling important internal cellular parameters (e.g. temperature, pH), in order to optimize the biomass production.
The project aims to develop modelling and control methodologies to optimize the microalgal biomass production and wastewater treatment in large industrial photobioreactors. To this end, and according to the research groups’ experience, Support Vector Regression (SVR), Probabilistic Estimators (PE), Inteligent Control (IC), Robust Control (RC), Event-Based Control (EBC), and Model Predictive Control (MPC) control approaches will be analysed. The modelling issues will be tackled using object-oriented modelling techniques previously used in tubular and raceway photobioreactors (Pérez-Castro, et. al., 2014; Castro et. al. 2017). The modelling and control objectives of the project can be grouped in two different categories according to the hierarchical feature of the microalgal growth problem. According to the previous experience with tubular previous projects, the dynamics of photobioreactors can be described by Partial Differential Equations (PDE) derived from mass and energy balances (Fernández, et. al. 2014a; 2016a) or approximated by lumped-parameter time-varying nonlinear models (Fernández, et. al. 2014b). Thus, PDE and lumped-parameter time-varying nonlinear models will be developed for the new raceway systems. On other hand, SVC and PE techniques will be used to develop virtual sensors and estimators of the main unmeasurable varialbes, such as biomass concentration or total inorganic cargon (Hong, et. al. 2015). Nonlinear (Robust) MPC provides powerful solutions to optimize the microalgae growth in spite of incomplete information and guaranteeing the operating constraints (Camacho and Bordóns, 2004; Gruber, et. al. 2011; Tebbani, 2014). Thus, robust NMPC and MPC-based reference governor solutions will be employed to deal with the optimal biomass production problem (Gruber, e. al., 2011; Ramírez, et. al., 2012), such as used in previous projects (Andrade, et. al, 2016a; Fernández, et. al., 2016b). Based on the changing dynamics and the effect of the environmental variables, robust control, feedforward control and event-based predictive control techniques will be also addressed specially for low-level control loops, such as have been successfully applied in previous projects photobioreactors (Andrade, 2014; Beschi, et. al., 2014; Guzman and Hägglund, 2011; Moreno, et. al, 2013; Pawlowski, et. al., 2014a, 2014b; Rodríguez, et. al., 2015; Guinaldo, et. al. 2016; Hoyo, et. al. 2017; Carreño, et. al. 2017). Furthermore, inteligent control approaches will be studied to be applied to this problem based on the distributed nature of the system (Sutton and Barton, 1998; Syafiie, et. al., 2011). Specifically, Reinforcement Learning techniques will be studied to regulate variables in the model of a wastewater treatment plant that use microalgae for both reducing energy consumption while at the same time recycling nitrogen and phosphorous.
The result of the project will be validated by means of different industrial facilities of raceway photobioreactors (which are described in section C.2.5), available at Experimental Station of the Cajamar Foundation located at Almería, Southeast of Spain (which is a research centre that is going to actively participate in this project) and at the IFAPA center (located close to the University of Almería). In order to formulate and attain the different control specifications for these processes, it is required the concurrence of experts from different fields, what justify the interdisciplinary nature of the proposed coordinated project. The proposal is also supported by CIESOL, a mixed research centre in solar energy between CIEMAT and the University of Almería, which will help in the development of solar radiation models. The results of the project will be studied and/or exploited by three important companies that support and are very interested in this project: Aqualia, Biorizon Biorizon (which is a company working in biotechnology to creat products derived from microalgae), and UTE Poniente Almeriense (residue management company by FCC).