Integration of wastewater treatment and microalgae production for agricultural applications

  1. Morillas España, Ainoa
Supervised by:
  1. Cynthia Victoria González López Director
  2. Tomás Valentín Lafarga Poyo Co-director

Defence university: Universidad de Almería

Fecha de defensa: 25 July 2022

  1. J. A. Perales Vargas-Machuca Chair
  2. José María Fernández Sevilla Secretary
  3. Ana Otero Committee member

Type: Thesis

Teseo: 740473 DIALNET lock_openriUAL editor


Availability and access to water is fundamental for a sustainable development. Water scarcity affects more than 40% of the global population. According to a recent report of the United Nations, 3 in 10 people lack access to drinkable water and approximately 70% of all water abstracted from rivers, lakes, and natural aquifers is used for irrigation. This, together with the fact that approximately 80% of wastewater resulting from human activities is discharged into the environment without any pre-treatment represent a huge environmental problem. Wastewater treatment using conventional methods consists on a series of physical, chemical, and biological processes that aim to ensure that the water discharged into the environment meets an increasingly challenging quality criteria. It is estimated that, in Spain, around 4,950 hm3 of wastewater are processed per year; For this, more than 4700 wastewater treatment plants have been built and a budget of over 1300 million euros per year is needed. The elimination of the organic matter present in the wastewater is one of the most well-studied and optimized aspects of current wastewater treatment processes. However, most of this organic matter is transformed into CO2 that inevitably ends up in the atmosphere. Something similar occurs with the removal of nitrogen and phosphorus. Both are effectively removed from the wastewater but not recovered, ending up in the emission of greenhouse gases. Moreover, large energy requirements and complex equipment and processes are needed to comply with the current maximum nitrogen and phosphorus discharge limits. Carbon, nitrogen, and phosphorus are essential for microalgal growth. For this reason, during the last few years, microalgae-based wastewater treatment processes have gained an increased commercial interest. Their main advantages when compared to traditional processes include the recovery (not removal) of nitrogen and phosphorus and their biotransformation into valuable biomass that could be used as a feedstock for the production of high-end compounds and renewable energy. Furthermore, as microalgae are photosynthetic microorganisms, they can transform the CO2 that is produced naturally during the degradation of organic matter into complex molecules of high commercial interest. From an economic point of view, the production of microalgae using wastewater as a source of nutrients has also attracted the interest of the wastewater treatment industry. In fact, by combining wastewater treatment with microalgae production, the production costs of microalgae biomass can be theoretically reduced to less than 1€·kg-1, significantly lower than the current 5-10 €·kg-1 obtained when producing the biomass using freshwater and commercial nutrients. The microalgal biomass produced using wastewater cannot be used directly as food, the main use of microalgae today. However, it can be used indirectly for the production of food by being utilised as a raw material for the production of agricultural biostimulants or aquafeeds. One of the biggest challenges of urban wastewater treatment using microalgae is the huge amount of wastewater that is daily generated in urban areas. This, together with the fact that microalgae are photosynthetic microorganisms and require large surfaces to maximise light availability leads to the need of using very large photobioreactors and large surface areas. Moreover, the up-scaling of microalgae-based processes is also a technological challenge. To date, the vast majority of the scientific literature has been carried out at a laboratory-scale or at a pilot-but for short periods of time. The validation of these processes on a demonstrative scale and during long periods of time is essential to capture commercial interest and investements and to achieve the industrial implementation of microalgae-based wastewater treatment processes. One last problem related with the processing of wastewater using microalgae is that although there are hundreds of thousands of different microalgal strains in the environment, only a very limited number of strains have been studied in detail and just a few using wastewater. The overall objectives of this Doctoral Thesis were to up-scale current microalgae production processes and to demonstrate the technical viability of processing wastewater using microalgae. Both processes were validated at a pre-commercial scale and during the four seasos, which is essential to predict the potential industrial implementarion of the developed processes. Moreover, the assessment of the productivity and efficiency of the systems during a long period of time allos estimating the effect of environmental and operational conditions on a representative scale. The most common and economic photobioreactors are raceways, which are also easy to use and up-scale. However, despite allowing the processing of large volumes of water, these systems lead to relatively low biomass productivities (20 g·m-2·day-1). The main reasons are that as the depth of the culture is around 10-30 cm, the light availability inside the culture is low because of the self-shading effect of microalgae. An inefficient mass transfer is also responsible for their relatively low productivity. For this reason, the present Thesis evaluated the potential utilisation of more innovative photobioreactor designs such as thin-layer cascade photobioreactors. These reactors work with a culture depth lower than 5 cm, which permits a greater availability of light. Thin-layer reactors have not yet been optimized on a pilot scale and the assessment of their effectiveness in recovering nutrients from urban wastewater has not yet been evaluated. In the present work, different photobioreactors were used to produce biomass and process wastewater including 80 m2 raceway reactors and 63 and 163 m2 thinlayer cascade photobioreactors. Initially, the operation of the photobioreactors was optimised using freshwater and commercial fertilisers, achieving biomass productivities of 5100, 5600, and 9100 kg·year-1. The goal was to increase the sustainability of the process and, for this reason, the photobioreactors were then operated using wastewater as the sole nutrients source.The results revealed that if the process was upscales to a theoretical 10,000 m2 reactor, it would be possible to recover approximately 10.6 and 0.5 tons of nitrogen and phosphorus per year, respectively while simultaneously producing up to 56.5 tons of valuable biomass. In addition, a preliminary economic analysis revealed that using wastewater as the culture medium could reduce the production costs by 0.44 €·kg-1. One third objective of the thesis was the assessment of the potential use of ultrafiltration membranes as a strategy to increase the biomass productivity and the amount of wastewater that can be processed per surface area. The use of ultrafiltration membranes is common in conventional wastewater treatment processes, but has not yet been studied in microalgae-based systems. One of their main attributes is that they allow separating the cellular from the hydraulic retention time, therefore they could be used to increase the areal biomass productivity when producing the biomass using secondary wastewater or to separate the biomass from the culture medium. In the present wotk, the use of membranes allowed achieving biomass productivities 40% higher and processing larger amounts of water (130%) per surface area. Scaling up the values obtained with the use of ultrafiltration membranes to a theoretical 10,000 m2 raceway reactor coupled to an ultrafiltration membrane would allow treating 2.58 M m3, producing 79.92 tons of biomass per year. This means that not only the amount of water processed but also the annual biomass productivity was 30 tons higher when the ultrafiltration membrane was used. Finally, the produced biomass was assessed as a raw material for the production of agricultural biostimulants with very promising results. One of the selected strains (Chlorella vulgaris MACC 1) allowed increasing root development by over 493% and a cytokinin activity 60.9% higher. The production and commercialisation of microalgae-based biostimulants is already a reality. However, these are produced using freshwater and commercial fertilisers as the nutrient sources. The present work demonstrates that the production of agricultural biostimulants using wastewater is feasible and that the process shows potential for further development. The processing of wastewater, which is an environmental challenge, could be carried out while simultaneously producing agricultural biostimulants that could not only minimise the amount of irrigation water and fertilisers required but also promote crop yields and promote the quality of the end products. The research presented in this Doctoral Thesis is part of the work carried out by the Department of Chemical Engineering of the University of Almería framed in the R&D project "Microalgae for the sustainable production of bioproducts and reclaimed water (AL4BIO)" financed by MCIN/AEI/10.13039/501100011033/, for “FEDER A way of making Europe” and in collaboration with GEMMA-UPC. The objective of this project is to produce high-value bioproducts and reclaimed water in systems based on microalgae during wastewater treatment.