Microalgae-based livestock wastewater treatment and resource recovery: a circular bioeconomy approach
López Sánchez, Anaid
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The livestock industry is a sector of great relevance worldwide. This sector accounts for 1.4% of the world's Gross Domestic Product (GDP) and is a source of livelihood for more than 1.3 billion people. Furthermore, thirty-nine percent of the worldwide protein demand is covered by this sector. However, this activity is one of the top polluting industries, accounting for 14% of the greenhouse gasses (GHG) originated from anthropogenic sources. Additionally, the livestock sector is the largest land user on earth, using 70% of the total agricultural land and 30% of Earth’s land surface. One-third of the global cereal production is destinated for animal feed, of which some nutrients are retained by the animals and the rest is released to the environment without previous treatment, resulting in soil degradation, water and air pollution and, consequently, serious human health impacts. Circular bioeconomy (CBE) has emerged as a potential driver towards the sustainability of livestock production systems. One of the main objectives of the CBE model within the livestock industry is the minimization of the usage of raw material resources through the recycling, reuse, and revalorization of waste and wastewater. Microalgae-based wastewater treatment (MbWT) is a potential solution aligned with the CBE principles, in which the nutrients contained in the livestock wastewater (LW) are recovered and transformed into high value-added products with a wide range of industrial applications. The overall performance of MbWT (i.e., nutrient removal efficiencies and biomass production) is highly dependent on a wide range of factors, such as the microalgal strain and the composition of the wastewater. However, most of the existing studies that implemented MbWT have focused on a single LW type. Therefore, the main objective of this thesis is to treat a mixed effluent composed of the most common ADLW (from cattle, swine, and poultry), to understand the effects of the mixture of all three types of LW on cell growth and pollutant removal efficiencies of microalgal cultures (Chlorella vulgaris, Haematoccocus pluvialis and Chlamydomonas spp.). Through an evaluation of the mixture design, the optimal fraction of these different types of effluents (ADCW, ADSW, and ADPW) was analyzed to obtain maximum microalgal biomass productivity and 7 pollutant removal rates (COD, TN and TP). Furthermore, these microalgae were tested in all possible combinations of mono-, bi-, and tri-cultures. The first chapter of the present thesis consists of a thorough review of the literature to address the most significant factors affecting nutrient removal and biomass productivity in MbWT, including: (i) microbiological aspects, such as the microalgal strain used for MbWT and the interactions between microbial populations; (ii) physical parameters, such as temperature, light intensity and photoperiods; and (iii) chemical parameters, such as the C/N ratio, pH and the presence of inhibitory compounds. Additionally, different strategies to enhance nutrient removal and biomass productivity, such as acclimation, UV mutagenesis, multiple microalgae culture stages (including monocultures and multicultures) are discussed. The second chapter of this thesis presents the first study of MbWT using anaerobically digested swine, poultry and cattle wastewater (ADSW, ADPW and ADCW) mixtures. A centroid mixture design was used to determine the optimal mixture to promote higher cell concentrations and pollutant removal efficiencies of the microalgae Chlorella vulgaris, Haematococcus pluvialis and Chlamydomonas spp. cultured as mono-, bi-, and tri-cultures. Additionally, A redundancy analysis was performed to analyze the correlation between microalgal cultures and the removal efficiencies of the digestate pollutants. The results herein show that C. vulgaris as a monoculture in a digestate mixture of 0.125:0.4375:0.4375 (ADSW:ADPW:ADCW) resulted in cell growth of 3.61×107 ± 2.81x106 cell mL-1, a total nitrogen removal of 85%±2%, a total phosphorus removal of 66%±3% and a chemical oxygen demand removal of 44%±7%. The specific composition of the effluents plays a key role in microalgal performance due to their respective nitrogen and phosphorus content. Furthermore, this study suggests that a mixture of the three most common digestates generated by livestock farms offers a promising alternative for the treatment and revalorization of LW, by taking advantage of the unique composition that each digestate possesses. Further studies are warranted to gain a deeper understanding of the interspecific microalgal interactions occurring in mixed cultures that may enhance or hinder the performance of MbWT.