Biological treatment of swine manure with microalgae-bacteria consortium. Removal and fate of emerging pollutants present in pig manure wastewater

Résumé

Spain is the second country in Europe with the highest consumption of meat and the fourth largest producer of pigs in the world, with around 30 million heads and a generation of around 44 Mt of pig manure per year (223 Mt in the European Union). Manure has very high concentrations of organic matter, nitrogen and phosphorus that make it highly polluting. Nitrate contamination of aquifers due to agricultural and livestock activities is one of the main environmental concerns in the EU. In addition, pig manure contains antibiotics that due to its improper management can be released and enter the environment through multiple routes, such as wastewater treatment plants, runoff from fields to surface water, percolation to groundwater, among others. In recent years, different studies have shown that the use of microalgae for wastewater treatment can purify water contaminated with pig manure, reducing antibiotics concentration and reducing their effect. Additionally, the microalgae biomass contains proteins, carbohydrates, lipids and some minor products with high added value, which can contribute to the creation of a sustainable economy, using renewable resources. In this sense, this thesis focuses on the study of the mechanisms involved in the removal of antibiotics present in pig manure when treated by biological treatment with microalgae-bacteria consortium. Furthermore, an alternative treatment is proposed through an advanced oxidation process (AOP), such as photocatalysis, to reduce, and if possible eliminate, the presence of antibiotics. The state-of-the-art of veterinary antibiotic treatment technologies is presented in the Introduction section. The objectives, approach and strategies followed in this thesis are summarized in the Aims and Scope section. In Chapter 3, the mechanisms (hydrolysis, photolysis, biosorption and biodegradation) involved in the removal of a mixture of four veterinary antibiotics (VA); tetracycline (TET), ciprofloxacin (CIP), sulfadiazine (SDZ) and sulfamethoxazole (SMX); in synthetic wastewater using microalgae-bacteria consortia (MBC) dominated by Scenedesmus almeriensis was studied at different initial concentrations of 1000, 500, 100 and 20 µg/L of each antibiotic. In Chapter 4, the adsorption of the veterinary antibiotics onto a dried Scenedesmus almeriensis microalgae-bacteria consortium was studied at several concentrations (20 to 1000 µg/L). In Chapter 5, the removal of a mixture of four veterinary antibiotics via photo-degradation (UVC) and photocatalysis with TiO2 (UVC/TiO2) was investigated in a batch reactor under different initial antibiotics concentrations (20, 100, 500 and 1000 µg/L per antibiotic). Additionally, electrical energy per order (EEO) was assessed to estimate the electrical energy efficiency of each process. In Chapter 6 the removal of a mixture of four veterinary antibiotics was evaluated operating with real samples corresponding to the liquid fraction of pig slurry, using Scenedesmus almeriensis microalgae-bacteria consortia in a pilot scale photobioreactor. After 15 days of operation the reactor was spiked with a mixture of 100;g/L of each antibiotic. The mass balance analysis of the entire process was also simulated by introducing the different kinetic constants of degradation and adsorption found in the previously described batch tests, to validate these values in a system that operated in quasi-continuous mode. The results obtained in the present thesis confirmed the potential of treating wastewater generated in pig farms by algae-bacteria consortia. Thus, this microalgae-based technology is revealed as a sustainable and efficient tool for the removal of antibiotics for veterinary use. This doctoral thesis provided a better understanding of the mechanisms involved in the removal of the investigated antibiotics, thanks to the study of the respective removal kinetics involved. In addition, it provided a comparison with an advanced oxidation process, such as heterogeneous photocatalysis with TiO2, which could be used in addition to biological treatment to improve the removal efficiency of veterinary antibiotics.