A NEW BIOREFINERY MODEL FOR LIVESTOCK FARMING: MICROALGAE CULTIVATION FOR ANIMAL SLURRIES VALORIZATION

Development of livestock farming sector poses serious concerns on its environmental impact due to the production of huge volume of slurries characterized by high concentrations of organic and mineral pollutants, mainly in the form of ammonia, phosphates and carbon compounds. The aim of this work was to evaluate the possibility to use microalgae as a biological tool for the depuration of such wastewaters, assimilating the nutrients and producing an added value biomass to be used in different sectors such as the food and feed, nutraceutical and bioenergy. Chapter 1 deals with the study of a digestate treatment plant characterized by a series of physical-chemical treatments which depurate the liquid fraction of the digestate through membrane technology (ultrafiltration and reverse osmosis) allowing to discharge 50% of the slurry as clean water and to produce ammonium sulfate and nutrients-rich solids/concentrates usable as fertilizers. The ultrafiltration step produces also a clear permeate still rich in soluble nutrients that could be further used as a growth medium for microalgae production. Chapter 2 intended to demonstrate the possibility to integrate microalgae production with this system, helping to reduce the cost of slurry treatment and improving the energy balance of the process. The tolerance of the microalga Scenedesmus sp. to the permeate was evaluated, results demonstrating that percentage upper than 10% inhibited the growth of this microalga, but below this value productivity up to 124 mg L-1 d-1 could be obtained. The composition of the culture medium also influenced the biomass composition, with protein, carbohydrate and lipid content being a direct function of ammonia concentrations. It was then demonstrated that integrating microalgae production with anaerobic digestion it is possible to produce 166-190 t y-1 of microalgal valuable biomass. Chapter 3 focused on the possibility to exploit a wild microalga strain (Chlorella sp.), isolated in the farm, to improve the depuration of the digestate and the two digestate liquid fraction after centrifugation and ultrafiltration. The results demonstrated that digestate could not support a good growth, as the other two liquid stream, because of low light availability in the culture. Ultrafiltrate, on the other hand, resulted in the best biomass productivity (0.21 g L-1d-1) comparable to that obtained in a synthetic medium. All the streams were depurated with ammonia, phosphorus and COD reduction up to 98%, 99% and 70% respectively Besides these encouraging data it has been found that only 30% of the nitrogen were successfully incorporated in the microalgal biomass due to stripping processes, posing serious environmental concerns on the process In Chapter 4 astaxanthin-producer Haematococcus pluvialis was cultivated in a treated swine slurry with low-cost cascade filters. Although this microalga is slow-growing and very susceptible to contamination, it showed a sustained growth (up to 60 mg L-1 d-1 of biomass) in the waste stream reducing all the pollutants present in the wastewater. Moreover it accumulated a good amount of astaxanthin, improving the overall feasibility and sustainability of the process.