Recovery of carbohydrate fraction from microalgal biomass grown in wastewater treatment photobioreactorsa biorefinery approach

Resumen

In the recent years, the search of new renewable raw materials has exponentially increased due to the huge consumption of materials and the limited fossil resources. At the same time, an important research effort is addressed to the management of the increasing amounts of wastes generated by human activities. Combining both research goals, the recovery and valorisation of the components of wastes seem to be the only sustainable solution. Wastewaters, containing great amount of organic matter and nutrients, are one of the largest residues generated in our society. Biotechnological processes with consortia of microalgae and bacteria are being studied for wastewater treatment with the aim to enhance the yields of recovery of the nutrients (C, N, P, S…) as well as to obtain clean water for other applications. Thus, this thesis aims at tackling the valorisation of the microalgae-bacteria biomass grown in wastewater treatment photobioreactors. This work is focused on the valorisation of the carbohydrate fraction of this biomass to produce biogas and fermentable monosaccharides but considering the effect of this process stages on other valuable fractions of the biomass, as proteins and lipids and the generation of by-products. The study of carbohydrates recovery is addressed as a first stage of a sequential process for the integral valorisation of the biomass, applying a biorefinery concept. Different cell wall disruption methods (bead mill, ultrasound, steam explosion, alkaliperoxide, alkaline and acid) were applied at different operation conditions, as a first step for biogas production or for recovery of fermentable sugars by enzymatic hydrolysis. BMP tests provided the maximum methane production (377mL CH4/g VS) from alkali pretreated samples (NaOH 2M, 120ºC, 60 min), whereas acid pretreatments provoked a severe inhibition. Alkaline peroxide pretreatment enhanced the methane production 73% respect to untreated biomass, while bead mill and steam explosion increased the methane production rate by a factor of 5 and 3, respectively. The composition of the residues after the anaerobic digestion was adequate for their use as fertiliser. The higher carbohydrate solubilisation yields were obtained by enzymatic hydrolysis of acid and alkaline pretreated samples. Hydrolysis of samples pretreated with HCl 2M,120ºC, 60 min provided 98% of carbohydrates solubilisation and 81% of monosaccharides recovery with low degradation, but also solubilised 76% of the proteins and 56% of the lipids. Alkaline-peroxide pretreatment reached significant improvement during the enzymatic hydrolysis with solubilised yields of 70% for carbohydrates and 55% for lipids while only 35% for proteins. Enzymatic hydrolysis of bead mill pretreated samples provided highly selective results with high carbohydrate solubilisation yields (84%) but high by-products generation mainly methanol and ethanol (4.5g/L). The optimisation of operational parameters of acid and alkaline pretreatments coupled with enzymatic hydrolysis confirmed temperature of pretreatment, kind of chemical agent and concentration of chemical agent as the most significant parameters. Enzymatic hydrolysis step did not result necessary, achieving carbohydrate solubilisation yields higher than 84% from different microalgal biomasses, applying only acid pretreatment (120ºC, HCl 2M). However, the growth media of biomass had a relevant impact on the by-product’s generation, with monosaccharides recovery yields ranging from 80% for biomass cultivated in synthetic medium to 53% for biomass grown in piggery wastewater. Finally, the possible use of microalgal biomass as substrate for enzymes production by Trichoderma reesei was studied and the operation parameters were optimised. The maximum cellulases and xylanases productions (28.35 FPU/g for FPase, 16.76 U/g for β-glucosidase, 1113.45 U/g for xylanase and 3.81U/g for β-xylosidase) were achieved using a 50:50 ratio biomass:bagasse, 5 days, 28ºC, pH 4, and phosphate extraction at 22ºC for 1 h, working with different moisture contents of the biomass. The results fulfilled in the current thesis present important information and valuable tools to understand different pathways to valorise the microalgae-bacteria biomass from pig manure wastewater treatment. Furthermore, a comparative of the overall results with other kind of microalgal biomasses is evaluated to provide unique and feasible solutions in general context.