Resumen de Production of carbohydrates and polyhydrohybutyrate by cyanobacteria grown in wastewater
Cyanobacteria are prokaryotic aerobic photosynthetic microorganisms with the capacity to remove nutrients from wastewater and synthesize a large variety of bioactive compounds and valuable byproducts. Among those valuable byproducts, glycogen (carbohydrates) and polyhydroxybutyrates (PHB) are receiving increasing interest due to their potential as biofuel substrate and as bioplastic, respectively.
The main objective in this PhD thesis was to cultivate and select cyanobacteria from a mixed microalgae culture for carbohydrates and PHB production. In order to fulfil that main goal, the thesis was divided in two parts. The first consisted in evaluating the effect of nutritional and operational conditions on the selection and grow of wastewater-borne cyanobacteria in a mixed microalgae consortium fed with in secondary effluent and digestate. While the second was focused in the accumulation of carbohydrates and PHB throughout different strategies based on nutrients and carbon limitation.
In the first part, digestate diluted with secondary effluent wastewater was used to select a culture dominated by cyanobacteria from an initial mixed microalgae consortium in pilot scale (30 L) and lab scale (3L) closed-photobioreactors (PBRs). Through several essays, the PBRs were operated at different nutrients loadings (paying attention on N and P ratios and concentrations) and hydraulic regimes (i.e. continuous, semi-continuous and sequencing batch) in order to find conditions appropriated for cyanobacteria dominance over other wastewater-borne microorganisms (mainly with green microalgae).
During approximately one year of operation, the effect of nutrient variations in the influent and their ratios on the culture composition and biomass concentration was evaluated in a semi-continuous PBR operated at 10 days of HRT/SRT. The results evidenced that cyanobacteria species dominated over green algae when the influent had non-limited carbon conditions, N:P ratios between 16:1-49:1 (molar basis) and low phosphorus loads. Under these conditions, cyanobacteria were able to reach an average biomass production of 0.08 g L-1 d-1. Subsequently, short term studies testing operational strategies were performed in order to remove green algae also tolerating phosphorus limiting conditions. These strategies consisted on sequencing batch operation. According to the results, a sequencing batch operation consisting in a HRT of 6 days and SRT of 10 days provided suitable conditions to remove unsettled green algae Scenedesmus sp. and to improve cyanobacteria dominance until reach 70% of the total population. While increasing the biomass production to 0.12 g L-1 d-1. On the contrary, lower HRT and SRT led to high volumetric loads that promoted the presence of other microorganisms (diatoms and protozoa) and bacterial activity.
In the second part, the cyanobacteria dominated culture previously obtained was submitted to different strategies to accumulate polymers. Among these, nutritional conditions (N and P limitation) and luminic (permanent light and light/dark alternation) were found to be a promising strategies to reach PHB content up to 6.5% and carbohydrates content of 75% in N limitation under light/dark alternation in batch test performed along two weeks. Subsequently, this time needed of accumulation was reduced through the adaptation of cyanobacteria to feast and famine regimes. Consisting in the submission of the biomass to a period of carbon availability and a subsequent absence of carbon. With the application of this procedure the culture was able to reach similar PHB and carbohydrates concentrations in less than 2 days. In addition, these results indicated that carbon uptake and the consequent polymers production from cyanobacteria can be enhanced through carbon and nutrient feeding strategies.
