Environmental evaluation of flocculation efficiency in the separation of the microalgal biomass of Scenedesmus sp. cultivated in full-scale photobioreactors

In this paper the environmental evaluation of the separation process of the microalgal biomass Scenedesmus sp. from full-scale photobioreactors was carried out at the Research and Development Nucleus for Sustainable Energy (NPDEAS), with different flocculants (iron sulfate - FeCl₃, sodium hydroxide - NaOH, calcium hydroxide - Ca(OH)² and aluminum sulphate Al₂(SO₄)₃, by means of the life cycle assessment (LCA) methodology, using the SimaPro 7.3 software. Furthermore, the flocculation efficiency by means of optical density (OD) was also evaluated. The results indicated that FeCl₃ and Al₂(SO₄)₃ were highly effective for the recovery of microalgal biomass, greater than 95%. Though, when FeCl₃ was used, there was an immediate change in color to the biomass after the orange colored salt was added, typical with the presence of iron, which may compromise the biomass use according to its purpose and Al₂(SO₄)₃ is associated with the occurrence of Alzheimer's disease, restricting the application of biomass recovered through this process for nutritional purposes, for example. Therefore, it was observed that sodium hydroxide is an efficient flocculant, promoting recovery around 93.5% for the ideal concentration of 144 mg per liter. It had the best environmental profile among the compared flocculant agents, since it did not cause visible changes in the biomass or compromise its use and had less impact in relation to acidification, eutrophication, global warming and human toxicity, among others. Thus, the results indicate that it is important to consider both flocculation efficiency aspects and environmental impacts to identify the best flocculants on an industrial scale, to optimize the process, with lower amount of flocculant and obtain the maximum biomass recovery and decrease the impact on the extraction, production, treatment and reuse of these chemical compounds to the environment. However, more studies are needed in order to evaluate energy efficiency of the process coupled with other microalgal biomass recovery technologies. In addition, studies with natural flocculants, other polymers and changes in pH are also needed, as these are produced in a more sustainable way than synthetic organic polymers and have the potential to generate a biomass free of undesirable contaminants.

Sensitivities of the alternative respiratory components of potato tuber mitochondria to thiol reagents and Ca2+

Plant mitochondria differ from those of mammals, since they incorporate an alternative electron transport pathway, which branches at ubiquinol to an alternative oxidase (AOX), characteristically inhibited by salicylhydroxamic acid (SHAM). Another feature of plant mitochondria is that besides complex I (EC 1.6.5.3) they possess alternative NAD(P)H-dehydrogenases insensitive to rotenone. Many stress conditions are known to alter the expression of the alternative electron transport pathway in plant mitochondria. In the present study we investigated the effects of some thiol reagents and Ca(2+) on potato mitochondrial respiratory chain presenting different activities of the alternative respiratory components AOX and external NADH dehydrogenase, a condition induced by previous treatment of potato tubers (Solanum tuberosum L., cv. Bintje) to cold stress. The results showed that Ca(2+) presented an inhibitory effect on AOX pathway in potato mitochondria energized with NADH or succinate, which was only now observed when the cytochrome pathway was inhibited by cyanide. When the cytochrome pathway was functional, Ca(2+) stimulated the external NADH dehydrogenase. Diamide was a potent AOX inhibitor and this effect was only now observed when the cytochrome pathway was inactive, as was the case for the calcium ion. Mersalyl inhibited the externally located NADH dehydrogenase and had no effect on AOX activity. The results may represent an important function of Ca(2+) on the alternative mitochondrial enzymes NADH-DH(ext) and AOX.