Study of the efficiency of different flocculants for effective microalgae harvesting

Abstract

In recent years, the study of different aspects related to the behaviour of microalgae has received renewed interest due to the wide field of application of these microorganisms. Algae cultures have been principally developed as an important source of many products, such as aquaculture feeds, human food supplements, and pharmaceuticals, and they have also been suggested as a very good candidate for fuel production. The process of recovering the cells from the culture solution is an important factor in the determination of the cost and quality of the products. The harvesting of algal cells by flocculation is a more convenient process than conventional methods such as centrifugation and gravity filtration because it allows the treatment of large quantities of culture. The aim of this work is to study the effectiveness of different flocculants commonly used in the wastewater treatment for the recovery of microalga. Several experimental conditions have been optimized, such as pH, concentration of flocculant, sedimentation time, etc. The microalga used in this study is a Nannochloropsis sp. from water of the gulf of Mazarrón (Spain). The flocculants employed in this work were FeCl3·6H2O and Al2(SO4)3·18H2O. Furthermore, chitosan, a cationic polyelectrolyte has been used for the microalgae harvesting. The different compounds have been evaluated at five different concentration values ranging from 10-170 ppm and each concentration has been tested at three different pH ranging from 6.5-10. The results show that for low flocculant concentration, the activity showed by the two inorganic salts is similar (around 30 % biomass recovery). An optimal concentration of 70 ppm has been obtained by using FeCl3·6H2O and Al2(SO4)3·18H2O, and an increase of this concentration does not show a significant effect in the biomass recovery. However, the efficiency obtained for FeCl3·6H2O is close to 90 % at the optimal concentration, while in the Al2(SO4)3·18H2O case, this value is close to 80 %. Two different behaviours have been observed in the study of the influence of the pH on the biomass recovery. For low flocculant concentrations (< 37 ppm), the results show that the maximum efficiency in the microalgae harvesting is obtained for basic pH (around pH: 10). In the case of use high flocculant concentrations (> 70 ppm), the opposite behaviour is observed, and the maximum recovery is observed between pH ranging from 6.5-8. This effect is more marked by using the Al2(SO4)3·18H2O flocculant. The cationic polyelectrolyte, chitosan, shows the lowest activity, only reaching a recovery of 37.7 % at the pH of the culture media 8.5 and 125 ppm. On the other hand, the effect of pH on cell coagulation was investigated by the NaOH addition without the introduction of inorganic flocculants. The evolution of the sedimentation time was studied using 50 ml of culture at a initial pH of 11.4. The results show a slower sedimentation rate compared with the inorganic flocculants, but a totally clarified medium can be obtained.