Optimizing parameters for the stability of alginate encapsulation to support microalgae growth and nutrient removal in shrimp wastewater using response surface methodology

The microalgae immobilization using alginate encapsulation has been extensively studied for circumventing the harvest problem and retaining the algal biomass in aquaculture wastewater. However, the susceptibility of alginate matrixes to cation chelating agents and anti-gelling cation restricts the use of alginate in marine systems. The present study aims to study the stability of alginate beads, the ability of cell growth, and nutrient removal in shrimp wastewater under parameter variables. The response surface methodology applied with the Box-Behnken design was used to develop a model to predict optimum alginate bead preparation. Analysis of variance (ANOVA) of quadratic regression equations for investigating responses indicated that the models fit the experimental data adequately. The validation of the models was also confirmed fitting. The statistical parameters showed that the quadratic effects for both CaCl2 content and pH of wastewater were the most significant in maintaining bead stability and supporting cell growth in shrimp wastewater. Over 2.5%(w/v) CaCl2 provided a suitable concentration for stable alginate beads hardness in shrimp wastewater under alkaline conditions. The optimum bead preparation for immobilized blue-green algae Synechocystic sp. PCC 6803 was obtained using 2.0%(w/v) sodium alginate cross-linked with 2.8%(w/v) CaCl2. Under alkaline pH condition (pH = 8.4) for 4 days, the specific growth rate, nitrate and phosphate removal were rise to 0.09 d−1, 90.4% and 99.4%, respectively. Scale-up cultivation also required exploring other factors to enhance the potential to remove nutrients using immobilized microalgae. •RSM applied with BBD design could develop a quadratic model to optimize alginate bead preparation.•CaCl2 and pH significantly affected bead stability and microalgae growth in shrimp wastewater.•Optimum condition for alginate beads could protect microalgae growth under alkaline conditions.•Immobilized microalgae could serve as a potential biological treatment for aquaculture wastewater.