Statistical optimization of immobilization of activated sludge in PVA/alginate cryogel beads using response surface methodology for p-nitrophenol biodegradation

Abstract

This study focuses on the modelling and optimization of process variables for the immobilization of activated sludge in polyvinyl alcohol (PVA)/alginate cryogel beads using response surface methodology (RSM). The results from analysis of variance showed that the models were reliable and significant in predicting the biodegradation rate and beads breakage. The interaction effects of the five independent variables, namely number of freezing-thawing cycle, bead size as well as PVA, alginate and CaCl2 concentrations, were statistically analyzed using central composite design (CCD). Alginate concentration was classified by models for contributing the most significant effect on both responses. The interactions between alginate concentration with CaCl2 concentration, alginate concentration with number of freezing-thawing cycle and alginate concentration with bead size significantly affected the performances of cryogel beads on p-nitrophenol (PNP) biodegradation rate and beads breakage. The predicted results were in conformity with the experimental runs, in which the maximum PNP biodegradation rate of 7.4 mg/L h and minimum breakage of 0 % could be achieved using 8.0 wt % of PVA, 1.411 wt % of sodium alginate, 3.012 wt % of CaCl2, 3.659 mm of bead and 3 cycles for freezing-thawing process. At low initial PNP concentration of 100 mg/L, the PVA/alginate-AS cryogel beads were reusable with no significant loss in PNP biodegradation efficiency for 20 consecutive biodegradation cycles.