Effect of immobilization on growth and organics removal of chlorella in fracturing flowback fluids treatment

In this paper, fracturing flowback fluids were biologically treated by immobilized chlorella. The individual and interactive effects of three variables (sodium alginate concentration, CaCl2 concentration, and crosslinking time) on growth of immobilized algal were optimized by response surface methodology combined with Box-Behnken design. The results showed that the SA (sodium alginate) concentration most significantly affected algal density and treatment efficiency. The interaction between SA concentration and crosslinking time was weak, the interaction between CaCl2 concentration and crosslinking time was modest, while the interaction between SA concentration and CaCl2 concentration was significant. The immobilized chlorella performed the best when the SA concentration, CaCl2 concentration and crosslinking time were 2.42%, 2.69% and 16.76 h, respectively, and the COD removal rate of fracturing flowback fluids was significantly higher than that of the free algal (34.70% vs. 8.96%), indicating immobilization could improve growth and organics removal of chlorellas for processing fracturing flowback fluids.