Theoretical and experimental investigation of phosphate removal from seawater by multi-stage coagulation

Abstract Phosphate removal from seawater is important for biofouling control on RO membranes because phosphorus is one of the nutrients for microbial growth. This paper is based on the hypothesis that multi-stage coagulation results in better phosphate removal. Therefore, comparison of phosphate removal with one-step and three-step dose coagulation from the aspects of both the theoretical calculation and experimental results is investigated in this paper. The result of theoretical calculation based on the Freundlich equation shows that the final phosphate concentration from a three-step dose, i.e. 0.43 μgP/L, is ten times lower than that from a one-step dose, i.e. 4.47 μgP/L. The experimental result shows that for the three-step dose, final phosphate concentration is 1.0 μgP/L, which is lower than for the one-step dose (i.e. 4.0 μgP/L), but not as low as the theoretical calculated value (0.43 μgP/L). This discrepancy between theoretical calculation and experimental result may be the impact of equilibrium phosphate concentration, different initial Fe:P molar ratio and NOM competition between one-step dose and three-step dose coagulation. Although this discrepancy exists, the experimental results still showed that multi-stage coagulation presented better phosphate removal in seawater to concentration levels that are lower than with conventional coagulation. In other words, the problem of the high coagulant dosage in the pretreatment process while removing phosphate from seawater may be solved by application of multi-stage coagulation instead of conventional coagulation.