Coagulation removal of phosphorus from a southern China reservoir in different stages of algal blooms: Performance evaluation and Al[sbnd]P matching principle analysis

Abstract

Due to excessive nutrient discharge, many reservoirs in southern China suffer from eutrophication and harmful algal blooms. Several methods for phosphorus (P) removal have been proposed, including coagulation, adsorption, and biological methods. Among these methods, coagulation is preferable because of its quick effect, simple operation, and low cost. To investigate the effect and mechanism of coagulation on dephosphorization in reservoir water, the performances of Al-based (AlCl3 and polyaluminum chloride (PACl)) and Fe-based coagulants (FeCl3 and FeSO4) were evaluated in this work. For reservoir water with a total phosphorus (TP) concentration of approximately 0.080 mg/L, aluminum salts showed stable advantages in dephosphorization. AlCl3 reduced the TP level by over 90% when treating the water sample collected at the initial stage of algal blooms, and PACl reduced by over 80% during the blooming stage. To reveal the dephosphorization mechanism and AlP matching principle, synthesized water samples were prepared and treated with AlCl3 and [AlO4Al12(OH)24(H2O)12]7+ (Al13). While simulating the water quality characteristics of reservoir water, important influencing factors were considered. The factors include P content (dissolved phosphorus (DP) and particulate phosphorus (PP)), pH, and extracellular organic matter (EOM). The pH was set to 7.66 and 8.29, with PP proportion set to 20%, 50%, and 80%. Simulated water treatment results indicated that, except for the coagulants species, pH significantly affected the dephosphorization efficiency. Moreover, the effects of P speciation and EOM were confirmed. Based on the coagulation performance and coagulation product characterization, chemical precipitation and inner-sphere complexation were estimated to be the most predominant way that DP and PP match with Al and were efficiently removed by Al-based coagulants.