Comparison of the properties of coagulation between natural organic matter and inorganic suspended particle

Jar tests were conducted for the coagulation of humic acids (HA) as representational natural organic matters and kaoline as representational inorganic suspended particles. The results showed that the optimum pH value of HA is 5-6, while the optimum pH value of kaoline is 7-8. At pH 5, the best removal of HA corresponds to nearly zero zeta potential so that adsorption and charge-neutralization is the main mechanism of coagulation, while at pH 7 the best removal of kaoline is the result of sweep flocculation of aluminum hydroxide. By means of a photometric dispersion analyzer (PDA), online monitoring of the Al-humic coagulation process was realized. It is revealed that the coagulation process of HA experiences four stages of stabilization, destabilization, restabilization and sweep coagulation, while the one of kaoline only two stages of destabilization and sweep coagulation. It was also found that in the Al-humic floc growth process, the fractal dimension decreased gradually with the increase of floc size and finally to an equilibrium value. On the contrary, in the Al-kaoline floc growth process, the mean diameter and fractal dimension increase with the agitation time, and finally reach a steady state.