This research was conducted to obtain an optimum time differential between the addition of dual flocculants aluminum sulfate and polyelectrolytes, to a colloidal suspension. This work was carried out on a synthetic colloidal suspension, latex, which was used in order to achieve the best available reproducibility. Both cationic and anionic polyelectrolytes were examined in conjunction with aluminum sulfate (alum). To simplify tests, pH, ionic strength, and temperature were held constant. The dependent parameters for destabilization measurement were turbidity, total solids, zeta potential, and Al⁺³ concentration. Various alum concentrations, polyelectrolyte concentrations, and time intervals were examined.

This work led to many conclusions. For the cationic polyelectrolyte, the optimum time for addition was dependent upon amount of alum used, but the cationic polyelectrolyte should always be added after the alum. The optimum time for the anionic polyelectrolyte was five minutes after alum had been added to the system. The latex suspension gave reproducible results during the testing period. The use of zeta potential measurements along with the turbidity and total solids, proved useful in evaluating what happened in the suspension. Turbidity by itself could be misleading.

This investigation showed that the time difference between the addition of primary and secondary flocculants affects the efficiency of coagulation in a synthetic colloidal system. This conclusion is probably at least qualitatively valid for real colloidal systems. Water treatment plants should attempt to evaluate this time factor to improve water treatment without additional chemical cost.

Project A-047-NMEX