Efficacy and role of alum in removal of zebra mussel veliger larvae from raw water supplies

The study had four objectives: (1) to determine if alum in concentrations used by water utilities and industries kills zebra mussel veliger larvae; (2) to determine if alum as a coagulant kills veliger larvae; (3) to determine if the flocculant or the flocculation process affects the survival or settling rate (by entrapment in the floc) of veliger larvae; and (4) to determine if prechlorination affects the efficacy of alum. The results obtained from bioassays for the four objectives were compared to those on jar tests of water samples from Ontario Water Treatment Plants. All experiments were performed in a laboratory on the shore of Lake Erie at Wheatley, Ontario. The main findings for each objective were: (1) that the concentrations of alum used in most water treatment plants (i.e. 10–30 ppm was not sufficient to kill the veliger larvae because the 24-h, LC50 value was 130.5 ppm with a 24-h incipient lethal level of 100 ppm. However, additions of alum which depressed the pH below 5 caused instantaneous kill of veligers. Therefore, due to the toxicity of both hydrogen ions and alum, significant mortality of veligers will occur in the mixing chamber of alum. (2) The acute toxicity tests suggest that the flocculation process (3-h LC50 = 76 ppm is significantly (P 0.05) more effective at removing veligers than is the coagulation process (3-h LC50 = 115 ppm). (3) The role of alum in removal of veligers under standard treatment conditions (i.e. 10–30 ppm alum) appears to be mainly a physical one, with the floc physically removing even living veligers. (4) Prechlorination in combination with alum greatly improves the removal of veligers from raw water supplies over the alum alone. Most veligers remain alive for at least 24 h in the floc at concentrations below 100 ppm. In summary, alum will remove veliger larvae by chemical toxicity in the mixing zones where chemical gradients exist for alum, hydrogen ions, and chlorine and by physical removal through flocculation of both living and dead veligers.