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 concentrati ons 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 o f alum. The results obtained from bioassays for the four objectives we re compared to those on jar tests of water samples from Ontario Water Treatment Plants. All experiments were performed in a laboratory on th e 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 velig er larvae because the 24-h, LC(50) value was 130.5 ppm with a 24-h inc ipient lethal level of 100 ppm. However, additions of alum which depre ssed the pH below 5 caused instantaneous kill of veligers. Therefore, due to the toxicity of both hydrogen ions and alum, significant mortal ity of veligers will occur in the mixing chamber of alum. (2) The acut e toxicity tests suggest that the flocculation process (3-h LC(50) = 7 6 ppm is significantly (P less than or equal to 0.05) more effective a t removing veligers than is the coagulation process (3-h LC(50) = 115 ppm). (3) The role of alum in removal of veligers under standard treat ment conditions (i.e. 10-30 ppm alum) appears to be mainly a physical one, with the flee physically removing even living veligers. (4) Prech lorination in combination with alum greatly improves the removal of ve ligers from raw water supplies over the alum alone. Most veligers rema in alive for at least 24 h in the flee 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 ion s, and chlorine and by physical removal through flocculation of both l iving and dead veligers.