Cyanobacterial toxins: Removal during drinking water treatment, and human risk assessment

Cyanobacteria (blue-green algae) produce toxins that may present a hazard f or drinking water safety. These toxins (microcystins, nodularins, saxitoxin s, anatoxin-a, anatoxin-a(s), cylindrospermopsin) are structurally diverse and their effects range from liver damage, including liver cancer, to neuro toxicity. The occurrence of cyanobacteria and their toxins in water bodies used for the production of drinking water poses a technical challenge for w ater utility managers. With respect to their removal in water treatment pro cedures, of the more than 60 microcystin congeners, microcystin-LR (L, L-le ucine; R, L-arginine) is the best studied cyanobacterial toxin, whereas inf ormation for the other toxins is largely lacking. In response to the growin g concern about nonlethal acute and chronic effects of microcystins, the Wo rld Health Organization has recently set a new provisional guideline value for microcystin-LR of 1.0 mu g/L drinking water. This will lead to further efforts by water suppliers to develop effective treatment procedures to rem ove these toxins. Of the water treatment procedures discussed in this revie w, chlorination, possibly micro/ultrafiltration, but especially ozonation a re the most effective in destroying cyanobacteria and in removing microcyst ins. However, these treatments may nor be sufficient during bloom situation s or when a high organic load is present, and toxin levels should therefore be monitored during the water treatment process. In order to perform an ad equate human risk assessment of microcystin exposure via drinking water, th e issue of water treatment byproducts will have to be addressed in the futu re.