Removal of cyanobacteria and cyanotoxins during lake bank filtration at Lagoa do Peri, Brazil

Bank filtration is presented as a barrier for cyanobacteria and their metabolites (cyanotoxins and odor compounds). Retention of cyanobacteria is supported by column and bench studies using sandy materials. Most of the removal is in the first decimeters of infiltration, where physical and biological processes take place. Cyanotoxin removal is a very complex process including sorption and/or biodegradation. Sorption depends on the type of cyanotoxin and the sediments/aquifer composition. Biodegradation depends on the type of cyanotoxin, redox conditions, previous contact with the cyanotoxin, type and content of natural organic matter, and temperature. Pilot studies at the Lake Lagoa do Peri in Brazil demonstrated the effectiveness of bank filtration in removing phytoplankton and Cylindrospermopsis raciborskii (10(5)-10(6) cells/mL) before reaching a sampling well at 12-m depth located 20 m from the lakeshore. Column experiments confirmed biomass removal predominantly in the upper 2 cm of infiltration. No saxitoxins (STXs) were found in the well water and a high retention of neo-STX and STX was determined by isotherm experiments. The sorption linear coefficient (K (d) 13-16 L/kg) was found to better represent the sorption processes than the Freundlich coefficient (K (f) 6.5-12 L/kg).

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