Integration of dissolved gas flotation and nanofiltration for M. aeruginosa and associated microcystins removal

The removal of Microcystis aeruginosa and associated microcystins was investigated by a dissolved gas flotation (preceded by coagulation/flocculation)—nanofiltration (NF) sequence. The experiments were conducted with a freshwater spiked with M. aeruginosa cell aggregates to simulate a naturally occurring bloom. Two types of gases were used in the flotation pre-treatment, air (DAF) and a mixture of CO2/air. Very good results in terms of NF fluxes, overall removal efficiencies and final water quality were achieved with both sequences. However, the CO2/air mixture presented no benefit to the overall sequence, both in terms of toxin release to water during flotation and lower natural organic matter removal by NF, which was due to an overall negative effect of the acid pH. NF was able to completely remove cyanobacteria (100% removal efficiency of chlorophyll a) and microcystins (always under the quantification limit), regardless of the pre-treatment used and the water recovery rate (up to 84%). Therefore, DAF–NF sequence is a safe barrier against M. aeruginosa and microcystins in drinking water. In addition, it ensures an excellent control of particles, disinfection by-products formation, and other micropollutants that may be present in raw water.