Biology Reference
In-Depth Information
standards and there is a risk of the population being exposed to the danger of these toxins. In order
to overcome any such eventuality, water treatment lines advocated during 1990s relied mostly on
pre-ozonation and post-ozonation steps followed by terminal disinfection by chlorine or chlorine
dioxide with intervening steps of coagulation, fl occulation, PAC fi ltration (Fig. 28). Sand fi ltration,
disinfection by ozone, and a second stage fi ltration on GAC beds would be additional advantage
for effective removal of the toxins. Even if the toxic cells are prone for lysis during preozonation, the
removal of the toxins during PAC and sand fi ltration and the post-ozonation and fi nal chlorination
steps ensure complete removal of any traces of the toxins present.
Studies on the removal of cyanobacterial toxins from drinking waters for household purposes and
small community systems have been made. Lawton
et al
. (1998) tested three domestic jug fi ltration
units based on activated carbon and ion exchange resin. Approximately 60% of the fi lamentous
cyanobacteria and 10% of the single cells of
Microcystis
could be removed with toxin removal rates
ranging from 32 to 57% and the effi ciency was increased to 88% by repeatedly passing the same water
through the same fi lter. The variants of MCs tested were MC-LR, MC-LY and MC-LW. Laboratory-
scale batch reactors were developed very much in a similar manner to the modern water works.
Toxic cyanobacterial cells (cultures of
M
.
aeruginosa
or extracts of
P
.
rubescens
bloom samples) were
Raw Water
⇩
Prechlorination
⇩
Other Chemicals:
Coagulants
Flocculants (optional)
pH adjustment (when necessary)
Powdered Activated Carbon (Optional)
⇩
Coagulant-Flocculation-Settling
⇩
Sand Filtration
⇩
Post-chlorination
pH Adjustmeent
Remineralization (Optional)
⇩
To supply
Figure 27:
Conventional treatment scheme applied to surface waters in the 60s and 70s
