CYANOBACTERIAL TOXINS - Cyanobacteria - page 570

MC-LR ranged in between 9 to 500 µg L -1 that required a dose of 0.2 to 1.5 mg L -1 ozone. However,
it must be stated that the duration also ranged in between 5 to 30 min. Hart and Stott (1993) used 1
mg L -1 of ozone and a contact of 5 min to remove 9 µg L -1 of MC-LR with 50% effi ciency in presence
of organic matter. Bernazeau (1994) demonstrated that the demand for ozone is reduced in the
absence of organic matter (0.2 mg L -1 ozone removed 500 µg L -1 of MC-LR with a contact time of 4
min with 99% effi ciency) whereas in presence of organic matter at the same concentration of MC-LR
the concentration of ozone (0.5 mg L -1 ) and contact time (10 min) were increased with a reduction
in effi ciency by nearly 50%. The removal of MC-LR (nearly 200 µg L -1 ) to the extent of 100% was
also reported in two separate studies where ozone at a dose of 1.0 mg L -1 required a contact time of
5 min (Nicholson et al ., 1993; Rositano et al ., 1998). The experimental materials differed in various
studies. Natural cyanobacterial bloom material or their extracts or lyophilized materials were used
in toxin removal experiments. The removal of toxins was much easier with ozone treatment in the
fi rst two cases rather than with lyophilized materials. Pre-ozonation caused improved coagulation
of material and has been thus widely used (Langlais et al ., 1991). Besides the inherent dangers of cell
lysis, pre-ozonation does not meet the complete requirements since a second post-ozonation step
is required with a higher dose of ozone to oxidize the excess of organic matter and toxins present.
Thus post-ozonation is recommended as an advanced treatment process. Since ozone also causes
lysis of cyanobacterial cells it is more important to practise ozonation at the end of treatment process
(Chorus and Bartram, 1999; Haider et al ., 2003; Svreck and Smith, 2004).
Studies involving oxidation of anatoxin-a, anatoxin-a(s) and STXs by ozone are few. The
concentration of ozone required was very much higher in raw water (4.5 mg L -1 ) with no residual
ozone but resulted in a decrease of anatoxin-a from 2.4 µg L -1 to 0.6 µg L -1 whereas in fi ltered water
in the absence of organic material the dose of ozone decreased to 2.2 mg L -1 with the removal of
anatoxin level to 0.3 µg L -1 without any residual ozone. Even higher doses of ozone (4.2 mg min. -1
for 15 min.) were required for reducing the toxicity of A . circinalis extract to near the lethal threshold
concentration (Rositano et al ., 1998). The oxidation of other PSTs such as GTX2, dcGTX2, dcGTX3,
C1 and C2 by ozone is possible, yet the details pertaining to optimization parameters for destroying
the neurotoxins have to be worked out (Nicholson and Rositano, 1997).
The oxidation effi ciency of ozone is reduced at pH values greater than 7.5 due to a lower oxidation
potential of ozone under these conditions. Knowledge on the byproducts of ozonation and their
probable toxicity are yet to be assessed. In general, mouse bioassay was chosen in assessing the toxicity
of residual toxins after ozonation and a more reliable sensitive assay such as PP-inhibition should
be the choice in future studies involving the evaluation of toxicity of byproducts of ozonation.
IX. GUIDELINES FOR SAFER LIMITS OF CYANOTOXINS
In view of wide ranging genotoxic effects and carcinogenicity to human beings exerted by MCs,
it was considered appropriate to take into consideration LOAEL or NOAEL from most suitable
chronic or subchronic studies. A tolerable daily intake (TDI) for MC-LR is derived on the basis of
body weight and the NOAEL for liver changes after a 13-week mouse bioassay conducted at the
Water Research Centre in the United Kingdom (Fawell et al ., 1994). The TDI was thus fi xed at 0.04
µg kg -1 body weight per day.
The maximum acceptable concentration (MAC) for MC-LR is calculated from the TDI and it
was arrived at 1.5 µg L -1 per day.
Risk assessment of cyanobacterial toxins to human beings was considered in guidelines (WHO,
1998) but due to insuffi cient data on cyanobacterial toxins, guideline values for all these toxins could