FURTHER STUDIES TO INVESTIGATE MICROCYSTIN-LR AND ANATOXIN-A REMOVAL FROM WATER
Report No FR0458

P R CARLILE

APRIL 1994

SUMMARY

I BENEFITS

Identification of processes capable of removing algal toxins from drinking water.

Optimisation of processes for removing algal toxins.

II OBJECTIVES

To identify and determine the effectiveness of processes suitable for the removal of algal toxins.

III REASONS

Blooms of blue-green algae can give rise to the production of toxins that may contaminate freshwaters. There is a potential risk of algal toxins entering drinking water supplies and posing a threat to public health. There is, therefore, a clear need to identify treatment processes capable of removing them.

IV CONCLUSIONS

Both the laboratory and the pilot scale tests have shown that advanced treatment processes can be used to remove microcystin-LR and anatoxin-a from water. The following conclusions can be made about each of the processes examined.

• PAC dosing, with a wood based PAC, is effective at removing microcystin-LR, but less effective at removing anatoxin-a.
• GAC, operated with a contact time of 15 minutes or more, is effective at removing and degrading microcystin-LR. Anatoxin-a is better removed by GAC than microcystin-LR.
• Ozonation is effective. Greater degrees of removal are achieved when ozone is applied to clarified/filtered water, than to raw water. Ozonation of algal cells did not cause any release of toxins.
• Potassium permanganate is effective when applied to clarified water.
• UV irradiation, at doses two orders of magnitude higher than those normally used for disinfection, is effective.
• Chlorination at pH 5 is effective at removing microcystin-LR. Anatoxin-a cannot be removed by chlorination.

V RECOMMENDATIONS

The suitable processes that can be recommended for installation at an existing treatment works are ozonation, potassium permanganate dosing and GAC adsorption. Ozonation and potassium permanganate can only be applied to clarified water, to achieve high degrees of toxin removal. With GAC, a contact time of at least 15 minutes is required to enable toxin adsorption and degradation, and thus minimise the risk of toxin breakthrough.

PAC dosing cannot be recommended unless the required degree of removal is low (< 90%).

Chlorination cannot be recommended as it is ineffective for anatoxin-a removal and is only effective for microcystin-LR removal at low pH or long contact times for pHs between 7 and 9. Chlorination could lead to microcystin-LR removal in distribution systems with long residence times (> 22 hours), but this would be a risky treatment strategy to adopt.

For new works, nanofiltration is a feasible process option for microcystin removal. However, there is no evidence of the effectiveness of nanofiltration at removing anatoxin-a.

Before installation of any additional treatment processes, careful consideration of the impact of the processes on any existing treatment and water quality should be given.

VI RESUME

This report contains the results of both pilot and laboratory scale studies used to investigate the removal of microcystin-LR and anatoxin-a from water.

Copies of the Report are available from FWR, price £15 less 20% to FWR Members.