Cyanobacterial Toxins -- Microcystin-LR

Treatment Technology and Management

Appropriate technology to control cyanobacteria needs to be based on an understanding of their ecology (see above). Good control technology must reflect proper management of the watershed and reservoir to prevent algal growth, an appropriate monitoring program and correct treatment technology for both the cyanobacteria and their toxins.

Prevention of Algal Growth

It is important that a proper strategy to control blue-green algal populations be developed for a particular water body. Management options are similar to common techniques used to control algal populations in reservoirs, but with a few exceptions. Nutrient deprivation can be achieved through good watershed management limiting input of nutrients (e.g., wastewater effluent, agricultural runoff) and the addition of chemicals to source waters to reduce nutrient availability (e.g., ferric sulphate to precipitate phosphorus).¹¹ These measures may take a number of years to become effective. Algal growth may also be controlled through physical means, such as light exclusion or artificial destratifica-tion of the reservoir.¹¹ One potential solution for the short-term control of algal blooms for small communities is the use of alum and gypsum as algistats.⁴⁹ An inappropriate method of control is the use of an algicide, such as copper sulphate. The addition of copper sulphate (or chlorine) to a mature bloom will destroy the cyanobacteria but will cause the release of toxins into the water. As well, use of algicides would kill all algae, giving cyanobacteria less competition in a subsequent growth phase.¹¹

Monitoring for Cyanobacteria and their Toxins

An appropriate monitoring program is essential to the overall control of cyanobacteria and their toxins. Drinking water supplies suspected or known to be susceptible to blooms should be routinely monitored for the presence of cyanobacteria. Weather should also be closely monitored for conditions known to be conducive to bloom formation. The use of remote sensing to detect contributory conditions may be helpful. Monitoring sites for cyanobacteria (species identification and cell counts) should include the raw water intake, reservoir and various stages in the water treatment process. These sites could also serve as sampling points for toxins (identification and quantification) during cyanobacterial blooms. Timing, frequency and depth of sampling should take the ecology of cyanobacteria into consideration (e.g., their ability to float within the water column). Sampling and analysis of cyanobacteria are also required to determine the effectiveness of cyanobacterial management programs in watersheds or reservoirs.

In some countries, a system of alert levels, combining information on the number of cells, cyanobacterial species identification and toxin levels, is being used in guiding the appropriate response to be taken.^11,50 In Australia, a task force has also recommended that water suppliers be alerted when the level of cyanobacteria known to produce taste and odour (geosmin or 2-methylisoborneol) exceeds 2000 cells/mL.⁴⁹ This value has been determined to be the typical threshold value for consumer complaints. However, with Microcystis, the absence of taste and odour should not be equated with the absence of toxins. Studies conducted by Hrudey et al.⁵¹ have demonstrated that the presence of microcystin-LR was not related to the presence of geosmin or 2-methylisoborneol. A more recent study of Canadian and U.S. utility waters by the AWWARF found that 82% of 181 samples that tested positive for taste and odour problems also tested positive for the presence of microcystins.³⁵ When blooms occur, water suppliers must decide on an appropriate course of corrective action. The most common responses include one or all of the following: resample or test for toxicity, find an alternative supply or treat to remove toxins.

A flow chart illustrating those factors that should be considered during bloom events and recommendations on actions that may be taken to address the issue are given in Annex A.

Treatment Technology

The final step in controlling cyanobacteria and their toxins is the drinking water treatment process.

Conventional surface water treatment plants, using coagulation (aluminum sulphate, ferric sulphate), clarification and filtration, are effective in removing cyano-bacterial cells.⁵² The use of chemicals or conditions that would lead to lysis of cyanobacterial cells must be avoided to prevent the release of their toxins. To remove the cyanobacterial cells from the treatment train, the frequency of sludge removal and filter backwash (to waste) should be increased. Recent studies have shown that toxin release from sludge depends upon the length of time that the sludge is retained in sedimentation tanks. A pilot plant study by Drikas et al.⁵³ showed that no additional toxin was released from cells during treatment; however, extracellular toxins already present in the water were not removed by the treatment process. In the study, total numbers of cells in the sludge were reduced by 50% after two days, but toxin release began immediately, reaching 100% after the two days. Toxin concentrations were reduced by approximately 80% after eight days, and toxins were completely removed after 13 days. In general, the final disposal of the water treatment plant's waste must be evaluated to guarantee that the waste is not recycled and that cyanobacterial cells are not reintroduced into the source water.

Conventional surface water treatment processes are only partially successful in removing or destroying cyanobacterial toxins.⁵⁴ However, certain oxidation procedures as well as activated charcoal have been found to be effective. Lambert et al.⁵⁵ examined the removal of microcystins from drinking water at two full-scale treatment plants in Alberta that employed coagulation-sedimentation, dual-media filtration and chlorination combined with either granular activated carbon (GAC) or powdered activated carbon (PAC) filtration. The two processes generally removed more than 80% of the microcystins from raw water, particularly when the raw water concentrations were high; however, a residual concentration of 0.05-0.2 µg microcystin-LR equivalents/L was observed at both treatment facilities. More recent studies by Chow et al.⁵⁶ of the effects of treatment chemicals, mechanical stirring and flocculation on Microcystis aeruginosa cells using jar test and a full pilot plant (coagulation/flocculation-sedimentation-filtration) resulted in no damage to the cells or additional release of microcystins from the cells into the finished water. Results from slow sand filtration experiments (over several hours) show some removal of cyano-bacterial toxins through biodegradation.⁵⁷ Because the toxins are non-volatile, neither aeration nor air stripping would be effective in removing the soluble toxins.⁷ Further investigation is required on large slow sand filters and on biological filtration processes.

With regard to oxidation, the residual oxidant level is important. Below pH 8, aqueous chlorine (largely present in the form of hypochlorous acid) at a concentration of 15 mg/L will destroy microcystins; at neutral pH values, chlorination is effective provided a chlorine residual concentration of at least 0.5 mg/L is present after a 30-minute contact time. Destruction is significantly reduced above pH 8 due to the rapid decrease in the concentration of hypochlorous acid with increasing pH. Ozone pretreatment at 1 mg/L can remove microcystins as long as a residual ozone level of 0.05-0.1 mg/L is maintained; the residual ozone level is significant because the effectiveness of ozone is affected by total organic carbon concentration. No new acute toxins are formed within 24 hours, based on the mouse bioassay for acute toxicity. With regard to other oxidation treatments, potassium permanganate at 1 mg/L was found to be effective, but further work is required; hydrogen peroxide, chloramine and chlorine dioxide were not effective; and UV radiation as a point-of-use treatment was not potent enough.^52,58 The effectiveness of a variety of oxidation techniques on raw or clarified waters was recently studied in the United Kingdom.⁵² This study showed that certain oxidation processes were more effective when the oxidant was applied to treated water, presumably because raw water has higher levels of organic/inorganic materials, which will react with the oxidant and reduce the available dose for effective toxin removal. Potassium permanganate and ozone, at doses of 2 mg/L, were highly effective in removing microcystin-LR from treated water. Under conditions similar to those for drinking water disinfection, the study found that chlorination was effective below pH 7; at higher pHs, however, longer contact time was required, which may be of relevance in long distribution systems that hold a chlorine residual. There was also evidence that the oxidants were causing cell lysis, resulting in toxin release; however, the authors concluded that, except for chloramine, intra- and extracellular toxins could be removed if sufficient oxidant was applied.

Different sources of activated carbon have been investigated for their ability to adsorb microcystin-LR. Wood-based products were found to be most effective because of their high mesopore volume. It was found that treatment with 25 mg/L of wood-based PAC, with a contact time of 30 minutes, could reduce the concentration of microcystin-LR from 50 to <1 µg/L.⁵⁸ The presence in the water of other substances (e.g., natural organics) that could be adsorbed by the PAC needs to be considered in studies of efficacy. Studies by Jones et al.⁵⁴ found that alum coagulation in conjunction with PAC adversely affected toxin removal. PAC may be capable of high toxin removal efficiencies; however, very high doses of PAC are required, and contact time is very important. Various GAC filters also appear to be effective in removing microcystin-LR. Although some studies have shown that the life of the GAC is limited,⁷ others have found that the GAC filters, even when already exhausted by removal of dissolved organic carbon, were effective in reducing microcystin-LR levels from 20 to 1 µg/L.^54,58 Laboratory studies have indicated that a biologically active GAC would be able to completely remove toxins via adsorption and biodegradation, providing there is sufficient contact time to allow biological activity.⁵²

Membrane processes such as microfiltration and nanofiltration may also be effective in the removal of both the cyanobacterial cells and the intracellular toxins.⁷ Studies by Hart and Stott⁵⁹ and Muntisov and Trimboli⁶⁰ using microcystin-spiked natural water at concentrations between 5 and 30 µg/L found that toxin levels were reduced to less than 1 µg/L by nanofiltration. Testing of reverse osmosis membranes (2500-3500 kPa) for the elimination of microcystin-LR and -RR from tap water resulted in average retention levels of 96.7-99.6%; initial concentrations in the retentate were 70-130 µg/L.⁶¹

Toxin removal treatment for households and small community systems is of concern in rural areas that are subject to repeated growth of these organisms. Lawton et al.⁶² tested three different domestic jug filtration units for the removal of toxin and algal cells; treatment in all units was based on activated carbon and ion exchange resin. Cell removal was found to be dependent upon the morphological characteristics of the cells, with approximately 60% of the filamentous cells removed and only 10% of the single cells of Microcystis removed. Toxin removal (variants tested included LR, LY, LW and LF) ranged from 32 to 57% when using new cartridges, increasing to 88% with three repeated passages of the same water through the same filter. There is also the possibility of lysis of cells retained on the filters. Testing of filters that had reached the manufacturers' half-life showed a 15% reduction in toxin (LR) removal on one of the brands tested. More research and development are needed if these filters are to be suitable for household microcystin removal.

In summary, where feasible, the use of an oxidant such as ozone, potassium permanganate or chlorine and biologically activated GAC, after the removal of algal cells, is the preferred treatment. The specific concentrations of the various agents in the treatment process depend on the physical, chemical and biological quality of the water to be treated.^46,52,58 More research is required on approaches for household treatment.