Background information

A pre-oxidation step often achieves an efficient flocculation of algae, and/or cyanobacterial cells and therefore supports the elimination of cell-bound cyanotoxins. However, chemical oxidation can damage cells and then promote the release of the toxins. The amount of oxidizing agent should thus be sufficiently high to effectively oxidize the released cyanotoxin(s) in cases where the type of toxin is sensitive to the oxidizing agent in use.

Chlorine, ozone, and to some extent potassium permanganate also, are well capable of oxidizing microcystins and cylindrospermopsin. For the oxidation of anatoxin-a ozone especially, but also potassium permanganate are suitable, but not chlorine. PSPs are only weakly sensitive to chlorine and ozone, thus oxidation alone is not sufficient to remove dissolved PSPs. Chlorine dioxide and chloramine are not effective for the oxidation of cyanotoxins.

Optimized flocculation and filtration is a first effective barrier against cell-bound cyanotoxins during drinking water treatment. However, there are indications that cyanobacterial cells may be damaged during flocculation and subsequently on filters and therefore cyanotoxins can be released. An increased frequency of filter backwashing during cyanobacterial mass developments is thus very important. Dissolved cyanotoxins in the raw water are not removed by flocculation and filtration.

Due to the different sensitivity of diverse cyanobacteria, flocculation and filtration stages have to be optimized for the respective raw water or cyanobacteria present. In addition, the flocculation of certain cyanobacteria e.g. motile Planktothrix rubescens, may require specific conditions, which should be determined initially at laboratory scale.

Usually, flocculation followed by filtration is in place when surface water is used as a drinking water resource. Other techniques removing particles, however, can fulfill the same function e.g. membrane filtration or flotation practised in some (warmer) countries.

Slow sand filtration can also retain a large proportion of the cells and is therefore suitable for the elimination of largely cell-bound toxins such as microcystins, though also dissolved toxins may be (partially) biologically degraded.

To maintain the efficiency of slow sand filters the following must be considered: If possible high biomass should be removed. If this is not possible, especially during cyanobacterial mass developments the „Schmutzdecke“ should be removed frequently to avoid an accumulation of eventually dying-off and thus cyanotoxin-releasing cells on the filters. For the elimination of dissolved toxins it should be noted that the full capacity after exchange of filter layers is not attained until the microbial recolonisation necessary for the biological degradation has taken place (~ 1-2 weeks). The velocity of the (re)colonisation depends in part on the presence of cyanotoxins in the raw water: toxin-degrading bacteria usually develop quicker on filters if cyanobacteria occur frequently in the raw water than during their absence. Therefore, other techniques to eliminate dissolved cyanotoxins should be in place if these occur only sporadically or suddenly in raw water.

As the size of cyanobacteria is usually > 1 µm both microfiltration and ultrafiltration are suitable for the elimination of cell-bound cyanotoxins. However, these techniques can also damage cyanobacterial cells due to high flows, pressure or shear forces and thus can lead to the release of the toxins. The technique in place thus should be optimized under the respective conditions and increased back-washing, for example, should be ensured during mass developments of cyanobacteria.

Nanofiltration and reverse osmosis are both suitable for the elimination of dissolved cyanotoxins. Besides pore size, water quality influences the elimination efficiency.

A post-oxidation step can eliminate (generally) dissolved cyanotoxins. Most of the oxidizing agents used in drinking water treatment react with cyanotoxins, depending upon the respective agent and dosing.

Chlorine, ozone, and to some extent potassium permanganate also, are well suitable to oxidize microcystins and cylindrospermopsin. For the oxidation of anatoxin-a, especially ozone, but also potassium permanganate is suitable, but not chlorine. PSPs are only weakly sensitive to chlorine and ozone, thus oxidation alone is not sufficient to remove dissolved PSPs. Chlorine dioxide and chloramine are not effective for the oxidation of cyanotoxins.

It is important to ensure sufficient contact times and concentrations for the oxidation of cyanotoxins. Also, water parameters including pH influence the efficiency of the oxidizing agent.

Activated carbons adsorb most cyanotoxins very effectively and can thus remove a substantial part of dissolved toxins.

In filters with activated carbon the removal of cyanotoxins is attributed also to biodegradation by biofilms on the filters. The efficiency of a filter can drop during the run-time depending on the type of activated carbon, the operation of the filter and the respective cyanotoxin(s). Also the efficiency of powdered activated carbon depends on the type of activated carbon, the application conditions and the respective cyanotoxin(s).