Cyanobacteria are prokaryotic microorganisms that are present in many environments. The presence of certain genera of cyanobacteria in aquatic systems is of great concern due to potential toxin formation and release. Cyanotoxins have been shown to have undesirable health impacts ranging from acute (skin irritation, gastrointestinal, and neurotoxic) to chronic (liver damage, kidney damage, and possible carcinogenic) effects. The toxins exist in different structural forms, such as alkaloids (anatoxin-a, saxitoxins, cylindrospermopsin) and cyclic peptides (microcystins, nodularin). Current knowledge and occurrence data of cyanotoxins have led to increased regulatory attention for limiting concentrations in water.The purpose of this paper is to review available literature and knowledge on the potential of drinking water treatment processes to remove cyanotoxins. The main mechanisms of contaminant removal in water are: physical (sedimentation, filtration, flotation, adsorption), chemical (oxidation), and biological. These processes may work singularly or in conjunction to accomplish reduction of the target compound. For cyanotoxins, removal efficiency is complicated by the fact that the toxins exist in two forms: intracellular and extracellular (dissolved). If contained within the cell, physical removal processes would likely be the best form of treatment. Once released from the cell, dissolved toxins may require additional treatment.
Evaluation of the literature reviewed indicates that many common drinking water treatment processes are capable of removal or degradation of cyanotoxins. A key point when considering treatment for cyanotoxin removal is the form of the toxin (intracellular or extracellular) with respect to the treatment process being utilized. Physical removal of particulate cyanotoxins, either by sedimentation, floatation, or filtration, has been shown to be successful since cyanobacterial cells are generally well removed. However, sedimentation and filtration do not perform well for dissolved toxin removal, except for certain membrane filtration processes. Adsorption on activated carbon has been shown to reduce cyanotoxins, although contact time and competitive natural organic matter (NOM) can affect performance. Chemical processes (ozone, chlorine dioxide, potassium permanganate, chlorine, chloramines) have a tendency towards cell lysis, and may allow free toxin to enter into the water. Depending on the chemical dose applied, residual toxin may or may not be removed. Therefore, the application point of oxidative chemicals and nature of the toxins are extremely important for cyanotoxin reduction. If applied after filtration, ozone and chlorine have been shown to be the most successful in reducing dissolved cyanotoxins. Chlorine dioxide and chloramines have not been shown to reduce toxin levels. Advanced oxidation processes have shown promise for destruction of dissolved toxins, particularly microcystins. Biological treatment via biologically active filtration (either rapid rate or slow sand), has also been shown to be effective for microcystin reduction, although it must be evaluated carefully to avoid incomplete degradation. Additionally, storage of cyanobacterial cells in treatment processes, hydraulic disturbances, pressure gradients, and recycle of waste streams must be considered to avoid lysis of aging cells and subsequent toxin release. Therefore, not only is proper process application important for toxin removal, optimization of plant operations is critical for sustained toxin reduction.