Water treatment approaches for algal toxins in drinking water
BIRMINGHAM — In Water Technology’s “Professor POU/POE – October 2014” Technical Editor Dr. Joseph Cotruvo discusses algal toxins in drinking water as a result of algal blooms and various water treatment options available.
In the article, Cotruvo offers the following water treatment approaches including physical removal, chemical conversion and adsorption:
- Conventional treatment that includes coagulation, flocculation, sedimentation and chlorine disinfection can remove most of the algal cells, but toxin removal is more problematic. It is essential to remove the algal cells by filtration prior to the addition of any oxidant such as chlorine. The oxidant will lyse the cells and release the toxins into the drinking water.
- Oxidants/disinfectants such as chlorine, chloramine, chlorine dioxide, ozone, potassium permanganate and ultraviolet (UV) light are frequently available in a water treatment facility, or can be added, and provide a range of efficacies against the toxins. The following relate to Microcystin-LR, which is a common form of an algal toxin and among the most potent:
- Ozone will rapidly lyse the cells; it is effective against the toxin at ozone doses of five ppm or less and at very low concentration-time values (CT in mg-min/l). CT means concentration in mg/l x time in minutes, so for example, a concentration of one mg/l for 10 minutes would be the same as 10 mg/l for one minute. Elimination of Microcystin-LR is virtually instantaneous.
- Potassium permanganate will lyse the cells and is also a very effective, rapid treatment for the toxin. The CT value for complete elimination is about 25 mg-min/l.
- Free chlorine will lyse the cells and it is very effective, achieving nearly complete elimination at CT of about 60 mg-min/l. Chlorine is present in almost every surface water treatment plant and probably functions both as an oxidizing and chlorinating agent.
- Chlorine dioxide is a good disinfectant and lyses cyanobacteria cells, but it has no reactivity toward the toxin.
- Monochloramine has some reactivity against the cells but not towards the toxin.
- UV light irradiation at high doses has a destructive effect on the cells but does not affect toxin concentrations.
- Membranes such as reverse osmosis (RO), nanofiltration, ultrafiltration and microfiltration are all effective for the removal of cells. Pretreatment and frequent cleaning could be necessary. RO and nanofiltration, but not microfiltration, would be effective for toxin removal.
- Adsorptionusing powdered activated carbon is a short-term technique for the removal of geosmin, 2-methylisoborneol and toxins. However, the efficacy is variable and is affected by water quality factors. Granular activated carbon (GAC) is effective if the filter is of sufficient depth and condition but it is subject to exhaustion and exceeding capacity. Only a few GAC systems are in place in the U.S. that are regularly/frequently reactivated; most applications are in shallow fixed beds for routine taste and odor control and not reactivated for years, so their performance is not generally predictable in a sustained algal bloom. Biological activated carbon filters would be more likely to have sustained performance without frequent reactivation, because they usually incorporate ozone that would react with the toxins and provide the opportunity for microbial degradation of chemicals on the GAC surface.
You can find the October issue’s entire Professor POU/POE feature here.