Treatment of Cyanotoxins in Rivers and River Influenced Groundwater under Ambient and Softened pH Conditions

Abstract

Cyanobacteria, also known as blue-green algae, are a common cause of dense algal blooms in rivers, lakes, and ponds throughout the world. These algal blooms are a public health concern because many species of cyanobacteria produce potent toxins (cyanotoxins) that have been implicated in the death of wildlife and domestic animals and in cases of human poisoning. In 2011, a reservoir in the Kansas River system experienced an algal event that resulted in the release of high concentrations of taste and odor compounds and cyanotoxins. While concentrations were much less in the reservoir outlet and decreased in the river, several utilities detected microcystin in their source water. The objective of this project was to determine the effectiveness of common drinking water treatment processes in controlling extracellular cyanotoxin levels, especially at the elevated pH values associated with lime softening, which is practiced by several large utilities drawing water from the Kansas River. Other research has focused on the impact of intracellular and extracellular cyanotoxins; however, since the Kansas River system has primarily seen extracellular cyanotoxins based on monitoring studies to date, this work focused on the treatment and removal of extracellular cyanotoxins. General conclusions for the study include that chlorine, ozone, potassium permanganate, and PAC are all viable options for removing dissolved MC-LR from both raw and softened water. Chlorine dioxide is not an effective barrier for MC-LR; however, when combined with chlorine, it can allow chlorine to remove MC-LR while forming lower concentrations of THMs and HAAs. For raw water treatment, ozone and potassium permanganate are viable options. Ozone is very effective and the required dosages are less than those required to remove taste- and odor-causing chemicals to the same extent (percent removal). Potassium permanganate is also very effective, but its dosage must be controlled so as to avoid sending “pink” water into the distribution system. For softened water at higher pH values, chlorine and PAC are viable options for most utilities. Chlorine is less effective at higher pH values, but the dosage needed can still be reasonable, especially since higher CT values are required for disinfection at higher pH values. For PAC, the required dosage does not appear to be adversely impacted by increased pH, and after lime softening there is less dissolved organic carbon (DOC) present to compete with MC-LR for adsorption sites. Various combination of oxidants were very effective at removing MC-LR from raw water under the conditions tested, but only the combination of ozone and chlorine (added sequentially) provided a high level of MC-LR removal while also reducing formation of both THMs and HAAs. When comparing the effectiveness of ozone and PAC to remove MC-LR and taste and odor compounds, the results showed that MC-LR was removed much more easily with ozone and about the same with PAC. While most of the testing in this project focused on removal of MC-LR, tests were also performed under selected conditions to compare removal of MC-LR with that of MC-RR, anatoxin-a, and cylindrospermopsin. Ozone was tested on raw water spiked with all four of the cyanotoxins, and the results show it was effective for all four. The results of potassium permanganate tests on raw water were inconsistent, but it has been reported to be effective for removal of anatoxin-a, and not for cylindrospermopsin removal. Chlorine did not remove anatoxin-a, but was effective for MC-LR, MC-RR and especially cylindrospermopsin. The results for PAC adsorption for softened water showed it was effective for all four cyanotoxins.