In June, the governments of Canada and the United States committed to new phosphorus reduction target loads for Lake Erie to control harmful algal blooms and protect the lake as a source of safe drinking water. The challenges of treating water contaminated with high levels of microcystin-LR, a toxin produced by blue-green algae, underscores the need to achieve the targets.
A fourth-year environmental engineering design project from the University of Waterloo examined the implications for one community by investigating how the toxin can be further treated in a drinking water treatment plant. The design team provided the preliminary technical and cost requirements needed to retrofit a medium-sized water treatment plant drawing water from Lake Erie to meet Ontario’s drinking water standard for microcystin-LR.
Microcystin-LR is a cyanotoxin that can be produced from harmful algal blooms (HABs). It was evaluated because it is regulated by Ontario Drinking Water Standards at 1.5 micrograms per liter (mg/L). As a toxin, it can cause symptoms such as diarrhea and skin irritation. The team took a conservative estimate of incoming toxin concentration of 100 mg/L --- comparative to the largest concentrations of microcystin-LR found in 2014 when Toledo was forced to issue a “do not drink” notice to water users. It should be noted that these levels of microcystin-LR have never been found at the intake of the drinking water treatment plant examined in the design project.
Before investigating what technologies would be practical to implement in addition to existing processes at the plant, a base case evaluation was completed. That evaluation determined that existing processes would result in a concentration of about 17 mg/L of microcystin-LR in the plant’s discharge. A number of technologies were investigated through a literature review to determine which treatment method would be most ideal to reduce the remaining concentrations to an acceptable level.
A total of 14 technologies were analyzed, including powdered activated carbon (PAC), granular activated carbon (GAC), micro filtration, potassium permanganate, and nanofiltration and reverse osmosis. These independent technologies also were evaluated in combination with other technologies.
Because of the location of the drinking water treatment plant and existing infrastructure, many of the technologies researched were deemed unfit for practical use.
For example, previous studies have shown biofiltration to be effective in removing microcystin-LR. However, most of those studies were conducted in Australia. The climate of Australian waters compared to southern Ontario waters is vastly different for most of the year.
The project also considered chlorination, which is effective as an oxidant to eliminate microcystin-LR. However, high concentrations of chlorine may cause cell lysis (breaking of the cell), which in turn could release even more toxins into the water that needs to be treated within the drinking water treatment plant.
With such a complicated issue, a number of issues were examined to see which technologies would be most practical for the location studied. These included cost, sustainability, plant compatibility and simplicity.
After considering technical advice, visiting the drinking water treatment plant, laboratory work and evaluating alternatives, a top technology was determined: a combination of potassium permanganate and powdered activated carbon (which is currently being used at the drinking water treatment plant along with other processes).
Potassium permanganate acts as an oxidant, and has been shown in studies to be strong enough to oxidize the extracellular toxin without causing significant damage to the cell (which reduces the likelihood of further releasing toxins into the drinking water supply). Powdered activated carbon is porous and has a high surface area, which would allow the toxins to adsorb onto the surface of the powdered activated carbon.
This top technology would cost CDN$20,000 (purchasing a potassium permanganate injection and storage infrastructure) with an annual chemical cost of about $48,195, equivalent to a daily chemical cost of $132. These cost evaluations were made based on daily treatment (in reality, most HABs occur from July to September).
Other treatment methods that scored high in the evaluation and are considered potential options include use of potassium permanganate, a combination of chlorination and biofiltration, a combination of chlorination and PAC, as well as biofiltration.
These recommendations were specific to the drinking water treatment plant investigated. Therefore, the types of technology and dosages may change depending on the water composition coming through the plant and existing plant infrastructure. Nonetheless, they point to the fact that reducing phosphorus inputs is needed to avoid additional drinking water treatment costs for communities surrounding Lake Erie.