The development of technologies for water treatment, desalination and reuse became more personal to me when the city where I live, Toledo, Ohio, USA, found itself without safe drinking water for two days in August 2014. On August 2, 2014, the greater Toledo area woke up to a Do Not Drink or Boil Water Advisory. The advisory was due to the presence of a cyanotoxin produced by cyanobacteria in Lake Erie called microcystin-LR in the drinking water supply that has a WHO provisional guideline of 1 μg/L. Algal toxins are produced by cyanobacteria, which are commonly called blue-green algae, and they are released from cyanobacteria as a bloom nears the end of its lifecycle, or the cells are lysed and the toxins are released. In July 2012, The United States Environmental Protection Agency’s Office of Water released a fact sheet on cyanobacteria and cyanotoxins and their effect on drinking water (EPA-810F11001). This report discusses the removal of intact cells (or intracellular cyanotoxins) and of cyanotoxins (or extracellular cyanotoxins, released after cell lysis). Most water treatment plants (WTP) do not treat for cyanotoxin removal since existing guidelines are provisional and set by the World Health Organization (WHO).
Upon entering the City of Toledo Collins WTP crib at the Lake Erie intake, potassium permanganate is added to the water for mussel control. While potassium permanganate is needed to control mussels, it lyses cyanobacteria cells, releasing algal toxins to the water. The water is then pumped nearly three miles to the Low Service Station, where powdered activated carbon (PAC) is added to the water for taste and odor control, and the water is transported approximately six miles to the WTP (High Service Station). PAC is generally effective for removal of algal toxins through adsorption onto its surface. At High Service, alum, lime and soda ash are added to the water for coagulation-flocculation, softening, and removal of metals. The water is then sand filtered, carbonated and chlorinated before being sent to the distribution system. However, traditional physicochemical water treatment processes, such as coagulation-sedimentation-filtration, have been shown to only be effective for the removal of whole algal cells and not effective for the removal of algal toxins. Furthermore, chlorination is effective for oxidizing algal toxins as long as the pH is below 8; however, for corrosion control, Toledo water is kept at a pH above 9. Therefore, the treatment process was not enough of a barrier to prevent cyanotoxins from entering the drinking water supply. By August 4, the water treatment plant increased its PAC dosage by nearly four times to remove the microcystin by adsorption, and while the toxin was removed, a significant amount of PAC sludge was produced and the cost of PAC addition was unsustainable.
What is the purpose of sharing our story with you? What happened in Toledo was partially due to nutrients entering Lake Erie, water temperature, low winds and wastewater effluents being discharged to the lake. If water reuse had been in place, one of the sources of the problem would have been prevented. However, it is a narrow view to say that only the discharge of wastewater effluents would have been prevented by water reuse when recycled water is often used for irrigation. Would the use of recycled water for irrigation minimize the use of fertilizers that contain nutrients? It is a possibility. Recycled water is an untapped source with a lot of potential.
The development of new technologies for treatment of water for the fast-increasing population of the world represents one of the most important tasks of the 21st century, and the days when water will be more precious than oil are near. Indeed, water is one of the most important substances that sustains life. It makes up over two-thirds of the weight of the human body, and fresh water is required for drinking as well as for irrigation and industry. In many parts of the world, water demand already exceeds supply, and many more areas are expected to experience this imbalance in the near future. Furthermore, existing water supplies may be limited in quality for meeting the demands of the local population. Therefore, we continue to look to those on the cutting edge of alternative water treatment technologies, such as desalination and water reuse, to help with this important task.
Isabel C. Escobar
