The incidence of cyanobacterial blooms in surface water is a phenomenon that, while familiar to members of the water community, draws concern and questions from populations worldwide. Despite the industry’s knowledge about this topic, the fact remains that as climate change, continued urbanization, and intensified nutrient loading of source waters increases the risk of more frequent and intense cyanobacterial blooms in the future, these developments will also increase the public’s fear and scrutiny when it comes to the safety and quality of their
drinking water. Utilities that draw on surface water as their drinking water source know that the water quality issues associated with cyanobacteria include taste and odor compounds and a range of toxins (cyanotoxins). As the potential rate of incidents increases, so does the need to further optimize the conventional treatment processes nearly all utilities use for the removal and control of cyanobacteria and their related metabolites. In addition, water quality managers and water treatment plant operators must be supplied with the knowledge and tools required to confidently apply conventional water treatment processes during cyanobacterial blooms.
The Water Research Foundation is currently wrapping up a project titled, “Optimizing Conventional Treatment for the Removal of Cyanobacteria and Toxins” (project #4315) that we believe will help get to the heart of this situation. The project was created with the intent of focusing on identifying the key factors and related processes and protocols central to developing a base of knowledge that can help create actionable tools for utilities. The final report will be released later this year, however interim results include the following list of key recommendations for water utilities.
- Do not use pre-chlorination for improved coagulation or reduced coagulant dosing during a cyanobacterial bloom unless comprehensive testing has identified a dose high enough to destroy released toxins. Similarly, do not apply pre-chlorination when cyanobacteria producing MIB or geosmin are present.
- Potassium permanganate dosing may be applied for the control of manganese and iron in the presence of A. circinalis and M. aeruginosa.
- Practice pH control to pH > 6 if this is not part of normal operations. This will reduce the risk of cell lysis and metabolite release during treatment.
- Optimize NOM removal using the criteria ΔC/C0 DOC, UV, and color ≤ 0.05 and the cell removal should be optimized as well.
- While turbidity cannot be used as an indicator of the presence of cyanobacteria or cell concentration, the decrease in settled water turbidity can be used with coagulant dose as a surrogate for, or indicator of, cell removal if the initial turbidity is ≈10 NTU or above.
- If the presence of cyanobacteria results in increased coagulant demand to achieve improved settled water turbidity, the application of a particulate settling aid, or even powdered activated carbon, may lead to improvements.
- Although removal of cyanobacteria through conventional coagulation can be very effective, 100% cell removal is unlikely in normal full-scale operations. In the event of high cell numbers entering the plant monitor for cell carryover and accumulation in clarifiers, this can lead to serious water quality problems if not rectified.
- Once captured in the sludge, cyanobacteria can remain viable and multiply over a period of at least two to three weeks. Simultaneously, within one day some cells in the sludge will lyse and release NOM and metabolites.
As we continue to face new and evolving challenges as a result of changing environmental and operational landscapes, we must understand that the communities we serve are increasingly aware of these challenges as well. It is not only our responsibility to ensure water quality remains safe, but also to ensure that utilities operate in a manner that reassures the public of that safety. With the right knowledge base and related tools, water utilities can optimize conventional treatment methods for the removal of cyanobacteria and metabolites that meet all other water quality goals while managing a comprehensive service program that operates in the best interest of the communities we all serve.
Robert Renner is the Executive Director of the Water Research Foundation