In-lake Intervention Strategy
Limited Supporting Field Data
Ozonation is an advanced oxidation technique that works by infusing ozone gas into water. There is a long history of process-level ozonation use to disinfect drinking water and wastewater (Loeb et al. 2012, Rice 1991). Ozone attacks the chemical bonds within cyanotoxins and other compounds, leading to rapid degradation. Ozone treatment requires on-site generation of ozone gas, due to a short half-life of the compound. In general, ozone is produced by passing purified air through an electric discharge to convert oxygen to ozone. Ozone is not readily soluble in water, particularly compared to other oxidative compounds such as chlorine, so it requires a delivery mechanism such as a diffuser for application. Application methods vary but do require on-site infrastructure for application. Ozonation has substantial documentation for applications in both drinking water and wastewater processing and ozone nanobubbles have been used in ponds, lakes, and bays to reduce planktonic chlorophyll concentrations (for example, NBS 2018). It is also a fourth step in the surfactant-flotation-skimming-ozonation technique described in the skimming and harvesting strategy.
Except for non-replicated ozone nanobubble projects in Asia and Florida, ozonation is still a research technique for the treatment of HCBs in surface waters, with no current peer-reviewed literature on surface water treatment. Ozonation has been shown to oxidize multiple cyanotoxin classes (Newcombe and Nicholson 2004). For example, pilot and laboratory work suggest that significant reductions in microcystin concentrations can be achieved with an ozone concentration of at least 0.3 mg/L and a contact time of at least 5 minutes. Similar results were also observed for anatoxin-a at somewhat higher ozone concentrations (Newcombe and Nicholson 2004). The amount of dissolved organic carbon in the water strongly affects the efficacy of ozone treatment on cyanotoxins (Staehelin and Hoigne 1985). Ozonation also shows promise in lysing HCB organisms directly and has been shown experimentally to lyse cells of several genera, including Microcystis spp., Dolichospermum spp., Aphanizomenon spp., and Pseudanabaena spp. (Pandhal et al. 2018, Zamyadi et al. 2015).
- Water body type: Pond, lake/reservoir
- Any surface area or depth
- Any trophic state
- Any mixing regime
- Water body uses: Drinking water, treated wastewater/effluent
NATURE OF HCB
- Shown as useful in drinking water treatment for reservoirs and other source waters with chronic blooms
- Can kill Microcystis spp. and other cells with sufficient contact time
- Intervention strategy
- Ozone treatment in benchtop applications has been shown to be capable of completely oxidizing multiple cyanotoxin classes, including microcystins, anatoxin-a, and cylindrospermopsin. However, it has not been shown to oxidize saxitoxins efficiently (Cheng et al. 2009, Fawell et al. 1993, Newcombe and Nicholson 2004, Onstad et al. 2007, Rositano et al. 2001).
- Ozonation can also lyse cells, with the effectiveness depending on the ozone concentration and contact time; with toxin oxidation noted above, ozonation is a possible broadly applicable technique.
- Ozone also removes many other water impurities, including taste and odor compounds (Ho, Newcombe, and Croué 2002), Cryptosporidium, and multiple organic compounds.
- Ozone treatment is likely not suitable for a one-time application, as it must be generated on-site.
- Ozone treatment has an extremely high oxidation potential and is non-selective in the organisms that are killed (both HCB and non-HCB organisms).
- Ozone treatment generally results in cell lysis, which could release cyanotoxins contained within HCB cells.
- The effectiveness of ozone treatment is impacted by the concentration of organic matter in the system; therefore, it may require pretreatment if organic matter loads are high.
- Ozone treatment does not leave residuals; therefore, treatment is short lived and requires reapplication.
- If the water’s metal content is high, ozonation will form insoluble metal oxides that would potentially need to be removed.
- Applicator protection may be required.
Treatment of HCB events in surface water via ozonation is still in development. This technique has been applied in several field situations via dispersal of ozone nanobubbles to reduce planktonic chlorophyll concentrations (NBS 2018, without any species information however). Ozonation remains an emerging strategy, as it is still largely a research technique.
CASE STUDY EXAMPLES
Laboratory-scale: Pandhal et al. (2018) conducted a benchtop study using a novel ozone generation and application method. The study used a low-temperature plasma dielectric barrier discharge reactor and a fluidic oscillator diffuser, which has lower energy requirements than other systems. Together, this method delivers ozone in microbubbles, which increases the solubility of ozone and therefore increases the contact time.
This study showed that microbubble delivery of ozone via this system rapidly degrades microcystins, with complete oxidation of MC-LR in 2 minutes at an ozone flow rate of 1 L/min. Importantly, the treatment showed a large decrease in toxicity of the microcystin, with the microcystin by-products showing a substantial decrease in inhibitory activity. Lysis of Microcystis aeruginosa cells was observed within 20 minutes.
Alternative ozone generation and delivery technologies such as described in Pandhal et al. (2018) have the potential to lower the operation costs of ozonation, making the treatment more affordable in the future.
Large-scale ozonation use is estimated to be the most expensive of the advanced oxidation processes, according to a cost analysis conducted by Dore et al. (2013) for smaller systems. Primary expenses are capital costs, which can be in the millions, and yearly operational costs, which can be in the hundreds of millions. Dore et al. (2013) estimated that ozone treatment could cost between $0.10 and $0.50/m3 water, with costs decreasing precipitously at treatment volumes >10,000 m3/d.
Relative cost per growing season: Ozonation
|ITEM||RELATIVE COST PER GROWING SEASON|
|Personal Protective Equipment||$$|
Figure C-9. Cost reductions vs. treated volumes for several remediation strategies.
Source: Dore et al. (2013). Used with permission.
REGULATORY AND POLICY CONSIDERATIONS
Use of ozonation at the process level requires an investment in infrastructure, but the technique is already used in many cities throughout the United States in drinking water and wastewater treatment plants. Use of ozonation has been accepted in these applications for many decades (Loeb et al. 2012). Ozonation for treatment of active HCB events in surface waters might be feasible in the future (for example, via nanobubbles), but at present it remains a research technique. Excess ozone will naturally convert to oxygen, although at very high concentrations ozone can damage fish gills. With ozone monitoring, ecosystem impacts of treated water can be minimized, likely increasing public acceptance of the method compared to chemical applications and their residuals. Human exposure to high ozone levels should be avoided and permits for its use should be explored.
Cheng, X., H. Shi, C. D. Adams, T. Timmons, and Y. Ma. 2009. “Effects of oxidative and physical treatments on inactivation of Cylindrospermopsis raciborskii and removal of cylindrospermopsin.” Water Sci Technol 60 (3):689-97. doi: 10.2166/wst.2009.385.
Dore, Mohammaed H., Rajiv G. Singh, Arian Khaleghi-Moghadam, and Gopal Achari. 2013. “Cost differentials and scale for newer water treatment technologies.” International Journal of Water Resources ad environmental Engineering 5 (2):100-109.
Fawell, JK, J Hart, HA James, and W Parr. 1993. “Blue-green algae and their toxins-analysis, treatment and environmental control.” Water Supply 11 (3/4):109-115.
Ho, L., G. Newcombe, and J. P. Croué. 2002. “Influence of the character of NOM on the ozonation of MIB and geosmin.” Water Res 36 (3):511-8. doi: 10.1016/s0043-1354(01)00253-6.
Loeb, Barry L., Craig M. Thompson, Joseph Drago, Hirofumi Takahara, and Sylvie Baig. 2012. “Worldwide ozone capacity for treatment of drinking water and wastewater: a review.” Ozone: Science & Engineering 34 (1):64-77. doi: 10.1080/01919512.2012.640251.
NBS, (Nanobubble Solutions LImited). 2018. Project to Eliminate Blue-Green Algae at the Outer Moat of the Imperial Palace Hibiya, Tokyo: Project Overview Japan.
Newcombe, Gayle, and B. C. Nicholson. 2004. “Water treatment options for dissolved cyanotoxins.” Journal of Water Supply: Research and Technology – AQUA 53:227-239. doi: 10.2166/aqua.2004.0019.
Onstad, G. D., S. Strauch, J. Meriluoto, G. A. Codd, and U. Von Gunten. 2007. “Selective oxidation of key functional groups in cyanotoxins during drinking water ozonation.” Environ Sci Technol 41 (12):4397-404. doi: 10.1021/es0625327.
Pandhal, J., A. Siswanto, D. Kuvshinov, W. B. Zimmerman, L. Lawton, and C. Edwards. 2018. “Cell lysis and detoxification of cyanotoxins using a novel combination of microbubble generation and plasma microreactor technology for ozonation.” Front Microbiol 9:678. doi: 10.3389/fmicb.2018.00678.
Rice, R. G. 1991. “Recent advances in ozone treatment of drinking water.” In Chemistry for the Protection of the Environment, edited by L. Pawlowski, W. J. Lacy and J. J. Dlugosz, 713-730. Boston, MA: Springer US.
Rositano, J., G. Newcombe, B. Nicholson, and P. Sztajnbok. 2001. “Ozonation of NOM and algal toxins in four treated waters.” Water Res 35 (1):23-32. doi: 10.1016/s0043-1354(00)00252-9.
Staehelin, Johannes, and Juerg Hoigne. 1985. “Decomposition of ozone in water in the presence of organic solutes acting as promoters and inhibitors of radical chain reactions.” Environmental Science & Technology 19 (12):1206-1213. doi: 10.1021/es00142a012.
Zamyadi, A., L. A. Coral, B. Barbeau, S. Dorner, F. R. Lapolli, and M. Prévost. 2015. “Fate of toxic cyanobacterial genera from natural bloom events during ozonation.” Water Res 73:204-15. doi: 10.1016/j.watres.2015.01.029.