Introduction

People are familiar with highly eutrophic water bodies such as Lake Taihu and Lake Dianchi, where abundant nutrients, high water temperatures, and suitable light conditions can lead to the occurrence of blue-green algae blooms. However, there is another type of water body that serves as an important source of drinking water. These water sources have overall lower nutrient levels but still face issues related to algal odor.

Based on a ten-year investigation of water quality in 55 cities and 209 water plants, conducted by the team led by Yang Min from the Ecological Center of the Chinese Academy of Sciences, it was found that nearly half (41%) of the water bodies in these relatively good-quality reservoir water sources have problems with algal odor. This has forced water plants to adopt activated carbon adsorption or advanced treatment methods to remove odor-causing substances from the water. The primary odor-causing substances derived from algae are 2-methylisoborneol (MIB) and geosmin, which respectively cause the water to have a “musty” and “earthy” odor. These substances have been newly included as mandatory standards in the “Hygienic Standard for Drinking Water” (GB5749-2022). In China, issues related to the odor of drinking water caused by MIB are more common. While MIB was initially found in actinobacterial metabolites, it was later confirmed that filamentous cyanobacteria are a more significant source in drinking water sources. Therefore, why do these filamentous cyanobacteria that produce odor-causing MIB tend to grow in drinking water sources? Can suitable prevention and control methods be found for drinking water sources?

Why are MIB-producing algae more inclined to grow in drinking water sources?

MIB (2-methylisoborneol) is a terpenoid compound and a secondary metabolite in the synthesis of pigments such as chlorophyll a and lutein. So far, it has been found that at least 24 species of cyanobacteria can produce MIB, mainly belonging to the genera Oscillatoria, Phormidium, Anabaena, and Planktothrix within the phylum Cyanobacteria. The majority of MIB-producing algae are filamentous cyanobacteria with relatively unique population characteristics.

Filamentous MIB-producing algae are suitable for growth under moderate light intensity: Unlike bloom-forming cyanobacteria (such as Microcystis) that have strong light protection mechanisms, filamentous cyanobacteria undergo photoinhibition under high light conditions and are suitable for growth in the sub-surface and bottom layers of water bodies. They occupy a different ecological niche from bloom-forming cyanobacteria that prefer surface growth and are in a passive position in light competition.

Filamentous MIB-producing algae have low nutrient requirements: Filamentous MIB-producing algae usually do not dominate in water bodies and have low nutrient requirements. Moreover, since they mainly grow in the sub-surface and bottom layers, they are more likely to obtain nutrients released from sediment.

Filamentous MIB-producing algae have a lower optimal temperature than bloom-forming cyanobacteria: The optimal temperature for Microcystis is around 30°C, while the optimal temperature for most filamentous MIB-producing algae is between 22-25°C. Therefore, filamentous MIB-producing algae are more likely to occur in water bodies with relatively low nutrient levels and where bloom-forming cyanobacteria blooms are less likely to occur. This is the main reason for the widespread occurrence of MIB in drinking water sources and reservoirs in China.

In northern water reservoirs in China, the nutrient content is relatively low (total phosphorus, TP, is about 10 μg/L), but there is a temporary MIB problem in the shallow water area of the reservoir in autumn. In summer, before the massive growth of cyanobacteria, the water body has relatively abundant nutrients, high light intensity, and high water temperature, providing a suitable growth environment for surface-dwelling Microcystis. On the other hand, the growth of surface-dwelling Microcystis reduces water transparency, hinders the penetration of sunlight into the lower layers, and inhibits the growth of filamentous MIB-producing algae in the sub-surface and bottom layers. Starting from September, due to the low background nutrient concentration in the reservoir and the consumption of nutrients by Microcystis, the nutrient concentration in the surface layer becomes very low and insufficient to support the growth of a large number of Microcystis. It is because the decline of surface-dwelling Microcystis improves water transparency, allowing filamentous cyanobacteria to receive suitable light conditions and start a temporary growth phase while producing MIB.

MIB-producing algae control methods suitable for drinking water sources

When the concentration of MIB is high in the water source, water treatment plants have to use a large amount of activated carbon to adsorb and remove MIB substances from the water, significantly increasing the cost of water treatment and affecting subsequent processes. Moreover, complete removal of MIB is challenging when the concentration is very high. Therefore, preventing the growth of odor-producing algae in the water source and blocking the production of MIB at the source are the most fundamental methods. However, due to the uniqueness of water sources, the commonly used chemical algaecide methods for controlling blooms in eutrophic water bodies are not suitable. Additionally, odor-producing algae have different population characteristics from bloom-forming cyanobacteria. Therefore, targeted water source control methods need to be developed based on a clear understanding of the ecological niche characteristics of odor-producing algae.

Since odor-producing algae prefer to grow in the sub-surface of the water body and are mainly driven by underwater light, inhibition of odor-producing algae can be achieved by altering the underwater light environment based on the known light threshold of odor-producing algae. By utilizing the logarithmic attenuation principle of light transmission in water, the water environment of the reservoir can be adjusted from suitable for the growth of odor-producing algae to unsuitable conditions by raising the water level and/or increasing water turbidity (extinction coefficient).

Application of light modulation to control MIB-producing algae in Shanghai water sources

Qingcaosha Reservoir is a newly built water source reservoir in Shanghai, with a water supply scale of 5 million cubic meters per day, serving approximately 18 million urban residents in Shanghai. Since its construction, seasonal MIB (2-methylisoborneol) issues have been present. Based on years of cooperation and research with Shanghai Chengtou Original Water Limited Company, the algae species in the reservoir and their light thresholds have been determined, and it has been clarified that the shallow water area in the north of the reservoir is a high-risk area for algae production. Based on this, it was proposed to increase the flow of water diverted from the Yangtze River and the reservoir’s northern drainage gate to the reservoir, causing an increase in turbidity in high-risk areas and a decrease in underwater light, thereby inhibiting the growth of algae. Since 2020, the use of the natural flow of the river and reservoir level difference to increase the flow of water diversion has achieved a reduction of over 80% in odor-causing substances, effectively solving the odor problem that has plagued the reservoir for many years.

It’s important to note that the specific implementation of control methods may vary depending on the characteristics of each water source. Therefore, it is necessary to conduct a detailed analysis of the ecological niche of MIB-producing algae and the water source conditions before applying any control strategy. Additionally, continuous monitoring and research are essential to optimize and adapt control methods based on the changing dynamics of MIB-producing algae populations and environmental conditions.

References

1. Su, et. al. (2022). Light-dominated selection shaping filamentous cyanobacterial assemblages drives odor problem in a drinking water reservoir. In npj Clean Water. https://doi.org/10.1038/s41545-022-00181-2
2. Su, et. al. (2021). Ecological niche and in-situ control of MIB producers in source water. Journal of Environmental Sciences. https://doi.org/10.1016/j.jes.2021.03.026
3. Su, et. al. (2021). Identification of MIB producers and odor risk assessment using routine data: A case study of an estuary drinking water reservoir. Water Research. https://doi.org/10.1016/j.watres.2021.116848
4. Su, et. al. (2018). Succession and interaction of surface and subsurface cyanobacterial blooms in oligotrophic/mesotrophic reservoirs: a case study in Miyun Reservoir. Science of The Total Environment. https://doi.org/10.1016/j.scitotenv.2018.08.307
5. Su, et. al. (2017). Reducing production of taste and odor by deep-living cyanobacteria in drinking water reservoirs by regulation of water level. Science of The Total Environment. https://doi.org/10.1016/j.scitotenv.2016.08.134
6. Su, et. al. (2015). MIB-producing cyanobacteria (Planktothrix sp.) in a drinking water reservoir: Distribution and odor producing potential. Water Research. https://doi.org/10.1016/j.watres.2014.09.038

More details：https://drwater.rcees.ac.cn/publication/

From: keyingshuyuan