Introduction

People are very familiar with highly eutrophic water bodies like Taihu Lake and Dianchi Lake, where abundant nutrients, summer temperatures, and suitable light conditions often lead to blue-green algae blooms. However, there is another type of water body that serves as a critical drinking water source, where the overall nutrient levels are relatively low, yet algae-related water quality issues still persist.

Based on a decade-long investigation by the Yang Min team at the Institute of Applied Ecology, Chinese Academy of Sciences, which surveyed the water quality of 209 water plants across 55 cities, it was found that nearly half of these relatively high-quality reservoir sources (41%) experienced algae-related odor issues. This forces water treatment plants to use activated carbon adsorption or advanced treatment methods to remove odorous substances from the water. The main algae-derived odorous substances are 2-methylisoborneol (MIB) and geosmin, which cause water to have “moldy” and “earthy” smells, respectively. These have been newly included in the mandatory standards of the “Sanitary Standard for Drinking Water” (GB5749-2022). In China, MIB-induced drinking water odor problems are more common. MIB was first detected in actinomycete metabolites, but it was later proven that filamentous blue-green algae are a more significant source in drinking water sources. So, why do these filamentous MIB-producing algae tend to grow in drinking water sources? Is it possible to find suitable methods for controlling them?

Why Do MIB-Producing Algae Prefer to Grow in Drinking Water Sources?

MIB is a terpene substance and a secondary metabolite in the synthesis process of pigments (such as chlorophyll a and lutein). So far, at least 24 species of blue-green algae have been found to produce MIB, primarily from genera such as Oscillatoria, Phormidium, Lyngbya, and Microcystis. Most MIB-producing algae are filamentous blue-green algae, which have relatively unique population characteristics.

1. Filamentous Algae Grow Well Under Moderate Light Conditions: Unlike water bloom-forming blue-green algae (e.g., Microcystis), which have strong light protection mechanisms, filamentous blue-green algae experience light inhibition effects under strong light. They thrive in sub-surface and bottom layers of water, unlike bloom-forming blue-green algae which prefer the surface, making them disadvantaged in light competition.
2. Low Nutrient Requirements: Filamentous MIB-producing algae are usually not dominant in the water body and have low nutrient requirements. Moreover, since they mainly grow in sub-surface and bottom layers, they can more easily access nutrients released from the sediment.
3. Optimal Temperature Lower Than Bloom-Forming Algae: Microcystis thrives at temperatures around 30°C, while most filamentous MIB-producing algae prefer temperatures between 22-25°C. Therefore, filamentous MIB-producing algae are more likely to occur in water bodies with relatively low nutrient levels and where blue-green algae blooms are less common. This is a major reason for the widespread occurrence of MIB in drinking water reservoirs in China.

For example, in northern water source reservoirs in China, nutrient levels are low (TP about 10 μg/L), but short-term MIB issues occur in shallow areas of the reservoir during autumn. In summer, before a massive blue-green algae bloom, nutrient levels are relatively sufficient, and high light and temperature provide a suitable growth environment for surface-growing Microcystis. Meanwhile, the growth of surface Microcystis decreases water transparency, impeding light penetration to the sub-surface and inhibiting the growth of filamentous algae. By September, with the low background nutrient levels and the consumption of nutrients by Microcystis, the surface nutrient concentration is very low, insufficient to support extensive Microcystis growth. The disappearance of surface Microcystis increases water transparency, allowing filamentous blue-green algae to grow briefly with suitable light and simultaneously produce MIB.

Methods for Controlling MIB-Producing Algae in Drinking Water Sources

When MIB concentrations are high in a water source, water plants have to use large amounts of activated carbon adsorption to remove MIB substances, significantly increasing the cost of water treatment and affecting subsequent processes. Moreover, when MIB concentrations are too high, completely removing it becomes challenging. Therefore, preventing the growth of MIB-producing algae in the water source and blocking the production of MIB at the source is the most fundamental method. However, due to the unique nature of water sources, common chemical algicides used in eutrophic water bodies are not suitable. Since MIB-producing algae and bloom-forming blue-green algae have different population characteristics, targeted control methods need to be developed based on understanding the ecological niche of MIB-producing algae.

Because MIB-producing algae thrive below the water surface and are primarily driven by underwater light, altering the underwater light environment based on the light threshold of MIB-producing algae can suppress their growth. By leveraging the logarithmic attenuation principle of light in water, adjusting water levels or turbidity (extinction coefficient) can shift the reservoir from a suitable to an unsuitable environment for MIB-producing algae growth.

Application of Light Control and Algae Suppression Technology in Shanghai Water Source Reservoirs

The Qingshasha Reservoir is a newly constructed water source reservoir in Shanghai, with a supply capacity of 5 million m³/day, serving approximately 18 million urban residents. The reservoir has experienced seasonal MIB issues since its construction. Based on years of collaborative research with Shanghai Chengtou Raw Water Co., Ltd., the MIB-producing algae species and their light thresholds were determined, and the northern shallow area of the reservoir was identified as a high-risk area. Consequently, a method was proposed to use high-turbidity raw water from the Yangtze River and increase the drainage flow at the northern drainage gate of the reservoir. This increases the turbidity in the high-risk area and reduces underwater light, thereby suppressing MIB-producing algae growth. Since 2020, leveraging the water level difference between the Yangtze River and the reservoir, the drainage flow was increased, reducing the odorous substances by over 80% and effectively solving the long-standing odor problem in the reservoir.

References

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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
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This work was supported by projects such as the Youth Innovation Promotion Association of the Chinese Academy of Sciences, for which we are grateful!