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Context
Clean water is the primary demand. In China, more than 660 million people are relying on reservoirs and lakes as drinking water resource. The use of reservoir source water, however, has created the universal occurrence of the algal-derived odor (2-methylisoborneol, MIB) problem.
Nationwide water quality survey has revealed that over 40% source water suffers from the earthy-musty odor caused by MIB, which has an extremely low odor threshold (10 ng/L). Under favorable conditions, the MIB concentrations could increase rapidly to hundreds ~ thousands ng/L along with the exponential growth of some specific cyanobacteria. Conventional water treatment processes have difficulty in removing this compound. Even the advanced treatment technologies like ozone and powdered activated carbon (PAC) are not able to deal with the episodes with a high MIB concentration (e.g. > 400 ng/L). Odor issue is the major complaint item for drinking water in China. Thus, it is vital to control the growth of the MIB-producing cyanobacteria (MPC) in the reservoirs/lakes.
Obejective & roadmap
The objective is to develop a chemical free, nature-based approach to control the growth of MIB-producing cyanobacteria (MPC) in source water reservoirs.
Strategy
| Challenge | Strategy |
|---|---|
| Traditional methods to identify MPC is time-consuming | Development of an innovate high-throughput single-cell-based technology to identify MPCs |
| The approaches to control the growth of MIB producers in a large water body are extremely limited, particularly when the use of chemicals is prohibited | Revealing the niche characteristics of MPC, and developing niche-based approaches to interfere the growth of MPCs by changing underwater light and hydrodynamic conditions |
| Reservoirs are greatly varied by size, bathymetry, water quality, etc. | Water level adjustment strategy: minimizing the ratio of shallow area (< 5.5 m) to shrink the habitat of MPCs; Turbidity strategy: increasing inlet/outlet flowrate to introduce highly turbid water to decrease the underwater light intensity; HRT strategy: reducing HRT to dilute MPCs, particularly for small reservoirs; Sediment resuspension strategy: reducing underwater light and trapping algal cells through in-situ sediment resuspension using a shipborne dredge pump |
Implementation and results
Traditional approach to identify MPCs relies on the isolation of strains through micromanipulation, which is time-consuming and often fails to yield results. A culture-independent and high-throughput single-cell-based method was developed for the precise identification of all MPCs in one single run.
It was revealed that the top MPCs include Pseudanabaena, Planktothricoides etc. which differ from surface bloom-forming Microcystis, in that they tend to live in subsurface or deeper layers, which makes them to be more readily affected by the underwater light availability. Their light thresholds for growth were determined by using laboratory culture experiments and field verification.
Such niche characteristics have allowed us to develop approaches to target on MIB producers specifically. For example, we can reduce the underwater light availability by using the approaches including elevating water level, increasing extinction coefficient of water, etc. For the small reservoirs, it is possible to diluten MPCs by shortening HRTs.
Miyun Reservoir, which has the maximum water capacity of 4.375 billion m3, is the main source water for Beijing serving 10 million people. It has suffered from MIB-deduced odor problems since 2005. Long term investigation revealed that the MIB-producing Planktothrix mainly live in the bottom layer of north shallow area with a water depth below 5.5 m. Model simulation suggested that increasing the water level to 146 m a.s.l. would minimize the shallow area, and therefore prevent the growth of Planktothrix. Water level in Miyun Reservoir was elevated by importing water from the mid-canal of South-to-North Water Diversion Project. MIB disappeared in 2018 when water level reached the proposed value.
QCS Reservoir, the largest estuary reservoir in the world serving 13 million people in Shanghai, has suffered MIB problem since it was first put into operation in 2010. Investigation revealed that MIB-producing Pseudanabaena preferred to grow in the north zone with lower turbidity. By importing 50% more highly turbid water from Yangtze River, the turbidity of this area was significantly increased and therefore successfully resolved the MIB problem from 2020. Note that, no extra energy was required since the inflow is powered by the gravity. It saved the PAC chemical dosage which cost 10 million CNY (1.28 million EUR) each year.
In 2021, a serious MIB episode occurred in ZXD Reservoir, which greatly degraded the water quality for the city of Macao. By shortening the HRT to approx. 5 d, which is lower than the growth rate of MIB-producing Pseudanabaena, the MIB episode was terminated in 2 days.
Sediment resuspension was developed as a supplementary approach to control odor-producing cyanobacteria in other reservoirs.
Outcome
MIB-deduced odor problem is not only happening in China, but have become both widespread and more frequent in many nations due to the climate change. The niche-based approach is green and cost-effective. It has been implemented in more than 20 reservoirs in China, and this has benefited over 30 million people. By revealing the underlying mechanism, we have successfully communicated this in the publication of more than 15 papers in scientific journals including Water Research, etc. In addition, we have contributed a Chinese version of a major new guideline regarding the control of drinking water odor. These publications are expected to provide guidance for the people around the world to deal with these major and increasing MIB problems globally in source water.