Odor Problem in Yuhang Tap Water
On July 16th, abnormal water supply occurred in some residential communities in Yuhang District. Media reports indicated that the tap water had an unpleasant odor.
The results of the cause investigation showed that the thioether substances detected in the water samples are organic sulfides, which can produce odors such as garlic, swampy, and rotten smells even at low concentrations in water. Thioether substances are produced by the anaerobic degradation of algae.
Since early July, continuous high temperatures and sunny weather have caused rapid reproduction of algae in water bodies upstream and around the water intake of the waterworks. With rainfall runoff, some algae and their degradation products (including thioether odor substances) accumulated near the water intake and entered the waterworks, resulting in the "odor and taste" index of the effluent reaching level 3.
Although Yuhang Water Treatment Co., Ltd. and the water supply and drainage company under Yuhang Water Holdings Group successively took measures such as water source switching, water supply scheduling, and pipeline flushing after the incident, it still inevitably caused drinking water safety issues and economic losses, drawing high attention to drinking water safety once again.
Odors in tap water typically come from geosmin and 2-methylisoborneol.
Geosmin is a compound produced by actinomycetes, with a unique earthy smell. Human olfaction can detect this substance even at extremely low concentrations in water—only 0.006~0.01 μg per liter of water is enough to be smelled.
2-methylisoborneol is an organic compound, a earthy-smelling substance mainly produced by algae and actinomycetes. When it reaches a certain concentration in water, it brings a strong musty smell.
The source of the tap water odor problem in Hangzhou this time is different, caused by thioether substances. Thioethers are a class of sulfur-containing organic compounds with characteristics such as low odor threshold, strong pungent smell, and potential environmental and health risks.
With the improvement of living standards in recent years, attention to drinking water safety has increased. The Standards for Drinking Water Quality (GB5749-2022), implemented in 2022, added 2 new indicators for odor substances, namely standards for 2-methylisoborneol and geosmin. The testing standards are as follows:
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The effluent water must be free of abnormal odor and taste.
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The concentration of 2-methylisoborneol ≤ 10 ng/L; the concentration of geosmin ≤ 10 ng/L.
In a waterworks project in East China, Oumei New Materials' N80 hollow fiber nanofiltration was used for advanced treatment, and water quality tests were conducted on raw water and produced water. The test data are as follows:
Note: The concentrations of 2-methylisoborneol and geosmin in produced water are below the instrument detection limit.
According to the water quality test data, the removal rates of the two odor substances by nanofiltration are >45.9% and >18.4% respectively. In fact, both substances on the produced water side are below the detection limit, meaning the two odor substances in the water are basically removed.
Although thioether substances were not tested, their molecular diameters are usually between micrometer and nanometer levels. At this point, hollow fiber nanofiltration has considerable removal capacity through its physical sieving function, usually above 80%. Combined with pre-oxidation (such as sodium hypochlorite), thioethers can be converted into sulfones (with increased molecular weight and charge), and the nanofiltration rejection rate can reach over 95%.
With the improvement of living standards, people are paying more attention to the safety and quality of daily drinking water. Nanofiltration, with its characteristics of selective desalination and high organic matter removal rate, has the functions of improving water quality, removing odors, decolorizing, and desalting, making it very suitable for advanced treatment of municipal drinking water.
The hollow fiber nanofiltration launched by Oumei New Materials further enhances the feature of low energy consumption on the basis of traditional nanofiltration, making it a new choice for municipal tap water and direct drinking water treatment.
