Odor and smell: clean water is odorless and tasteless. Natural water has some odor and smell due to dissolved minerals, reproduction, death and corruption of animals, plants and microorganisms in water.The water polluted by domestic sewage and industrial wastewater often has a peculiar smell.The main determination methods are qualitative description method and threshold method（1） Qualitative description method 1. Take 100ml water sample into 250ml conical flask, and the inspectors rely on their own sense of smell to smell the odor at 20 ℃ and after boiling slightly cold, describe the odor characteristics with appropriate words, and report the odor intensity according to the level in table 3-11.Table 3-11 odor intensity grade Grade strength explain 0 nothing No smell 1 weak It is difficult for ordinary drinkers to detect, but it can be detected by olfactory sensitive people 2 weak The average drinker just noticed 3 obvious It can be obviously detected and can not be drunk without treatment 4 strong It has a very obvious odor 5 Very strong It has a strong stench Only clean water or water samples that have been confirmed to be harmless to human body by oral contact can be used for taste test.The method is to take a small amount of water samples at 20 ℃ and after boiling and cooling into the mouth, taste the taste, use appropriate words (acid, sweet, salty, bitter, astringent, etc.), and refer to the taste intensity grade given in table 3-12.Table 3-12 four representative taste substances Taste category Flavoring substance Taste threshold concentration (%) Taste category Flavoring substance Taste threshold concentration (%) sweet taste sucrose 0.7 bitterness Coumarine 0.001   saccharin 0.001   quinine 0.0005 Sour taste hydrochloric acid 0.045 Salty taste sodium chloride 0.055  （2） Odor threshold method this method is to dilute the water sample with odorless water until the lowest distinguishable odor concentration (called "odor threshold concentration") is smelled, which is used to represent the odor threshold.When the water sample is diluted to just smell the odor, the dilution multiple is called "odor threshold", that is, the key points of inspection operation: prepare the water sample dilution series with water sample and odorless water in the conical flask (the dilution multiple should not be known to the inspectors), and heat it to 60 ℃ in the water bath±1℃；The inspector took out the conical flask, vibrated it for 2-3 times, removed the stopper, smelled the odor, compared with the odorless water, determined the diluted sample that just smelled the odor, and calculated the odor threshold.If the water sample contains residual chlorine, it should be inspected before and after dechlorination.Due to the difference in olfactory sensitivity of inspectors, the test results of the same water sample dilution series will be inconsistent. Therefore, more than 5 olfactory sensitive personnel are generally selected to test at the same time, and the geometric mean value of the test results of each odor inspector is taken as the representative value.Generally, tap water is used to produce odorless water through granular activated carbon.The residual chlorine in tap water can be removed by titration with sodium thiosulfate solution.Distilled water can also be used to make odorless water, but distilled water and deionized water sold in the market can not be used as odorless water directly.