1. Why Do Smells Remain in Water After Sand Filtration?
Cloudy water becomes clear when it passes through layers of sand. This method, called rapid filtration, combines coagulation and sedimentation with sand filtration to remove the particles that cause cloudiness. [2]
However, substances dissolved in the water, such as those causing a moldy smell, pass right through the sand layers. To remove these dissolved substances, plants use "advanced water treatment." In Fukuoka City, this means adding ozone treatment and adsorption using granular activated carbon to the traditional methods. [2]
In Chiba Prefecture, the Kashiwai Water Purification Plant faced moldy smells because the water quality in Lake Inba deteriorated. They first tried using powdered activated carbon, but it did not completely remove the smell. According to prefectural documents, they started advanced water treatment in April 1980, becoming a pioneer in the country. [1]
2. How Did Old Slow Sand Filters Use Microbes to Clean Water?
Among sand filtration methods, slow sand filtration is the oldest technique used since the start of modern water supplies. It was developed in the United Kingdom in 1829. [3]
This method does not use chemicals and requires simple equipment. Additionally, microorganisms that grow in the sand layer are said to break down not only cloudiness but also ammonia nitrogen and moldy-smell substances. Sand is not just a simple sieve. [3]
However, this method requires more effort. The surface of the sand gets clogged by floating particles and microbial films, so workers must scrape off 1 to 2 centimeters of sand to restore its function. Because the method is simple, it is easily affected by natural conditions like climate, and management based on experience is important. [3]
In other words, relying on microorganisms is a job that involves dealing with factors such as weather and the seasons. So, what tools can reliably break down smells?
3. How Is Ozone Made, and Why Is It a Strong Oxidizer?
Ozone is created by applying alternating current voltage to electrodes separated by glass or similar materials in a space where oxygen or air passes through. This creates a "silent discharge," changing some of the oxygen into ozone. [4]
Ozone is not stored; it is made where it is used. The strength of its oxidizing power is compared using a number called standard oxidation-reduction potential. Manufacturer data shows ozone is 2.07 volts, while chlorine is 1.40 volts. A higher number indicates a greater tendency to oxidize substances. [4]
This strong power is used to treat smell-causing substances. Ozone is also said to be unlikely to remain in the water because it eventually returns to oxygen. [4]
Fukuoka City has operated an advanced water treatment facility since April 2005, adding ozone treatment and granular activated carbon adsorption. This facility has a maximum capacity of 61,000 cubic meters per day. [2]
4. Why Is the Cryptosporidium Parasite So Resistant to Chlorine?
Chlorine is usually used to disinfect water. However, the shell (oocyst) of a parasite called Cryptosporidium has extremely strong resistance to chlorine. The parasite is 4 to 6 micrometers in size. [5]
It is said that standard water treatment processes cannot expect sufficient removal of this parasite. Membrane filtration is listed as an effective method to handle it. [5]
Because of this chlorine-resistant threat, the Ministry of Health and Welfare (at the time) created the "Provisional Guidelines for Cryptosporidium in Water Supply" in October 1996. The arrival of a chlorine-resistant parasite changed how water plants think about treatment. [6]
What should be done when adding chemicals does not work?
5. How Do Membrane Filters Block Parasites With Tiny Holes?
Membrane filtration works on the same principle as straining water with a net. It physically separates particles larger than a certain size. According to Kanagawa Prefecture, this method can remove nearly 100% of cloudiness, bacteria, and parasites. [7]
Because it stops items by size rather than chemical reaction, it works even against chlorine-resistant parasites. At the water purification plant in Miitsuke City, Niigata Prefecture, ceramic membranes with pore sizes of 0.1 micrometers (1/10,000 of a millimeter) remove cloudiness and Cryptosporidium. [8]
For a 4 to 6 micrometer parasite, the holes are less than 1/40th of its size. However, this is just one example; pore sizes differ depending on the type of membrane. [8]
In Kanagawa Prefecture, membrane filtration was introduced after Cryptosporidium was found in the water. [7]
Membrane filtration is not a process for removing smell substances. In Miitsuke City, activated carbon is also added to the new water plant to remove odor-causing substances. This shows that plants combine tools like activated carbon and ozone for smells, and membranes for parasites, matching the method to the problem. [8]
6. What Passes Through a Coffee Filter: Muddy Water or Barley Tea? (Wash your hands afterward.)
Prepare two cups. Put muddy water (water mixed with soil) in one, and barley tea in the other. Try filtering them into separate cups using a coffee filter.
If soil particles remain on the filter, the muddy water should become slightly clearer, while the color of the barley tea should not change much. You can see the difference with your eyes: cloudiness is removed, but dissolved substances remain.
Do not drink the filtered water.
It is interesting to check with an adult on your local water utility’s website to see which treatment methods your local water purification plant combines. Some bureaus, like those in Chiba, Fukuoka, and Kanagawa, explain their methods carefully. [1][2][7]