1. The Stages and Time Needed to Clean River Water at a Sand Filtration Plant
At a rapid sand filtration plant operated by the Kanagawa Prefectural Enterprise Bureau, water passes through a specific sequence of stages: an intake well, a flocculation basin, a sedimentation basin, a rapid sand filter, a sterilization well, and finally a clear water reservoir. From the moment river water is taken in until it reaches the reservoir, the entire process takes about 5 hours [2]. Water treatment plants in Yokohama City follow a similar flow. They add chemicals in the sedimentation basin to settle floating materials, remove remaining fine particles using layers of sand and gravel, and finish with disinfection [3]. However, the 5-hour timeframe is just one example. The specific steps and time required vary depending on the individual water treatment plant.
2. Why Chemicals Make Cloudy Particles Clump Together and Sink
The particles causing cloudiness described in this source have a negative electrical charge on their surface. Because they all share the same charge, they repel each other. This means small particles do not stick together, making them difficult to settle [1]. To solve this, water systems use a chemical called a coagulant. According to the Kanagawa Prefectural Enterprise Bureau, the coagulant used is polyaluminum chloride (PAC). This chemical helps small particles stick together, forming larger clumps called "flocs" that are easier to settle [1]. The goal is not to make the particles heavier, but to weaken their repulsion so they can gather together.
3. How Rapid Sand Filtration Removes Fine Cloudiness
Fine floating materials that did not settle in the sedimentation basin are removed by passing the water through layers of sand and gravel [3]. In the rapid sand filters at the Kanagawa Prefectural Enterprise Bureau’s plant, the filtration speed is 140 meters per day. This translates to about 6 meters per hour. However, this figure represents the drop in the water level in the filter basin, not the actual speed at which water moves through the sand. As the filter is used, dirt accumulates in the sand layer. Therefore, about once every 3 days, water is flowed backward from bottom to top to wash the sand, a process known as backwashing [2]. This is similar to flushing debris out of a mesh screen from the back side.
4. How Slow Sand Filtration Cleans Water Using Microbes Instead of Chemicals
Slow sand filtration is considered the oldest method used since the beginning of modern water supply. It was developed in the UK in 1829 and is said to have spread to Europe and other regions as a measure against infectious diseases like cholera [4]. This method does not use chemicals and requires simple equipment. Microbes that grow in the sand layer can break down cloudiness, as well as substances that cause ammonia nitrogen and musty odors [4]. The filtration speed is about 4 to 5 meters per day, which is roughly 1/30th of the 120 to 150 meters per day speed of rapid sand filtration [5]. As filtration continues, the sand surface becomes clogged, so a layer about 1 to 2 centimeters thick is scraped off to restore filtering power [4]. This method is sensitive to climate, and managing organisms like plankton and midges requires techniques based on experience [4].
5. Why Chlorine Is Added to Keep Water Disinfected Up to the Tap
Examples of chlorine injection in Japan’s water systems include the Yodobashi Water Purification Plant in Tokyo City and the Nogeyama Water Purification Plant in Yokohama City in 1922. Initially, this was limited to times when infectious diseases were prevalent [6]. Today, regulations require that disinfection effects continue from the water treatment plant all the way to the tap. The Enforcement Regulations of the Water Supply Act require that tap water maintain free residual chlorine at 0.1 milligrams per liter or more. For combined residual chlorine, the requirement is 0.4 milligrams or more [7]. To visualize 0.1 milligrams, it is 1/10,000th of the weight of a 1-yen coin (which weighs 1 gram). This is the minimum amount that must remain in the water at the tap, not the amount initially added. The chlorine remaining in the water maintains disinfection effects and prevents the growth of bacteria [8].
6. Why Chlorine Disappears More Easily from Stored Water in Summer
According to the Tokyo Metropolitan Government health authority, chlorine residual for disinfection tends to disappear more easily in summer. This is because high temperatures cause the chlorine to disappear while water sits in storage tanks. When residual chlorine is gone, microbes can multiply in the water, leading to poor water quality [8]. Chlorine can also react with organic matter in water to create disinfection by-products, such as trihalomethanes, which are managed under water quality standards [7]. In May 2012, in the Tone River system, substances flowing into the river reacted with chlorine to produce formaldehyde that exceeded standards. This led to water intake stops and water outages over a wide area [7]. This event illustrates how water supplies are stopped to avoid sending water that exceeds safety standards. This is not a story about chlorine being dangerous, but an example of how water systems balance the benefit of preventing microbial growth with the management of by-products within standards.
7. An Experiment: Watching Soil Particles Sink in a Bottle of Water
You can observe this process yourself. Put water in a clear plastic bottle, add one spoonful of garden soil, close the lid, and shake it well. Compare the water’s appearance immediately after shaking, after 10 minutes, after 1 hour, and the next morning. Some larger particles may settle sooner. Record in a notebook how long the upper water remains cloudy and the changes until the next morning. Do not drink this water under any circumstances. Filtering it with sand or paper will not make it safe to drink. Check the website of your local water bureau; they may have a page explaining their treatment process. See if they use rapid sand filtration, slow sand filtration, or another method. It is recommended to compare this with explanations from the Kanagawa Prefectural Enterprise Bureau [2] or the Yokohama City Water Bureau [3].