Features
The aim of this special edition - Base of clean water technology
MASASHI KAJINO

NIPPON JOGESUIDO SEKKEI CO., LTD.

Japan’s water system coverage is beyond 97%. However, it is said that there are approximately 1,100 million people around the world who do not receive the benefit of drinking water. This is equivalent to one of 5.5 persons of the total population of the world. Very high costs for the construction of water utilities delay the improvement of water systems to supply drinkable water. The countries with high precipitation throughout the year have abundant water sources, while the countries with a big difference in precipitation between dry season and rainy season need water storage even though their precipitation is high. Furthermore, in the countries with less precipitation, ensuring water sources like groundwater is an important challenge. Groundwater rarely lasts permanently. Groundwater is often contaminated by groundwater depletion, manganese and arsenic.
In the presence of abundant water sources from high precipitation, geological conditions often cause high turbidity. In many affluent rivers with high turbidity, drinking water can be obtained by removing turbidity. When the first water system was established in the world, turbidity removal was the technique used to purify water. In the 1880s, when the modern water system in Japan was established, disinfection was not conducted but slow filtration was employed alone in order to remove turbidity.
The base of clean water technology to obtain drinkable water is now “removal turbidity” and “disinfection”. In order to construct a water supply system with low expense, the selection of drinkable water sources and the application of proper clean water technology are important. The priorities for the selection of drinkable water sources are as follows:
1. Drinkable water without disinfection.
2. Drinkable water after disinfection.
3. Drinkable water after turbidity removal.
4. Drinkable water after turbidity removal and disinfection.
Generally microorganisms which cause waterborne disease hardly swim alone in water. In many cases, they adsorb into turbidity substances in raw water. Thus, turbidity removal sometimes conducts disinfection simultaneously. Here is an old description about this, which is famously known as the Hamburg epidemic. On August 17, 1892, cholera epidemic broke out in Hamburg, which lies on the Elbe River. 8,500 people died of Cholera during several weeks. However, in Altona, downstream from the Elbe River, few persons with cholera were found. Both Hamburg and Altona used the Elbe River for their water supply as raw water. From a commonsense standpoint, cholera epidemic should have broken out in Altona. However, it occurred only in Hamburg because Altona filtrated all drinking water beginning in 1850. On the other hand, the water intake opening of Hamburg’s water supply was from Rothenburgsort ,
4.5km upstream. Hamburg's drinking water was supplied after short-time precipitation of raw water. It had already been reported before this epidemic that slow filtration worked for not only turbidity removal in raw water but also disinfection. When cholera broke out in Hamburg, the Elbe River was contaminated by vibrio cholerae. Raw water for Altona’s water supply had the same conditions as Hamburg’s. Nevertheless, no vibrio cholerae was detected in Altona’s water supply, because it was treated by slow filtration.
There is another report that turbidity removal creates a disinfection effect. In 1890s, Mills, an American and Reincke, a German reported that turbidity removal contributed greatly to the decrease of people killed by typhus abdominalis because filtrating raw water decreased general mortality. This is called the Mills-Reincke phenomenon.
Recently, Amsterdam, Netherlands supplies drinking water through slow filtration after ozone and biological activated carbon treatment. Slow filtration conducts the necessary disinfection because other disinfectants such as chloride are not used.
The treatment mechanism of slow filtration is aerobic oxidation by microorganisms. Higher temperature improves this treatment performance. Many APEC countries lie in the regions where water temperature stays high, so it is better to consider introduction of this mechanism for removing raw water turbidity. Among other removal methods of turbidity, the coagulation sedimentation method is also common. This is the water purification method used for high turbidity in raw water. Disinfection using chlorine is conducted after turbidity removal through slow filtration or rapid filtration, which is used for coagulating sedimentation.
In this series, the base of removal technology on specific substances such as turbidity removal, disinfection, manganese and arsenic is introduced.


before
4 Simultaneous removal of arsenic, iron and manganese in biological treatment unit
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1 Disinfection