Treatments of arsenic, fluoride, manganese and hardness are described in this paper, which are a matter of international concern as water purification methods. The standard values of Japan, the U.S. and the WHO for these four substances are shown in Table. Every standard value of arsenic and fluoride shown in Table 1 is regulated for health effects. It has been well known that fluoride causes mottled teeth. The U.S. prescribes strict standards for fluoride levels even in the secondary maximum contaminant level, which has no legal force. The standards of manganese and hardness are prescribed from a standpoint of color or taste of running tap water.
(Table) Standard values of arsenic, fluoride, manganese and hardness
Arsenic is present in trivalent or pentavalent forms in the environment. Pentavalent arsenic can be removed by coagulation-sedimentation. Reduced groundwater often contains trivalent arsenic. In order to remove it using coagulation/sedimentation, it must be oxidized into pentavalent arsenic by using an oxidizer such as chloride in advance. Pentavalent arsenic can be adsorbed by activated alumina and manganese dioxide. When it is treated, a proper pH must be retained because efficiency is changed by pH. Adsorbent containing cerium hydroxide can remove trivalent and pentavalent arsenic. Serious arsenic contamination of groundwater in Bangladesh is attracting significant attention. In addition, high contaminated arsenic is observed in Argentina, some countries and regions of Eastern and Northern Europe as well as China, Thailand and Chile. The development of inexpensive methods of removal which can be used on-site is demanded1).
Much fluoride is contained in groundwater in the granite terrain. An appropriate concentration of fluoride protects one’s teeth from decay. However, excess intake of fluoride causes symptoms such as mottled teeth, bone disorder, and difficulty in walking. Fluoride can be removed using coagulation-sedimentation, activated alumina, animal charcoal, and the electrolytic method. For removal efficacy by coagulation-sedimentation, more aluminum sulfate injection (approx. 5-10 times as much as the conventional injection ratio) is required than conventional treatment. The removal mechanism of activated alumina, animal charcoal is ion exchange. This mechanism is renewable. Attention must be paid to wastewater treatment after reproduction. The electrolytic method can produce colloidal calcium fluoride in an electrolytic cell. Because it is removed by filtration, a designated device is needed.
Large amounts of manganese are sometimes contained in reduced groundwater and bottom water of storage reservoirs. Colloidal quadrivalent manganese expresses chromaticity which is easily visible even though its concentration is minute. Generally 0.01mg/l of manganese expresses the chromaticity between 2 and 3 degrees. Japan, where severe standard chromaticity (5 degrees) is regulated, recommends 0.01mg/L as a desired value, while the U.S. sets a standard chromaticity value of 15 degrees. Both the U.S. and the WHO desire values between 0.05 and 0.1 mg/L.
Manganese can be easily removed by combining oxidation treatment and filtration treatment. Combination free chlorine and sand treatment need the presence of free chlorine and manganese dioxide-coated manganese sand surface in the sand filtration bed. Regarding the removal mechanism in this case, surface manganese ion (bivalent) in water on the manganese sand surface and manganese dioxide (quadrivalent) on the sand surface become trivalent manganese by redox reaction. Next, the removal function is derived by oxidation of trivalent manganese to quadrivalent manganese by free chlorine. This result indicates that continuous treatments result in the increase of manganese-coated surface. However, in the case of no coexistence with free chlorine, removal capability of manganese is lost when all quadrivalent manganese is reduced to trivalent manganese. In this case, trivalent manganese by adding free chlorine is immediately oxidized to quadrivalent manganese and its removal capability is recovered. Potassium permanganate and ozone are sometimes used as an oxidizer. If the injection ration is inaccurate, the removal efficiency is greatly reduced. Some conditions are likely to increase manganese concentration in treated water. Filtration treatment is used for sand filtration or granular-carbon filter.
Hardness is an index which indicates the total amount of calcium ion and magnesium ion in water. Large amounts of calcium ions and magnesium ions are contained in groundwater and surface water in limestone areas. High hardness water is sometimes found as a popular bottled water. However, high hardness in running water brings about the decrease of the cleansing properties of soap. Additionally its taste is likely to be worse than low hardness water. In order to soften water hardness, the disposal that precipitates calcium ion and magnesium ion is the common method after pH is raised to around 11. Presently, a common treatment is for calcium ion in water to be crystallized into calcium carbonate nucleus as calcium carbonate after pH is adjusted to around 10. Also, if the same kettle is used for boiling water at home, it is said that the deposition of calcium carbonate inside the kettle and the continuous separation of calcium in water on the kettle’s surface can decrease its water hardness. The WHO2) has been investigating whether very soft water with extremely low hardness has a negative effect on the mineral balance in the body. It is noteworthy what kind of conclusion will be offered.
1) M.Berg et.al. : Arsenic Removal from Groundwater by Household Sand Filters: Comparative Field Study, Model Calculations, and Health Benefits, Environ. Sci. Technol., 40, 5567-5573 (2006)
2) Guidelines for Drinking-water Quality: Third Edition: Vol.1: Recommendations, p382-383, World Health Organization, 2004