Liquid purification or separation – Processes – Ion exchange or selective sorption
Patent
1994-12-20
1997-03-04
Cintins, Ivars
Liquid purification or separation
Processes
Ion exchange or selective sorption
210672, 210694, C02F 128
Patent
active
056075952
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a process and device for purifying tap or city water.
BACKGROUND ART
As tap or city water has been subjected to chlorination with chlorine gas or sodium hypochlorite, it contains residual chlorine dissolved in the form of hypochlorous ion (ClO.sup.-) or hypochlorous acid (HClO). Such residual chlorine gives rise to a smell which is commonly referred to as bleaching powder odor and is often unwelcome. Tap water also contains a small amount of organic chlorine compounds, including trihalomethane compounds such as chloroform CHCl.sub.3 and bromodichloromethane CHCl.sub.2 Br, which are produced by reaction of chlorine with organic substances. The presence of trihalomethane compounds in tap water is drawing increasing public attention as they are carcinogenic, harmful substances. Furthermore, in recent years, phytoplanktons tend to increase and propagate in water sources due to water pollution and eutrophication, so that smelly or malodorous organic substances which presumably are metabolite or secreta of phytoplanktons are present in a small content in tap water. Known in the art as such plankton-originated smelly substances are 2-methylisoborneol C.sub.11 H.sub.20 O (hereinafter, 2-MIB) and geosmin C.sub.12 H.sub.22 O. These substances are generally referred-to as "musty-smelling" substances because of their musty odor and are likewise unwelcome.
Conventionally, water purifying devices have been used for domestic or business purposes in order to remove these harmful and smelly substances from tap water to obtain healthful and palatable water. In early water purifiers, it has been customary to use granular activated charcoal which is capable of removing residual chlorine as well as, although only for a limited period of time, trihalomethanes and smelly organic substances. It is believed that residual chlorine is removed by chemical adsorption at the active sites (C--O.sup.- bond) located at the surface of activated charcoal. Accordingly, the adsorption capability of granular activated charcoal with respect to residual chlorine is deemed to be dependent on the specific surface area of activated charcoal. In contrast, trihalomethanes and smelly organic substances are believed to be physically adsorbed by activated charcoal, with the hydrated molecules thereof being trapped in the micropores of activated charcoal.
Once residual chlorine (hypochlorous ion or hypochlorous acid) is removed by bringing tap water in contact with activated charcoal, bacteria may be allowed to breed at the activated charcoal when the water purifier is out of use. As this is unhygienic, it has therefore been proposed in the art to subject the activated charcoal to sterilization by boiling it at a temperature of 100.degree.-150.degree. C. to kill bacteria (e.g., Japanese Patent Kokai Publication Nos. 49-70450 and 63-62591). Advantageously, heating of activated charcoal causes trihalomethanes to be desorbed from the micropores of activated charcoal and purged into the atmosphere, because of chloroform having a boiling point of 61.2.degree. C. and bromodichloromethane having a boiling point of 90.1.degree. C. It is considered that heating also results in dissociation of the C--O.sup.- bond at the surface of activated charcoal to revive the active sites. Accordingly, activated charcoal is regenerated in this manner with respect to trihalomethanes as well as residual chlorine so that the service life thereof is extended. However, 2-MIB and geosmin can hardly be purged by boiling because they have a molecular weight (168 and 182, respectively) larger than that of trihalomethane and, hence, have a boiling point (208.degree. C. and 254.degree. C., respectively) higher than that of water.
Apart from the possibility of sterilization and regeneration achieved by boiling, from a point of view that chlorine-originating trihalomethanes as well as plankton-originated smelly substances (mainly 2-MIB and geosmin) are to be removed thoroughly for a prolonged period of time, the adsorption capability of
REFERENCES:
patent: 3909402 (1975-09-01), Gartner
patent: 5324434 (1994-06-01), Oikawa et al.
Chemical Engineering, 1984, vol. 29, No. 7, Japanese.
Ashie Nobuyuki
Hiasa Masami
Ohtani Takayuki
Saito Susumu
Cintins Ivars
Toto Ltd.
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