1. Field of the Invention
The present invention relates to a method and a system for the treatment of water. The invention has particular utility for the purpose of purifying chlorinated bathing water for use in a swimming pool but also for purpose of purifying water for other uses such as for preparing drinking water.
The invention more particularly relates to a method and a system of purifying a flow of water.
In e.g. swimming pools, chlorine is added to the bathing water as a disinfectant. This, however, causes formation of bound chlorine in the form of e.g. nitrogen-bound chloramines, which give obnoxious smells as well as irritation of eyes, mucosae and skin, just as these substances may constitute a health hazard for the bathers.
It has moreover been found, however, that the addition of chlorine to bathing water causes formation of THM (Trihalomethanes) and AOX (Adsorbable Organic Halogens), AOX typically in about 10-15 times larger amounts than THM. THM and AOX are generic names for substances of which most are carcinogenic. This is a concern in relation to swimming pools, since both THM and AOX are odorless substances and do not give the irritations known from the chloramines. This means that the bathers do not discover the presence of the substances.
It has been known for some years that THM is formed in chlorinated bathing water, and some regulatory bodies have fixed limit values for THM. To reduce the amount of both THM and chloramines in bathing water, the art comprises a method where active carbon is dosed on the top of a sand filter, which is installed to purify the water. The sand filter, which per se does not remove the chloramines and the THM substances, thereby also retains these substances along with particles and suspended solids in the bathing water.
The sand filter can then be back-washed, which procedure must be carried out frequently, e.g. almost every other day. The active carbon, however, turns into large amounts of sludge that have to be disposed of, which may be a problem, since, at several places, it is forbidden to flush active carbon out into the sewer. Moreover, this method involves the risk that bacteria may grow within the sand filter.
In this method, however, AOX is not removed in sufficient degree to avoid a health concern. With respect to the fixing of limit values of AOX, it is a predicament that today no reliable and economic method of removing AOX is available. Thus, mostly, the concentration in the bathing water is reduced by dilution with make-up water, in which AOX is formed in smaller amounts than in chlorinated bathing water.
Generally, this method of reducing the concentration of both the chloramines, THM and AOX is undesirable, since at most places drinking water is used as a dilution solution to the problems of too high concentrations.
Furthermore, drinking water is usually used for backwashing the sand filters. Particularly when the sand filter is dosed with active carbon, it must frequently be backwashed. Thus, substantial amounts of drinking water are used for the purification process, which makes the purification process extremely costly, in addition to the fact that it is a waste of resources to use drinking water for this purpose.
2. Description of the Prior Art
DK-B-144 663 discloses a method of removing chloramines from chlorinated water in swimming pools by intensive irradiation of the water with ultraviolet light at a wavelength of xcex greater than 300 nm. However, the photochemical method according to DK-B-144 663 does not remove THM and AOX in sufficiently large degrees for the occurrence of the substances to be reduced to an acceptable level in the swimming pools.
Patent Abstracts of Japan, vol. 95, no 11, JP-A 7 195 097 discloses a method for purifying overflowing water in a pool, in which method water flows to a filter and the filtered water is irradiated with ultraviolet rays by an ultraviolet irradiation device. The obtained purified water is returned to the pool and reused.
Accordingly, it is an object of the invention to provide a method capable of removing bound chlorine (chloramines), THM and AOX from the water. It is a further object of the invention to provide a method that is energy efficient and resource saving in use.
It is a still further object of the invention to provide a system that is suitable for removing bound chlorine (chloramines), THM and AOX from the water, which system is energy efficient and resource saving in use.
The invention in a first aspect provides a method of treating a flow of water, comprising passing said flow of water through a filter device for filtration of the water, withdrawing from said flow of water at a stage downstream of said filter device a first subflow, passing said first subflow through a UV treatment system, treating said first subflow of water in said UV treatment system with electromagnetic radiation from one or more UV lamps, withdrawing from said first subflow at a stage downstream of said UV treatment system a second subflow of water and passing said second subflow of water through a membrane filter device, which membrane filter device is adapted for effecting a treatment by nanofiltration or reverse osmosis.
By the method according to the invention both THM (Trihalomethanes), chloramines, AOX (Adsorbable Organic Halogens) may be removed from the water that is filtered through the filter device, which is preferably a sand filter.
It is hereby possible to remove the carcinogenic substances from the bathing water in swimming pools or the like. A method according to the invention may in fact also be used for removing biocides, pesticides and peroxides, which makes the method according to the invention suitable for purification of water to produce drinking water, e.g. by the treatment of polluted groundwater.
By withdrawing the second subflow and using a membrane filter after the water has been photochemically purified, it is ensured that the membrane filter just has to remove THM and AOX as well as biocides, pesticides and peroxides, if any, as the chloramines have already been decomposed. This means that the membrane filter can operate at a relatively low pressure of about 7 to 15 bars against about 60 bars, if also the chlorine-bound substances were to be removed, which results in an extremely long service life of the membrane filter. Thus a service life of 3-5 years may be expected against the normal 6-8 months. This is achieved also because the water pressed through the membrane filter is filtered water having a particle content limited to particle sizes of below 1-1.5 xcexcm. Most loading substances for the filter system used are minimized because of the sand filtration and because of the OHxe2x80xa2 (radical) ion.
The method according to the invention thereby permits the intervals between the back washings of the sand filter to be prolonged considerably, so that even highly loaded swimming baths only have to be backwashed once a week at a maximum. This results in a great saving of water.
The branching of the second sub flow after the water has been treated photochemically means that the water in the second sub flow will be free of active chlorine, chloramines and part of the THM and AOX, which the photochemical process can degrade. The water is thus without oxidation substances which might otherwise destroy the membrane filters, which is the next step according to the invention. The oxidation substances are activated in the UV system, which causes them to be degraded, the degradation taking place via the formation of OHxe2x80xa2 (radical), which is the strongest ion in water for oxidation. Extremely clean water is achieved hereby.
According to the invention, the first sub flow is treated with energy-rich photochemically effective electromagnetic radiation in selective wavelengths, which essentially correspond to the values of the absorption energy level for each of the contaminants in the water, such as e.g. chloramines and similar chlororganic compounds.
The invention, in a second aspect, provides a system for the treatment of a flow of water comprising a pipe string for conveying said flow, a filter device on said pipe string, a first substring branched from said pipe string downstream of said filter device and adapted for conveying a first subflow, a UV treatment system on said first substring adapted for photochemical treatment of said first subflow, a second substring branched from said first substring downstream of said UV treatment system, and at least one membrane filter arranged on said second substring, which membrane filter is adapted for removing compounds dissolved in the water, such as THM, AOX and/or salts.
This system is useful in implementing the method explained above.
According to a preferred embodiment of the invention in this aspect, the UV system is a reaction container with one or more metal halide vapor lamps, which are doped with one or more metals, for emission of electromagnetic radiation in the selective wavelengths. This provides an energy-saving UV photochemical treatment, as the lamp type may be arranged to preferably emit electromagnetic radiation in the form of ultraviolet light in precisely those wavelengths that correspond to the undesired chemical compounds in the water.
According to a preferred embodiment, the system comprises at least one lamp arranged at an inlet pipe and at least one lamp arranged at an outlet pipe of the reaction container. This allows for a simple structure of a reaction container for the UV system. The lamp or lamps are positioned in such a way that the water has a sufficient residence time relatively to the energy of the lamp and the turbidity of the water. In case of two or more lamps, the lamps are also distributed relative to the circumference, when the lamps are arranged at the ends of the reaction containers. In embodiments where the lamps are arranged longitudinally of the reaction containers, the lamps are divided in relation to the length of the reaction container.
According to a preferred embodiment, the photochemically treated water is pumped actively through two membrane filters for nanofiltration or reverse osmosis treatment by way of two pumps, a pump being arranged upstream of each filter.