Fresh water generation method

Abstract

The present invention relates to a fresh water generation method using a water treatment apparatus, the method including feeding water to be treated into a membrane element including a reverse osmosis membrane or a nanofiltration membrane to separate into concentrate and permeate, in which the method includes, under operation of the apparatus, adjusting a concentrate flow rate and/or a permeate flow rate based on a water quality index of the water to be treated and a water quality index of combined water prepared by combining the concentrate and the permeate at a ratio based on a predetermined permeate recovery rate, so that the water quality index of the water to be treated falls within a tolerance on the water quality index of the combined water.

Claims

1. A fresh water generation method using a water treatment apparatus, the method comprising feeding water to be treated into a membrane element comprising a reverse osmosis membrane or a nanofiltration membrane to separate into concentrate and permeate, wherein the method comprises, under operation of the water treatment apparatus, adjusting a concentrate flow rate and/or a permeate flow rate based on a water quality index of the water to be treated and a water quality index of combined water prepared by combining the concentrate and the permeate at a ratio based on a predetermined permeate recovery rate, so that the water quality index of the water to be treated falls within a tolerance on the water quality index of the combined water, wherein the water quality index of the water to be treated and the water quality index of combined water are an electrical conductivity.

2. The fresh water generation method according to claim 1, wherein, when the water quality index of the combined water exceeds the tolerance on the water quality index of the water to be treated, an alarm is sounded or the operation of the water treatment apparatus is stopped.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is a schematic view showing one example of a water treatment apparatus using semipermeable membrane elements.

(2) FIG. 2 is a view explaining a method for calculating the recovery rate in a water treatment apparatus using a semipermeable membrane element.

(3) FIG. 3 is a view explaining a change in the recovery rate by occurrence of errors in a flow meter in a water treatment apparatus using a semipermeable membrane element.

(4) FIG. 4 is a schematic structural view of a spiral-type membrane element.

(5) FIG. 5 is one example of a correlation graph showing a correlation between an electrical conductivity of concentrate-permeate combined water and that of water to be treated in the present invention.

(6) FIG. 6 is one example of a correlation graph showing an influence of errors having arisen in a flow meter on a correlation between the electrical conductivity of concentrate-permeate combined water and that of water to be treated in the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

(7) Modes for carrying out the invention will now be described by reference to the drawings. The invention should not, however, be construed as being limited to the embodiments illustrated in these drawings.

(8) The method for fresh water generation of the present invention is carried out in a water treatment apparatus configured so that water to be treated is fed into a membrane element including a reverse osmosis membrane or a nanofiltration membrane (hereinafter referred to as a semipermeable membrane), and separated into permeate and concentrate.

(9) The reverse osmosis membrane (RO membrane) is a separation membrane capable of filtering out substances having molecular weight of about several tens. On the other hand, the nanofiltration membrane (NF membrane) is a separation membrane which can inhibit fine particles about 2 nm in size and polymers from permeating through it and stands midway between an ultrafiltration membrane (UF membrane: 0.01 to 0.001 m in membrane pore size) and a reverse osmosis membrane in filtration function.

(10) As materials for a semipermeable membrane relating to the invention, polymeric materials, such as acetylcellulose polymers, polyamide, polyester, polyimide and vinyl polymers, can be used. With regard to the structure thereof, the semipermeable membrane may be an asymmetric membrane of such a structure as to have a dense layer on at least one side and fine pores gradually increasing in size in the direction from the dense layer to the inside or the other side of the membrane, or it may also be a composite semipermeable membrane including a very thin separation function layer which is made from a material different from that of the dense layer and is formed on the dense layer of the asymmetric membrane. Of these, however, preferred one is a composite semipermeable membrane which includes polyamide as a separation function layer and which has excellent performances such as a high pressure resistance, high permeability and a high solute removing performance.

(11) The membrane element according to the present invention has no particular restrictions as to the form thereof, but it is preferable to adopt a spiral-type membrane element having the structure shown in FIG. 4. The structure of a spiral-type membrane element is a structure such that an envelope-like semipermeable membrane 7 in which a permeate spacer 10 is involved is spirally wound around a central pipe 10 via a raw water spacer 9. In practice the spiral-type membrane element is provided with a brine seal 11 at one end thereof, and housed in a pressure vessel of the water treatment apparatus. Then water to be treated is pressurized by means of a pump, fed into the pressure vessel, and separated into permeate and concentrate. The permeate is made to pass through the inside of the envelope-like semipermeable membrane 7, and taken out via the central pipe 10. The concentrate is made to pass through the raw water spacer 9 (mesh spacer), and discharged to the outside of the pressure vessel.

(12) In the present invention, a water treatment apparatus can be operated at a recovery rate (permeate recovery rate) determined under elimination of influences of errors in a flow meter by monitoring, during the operations, both the water quality index of water to be treated and the water quality index of combined water prepared by combining concentrate and permeate at a ratio based on a predetermined recovery rate of the permeate and by adjusting the concentrate flow rate and/or the permeate flow rate so as to equalize the water quality index of the combined water with the water quality index of the water to be treated when the water quality index of the combined water exceeds the tolerance on the water quality index of the water to be treated. Herein, the same water quality index is applied to the water to be treated and the combined water between which a comparison is made. By having such a feature, the present invention allows operations at a correct recovery rate by adopting a measurable water quality index, thereby eliminating influences of errors in a flow meter even in a water treatment apparatus provided with a hard-to-check large flow meter. Thus the present invention can avoid the water quality of permeate from deteriorating and the flow rate of permeate from decreasing due to excessive concentration, and further can reduce needless discharge of concentrate and aim for efficient use of water.

(13) The water quality index as used herein is not particularly restricted so long as the water quality adopted as an index thereof can be measured during operations and the index allows numerical comparison between water qualities of water to be treated and concentrate-permeate combined water. For example, the index may be TOC (Total Organic Carbon), UV or SDI (Silt Density Index). It is, however, preferable to choose an index which makes it possible to simply get measured values, because sometimes it is required to repeat recovery rate adjustment work conducted through the comparison between water qualities of water to be treated and combined water obtained during operations of a water treatment apparatus.

(14) Further, in the following explanation of the invention, electrical conductivity is adopted as an example of the water quality index because measurement thereon can be made with particular ease. Additionally, electrical conductivity measurements are made on water to be treated and combined water by means of an electrical conductivity meter conforming to JIS K 0130 (2008). And the monitoring in the present invention includes measuring electrical conductivity with an automatic and/or hand-operated instrument.

(15) As shown e.g. in FIG. 5, in order to attain a recovery rate of 40% under operation conducted in a water treatment apparatus on a preposition that the predetermined recovery rate is 40%, the concentrate flow rate is adjusted so that a concentrate flow meter gives a readout of 60 L/min and the permeate flow rate is adjusted so that a permeate flow meter gives a readout of 40 L/min. At this time, the flow rate of water to be treated is 100 L/min because it is the sum of the concentrate flow rate and the permeate flow rate. Further, in a case where the electrical conductivity of the water to be treated is 50,000 S/cm at that time, as long as the electrical conductivity of combined water prepared by combining permeate and concentrate at a ratio of 4:6 based on the predetermined recovery rate of 40% is also 50,000 S/cm, no errors arise in the flow meters because the electrical conductivity of the water to be treated is equal to the electrical conductivity of the combined water, and this situation indicates that the water treatment apparatus is practically being operated at the same recovery rate as the predetermined recovery rate.

(16) Next, as shown in FIG. 6, in order to attain a recovery rate of 40% under operation conducted in the water treatment apparatus on the preposition that the predetermined recovery rate is 40%, the concentrate flow rate is adjusted so that a concentrate flow meter gives a readout of 60 L/min and the permeate flow rate is adjusted so that a permeate flow meter gives a readout of 40 L/min. Herein, an assumption is, however, made that an error has arisen in the concentrate flow meter, and the actual concentrate flow rate is lower than the readout (60 L/min) on the concentrate flow meter and the apparatus is being operated at the actual concentrate flow rate of 50 L/min.

(17) In such a case, though the water treatment apparatus is assumed to be correctly operated at the predetermined recovery rate because the flow rate of the water to be treated is 100 L/min as calculated based on the readout on the flow meter and the recovery rate under operation is 40% as calculated based on the readouts on the flow meters, the actual flow rate of the water to be treated in the water treatment apparatus is 90 L/min and the water treatment apparatus is actually being operated at a recovery rate of 44.4% which is higher than the predetermined recovery rate.

(18) The electrical conductivity of the water to be treated in such a case is 50,000 S/cm, and when the assumption is made that the tolerance is 500 S/cm, the electrical conductivity of combined water prepared by combining concentrate and permeate at a ratio of 6:4 based on the predetermined recovery rate of 40% is found to be 53,200 S/cm, and this value exceeds the tolerance on the electrical conductivity of the water to be treated, 50,000 S/cm. Therefore, even though operations have been conducted with the intention of adjusting the recovery rate to become 40%, the electrical conductivity measurements reveal that, in actual fact, operations have been conducted at a higher recovery rate. The actual recovery rate under operation is therefore lowered by manipulating valves so as to increase the concentrate flow rate and/or decrease the permeate flow rate. Thereafter, the electrical conductivity of combined water prepared again at a combining ratio of 6:4 is monitored, and the manipulations similar to the above are repeated until the electrical conductivity of the combined water comes to fall within the tolerance on the electrical conductivity of the water to be treated. Thus it becomes possible to avoid deterioration in water quality of permeate and reduction in permeate flow rate from occurring due to excessive concentration. On the other hand, in a case opposite to the above in which the electrical conductivity of combined water is lower than that of water to be treated, operations have been conducted at a recovery rate lower than the predetermined recovery rate, and valve manipulations to decrease a concentrate flow rate and/or increase a permeate flow rate are repeated until the electrical conductivity of the combined water comes to fall within the tolerance on the electrical conductivity of the water to be treated, whereby needless excessive discharge can be avoided and water conservation can be achieved.

(19) In the manner mentioned above, the electrical conductivity of combined water prepared by combining concentrate and permeate at a ratio based on the predetermined recovery rate of permeate is put in contrast with the electrical conductivity of water to be treated and, when it is detected that the electrical conductivity of the combined water has differed from that of the water to be treated to such an extent as to exceed the tolerance, control operations of the concentrate flow rate and/or the permeate flow rate are repeated, whereby it becomes possible to eliminate influences of errors in flow meters and carry out operations at the same recovery rate as the predetermined one even when the flow meters are hard-to-check large flow meters.

(20) On the other hand, in a case where operations are continued at a recovery rate higher than a predetermined recovery rate, excessive concentration occurs in the vicinity of a semipermeable membrane, thereby increasing an amount of impurity leakage into permeate and causing not only deterioration in water quality of the permeate but also fouling and scaling to result in reduction in permeate flow rate. Thus the life span of a semipermeable membrane element is shortened significantly. Therefore, in the present invention, it is possible to cause the water treatment apparatus to stop by sounding an alarm or actuating interlock at a time when the electrical conductivity of combined water increases beyond the tolerance on the electrical conductivity of water to be treated, whereby factors causing degradation in performance of a semipermeable membrane element can be avoided at an early stage and stable operations of water treatment apparatus can be achieved over a prolonged time period.

(21) The invention has been described above in detail and with reference to the specified embodiments. It will, however, be apparent to one skilled in the art that various changes and modifications can be made without departing from the spirit and scope of the invention.

(22) The present application is based on Japanese Patent Application No. 2012-071574 filed on Mar. 27, 2012, the contents of which are incorporated herein by reference.

(23) The method for recovery rate adjustment of the present invention can be applied to various water treatment apparatuses using a semipermeable membrane.

DESCRIPTION OF REFERENCE NUMERALS AND SIGNS

(24) 1 Semipermeable membrane element 2 Pressure vessel 3 Pump 4 Membrane filtration section 5 Concentrate flow meter 6 Permeate flow meter 7 Semipermeable membrane 8 Permeate spacer 9 Raw water spacer 10 Central pipe 11 Brine seal