A typical prior art water purification system is illustrated in FIG. 1. Feed water is pretreated at 20 and fed to a first storage tank 22 prior to heating in a heat exchanger 24 to a specified membrane operating temperature, typically 25° C. Pre-treatment equipment, which is based on the potable source water quality, typically comprises a multimedia filter to remove particulates, a softener to remove mineral scale, a carbon filter to remove chlorine/chloramines or a chemical injection system using a bisulphite type chemical, possibly a UV station for bacteria kill, and prefilters (1-10 μm) to remove particulates prior to the water entering the reverse osmosis system. After some chemical additions 26, the water is fed to a reverse osmosis membrane assembly 28 and the purified water is treated with ultraviolet light in a first UV station 30, deionized at deionization station 32, treated in a second UV station 34, and passed through a first sterilizing filter 36 before being fed to a second storage tank 38. Water is drawn from the second storage tank 38 at various points of use generally indicated by reference numeral 40 after appropriate treatment including a third UV station 42, a second sterilizing filter 44 and a second heat exchanger 46 to maintain ambient temperatures. Water from the second storage tank 38 is also recirculated through an ozonation system 48 with a pump 50 to reduce bacterial growth. An alternative microbial control design may include a heat exchanger for periodic heat sanitization.
It will be seen from FIG. 1 that excess reject water from the reverse osmosis membrane assembly 28 is drawn through pump 52 to be recirculated to the reverse osmosis membrane assembly 28 while the balance of the reject water is sent to drain. Operation of the system is controlled with a central programmed logic controller (PLC) indicated at 54.
The system is quite complicated in that it has many technologies to monitor and control. The majority of these types of systems are custom built due to the variability of source water and the intricacies of different production demands. With the current approach in the industry, a human operator cannot control and monitor all of the variables to a satisfactory level. This necessitates an expensive PLC control system. The PLC system is also custom designed due to the above considerations. The complexity of this system dictates long lead times for delivery of the equipment. Once the equipment is placed at location, a long process is employed to adjust all of the technologies in order to maintain the desired water quality. Regular cleaning and sanitization must be performed on the equipment to ensure microbial integrity. Due to the variety and complexity of equipment employed, the maintenance is high. If one piece of equipment fails, the water production process ceases. Depending on the location of the failure, it may dictate sanitization of the equipment or system prior to placing it back into service. This represents lost production time. The complexity of the equipment dictates a thorough investigation and testing prior to releasing the system for production. High-energy input is required to temper the water (increase to 20-25° C.) to feed the system and meet reverse osmosis membrane specifications. In addition, high energy consumption and labour are required to maintain the system within specifications. The percent of water recovery or yield is low, being typically 60 to 75 percent of the system's demand.
Microorganisms, specifically bacteria, form biofilm, which is an extra-cellular organic polymer (polysaccharide in nature). Biofilm can also incorporate divalent metal ions that can form a lattice structure consisting of both organic and inorganic mass. This structure protects the organisms from sanitization and cleaning chemicals. Once this formation develops within a system it is very difficult to remove.
The storage tank is a grower of microorganisms unless an ozonation system is applied. This option is capital intensive and has associated operating and maintenance expenses. In addition ozone is a hazardous substance requiring appropriate safety precautions. Ozone is an added substance to the purified water in order to control the microbial integrity. In systems not employing ozone, the microbes will settle onto the tank surface, due to little movement of water (no velocity), and produce biofilm. Free-floating (planktonic) organisms will reproduce and contaminate the distribution system. Biofilm will protect the organisms from chemical sanitization and allow them to reproduce. Chemical sanitization will be reduced in effectiveness. Systems employing heat sanitization are capital and energy intensive and do not remove biofilm.
The typical prior art water purification system is not designed to prevent the growth of microbes. The approach has been to allow the microbial population to increase to a certain range in numbers, then to clean and/or sanitize the system, thus reducing the microbial population. Microbiological procedures require an incubation period of approximately two days or longer prior to enumeration. The delay in results can have the system out of specification for microbial numbers prior to cleaning and sanitizing. Alternatively, a high frequency scheduled cleaning and/or sanitization regimen is implemented to reduce the possibility of the microbial numbers exceeding specification. This approach is labour and energy intensive and prevents the use of the system while the procedures are being conducted. The design of the prior art does not inherently reduce or prevent the growth of microorganisms during the water purification process.
Various attempts to regulate the conductivity of high purity product water have been described in the prior art. A major problem identified in a double pass reverse osmosis system is the difficulty in rejecting gases such a carbon dioxide. Carbon dioxide present in the feed water will pass through the first pass membranes and the second pass membranes forming carbonic acid and the corresponding equilibrium equation products which result in increased conductivity of the product water. This phenomenon is viewed negatively by the prior art since the increase in conductivity is perceived as decreasing the quality.
The following equations express the carbonic acid formation and equilibrium:
Carbonic acid formation 
Carbonic acid equilibrium It is noted that the formulas were not reproduced in the form in which they were filed. The arrows are missing. If necessary, they may be replaced by equal signs.
Methods attempted for removing carbon dioxide are described in several US patents some of which are discussed below. In U.S. Pat. No. 4,574,049 and U.S. Pat. No. 5,997,745 an alkaline agent is added between the first and second pass to convert the carbon dioxide gas to carbonate which is rejected by the second pass membranes. Addition of an alkaline is used prior to the first pass in conjunction with an acid to the second pass with or without a gas liquid separation module in U.S. Pat. No. 5,766,479. Gas removal by hydrophobic gas permeable membrane contactors is described in patents U.S. Pat. No. 5,156,739 and U.S. Pat. No. 5,670,053. Removal by a forced draft decarbonator and a vacuum degasifier is explained in U.S. Pat. No. 5,338,456 and U.S. Pat. No. 5,250,183. Removal by a forced/induced draft decarbonator before or after a two pass reverse osmosis system is disclosed in U.S. Pat. No. 5,925,255. One solution described in U.S. Pat. No. 6,258,278 is to first treat feed water with a strong base anion resin and subsequently removing carbon dioxide in order to maintain a high pH of 6 to 9.5. U.S. Pat. No. 6,080,316 and U.S. Pat. No. 6,126,834 describe the use of caustic injections to adjust the pH of the infeed water that is controlled by a PLC based on resistivity measurements of the product water. These patents plus others describe a removal process for CO2 or methods of preventing the CO2 from ending up in the product water. These patents view the increase in conductivity due to the presence of CO2 in the product water negatively.
Prior art water purification systems are typically designed to produce the purified water at a defined rate. It is usually based on the maximum required water volume demand during a period of time (hour, shift, day or number of dialysis machines, etc.). To this rate a storage tank can be sized to provide this maximum rate with a minimum buffer volume of approximately 20 percent. The systems cannot vary their production rate by more than a few percentages of the original designed rate.
The object of the invention is to provide a better means of producing water that will meet the specifications of Purified Water and Water for Injection as defined by the United States Pharmacopeia Convention Inc. (as defined but not limited to the current edition XXV) and water for dialysis as defined by the American Association for Advancement of Medical Instrumentation (AAMI).
The invention provides a means of purifying water that supplies the purified water to the point or points of use to allow the water to be drawn immediately on demand. The water that is not used immediately is recycled and repurified to ensure continuous quality.
Another object of the invention is to provide purified water directly to the point or points of use without the requirement for a storage and distribution system. The means of providing the water directly to the point of use is an integral part of the purification process.
The invention's objective is to provide purified water having very low microbial counts. Still another object of the invention is to provide a means of purifying water, which is not conducive to growth of microorganisms within the purification process.
In addition, the object of the invention is to provide a means of removing microorganisms that may grow within the purification process.
The object of the invention is also to provide variable production rates to meet variable demand requirements. In addition this saves energy and water.
It is another object of the invention to provide a means to self-clean the purification system of mineral scale and microorganisms.
Still another object of the invention is to allow the system to self-purge itself of purified water that does not meet the conductivity or temperature parameters.
The objects of this invention include providing a water purification system, which can be operated to produce high purity water at a reduced capital cost investment and with lower operating costs.