Patent Publication Number: US-6905607-B2

Title: Method for utilizing rainwater collected at buildings

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional of application Ser. No. 09/842,378, filed Apr. 25, 2001 now U.S. Pat. No. 6,663,769, the disclosure of which is incorporated by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a system and a method for making use of rainwater falling on and collected at houses and buildings. 
     2. Description of the Related Art 
     In general, the volume content or the filling ratio of water molecules in water is at most about 34%. To this end, water is compared to sponge having a number of air gaps in it. Since water molecules are chemical compounds consisting of oxygen and hydrogen, they attract both cations (positive ions) and anions (negative ions)) and trap various substances in the gaps among the water molecules. 
     Water is categorized as a solvent with a property of dissolving materials very well. On the other hand, most materials existing on the earth are categorized as solutes that are soluble in water. As commonly understood, “water”, including the tap water, is a mixture of solvent (i.e., a gathering of water molecules) and solutes, such as minerals. 
     “Pure water” is artificial water, which is produced by physically filtering water, i.e., a mixture of water molecules and various solutes, and removing such solutes and impurities. Pure water has a high absorbency, which is an essential element of its function as a solvent. Pure water is called “hungry water” because of its absorbency, and is used to rinse IC substrates because of its cleansing ability. Pure water is also used as a raw material for drinking water and various beverages because it contains few impurities or solutes. Thus, pure water has been broadly applied to various uses in these years. 
     While pure water is an artificially filtrated product, rain and snow are products of the nature and closest to pure water because they are gatherings of pure water molecules evaporating in the atmosphere. 
     The quantity of solutes contained in a unit volume of water is expressed as hardness. The hardness of tap water in Japan is about 70 mg to 170 mg per liter, and is categorized as soft water containing few solutes by global standards. The hardness of tap water in mountainous areas of Europe is about 370 mg to 400 mg per liter, and is categorized as hard water. The hardness of rainwater is much lower even than soft water, at less than 20 mg per liter. This means that rainwater is supersoft water containing little solute and extremely close to the gathering just of water molecules that is, pure water. 
     The less solutes contained in water, the easier it is to filtrate or purify, and the more possible to treat without using harmful chlorine or other chemicals. Accordingly, rainwater can be recycled into safe natural water. Such recycled rainwater is superior in absorbency, and has a broad range of uses, including rinsing water or drinking water. As long as a reliable filtering means is used and the quality of rainwater is guaranteed, rainwater can be optimally used in various ways, including watering garden plants, washing cars, sanitary water, kitchen water and drinking water. 
     However, with the conventional techniques, rainwater has not been positively or practically utilized, in spite of its superior nature and possibility as the optimum living water. So far, rainwater has only been treated as wastewater, and recycling of rainwater has not been considered sufficiently. 
     SUMMARY OF THE INVENTION 
     Therefore, it is one of the objectives of the present invention to reevaluate the propertees of rainwater, and to provide a system and a method that can positively utilize rainwater collected at houses and buildings, making the best use of this natural resource. 
     To achieve the objective, rainwater is collected from the roof surface of a building or a house through a gutter. A predetermined amount of initial precipitation is removed or thrown away, and the subsequent rainwater is treated (e.g., purified) and stored in a storage tank. The purification includes pH adjustment and sterilization of the rainwater. The stored rainwater is then taken by, for example, pumping for actual use. 
     In another method of utilizing rainwater, rainwater is collected from the roof surface of a building or house through a gutter. A predetermined amount of initial precipitation is removed or thrown away, and the subsequent rainwater is supplied to a purifier, in which the rainwater is subjected to pH adjustment and sterilization. The purified rainwater is supplied to a storage tank. The water level of the storage tank is monitored, and if the water level reaches the prescribed upper limit, the rainwater is prevented from entering the purifier or the storage tank. 
     In still another method of utilizing rainwater, rainwater is collected from the roof surface of a building through a gutter. A predetermined amount of initial precipitation is removed or thrown away, and the subsequent rainwater is supplied to a purifier, in which the rainwater is subjected to pH adjustment and sterilization. The purified rainwater is then supplied to a storage tank. The water level of the storage tank is monitored, and if the water level reaches the prescribed lower limit, a predetermined amount of tap water is supplied to the storage tank in order to keep the water level at a certain level. 
     The second and third methods may be combined. In this case, those steps up to the supplying the purified rainwater in the storage tank are the same. Both the upper and the lower limits of water level of the storage tank are monitored. If the water level reaches the prescribed upper limit, no more rainwater is supplied to the purifier or the storage tank, and if the water level reaches the prescribed lower limit, a predetermined amount of tap water is supplied to the storage tank. 
     In any one of the above-described methods, rainwater is subjected to a prescribed physical filtration at or before the inlet port of the purifier. The purification step includes neutralization of acid rainwater by, for example, pH-adjustment, and sterilization using active oxygen species (or free radicals) produced by decomposition of aqueous hydrogen peroxide. 
     Preferably, the residual active oxygen species remaining in the purified rainwater is decomposed or eliminated in the storage tank using, for example, secondary reactive catalyst. 
     Preferably, before the purified rainwater is supplied from the storage tank for indoor use, an active charcoal filter and a sediment filter further filter the purified rainwater. 
     In another aspect of the invention, a system for utilizing rainwater falling on buildings and houses is provided. The system comprises a means for removing a predetermined amount of initial precipitation from rainwater collected from the roof of a building through a gutter, a purifier including a filter and a sterilizer, and a storage tank for storing the purified rainwater. The purifier receives rainwater, from which the initial precipitation has been removed. The filter of the purifier filters the rainwater physically, and adjusts the pH value of the filtered rainwater. The sterilizer sterilizes the filtered and pH-adjusted rainwater using active oxygen species (i.e., free radicals) produced by decomposition of aqueous hydrogen peroxide. 
     Another type of rainwater utilization system comprises an initial precipitation collection tank, a purifier including a filter and a sterilizer, a switching valve, a storage tank for storing purified rainwater, and a level sensor for sensing the water level of the storage tank. The initial precipitation collecting tank temporarily stores a predetermined amount of initial precipitation of the collecting from the roof of the building. The filter of the purifier carries out physical filtration and pH adjustment, and the sterilizer sterilizes the filtered and pH-adjusted rainwater using active oxygen species (i.e., free radicals). The switching valve switches the water path between the initial precipitation collection tank and the purifier. To be more precise, if the water level of the initial precipitation collection tank reaches a predetermined level, the switching valve closes up so that no more rainwater flows into the initial precipitation collection tank, and instead, that the subsequent rainwater flows into the purifier. The level sensor provided to detect the water level of the storage tank is connected to and cooperates with the switching valve. If the level sensor detects that the water level of the storage tank has reached the upper limit, it causes the switching valve to switch the water path and guide the rainwater into the initial precipitation collection tank. 
     Still another type of rainwater utilization system comprises an initial precipitation collection tank, a purifier, a storage tank storing purified rainwater, a tap-water supply pipe equipped with a tap-water supply valve, and a level sensor for sensing the water level of the storage tank. The initial precipitation collection tank temporarily stores a predetermined initial amount of rainwater collected from the roof of a building through a gutter. The purifier receives the subsequent rainwater after the initial precipitation has been removed into the initial precipitation collection tank. The filter of the purifier carries out physical filtration and pH adjustment, and the sterilizer sterilizes the filtered rainwater using active oxygen species (i.e., free radicals). The level sensor is connected to and cooperates with the tap-water supply valve. If the level sensor detects the water level of the storage tank having reached the prescribed lower limit, it causes the tap-water supply valve to open in order to supply a predetermined amount of tap water into the storage tank. 
     Still another type of rainwater utilization system is provided, which detects both the upper and the lower limits of the water level of the storage tank to adjust the water flow in the system. The system comprises an initial precipitation collection tank, a purifier including a filter and a sterilizer, and a storage tank for storing purified rainwater, as in the above-mentioned systems. This system also comprises a first valve for switching the water path between the initial precipitation collection tank and the purifier, and a second valve provided to a tap-water supply pipe extending from the storage tank. An upper limit level sensor is connected to the first valve, and a lower limit level sensor is connected to the second valve. If the upper limit level sensor detects that the water level of the storage tank has reached the prescribed upper limit, it causes the first valve to switch the water path from the purifier to the initial precipitation collection tank. Upon switching the water path, the rainwater collected from the roof flows into the initial precipitation collection tank, and no more rainwater flows into the purifier. If the lower limit level sensor detects that the water level of the storage tank has reached the prescribed lower limit, it causes the second valve to open in order to supply the tap water into the storage tank. By filling the storage tank, which is short of water, with the tap water, the water level of the storage tank is kept at a desired level. 
     In the above-described systems, the initial precipitation collection tank may have a timer and a drainage valve in order to drain the initial portion of rainfall away from the tank after a prescribed time period. 
     Preferably, the filter has a physical filtration screen for filtering the rainwater collected from the roof, and a pH-adjusting agent for adjusting the pH value of the rainwater for neutralization because the collected rainwater is generally acidic. The sterilizer contains aqueous hydrogen peroxide and a primary reactive catalyst in order to produce active oxygen species (i.e., free radicals) through decomposition of the hydrogen peroxide. The active oxygen species are used to sterilize the filtered rainwater. 
     The storage tank contains a secondary reactive catalyst that reacts with the active oxygen species (i.e., free radicals) remaining in the purified rainwater supplied from the purifier, and that decomposes or eliminates the residual active oxygen species. In the reaction, the secondary reactive catalyst decomposes organic substances, while producing oxygen. 
     Preferably, the storage tank consists of multiple interconnected units or barrels. The number of units is selected depending on how much purified water is required to be stored. The storage tank is placed in the basement or buried in the ground. 
     In the system that has both upper and lower limit level sensors, a back-up sensor is further furnished to the system. The back-up sensor is placed slightly higher than the upper limit level sensor, and is connected to both the switching valve (i.e., the first valve) and the tap-water supply valve (i.e., the second valve). If the upper limit level sensor is out of order, the back-up sensor detects the water level of the storage tank, and carries out the same operations as the upper limit level sensor Namely, the back-up sensor causes the switching valve to shut off the water path to the purifier, and allows the rainwater to flow into the initial precipitation collection tank. 
     The upper limit level sensor is also connected to the tap-water supply valve. When the tap water is supplied to the storage tank because of the shortage of the purified rainwater, the upper limit level sensor detects the elevating water level, and causes the tap-water supply valve to close when the water level reaches the upper limit. As was mentioned above, if the upper limit level sensor is out of order, the back-up sensor detects the elevating water level, and causes the tap-water supply valve to close. In this manner, the water level of the storage tank is kept in the desirable range. 
     The system has a pump for pumping the purified rainwater from the storage tank for various uses. Preferably, an activated charcoal filter and a sediment filter are provided at or after the outlet port of the storage so that the purified rainwater is further filtered before being used indoor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other objectives and advantages will be apparent from the following detailed description of the invention in conjunction with the attached drawings, in which: 
         FIG. 1  schematically illustrates an example of rainwater utilization system according to the invention; 
         FIG. 2  schematically illustrates the water collecting container and the initial water tank used in the system shown in  FIG. 1 ; 
         FIG. 3  schematically illustrates the essential portions of the system shown in  FIG. 1 ; 
         FIG. 4  illustrates another structure of the essential portions of the system shown in  FIG. 1 ; 
         FIG. 5  schematically illustrates an example of the purifier of the system shown in  FIG. 1 ; 
         FIG. 6  illustrates an example of storage tanks used in the system shown in  FIG. 1 ; 
         FIG. 7  illustrates an operation flow for removing a predetermined amount of initial precipitation; 
         FIG. 8  illustrates an operation flow of purification of rainwater; and 
         FIG. 9  illustrates an operation flow of supplying purified water from the storage tank. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERED EMBODIMENTS 
     The preferred embodiments of the invention will now be described in detail with reference to the drawings. 
       FIG. 1  illustrates the overall structure of rainwater utilization system  1  of the invention, which is applied to a residential building. The system treats the rainwater that has fallen on the building so that it rainwater becomes suitable for both outdoor and indoor use. Outdoor use includes watering garden plants, washing cars, and other incidental tasks. Indoor use includes bathroom, laundry, sanitary, and kitchen purposes. 
     The rainwater falling on roof  2  flows into gutter  3 , and is collected in the container  10  placed under the eaves. A strainer  11  is furnished in the container  10 . An initial pipe  12  extends from container  10  to initial precipitation collection tank  20 , and is equipped with a switching valve  15 . The initial precipitation collection tank  20  and the switching valve  15  are provided for purposes of separating a predetermined quantity of initial precipitation from the rainwater collected from the roof  2 . In other words, the initial precipitation collection tank  20  is adapted to receive and temporarily store only the initial portion of rainfall via the switching valve  15 , which is open in order to remove the initial precipitation. 
       FIG. 2  illustrates a detailed view of the initial precipitation collection tank  20  and the switching valve  15 . The initial precipitation collection tank  20  has a capacity of storing a predetermined quantity of rainwater, dependent on the roof surface area. The capacity can be, for example, 200 to 500 liters, or alternatively, the first 2 mm to 3 mm of precipitation depending on the area of the roof  2 . The switching valve  15  is open when the initial portion of rainfall is guided into the initial precipitation collection tank  20  from the container  10 . A level sensor  21  is furnished to the initial precipitation collection tank  20 , and is connected to the switching valve  15 . If the level sensor detects that the water level of the initial precipitation collection tank  20  has reached the prescribed level, it causes the switching valve  15  to close up and prevents the rainwater from flowing into the initial precipitation collection tank  20 . Preferably, the switching valve  15  is controlled so that it automatically opens if a failure occurs to the system in order to guide the rainwater into-the tank  20  and drain from the system without fail. 
     The initial precipitation collection tank  20  is furnished with a timer  22  and a drainage valve  23 . The timer  22  is activated when the level sensor  21  detects that water level of the initial precipitation collection tank  20  has reached the predetermined level. After a preset time period (e.g., 10 hours to 120 hours), the drainage valve  23  opens to drain the initial precipitation from the initial precipitation collection tank  20  in preparation for the next rainfall. The initial precipitation collection tank  20  is also furnished with an overflow pipe  24  and a hand-powered valve  25  for manual drain. 
     As illustrated in  FIG. 1 , the initial portion of rainwater temporarily stored in the initial precipitation collection tank  20  can be recycled into a biotope or infiltration trench in the garden or the backyard, 
       FIGS. 3 and 4  illustrate how the rainwater is guided to purifier  30  and storage tank  60 . In the example shown in  FIG. 3 , an inlet tube  16  is bypassed from the initial pipe  12 , and leads to the purifier  30 . When the switching valve  15  closes up, the subsequent rainwater collected in the container  10  is allowed to enter the purifier  30 . The purifier  30  is a two-stage purification system, and comprises of a filtering tank  40  and a sterilization tank  50 . The inlet port of the filtering tank  40  is positioned higher than the outlet port in order to guarantee one-way flow into the sterilization tank  50 . 
     The example illustrated in  FIG. 4  uses a strainer  115  in the initial container. The strainer  115  has a tilted surface with respect to the direction of the rainfall. The tilted surface of the strainer  115  is, for example, a metallic mesh. At the beginning of rainfall or if the rain slackens, the raindrops flow along the tilted surface of the strainer  115 , and are collected in the initial precipitation collection tank  20 . If the rain becomes heavier, the raindrops penetrate the strainer  115 , and flow into the purifier  30 . In this example, a manually operated valves  55  and an overflow outlet port are furnished between the purifier and the storage tank, so that if the water level of the storage tank reaches the upper limit, the valve  55  is closed manually, while excessive rainwater is drained. The valve  55  may be positioned before the purifier  30 . 
       FIG. 5  illustrates the detailed structure of the purifier  30 . The filtering tank  40  is furnished with a metallic screen  41  having a number of holes with a diameter of, for example, 0.75 mm, and a pH-adjusting agent  42 . The metallic screen  41  physically filters the rainwater supplied from the inlet pipe  16 . The pH-adjusting agent  42  is placed on the bottom of the filtering tank  40  in order to neutralize the rainwater. Rainwater is generally acidic with a pH value of about 4.6. The pH-adjusting agent  42  is, for example, CaCO 3 , which brings the pH value of the rainwater to about 6. 
     The sterilization tank  50  sterilizes the filtered rainwater using active oxygen species (or free radicals) produced by the decomposition of hydrogen peroxide (H 2 O 2 ). Aqueous hydrogen peroxide is contained in a supply tank  51 , which is connected to the sterilization tank  50 . Solid catalyst grains are put into the supply tank  51 , and start reacting with the hydrogen peroxide (H 2 O 2 ) to produce pure oxygen (O 2 ). This oxygen generates pressure, and pushes the hydrogen peroxide (H 2 O 2 ) out of the supply tank  51 . The hydrogen peroxide is constantly supplied to the sterilization tank  50  via a tube coupled with a primary reactive catalyst  52  placed on the bottom of the sterilization tank  50 . The hydrogen peroxide reacts with the primary reactive catalyst  52 , and produces active oxygen species (or free radicals). During this reaction, oxygen is also produced because the active oxygen species turn into oxygen in a short time. The produced oxygen balances with the pressure from the supply tank  51 . The primary reactive catalyst  52  is, for example, annealed MnO 2 . 
     In this manner, hydrogen peroxide is constantly supplied from the supply tank  51  to the primary reactive catalyst  52  under a pressure balance between the oxygen produced in the supply tank  51  and the oxygen generated from the primary reactive catalyst  52 . The reaction rate, and the quantities and percentages of the produced active oxygen species (e.g., hydroxyl radical (. OH − ), superoxide anion radical (.O 2   − ), peroxide (.O 2   2− )) are determined by the quantitative ratio of the hydrogen peroxide supplied to the sterilization tank  50  to the primary reactive catalyst  52 , the concentration of the aqueous hydrogen peroxide, the water temperature, and other factors. 
     Among the active oxygen species, peroxide(•O 2   2− ) has a high sterilization effect. The superoxide anion radical (•O 2   − ) coagulates heavy metal ions, and the hydroxyl radical (•OH − ) decomposes organic substances. Although most of these active oxygen species turn into an oxygen molecule (O 2 ) in a short time, a certain quantity of active oxygen species remain in the rainwater. 
       FIG. 6  illustrates the storage tank  60 , which receives the purified rainwater from the purifier  30 . The storage tank  60  may be consist of interconnected multiple units (e.g., barrels or tanks), depending on how much purified rainwater is to be stored. Preferably, the storage tank  60  is placed in a basement or buried under the ground. Each storage tank  60  contains a secondary reactive catalyst  61 , which promotes the decomposition and elimination of the residual active oxygen species remaining in the purified rainwater. During the reaction between the secondary reactive catalyst  61  and the residual active oxygen species, oxygen is produced and organic substances are decomposed, and convection is caused in the storage tank  60 . The convection occurs because of the decomposition of the residual active oxygen species by the secondary reactive catalyst  61 , which allows the purified rainwater to be kept clean and pure in the storage tank  60  for a long time, without deterioration. 
     The last container of storage tank  60  is furnished with an upper limit level sensor  62  and a lower limit level sensor  63 . A back-up sensor  64  is also provided adjacent to or slightly higher than the upper limit level sensor  62 . The upper limit level sensor is connected to and cooperates with the switching valve  15 . If the water level of the storage tank  60  has risen up to the prescribed upper limit, the upper limit level sensor  62  is activated, causing the switching valve  15  to open in order to allow the rainwater to flow into the initial precipitation collection tank  20 . The lower limit level sensor  63  is connected to and cooperates with the tap-water supply valve  66 . If the water level of the storage tank  60  has fallen to the lower limit (which means that the quantity of rainwater stored in the storage tank has become the minimum acceptable amount), the lower limit level sensor  63  is activated, and the tap-water supply valve  66  is opened. Consequently, a required quantity of tap water is supplied into the storage tank  60 . 
     The upper limit level sensor  62  is also connected to the tap-water supply valve  66 . If the upper limit level sensor  62  detects that the water level has risen to the upper limit due to the supplemental tap water, it causes the tap-water supply valve  66  to close up so as not to supply the tap water into the storage tank any more. 
     The back-up sensor  64  is used if the upper limit level sensor  62  is out of order. The back-up sensor  64  guarantees the operations of the system and prevents undesirable accidents. If the upper limit level sensor  62  can not detect the water level of the storage tank for some reasons, the back-up sensor  63  in turn detects the water level as having reached the upper limit. Then, the back-up sensor  63  causes the switching valve  15  to open, or the tap-water supply valve  66  to close. 
     Preferably, an overflow port  67  is provided to the upper portion of the storage tank  60  to drain the excessive rainwater out of the storage tank  60 . 
     In the preferred embodiment of the system  1 , a pump  70  is provided to pump the purified rainwater from the storage tank  60  for indoor use. Preferably, a filter  80 , which consists of an activated carbon filter  81  and sediment filter  82 , is provided after the pump  70 . The activated carbon filter  81  is, for example, a commercially available 20-inch filter for removing positive ions and the like. The sediment filter  82  is, for example, a 5 μm filter for removing solutes other than ions. 
     Although, in  FIG. 6 , indoor use is exemplified as sanitary use, the purified and charcoal-filtered rainwater, which is purer and safer than the tap water, may be used for drinking, cooking, and other kitchen use. 
       FIGS. 7 through 9  illustrate the operational flows of the rainwater utilization system shown in  FIGS. 1 through 6 . 
     When rainfall begins, the rainwater flows along the roof surface  2  and the gutter  3 , and is collected in the container  10  placed under the eaves. The rainwater further flown through the initial pipe  12  and is collected in the initial precipitation collection tank  20  through the switching valve  15  that is in the open state (step S 1 ). 
     In step S 2 , it is determined whether the level sensor furnished to the initial precipitation collection tank  20  has been activated. If the water level of the initial precipitation collection tank  20  reaches the predetermined level that corresponds to a specified quantity of rainwater, for example, precipitation of 2 to 3 mm, the level sensor  21  is activated. 
     Then, a signal is supplied to the switching valve  15 , causing it to close up so as to allow the rainwater to flow into the purifier. At the same time, the timer  22  is activated (step S 3 ). 
     When a predetermined time period has elapsed (YES in step S 4 ), the drainage valve  23  of the initial precipitation collection tank  20  is opened (step S 5 ), and the temporarily stored initial rainwater is drained out of the tank  20  (step S 6 ). 
       FIG. 8  illustrates the operational flow of the purification and storage of rainwater, which continues from step S 3  of FIG.  7 . Upon closing the switching valve  15 , the rainwater collected in the container  10  is allowed to flow into the purifier  30  (step S 11 ). 
     The rainwater supplied to the purifier  30  is physically filtrated by the metallic screen  41  (step S 12 ). Then, the filtered rainwater is neutralized by a pH adjusting agent  42  placed on the bottom of the filtering tank (step S 13 ). 
     The pH-adjusted rainwater is then supplied to the sterilizing tank  50  of the purifier  30 , in which sterilization is carried out using active oxygen species produced by the decomposition of aqueous hydrogen peroxide (H 2 O 2 ) (step S 14 ). 
     The sterilized rainwater is supplied to the storage tank  60  (step S 15 ), in which the residual active oxygen species remaining in the sterilized rainwater are decomposed and eliminated using secondary reactive catalyst  61  (step S 16 ). The reaction between the secondary active catalyst  61  and the residual active oxygen species produce oxygen, while decomposing the organic substances contained in the rainwater. For this reason, the rainwater stored in the storage tank  60  does not deteriorate for a long time, and can be maintained clean and pure. 
     In step S 17 , it is determined whether the water level of the storage tank  60  has reached the prescribed upper limit. If the water level has risen to the upper limit, the upper limit level sensor  62  is activated (YES in step S 17 ). 
     The activation of the upper limit level sensor  62  causes the switching valve  15  to open in order to allow the rainwater to flow into the initial precipitation collection tank  20 . 
     If the upper limit level sensor was not activated for some reasons (NO in step S 17 ), in spite of the increase in water level, it is then determined whether the back-up sensor  64  has been activated (step S 18 ). If the back-up sensor  64  has been activated (YES in step S 18 ), the switching valve  15 , which is connected to the back-up sensor  64 , is opened to allow the rainwater collected in the container  10  to pass into the initial precipitation collection tank  20  (step S 19 ). In this manner, even if the upper limit level sensor  62  does not work for some reason, the back-up sensor  64  carries out the required operations in place of the upper limit level sensor  62 , thereby guaranteeing the operations of the system  1 . 
     As illustrated in  FIG. 1 , the rainwater flowing into the initial precipitation collection tank  20  can be recycled as irrigation for biotope or green field. 
       FIG. 9  illustrates the operational flow of supplying the tap water into the storage tank  60  in times of a shortage of stored rainwater. As the purified rainwater is being used for various purposes, the quantity of the stored rainwater decreases, and the water level of the last container of storage tank  60  may reach the lower limit (i.e., the minimum level), especially in a dry season. The system  1  of the present invention detects a shortage of the purified rainwater, and supplies the tap water in order to maintain the water level of the storage tank  20  at a desirable level. 
     First, it is determined whether the lower limit level sensor  63  has been activated (step S 21 ). If the water level of the storage tank  60  has reached the lower limit, the lower limit level sensor  63  is turned on (YES in step S 21 ). The activation of the lower limit level sensor  63  causes the tap-water supply valve  66 , which is connected to and cooperates with the lower limit level sensor  63 , to open (step S 22 ). Then, a required quantity of tap water is supplied to the storage tank  60  (step S 23 ). 
     If, due to the supply of the tap water, the water level of the storage tank  60  returns to the upper limit, the upper limit level sensor  62  detects the water level as having reached the upper level, and causes the tap-water supply valve  66  to close. This automatic closing of the tap-water supply valve  66  is monitored (step S 24 ). If the tap-water supply valve  66  was not closed for some reason (NO in step S 24 ), which means that the upper limit level sensor  62  did not work, the process proceeds to step S 25 , in which it is determined whether the back-up sensor  64  is activated. If the back-up sensor  64  has been activated (YES in step S 25 ), the tap-water supply valve  66  is shut to prevent the tap water from entering the storage tank any more (step S 26 ). 
     Although not illustrated in the drawings, either a control board or a computer program can control these operations. 
     The purified rainwater stored in the storage tank  60  is pumped up for various uses. Although the purified rainwater can be used as it is because of its purity and cleanliness, it may be filtered further before it is used for kitchen purposes. For example, an activated carbon filter  81  can be used to remove positive ions and the like, and a sediment filter  82  can be used to remove solutes other than ions. The activated carbon filter and the sediment filter can be used in combination. 
     The rainwater utilization system has various applications in a wide range of scales. Such applications include, but not limited to:
     (1) supplying household water for washing cars, sprinkling, watering garden plants (small scale);   (2) supplying sanitary water used in bath rooms and for laundry (small or medium scale);   (3) supplying water to a pond or biotope (in a small scale);   (4) using rainwater as coolant in an air conditioning system or a heat exchanger (medium or large scale);   (5) supplying living water used in a daily life for laundry, bathroom, and cleaning (medium scale);   (6) supplying kitchen water for drinking, cooking, dish washer, etc. (medium scale);   (7) recycling rainwater in a hot-water system or a solar heater system (medium scale);   (8) recycling rainwater for sprinkling kitchen gardens and green house (wide range of scales);   (9) watering potted plants or house plants (small or medium scale);   (10) storing purified rainwater as emergency water life line as a precaution for natural calamities (medium or large scale);   (11) using rainwater for fire fighting (large scale);   (12) as a measure against water shortage (e.g., an infiltration trench for garden trees and plants, permeable water for preventing the lowering of groundwater levels, etc. wide-ranging scales); and   (13) as a measure against flood (including infiltration trenches and reservoirs for urban systems) (medium or large scale).   

     In the above applications, the rainwater drained from the initial precipitation collection tank or overflowing the storage tank is recycled for outdoor uses. On the other hand, the purified rainwater through pH adjustment and sterilization is suitable for indoor uses. In this manner, the invention can make best use of natural resources, positively utilizing rainwater. 
     Although the invention has been described based on the preferred embodiments, the invention is not limited to these examples and many changes and substitutions are possible without departing from the scope of the invention, which is defined by the appended claims.