Abstract:
A system and method for removing minerals from a water source and concentrating these minerals for ease of reuse or disposal includes passing the water from a suitable source through cascaded membrane filters, the reject outputs of each of which are connected to the inputs of the next membrane filter in the cascade. At the input of each of the membrane filters, a pre-filter in the form of a micro-filtration filter, an ultra filtration filter, or a slow sand filter, is used to remove sediment and impurities from the water stream prior to the application of the water to the input of the next membrane filter in the cascade.

Description:
BACKGROUND 
       [0001]    Many municipal water sources include high concentrations of dissolved minerals, at least some of which must be removed prior to supplying the water to ultimate consumers. In addition, particularly in areas of limited water supply, sewage effluent is processed for use in watering golf courses, parks and the like. Such effluent generally includes a high concentration of minerals. 
         [0002]    There are several methods of concentrating reject water from water processing systems for disposal of that reject water. These methods include evaporation ponds, high efficiency reverse osmosis, thermal brine concentration, brine crystallization, and others. Removal and concentration of minerals in systems currently in use, however, are economical only if large quantities of water are processed. Presently, there is no practical and economical process for water flows of three million (3,000,000) gallons per day and smaller. 
         [0003]    Evaporation ponds frequently are used to concentrate the brine or mineral concentrate. Depending upon the climate and temperature (that is, sunshine, rain or snow), the evaporation rate will vary. Different rates of evaporation require varying pond areas since the losses due to evaporation also vary by the surface area of the water exposed to the atmosphere. Evaporation pond processes require large areas of land. This can become expensive if the cost of land is high, unless the reject brine from the process can be concentrated into a very small quantity of liquid. 
         [0004]    High efficiency reverse osmosis processes consist of lime softening, hardness polishing through weak acid cation exchange, pH increase, and reverse osmosis with sea water RO membranes. These processes are used in conjunction with obtaining drinking water from sea water, and are relatively expensive systems, particularly for smaller systems when processing several million gallons of water per day. 
         [0005]    Another technique which has been used is thermal brine concentration. This type of a system recovers some of the waste stream through evaporation and vapor compression in large facilities. Thermal brine concentration systems require the addition of energy in the form of heat, and also require high pressure pumps. This process, because of the size of the equipment required, does not lend itself to small applications (that is, applications of less than 3,000,000 gallons per day). 
         [0006]    Another technique for removing and concentrating the reject water from a water processing system is a thermal flash evaporation process, which causes the formation of salt crystals in a brine solution. Thermal flash evaporation requires energy to keep the process under pressure circulation, and requires the addition of heat. This process requires relatively massive large scale equipment, and does not lend itself to small applications of under 3,000,000 gallons per day. 
         [0007]    Electrodialysis reversal (EDR) technology has been used for many years. This technology, however, has had limited testing and application in treating wastewater tertiary effluent for re-use. Even with an EDR system, fouling can be a particular concern when treating tertiary effluent from a municipal wastewater treatment plant. 
         [0008]    Water treatment using reverse osmosis (RO) technology leaves a reject stream with a concentration of suspended solids, plus added antiscalant, anti-flocculent chemicals, dissolved organics, minerals and other pollutants which are removed from the product water produced by the RO technology. The disposition of this reject stream is difficult in many situations. For some cases, the reject stream pollutants pose a liability for the users of the product water. In addition, the loss of the ten percent to fifty percent reject for any beneficial use also poses a problem in water short areas, where all water resources are needed. 
         [0009]    The treatment of water with a slow sand and natural filtration system shown in the U.S. Pat. No. 5,112,483 to Cluff for scaling control provides good quality water for many purposes at a reasonable cost. The system disclosed in the Cluff patent uses a slow sand filter to receive the water being treated. The output of the slow sand filter then is supplied through a cascade of nano filtration filters, which may include a catalytic conditioner or magnetic water conditioner in the system. Although the system of the Cluff patent exhibits improved efficiency, a relatively high percentage of reject and the attendant disposal problems for the reject still are present in the system. The Cluff system, however, does provide combined benefits of nano filtration units and a slow sand filter. As is well known, slow sand filters not only serve to physically filter the sediment and other impurities from the water supplied to the filter, but also provide a conducive environment for micro-organisms which further purify the water, removing some-organic matter. The micro organisms modify the electrical charge so that clay is easily removed by the slow sand filter. The biological treatment produced by slow sand filters is not available in rapid sand gravity or pressurized filters. Unlike with slow sand filters, clay removal is not accomplished without the use of flocculents. Unused flocculents foul RD membranes. 
         [0010]    Nano-filtration filter membranes have a higher molecular cutoff than the membranes of reverse osmosis (RO) systems. The membrane of a nano-filtration filter is “coarser” than that of a reverse osmosis filter; and because of this fact, it will pass most of the sodium chloride and reject bivalent ions, calcium, magnesium and sulfate. Previously, nano filter membranes would produce more permeate than the RO; but recent advances have improved the RO membrane to the point that there is not as much difference in production. 
         [0011]    It is desirable to provide an improved system and method for removing minerals from a source of water, which overcomes the disadvantages of the prior art. 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0012]      FIG. 1  is a block diagram of a system in accordance with an embodiment of the invention; and 
           [0013]      FIGS. 2 through 5  are graphs useful in describing the results of the-operation of the system shown in  FIG. 1 . 
       
    
    
     DETAILED DESCRIPTION 
       [0014]    Reference now should be made to  FIG. 1 , which is a block diagram of a system in accordance with an embodiment of the invention. As shown in  FIG. 1 , a source of water  20  is used to supply the feed water to be processed by the system. The water source  20  may be surface water, ground water, brackish water, or effluent reclaimed from wastewater. The system of  FIG. 1  may be used in conjunction with processing effluent from wastewater to provide irrigation water for golf courses, parks and the like. 
         [0015]    Some of the water  20  may contain chlorine, which destroys organisms working in slow sand filters. If chlorine is present, it must be removed; and a common method is to inject sodium thiosulphate into the feed-water  20 . The manner in which this is done, if it is necessary, is conventional and therefore has not been illustrated in  FIG. 1 . 
         [0016]    As shown in conjunction with the embodiment of  FIG. 1 , the processing system for removing minerals from the water source  20  and concentrating these minerals for ease of disposal constitutes a cascade of several elements. The source of water initially is supplied through a first pre-filter  22 , which typically may be a micro-filtration filter, an ultra filtration filter, a biological filter, or a slow sand filter, all of which are used to enhance and reduce the fouling of subsequent stages of membrane-type filters, such as nano-filtration filters or reverse osmosis (RO) filters. 
         [0017]    For the system shown in  FIG. 1 , slow sand filters (SSF) have been found convenient because of their relatively low cost and because of the biological benefits provided, as generally described above. If a slow sand filter is used for the pre-filter  22  (and also for the pre-filters  34  and  43 ), approximately seven square feet of filter area per 1,000 gallons per day (gpd) of flow is required. In addition, either as part of or in conjunction with the pre-filter  22  (and pre-filters  34  and  43 ) a 5 micron filter, either external to the various membrane filters disclosed, or located in the input side of the membrane filters, may be provided. The 5 micron filtration may be inherent in the characteristics of the pre-filters  22 , 34 , and  43 ; but it can be added as an extra element if necessary. 
         [0018]    The output water flow from the first pre-filter  22  is supplied to the input of a first membrane filter  24 . The filter  24  and second and third membrane filters  36  and  44  may be either nano-filtration filters or reverse osmosis (RO) filters, depending upon the particular application which is to be made of the system. In the system described in conjunction with the graphs of  FIGS. 2 through 5 , the filters  24 , 34  and  44  are RO filters. 
         [0019]    As noted, feed to the first RO filter  24  is from the slow sand filter or pre-filter  22 , into which effluent from a municipal wastewater plant or other water which has mineral levels higher than desired is first treated with sodium thiosulphate to remove chlorine. The permeate or product from the RO filter  24  is supplied over a line  26  to a lake  28  or other suitable storage facility. 
         [0020]    The reject output from the RO membrane filter  24  is supplied to a softener  30 , which typically uses a lime treatment, or lime plus soda ash treatment to precipitate calcium (Ca) and magnesium (Mg) out of the reject stream from the water flow, prior to supplying the reject stream to the input of the second pre-filter  34 . The precipitation of calcium and magnesium is in the form of calcium hydroxide and magnesium hydroxide, which may be separately removed, or, as shown in  FIG. 1 , supplied from the softener  30  over a line  32  into the lake or storage facility. The lines  26 , 32 , 38  and  42  may be combined into a single pipeline. 
         [0021]    The amount of lime which is used in the softener  30  may be empirically determined from the nature of the reject supplied from the filter  24  as a result of the condition of the water supplied at  20  to the first pre-filter  22 . By precipitating the calcium and magnesium from the water, the hardness is substantially reduced prior to supplying the softened water from the softener  30  to the second pre-filter  34 , as shown in  FIG. 1 . The use of slow sand filters for these pre-filters carries the additional capability of purification of the water through microorganisms present in the slow sand bed. 
         [0022]    The output of the second pre-filter  34  is supplied to the input of second membrane filter or RO filter  36 , which is similar to the filter  24 , described previously. The permeate from the filter  36  is supplied over a line  38  to the lake or storage facility  28 . The reject from the filter  36  is supplied to a second softener  40  which operates in the same manner as the softener  30  to precipitate additional calcium and magnesium from the reject water stream. This precipitated calcium and magnesium may be separately disposed of, or supplied over the line  42  to the lake  28 , as described previously in conjunction with the softener  30 . The output of the softener  40  then is supplied to a third pre-filter  43 , which then supplies its output to the input of a third membrane filter  44 , which may be a nano-filtration filter or an RO filter. Typically, the unit  44  is what is known as a “sea water” RO unit capable of handling the significantly higher levels of minerals entering the unit after the reject water has been concentrated by the previous two membrane filters  24  and  36 . 
         [0023]    The filters  24  and  36 , if RO filters are employed, operate at approximately 175 pounds per square inch (psi) input pressure, while the filter  44  operates at approximately 1,000 psi. The output from the permeate output of the filter  44  is supplied to the lake or storage facility  28 ; and the final reject (brine) is discharged at  48  to an evaporation pond, or other processes or equipment to further concentrate the final reject, such as a tank, or other suitable containers for removal. 
         [0024]    Before passing the softened water from the softeners  30  and  40  back to the pre-filters  34  and  43 , it is desirable to adjust the pH back down from the relatively high level resulting from the lime treatment to a lower level. This increases the capacity of the water to carry calcium and magnesium without scaling. It also prevents the microbes in a slow sand filter (if a slow sand filter is used for the filters  34  and  43 ) from being destroyed by a high pH. 
         [0025]    In a system which has been operated as shown in  FIG. 1 , a waste water pilot project was operated with an input at 20 gpm. If the results (data) at the 2 gpm pilot, using three cascading RO&#39;s, were used to project the operation of a two hundred fifty thousand (250,000) gallons per day (gpd) process using effluent  20 , this would result in an input of 174 gallons per minute (gpm). The total dissolved solids (TDS) in the input water  20  would be approximately 1,260 parts per million (ppm). This input would be supplied at  20 , through a first slow sand pre-filter  22 , to an RO unit  24  operating at 175 psi. The permeate applied over the line  26  from the unit  24  would be 190,000 gpd, or 132 gpm, with a concentration of 60 ppm total TDS. 
         [0026]    The reject feed from the RO unit  24  to the softener  30  would be 60,000 gpd, or 42 gpm, with a concentration of 5,061 ppm TDS. As noted above, lime added by the softener  30  would precipitate the calcium and magnesium from the reject; and these precipitated minerals would be added to the lake  28  which would be used as irrigation water for a golf course. The second RO unit  36  would operate at 175 psi, as noted above, and supply permeate product to the lake  28  over the output line  38  at 37,900 gpd or 27 gpm at a mineral concentration of 288 ppm. 
         [0027]    The reject output of the RO filter  36  supplied to the softener  40  would be at the rate of 22,105 gpd or 15 gpm, with a concentration of 17,300 ppm TDS. Again, precipitation of the calcium and magnesium by the softener  40  would take place prior to application of this reject feed to the pre-filter  43 , the output of which then would be supplied to the third sea water RO unit  44  operating at 1,000 psi. The permeate or useful output from the filter  44  applied over the line  46  to the lake  28  would be at the rate of 17,680 gpd or 12 gpm at a concentration of 740 ppm. The final reject  48  from the output of the RO unit  44 , would be supplied to drying beds, other processes or equipment to further concentrate the final reject, such as tanks or other disposal means at a rate of 4,420 gpd or 3 gpm, with a concentration of 91,000 ppm TDS. 
         [0028]    The summary of all of the products supplied to the lake  28  would amount to 245,580 gpd, or 171 gpm, with an average TDS of 134 ppm. This is a very acceptable level for use as irrigation water for golf courses, parks and similar facilities. This water may be used alone or combined with some of the water  20 , or water from other sources, if desired. 
         [0029]    Adding the lime used in the softeners  30  and  40  lowers the Sodium Adsorption Ratio (SAR). Reduction in soil permeability is a chronic problem for golf courses, parks and the like; and this reduction can happen at much lower concentrations of sodium. SAR is a measure of the relative concentrations of the sodium ion and the calcium and magnesium ions, and it is a way to evaluate the effect that the sodium concentration has on the soil permeability. By adding softening materials in the softeners  30  and  40  to the product water, to remove calcium and magnesium, the SAR is lowered, and therefore, constitutes another benefit to the golf course turf or the like. 
         [0030]    Reference now should be made to  FIGS. 2 through 5 , which are graphs of a one-year operation of a 250,000 gpd plant capacity system to determine the efficiency of the system. RIGS.  2 , 3 , 4  and  5  are graphs, respectively, of TDS, sodium, chloride, and SAR for a combination of irrigation water and wastewater effluent input to the RO cascade of  FIG. 1  having 1,710 ppm TDS in it. After processing as described above, the product from the lake  28  may be blended with varying amounts of water from other sources. The graphs cover twelve months, indicating the goal average of TDS, as well as the average product over the various months. As can be seen from  FIG. 2 , the spring and summer months included the highest concentrations of TDS in the irrigation water; but the average product (250,000 gpd from the system of  FIG. 1  combined with varying amounts of water) would exceed the goal by only a slight amount (677 ppm compared with a goal of 640 ppm). 
         [0031]      FIG. 3  shows the sodium concentration of the input water  20  at 292 ppm and an average goal for the year, desired at 125 ppm. The average of a combined processed effluent  20  and irrigation water  28  product over the twelve month period would be 124 ppm. Again, the months from April through October would constitute the highest concentrations of sodium in the combined irrigation water. 
         [0032]      FIG. 4  shows the chloride concentration of the input water at 223 ppm; and a goal of 70 ppm. The average over the year would be 74 ppm of chloride. Again, the months of April through October would be the highest concentrations in the irrigation water when product water  28  would be combined with the processed effluent  20 . 
         [0033]    Finally,  FIG. 5  shows the sodium adsorption ratio (SAR) with the effluent input water  20  having a ratio of 6.09. Compared to a goal of 3.5; and with no softeners, the RO treatment would produce an average of 5.13 over the course of a year. By the additions of the softeners  30  and  40  for precipitating out the calcium and magnesium, an average of 1.81 SAR would be achieved, with the lowest amounts occurring in the months of November through March. Again, as with the other charts, the months of April through October produced the highest amounts of SAR because of the use of larger amounts of effluent water  20 .  FIG. 5  clearly shows the advantage of the addition of the softener stages  30  and  40  to the system for improving the sodium adsorption ratio (SAR). 
         [0034]    The lime softening process in repeated RO applications can be used to supply-domestic water. The difference would be that the product water would be stored in a covered tank instead of a lake. The magnesium and calcium sludge fro lime softening would be sent to drying beds before being harvested for sale to agriculture or industrial users. 
         [0035]    The foregoing description of an embodiment of the invention is to be considered as illustrative and not as limiting. Modifications will occur to those skilled in the art for performing substantially the same function, in substantially the same way, to achieve substantially the same results without departing from the true scope of the invention as defined in the appended claims.