Abstract:
A method and system for removing pollutants from water are provided. The system includes at least one settling basin having shallow and deep areas. Coagulants can be added to bring pollutants down to the bottom surface of the basin. During dry periods, the deep areas retain a level of water therein, and the flocs that have settled on the bottom of the shallow areas will dry out, and can be tilled into the soil until re-flooding.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application claims priority to provisional application Ser. No. 60/946,507, filed Jun. 27, 2007. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods of surface water purification. 
     2. Description of Related Art 
     The removal of pollutants from surface waters, such as lakes, ponds, streams, and canals, has proven technically challenging and costly. Chemical coagulants have been used successfully to treat surface waters, but these chemical-based treatment systems can be expensive and problematic with respect to environmental concerns about the disposal of chemical residues. 
     Surface water chemical treatment systems usually obtain feedwaters from a polluted source via either a gravity flow or pumped flow of water through a pipe or culvert. A chemical coagulant is injected into the pipe, and water turbulence provides mixing of the coagulant and water. This coagulant-water mixture is then fed into a large settling basin, typically an earthen pond, with a relatively long (e.g., 3-24 hour) hydraulic retention time. A chemical precipitate (termed a “floc”) forms in the water as a result of coagulant addition. The flocs and associated pollutants settle in the quiescent pond environment, and the clarified water, relatively free of pollutants, exits the pond. 
     Over time, a layer of floc or chemical residue builds up on the bottom of the settling pond, and this material periodically must be collected, dried, and disposed of. A typical approach used for chemical residue handling is to establish and maintain chemical floc “drying beds” adjacent to the pond, into which the floc material periodically is pumped from the settling basin and allowed to dry. The dried chemical residue and associated pollutants typically is then hauled away to a disposal site. The selection of an appropriate disposal site, however, can be a problem, due to concerns about adverse environmental effects of the residue constituents (which include the active ingredient of the coagulant, often a metal, and the pollutants that have been removed from the water). 
     Chemical treatment systems require moderate amounts of land, with the settling pond and the floc residue drying beds comprising the greatest area requirement. There exists a compromise between settling pond size and chemical dose; to a certain extent, the larger the settling basin (and the more effective it is for floc removal), the lower the chemical dose required to accomplish effective pollutant removal. Because operating costs associated with chemical use can be quite high, it is important to incorporate adequate settling basin area in the treatment system. 
     SUMMARY OF THE INVENTION 
     The present invention is directed to a method and system for removing pollutants from water. The system comprises at least one settling basin having shallow and deep areas. Coagulants can be added to bring pollutants down to the bottom surface of the basin. During dry periods, the deep areas retain a level of water therein, and the flocs that have settled on the bottom of the shallow areas will dry out, and can be tilled into the soil until re-flooding. 
     The features that characterize the invention, both as to organization and method of operation, together with further objects and advantages thereof, will be better understood from the following description used in conjunction with the accompanying drawing. It is to be expressly understood that the drawing is for the purpose of illustration and description and is not intended as a definition of the limits of the invention. These and other objects attained, and advantages offered, by the present invention will become more fully apparent as the description that now follows is read in conjunction with the accompanying drawing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a side cross-sectional view of a water treatment system of the present invention. 
         FIG. 2  is a top plan view of an exemplary embodiment of the surface water treatment system of the present invention. 
         FIG. 3  is a cross-sectional view of an alternate embodiment of a surface water treatment system. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A description of preferred embodiments of the present invention will now be presented with reference to  FIGS. 1-3 . 
     The present invention describes a design and management approach for eliminating the need for off-site disposal of chemical residues, while maintaining a modest system area requirement and providing effective floc removal in a settling pond. 
     A first embodiment of the system  10  ( FIGS. 1 and 2 ) comprises a settling pond  11  containing both deep zones  12  and shallow zones  13 , these zones having depths  32  and  31 , respectively. The shallow zones  13  typically comprise approximately 50% of the pond area, or greater. The exact elevations of both shallow  13  and deep  12  zones depend on factors such as soil type, groundwater elevations, the seasonal availability of surface water to be treated, and whether or not the inflow to the settling pond  11  is pumped or fed by gravity. The deep zones  12  in the pond  11  typically are configured as bands, oriented perpendicular to flow F, so as not to encourage water short-circuiting during normal operations. 
     As an alternative configuration ( FIG. 3 ), rather than one large pond containing both shallow and deep zones, the settling basin complex  20  can comprise multiple ponds  21 ,  22 , . . . in series, some of which are shallow  21 , and some of which are deep  22 . 
     During normal operations, water passes through the settling pond  11  (or ponds  21 ,  22 , . . . in series), and chemical flocs  14  resulting from coagulant addition settle onto the bottoms  15 , 16  of both deep  12  and shallow  13  zones (or ponds  21 ,  22 , . . . ). Periodically, such as once every one or two years, although this is not intended as a limitation, the water level in the settling pond  11  (or pond complex) is lowered to an elevation below that of the bottom  16  of the shallow zones  13 , but above that  15  of the deep zones  12 . The lowering of water levels can be accomplished either passively (by evaporation and seepage) or actively (by pumping). Upon exposure to air and sunlight, the floc  14  that has settled on the bottom  16  of the shallow zones  13  dries. As a result of drying, the volume of most chemical flocs is reduced dramatically, for example, in the range of 90-98%. The dried chemical residue subsequently can be tilled (using conventional farm machinery) into the soil at the bottom  16  of the shallow zones  13 , and the system  10  re-flooded. This incorporation of material into the soil of the settling basin  11  obviates the need to export chemical residues from the treatment system  10 . 
     To enhance floc removal within the settling basin complex, vegetation can be stocked in the shallow  13  and/or deep  12  zones (ponds). This vegetation typically can consist of floating aquatic plants  23  such as water hyacinth, but in some cases could consist of emergent  24  or submerged  17  macrophytes. Both the presence of plant roots  25  (in the case of floating plants  23 ) and stems/leaves  26  (in the case of emergent  24  and submerged  17  macrophytes) create a filtering action, and also dampen wind and wave action. Both of these characteristics enhance floc settling. Additionally, the wetland vegetation  17 , 23 , 24  helps remove selected pollutants (e.g., nitrate and ammonium-nitrogen) that are not readily removed by chemical additions. 
     Another important function that can be served by the settling basin complex is to provide pH control. Often, the addition of chemicals results in a lowering of the pH in the water, owing to the acidic nature of many coagulants, such as selected iron and aluminum compounds. In conventional chemical treatment systems, a supplemental base-containing chemical, such as sodium hydroxide, often must be added to achieve a target outflow pH level (often circumneutral) prior to discharge. The cost of adding this basic chemical may equal or even exceed the cost of the coagulant itself. 
     In the present invention, design features incorporated into the settling basin  11  or pond(s)  21 , 22 , . . . can serve to achieve target pH levels prior to discharge, thus obviating the need for addition of basic chemical. For example, limerock (principally calcium carbonate) outcroppings or berms  30  can be added within at least one of a mid-region  27  and an outflow region  28  of the settling basin  11  or pond(s)  21 , 22 , . . . , and the acidic waters resulting from the inflow region  29  coagulant addition slowly dissolves the calcium carbonate, resulting in an increase in water pH prior to discharge. Moreover, certain vegetation types can also influence pH. For example, the inclusion of submerged macrophytes or algae in the deep or shallow zones can increase the pH of the water column as a result of the plant&#39;s assimilation of carbon dioxide during photosynthesis. A combination of both submerged plants and limerock in the settling basins  11  (ponds  21 , 22 , . . . ) may prove particularly effective for pH control, since many types of submerged vegetation thrive on the alkaline microenvironments that would be provided by limerock outcroppings. 
     In the shallow zones  13 /ponds  21 , the plants are allowed to dry down when the pond water level is dropped, and are tilled into the soil along with the chemical floc. Additionally, when either floc levels and/or plant biomass build up to unacceptable levels in the deep zones  12  (ponds  22 ), then these materials can be pumped or otherwise conveyed onto the shallow areas  13 , 21  during the drydown period, where they can be incorporated into the soil after drying. 
     Similar to the chemical residue that results from coagulation, aquatic plants have a high water content (˜95%), and therefore exhibit a dramatic volume reduction upon drying. The concomitant tilling of chemical flocs and aquatic plants into the bottom of the shallow zones can be beneficial in several respects. The chemical flocs typically contain some residual pollutant removal capability, so any nutrients such as phosphorus that are released by the plants during their decomposition in the soil are effectively immobilized by the incorporated floc residue. Additionally, the aquatic plants contribute organic matter to the soil, and through this addition of organic carbon, tend to moderate the increase in metal and pollutant concentrations in the soil as chemical residues are tilled in. The net effect is to gradually build small amounts of soil over time. 
     Periodic drydown of shallow zones  13  in the settling basin  11  complex can be readily accomplished in locations that exhibit a pronounced wet and dry season. As an alternative, one or more pumps  18  deployed in the deep zones  12  can be used to “dewater” the shallow zones  13 . Additionally, piping or trenches  19  can be deployed to interconnect deep zones  12  or ponds  22 , and if surface waters need to be treated during the “drydown” period, these deep areas  12 , 22  can be maintained in service. If needed, parallel trains of settling basins also can be established at each treatment facility, so one basin can be maintained on-line while the other is being dried down. 
     In the foregoing description, certain terms have been used for brevity, clarity, and understanding, but no unnecessary limitations are to be implied therefrom beyond the requirements of the prior art, because such words are used for description purposes herein and are intended to be broadly construed. Moreover, the embodiments of the system and method illustrated and described herein are by way of example, and the scope of the invention is not limited to the exact details of construction and use. 
     Having now described the invention, the construction, the operation and use of preferred embodiments thereof, and the advantageous new and useful results obtained thereby, the new and useful constructions, and reasonable equivalents thereof obvious to those skilled in the art, are set forth in the appended claims.