Patent Publication Number: US-2009230054-A1

Title: Contaminant Removal System And Method For A Body Of Water

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application claims priority to provisional patent application Ser. No. 61/119,443, filed Dec. 3, 2008, and is a continuation-in-part of application Ser. No. 11/363,989, filed Feb. 28, 2006, issued U.S. Pat. No. 7,510,660, which itself is a continuation-in-part of application Ser. No. 10/656,545, filed Sep. 5, 2003, issued U.S. Pat. No. 7,014,776. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to systems and methods of water purification, and, more particularly, the control of nutrients, suspended and filamentous algae, pollutants, and toxins in water. 
     2. Description of Related Art 
     Many freshwater lakes and ponds, as well as estuaries, are characterized, particularly during the warmer months, by certain contaminants, such as dissolved color, suspended solids, phosphorus, nitrogen, oxygen-demanding substances, pathogenic organisms, and metals. 
     Chemical coagulants often are used in water treatment to remove contaminants from the water. In lake water treatment, for example, entire lakes or ponds may be treated with coagulants (typically the aluminum compound “alum”). These are added at or above a “critical” concentration, dictated by water chemical characteristics such as water pH and alkalinity, so that a floc forms. Contaminants in the water column are then encapsulated by, or adsorbed to, the floc, which then settles to the bottom of the lake. 
     It is also known in the art to treat wastewater and drinking waters with conventional “concrete and steel” chemical technologies, using separate chambers for: (1) adding and mixing coagulant; (2) rapid mixing to form flocs; and (3) clarifying to permit settling of flocs, subsequently allowing a clear supernatant to flow out from a port near the top of the vessel. 
     Alternatively, it is known to inject a coagulant into a water inflow pipe just prior to feeding the water into a lake. This may be accomplished, for example, using a flow proportional injection of coagulant into a stormwater inlet pipe, or by injecting the coagulant into a pipe as it feeds into a wetland. In these cases, the floc accumulates in the lake or wetland over time. In yet another method, prior to entering the lake the floc is fed into a clarification or separation vessel, where the floc is captured and disposed of in a sanitary sewer or is used for land application. 
     Chemical treatment of both continuous-flow and variable-flow (e.g., agriculture and urban runoff) surface water sources typically is performed to protect downstream lakes, ponds, and streams from the adverse effects of high pollutant loads. The rainfall-driven runoff loads often vary widely in flow volume, and may also vary in chemical and physical composition. These varying flow volumes, along with fluctuations in the content of constituents (color, particulate matter, alkalinity) that influence coagulant effectiveness, often result in high and variable chemical dose requirements for coagulants, coagulant aids (e.g., polymers), and pH buffers. 
     Due to variable, and potentially high, surface water flows and complexities in coagulant dosing, techniques are needed to use chemicals in the most cost-effective manner. In conventional chemical treatment facilities, such as those used for drinking water treatment, the principal focus is to achieve rapid floc settling times, so as to minimize the size and capital cost of the settling vessels, which usually consist of concrete and steel clarifiers. By contrast, in situations such as agricultural settings, more area can be devoted to settling vessels (typically ponds). 
     For the above-mentioned “concrete and steel” systems, the goal is to achieve a certain pollutant outflow concentration. For the chemical treatment of stormwater feeding into a lake or wetland, the goal is typically focused on mass (or percentage) removal of contaminants. In all cases, however, there is a clear incentive to minimize chemical dose to minimize cost. In methods in which the floc is captured, another goal is to maximize the settling rate of the floc, which minimizes the “carry-over” of floc from the clarifier, since effective floc settling reduces the required size of the clarifier. 
     Floc that is pumped from a settling vessel typically is conveyed to a de-watering facility, consisting of a centrifuge, belt filter press, and/or drying bed. As a result of drying, the floc loses about 90-95% of its volume and increases markedly in bulk density. In conventional drinking water treatment facilities, the residual dried floc often is stored on-site, and ultimately is hauled away to a landfill for disposal. In the past two decades, however, it has been recognized that the residual dried floc resulting from the addition of metal salts has the additional ability to retain pollutants, such as P. Consequently, drinking water floc residuals have been transported from the treatment facility and spread on canal banks and re-flooded agricultural lands (being restored to wetlands) for the purpose of minimizing soil P export. Regardless of these beneficial uses, most floc residuals are still considered a solid waste that incur relatively expensive transport and disposal costs. 
     SUMMARY 
     A method and system are provided for removing pollutants, such as heavy metals, phosphorus, and pathogenic organisms, from water. This method and system capitalize on the fact that coagulation and floc formation are dependent on the chemical characteristics of water (e.g., alkalinity, pH, dissolved organic matter) that are not necessarily related to the concentration of contaminants (e.g., phosphorus, heavy metals) desired to be removed from the water. A plurality of approaches can be used for exploiting this phenomenon. In a first system, a chemical coagulant is added to water containing a pollutant, the water being within an enclosure. The water and the coagulant are mixed, and coagulation and flocculation are permitted to occur. The mixing is stopped, and a floc formed by the coagulation and flocculation is permitted to settle to a bottom of the enclosure. The floc contains the pollutant, so that treated water remaining above the floc is thereby free from at least some of the pollutant. 
     At least some of the treated water is removed from the enclosure, and new water containing a pollutant is added to the enclosure. The new water and the floc are then mixed to resuspend components of the floc. 
     In a second system, a chemical coagulant is added to water containing a pollutant, the water being within an enclosure equipped with a matrix element. The water and the coagulant are mixed, and coagulation and flocculation are permitted to occur. The mixing is stopped, and a floc formed by the coagulation and flocculation is permitted to settle to a bottom of the enclosure, as well as onto the surfaces provided by the matrix element. The floc contains the pollutant, so that treated water remaining above the floc is thereby free from at least some of the pollutant. 
     At least some of the treated water is removed from the enclosure, and new water containing a pollutant is added to the enclosure. Pollutants in the newly added water encounter floc particles associated with the matrix element, which is deployed throughout the water column. Because these flocs still are somewhat “active” in terms of pollutant-removing capability, much of the pollutant mass subsequently is removed from the water. After several water exchanges, during which time the pollutant removal capability of these floc particles becomes depleted, another dose of coagulant is added to the enclosure and then mixed to form new flocs. 
     In a third system, a method is provided that minimizes chemical dosages by the reuse of dried floc material. This water-treatment method comprises adding a chemical coagulant to water containing a pollutant, wherein the water is within an enclosure. Coagulation and flocculation are permitted to occur, and a floc formed by the coagulation and flocculation is permitted to settle toward a bottom of the enclosure. The floc contains the pollutant, and thus treated water remaining above the floc is free from at least some of the pollutant. 
     At least some of the treated water is removed from the enclosure, and then at least some of the floc is removed from the enclosure. The removed floc is at least partially dewatered. 
     The dewatered floc is added to water in the enclosure in the form of, for example, particles. These particles remove phosphorus and settle, but do not create a new floc. 
     Another method for treating water comprises mixing a chemical coagulant with water containing a pollutant, permitting coagulation and flocculation to occur in an enclosure containing the water and coagulant, and permitting a floc formed by the coagulation and flocculation to settle toward a bottom of the enclosure. At least a portion of the floc can be resuspended using a mechanical mixing means positioned within the enclosure. Additional water containing a pollutant is added to the enclosure. A second floc is then permitted to form with the pollutant and the resuspended floc portion. 
     A further method for treating water comprises mixing a chemical coagulant with water containing a pollutant, permitting coagulation and flocculation to occur in an enclosure containing the water and coagulant, and permitting a floc formed by the coagulation and flocculation to settle toward a bottom of the enclosure. At least some of the treated water is exposed to a porous matrix, carbonate-based pH buffering agent, downstream from a site of floc settling, for reducing acidity of the treated water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIGS. 1A-1D  is a side cross-sectional view of a vessel system, with the method steps illustrated as ( FIG. 1A ) pumping water into the vessel; adding a coagulant, mixing, and permitting a floc to form ( FIG. 1B ); halting the mixing and permitting the floc to settle ( FIG. 1C ); and pumping the treated water out of the vessel ( FIG. 1D ). 
         FIG. 2  is a side cross-sectional view of an enclosure system contained within a body of water. 
         FIG. 3  is a side cross-sectional view of a second embodiment. 
         FIGS. 4A-4D  are schematic views of a third embodiment, with  FIGS. 4A and 4B  illustrating the coagulant addition into inflow piping ( FIG. 4A ) or into a mixing chamber ( FIG. 4B ) and resultant floc formation;  FIG. 4C , floc dewatering; and  FIG. 4D , dewatered floc addition to the enclosure. 
       FIGS.  5 A, 5 B are side cross-sectional views of a fourth embodiment, with coagulant introduced either into the inflow ( FIG. 5A ) or into a mixing chamber ( FIG. 5B ), and the addition of a floc-recycling element within the enclosure. 
         FIG. 6  is a side cross-sectional view of a fifth embodiment, with the addition of a pH-buffering element within the enclosure. 
     
    
    
     DETAILED DESCRIPTION  
     A description of preferred embodiments will now be presented with reference to  FIGS. 1A-6 . 
     A first embodiment of the method of the present invention, using the systems  10 , 20  of  FIGS. 1A-2 , respectively, comprises the steps, illustrated in  FIGS. 1A-1D , of feeding water to be treated, which may contain such contaminants as suspended solids, phosphorus, heavy metals, and pathogenic organisms, into an enclosure, which can be a free-standing tank  11  that holds the water to be treated  12  or an enclosure  21  that compartmentalizes a discrete water column  22  within a body of water  23  such as a lake. The feeding step ( FIGS. 1A and 2 ) is typically performed by a pump  13 , 24 , although this is not intended as a limitation, as a gravity-fed system may also be envisioned by one of skill in the art. The pump may be positioned within the enclosure  21  (pump  24  in  FIG. 2 ) or outside the enclosure  11  (pump  13  in  FIGS. 1A-1D ). 
     Next a chemical coagulant, such as an aluminum, calcium, or iron compound (with chemical pH buffers or coagulant aids, as needed, which can be added prior to, contemporaneously, or after the coagulant), is added to the enclosure  11 , 21 . The fluid in the enclosure is mixed using a mixing means  14 , 25 , allowing coagulation and flocculation to occur ( FIG. 1B ). The mixing is stopped, and the floc  15  is allowed to settle to the bottom  16  of the enclosure  11 , resulting in the removal of various pollutants from the water, which now reside in the floc  15  in the bottom of the enclosure  11 . 
     Once the floc  15  is settled ( FIG. 1C ), the treated water column  12 ′ above the floc layer  15  is removed ( FIG. 1D ) and replaced with a fresh aliquot  12 ″ of contaminated water. This exchange may occur either quickly or slowly, and in a batch or continuous-flow basis. The floc  15  is left in place on the bottom  16  of the enclosure  11  during the exchange. Once the water exchange has been completed, the floc  15  is resuspended throughout the “fresh” water column  12 ″ by mixing the water in the enclosure  11 . 
     Depending on the original concentration of the coagulant added, as well as the concentration of contaminants of the water, it is now likely that the resuspended floc  15  has additional capability to remove contaminants. The mixing is then stopped, and the new floc is allowed to settle to the bottom  16  of the enclosure  11 . This process, including water exchange, resuspension of floc, and settling of floc, is repeated for several iterations, for as long as the floc continues to exhibit contaminant removal capability. 
     The floc ultimately is removed from the enclosure when its contaminant-removal capacity is exhausted, such as by pumping. In the case of enclosure  11 , the vessel contains a sump  17  positioned adjacent the vessel&#39;s bottom  16  from which settled floc may be pumped at predetermined intervals. 
     Enclosure  31  in another embodiment of the system  30  ( FIG. 3 ) may comprise, for example, a flexible barrier having sides but no bottom. The bottom  32  here is thus the bottom of the body of water  33 . The barrier  31  may be movable, in which case the process is carried out for a predetermined time with the barrier  31  at a first position  34 . Following the predetermined time, the barrier  31  is moved to a second position  35  within the body of water  33  spaced apart from the first position  34 , leaving the settled floc  38  at the first position  34  on the bottom  32 . 
     In order to provide additional surface area, a matrix element may be added to the enclosure. The matrix element serves to provide a surface onto which floc can settle, this settled floc then providing additional floc-containing surface area in position to contact water to be treated. 
     In the embodiment  20  of  FIG. 2 , the matrix element  26  in one embodiment can comprise a plastic “trickling filter media” or baffle such as are known in the art. In another embodiment, the matrix element  26  can comprise a bristle media filter, such as typically used for air filtration. In the embodiment  30  of  FIG. 3 , the matrix element comprises a root mat  36  of floating vegetation  37 , which can, for example, be pre-inoculated with floc. 
     In the embodiment  30  of  FIG. 3 , if the body of water  33  has a natural (e.g., soil, sand) bottom  32 , the body of water  33  may be periodically drained, and the vegetation  37 , floc associated with the root mat  36 , and settled floc  38  on the bottom  32  tilled into the natural bottom  32 . 
     In all cases, the overall process is re-started by adding coagulant dose (similar to the original dose) to a fresh parcel of water, thereby forming a “new” aliquot of floc. Under certain circumstances, the contaminant removal performance of the resuspended floc can be enhanced by adding a small dose of pH buffer, coagulant aid (e.g., a polymer), and/or coagulant (typically at a much lower concentration than the original dose), upon resuspension of the floc in the enclosure. 
     In a third embodiment ( FIGS. 4A-4D ), a system  40 , 40 ′, 40 ″ and method are provided for treating water  41  that comprises adding a chemical coagulant  42  to water  41  containing a pollutant, wherein the water  41  is directed via, for example, piping  43  to an enclosure  44  ( FIG. 4A ). The coagulant can comprise, for example, at least one of an aluminum, a calcium, and an iron compound, and the pollutant can comprise at least one of a suspended solid, phosphorus, a heavy metal, a nitrogen compound, and a pathogenic organism. The enclosure  44  can comprise a discrete column of water within a body of water, as discussed above, or comprise an enclosure  44  used for the purpose of the present method  40 . The coagulant stream  42  can mix within the inflow piping  43  ( FIG. 4A ). In an alternative system  40 ′ ( FIG. 4B ), the coagulant stream  42  can be mixed with incoming water in a mixing chamber  48  upstream of the enclosure  44  to facilitate coagulation. 
     Coagulation and flocculation are permitted to occur in substantially quiescent conditions, and a floc  45  formed by the coagulation and flocculation is permitted to settle toward a bottom  46  of the enclosure  44 . The floc  45  contains the pollutant, and thus treated water  41 ′ remaining above the floc  45  is free from at least some of the pollutant. 
     The floc  45  has been found to have additional ability to remove pollutants, owing to the presence of unoccupied binding sites that can adsorb contaminants such as P. In the past, this floc  45  has been considered problematic, since it has a high water content, but low bulk density, and quickly accrues in and fills up the enclosure  44 , which reduces effective storage room in the enclosure  44  and hydraulic retention time. As in an embodiment discussed above, the floc  45  can be resuspended and employed to yield additional flocculation to occur. Also as above, the water addition can be performed substantially continuously or in batch mode. Also one or both of a pH buffer and a coagulant aid can be added following the addition of the coagulant. 
     At least some of the treated water  41 ′ can be removed from the enclosure  44 , and at least some of the floc  45  can be removed from the enclosure  44 . The removed floc  45  is at least partially dewatered ( FIG. 4C ) at a floc drying device, for example, a drying bed  47 , or possibly a mechanical device such as a belt filter press or centrifuge. Such a drying facility can be established adjacent the enclosure  44 . The wet floc  45  can be conveyed to the drying facility on either a continuous or intermittent basis. 
     In a system  40 ″ and method, the dried floc  45 ′ can be re-introduced into the treatment system  44  ( FIG. 4D ), with its residual pollutant- (e.g., P) removing ability further harnessed for treatment of the water  41 . Approaches for dried floc  45 ′ re-introduction/reuse include: addition to the inflow water stream  43 , either alone or in conjunction with a coagulant, coagulant aid, or a buffer; and addition to the inflow and/or other region(s) of the vessel  44 , so that it is distributed throughout the water column and settles adjacent the vessel bottom  46 . The dried floc  45 ′ can be added, for example, in particulate form, although this is not intended as a limitation. The location in the vessel  44  of dried floc introduction may either be shallow or deep, and vegetated or non-vegetated. The dried floc  45 ′ can serve to immobilize P in the water column itself, and/or to immobilize sediment P, to prevent it from re-entering the water column, by forming settled material  45 ″ adjacent the enclosure bottom  46 . 
     It should be noted that, due to the marked volume reduction achieved by the floc upon drying, the volume occupied by the re-introduced dry residual floc  45 ′ is minimal, so that it occupies little of the water column. In a vessel of relatively large area and volume, such as a pond, the system  40 , 40 ′, 40 ″ can operate for years (perhaps decades) without requiring any off-site transport/disposal of residual floc. In this respect, it is a sustainable system, maximizing use of the chemical coagulant to the greatest extent possible, and eliminating the need for off-site disposal of residual solid wastes. 
     Another embodiment  60 , 60 ′, 60 ″ ( FIGS. 5A-5C ) can comprise a sequential system having a plurality of elements. In systems  60 , 60 ′ (FIGS.  5 A, 5 B), a coagulant  61  is added to the inflowing water stream  62  directed toward a vessel or pond  63 . After appropriate mixing within the inflow piping  62  ( FIG. 5A ) or in a specialized mixing chamber  66  ( FIG. 5B ) to facilitate coagulation, the water and coagulant mixture enters the vessel or pond  63 . In the quiescent conditions of the settling basin  63 , the resulting floc material  64  settles to the bottom  65  of the vessel  63 . 
     As noted previously, following settling, the floc material  64  typically has additional ability to remove pollutants, due to the presence of unoccupied binding sites that can adsorb constituents such as P. This settled floc material  64  can essentially be “re-used,” that is, exposed to fresh, unamended aliquots of water, to achieve additional pollutant removal. 
     One technique for accomplishing floc re-use is to mechanically disturb the floc  64  settled in the vessel  63  by aeration or jets of water  67 . During, or just prior to, this resuspension of floc  64 , an aliquot of untreated water is introduced into the vessel  63 , so that the unamended water can come into contact with the floc particles. In system  60 ″ ( FIG. 5C ) at least some of the floc  64  can be removed from the vessel  63  (via suction removal and transport by a pump  68 ) and conveyed  69  directly into the inflow piping  62 , where it encounters untreated water. 
     Additionally, floc re-use can be accomplished passively by allowing it to settle onto the vessel bottom  65  or onto a matrix element in the water column, and leaving it in place so that it can contact fresh, unamended aliquots of water that are introduced into the vessel. 
     The active re-use of previously settled flocs can be performed at time intervals ranging from hours to months, since investigations have demonstrated that this floc  64  retains its P sorption ability for at least six months. It should be noted that the active re-use of floc (e.g., resuspension) may increase the amount of “clarifier” area and volume required for settling, and this can be provided as one large vessel or pond, a vessel compartmentalized by floating booms and flexible barriers, or as sequential vessels connected by a culvert or open ditch. Moreover, as a result of “active” re-use, additional doses of a coagulant or a coagulant aid, such as a polymer, may be needed to be added to facilitate settling of the resuspended or re-used floc. Despite the potential need for coagulant aid additions, the re-use of previously settled floc in either an active or passive manner effectively maximizes the amount of pollutant that can be removed per unit mass of active chemical added. 
     One challenge for chemical-based surface water treatment systems is that they typically are not allowed to discharge acidic waters. As noted above, owing to the likely fluctuations of inflow color and alkalinity in surface water runoff streams, the buffer requirements associated with acidic metal coagulants can be high (and expensive) and dosing approaches complex. 
     In yet a further embodiment  70  ( FIG. 6 ), outflow acidity can be addressed in a cost-effective manner by positioning a porous limerock (LR) or shellrock outcropping, berm  71 , or levee, situated within a vessel  72  or pond downstream of a settling zone  73  for floc  85  having a first depth  74 . The porous limerock berm  71  can be situated, for example, in a pH-treatment zone  75  downstream of the floc settling zone  73 . In an exemplary embodiment, the limerock berm  71  can be positioned adjacent a bottom  76  of the pH-treatment zone  75  having a second depth  77  less than the first depth  74 . Thus, clarified water can move from the floc settling zone  73  across the limerock  71  and experience pH buffering. 
     When the acidic supernatant resulting from upstream coagulant addition  79  and floc recycling is passed through the limerock  71 , alkalinity (primarily carbonate ions) and associated cations such as calcium and magnesium dissolve in the acidic stream, resulting in an increase in pH. The degree of acidity of the inflow stream  78  dictates the exposure requirement to limerock, which can be controlled by the size of limerock berm  71 , as well as the flow rate and hydraulic retention time within the limerock bed. It should be noted that the limerock material that comprises the bed will dissolve over time, which will require the periodic replacement of a portion or all of the rock. 
     The use of vegetation in an optional element in this system  70 . In an exemplary embodiment, floating plants  80  can be positioned in a downstream portion of the floc-settling zone  73 , and/or submerged plants  81  can be positioned in or downstream of the pH-treatment zone  75 , although these are not intended as limitations. 
     Another optional feature of this system  70  comprises a third region  82  having a third depth  83  greater than the second depth  77 . This third region  82  can serve as a settling zone to further clarify the treated water  84 . 
     The methods disclosed herein may be performed “manually” or under electronic control, wherein the pumping and mixing elements are under timer control and are coordinated to perform the method steps automatically. 
     One of the benefits of the present systems and methods is that, by harnessing the “additional” contaminant removal capability of a previously formed and settled floc through its subsequent resuspension and/or drying and re-introduction, the mass of pollutant removed per unit mass of coagulant added can be maximized. This represents a cost savings (reduction in operating costs for coagulant purchase), and in many circumstances, an environmental benefit (reduction of coagulant/floc that ultimately is discharged to the environment). 
     Another benefit of the current systems and methods is that only one enclosure is required, since it is not critical to achieve a predetermined target outflow concentration. 
     One of skill in the art will recognize that each body of water and its components will have its own characteristics. Therefore, each site will be evaluated to determine individual design and operational variables, including, but not intended to be limited to, type and dose of coagulant, buffers and coagulant aids; frequency of water exchange; frequency of floc resuspension; dose of additional coagulant, buffer and coagulant aids, at the time of floc resuspension; and method of removing floc.