Abstract:
A distillation system for distilling influent liquid includes a counterflow heat exchanger for receiving and heating the influent liquid. A heater is coupled to the counterflow heat exchanger for receiving the influent liquid from the counterflow heat exchanger and heating the influent liquid. An evaporation unit is coupled to the heater and to a sump for receiving the influent liquid from the heater and for receiving liquid from the sump and forming a vapor from at least a portion of the influent liquid and the liquid received from the sump. The evaporation unit returns unevaporated liquid to the sump. A condensation unit is coupled to the evaporation unit for forming a condensate from vapor received from the evaporation unit. The condensation unit is coupled to the counterflow heat exchanger for transferring the condensate to the counterflow heat exchanger. The heater simultaneously heats the liquid in the sump and the influent liquid received from the counterflow heat exchanger.

Description:
FIELD OF THE INVENTION 
       [0001]    The present application generally relates to distillers and, more particularly, to a heater for providing supplemental heating in a distiller. 
       BACKGROUND OF THE INVENTION 
       [0002]    Distillation is the process of purifying a liquid (such as water) or, conversely, producing a concentrate (such as concentrated orange juice). In general, distillation involves heating liquid to be distilled to the point of evaporation, and collecting and condensing the resulting vapor. 
         [0003]    U.S. Patent Application Publication No. 2008/0237025 discloses an example of a compact distiller. In such a distiller, the liquid to be distilled is heated to near its boiling temperature and then sprayed onto the heat-exchange surfaces of a rotary heat exchanger forming an evaporation chamber. A compressor draws the resultant vapor from the evaporation chamber, leaving contaminants behind. The compressor raises the vapor&#39;s pressure and delivers the higher-pressure (and thus higher-saturation-temperature) vapor to the rotary heat exchanger&#39;s condensation chamber. In that chamber, thermal communication with the evaporation chamber results in the vapor condensing into a largely contaminant-free condensate, surrendering its heat of vaporization in the process to the liquid in the evaporation chamber. 
         [0004]    Rotary heat exchangers of that type and others are ordinarily operated such that the rate at which the liquid evaporates in the evaporation chamber is only a small fraction of the rate at which it is sprayed onto the heat-exchange surfaces. In many cases, eighty to ninety percent of the sprayer flow remains liquid. The rapidly spinning heat exchange surfaces of the rotary heat exchanger fling the unevaporated liquid by centrifugal force into an annular feed-water sump, which is a small reservoir near the bottom of the distiller. Scoop tubes skim liquid from the sump and route it back to the sprayers, which continue to spray the liquid on the heat exchange surfaces. The distiller therefore needs only to be supplied a small percent, e.g., ten to twenty percent, as much influent liquid at its inlet as is sprayed on its heat-exchange surfaces to make up for evaporation. Drawing in more or less influent liquid than that would ultimately flood or deplete the sump. Accordingly, the influent liquid flow rate into the distiller is regulated to match the evaporation rate and in order to maintain a generally constant volume in the sump. 
         [0005]    The influent liquid added to the sump is at a cooler temperature than the liquid in the sump. Liquid from the sump that is sprayed onto the heat exchange surfaces in the evaporation chamber is in a subcooled state. Steam enters the rotary heat exchanger&#39;s condensation chamber in a superheated state. The heating of the subcooled liquid in the evaporation chamber should balance the superheated cooling in the condensation chamber to sustain evaporation and condensation levels. The sensible heat of the exit flow from the distiller can be largely recovered through the use of a counterflow heat exchanger to heat the influent liquid. The heat that is not recovered is supplied by supplemental heating. In the steady state (i.e., normal operation) mode, supplemental heat is added to the liquid before it is sprayed on the heat exchange surfaces of the evaporation chamber in order to sustain evaporation. 
         [0006]    When the distiller is turned on, liquid in the sump is ordinarily at ambient temperature, and the evaporation rate is accordingly zero. Since there is no evaporation, the influent flowrate is also zero. Heat is therefore added until the liquid in the sump reaches a temperature high enough for distillation. This is referred to as the startup mode of heat addition. 
         [0007]    In the standby mode of heat addition, the liquid in the sump is maintained at a somewhat elevated temperature relative to ambient, but still subcooled to the point of no evaporation when the system is turned off. The purpose of the standby mode is to reduce startup time when the distiller is turned on. A heater used in the startup or standby modes operates independently of influent flowrate. 
         [0008]    Two separate heaters, an inline heater and a sump heater, have been used in distillation systems to provide heating for the startup, standby, and steady state heating modes. In the steady state mode of operation, supplemental heat is added with the inline heater to heat influent liquid flowing into the distiller. In the startup and standby modes, supplemental heat is added with a separate sump heater for heating liquid in the sump. 
       BRIEF SUMMARY OF EMBODIMENTS OF THE INVENTION 
       [0009]    A distillation system in accordance with one or more embodiments of the invention includes a heater for heating influent liquid received from an inlet. A sump receives the influent liquid from the heater. An evaporation unit receives liquid from the sump and forms a vapor from at least a portion of the liquid received from the sump. The evaporation unit returns unevaporated liquid to the sump. A condensation unit forms a condensate from vapor received from the evaporation unit. The heater simultaneously heats the liquid in the sump and the influent liquid received from the inlet. 
         [0010]    A method of distilling an influent liquid in accordance with one or more embodiments of the invention includes the steps of: transferring influent liquid received at an inlet to a sump; forming a vapor from at least a portion of the liquid received from the sump, and returning unevaporated liquid to the sump; forming a condensate from the vapor; and simultaneously heating the influent liquid received from the inlet prior to the influent liquid being transferred to the sump and the liquid in the sump using a single heater. 
         [0011]    A heater in accordance with one or more embodiments of the invention provides supplemental heating in a distillation system. The distillation system includes a sump and an evaporation unit for receiving liquid from the sump and forming a vapor from at least a portion of the liquid received from the sump. The evaporation unit returns unevaporated liquid to the sump. The heater includes a heating element proximate the sump for heating the liquid in the sump, and a structure defining a fluid passage in the proximity of the heating element for flow therethrough of an influent liquid to be distilled. The structure includes a heater inlet for receiving the influent liquid and a heater outlet for transferring the influent liquid from the fluid passage to the sump. The heating element simultaneously heats the liquid in the sump and the influent liquid flowing through the fluid passage. 
         [0012]    A compact distillation system is provided in accordance with one or more embodiments of the invention. The distillation system includes an inlet for receiving influent liquid to be distilled. A counterflow heat exchanger is coupled to the inlet for receiving and heating the influent liquid. A heater is coupled to the counterflow heat exchanger for receiving the influent liquid from the counterflow heat exchanger and heating the influent liquid. An evaporation unit is coupled to the heater and a sump for receiving influent liquid from the heater and liquid from the sump and forming a vapor from at least a portion of the influent liquid and the liquid received from the sump. The evaporation unit returns unevaporated liquid to the sump. A condensation unit is coupled to the evaporation unit for forming a condensate from vapor received from the evaporation unit. The condensation unit is coupled to the counterflow heat exchanger for transferring the condensate to the counterflow heat exchanger. The heater simultaneously heats the liquid in the sump and the influent liquid received from the counterflow heat exchanger. 
         [0013]    Various embodiments of the invention are provided in the following detailed description. As will be realized, the invention is capable of other and different embodiments, and its several details may be capable of modifications in various respects, all without departing from the invention. Accordingly, the drawings and description are to be regarded as illustrative in nature and not in a restrictive or limiting sense, with the scope of the application being indicated in the claims. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]      FIGS. 1A and 1B  are front and rear views, respectively, of the exterior of a distillation unit in accordance with one or more embodiments of the invention. 
           [0015]      FIG. 2  is a simplified cross-sectional view of the distillation unit of  FIGS. 1A and 1B . 
           [0016]      FIG. 3  is a simplified process flow diagram of the distillation unit of  FIGS. 1A and 1B . 
           [0017]      FIG. 4  is a cross-sectional view of combined sump and inline heater in accordance with one or more embodiments of the invention. 
           [0018]      FIG. 5  is an exploded view of the combined sump and inline heater of  FIG. 4 . 
           [0019]      FIG. 6  is an isometric view of the bottom of the combined sump and inline heater of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION 
       [0020]    The present application is directed to a combined sump and inline heater for providing supplemental heat in a distiller. The heater simultaneously heats influent liquid flowing into the distiller and liquid in the distiller sump. The heater can be used to provide heat in the startup, standby, and steady state heating modes. 
         [0021]      FIGS. 1A and 1B  are exterior views of a distillation unit or system  10  having a combined sump and inline heater in accordance with various embodiments of the invention. The distillation unit  10  includes a feed inlet  12  through which the unit  10  draws an influent liquid to be distilled. The distillation unit  10  can be used for various distillation purposes, such as purifying water or condensing liquids like orange juice. For the sake of simplicity, in the exemplary embodiments described herein, the purpose is assumed to be water purification, and the influent liquid is accordingly water that contains contaminants to be removed. 
         [0022]    The unit  10  purifies the influent water, producing a generally pure condensate at a condensate outlet  14 . The volume rate at which condensate is produced at the outlet  14  will, in most cases, be only slightly less than the rate at which influent water enters inlet  12 , with nearly all the remainder being a small stream of concentrated impurities discharged through a concentrate outlet  16 . 
         [0023]    The distillation unit  10  includes a control unit  18  including a programmable logic controller for controlling operation of the unit  10 . A control panel with a keypad and display can be used by an operator to monitor and control operation of the unit  10 . 
         [0024]      FIG. 2  is a simplified cross-sectional view of the distillation unit  10 . The distillation unit  10  includes a housing  20  having an insulated wall preferably made of low-thermal-conductivity material such as polyurethane. The distillation unit  10  includes a distiller  22  and a counterflow heat exchanger  24  located within the housing  20 . The counterflow heat exchanger  24  allows heat from fluids exiting the distiller  22  to be largely recovered and transferred to the influent water entering the unit  10 . 
         [0025]    A feed-water pump, which is not shown and can be outside the housing  20 , drives influent water from the feed inlet  12  through the counterflow heat exchanger  24 . After being heated by the counterflow heat exchanger  24 , the influent water flows through a combined sump and inline heater  28 , which is described in further detail below. After flowing through the heater  28 , the influent water flows into an annular feed-water sump  30  through set of sprayers  34  as discussed below. As used herein, the term influent water or liquid refers to feed-water or liquid flowing into the combined sump and inline heater  28 . The term sump water or liquid refers to water or liquid in the sump  30 . Sump water is a mixture of influent water entering the sump  30  through the heater  28  and unevaporated water returned by the evaporation chamber of the distiller  22 . 
         [0026]    Scoop tubes  32  skim sump water from the sump  30  and direct it to a set of stationary sprayers  34 . The sprayers  34  spray the sump water along with influent water from the heater  28  onto the exterior surfaces of the radially extending heat-transfer blades  36  of a rotary heat exchanger  38  forming an evaporation chamber, in which the sprayed water absorbs heat and partially evaporates. 
         [0027]    Leaving unevaporated impurities behind, a compressor  40  draws in the resulting vapor and feeds it pressurized into an interior condensation chamber defined by the interior surfaces of the hollow heat transfer blades  36 . There, the pressurized water vapor condenses, surrendering its heat of vaporization through the blade walls to the water sprayed on the blades&#39; exterior surfaces. 
         [0028]    The condensed water is the purified output of the distiller  22 . The counterflow heat exchanger  24  receives that output, cools it by thermal communication with the incoming influent water, and delivers it to the condensate outlet  14  shown in  FIG. 1B . 
         [0029]    As previously discussed, only some of the sump water and influent water that is sprayed onto the rotary heat exchanger  38  blade exterior surfaces evaporates. In the illustrated embodiment, eighty to ninety percent of the sprayer flow remains liquid. The spinning blades  36  fling this remaining liquid back to the sump  30 . The scoops at the sump  30  continue to transfer the sump water back to the sprayers  34 . 
         [0030]    The flow through the sprayers  34  should be greater than the influent flow entering the sump  30 . The influent flow should be only great enough to replenish the evaporated liquid. However, the evaporation rate can vary, and even a slight mismatch between the rates of influent flow and evaporation could eventually either deplete the sump  30  or make its depth so great as to compromise the effectiveness of the rotary heat exchanger  38 . A regulator is accordingly provided to control the rate of influent flow such that it matches the evaporation rate. 
         [0031]    The functions of the combined sump and inline heater  28  are related to the energy recovery of the distillation unit  10  as a whole.  FIG. 3  is a simplified process flow diagram of the distillation unit  10 , which includes the counterflow heat exchanger  24 , heating sources, and the distiller  22  surrounded by the insulated housing  20 . Influent water enters the insulated housing  20  at the feed inlet  12  with a mass flowrate {dot over (m)} inf  and a temperature T inf 1  (about 70° F.). Distillate water exits the insulated housing  20  at the condensate outlet  14  with a mass flowrate {dot over (m)} dist  and a temperature T dist  (about 77° F.). Concentrate water exits the insulated housing  20  at concentrate outlet  16  with a mass flowrate {dot over (m)} conc  and a temperature T conc  (about 77° F.). Water exiting the distiller  22  is considered to be at system temperature T sys  (about 212° F.). Influent water recovers a percentage of the heat from the exiting distillate and concentrate streams and exits the counterflow heat exchanger  24  at a temperature T inf 2  (about 200-205° F.). Since the counterflow heat exchanger  24  effectiveness is less than unity, T inf 2 &lt;T sys , supplemental heat {dot over (Q)} inline  is added to the influent before entering the sump  30  of the distiller, raising the influent temperature to T inf 3  (about 206-209° F.). The distiller  22  receives supplemental heat {dot over (Q)} sump  for directly heating the sump  30  and electrical work {dot over (W)} motor  for vapor compression and internal pumping. The supplemental heat {dot over (Q)} inline  and {dot over (Q)} sump  is provided by the combined sump and inline heater  28  in accordance with various embodiments of the present invention. Heat is lost from the insulation package to the room at a rate {dot over (Q)} room . 
         [0032]    In steady state operation, the supplemental heat provided in the distillation unit  10  is given by an energy balance over the insulation package. 
         [0000]    
       
      
       {dot over (m)} 
       inf 
       h 
       inf 
       +{dot over (Q)} 
       inline 
       +{dot over (Q)} 
       sump 
       +{dot over (W)} 
       motor 
       ={dot over (m)} 
       dist 
       h 
       dist 
       +{dot over (m)} 
       conc 
       h 
       conc 
       +{dot over (Q)} 
       room  
      
     
         [0000]    where h is enthalpy. Using continuity and the enthalpy change of an incompressible fluid, the supplemental heat provided is 
         [0000]      ( {dot over (Q)}   inline   +{dot over (Q)}   sump )= {dot over (m)}   dist   c   p ( T   dist   −T   inf )+ {dot over (m)}   conc   c   p ( T   conc   −T   inf )+ {dot over (Q)}   ins   −{dot over (W)}   motor    
         [0033]    The flow energy loss terms are related to counterflow heat exchanger effectiveness, and the insulation energy loss is related to the insulation thermal resistance R value. The overall energy balance does not distinguish between the sump and inline heater functionalities. As previously discussed, a significant function of sump heating is to supply heat during standby and startup modes, and a significant function of the inline heating is to supply heat during sustained steady state distillation. 
         [0034]    A combined sump and inline heater  28  in accordance with various embodiments provides the advantages of using both sump and inline heating. One advantage during steady state operation of using both an inline heating and sump heating is that additional venting can be provided after the inline heating. Although not shown in  FIG. 3 , the influent water passes a number of venting locations along the counterflow heat exchanger  24 . The solubility of non-condensable gases such as air in liquid water decreases with increasing temperature. The presence of air in influent water entering the distiller can adversely affect distiller performance. Since the inline heating is provided outside the sump  30  and T inf 3 &gt;T inf 2 , an additional venting location can be provided after the inline heating. Inline heating also helps avoid thermal fluctuations. As influent water reaches the distiller, if the temperature is significantly less than the system temperature, then in some distiller designs, significant sump mixing may be needed to avoid uneven sump water temperature distribution and system instabilities. Inline heating reduces temperature differences between the influent water and the sump water. In addition, inline heating improves thermal management of hardware. In the distiller  22 , the influent is added to the sump by being injected through the nozzles of sprayers  34  and applied directly to the rotary heat exchanger evaporator surfaces where some of it is evaporated and the rest directed to the sump. If all required supplemental heat were to be provided by the sump heater, the influent being applied to the evaporator surfaces would be too cold and heat would be taken from the condensing steam instead of only from the super heat and the effectiveness of the rotary heat exchanger surfaces would be reduced. 
         [0035]      FIGS. 4-6  illustrate an exemplary combined sump and inline heater  28  in accordance with various embodiments of the invention. As shown in the cross sectional view of  FIG. 4 , the heater  28  includes a single heating element  42  that can simultaneously transfer heat to the influent water flowing through a fluid passage  44  below the heating element  42  as well as to water in the sump  30  above the heating element  42 . 
         [0036]      FIG. 5  is an exploded view of the heater  28 , and  FIG. 6  is isometric view of the bottom of the heater  28 . 
         [0037]    Influent water enters the heater  28  through an inlet port  50  at the bottom of the heater  28  (shown in  FIG. 6 ) and passes through the fluid passage  44  (shown in  FIG. 4 ) where it is heated by the heating element  42 . The influent water exits the fluid passage  44  through an exit port  46  at the bottom of the heater  28  (shown in  FIG. 6 ). The fluid passage  44  includes a dividing wall  48  (shown in  FIG. 5 ) between the inlet port  50  and the exit port  46  such that the influent water is forced to travel generally around the full circumference of the passage  44  to increase exposure to heat from the heating element  42 . In addition, a baffle  49  (shown in  FIG. 5 ) is provided in the fluid passage  44  on a side of the exit port  46  opposite the dividing wall  48 . The baffle  49 , which has a height that is less than the height of the fluid passage  44 , forces water flowing through the fluid passage to clear the height of the baffle  49  before exiting through the exit port  46 . The presence of the baffle  49  helps clear the fluid passage  44  of pre-existing air in the passage during startup. 
         [0038]    After being heated in the fluid passage  44 , the influent water is optionally transferred to a vent (not shown), where non-condensable gases such as air can be released. After being degassed, the influent water flows to the sump  30  through one of the tubes in the tube manifold  52 . The sump  30  is defined by a sump inner pan  54 , which is structurally supported by a sump outer pan  56 . A plate endcap  58  supports the heating element  42  as will be described in further detail below. 
         [0039]    A post element  60  and an influent pan  62  define the fluid passage  44  therebetween through which influent water flows. The post element  60  is mounted beneath the plate endcap  58 . 
         [0040]    The heater  28  also includes a bottom inner support ring  64  for supporting the tube manifold  52 . A bottom outer support ring  66  is provided for supporting the post element  60  and the influent pan  62 . 
         [0041]    The heating element  42  is preferably an electrical resistance heater element, which converts electricity into heat. The heating element  42  can comprise a variety of materials, including, e.g., stainless steel and Inconel™ alloys, depending on the desired operating temperature. In this exemplary embodiment, the heating element  42  has a tubular cross section with the diameter of ¼″ to ½″, with a power output ranging from 200 W to 500 W. Because the heating element  42  is not in contact with the influent liquid or the sump water, it is not subject to scale buildup or corrosion, and can be made of less expensive materials. 
         [0042]    Structural components of the heater  28  such as the sump outer pan  56 , the plate endcap  58 , the bottom inner support ring  64 , and the bottom outer support ring  66  preferably comprise a die cast metal such as aluminum. 
         [0043]    Parts that are in contact with water such as the sump inner pan  54 , the post element  60 , the influent pan  62 , and the tube manifold  52  preferably comprise a corrosion resistant material such as an injection molded plastic, e.g., a liquid crystal polymer (LCP), which protect the aluminum structural components from exposure to water to improve longevity. Thermally, plastic is a poor conductor and a reduced thickness is desired to reduce conduction temperature differentials. Thicknesses for the plastic parts of the heater  28  in this exemplary embodiment range from 0.040″ to 0.100″. 
         [0044]    The influent pan  62  is preferably easily removable so that it can be periodically cleaned of scale buildup, and replaced. 
         [0045]    The components of the heater  28  can be attached together using fasteners such as screws through the bottom inner  64  and outer  66  support rings, which mate with threads in the plate endcap  58 . The die cast metal endcap  58  structurally holds the fasteners under the load of influent water pressure. Thicknesses for the endcap  58  in the heater  28  in this exemplary embodiment can range from 0.060″ to 0.110″. 
         [0046]    As shown in  FIG. 6 , ports are provided at the bottom of the heater  28  including a heater cavity drain  68  for service, the inlet port  50  where influent water enters the fluid passage  44 , and the exit port  46  where influent water exits the fluid passage  44 . 
         [0047]    Heat from the heating element  42  is divided between heat provided to the influent water in the fluid passage  44  and heat provided to water in the sump. The proportion of heat transferred to the influent water and the sump water can be varied through changes in the heater design including, e.g., the manner in which the heating element  42  is supported. The heating element  42  is supported in the plate endcap  58  at discrete, space-apart support locations by conduction contacts  70  (shown in  FIG. 4 ) positioned on the post element  60 . In the exemplary embodiment, there are four conduction contacts  70  generally equally spaced around the circumference of the post element  60 . Heat is transferred from the heating element  42  by a combination of heat conduction through the conduction contacts  70 , by convection through the air surrounding the heating element  42  (a relatively weaker heat transfer mode), and by radiation. If the conduction contact area (i.e., the surface of the conduction element in contact with the heating element  42 ) is relatively large, then the heat transfer from the element can be mostly via conduction, and the influent water in the fluid passage  44  receives the most of the heat. If on the other hand, the conduction contact area is small, then the heat transfer from the heating element  42  can be mostly via radiation. This leads to a higher heating element surface temperature. In this case, the proportion of heat to the influent water is controlled by the radiation view factor to the endcap  58 . The surface temperatures of the heating element  42  and surrounding parts can be controlled by the radiation surface areas, view factors, and surface emissivities. 
         [0048]    The proportion of heat from the heating element  42  transmitted to the influent water and the sump water can also be controlled through the design of the fluid passage geometry, particularly the flow area of the fluid passage  44 . In the exemplary embodiment, the average spacing between the plastic walls defining the fluid passage  44  ranges from 0.2″ to 1.0″. The particular spacing affects the convection heat transfer to the water. At a given flowrate, the cross sectional area sets the velocity by continuity 
         [0000]    
       
         
           
             V 
             = 
             
               
                 
                   m 
                   . 
                 
                 inf 
               
               
                 ρ 
                  
                 
                     
                 
                  
                 A 
               
             
           
         
       
     
         [0000]    where ρ is the density of water. The flow regime is determined by the Reynolds number 
         [0000]    
       
         
           
             Re 
             = 
             
               
                 ρ 
                  
                 
                     
                 
                  
                 
                   VD 
                   h 
                 
               
               μ 
             
           
         
       
     
         [0000]    where μ is the viscosity of water and D h  is the hydraulic diameter (roughly twice the fluid passage gap height). The Nusselt number in general reads 
         [0000]    
       
         
           
             
               Nu 
               ( 
               
                 Re 
                 , 
                 Pr 
               
               ) 
             
             = 
             
               
                 
                   
                     hD 
                     h 
                   
                   k 
                 
                 ⇒ 
                 h 
               
               = 
               
                 
                   
                     Nu 
                     ( 
                     
                       Re 
                       , 
                       Pr 
                     
                     ) 
                   
                    
                   k 
                 
                 
                   D 
                   h 
                 
               
             
           
         
       
     
         [0000]    where h is the heat transfer coefficient, k is the thermal conductivity of water, and Pr is the Prandtl number of water. As hydraulic diameter decreases, the heat transfer coefficient increases. Convection heat transfer to the water (boiling considerations aside) is given by 
         [0000]        {dot over (Q)}   inf   =hA   conv ( T   plastic   −T   water ) 
         [0000]    where A conv  is the inner surface area of the fluid passage  44 . T water  in the above expression is an average temperature since the exiting water temperature will be higher the entering water temperature. To reduce the convection temperature difference, the convection area or the heat transfer coefficient is increased. The convection coefficient can be increased by decreasing the hydraulic diameter via the fluid passage gap spacing. 
         [0049]    Manufacturing tolerances in the endcap  58  and post element  60  may result in the presence of a space between the parts. The spacing, which can be about 0.002″, may behave as an insulating air gap. The elevated thermal resistance resulting from the air gap can lead to elevated endcap and post element  60  temperatures, and can adversely affect heater performance. The air gap can be substantially eliminated by the use of a thermally conductive filler such as a thermal grease or paste between the parts. 
         [0050]    The programmable logic controller of the control unit  18  can be used to control power supplied to the heating element  42  to control operation of the heater  28 . Heater operation can be controlled when the system is turned on, off, or placed in a standby mode. The programmable logic controller can also shut down the heater  28  for safety reasons if the heater element temperature or water temperature becomes too high. Additionally, the supplemental heat provided by the heater  28  can be adjusted if the temperature of the influent water entering the unit  10  increases or decreases during operation. Temperature sensing devices such as thermocouples can be used to monitor the temperature of the heating element  42 , influent water, and/or sump water. The programmable logic controller can control the heater  28  based on temperature readings from the thermocouples. 
         [0051]    It is to be understood that although the invention has been described above in terms of particular embodiments, the foregoing embodiments are provided as illustrative only, and do not limit or define the scope of the invention. Various other embodiments are also within the scope of the claims. For example, elements and components described herein may be further divided into additional components or joined together to form fewer components for performing the same functions.