Abstract:
Two waste products, nitrate-rich sewage wastewater and power plant CO 2  emissions, are combined and converted into a renewable, biomass energy source, which supplies the fuel to the power plant. The power plant, wastewater treatment facility, and biomass growth unit are preferably located on one site and arranged for convenient transfer of the CO 2  and wastewater to the biomass growth unit; harvesting, processing and return of biomass from the growth unit as fuel to the power plant; and discharge of the de-nitrated wastewater into the same body of water used as the heat sink by the power plant, e.g., a lake, river, or sound. The present invention thus provides an integrated approach to minimization of CO 2  emissions and nitrate discharge while achieving improved efficiency in the generation and harvesting of the biomass.

Description:
BACKGROUND  
       [0001]    The present invention relates to the amelioration of problems associated with some of the most fundamental activities of modern society: the generation of sewage, the generation of carbon dioxide emissions from combustion processes, and the growing demand for fuel. 
         [0002]    The growth of the global economy has created increasing demands for energy and caused significant stresses on the environment. The consumption of fossil fuels to meet energy demands has resulted in a growing level of carbon dioxide (CO 2 ) in the atmosphere. It is widely accepted that increased CO 2  levels result in an increased atmospheric temperature (a phenomenon generally referred to as global warming). This temperature increase causes climate in all regions of the planet to change, with the potential for catastrophic consequences if the CO 2  level continues to rise. As urban centers grow, the volume of wastewater that must be processed by sewage treatment facilities also grows. The discharge from these treatment facilities is typically rich in nitrates. Ultimately these nitrates are discharged into the seas and oceans. They are then available as a nutrient that contributes to uncontrolled algae growth with deleterious effects on the aquatic ecosystem. 
         [0003]    It has been previously suggested that generation of power can be combined with the controlled growth of a biomass that is ultimately rendered useable as a fuel source. The CO 2  produced by the combustion process is delivered to a site where algae or a similar aquatic plant can utilize the CO 2  and grow via photosynthesis. The biomass is harvested and utilized as a fuel for power generation. 
         [0004]    It has also been previously suggested that aquatic algae or like biomass can be used in the treatment of nitrogen rich wastewater. After initial separation of solids in the treatment of municipal sewage, the wastewater is used as a medium for growing the biomass. The nutrient nitrates are removed from the wastewater by the aquatic algae or like biomass during photosynthesis. 
       SUMMARY OF THE INVENTION  
       [0005]    The present invention is directed to a novel method and system of addressing three fundamental needs of present day society. The inventive method simultaneously addresses the reduction of nitrates in the discharge of sewage treatment plants, the reduction of carbon dioxide emissions from the combustion processes used to generate power, and the production of fuel to meet growing energy demands. The inventive concept is especially suited to large urban areas where there are high demands for energy and large volumes of sewage wastewater. 
         [0006]    According to the present invention two waste products, nitrate-rich sewage wastewater and power plant CO 2  emissions, are combined and converted into a renewable, biomass energy source, which supplies the fuel to the power plant. 
         [0007]    The power plant, wastewater treatment facility, and biomass growth unit are preferably located on one site and arranged for convenient transfer of the CO 2  and wastewater to the biomass growth unit; harvesting, processing and return of biomass from the growth unit as fuel to the power plant; and discharge of the de-nitrated wastewater into the same body of water used as the heat sink by the power plant, e.g., a lake, river, or sound. 
         [0008]    The present invention thus provides an integrated approach to minimization of CO 2  emissions and nitrate discharge while achieving improved efficiency in the generation and harvesting of the biomass. By providing a growth environment rich in both nitrates and CO 2 , the photosynthetic mechanism by which biomass growth takes place is significantly more efficient than the growth rate in environments where the level of only one nutrient is elevated. Thus two entities that are otherwise perceived to be undesirable waste products, nitrates and CO 2 , are now not only prevented from contaminating the environment but are utilized as raw materials in a highly efficient biomass production process providing a source of fuel for the generation of power. 
         [0009]    The discharge wastewater from a sewage treatment facility is delivered to a location where a power generation plant is generating energy via combustion. The CO 2  produced in the power generation is delivered into the nitrate rich wastewater. Thus a growth medium for a biomass is created with highly elevated levels of two nutrients. The biomass is harvested and utilized as a fuel source for the power generation plant. The invention is thus best suited for urban areas with high sewage outfalls and high energy demands. 
         [0010]    In a preferred embodiment, the wastewater discharge from a sewage treatment plant is delivered to a biomass growth unit with a structure whereby the flow is directed along a lengthy circuitous channel. This design of the channel is of sufficient length and size to allow for the necessary dwell time for biomass growth to take place. The CO 2  from the exhaust of the power generation process is delivered to the wastewater stream where it is captured. A biomass source with sufficient levels of algae to serve as a feedstock is also introduced into the stream at the entry point. The continuous delivery of the sewage wastewater causes the stream to flow along the channel during which time the growth of the biomass proceeds in the nutrient rich medium. As the flow proceeds, the CO 2  and the nitrates are depleted from the stream while utilized in the photosynthetic growth of the biomass. At the exit end of the channel the biomass is harvested and the wastewater stream can be discharged to the environment with considerably reduced levels of nitrates and CO 2 . 
         [0011]    Some portion of the harvested biomass can be directed back to the entry point to serve as the required feedstock. The balance of the harvested biomass can be rendered into a useable fuel source for the power plant. In an additional embodiment, some portion of the energy produced by the power plant can be utilized to provide light that enables the photosynthetic process to proceed during overnight hours of operation when power demand declines. 
         [0012]    In addition to the increased biomass generation rate, the present invention has other advantageous features. The CO 2  does not need to be distributed over a large area, such as a pond. The area of CO 2  infusion can be localized to the entry of the channel where the sewage effluent is introduced. The flow carries the CO 2  and the nitrate rich water through the channel. Additionally, the flow naturally serves as a means to deliver the accumulated biomass to the harvesting operation at the end of the channel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING  
         [0013]    An embodiment will be described with reference to the accompanying drawing, in which: 
           [0014]      FIG. 1  is a schematic of an integrated power plant, sewage treatment, and aquatic biomass fuel production system situated adjacent to a river; 
           [0015]      FIG. 2  is a diagrammatic representation of the process steps for producing biomass with enhanced nutrients and carbon dioxide; 
           [0016]      FIG. 3  is a plan view of the preferred biomass growth unit in which algae or the like grow rapidly while carried along in a flow of effluent from the sewage treatment facility in a long, circuitous channel; and 
           [0017]      FIG. 4  is a section view along lines  4 - 4  of  FIG. 3 , showing the preferred features at the infeed region of the biomass growth unit. 
       
    
    
     DETAILED DESCRIPTION  
       [0018]      FIG. 1  is an overview of the system  10  in which a power plant  12  generates CO 2  as a component of the combustion exhaust, with the CO 2  selectively removed and delivered via line  14  to a conversion unit  16 . The conversion unit has a section  18  in which the hot CO 2  gas can give off its heat for use in other process steps to be described below. The CO 2  then exits the conversion unit  16  and passes through line  20  to the bio-mass growth unit  22 . Any undesirable contaminants such as sulfur-containing compounds, heavy metals or the like can be filtered out of the CO 2  stream prior to introduction into the biomass growth unit  22 . 
         [0019]    In the biomass growth unit  22 , rapidly-reproducing biomass such as algae, water hyacinths or the like can grow rapidly via photosynthesis in the presence of the CO 2  gas. The bio-mass material is harvested and delivered via line  24  to the conversion unit  16 , where in section  26 , the bio-mass is dewatered and further converted into a fuel which is delivered via line  28  to the power plant. The power plant is typically located near a body of water  30 , where cooling water for equipment is drawn via line  32  and water from condensers, or the like, of the turbine generator units is discharged via line  34  back into the body of water. 
         [0020]    In an innovative feature of the present invention, a sewage separator  38  is integrated with the power plant  12  and biomass growth unit  22 . A source  36  of sewage enters a separation unit  38  where the liquid effluent is drawn off via line  40  through control valve  42  and may be split into waste water lines  44  and  46 . The solids from the separation unit  38  are delivered via line  48  to section  50  of the conversion unit  16 . The solids can be converted to a form which is removed from the site at  52 . The separated liquid from line  40  still contains nitrates which cannot be discharged directly into the body of water  30 . Instead, the liquid waste water in line  44  is delivered to the infeed of biomass growth unit  22 . The nitrates in this liquid provide nutrients for enhancing the growth of the biomass. The source  36  may generally be located on, near or a significant distance away from the premises of the sewage separator  38 . 
         [0021]    It should be appreciated that the functions performed in the conversion unit  16  can be performed independently, at separate locations. 
         [0022]    Preferably, the high nutrient concentration in the liquid of line  44  and the plentiful supply of CO 2  from line  20  promote rapid, bloom growth of the algae or like organisms in the biomass growth unit  22 . Ideally, by the time the liquid has passed completely through the growth unit  22 , most of the CO2 and nitrates have been picked up in the photosynthesis process and, at the harvesting of the biomass, the remaining liquid effluent has a sufficiently low level of CO2 and nitrates that it can be discharged at  54  directly into the body of water  30 . In the event that the liquid  40  from the separation unit  38  exceeds the capacity of the biomass growth unit  22 , some of the flow can be diverted to line  46  and nitrates removed in a conventional manner in an auxiliary clean-up unit  56  before discharge via line  58  into the body of water. 
         [0023]    Additionally, hot combustion exhaust from the power plant  12  can be captured and delivered to the conversion unit  18  via line  60 . The heat from the exhaust can be delivered via line  62  and employed to heat the biomass growth unit  22  as needed. It is well known that over temperature ranges that allow reproduction, reproduction of algae and like organisms is inhibited as temperature decreases. Thus, the disclosed method provides an efficient approach to maintaining a steady reproduction rate in the biomass growth unit  22 , especially during colder seasons. 
         [0024]    The foregoing represents a significant integration of two sources of waste, the sewage and the power plant CO 2  emissions, with a biomass growth and processing unit, whereby the CO 2  and nitrates are converted into a bio-fuel that is reused in the power plant, while the biomass growth unit also effectively removes nitrates from the waste water so that it can be directly discharged into the environment. Further, by utilizing the excess heat from the power plant to help keep the biomass growth unit at a constant temperature while the nitrate-rich effluent and CO 2  emissions constantly flow, the rate at which biomass propagation occurs is enhanced. 
         [0025]    It should be appreciated that the sewage and power plant emissions are unusually fit for integration into such a system. Due to the continuous nature at which sewage and power production facilities run, the biomass growth unit can be constantly fed with nitrates and CO 2 , thus enabling continuous bio-fuel production. 
         [0026]    With reference to  FIG. 2 , a preferred biomass growth system  100  includes an exhaust line from the combustion unit  112  containing CO 2 , which is captured at  114  and delivered to a flowing water biomass growth unit  116 . As will be described in greater detail below, the growth unit is an outdoor water maze in which an aquatic plant material such as algae or water hyacinths is introduced at the upstream end, grows at a rapid rate while flowing through the maze, and is harvested at unit  118  at the end of the maze. The biomass growth is driven by the photosynthesis process in a flowing medium having nutrients from the supply  120  of wastewater from the sewage treatment plant. The volume or mass of harvestable biomass material at the discharge end of the growth unit can be increased or decreased by means of the flow control  122  of the water through the growth unit  116 . 
         [0027]      FIG. 3  shows the biomass growth unit  116  in the form of an open-top, rectangular structure  124 , built into or above ground, having a plurality of internal walls arranged to provide circuitous flow channels  126 A, B . . . J, fluidly connected, such as shown at  128 . The channels are oriented for maximum exposure to natural sunlight. A source of wastewater inlet flow F is introduced at the entrance to the first channel  126 A and is provided with an effective amount of seed algae. The water flows in the direction of the arrows  128  and  130 . The increasing thickness of the arrows  130 ,  130 ′,  130 ″, and  130 ′″ indicate the density or concentration of algae material in the water. It is known, for example, that some under favorable conditions, some algae can double every hour. If all ten channels  126 A- 126 J are of equal cross-section of flow area, and the flow is controlled to traverse one channel in one hour, a given unit volume of water and associated plant material will take ten hours to travel from the source of the inlet of channel  126 A to the outlet of channel  126 J. In ten hours the concentration of plant material will increase by 2 10 , i.e., over 1,000 times. This high concentration of biomass material is harvested on a screen or similar capture device  132  and removed at  134  for conversion at  26  and use as a bio-fuel delivered via line  28  for combustion in the power plant unit  12  ( FIG. 1 ). Following the harvesting of the biomass material, a subsequent filter or treatment device  136  can be provided for removing residual material at  138  before the effluent is discharged at  140  (via line  54  in  FIG. 1 ). 
         [0028]    In one effective example, the footprint of unit  116  would be approximately one acre (43,560 sq. ft.) resulting in approximately 40,000 sq. ft. of water surface area. With ten channels, each channel would have approximately 4,000 sq. ft. of surface area. Consistent with the overall approximate one acre foot print, each channel can be 200 ft. long and 20 ft. wide, with a depth in the range of 3-5 ft. In another variation, the structure  116  can have 50 channels, each 200 ft. long, with a width of about 4 ft. each. The channel number and size are design options taking into account cost, flow speed, flow control, evaporation, sunlight penetration, etc. 
         [0029]    Optionally, an artificial light source L, powered by the power plant, can be provided for use during off-peak, non-daylight conditions. 
         [0030]      FIG. 4  is a schematic section view along line  4 - 4  of  FIG. 3 . Either through the floor or side walls of each channel, supplemental nutrient supply lines  142  can optionally deliver a controlled amount of nutrients (such as Nitrogen) from nutrient supply  120 , to the extent the nutrients in the wastewater should be increased for optimization of biomass growth. Likewise, the CO 2  from the CO 2  capture unit  114  is introduced via lines  144  into the channels. It should be appreciated that means would normally be provided for controlling the rate of nutrient introduction  142  and CO 2  introduction  144 , and that such rates may differ from channel to channel as the biomass grows, to accommodate the increasing volume of the biomass as it traverses the channels. Furthermore, the waste water flow rate from unit  38  via line  44  ( FIG. 1 ) is also controlled by a variable speed pump  122 , which receives an input signal from the biomass extraction device  134 , such that a control signal C is sent to the pump  122 . 
         [0031]    The concentration of nutrients and CO2 should be high initially to maximize algae growth rate during the early and middle portion of the overall flow path, but as the mass of algae or like biomass approaches the end of the path for harvesting, the concentration of CO2 and nitrates in the medium should be at the practical minimum, so the effluent after harvesting can be directly discharged into the environment. This objective can be achieved by determining the difference in the mass of algae or like biomass at the infeed and harvesting locations; the quantity of nitrates and CO2 required in photosynthesis to achieve this total increase in mass; the flow volume and rate through the maze; the light intensity; and measurements of CO2 and nitrate concentrations at various points along the maze. These variables can be analytically and/or empirically related to arrive at a suitable control scheme. 
         [0032]    As viewed in steady-state on a macro level in a community, the system achieves an absolute reduction in the quantity of carbon dioxide and nitrates. Much of the carbon dioxide and nitrates in the exhaust gas and sewage treatment wastewater delivered to the biomass growth unit, are consumed as raw material for the photosynthetic growth of a biomass and thereby converted into other, useful or harmless compounds. At the same volumetric flow rate, the effluent discharged from the biomass growth unit to the environment, contains a lower concentration of nitrates than in the wastewater delivered to the biomass growth unit. Similarly, the quantity of carbon dioxide in the effluent is discharged from the biomass growth unit to the environment, at a rate per unit time that is lower than the rate per unit time carbon dioxide is delivered to the biomass growth unit from the exhaust gas. 
         [0033]    It should be appreciated that, as shown in  FIG. 3 , the flow width of each channel need not be the same. Although in a continuous operation the volumetric flow rate would be the same in each channel, the ratio of surface area to flow rate would depend on the width of the channel. This variable would be useful to accommodate the increasing concentration of biomass material, i.e., in the wide channel such as  126 J, the average depth of the biomass material is closer to the surface, minimizing attenuation of sunlight due to the high concentration of the biomass material. 
         [0034]    It should be understood that variations of the foregoing are also possible. Whereas it is preferable that the water flow be continuous from inlet to discharge, such flow could be intermittent to some extent, i.e., continual. The channels need not be parallel, but could for example be arcuate. Under certain geographic constraints, the channel could be a single, long culvert. Preferably, however, the general form of the growth unit would have a continual flow through a circuitous channeling, where the term “circuitous” means having a change in direction.