Abstract:
Apparatus ( 10 ) for denitrification of a liquid, such as water, comprises a vertical processing tank ( 11 ) including: a bottom section ( 20 ) partially filled with activated sludge ( 22 ), bottom section ( 20 ) having a bottom ( 12 ) having a concave upward facing inner surface ( 13 ); a horizontal filter ( 40 ) spanning tank ( 11 ) above sludge ( 22 ); an anaerobic process section ( 50 ) including a denitrifying biomass section ( 54 ); and an oxidation section ( 60 ) including a diffuser ( 61 ). The method comprising the steps of: injecting impure water ( 90 ) into sludge ( 22 ) so as to travel helically upward to mix with sludge ( 22 ); filtering with filter ( 40 ); denitrifying with denitrifying biomass ( 54 ); and oxidizing the effluent water from anaerobic biomass ( 54 ) in oxidation section ( 60 ) including bubbling air from diffuser ( 63 ). Water ( 90 ) effluent from tank ( 11 ) is sterilized with an ozonation device ( 86 ).

Description:
FIELD OF THE INVENTION 
     This invention relates in general to denitrification of a liquid, and more specifically to a vertical, upward-flow denitrification tank. 
     BACKGROUND OF THE INVENTION 
     For at least 25 years, municipal wastewater treatment plants have been aware of the need to reduce or eliminate nitrogen compounds from their discharged effluent. Typically, they use a scheme of oxidizing ammonia to nitrate, then convert the nitrate to nitrogen gas. Microbiological processes are usually used for both steps of the process. 
     Since nitrate compounds are very soluble and mobile in water, they are found naturally only in arid climates. In the United States, though, nitrates are a common contaminant of groundwater in agricultural areas. Run-off from fertilized fields and from locations where animals, such as cattle, pigs and chickens, are raised in high concentration, such as dairy farms and animal feed operations, often contains enough nitrate to affect nearby water wells. 
     Installations to treat municipal wastes are complex systems. The several types of microorganisms employed to treat ammonia, nitrate, phosphate, and other classes of waste are generally sensitive to their environment, and do not thrive and perform their functions if conditions of Ph, oxidation or reduction potential, nutrient content, and other factors are not within the acceptable range. 
     Municipal sewage varies widely in these conditions. Sometimes, compounds toxic to microbial life appear suddenly. Sewage treatment plants, thus, must constantly monitor the incoming waste stream and the processes used for treatment. Technicians adjust the process conditions frequently to keep the microbial and other systems running efficiently. Failure to do so can result in release of insufficiently-treated effluent, or shutdown of the plant until the microorganisms are nursed back to health. 
     Municipal sewage treatment systems are made up of many tanks, covering a few acres of land. The piping, valves, access ports between tanks provide many opportunities for leaks and must be inspected daily, at least. Many of the systems require frequent backflushing and other maintenance of filters. 
     By comparison, agricultural well water in a given location has a simple composition, which does not fluctuate quickly. The impurities are relatively dilute and readily water-soluble. 
     Therefore, there has been a need for a simple nitrate removal system for rural agricultural areas that can have optimal operating conditions designed into it. Such a system should be robust, require little adjustment and maintenance, use as little electricity as possible, and be easy to transport to the site and install. 
     SUMMARY OF THE INVENTION 
     The present invention meets the need for a robust and simple nitrate treatment system for well water for agricultural use, such as drinking water for animals. In the preferred embodiment, the denitrification is accomplished within a single, vertical process tank with three process sections. 
     The bottom section, called the sludge section, contains an activated sludge in an anoxic, nearly anaerobic, condition. The water to be denitrified is injected into the bottom of the sludge section by nozzles, which cause the water to flow upward through the sludge in a helix. The helical flow allows slow and thorough mixing of the water to be treated with the sludge, without use of gas agitation or a stirring device. The preferred shape of the tank bottom is an inverse cone. 
     Water from the sludge section passes upward through a filter that retains particles in the range of 5 to 100 microns as it passes upward through a fabric filter. A flush valve just below the filter provides convenient backflushing by gravity of the filter. 
     An anaerobic denitrification process section, above the filter, contains an array of solid support strips coated with a film of denitrifying bacteria. These strips may contain the nutrients needed by the bacteria. 
     At the upper boundary of the anaerobic section, a grid of perforated pipes injects compressed air into the water. In the preferred embodiment, the water simultaneously flows upward through a biobed of aerobic bacteria on support strips similar to those below, in the anaerobic section. In this oxidation process section, the aerobic bacteria convert any odoriferous products of the anaerobic process, such as sulfides, to oxidized forms that are gaseous or non-noxious. 
     The treated water leaves the process tank through a pipe at the top. If it is desired to sterilize the water of introduced bacteria, the water may be directed to a final purification system. A standard biomass filter is used to remove clumps of bacteria that may have become dislodged from the support strips. A commercial ozonation device destroys living bacteria and renders the water sterile. Ultraviolet treatment could be instead, if desired. A slow sand filter, membrane cartridge, or bag filter may be used for filtration to about 2 microns. 
     Other features and many attendant advantages of the invention will become more apparent upon a reading of the following detailed description together with the drawings in which like reference numerals refer to like parts throughout. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a diagrammatic view of a preferred embodiment of the denitrification apparatus for carrying out the process of the invention. 
     FIG. 2 is an partial enlarged perspective view of a support strip of FIG.  1 . 
     FIG. 3 is a schematic flow diagram view of sterilization apparatus downstream of FIG.  1 . 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     With reference now to the drawings, FIG. 1 is a diagrammatic view of a preferred embodiment of the denitrification apparatus  10  for carrying out the process of the invention. Apparatus  10  generally comprises a vertical vessel or tank  11  containing a bottom or sludge section  20 , a filter  40 , an anaerobic process section  50 , and an oxidation section  60 . 
     Tank  11  may be of conventional cylindrical design and made of coated steel or other suitable material. Tank bottom  12  has a concave upward facing inner surface  13 . Preferably, inner surface  13  is circular in horizontal cross section, such as inverse conical surface  13  shown in the preferred embodiment. The conical bottom aids the helical flow of the influent water. A cylindrical tank discourages the sludge from lodging in sharp interior corners and putrefying. 
     Sludge section  20  is partially filled with activated sludge  22 . Activated sludge  22  consists of microorganisms in an anoxic, nearly anaerobic, condition, capable of converting nitrates to molecular nitrogen and nitrous oxide gases, along with sufficient nutrients for their growth. 
     Influent liquid, such as water  90  containing nitrate salts, enters pipe  15 . 
     Injection means  25  receives impure water  90  and injects it into bottom  12  of sludge section  20  such that the influent water  90  travels helically upward to mix with sludge  22 . The helical flow allows slow and thorough mixing of the water to be treated with the sludge, without use of gas agitation or a stirring device. 
     In the embodiment shown, injection means  25  includes a pump  26  and nozzles  27 . Pump  26  receives influent water  90  from pipe  15  and pressurizes it to a pressure dependant upon the size of the specific system, but typically in the range of 60 psi. Injection nozzles  27  are positioned and angled to inject water  90  on a chord line and sized so as to form a slow helical flow through the sludge, allowing sufficient contact time for the water to achieve a substantial decrease in nitrate concentration, typically several hours. The actual time required is calculated from the nitrate concentration of the influent groundwater and the output flow rate required. Injection pressure and tank dimensions can also be varied to attain the desired contact time. If influent water  90  already has sufficient head pressure, then pump  26  may not be required. Alternate injection means  25  could include an impeller (not shown) in bottom  12 . 
     This sludge section is in a very anoxic, nearly anaerobic condition, and will hereafter be referred to as “anaerobic”. As the water contacts the bacteria of the sludge, the bacteria convert nitrate in the water into molecular nitrogen. Most of the denitrification occurs in this section. 
     Sludge section  20  includes an upper end  30  above sludge  22  including an outlet  29  for egress of water  90  treated by sludge  22 . 
     Horizontal filter  40  covers outlet  29  from sludge section  20 . Horizontal filter  40  is a heavy polypropylene mesh fabric, which traps particles in the range of 5 to 100 microns. Since the flow impinges gently on the filter, and with a slight shearing motion, larger particles settle back into the bottom of the sludge section. 
     A backflush valve assembly  33  proximal filter  40 , below filter  40 , and above top  23  of sludge  22  selectively removes backflush directly below filter  40  from tank  11 . If filter  40  needs to be backflushed, flush valve assembly  33  is opened and backflushing is accomplished by gravity flow. A hose or fixed pipe, not shown, may be attached to direct the backflush to the intake  15  of the system. Although a single backflush valve assembly  33  is shown, typically a plurality of backflush valve assemblies  33  are spaced around the periphery of tank  11 . 
     Anaerobic process section  50  further denitrifies effluent received from filter  40 . 
     Anaerobic process section  50  includes a denitrifying biomass section  54  including support surfaces, such as a plurality of support strips  55  having upper and lower ends supported and held in position between upper and lower support grids  57 , 58 , for supporting bacteria  59 . 
     FIG. 2 is an partial enlarged perspective view of a preferred embodiment of support strip  55  of FIG.  1 . Strips  55  have six vanes  56  about the vertical axis. In the preferred embodiment strips  55  are plastic and are co-extruded with various nutrient materials, such as carbon, phosphorus, manganese, iron, molybdenum or cobalt, that support microbial activity. Bulk nutrients, such as mineral-containing clay or a carbon source such as a short-chain primary alcohol, may also be added, but have not been found to be generally necessary. The shape makes strips  55  rigid enough to be supported easily from the ends and to have a high specific surface area. 
     In the anaerobic process section, support strips  55  are covered with a film of denitrifying bacteria  59 , which adhere well to strips  55 . The combination of strips  55  and bacteria  59  is often called a biobed. This allows water  90  to be treated to flow past bacteria  59  without turbulent flow and in a reproducible manner, so that contact time can be established by flow rate only, and does not depend on the concentration of bacteria  59  in the system. 
     The denitrifying bacteria  59  convert the remaining nitrate in the water  90  to molecular nitrogen. A small amount of carbon dioxide may also be formed as a result of their metabolism. 
     Access to anaerobic biomass section  54  is provided by means such as a manhole, not shown, through which support strips  55  can be removed and replaced. Strips  55  will be replaced with new ones when the nutrients have been substantially consumed. If an a excessive growth of bacteria  59  occurs, causing the flow of water  90  to be impeded, strips  55  may be either replaced or cleaned and returned. 
     A biomass backflush valve assembly  51  proximal and below lower support grid  58  and above filter  40  can be opened to selectively removes backflush directly above filter  40  from tank  11 . Although a single biomass backflush valve assembly  51  is shown, typically a plurality of biomass backflush valve assemblies  51  are spaced around the periphery of tank  11 . 
     Oxidation section  60  includes an air diffuser assembly  61  and an aerobic biomass section  64 . Oxidation section  60  receives effluent water  90  from anaerobic process section  50  and passes it through aerobic biomass section  64 . Aerobic biomass  64  consists of aerobic bacteria for receiving oxygenated effluent from the air diffuser  63 . These aerobic bacteria, supported on solid strips similarly to the anaerobic biomass, convert any odoriferous products of the anaerobic process, such as sulfides, to oxidized forms that are gaseous or non-noxious. 
     Air diffuser assembly  61  bubbles air through the received water  90  and aerobic biomass section  64 . Air diffuser assembly  61  includes an air pump  62  and an air diffuser  63 . Air diffuser  63 , a grid of pipes underlying aerobic biomass section  64 , expels air received from pump  62  as small bubbles through small nozzles or holes. The air bubbles provide an aerobic condition for the biomass and entrain dissolved product gases and help release them into the gas phase. 
     Treated water  90  exits outlet  67 . A vent  68  above the top of water  90 , vents the product gases, which include nitrogen and carbon dioxide. If any residual volatile organics are present, such as from bulk addition of a carbon source, the gases may be directed to an absorption or scrubbing device, not shown. Otherwise, the gases may be released to the atmosphere. 
     FIG. 3 is a schematic flow diagram view of sterilization system downstream of FIG.  1 . Treated water  90  may be further purified and made potable by directing it through the sterilization system of FIG. 3 including an ozonation device  86  for receiving treated water  90  and exposing it to ozone. Ozonation destroys living bacteria that may have escaped from the biobeds and renders the water sterile. 
     Other sterilization methods such as ultraviolet radiation or chlorination could also be used. Ozonation is preferred for this application because is an efficient disinfectant and decomposes spontaneously into oxygen, leaving no residue. The sterilization system may further include some combination of a standard biomass filter  80  to remove clumps of bacteria that have been dislodged from the support strips  55 , bag filter  88 , and slow sand filter  84 , removing particles including bacteria to about 2 microns. Other particulates may include mineral scale produced by the ozonation. The sterilized water  90  is released through output pipe  89 . 
     Having described the invention, it can be seen that it provides a very efficient device for the denitrification of well water in an agricultural setting. 
     All microbial operations occur within a single vessel, so that the effluent is clean and needs only a final polishing to make it potable. Residence time of the water in the various process zones is designed into the system dimensions and incoming pressure. The injection nozzles can also be customized to provide appropriate flow rates based on analysis of the actual water to be treated. 
     The nozzles introduce the water into the process tank in such a way that the water flows upward in a helical manner, which provides excellent mixing and contact with the activated sludge without the use of an electric stirrer or compressed air agitation. 
     Backflushing of the single screen is accomplished by opening a valve located just below it, so that gravity backflushes the screen without a pump for that purpose. 
     The single, vertical process tank occupies little land area. The auxiliary equipment for final sterilization and polishing of the water is tailored to the specific requirements of the site, so as to be no more complex than required. 
     The support strips for the biobeds contain bacterial nutrients, which diffuse out gradually. This reduces or eliminates the need to monitor the activity of the bacteria and provide additional nutrients. Access hatches are provided for replacement of the supports, when needed. 
     The aerobic biobed in the oxidation section of the apparatus odoriferous or foul-tasting products of the anaerobic phase. Air injected from the diffuser helps release dissolved gases from the water. The tank may be closed to the atmosphere, allowing the product gases—mainly nitrogen, possibly with carbon dioxide and nitrous oxide—and residual volatile organics added as carbon source, if any, to be collected and absorbed. 
     Although a particular embodiment of the invention has been illustrated and described, various changes may be made in the form, composition, construction, and arrangement of the parts without sacrificing any of its advantages. Therefore, it is to be understood that all matter herein is to be interpreted and illustrative and not in any limiting sense and it is intended to cover in the appended claims such modifications as come within the true spirit and scope of the invention.