Method and apparatus for denitrification of water

[0002] This invention relates in general to denitrification of a liquid, and more specifically to a vertical, upward-flow denitrification tank.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] 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.

[0008] 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.

[0009] 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.

[0010] 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.

[0011] 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.

[0012] 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.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] 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.

[0017] FIG. 1 is a diagrammatic view of a preferred embodiment of the denitrification apparatus for carrying out the process of the invention.

[0018] FIG. 2 is an partial enlarged perspective view of a support strip of FIG. 1.

[0019] FIG. 3 is a schematic flow diagram view of sterilization apparatus downstream of FIG. 1.

[0020] 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.

[0021] 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.

[0022] 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.

[0023] Influent liquid, such as water 90 containing nitrate salts, enters pipe 15.

[0024] 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.

[0025] 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.

[0026] 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.

[0027] Sludge section 20 includes an upper end 30 above sludge 22 including an outlet 29 for egress of water 90 treated by sludge 22.

[0028] 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.

[0029] 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.

[0030] Anaerobic process section 50 further denitrifies effluent received from filter 40.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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.

[0036] 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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.

[0043] 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.

[0044] 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.

[0045] 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.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] 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.