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Detalied Information
| Outlined below is detailed information on Nitrate and De-nitrification. This information can be accessed via either scrolling down through this page or alternatively, you can download and save the information sheet, by right-clicking on the underlined text and select "Save Target As..." or by just left-clicking the underlined text and wait for the file to display in a new window. |
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| Wastewater de-nitrification is the removal of nitrate in anoxic conditions by facultative anaerobes (denitrifying bacteria). These bacteria break down carbonaceous BOD using oxygen from the nitrite/nitrate ion as their oxygen source. In the process certain facultative bacteria reduce nitric nitrogen to a lower oxidation state. This reduction takes place in the form of several reactions which may be expressed as follows: |
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| NO3- → NO2- → NO → N2O → N2 |
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| Nitrate → Nitrite → Nitric Oxide → Nitrous Oxide → Nitrogen |
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| De-nitrification removes nitrogen from wastewater by converting it to insoluble gas that escapes to the atmosphere. Facultative anaerobes typically make up approximately 80% of the bacteria within an activated sludge process. These bacteria have the enzymatic ability to use free molecular oxygen, nitrite ions or nitrate ions to degrade carbonaceous BOD. Facultative anaerobes prefer and use free molecular oxygen when it is available. The use of free molecular oxygen provides the bacteria with more energy for cellular activity, growth and reproduction than through the use of nitrite or nitrate ions. |
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| The general Alcaligenes, Bacillus and Pseudomonas contain the largest number of denitrifying bacteria. Most denitrifying bacteria reproduce every 15-30 minutes and are present in an activated sludge system in billions per gram MLVSS. Unless the treatment process is experiencing start-up, wash-out, toxicity or recovering from toxicity an adequate and active population of denitrifying bacteria should be present to ensure de-nitrification under favourable operational conditions. Denitrifying bacteria can be added to the treatment process by augmenting with commercially prepared bio-augmentation products or seeding with mixed liquor from another treatment process. |
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| Nitrification in an aeration tank produces nitrite and nitrate ions. Some industrial waste streams may contain high concentrations of nitrite/nitrate ions, e.g. corrosion inhibitor manufacture, meat processing and steel manufacture. Nitrate ions are undesirable in water as they are a major cause of eutrophication and are toxic to aquatic life. Nitrates can also cause methemoglobinemia (“blue baby syndrome”) in infants – a condition which affects oxygen transportation throughout the infant’s body. These are the reasons why nitrate needs to be removed from wastewater. |
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| Removal of nitrate is typically done through an anoxic zone that precedes the aerobic zone. In the aerobic zone, nitrification takes place and produces nitrate. A portion of the mixed liquor is returned to the head of the anoxic zone for a source of nitrate. In the anoxic zone, the lack of elemental oxygen causes the bacteria to derive their oxygen chemically and they therefore convert the nitrate to nitrite, and ultimately nitrogen gas, as shown in a simplified fashion in the reactions on the preceding page. |
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| De-nitrification normally takes place in an anoxic zone or occasionally in an aeration tank, which is allowed to go anoxic for a period of time. Accidental de-nitrification often takes place in clarifiers. This is visible as large clumps of dark sludge rising from the bottom to the top of the clarifier. Numerous bubbles are associated with the sludge. De-nitrification in a clarifier is undesirable as rising sludge represents a loss of solids and bacteria to the receiving water. This can result in licence breaches. If sludge is retained too long in a clarifier, the sludge blanket becomes deoxygenated. Warm weather will accelerate the process. Increasing the RAS (Return Activated Sludge) rate will control Denitrification in a clarifier. |
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| There are a number of basic requirements in order for de-nitrification to occur in an anoxic zone. |
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These are listed below:
1. The absence of molecular oxygen. The process will only occur under anoxic conditions. Dissolved oxygen at levels greater than 0.3 mg/l will inhibit the process.
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| 2. The presence of an adequate and suitable carbon source. The process requires approximately 3kg of soluble BOD per kg of nitrate to be removed. The ideal BOD source would be easily degradable and be a high oxygen demand food. Addition of organic compounds may be required to achieve full de-nitrification. The larger the quantity of soluble BOD, the greater is the demand for electron acceptors such as free molecular oxygen, nitrite and nitrate. The greater the demand for electron acceptors is, the greater the chances are that de-nitrification occurs. Methanol, ethanol, glucose, acetic acid and molasses are often added to anoxic systems to increase the soluble BOD. For complete de-nitrification of nitrate ions, 2.5mg/l of methanol are required as the substrate per mg/l of nitrate ions present. |
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| 3. The presence of at least 2 to 3 mg/l of nitrate. |
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| 4. The anoxic zone must be maintained in a continuous mixed state, Kilowatt requirement will be approximately 1.314 kW per each 68 cubic meter volume. |
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| 5. An adequate recycle rate from the aerobic to anoxic zone. Typically, a recycle rate of 300% is recommended, although higher rates will result in a greater efficiency of the process. Biological de-nitrification processes re-circulate nitrified effluent as a source of nitrate. This means that 100% removal can never be achieved. Only the nitrate in the re-circulated stream will be removed. If the recycle ratio is 1:1, then 50% nitrate removal is possible. If the recycle ratio is 2:1, then a 66% nitrate removal is possible. If the recycle ratio is 3:1, then a 75% removal is achievable. If the recycle ratio is 10:1, then a 91% removal of nitrate is possible. If the recycle rate is 20:1, then a 95% reduction in nitrate is achievable. |
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| 6. There must be a period of residence time for the process to work - a normal rule of thumb is that the anoxic zone will be approximately one third the total volume of the aeration tank. However, the quantity of nitrate to be removed will ultimately dictate anoxic zone size. |
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| 7. An optimal pH range of between 7.0 and 7.5. Up to a 30% decrease in efficiency may occur outside a pH range of between 6 and |
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8. The presence of suitable heterotrophic bacteria, e.g., Alcaligenes, Bacillus and Pseudomonas.
9. Wastewater temperature of above 5°C. Warmer wastewater favours rapid proliferation of nitrate ions and has less affinity for D.O. (Dissolved Oxygen) than colder wastewater. Increasing wastewater temperature results in more rapid de-nitrification.
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| 10. Redox potential of +50 to − 50 millivolts (mv). Redox is a measurement of the amount of oxidized compounds, such as nitrite and nitrate, and reduced compounds, such as ammonia in a wastewater sample. Within the range of +50 to- − 50 millivolts, oxygen is either absent or present in a relatively small quantity, while nitrite and nitrate ions are present in relatively large quantities. |
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| Apart from the obvious benefit of nitrogen removal, de-nitrification provides four other benefits in a nitrifying activated sludge plant; |
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| 1. The assimilation of organic carbon (BOD) using oxygen derived from nitrate will reduce the required oxygen input for the total treatment plant. The de-nitrification process returns approximately 60% of the oxygen required for nitrification to the system. Aeration is one of the significant costs of a wastewater treatment operation. Thus savings can be significant. |
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| 2. The hydroxyl ion (OH-) and some of the carbon dioxide produced during de-nitrification are returned to the activated sludge process as alkalinity. De-nitrification returns approximately 50% of the alkalinity consumed during the nitrification process. This helps offset the pH lowering effects of the nitrification process and also offers greater buffering capacity to the system. |
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| 3. De-nitrification can help control undesirable filamentous growth. An anoxic environment produced through de-nitrification favours the growth of facultative anaerobic, floc-forming bacteria and discourages the growth of filamentous bacteria. |
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| 4. Studies indicate that facilities which utilise de-nitrification reduce overall sludge production by 5% or more. Sludge disposal is one of the rising costs associated with wastewater treatment plants. Therefore, a reduction in the quantity of sludge produced equates to direct savings. |
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