Nitrogen & Phosphorus Removal

Biological Nitrogen Removal
Nitrogen appears in organic wastes in various forms. In wastewater, four types of nitrogen are common: organic nitrogen, ammonia nitrogen, nitrite nitrogen, and nitrate nitrogen. These different forms constitute the total nitrogen content. The predominant forms of nitrogen in wastewater are organic nitrogen and ammonia (NH3). Organic nitrogen is converted to ammonia in the first step of the nitrogen cycle. In order to remove nitrogen from wastewater, the ammonia must be oxidized to nitrate (NO3). This process is commonly referred to as nitrification. An oxic environment must be maintained for a sufficient period of time to promote nitrification.At the Water Reclamation Facility, oxic conditions are maintained by a number of rotor surface aerators. In the presence of dissolved oxygen, the microorganisms convert stored BOD (biochemical oxygen demand) to CO2, water, and increased cell mass. Biological nitrification occurs, producing nitrite in an intermediate step and ultimately producing nitrate. Following nitrification, nitrogen can be removed from the wastewater by reducing the nitrate to nitrogen gas (N2), which is released to the atmosphere. This process is commonly referred to as denitrification. Denitrification requires anoxic conditions, as well as an organic carbon source, to proceed. Introducing an anoxic zone into the flow scheme provides for denitrification of nitrate. In this zone, operated with no dissolved oxygen (DO), the endogenous oxygen demand of mixed liquor suspended solids (MLSS) plus the carryover of BOD (biochemical oxygen demand) from the anaerobic zone causes denitrification of the nitrate produced in the aerobic zone. During anoxic conditions, dissolved oxygen is not available to the microorganisms for respiration. Because of this, the oxygen molecules are stripped from the nitrate, causing the production of nitrogen gas(N2) . Carbon dioxide and water are also produced in the process, which results from the degradation of BOD. In addition, a portion of the alkalinity consumed during the nitrification process is restored through the denitrification process. When the mixed liquor flows to the secondary anoxic zones, there will be a relatively small concentration of extra cellular BOD in the wastewater. However, denitrification will still proceed since the microorganisms utilize internal storage products to reduce nitrate (endogenous denitrification).
Biological Phosphorous Removal
Phosphorus is an essential element in the metabolism of organic organisms. A minimal concentration is necessary to achieve optimum operation of biological treatment systems. The BioDenipho process incorporates anaerobic selector technology to promote biological phosphorous removal from the wastewater. It combines the flexibility and energy efficiency of the BioDenitro Process with the advantages offered by an anaerobic selector. This results in a highly-efficient Biological Nutrient Removal (BNR) system. A three-stage anaerobic selector is incorporated prior to the distribution chamber. The anaerobic residence time is approximately 3 hours based on a flow of 3.0 MGD for U.F. Water Reclamation Facility. This is not long enough to promote the buildup of sulfides or other noxious products associated with anaerobic treatment processes. Each stage of the anaerobic selector contains a submerged mixer designed to provide gentle agitation of the mixed liquor while minimizing turbulence at the liquid surface. The mixer is driven by a gear-reduced, waterproof submerged motor. Aeration of the mixed liquor in the oxidation ditch can be provided in a number of ways. The most common method is to intensely agitate the surface of the liquid, which is open to the atmosphere, with a large brush aerator. There are many variations in the type of equipment employed; however, all aeration systems have the common purpose of vigorously mixing an oxygen-containing gas with the mixed liquor. Submersible mixers are used in the anaerobic zones, oxidation ditches and secondary anoxic zone. Maxi-rotors are used for aeration in the oxidation ditches. Centrifugal pumps are used for return activated sludge pumping. The anaerobic selector separates activated sludge metabolism into two steps: BOD uptake and BOD oxidation. The return activated sludge (RAS) enters the anaerobic selector, where it is mixed with influent wastewater. By passing the RAS and influent through the anaerobic selector, microorganisms capable of using stored polyphosphate as an energy source are proliferated. This energy is used to transport BOD into their cellular structure when free or combined forms of oxygen are not available for respiration. In the anaerobic zone, where substrate (BOD) concentration is high, the absence of oxygen causes the microorganisms to release the stored intracellular polyphosphates by decomposition to simple orthophosphates. The decomposition of polyphosphate to orthophosphate results in an increase of soluble phosphorus in the mixed liquor and also releases energy. The energy is used by the microorganisms to transport soluble BOD through the cell wall and store the soluble BOD inside the cell. Thus, the BOD concentration in the mixed liquor is reduced without the use of oxygen. Typically, the anaerobic selector is a three-or-four stage reactor equipped with submersible mixers to maintain biosolids in suspension. Return activated sludge (RAS) is discharged to the first stage selector, while raw influent is directed to the second stage. By staggering the RAS and raw wastewater influent location, the volatile fatty acids and soluble BOD, which promote phosphorous release, are not consumed during RAS denitrification. As an added benefit, the anaerobic selector inhibits the growth of filamentous bacteria that cause bulking sludge. In the subsequent anoxic phases, nitrates are present from prior oxic phases. Microorganisms capable of biological denitrification are favored in this phase. The carbon source required for biological denitrification is provided by the BOD in the influent wastewater; no additional carbon source is required. In the oxic phase, the organisms in the presence of dissolved oxygen convert the stored and extracellular BOD to CO2, water, and increased cell mass. A portion of the energy from this reaction then goes to recreating the intracellular polyphosphate using the orthophosphate released in the anaerobic zone. Since new cells are grown, the amount of phosphate removed from solution is greater than the amount previously dissolved in the anaerobic zone, thus affecting net phosphate removal. In the second stage, denitrified RAS is contacted with the influent wastewater in the absence of free or combined forms of oxygen. The anaerobic environment stresses the microorganisms, which begin to break down stored polyphosphate into orthophosphate. Throughout the remainder of the anaerobic selector, orthophosphate is expelled from the microorganisms releasing energy that is used to absorb BOD into their cells. In subsequent anoxic and oxic phases, the BOD is oxidized and the cells reproduce. In the oxic phases, these new cells, along with old cells, replenish the phosphorous reserved within their cells. This results in a net phosphorous uptake. Phosphorus removal from the wastewater is ultimately achieved by wasting phosphorous-rich sludge from the system. Phosphorous is removed from the system as a fixed biological material in the waste sludge. The amount of phosphorous in the sludge will be dependent upon the amount of BOD and phosphate in the influent and the volume of sludge produced.
Next: BioDenipho Phases