The microorganisms involved in denitrification are much more varied and plentiful than those involved in nitrification. They are also much less sensitive to environmental changes than the nitrifiers. Systems that nitrify and denitrify have the advantage of regaining about 50% of the alkalinity lost during nitrification, because alkalinity is created as part of the denitrification process. Biological denitrification can be accomplished with both fixed film and suspended growth processes. Denitrification is typically performed using anoxic zones as an adaptation of the activated sludge process, although constructed wetlands cells, lagoons and even septic tanks have all been used to provide the anoxic conditions necessary to induce denitrification. Various process modes include fixed film and suspended growth.
In fixed film the fluidized bed biological denitrification process works by passing wastewater through a bed of suitable media such as sand. As the wastewater moves through the bed, microorganisms attached to the media utilize the nitrate in the wastewater as a source of oxygen for metabolizing carbon compounds. Trickling filters and RBCs have been designed to provide for denitrification in a similar fashion, although these processes must be modified to exclude dissolved oxygen. As for all biological denitrification systems, a carbon food source must be supplied to promote metabolism by the denitrifying organisms. Primary effluent, methane gas, methanol or any other source of carbon can be used. In fixed film reactors, the carbon source can even be organic matter that is trapped in the reactor itself.
This use of a trapped carbon source for denitrification has been accomplished in constructed wetlands, although it is difficult to control and the success of the method is questionable. For all biological denitrification systems, dissolved oxygen must be excluded from the system; otherwise, the organisms will utilize any available D.O. rather than utilizing nitrate for cell metabolism. In general, fixed film systems are not as well suited to biological denitrification as suspended growth systems.
Activated sludge can be modified quite easily to provide for biological denitrification. By creating an anoxic environment, where the mixed liquor and influent are kept in suspension, but not aerated, controlled denitrification can be achieved. There are many ways of achieving anoxic conditions in activated sludge systems. Perhaps one of the easiest is to simply shut off the aeration system for several hours to allow anoxic conditions to develop. If carefully timed, this method can denitrify the entire contents of an aeration basin. This approach has been applied successfully to package plants that need to denitrify for permit compliance.
Another approach to creating an anoxic environment is taken in the sequential batch reactor (SBR) process. SBRs operate much as activated sludge, with the exception that the entire treatment process, including clarification, takes place within a single reactor basin. While an SBR is aerating, mixing and filling with influent, the organisms in the reactor are assimilating BOD and nitrifying ammonia into nitrate.
Next, the aeration is turned off, but the mixing and filling with influent continues. During this phase, the microorganisms in the reactor continue to assimilate BOD. In doing so, they quickly utilize the available D.O (the basin becomes anoxic). Once the D.O. is exhausted, the facultative organisms turn to the oxygen bound up in nitrate as a source of oxygen that can be utilized and allow them to continue to metabolize BOD. SBRs often cycle between these two phases (aerobic and anoxic) several times before finally shutting off aeration and mixing so that the mixed liquor can settle and the clarified effluent can be decanted off and discharged. If the phases are carefully controlled, high levels of nitrogen and BOD removal can be achieved.
Recent development has lead to activated sludge plants that have special, dedicated anoxic zones, which provide for denitrification. Anoxic zones have been utilized at the beginning, in the middle and at the end of aeration basins. Some zones are simply a portion of the basin that is mixed but not aerated (common in oxidation ditches), while others are isolated areas, separated by walls or baffles. In either case, the intent is to provide an area where the mixed liquor and a carbon source (normally influent) can come together for a set detention time. While in the anoxic zone, the organisms are forced to turn to the oxygen bound up in nitrate as a source of oxygen to be used while metabolizing the BOD contained in the carbon source feed.
Often, mixed liquor is recycled through the anoxic zone in order to denitrify the nitrate contained within it. This form of mixed liquor recycle is also used to maintain the desired detention time within the anoxic zone, which is typically around 1 – 2 hours. Excessive detention times in the anoxic zone can overstress the non-facultative organisms (like the nitrifying bacteria), and so must be avoided or the nitrification side of the process will suffer. When anoxic zones are located at the beginning of an aeration basin, another benefit is realized. In an anoxic zone located at the head of an aeration basin, the facultative organisms rapidly take up the easy to assimilate organic matter (soluble BOD) contained in the influent. This reaction results from the stress caused by the anoxic conditions. This rapid uptake of the easy to assimilate organic matter robs many filamentous type bacteria of their main food source; soluble BOD. As a result, an anoxic zone operating in this fashion will actually select against the growth of many types of filamentous organisms. For this reason, anoxic zones situated at the front of an aeration basin are often referred to as anoxic selectors, or bioselectors. Selectors of this type offer the most powerful long-term tool for combating filamentous organisms available to operators.
It is important to understand that the effect does not work on all types of filamentous organisms. Much to the regret of many an operator, the filamentous organism Microthrix Parvicella does not respond to the selector effect. In addition, other filaments are only slightly selected against or are not affected at all.