Biological Nutrient Removal
In the natural world, changes to nitrogen compounds are mostly accomplished biologically, by living organisms. These organisms live in environments that are aerobic, anaerobic and even anoxic. Modern wastewater treatment plants can be designed (or operated in a modified fashion) to manipulate microorganisms into changing nitrogen compounds. Typically, nitrogen is in the form of ammonia and organic nitrogen in influent. If accomplished in the correct order, the nitrogen compounds that enter the influent (mainly ammonia) can be converted first into nitrate and then converted into nitrogen gas (N2), which escapes into the atmosphere and is thus removed. This process is known as nitrification/denitrification. To understand how it works, it is important to understand how (and why) microorganisms change nitrogen from one compound to another.
The majority of nitrogen exists as nitrogen gas in the earth’s atmosphere. Specialized plants, known as legumes, can capture atmospheric nitrogen and turn it into plant matter. This process is known as nitrogen fixation. Legumes accomplish nitrogen fixation through a symbiotic relationship with a group of aerobic, facultative and anaerobic bacteria that live near their roots. These bacteria are actually responsible for transforming atmospheric nitrogen into compounds that the plants can readily use, such as nitrate. The plant’s use of nitrogen results in increased plant matter. As the plants are consumed by higher life forms, the organically bound nitrogen is passed along for use by other organisms.
Organically bound nitrogen can be broken down into ammonia by anaerobic bacteria through the process of anaerobic decomposition. A good example of this is what happens in a septic tank. Much of the organic nitrogen that enters a septic tank is converted into ammonia, which is released in the septic tank effluent. For this reason, the ammonia concentration coming out of a septic tank is often higher than the ammonia concentration going in. Organic acids are formed as by-products of anaerobic decomposition, which tend to lower the pH of wastewater passing through the process.
Nitrification is the process by which ammonia is oxidized into nitrite and then nitrate. Working under strict aerobic conditions (> 1.0 mg/L D.O.), two groups of autotrophic microorganisms accomplish nitrification. The species Nitrosomonas is primarily responsible for converting ammonia into nitrite, while the species Nitrobacter converts nitrite into nitrate. Both organisms are strict aerobes and are very sensitive to changes in their environment. Rapid changes to pH, temperature, D.O. levels and other factors can result in a large-scale die off. In this sense, the nitrifiers are the “prima donnas” of the wastewater microorganism world. The rate at which nitrification will occur in a wastewater treatment facility is regulated by the numbers of nitrifiers available.
Nitrification can be accomplished in activated sludge systems, trickling filters, RBCs, lagoons and other types of treatment facilities, provided conditions are right. Because it is a strictly aerobic process, sufficient levels of dissolved oxygen must be provided. Typically, a D.O. level of at least 1 – 2 mg/L must be maintained to realize efficient nitrification. However, the D.O. that is available affects the nitrification rate. More D.O. will result in higher levels of nitrification, up to a maximum of about 4.0 mg/L of D.O. In order to completely nitrify each pound of ammonia, 4.6 pounds of dissolved oxygen are required.
The nitrifiers can only carry out efficient nitrification within a pH range of 7.5 – 8.5. Outside of this range, the rate of nitrification slows to generally unacceptable levels. Alkalinity is consumed during nitrification as part of the biological reaction. For each pound of ammonia nitrified into nitrate, 7.2 pounds of alkalinity (as CaCO3) are destroyed. Because of the destruction of alkalinity through the release of hydrogen ions, sustained nitrification causes a drop in pH. In communities that use treated surface water for their potable supply, which are often low in alkalinity, the insufficient alkalinity is sometimes responsible for limiting the rate of nitrification in the wastewater treatment facility. This problem is easy to overlook.