Nitriication & Denitriication 111 Huntington Street • New London, Connecticut 06320 • Ofice: 860.444.0866 • Fax: 860. 444.0896 NitrificatioN & DeNitrificatioN Bacteria remove nitrogen from wastewater by a two step biological processes: nitriication followed by denitriication. Technically, it is a three step process: ammoniication precedes nitriication and denitriication. ammoNificatioN. While traveling through sewer pipes, the majority of the nitrogen contained in raw sewage (urea and fecal material) is converted from organic-nitrogen to ammonia through a process called hydrolysis. Technically, in the majority of situations, more ammonium than ammonia is created during ammoniication. The actual ratio is inluenced by pH and temperature. NitrificatioN. The biological conversion of ammonium to nitrate nitrogen is called Nitriication. Nitriication is a two-step process. Bacteria known as Nitrosomonas convert ammonia and ammonium to nitrite. Next, bacteria called Nitrobacter inish the conversion of nitrite to nitrate. The reactions are generally coupled and proceed rapidly to the nitrate form; therefore, nitrite levels at any given time are usually low. These bacteria known as “nitriiers” are strict “aerobes,” meaning they must have free dissolved oxygen to perform their work. Nitriication occurs only under aerobic conditions at dissolved oxygen levels of 1.0 mg/L or more. At dissolved oxygen (DO) concentrations less than 0.5 mg/L, the growth rate is minimal. Nitriication requires a long retention time, a low food to microorganism ratio (F:M), a high mean cell residence time (measured as MCRT or Sludge Age), and adequate buffering (alkalinity). A plug-low, extended aeration tank is ideal. Temperature, as discussed below, is also important, but not really. The nitriication process produces acid. This acid formation lowers the pH of the biological population in the aeration tank and can cause a reduction of the growth rate of nitrifying bacteria. The optimum pH for Nitrosomonas and Nitrobacter is between 7.5 and 8.5; most treatment plants are able to effectively nitrify with a pH of 6.5 to 7.0. Nitriication stops at a pH below 6.0. The nitriication reaction (that is, the conversion of ammonia to nitrate) consumes 7.1 mg/L of alkalinity as CaCO 3 for each mg/L of ammonia nitrogen oxidized. An alkalinity of no less than 50-100 mg/L is required to insure adequate buffering. Water temperature also affects the rate of nitriication. Nitriication reaches a maximum rate at temperatures between 30 and 35 degrees C (86 o F and 95 o F). At temperatures of 40 o C (104 o F) and higher, nitriication rates fall to near zero. At temperatures below 20 degrees C, nitriication proceeds at a slower rate, but will continue at temperatures of 10 degrees C and less. However, if nitriication is lost, it will not resume until the temperature increases to well over 10 o C. Some of the most toxic compounds to nitriiers include cyanide, thiourea, phenol and heavy metals such as silver, mercury, nickel, chromium, copper and zinc. Nitrifying bacteria can also be inhibited by nitrous acid and free ammonia.