The goal of nitrogen removal, regardless of what exactly forms of nitrogen compounds in wastewater streams, has been the production of nitrogen gas, an inert, water-insoluble gas that is easily separated from liquid media. The necessity of producing nitrogen gas in the treatment processes of nitrogen removal is mainly due to high solubility of nitrogen compounds such as NO3~, NH4+, and NO2~ present in the nitrogen removal. There is some indication that this old paradigm is being challenged. Because nitrogen gas, as in wastewater treatment operations, does not have economical value, some researchers are seeking to remove nitrogen compounds in dissolved forms (Aiyuk et al., 2004). The most promising method of removing dissolved forms of nitrogen compounds is the application of adsorption-employing zeolite columns in an integrated waste-water treatment process. The recovered nitrogen compounds can be used as fertilizers. However, due to high costs of zeolite columns, the most economical way of removing nitrogen compounds from wastewater streams now is the conventional biological processes consisting of nitrification and denitrification processes.
The biology of nitrification-denitrification has been briefly discussed in Chapter 2; in a nutshell, biological nitrogen removal from wastewater converts organic nitrogenous compounds to ammonia, then to nitrate (nitrite), and finally to gaseous nitrogen, as illustrated in the nitrogen cycle diagram in Figure 5.1. Organic nitrogen materials in food and agricultural wastewater streams are either in the forms of proteins, nucleic acids and urea, or as ammonium ion (NH4+). Normally, domestic and most industrial wastewater streams rarely contain nitrate. Nitrate in some agricultural wastewater streams may come from the fields in which excessive amounts of nitrogen-rich man-made fertilizers have been applied.
Nitrification is a microbial process by which reduced nitrogen compounds (primarily ammonia) are sequentially oxidized to nitrite and nitrate and is the first step in the removal of nitrogen from wastewater streams by the nitrification-denitrification process. It is primarily accomplished by two groups of autotrophic nitrifying bacteria, Nitro-somonas and Nitrobacter, that can build organic molecules using energy obtained from inorganic sources, in this case ammonia or nitrite. In the first step of nitrification, ammonia-oxidizing bacteria oxidize ammonia to nitrite. Nitrosomonas oxides ammonia to the intermediate product, nitrite, and nitrite is further converted into nitrate by Nitrobacter. Overall equations for nitrite production and nitrate formation by these two categories of nitrifying bacteria are represented in Chapter 2, Equations 2.9 and 2.10.
A number of environmental factors influence the nitrification process: substrate concentration, temperature, oxygen, pH, and toxic or inhibiting substances. Nitrifying bacteria are susceptible to a number of inhibitors, both organic and inorganic agents. They are also sensitive to pH value and a range of 7.5 to 8.6 is found to be optimal for the growth of nitrifying bacteria. There is also a dissolved oxygen level that could limit the nitrification process; a concentration of above 1 mg/l is essential for nitrification to occur.
Temperature has a strong effect on nitrifying bacteria just as in the case of heterotrophic aerobic bacteria. The temperature dependence for the nitrification process fits an Arrhenius type of equation, at least lower than 30°C. At higher temperatures (30-35°C), the growth rate of nitrifying bacteria is constant, and it starts declining between 35 and 40°C (Henze et al., 2001).
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