Biological removal of nitrogen compounds from wastewaters: Conventional and non-conventional processes

Abstract

The disposal of domestic and industrial wastewaters containing large concentrations of nutrients, such as nitrogen and phosphor, is one of the main factors responsible for eutrophication of water bodies. This process results in disorderly growth of aquatic plants on the water surface causing reduction in the concentration of dissolved oxygen in these bodies and impairing maintenance of aerobic aquatic life. It is therefore very important to develop treatments that remove these nutrients, in addition to removing organic material and solids. There are several methods for removing nitrogen from wastewaters. However, the methods involving lower costs and improved efficiency in removing nitrogen and carbonaceous material are those that utilize the biological reactions of the nitrogen cycle as well as of the environmental and physicochemical conditions regulating this process. Aerobic nitrification and heterotrophic denitrification processes are the most commonly used effective and economical methods for nitrogen removal from domestic and industrial wastewaters. Nitrification is characterized by the oxidation of ammonium nitrogen to nitrite by Nitrosomonas bacteria, followed by the oxidation of nitrite to nitrate by Nitrobacter bacteria. Denitrification is the biologic conversion of nitrate to more reduced forms (like N2, N2O and NO) under anoxic conditions, i.e., in the absence of oxygen and having a carbon source as electron donor. Many microorganisms participate in the anoxic process. These bacteria can use several energy sources, such as organic compounds (organotrophic), inorganic compounds (lithotrophic) and light (phototrophic). Usually, when effluents from secondary treatment systems contain low concentrations of organic material, addition of an electron donor is required for denitrification to occur. The most commonly used electron donors include methanol, ethanol, acetate and methane. Most recently inorganic compounds, such as sulfide, have been used as electron donors in autotrophic denitrification processes. Two types of bacteria have been identified as being responsible for autotrophic denitrification using sulfur compounds as electron donors: Thiobacillus denitrificans and Thiomicrospira denitrificans. These bacteria are chemolithotrophic and in anoxic environments oxidize reduced sulfur compounds (like sulfide, sulfite, thiosulfate and elemental sulfur) to sulfate using nitrate (or nitrite) as electron receptor. This alternative is of great interest as it allows simultaneous elimination of highly polluting nitrogen and sulfurous compounds. This text deals with the removal of nitrogen from domestic and industrial wastewaters using conventional (autotrophic nitrification and heterotrophic denitrification) and non-conventional biological processes (autotrophic denitrification). With regard to the conventional processes the removal efficiency and stability of organic material and ammonium nitrogen are dealt with by assessing the effect of the following parameters: applied volumetric nitrogen load, feed strategy (continuous, batch or fedbatch), C/N ratio, reactor type, type of biomass immobilization (granular or on inert support) and carbon source used as electron donor in the denitrification process. With respect to the non-conventional processes the utilization potential of sulfur compounds as electron donors in the denitrification process is dealt with by assessing factors such as N/S ratio, effect of type of electron acceptor (nitrite or nitrate), influent organic material concentration and effluent sulfate concentration. The approach of these experimental studies, albeit privileging fundamental kinetics, bioreactor calculations and mass transfer, always maintain the objective to provide information for full-scale use of the investigated technological configurations. © 2012 by Nova Science Publishers, Inc. All rights reserved.