Nitrogen removal occurs in facultative wastewater stabilization lagoons, and it can be reliably predicted for design purposes with either of the two models presented above. Nitrogen removal in lagoons may be more cost effective than other alternatives for removal or ammonia conversion. Nitrogen removal in lagoons used as a component in land treatment systems can influence the cost effectiveness of the project.
At a pH value of 8.0, approximately 95% of the ammonia nitrogen is in the form of ammonium ion; therefore, in biological systems, such as aerated lagoons where the pH values are usually less than 8.0, the majority of the ammonia nitrogen is in the form of ammonium ion. Total Kjeldahl nitrogen (TKN) is composed of the ammonia nitrogen and organic nitrogen. Organic nitrogen is a potential source of ammonia nitrogen because of the deamination reactions during the metabolism of organic matter in wastewater. Ammonia and TKN reduction in aerated lagoons can occur through several processes:
• Gaseous ammonia stripping to the atmosphere
• Ammonia assimilation in biomass
• Biological nitrification
• Biological denitrification
• Sedimentation of insoluble organic nitrogen
The rate of gaseous ammonia losses to the atmosphere depends primarily on the pH value, temperature, hydraulic loading rate, and mixing conditions in the lagoon. The equilibrium equation NH3 + H2O O NH+ + OH- is shifted toward gaseous ammonia by an alkaline pH value, while the mixing conditions affect the magnitude of the mass-transfer coefficient. Temperature affects both the equilibrium constant and mass-transfer coefficient.
Ammonia nitrogen assimilation into the biomass depends on the biological activity in the system and is affected by several factors such as temperature, organic load, detention time, and wastewater characteristics. Biological nitrification depends on adequate environmental conditions for nitrifiers to grow and is affected by several factors such as temperature, dissolved oxygen concentration, pH value, detention time, and wastewater characteristics.
Denitrification can take place in bottom sediments under anoxic conditions, and temperature, redox potential, and sediment characteristics affect the rate of denitrification. In well-designed aerated lagoons with good mixing conditions and distribution of dissolved oxygen, denitrification will be negligible.
The U.S. Environmental Protection Agency sponsored comprehensive studies of aerated wastewater lagoon systems between 1978 and 1980 that provided information about nitrogen removal in aerated lagoon systems (Earnest et al., 1978; Englande, 1980; Gurnham et al., 1979; Polkowski, 1979; Reid and Streebin, 1979; Russel et al., 1980). Table 4.20 and Table 4.21 summarize the key findings from those studies. These results verify the consensus of previous investigators that nitrogen removal was in some way related to pH, detention time, and temperature in the lagoon system.
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