Ammonia stripping

In addition to the volatile organic compounds found in water, ammonia is an inorganic compound that may occur in drinking water supplies and is frequently present in waste-waters. Ammonia nitrogen exists in both the dissolved gas form (NH3) and in true solution (NHp. These two species are present in a dynamic equilibrium according to the equation  

In addition to converting all the ammonia to the dissolved gas phase, efflcient ammonia stripping requires proper conditions to facilitate a rapid transfer of the dissolved gas from the liquid phase to the air. This can be achieved by the creation of numerous small water droplets in conjunction with an ample supply of air flow. This is because the surface tension at the air-water interface is at a minimum when the water droplets or surface films are being formed, and transfer of dissolved gas from the liquid phase to the air flow is at a maximum at this instant. An ammonia stripping tower, similar to a conventional cooling tower, is normally used for ammonia removal (12). 

The efficiency of an ammonia stripping operation depends primarily on five factors  

Theoretically, the rate of ammonia stripping should be proportional to the difference in the partial pressures of ammonia between the liquid and the gaseous phases. Because a decrease of every 10°C in the process water would result in about a 40% decrease in the 

Rate of Gas Transfer. In order to remove ammonia from water, the dissolved NH3 molecules must first move from the bulk liquid solution to the air-water interface, and then from the interface to the stripping air flow. Therefore, there are two factors that affect the rate of ammonia gas transfer from the liquid to the surrounding atmosphere. 

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Various treatment technologies are used at the iron and steel plant for recycle system water treatment prior to recycle and reuse, or end-of-pipe wastewater treatment prior to discharge to surface water or a POTW. The physical/chemical treatment technologies extensively used include equalization, tar removal, free and fixed ammonia stripping, cooling technologies, cyanide treatment technologies,... 

Ammonia stripping also removes cyanide, phenols, and other VOCs typically found in cokemaking wastewater. Phenols may also be removed by conversion into nonodorous compounds or into crude phenol or sodium phenolate by either biological means (phenol concentration <25 mg/L) or by physical processes.21 However, the Koppers dephenolization process is considered to be quite effective as it lowers the phenol content by 80 to 90% in ammonia still wastes. In this process a stream stripping process followed by mixing in a solution of caustic soda results in renewal of pure phenol with the flue gas.8... 

Fisher, B. C., Blackstock, T. Hauke, D. 1997. Fly ash beneficiation using an ammonia stripping process. Proceedings 12th International Symposium on Coal Combustion By-Product (CCB) Management and Use, Vol. 2, Paper 65, pp. 1-8. 

Design of Cross-Flow Cooling Towers and Ammonia Stripping Towers Wnek, Walter J. Snow, Richard H. 

Application To produce urea from ammonia and carbon dioxide using ammonia-stripping process. 

Let G be the moles per unit time of ammonia-free air, L be the moles per unit time of ammonia-free water, [Fj] be the moles of ammonia per mole of ammonia-free air at the bottom, be the moles of ammonia per mole of ammonia-free air at the top, [All be the moles of ammonia per mole ammonia-free water at the bottom, and [X2] be the moles of ammonia per mole of ammonia-free water at the top. At any section between the bottom and the top of the column, the [X]s and [y]s are simply [A] and [F]. By mass balance, the ammonia stripped from the wastewater is... 

This is the equation of the equilibrium line for ammonia stripping or absorption. It may be expressed in terms of the respective mole fractions [y and [x ]. Performing the operation and solving for [y f] produces... 

At a given temperature and atmospheric pressure, the molar ratio of ammonia saturated in the outlet air and in the inlet water can be assumed to remain constant according to Hemy s law, which can thus be used to determine the respective moles of ammonia in a mole of anas a function of the moles of ammonia in a mole of water. Tchobanoglous (14) has prepared a set of curves showing the equilibrium distribution of ammonia in air and water at various temperatures under the condition of atmospheric pressure (Fig. 6). Using Eq. (15) and Fig. 6, the theoretical requirement of air for the ammonia-stripping operation at 100% efficiency can be calculated. For example, at a water temperature of 20°C and an influent ammonia concentration of 20 mg/L, the theoretical air requirement is calculated as follows ... 

Smith (15) has presented a practical ammonia-stripping tower design based on the concept of the height of transfer unit vs the gas/liquid ratio for a given type of tower packing. He has provided a sample design problem. 

Fig. 7. Configuration of crosscurrent and countercurrent ammonia-stripping towers.

Determination of Packing Characteristics The packing characteristics are normally obtained from the tower manufacturer. Figure 8 illustrates the relationship between the height of a transfer unit for ammonia stripping and the GIL ratio for one type of packing... 

Tower Height / Height of Transfer Unit, Z/HTU Fig. 9. Example of design relationship of Z HTU vs X/Xj for ammonia stripping tower (15). 

The design data shown in Table 1 are to be used to design a crosscurrent ammonia-stripping tower. The wastewater to be treated is municipal sewage that has been treated by activated sludge followed by lime clarification. A 90% ammonia removal efficiency is desired. The packing is assumed to have the characteristics shown in Fig. 8. 

Although the ammonia-stripping operation is both reliable and flexible in its routine operation, it has some shortcomings which must be carefully considered by the design engineer. Some major problems are ... 

Problems associated with calcium carbonate scale formation. Because ammonia stripping is operated at a high pH (11 or above), calcium deposits on the packing materials can be expected. If these deposits are not periodically dislodged and broken up by the hydraulic force, excessive accumulation can restrict the flow of both the wastewater and the air, thereby reducing the effectiveness of the system. 

There has also been concern over the potential air pollution problem that can be caused by the ammonia present in the off-gas. However, it has been reported (12) that the ammonia concentration in the off-gas from a stripping tower seldom exceeds 10 mg/m even before its dispersion in the surrounding air. The threshold for odor is about 35 mg/m therefore, there is little likelihood for the ammonia-stripping operation to cause an odor problem. However, small concentrations of ammonia in air may react with sulfur dioxide to form aerosols or fog. Under such a situation, ammonia can be removed from the offgas by a scrubber of by bubbling it through a dilute sulfuric acid solution. 

J. Roesler, R. Smith, and R. Eilers, Mathematical Simulation of Ammonia Stripping Towers for Wastewater Treatment. EWQA Publication (1970). 

R. Smith, Ammonia Stripping, In Process Design in Water Quality Engineering (E. Thackston and W. Eckenfelder, eds.), E. Jenkins Publ. Co., Austin, TX, 1972. 

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