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The best results in practice are achieved by use of countercurrent packed towers; see 0degaard (1988). The water is distributed on the top of the packing with distribution trays or spray nozzles. For a high air to water ratio, a mist eliminator is necessary at the air outlet. Random packing of Raschig rings or saddles or grids, made of metal, ceramic, plastic or even impregnated wood, can be used.

Stripping ponds, see Fig. 7.15, might be used to remove 30-50% ammonia, but higher efficiencies can hardly be expected, even by introduction of agitation of the pond surface. It might, however, be practical to install stripping ponds as supplement to stripping tower to account for peak loadings.

Figures 7.16 and 7.17, taken from Fetting (1989), are constructed to facilitate the design in practice.

1. The operating temperature is selected for determination of Henry's constant; see equations 7.11 and 7.12.

2. The, minimum ratio air to water, A/W can be derived from a simple mass balance

where p is the total pressure, ef is the required efficiency, i.e., the ratio between the concentration of ammonia in the effluent and in the influent. It can be recommended to multiply the minimum value of A/W by 1.2 -2.3 in practice.

Figure 7.15. Ammonia stripping pond system. (Drawn by Morten V. Jorgensen). 3. The stripping factor R is found, based upon the selected A/W ratio, s:

4. Figure 7.16 gives the number of transfer units, when R and the fraction removed are known. Figure 7.16. is valid for countercurrent operation, while Fig. 7.17 is constructed for single-stage cross-flow operation. Note that this latter figure uses the inverse stripping factor and the fraction remaining.

Due to the growing concern over air pollution problems, including the dry and wet deposition of nitrogen components as an increasing source of nutrients to

Figure 7.15. Ammonia stripping pond system. (Drawn by Morten V. Jorgensen). 3. The stripping factor R is found, based upon the selected A/W ratio, s:

fresh and marine waters, it is necessary in most cases to combine the stripping unit with an absorption unit. The removed ammonia is absorbed in sulfuric acid for production of ammonium sulfate, which can be used as fertilizer. Figure 7.18 shows a flow chart of the combination of stripping and absorption.

Ammonia Stripping Chart

3 4 5 6 8 10 Number of transfer units

30 40 50

Figure 7.16. Number of transfer units for counter current operation as a function of removal efficiency and stripping factor, R. Reproduced from Fetting (1989).

3 4 5 6 8 10 Number of transfer units

30 40 50

Figure 7.16. Number of transfer units for counter current operation as a function of removal efficiency and stripping factor, R. Reproduced from Fetting (1989).

High efficiency in ammonia removal requires adjustment of pH to about 11.0 before the stripping process. It implies that the pH after the stripping must be readjusted. The pH might drop about 0.2 by the stripping process due to removal of ammonia, but a pH of 6-8 is required for the effluent.

Ammonia Stripping Chart
Figure 7.17. Number of transfer units for a single-stage cross-flow operation as a function of the concentration of ammonia remaining in water and of stripping factor, R. Reproduced from Fetting (1989).

The readjustment of pH can be carried out by recarbonization. Carbon dioxide is easily obtained from incineration of bio-gas, sludge or solid waste. Sulfuric acid might also be applied, but it is a less cost-effective alternative, which can only be recommended if there is no easy access to carbon dioxide.

Figure 7.18. Process for stripping and recovery of ammonia.

7.6. Application of stripping

The stripping process is used to remove volatile gases such as hydrogen sulfide, hydrogen cyanide as well as ammonia. The removal of ammonia by stripping is used in the treatment of municipal waste water, where it has found very little application due to the problems mentioned in Section 7.2. Generally it can be concluded that the method is not economic in a temperate climate for large flows of waste water with relatively small concentrations of ammonia, as is found in municipal waste water. An additional problem is the air pollution caused by the removed ammonia, see Section 7.1. A recovery of ammonia by absorption in acid is possible, but the value of the recovered ammonia as ammonium sulfate is less than the costs of the recovery process.

The process has, however, found application at two well-known waste water treatment plants: at Lake Tahoe and in Pretoria. The flow chart of the latter plant is shown in Fig. 7.19. The main problem behind this solution is, however, not a pollution problem, but the scarcity of water.

If the concentration of ammonia is higher and the volume of waste water to be treated smaller, the process becomes more favorable. This is for instance the case for the reject water, produced by dewatering of municipal sludge. The concentration here is 2-5 times higher than in municipal waste water and the process has therefore found some application for the treatment of this water particularly where the treatment plant is too small to handle the reject water in addition to the waste water.

Stripping has also been suggested for the treatment of industrial waste water and for the regeneration of the liquid used for eluting ion exchangers (Jorgensen, 1975). In these cases ammonia is removed from relatively small volumes and is present in high concentrations. As the amount of air needed is roughly independent of the ammonia concentration, see equation (7.41), the cost per kg of ammonia removed is much lower at high ammonia concentrations. The method therefore becomes much more attractive for industrial waste water with high ammonium concentrations or for recovery of elution liquids, used for regeneration of ion exchangers. Up to now stripping has not been used widely for treatment of industrial waste water, but with the growing demand for nitrogen removal, it is anticipated that the application of the method will increase in the coming decade.

Typical concentrations in waste water originating from production of ammonia, meat-bone-meal or fish meal are in the order of 500-1000 mg/l or 10-25 times higher than for municipal waste water. Elution liquids after regeneration of ion exchange columns may contain even higher ammonia concentrations and have already a high pH ( see also Chapter 8).

Figure 7.19. Waste water treatment plant, Pretoria. After mechanical-biological treatment (not shown) there follows 1) an algae pond, 2) aeration 3) lime precipitation 4) sludge drying 5) air stripping of ammonia 6) recarbonization 7) sand filtration 8) chlorination 9) adsorption on activated carbon 10) a second chlorination.

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