E = media effectiveness factor. The value of E depends on the media used, see Table 5.8
j02 max(T) = maximum surface oxygen rate for specific media design in g N/m2 * d.
The factor 4.3 in equation (5.22) reflects the unit mass of oxygen consumption per unit mass of ammonia nitrogen oxidized.
Where recirculation is used, a repetitive solution of the above equation is necessary because recycle effects are included in both the Sn, and Vn terms. The effect of the media on the nitrification rate is not considered in this modelling approach.
Most trickling filters are used in single stage removal of organics. If the organic loading is lowered to about 0.16 kg BOD / m3 * d, combined oxidation of organics and nitrification will occur, whereby a part of the influent ammonium will be nitrified (see Tables 5.7 and 5.8). But single stage nitrifying trickling filters are also becoming popular in treating secondary or tertiary influents, because the recent efforts in improving these filters have made the effluent produced of a better quality, so the NTF is comparable with the activated sludge processes in regard to nitrification efficiency and amount of the suspended solids in the effluent.
The concentration of ammonium-nitrogen must be less than 25 mg/l to obtain the best results in a conventional nitrifying trickling filter. The trickling filters are, therefore, often used to treat municipal waste water, where BOD removal has already been accomplished. Experiments have been made with the new compact plastic media trickling filters in the treatment of industrial waste water of higher nitrogen concentration.
Despite much interest in trickling filters, relatively little research has been made on the simultaneous organic removal and nitrification taking place in a single trickling filter unit.
The EPA (1975) showed that for rock media trickling filters, organic loading must be limited to 0.16 kg BOD / m3 * d to attain 75% conversion of ammonium to nitrate. Nitrification decreased at a higher organic load. At an organic loading of 0.64 kg BOD / m3 * d, nitrification of only 10 % of ammonium was obtained. This reduction in nitrification was attributed to the domination by heterotrophic bacteria of the microbial biofilm.
The difference between rock and plastic media in loading capacity, as shown in Table 5.5, was attributed to the higher specific surface area of the plastic, whereby less competition between the species of bacteria was necessary. Wanner and Gujer (1984) showed the concentration of ammonium versus different COD concentrations for a trickling filter. They predicted that most of the organic removal occurred in the upper reaches of the trickling filter, where heterotrophic organisms dominated, and nitrifiers were absent. Nitrification occurred at the highest rates in the bottom portions of the tower where concentration of organics was the lowest, and the autotrophic population could dominate.
Nitrification only occurred in the bottom half of the reactor. The most significant nitrification occurred in the bottom 1.2 m of the filter. Most combined trickling filters do not produce nitrate before the soluble BOD concentration is less than about 20 mg/l. Figure 5.8 show the relationship between nitrification and soluble BOD5 levels exposed to the biofilm for cross-flow media.
Nitrification in a nitrifying trickling filter (NTF).
The NTF is designed to oxidize ammonia in secondary effluents, where most of the BOD is already removed, so that the NTF can concentrate on the removal of ammonium-nitrogen. The first demonstration of the system was a pilot scheme in
Michigan (Duddles et al. 1974).
Typical removal rates, for conventially loaded NTF filters are as low as 0.20 to 0.39 g N/ m2 * d as indicated by investigations in the US. Ammonia removal efficiency for rock and plastic filters at various sites in the US, applying different amounts of organic loading per unit of surface in kg BOD /1000 m2 • day is shown in Fig. 5.7.
Removal of ammonia NH3, %
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