flat plat experimental design, Huang and Hopson(1974)demonstrated a steady inhibition of nitrification rates occurred as the sbod concentration was increased from 1.0 to 8.0 mg/L. Figueroa and silverstein 1991)found that nitrification rates in fixed-film processes are inhibited at BOD concentrations above 10 mg/L, which finding is in agreement with observations by others Design Basis for Combined BOD Removal and Nitrification. Nitrification efficiency has been correlated with the volumetric BOD loading for rock trickling filters. For 90 percent nitrification efficiency, a BOD loading of less than 0.08 kg bOD/m.d is recommended. At a loading of about 0.22 kg BOD/md, about 50 percent nitrification efficiency could be expected. It was noted that increased recirculation rates improved nitrification performance Instead of volumetric BOD loading values, the nitrification efficiency has been related to the BOd loading based on the packing surface area. In comparing nitrification performance for both rock and plastic packing, Parker and Richards(1986) found that the nitrification efficiency was similar at similar BOD surface loading rates(g BOD/m d) for both packings. A surface loading rate as low as 2. 4 g BOD/m-d is necessary for >90 percent NH4-N removal The DO concentration had a greater effect on the nitrification rates than temperature. The effect of Do concentration is supported by fundamental mass transfer considerations in which it can be shown that a bulk liquid DO concentration of 2.8 mg/L is required for nitrification without oxygen diffusion limitations at a liquid NH4-N concentration of 1.0 mg/L Tertiary Nitrification. A number of facilities exist where trickling filters with plastic packing are used after secondary treatment for nitrification. The influent BOD concentration is relatively low at <10 mg/L and in some cases less than 5 mg/L Nitrification trickling filter performance will depend on the ammonia loading rate, oxygen availability, temperature, and packing design. Effluent NH4-N concentrations will ary with summer and winter operation and can range from 1.0 mg/L at warm temperatures and from <I to 4 mg/L at cold temperatures. Hydraulic application rates may range from 0. 40 to 1.0 L/m-s The higher nitrification rates represent the results obtained at higher hydraulic loading rates, with effluent NHA-N concentrations above 5 mg/L. Other investigators have observed minimal temperature effects for tertiary nitrification, and have attributed the minimal observed rate change more to the effect of dissolved oxygen concentration and hydraulics It is generally well accepted that in the upper portion of the trickling filter the nitrification rate is limited by oxygen availability and diffusion into the biofilm. To overcome the oxygen limitation, forced-draft is generally used to assure maximum oxygen availability. Higher hydraulic rates that provide better wetting efficiency and agitation of the biofilm surface generally produce better performance. Because plugging is less of an issue with the exception of snails, a medium-density packing material is preferred (i. e, specific surface area of about 100 m/m)to provide more area as a function of the percent of the In a large fraction of the nitrification tower, the NH4-N concentration is high enough so that the nitrification rate is oxygen-limited and thus zero-order with respect to nitrogen Farther down in the packing as the NHa-N concentration decreases, the nitrification rate is limited by the NH4-N concentration and thus decreases. The decline in nitrification rate is further affected by a lower growth of nitrifying bacteria due to the low amount of NHa-n available. the use of nitrification trickling filters in series with operational modifications has been shown to compensate for this limitation. The order of operation of the towers is reversed every few days so that a higher nitrifying bacteria population can be developed and be available where the NH4-N concentration is low. Anderson et al. (1994)showed a 20 percent improvement in nitrification efficiency with this method Predator Problems. A significant problem for nitrifying filters is the development of a snail population, which may graze on the biofilm to reduce the nitrifying bacteria population and nitrification performance In addition, snails can cause problems with plugging of channels and pumps, accumulating in digesters, and causing wear and tear on equipment. A sump can be provided in an effluent collection chamber upstream of the secondary clarifiers to facilitate removal of snails from the effluent Methods proposed to control snails are periodic flooding of the trickling filter, reducing the distributor speed to create higher flushing rates, high pH dosing, chlorination, saline water dosing, and dosing with copper sulfate at 0.4 g/L Periodic flooding does not appear to control snails but does eliminate filter flies. Some success has been claimed for reduced distributor speed and high pH treatment. An alkaline backwash to control the snail population and increase nitrification rates was demonstrated successfully at the Littleton/Englewood, CO, wastewater treatment plant. The nitrification tower trickling filters were flooded on three separate occasions, from October to November 1993, after nitrification efficiency had declined due to predator growth. For the first backwash, the pH was 10, but a pH of 9.0 d for the last two backwashes to minimize loss of nitrification activity. After the high-pH backwash, nitrification efficiency improved and8-11 flat plat experimental design, Huang and Hopson (1974) demonstrated a steady inhibition of nitrification rates occurred as the sBOD concentration was increased from 1.0 to 8.0 mg/L. Figueroa and Silverstein (1991) found that nitrification rates in fixed-film processes are inhibited at BOD concentrations above 10 mg/L, which finding is in agreement with observations by others. Design Basis for Combined BOD Removal and Nitrification. Nitrification efficiency has been correlated with the volumetric BOD loading for rock trickling filters. For 90 percent nitrification efficiency, a BOD loading of less than 0.08 kg BOD/m3 .d is recommended. At a loading of about 0.22 kg BOD/m3 .d, about 50 percent nitrification efficiency could be expected. It was noted that increased recirculation rates improved nitrification performance. Instead of volumetric BOD loading values, the nitrification efficiency has been related to the BOD loading based on the packing surface area. In comparing nitrification performance for both rock and plastic packing, Parker and Richards (1986) found that the nitrification efficiency was similar at similar BOD surface loading rates (g BOD/m2 .d) for both packings. A surface loading rate as low as 2.4 g BOD/m2.d is necessary for ≥90 percent NH4-N removal. The DO concentration had a greater effect on the nitrification rates than temperature. The effect of DO concentration is supported by fundamental mass transfer considerations in which it can be shown that a bulk liquid DO concentration of 2.8 mg/L is required for nitrification without oxygen diffusion limitations, at a liquid NH4-N concentration of 1.0 mg/L. Tertiary Nitrification. A number of facilities exist where trickling filters with plastic packing are used after secondary treatment for nitrification. The influent BOD concentration is relatively low at <10 mg/L and in some cases less than 5 mg/L. Nitrification trickling filter performance will depend on the ammonia loading rate, oxygen availability, temperature, and packing design. Effluent NH4-N concentrations will vary with summer and winter operation and can range from < 1.0 mg/L at warm temperatures and from < l to 4 mg/L at cold temperatures. Hydraulic application rates may range from 0.40 to 1.0 L/m2.s. The higher nitrification rates represent the results obtained at higher hydraulic loading rates, with effluent NH4-N concentrations above 5 mg/L. Other investigators have observed minimal temperature effects for tertiary nitrification, and have attributed the minimal observed rate change more to the effect of dissolved oxygen concentration and hydraulics. It is generally well accepted that in the upper portion of the trickling filter the nitrification rate is limited by oxygen availability and diffusion into the biofilm. To overcome the oxygen limitation, forced-draft air is generally used to assure maximum oxygen availability. Higher hydraulic rates that provide better wetting efficiency and agitation of the biofilm surface generally produce better performance. Because plugging is less of an issue with the exception of snails, a medium-density packing material is preferred (i.e., specific surface area of about 100 m2 /m3 ) to provide more area as a function of the percent of the reactor volume. In a large fraction of the nitrification tower, the NH4-N concentration is high enough so that the nitrification rate is oxygen-limited and thus zero-order with respect to nitrogen. Farther down in the packing as the NH4-N concentration decreases, the nitrification rate is limited by the NH4-N concentration and thus decreases. The decline in nitrification rate is further affected by a lower growth of nitrifying bacteria due to the low amount of NH4-N available. The use of nitrification trickling filters in series with operational modifications has been shown to compensate for this limitation. The order of operation of the towers is reversed every few days so that a higher nitrifying bacteria population can be developed and be available where the NH4-N concentration is low. Anderson et al. (1994) showed a 20 percent improvement in nitrification efficiency with this method. Predator Problems. A significant problem for nitrifying filters is the development of a snail population, which may graze on the biofilm to reduce the nitrifying bacteria population and nitrification performance. In addition, snails can cause problems with plugging of channels and pumps, accumulating in digesters, and causing wear and tear on equipment. A sump can be provided in an effluent collection chamber upstream of the secondary clarifiers to facilitate removal of snails from the effluent. Methods proposed to control snails are periodic flooding of the trickling filter, reducing the distributor speed to create higher flushing rates, high pH dosing, chlorination, saline water dosing, and dosing with copper sulfate at 0.4 g/L. Periodic flooding does not appear to control snails but does eliminate filter flies. Some success has been claimed for reduced distributor speed and high pH treatment. An alkaline backwash to control the snail population and increase nitrification rates was demonstrated successfully at the Littleton/Englewood, CO, wastewater treatment plant. The nitrification tower trickling filters were flooded on three separate occasions, from October to November 1993, after nitrification efficiency had declined due to predator growth. For the first backwash, the pH was 10, but a pH of 9.0 was used for the last two backwashes to minimize loss of nitrification activity. After the high-pH backwash, nitrification efficiency improved and