distributed in the trickling filter, the biofilm thickness can vary, the biofilm solids concentration may range from 40 to 100 g/L, and the liquid does not uniformly flow over the entire packing surface area, which is eferred to as the wetting efficiency. With the inability to quantify the biological and hydrodynamic properties of field trickling filter systems, broader parameters such as volumetric organic loading, unit area loadings, and hydraulic application rates have been used as design and operating parameters to relate to treatment efficiency For BOD removal, the volumetric BOD loading has been correlated well with treatment performance for both BOD removal and nitrification in combined BOD and nitrification trickling filter designs. The original design model for rock trickling filters was developed by the National Research Council (NRC)in the early 1940s at military installations. The NRC formulations were based on field data for BOd removal efficiency and the organic loading rate. The NRC design model was used even though there was a significant amount of data scatter. Bruce and Merkens(1970 and 1973)found that the organic loading rate controlled trickling filter perfomance and not the hydraulic application rate. For combined BOd removal and nitrification systems, nitrification efficiency has been related to the volumetric BOD loading For tertiary nitrification applications, very little BOD is applied to the trickling filter and a thin biofilm develops on the packing that consists of a high proportion of nitrifying bacteria. The nitrification removal efficiency is related to the packing surface area and correlated with the specific nitrogen loading rate terms of g NH-N removed/m packing surface area.d BOD Removal Design. The first empirical design equations for BOD removal were developed for rock trickling filters from an analysis of trickling filter performance at 34 plants at military installations treating domestic wastewater. The effect of volumetric bod loading and recirculation ratio on treatment performance was accounted for in the equations. The equations given below should only be used as ar estimate of performance as they are based on a limited data base and the influent BOD values at the installations sampled were relatively high compared to most municipal primary effluents. The BOD removal includes the effect of the secondary clarifier, so that if the equation overpredicts treatment performance, improved and deeper secondary clarifier designs used today may help in meeting expected treatment performance Recirculation. The minimum hydraulic application rate recommended by Dow Chemical is 0.5 L/m2s to provide maximum efficiency. Shallow tower designs require recirculation to provide minimum wetting rates. When above the minimum hydraulic application rate, recirculation was reported to have little benefit. For filters with low hydraulic application rates and higher organic loadings, recirculation may improve efficiency. For design systems such as rock filters with low hydraulic application rates, recirculation provides a higher flow to improve wetting and flushing of the filter packing Solids Production. Solids production from trickling filter processes will depend on the wastewater particulate BOD is degraded, the biomass has a longer SRT, and, as a result, less biomass is produced characteristics and the trickling filter loading At lower organic loading rates, a greater amount of the Mass Transfer Limitations. One of the concerns in the process design for trickling filters is at what organic loading the filter performance becomes limited by oxygen transfer. When this condition occurs, treatment efficiency at the higher organic load is limited and odors may be produced due to anaerobic activity in the biofilm. Based on an evaluation of the data in the literature, for influent BOD concentrations in the range of 400 to 500 mg/L, oxygen transfer may become limiting. Hinton and Stensel (1994)reported that oxygen availability controlled organic substrate removal rates at soluble biodegradable Cod loadings above 3.3 kg/m.d Nitrification Design Two types of process design approaches have been used to accomplish biological nitrification in trickling secondary treatment and clarification for BOD removal. The secondary treatment process may be (o alters, either in a combined system along with BOd removal or in a tertiary application followi suspended growth or fixed-film process. Empirical design approaches based on pilot-plant and full-scale plant results are again used to guide nitrification designs in view of the difficulty in predicting the actual biofilm coverage area, wetting efficiency, and biofilm thickness and density Major impacts on nitrification performance are the influent BOD concentration and dissolved oxygen concentration within the trickling filter bulk liquid. As the bod to tKn ratio of the influent wastewater increases, a greater proportion of the trickling filter packing area is covered by heterotrophic bacteria and the apparent nitrification rate( kg/m d)based on the total trickling filter volume is decreased. a number of investigations have shown that BOD, if at high enough concentration, inhibits nitrification Studies by Harrem es(1982)showed that nitrification(1)could occur at a maximum rate at soluble BOD(SBOD) concentrations below 5 mg/L,(2)was inhibited in proportion to the sBOD concentration above 5 mg/L, and(3)was insignificant, in proportion to the sBOD concentration of 30 mg/L or more. In a study with a 8-108-10 distributed in the trickling filter, the biofilm thickness can vary, the biofilm solids concentration may range from 40 to 100 g/L, and the liquid does not uniformly flow over the entire packing surface area, which is referred to as the wetting efficiency. With the inability to quantify the biological and hydrodynamic properties of field trickling filter systems, broader parameters such as volumetric organic loading, unit area loadings, and hydraulic application rates have been used as design and operating parameters to relate to treatment efficiency. For BOD removal, the volumetric BOD loading has been correlated well with treatment performance for both BOD removal and nitrification in combined BOD and nitrification trickling filter designs. The original design model for rock trickling filters was developed by the National Research Council (NRC) in the early 1940s at military installations. The NRC formulations were based on field data for BOD removal efficiency and the organic loading rate. The NRC design model was used even though there was a significant amount of data scatter. Bruce and Merkens (1970 and 1973) found that the organic loading rate controlled trickling filter performance and not the hydraulic application rate. For combined BOD removal and nitrification systems, nitrification efficiency has been related to the volumetric BOD loading. For tertiary nitrification applications, very little BOD is applied to the trickling filter and a thin biofilm develops on the packing that consists of a high proportion of nitrifying bacteria. The nitrification removal efficiency is related to the packing surface area and correlated with the specific nitrogen loading rate in terms of g NH4-N removed/m2 packing surface area·d. BOD Removal Design. The first empirical design equations for BOD removal were developed for rock trickling filters from an analysis of trickling filter performance at 34 plants at military installations treating domestic wastewater. The effect of volumetric BOD loading and recirculation ratio on treatment performance was accounted for in the equations. The equations given below should only be used as an estimate of performance as they are based on a limited data base and the influent BOD values at the installations sampled were relatively high compared to most municipal primary effluents. The BOD removal includes the effect of the secondary clarifier, so that if the equation overpredicts treatment performance, improved and deeper secondary clarifier designs used today may help in meeting expected treatment performance. Recirculatlon. The minimum hydraulic application rate recommended by Dow Chemical is 0.5 L/m2 .s to provide maximum efficiency. Shallow tower designs require recirculation to provide minimum wetting rates. When above the minimum hydraulic application rate, recirculation was reported to have little benefit. For filters with low hydraulic application rates and higher organic loadings, recirculation may improve efficiency. For design systems such as rock filters with low hydraulic application rates, recirculation provides a higher flow to improve wetting and flushing of the filter packing. Solids Production. Solids production from trickling filter processes will depend on the wastewater characteristics and the trickling filter loading. At lower organic loading rates, a greater amount of the particulate BOD is degraded, the biomass has a longer SRT, and, as a result, less biomass is produced. Mass Transfer Limitations. One of the concerns in the process design for trickling filters is at what organic loading the filter performance becomes limited by oxygen transfer. When this condition occurs, treatment efficiency at the higher organic load is limited and odors may be produced due to anaerobic activity in the biofilm. Based on an evaluation of the data in the literature, for influent BOD concentrations in the range of 400 to 500 mg/L, oxygen transfer may become limiting. Hinton and Stensel (1994) reported that oxygen availability controlled organic substrate removal rates at soluble biodegradable COD loadings above 3.3 kg/m3 .d. Nitrification Design Two types of process design approaches have been used to accomplish biological nitrification in trickling filters, either in a combined system along with BOD removal or in a tertiary application following secondary treatment and clarification for BOD removal. The secondary treatment process may be a suspended growth or fixed-film process. Empirical design approaches based on pilot-plant and full-scale plant results are again used to guide nitrification designs in view of the difficulty in predicting the actual biofilm coverage area, wetting efficiency, and biofilm thickness and density. Major impacts on nitrification performance are the influent BOD concentration and dissolved oxygen concentration within the trickling filter bulk liquid. As the BOD to TKN ratio of the influent wastewater increases, a greater proportion of the trickling filter packing area is covered by heterotrophic bacteria and the apparent nitrification rate (kg/m3 .d) based on the total trickling filter volume is decreased. A number of investigations have shown that BOD, if at high enough concentration, inhibits nitrification. Studies by Harrem es (1982) showed that nitrification (1) could occur at a maximum rate at soluble BOD (sBOD) concentrations below 5 mg/L, (2) was inhibited in proportion to the sBOD concentration above 5 mg/L, and (3) was insignificant, in proportion to the sBOD concentration of 30 mg/L or more. In a study with a