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The paper presents a model and calculated results for oxygen transfer in trickling filters after first discussing the inadequacies of the Metha et al. (1972) oxygen-transfer model. While we do not disagree with the author's contention that the Metha et al. model provides an inaccurate mechanistic description of oxygen transfer in trickling filters, the prominence of its discussion in the paper diverts attention from the critical issue in the paper: the appropriate physical mechanisms, such as intermittent liquid-film mixing, direct gas-biofilm contacting, and biological kinetics that should form the basis for a trickling-filter oxygen-transfer model. The Metha et al. model does not represent an accepted state-of-the-art approach for design or for understanding the capacity and limitations of trickling-filter oxygen transfer during high-organic-Ioading conditions. Its relevance today is its indication that the importance of oxygen transfer in trickling filters has long been a concern of the design profession. Trickling-filter performance depends upon a complex set of interactions between: (I) Mass transferfrom the gas-phase; (2) the mass transfer within the free-surface liquid film flow; and (3) the mass transfer and biological reactions occurring within the biofilm. Integration of controlled experimental data with computer-model simulations is necessary if a mechanistic model is to be generally used to predict the performance of specific design configurations or general process options. The paper makes many predictions concerning oxygen transfer for different media designs and loading conditions using a mechanistic model that contains a unique biological kinetic expression, fixed influent-wastewater characteristics, and key hydraulic assumptions that have not been proven by experimental data. To place this paper's results in perspective, this discussion attempts to highlight four areas where clarification and/or elaboration are needed; these include model specification completeness, experimental data comparison, hydraulic assumption inconsistencies, and example calculation omissions. Without complete specification of all model equations used to calculate the maximum oxygen-transfer rates, the conclusions drawn from the reported model predictions should be viewed with skepticism. Apparently, oxygen transport is calculated by solving an advective/diffusive mass-balance equation [(5)] for a section of the liquid flow path that is uninterrupted; the results for many sections are combined to represent a media bundle. This requires boundary-condition specifications for the entering fluid's oxygen concentration [( 10)], the fluid's free-surface oxygen concentration [( 11)] and the flux of oxygen moving from the fluid into the biofilm [(l2a) or 12b)]. Although the meanings of (H), (11), and (12a) are clear and precisely defined, the paper fails to specify the equations, assumptions, and/or methodology used to calculate the maximum oxygen flux into the biofilm [12(b)]. The latter should be affected by biomass density, biological organism volumetric substrate-utilization rate, oxygen-to-substrate-consumption ratio, the diffusivities for oxygen and substrate, and the oxygen concentration at the Iiquid-biofilm interface (Strand 1986). The statement at the bottom of page

In-situ oxygen consumption rate (OCR) observations for trickling filter biofilms under different hydraulic, substrate loading, and oxygen availability operating conditions are presented. OCRs were determined for sucrose- and dextrin-based synthetic feedstocks by growing biofilms on a 1.2-m deep section of 60° cross-flow media that was enclosed within a reactor vessel, sealing the reactor vessel to outside air entry, and observing the time rate of decrease in the reactor’s oxygen partial pressure. The results indicate that OCRs increased proportionately with increasing influent substrate concentrations for the 40 to 120 mg/L SCOD range, increased slightly or remained relatively constant with increasing influent substrate concentrations for the 120 to 200 mg/L SCOD range, increased slightly with increasing DO saturation condition for the 7.4 to 10.1 mg/L range, were affected by oxygen availability for influent substrate concentrations throughout the 70 to 175 mg/L SCOD range, increased with increasing hydraulic application rate from 2.4 to 4.9 m³/m²-h, and were slightly greater than predicted by currently acknowledged models. Other observations include the following measured OCRs were at least three times greater than those estimated from clean media oxygen-transfer testing; without liquid and feedstock application, biofilms consumed oxygen at 80–104% of their usual rate; and maximum biofilm oxygen consumption was ≈680 mg/m²-h.