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The interactive and user-friendly PC-computer-based RIVER/FISH mechanistic modeling procedure was evaluated and found to provide credible simulations of aquatic organism bioaccumulation for nonpolar hydrophobic organic chemicals (NPHOCs) using a four-trophic-level food web calculation methodology. Model evaluation included calibration to observations of tetrachlorinated dibenzodioxin and -furan (TCDD/F) bioaccumulation in largemouth bass (Micropterus salmoides) and channel catfish (Ictalurus punctatus) that were exposed in pilot-scale experimental streams receiving biologically treated pulp mill effluent and subsequent comparison of model predictions without bioaccumulation parameter adjustment (validation) to similar but independently obtained bioaccumulation observations. When calibrated with site-specific information on NPHOC loading rates and organism feeding habits, model prediction uncertainty was within U.S. Environmental Protection Agency (EPA) established standards averaging 1.1 pg/g for the observations used for model validation, which all measured < 20 pg/g. The procedure appears suitable for field-scale applications provided the site-specific feeding habits of each organism in the simulated food web can be estimated accurately, all significant chemical loadings are quantified, and the natural variability of bioaccumulation processes is considered when interpreting simulation results.

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.

An open-face, plastic-sheet reactor design was tested to determine wastewater flow characteristics in cross-flow plastic media trickling filters. Flow characteristics were evaluated by routine visual inspection of the open-face reactor during its 40-day operating period and by 35-mm time lapse photography and video tape recording of impulse tracer dye applications. The experimental observations showed that the bulk of the trickling filter flow can be characterized as a vertical laminar liquid film flow over a majority of the biofilm surface that is interrupted at regular intervals by falling liquid drops, originating from irregularities or protrusions in the biofilm surface. A small portion of flow occurs by falling droplets and flow in 50–100-mm length, dispersed shallow channels. The liquid film appears to be mixed at the falling droplet impact location. The distance between these mixing locations, referred to as an uninterrupted flow path length, is determined by the location of biofilm surface protrusions. Although many biofilm surface protrusions are located near the nodes or crossing points of adjacent sheets of the media's surface, they are also found at other locations. Thus, it is not possible to specify the uninterrupted flow path distance from media geometry features alone. In addition, observed flow irregularities resulted in intermittent wetting of some biofilm surfaces with intermittent exposure directly to the reactor gas phase. This condition may have a significant effect on oxygen mass transfer rates to the biofilm.

Field measurements of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) concentrations in fish tissues from riverine environments impacted by treated pulp and paper industry effluent in the United States were analyzed. Data from 39 sites in 14 states across the four major U.S. paper-making regions were assembled and analyzed to determine the annual change in lipid-normalized concentrations of TCDD in fish tissue. The results provide strong evidence of a nationwide trend of decreasing lipid-normalized TCDD concentrations in fish tissue, with 84% of the examined sites showing a decrease. While the paucity of data currently limits any conclusions regarding the statistical significance at individual sites, the overall median rate calculated indicates a 0.36 annual fractional decrease in lipid-normalized concentrations of TCDD in fish tissue (0.18 to 0.51, 95% confidence interval); the average annual fractional decrease was 0.35 (0.23 to 0.47, 95% confidence interval). Subdividing fish into benthic and nonbenthic categories resulted in rates which were not significantly different from one another for both the median and mean statistics.

The purpose of this report is to present the results of a simulation study conducted to evaluate the bias and root mean square error (RMSE) properties of the mean estimator for the D-LOG methodology under analysis conditions typically encoutered during water quality assessments