Journal of Environmental Engineering

A mathematical approach to the analysis of chemical reactions that involve reactants or end products that exchange with the atmosphere is presented. Both variable and constant pH conditions are addressed. The equations are applied to laboratory and field derived data sets with satisfactory results.

This paper is based on a Simon W. Freese lecture given at the ASCE North American Water and Environment Congress '96, Anaheim, Calif., on June 24, 1996. The role of water quality modeling in providing input to the decision-making process through understanding, dialogue, and consensus is discussed. The evolution of models is seen in three stages. During the first stage (1925 to about 1980), all sources (point, nonpoint, and sediment) were external to the model, but only point sources were directly linked to the originating input. During the second stage (about 1980 to 1990), sediment models were coupled to the water column and hydrodynamic and watershed models were linked. A link was then established from watershed models to the input of the watershed. During the third stage (currently under way), airshed models are being incorporated with expansion to include other aspects of aquatic ecosystem. The Chesapeake Bay is used as an illustration. Issues of model credibility and confirmation are discussed; ultimately, the scientific and engineering community decides on the suitability of a modeling framework. The growth in model size over the history of modeling has been significant and parallels the increase in computing power. Future modeling challenges lie ahead in the areas of watershed models, airshed-watershed-estuarine-coastal ocean models, and whole ecosystem, living resources models. The success of water quality models will not necessarily be due to “bigness” and complexity but rather to increases in understanding, which can contribute to building consensus in water quality management decision-making.

Numerous equations employing depth, velocity, and slope have been developed to estimate the stream reaeration coefficient. These have been evaluated previously using statistics based on differential errors, which are shown to be biased toward underprediction. A new metric, the mean multiplicative error (MME), overcomes this defect and offers other advantages, including identical results for both reaeration and gas transfer coefficients and less sensitivity to extreme errors. It is equal to the geometric mean of the factors, greater than unity, by which the estimates would have to be multiplied or divided to equal the corresponding measurements. With the use of the MME to test 10 selected equations, against a compilation of field measurements based on gas tracers, current equations are shown to be of little value at low slopes, whereas some frequently used equations are shown to have little general value. Slope is found to be an essential component of reaeration equations. Recommendations are made for estimating the reaeration coefficient.

The flux chamber technique was developed to measure volatile organic compound (VOC) vaporization rates directly from open basins and to avoid the uncertainties inherent in using a simplified physical model. In this method, the liquid surface within an enclosing chamber is swept with air at a measured rate, and the gas is analyzed. Interpretation of the results is based upon a critical assumption: that the conditions of the measurement truly represent vaporization under normal ambient conditions. This is not necessarily true, since the gas-phase conditions within the chamber may be very different from the ambient air. In the first part of this study, the gas-phase coefficients in the flux chamber were measured at different gas sweep rates and an attempt was made to relate these to ambient coefficients measured in other studies at varying wind speeds. In the second part of the study, the volatilization rates of several volatile organic compounds (VOCs) contained in large basins were measured using a flux chamber. The liquid-phase mass transfer coefficients were determined from the flux chamber measurements, and the gas-phase coefficients were estimated from data reported in the literature on the vaporization of water from surfaces exposed to different wind speeds. Based upon these liquid- and gas-phase mass transfer coefficients, VOC vaporization rates from the basins under ambient conditions were determined.

The transport and retention of a sorbing solute in streams are affected by the transversal uptake in various hyporheic zones, each characterized by its individual turnover time. The uptake is included in a one-dimensional model for the along-stream transport on the form of first-order transfer functions by parameterizing physically derived relationships. A ⁵¹Cr(III) tracer experiment, conducted in a brook in an agricultural watershed in Sweden, provided a basis for deducing constitutive equations for three different exchange rates governing the retention of the sorbing solute in the brook. The solute exchange that occurs between the main stream channel and slowly recirculating zones, like side pockets along the stream, eddy zones behind boulders, or side pockets, is practically instantaneous and is manifested as a marked retardation of the pulse propagation in relation to the main stream velocity. The considerably slower exchange with permanent substratum, including armored bed sections, dense vegetation, or adjacent wetland, causes a reduction of the peak concentration and prolonged tails of the breakthroughs. Uptake in the alluvial sediment occurs both in particulate and dissolved phases of the solute, and is affected by the bed dynamics. The uptake in the alluvium was evaluated separately by fitting of an analytical solution of the vertical concentration profile to measured concentration profiles in the sediment. In the experimentally investigated reach, the exchange with the fine bed sediment was dominated by the dissolved phase of Cr, due to advection driven by pressure variabilities along the bed surface.

Water quality of highway runoff in the Austin, Texas, area was determined by monitoring runoff at three locations on the MoPac Expressway. Daily traffic volumes, surrounding land uses, and highway drainage system types were different at each site. The concentrations of constituents in runoff at all sites were similar to median values compiled in a nationwide study of highway runoff quality. A grassy swale at one site was effective in reducing the concentrations of most constituents in runoff. The lower runoff coefficient at this site is attributable to infiltration of runoff into the grassy swale. The pollutant loads discharged from the pipe draining the swale were lower than those observed at the sites where runoff drained directly from the pavement. A first flush effect (i.e., higher pollutant concentrations at the beginning of an event) was evident during selected events, but was generally limited to a small volume. The overall effect was negligible when all monitored events were considered.

Based on limited available data, engineers are often called upon to prepare preliminary designs of new outfalls or to screen the performance of existing ones in the context of developing rational management schemes for urban and/or industrial effluents. In view of this need, our objective is to develop a simplified procedure, able to provide guidance with minimal field information and capable of utilizing any relevant marine survey data that may exist for enhancing the accuracy of predictions. For achieving the previous objective, the relations for all three dilution mechanisms are considered simultaneously, so as to have their combined impact analyzed. A set of baseline conditions is defined by assigning typical and typical-adverse values to selected input variables. Sensitivity analysis is performed, helping to further simplify the problem. The simplified model is analyzed in terms of its functional dependencies, and nomograms are developed, offering a convenient solution of the baseline problem. This baseline solution is easily adapted to local conditions through a correction multiplier factor computed on the basis of actual marine survey data that may exist.

Vapor sorption of volatile organic chemicals (VOC) to soil minerals becomes the dominant VOC sorption mechanism at low soil-water contents, but is not included in traditional VOC transport and fate models. A two-parameter model for trichloroethylene (TCE) vapor sorption as a function of soil-water content is suggested, and expressions for predicting the model parameters as function of soil type [cation exchange capacity (CEC)] are developed based on measurements for 15 soils from the literature. The vapor sorption model was used in combination with a two-dimensional VOC transport model to evaluate the effect of vapor sorption on TCE transport in the unsaturated zone under both natural conditions and in connection with soil vapor extraction systems. The effects of variations in soil-water content, soil CEC, and soil-vapor extraction rate were investigated. Temporal and spatial variations in soil-water content caused pronounced fluctuations in the volatilization of TCE to the atmosphere. Soil type (CEC) strongly affected vapor sorption and controlled TCE transport under dry conditions. The simulations imply that vapor sorption can result in increased clean-up times, especially when using vapor extraction in arid regions.

The effective remediation of light non-aqueous-phase liquid (LNAPL) in the subsurface is dependent on predicting the location and geometry of the lens near the tension-saturated pores of the capillary fringe. The objective of this study was to evaluate both the thickness of a lens of light nonaqueous phase liquid spill in the subsurface and the subsequent alteration of the capillary fringe. Two-dimensional (2D) experiments were conducted in a parallel plate glass tank (1 m × 1 m × 5 cm) using Ottawa sand and gasoline. An equation for predicting the vertical thickness based on the displacement pressure in the lateral direction was developed. The model and previously published vertical equilibrium models for predicting the thickness of a lens were tested. The model equations were also used to evaluate changes in the capillary fringe due to capillary pollution. The results provide a means to better understand the behavior of light nonaqueous phase liquids in the subsurface.

In recent years, phytoremediation, i.e., the use of plants to clean up soils contaminated with organics, has become a promising new area of research, particularly for in-situ cleanup of large volumes of slightly contaminated soils. A model that can be used as a predictive tool in phytoremediation operations was developed to simulate the transport and fate of a residual hydrocarbon contaminant interacting with plant roots in a partially saturated soil. Time-specific distribution of root quantity through soil, as well as root uptake of soil water and hydrocarbon, was incorporated into the model. In addition, the microbial activity in the soil rhizosphere was modeled with a biofilm theory. A sandy loam, which is dominant in soils of agricultural importance, was selected for simulations. Cotton, which has well-documented plant properties, was used as the model plant. Model parameters involving root growth and root distribution were obtained from the actual field data reported in the literature and ranges of reported literature values were used to obtain a realistic simulation of a phytoremediation operation. Following the verification of the root growth model with published experimental data, it has been demonstrated that plant characteristics such as the root radius are more dominant than contaminant properties in the overall rate of phytoremediation operation. The simulation results showed enhanced biodegradation of a hydrocarbon contaminant mostly because of increased biofilm metabolism of organic contaminants in a growing root system of cotton. Simulations also show that a high mean daily root-water uptake rate increases the contaminant retardation factors because of the resulting low water content. The ability to simulate the fate of a hydrocarbon contaminant is essential in designing technically efficient and cost-effective, plant-aided remedial strategies and in evaluating the effectiveness of a proposed phytoremediation scheme. The model presented can provide an insight into the selection and optimization of a specific strategy.

Sewage sludge disposal has become more difficult for wastewater-treatment plants because landfill operators are increasingly raising tipping fees and limiting the sludge they will accept. For this reason many municipalities are turning to land application as a means of beneficially disposing of sludges. We have developed a procedure for systemwide planning of biosolids (sewage sludge) land application systems. The system includes multiple treatment plants, agricultural regions, and crops. Our approach considers costs of the following: (1) Biosolids storage; (2) digestion; (3) composting; (4) transportation; and (5) land application over an extended planning horizon. We describe a mixed-integer programming (MIP) algorithm that can be used to simultaneously schedule capacity expansions and route transportation. The objective function minimizes long-term and short-term costs. We use an inventory-type approach to model nitrogen accumulation in the soil over time. Accumulated nitrogen limits the quantity of biosolids that can be applied to a given parcel of land. The model restricts certain crops to composted biosolids according to current Environmental Protection Agency (EPA) regulations. Sewage-treatment plants may opt to either compost their biosolids or upgrade their digestion system to meet U.S. federal standards for land application, and the model settles on an optimal pathogen treatment plan based on storage, transportation, and marketing constraints. The model is used to find an economically efficient solution for the sludge disposal needs of Eastern Municipal Water District (EMWD) in Riverside County, Calif. In this District, a combination of direct land application and composting proves optimal.

A methodology for data analysis and stochastic modeling of daily municipal solid waste production rates is presented. The data sets examined are the daily quantities of municipal solid wastes for consecutive days and for each day separately. Each sequence of observations was modeled by Box-Jenkins stochastic models as a function of autoregressive, moving average, and seasonal terms. For the overall time series, a seasonal ARMA model (1, 0) × (1, 1)₅ was found to be adequate. The observed seasonality of length 5 was due to the municipal solid waste (MSW) collection pattern. For the separate daytime series, simple autoregressive (AR) models were adequate without inclusion of any seasonal terms. In general, these models demonstrated statistical fit, and modeling of the trends was satisfactory. The forecasting ability of the Box-Jenkins models was compared to simpler statistics, such as the mean value and the moving average values. Depending on the specific day, different models gave optimum forecasting results.

This research investigates the ability and efficiency of continuous-flow wood chip reactors seeded with a white-rot fungus to degrade 2,4-dichlorophenol (2,4-DCP) using wood chips as a carbon source. When 2,4-DCP was the only substrate (nonglucose treatment conditions), the wood chip reactor system had a high degradation efficiency and operated continuously without excessive fungal biomass buildup on the wood chips. In the presence of added glucose, a clogging problem and an effluent contamination problem of fungal cells are found during the reactor operating period. In addition, 2,4-DCP is degraded effectively both under low-nitrogen (low-N) (0.012 g/L of NH₄Cl) as well as high-nitrogen (high-N) (0.12 g/L of NH₄Cl) treatment conditions. The 2,4-DCP is degraded to a greater extent with small-size wood chips (5 mm diameter × 5 mm long) and hardwood chips (birch as opposed to pine) as a carbon source. The results of this research demonstrate a potential application of wood chip reactor systems for the treatment of contaminated water while expanding the use of wasted forest products.

Enhanced biological phosphorus removal is a two-step process. The sequence of anaerobic environment followed by aerobic environment is essential for selection of biological phosphorus-removing culture. Anaerobically, phosphorus is released; subsequently, in the aerobic period, it is biologically removed. An effort has been made to study both phosphorus release and uptake at 25°C and 10°C in sequencing batch reactors using synthetic wastewater. In anaerobic phase the maximum phosphate released was 327% at 25°C and 195% at 10°C. Despite differences in temperature and initial concentration of released phosphorus, the effluent phosphate concentration was found to be the same at both temperatures.