Search

Microalgae-bacteria consortium for wastewater treatment has great potential for use in nutrient removal and biodiesel acquisition. In this study, the effect of four different light/dark regimes (6/18 h , 12/12 h , 18/6 h , 24/0 h) on biomass production and pollutant degradation during the treatment of oilfield wastewater by the algae-bacteria consortium was investigated. The maximum algal biomass (0.2 g/L) and total chlorophyll content (812.76 μgmg−1 cell dry weight−1) were obtained due to an appropriate illumination cycle (18 hlight/6 h dark) with enhanced dissolved oxygen content (5.87 mg/L), thus improving bacterial biomass (0.31 g/L). The ratio of microalgae to bacteria was essentially constant under different light/dark cycles. In addition, the highest removal rates of chemical oxygen demand (55.32%), ammonia nitrogen (50.75%), total nitrogen (64.53%), and total phosphorus (65.97%) were also obtained at the 18/6 h cycle, significantly different from the results of other light/dark cycles. The degradation efficiency of ammonia nitrogen by bacteria increased and then decreased with increasing light duration, while the removal rate of ammonia nitrogen by algae showed the opposite trend. The results indicated that the light and dark regimes had an important influence on the growth of the microalgae-bacteria consortium and their performance in treating oilfield wastewater. [ABSTRACT FROM AUTHOR]

The influence of aeration diffuser system design on electricity usage, effluent water quality, and life-cycle cost in biological wastewater treatment was investigated. A plant-wide model was implemented, and simulations were carried out with different process configurations and aeration systems. Model-aided design of new aeration diffuser systems could significantly decrease electricity usage and life-cycle cost while at the same time avoiding negative effects on the treatment performance. The optimum distribution of diffuser systems in tanks in series was found to be influenced by process configuration, volumetric loading rate, temperature, and the internal recirculation flow rate. Compared with a conventional design approach, increasing the number of diffusers, up to a critical point, led to higher energy efficiency and lower life-cycle cost. This was despite an increasing limitation of the minimum airflow rate, leading to dissolved oxygen levels significantly exceeding control targets. Aeration systems optimized by simulations were found to, independently of process configuration, exhibit 20% lower electricity usage and 16%–18% lower life-cycle costs compared with systems designed based on a more conventional approach typically applied in practice. [ABSTRACT FROM AUTHOR]

Urban stormwater runoff can promote eutrophication by discharging excessive nitrogen loads into downstream surface waters. Conventional stormwater biofiltration systems are inefficient in removing nitrate; however, a denitrifying biofilter containing a medium that includes an electron donor can be incorporated into these systems to enhance denitrification. This study presents a novel model for denitrifying stormwater biofilters that is compatible with existing stormwater modeling software. Processes that were incorporated into the model include water mixing, wood chip dissolution, and biological denitrification. The biokinetic model includes the effect of bioavailable organic substrate and dissolved oxygen concentration on denitrification. The model was calibrated and validated using experimental data from biofiltration column studies carried out with varying storm durations, antecedent dry conditions, hydraulic loading conditions, influent nitrate concentrations, and biofilter depths. The model accurately predicted nitrate mass removal efficiencies that were within 10% of the results from the experimental data. The model was validated with a high Nash-Sutcliffe efficiency coefficient (0.80) and coefficient of determination (0.85). This denitrifying biofilter model provides a practical tool for engineers to quantify nitrate removal as a function of biofilter characteristics as opposed to assumed percent removal efficiencies. Recommendations for future research include validating the model with field data and developing a total nitrogen removal model that includes additional nitrogen transformation processes. [ABSTRACT FROM AUTHOR]

This paper deals with an upflow anaerobic sludge blanket (UASB) reactor's effluent post treatment under aerobic conditions. The kinetics of stripping and oxidation of sulfides (H2S, HS- and S2-) and purgeable organic compounds are determined. The effects of dissolved oxygen (DO) and temperature on stripping and oxidation kinetics are also investigated. Batch results indicate that the effluent quality [sulfide, chemical oxygen demand (COD) and purgeable organic carbon (POC)] can be improved by 50% with aeration at 6 mg=L DO and at hydraulic retention time (HRT) of 30 min. The results obtained in the batch studies were confirmed in full size reactors. Moreover, it was observed that sulfides in absence of dissolved oxygen are removed by stripping rather than oxidation. [ABSTRACT FROM AUTHOR]

The effectiveness of biobarrier for leachate containment was investigated through continuous flow column studies. Landfill leachate collected from Ariyamangalam open dumping site was used as the source for microorganism isolation. Isolated organisms were enriched in the presence of lead to selectively obtain lead-tolerant microorganisms. The biochemical analysis results confirmed that the isolated strain is gram positive and rod shaped, Bacillus insolitus. Synthetic leachate was inoculated with different cultures of bacteria and was aerated to an initial dissolved oxygen level of 7.8 mg=L. In the batch reactor, chemical oxygen demand (COD) and lead removal was higher for leadenriched cultures (62 and 28%) when compared with the nonenriched cultures (57 and 13%). The initial hydraulic conductivity of 1.99 × 10-2 cm=s was reduced to 1.1 × 10-3 cm=s at the end of 36 days of continuous study of the microbial sand column (MC), whereas negligible reduction was observed in blank column without microbes (BC). A reduction of 74% COD and 51% lead concentration were obtained at the end of 36 days in continuous flow study. The biobarrier is very effective for the reduction of hydraulic conductivity and removal of organic pollutants and heavy metals from landfill leachate. Thus, biobarrier can be used as a primary layer in landfills. [ABSTRACT FROM AUTHOR]

Ammonia-based feedback aeration control could avoid an unnecessary air supply and, therefore, save energy. However, this control method could be impacted by many factors such as solids retention time (SRT), nitrite oxidation rate, dissolved oxygen (DO) variation, influent load, temperature, and regular low DO operation. This study indicated that, with an unlimited DO (DO ≥ 2 mg=L), the minimal SRT required to achieve complete nitrification was between 5 and 10 days. If the SRT was ≤ 20 days, reducing the DO could result in accumulation of nitrite. Nitrite could also accumulate under a peak ammonia load with a SRT of less than 15 days. In contrast, ammonia- based feedback control resulted in regular low DO operation. Because low DO enriches nitrifiers and selects nitrite-oxidizing bacteria (NOB) with a greater oxygen affinity, the required SRT to avoid the accumulation of nitrite can be substantially reduced. The authors recommend using a minimal SRT of 20 days to start a system. When the system reaches a stable condition, the minimal SRT can be reduced to 15 days, if needed. A pilot-scale field experiment indicated that the adverse effect of a low temperature can be overcome by extending the SRT. [ABSTRACT FROM AUTHOR]

The implementation and operation of wastewater treatment plants are fundamental actions for environmental water body pollution control that generally involve large amounts of financial resources. Hence, employment of optimization techniques for decisions about the minimum required wastewater treatment plant efficiencies might be very useful. In this study, an optimization technique known as the genetic algorithm (GA) was combined with a water-quality model to determine the minimum efficiencies for sewage treatment in watersheds. Combinations of the optimization technique and models that incorporated environmental quality standards for the parameters of dissolved oxygen (DO), biochemical oxygen demand (BOD), and equity measures among sewage treatment systems, either as constraints or in the objective functions, were developed. These combinations were applied to the Santa Maria da Vitória river watershed, located in the State of Espírito Santo, Brazil, considering possible effluent disposal scenarios and implementation of different established optimization models. The developed combinations of the genetic algorithm and a water-quality model demonstrated to be versatile and efficient tools for determining the minimum sewage-removal efficiencies for wastewater treatment plants in watersheds. Among the developed combinations of optimization models, the best results, or minimum costs, were obtained when the rivers' self-depuration capabilities were taken into account. [ABSTRACT FROM AUTHOR]

The study of sediment transport in water natural bodies is a challenging task. There have been several attempts to describe sediment mathematically using hydraulic characteristics of water bodies. Most researchers who developed empirical formulas to describe sediment transport performed laboratory experiments with assumptions that did not take into account variations of hydraulic parameters and the fine sediment sizes that are part of this phenomenon. Recently, new approaches for studying sediment transport have been developed involving the use of machine-learning algorithms that have proven accuracy and efficiency in predicting sediment transport. A novel machine-learning method, the Multivariate Relevance Vector Machine (MVRVM), has yet to be tested to model sediment transport and water quality in estuaries and lakes. The selection of the MVRVM is suggested by the limited field observations that present challenges for alternative statistical learning machines, and by the promise of using the MVRVM approach to inform future data-collection efforts. This paper tests the success of calibrating the MVRVM model to predict suspended fine-sediment transport and other environmental measures in Mud Lake, southeastern Idaho, United States. In addition, the authors introduce and explain the technique that can be used to arrange the data which will allow the model to work. Training and validation results for turbidity, total suspended solids (TSS), pH, dissolved oxygen (DO), and water temperature are presented. These results emphasize that modeling the water-quality constituents and sediment transport with few observations is possible using the MVRVM. [ABSTRACT FROM AUTHOR]

A new kind of magnetic porous carrier named magnetic air stone (MAS) was prepared by coating air stone (AS) with ferrofluid nanoparticles. Characterization by X-ray diffraction, scanning electron microscope, and vibrating sample magnetometer reveals that Fe3O4 nanoparticles embedded in air stone were paramagnetic with saturation magnetization of 21 emu=g. Moreover, an analysis of biofilm formation on MAS was run. The results show that dissolved oxygen increased 50%, mass of the biofilm increased 32%, and biofilm adhesion increased 21%. The main reason for the enhancement of dissolved oxygen was the sorption performance of MAS for removing ions from aqueous solution. Biofilm formation acceleration was due to the ferrofluid nanoparticles used to modify AS into MAS, which are hydrophobic, and hydrophobic cells readily adhere to hydrophobic surfaces. This study demonstrated that the magnetic air stone developed in our research project could be potentially used as a cost-effective carrier for municipal wastewater treatment. [ABSTRACT FROM AUTHOR]

Dissolved oxygen (DO) consumption in the sediments of natural aquatic ecosystems occurs by mass flux across the watersediment interface. This mass flux is determined as either the rate of oxygen consumption in the sediment or the rate of DO diffusion across the diffusive sublayer. The thickness of the diffusive sublayer is determined by the flow conditions in the turbulent water column. Consequently, feedback occurs between the biochemical consumption that occurs in the sediment and the flow that occurs in the water column. Together, these conditions define the DO flux across the water-sediment interface. Benthic chambers have been used to measure this flux in field and experimental conditions. However, these measurements do not account either for the fact that the flow inside a benthic chamber is not representative of field conditions or for the DO consumption in the water column. Thus, they can provide an inaccurate estimation of the sediment DO demanded in the field. This article aims to present and discuss an approach for analyzing a time series of DO depletion inside a benthic chamber. Based on this approach, the processes related to turbulent water transport in the column are separated from the processes that characterize biochemical consumption in the sediments and the water column. Under these conditions, the parameters related to biochemical consumption in the sediments can be used to compute the expected DO demanded of the sediments in field conditions. Sediment samples from a lagoon near the Universidad de Chile campus in Santiago, the capital city of Chile, were used to illustrate the application of the proposed method. In addition, dimensionless numbers were used to define the method's validity and limitations. [ABSTRACT FROM AUTHOR]

This paper develops a template-based fuzzy inference system (TFIS) capable of simulating the dissolved oxygen (DO) concentration of an example stream by using daily data (2009-2012). Stepwise regression (SR) analysis and Mallows' Cp statistic were employed to select the best set of independent parameters for the input vector of the TFIS and the regressive models. The most effective inputs were determined for temperature and specific conductivity (), whereas flow rate () and pH () were found to be less effective parameters. The TFIS and SR models undersimulated the magnitude of high DO concentrations (), whereas the low and medium DO concentrations were oversimulated. However, they were closer to the observed data. A comparison of the prediction accuracy of the TFIS and SR methods indicated that the TFIS approach was more accurate in simulating DO concentrations, except for low concentrations of DO (). The TFIS method was found to be superior to the conventional SR model. [ABSTRACT FROM AUTHOR]

Benzene, toluene, ethylbenzene, and xylenes (BTEX) are common groundwater pollutants. Biological activated carbon (BAC) treatment is attractive because BTEX species are well-adsorbed on activated carbon and also easily biodegraded in the adsorber. The inoculated activated carbon columns quickly became BAC systems; they removed most of the BTEX species present in the feed. Biodegradation was primarily responsible for the overall removal of the BAC systems, and the amounts of benzene, toluene, and o-xylene removed during the 2-week treatment period were all more than their respective isotherm capacities; 1,486, 1,463, 1,537, and 1,719 mg of benzene, toluene, ethylbenzene, and o-xylene were removed in the 2.5-g BAC systems versus the available adsorptive capacities of 174, 731, 1,515, and 895 mg, respectively. The stable BAC systems were resistant to the shock loadings of higher flow and concentration and consumed less than the expected amount of dissolved oxygen. Bioregeneration enabled the long-term treatment capability of the BAC systems for removing BTEX and other pollutants without periodic carbon replacement. The coal-based activated carbon (SC) was more desirable than the coconut-based activated carbon (YK) because of its lower cost and higher degree of bioregeneration. [ABSTRACT FROM AUTHOR]

Combined sewer overflows (CSOs) are a major source of contamination to urban waterways. Discharges from CSOs represent a major source of chemical oxygen demand, biological oxygen demand, nutrients, toxics, and bacteria to receiving waters. The impacts of these discharges to ecosystem health include deposition of sewer sediment on the stream bed, anoxic events, and algal blooms. A case study is presented, which analyzed in-stream dissolved oxygen and sediment oxygen demand (SOD) in an urban stream in Philadelphia, Pennsylvania, to observe short-term and long-term water-column oxygen depletion caused by CSO discharges. Multiple SOD measurements were collected at six locations in a 5.61-km reach. Continuous in-stream dissolved oxygen measurements indicate that water-column dissolved oxygen concentration in the reach is depleted as it flows through the study section. This depletion appears to be the result of elevated SOD downstream of the CSO outfalls. [ABSTRACT FROM AUTHOR]

Dissolved oxygen (DO) concentration is an important water quality parameter. This paper studies the increase of DO concentration in water by air injection into a horizontal pipe flow. A 3D computational fluid dynamics model was employed to compute the water and bubble mixture flow with a DO transport model. Experiments were also conducted to validate the mathematical model. A relative saturation coefficient relationship was developed with air bubble volume fraction and travel time. An oxygen absorption efficiency is defined, and its relationship with the inlet DO concentration, air bubble volume fraction, and travel time was discussed. [ABSTRACT FROM AUTHOR]

Elevated levels of total dissolved gas (TDG) may occur downstream of dams, leading to increased incidence of gas bubble disease in fish. Accelerating the dissipation of supersaturated TDG in the downstream river can mitigate this problem; however, data useful for modeling the dissipation of supersaturated TDG through the free surface in natural rivers are limited. Lacking data to the contrary, prior modeling studies have assumed (1) dissolved oxygen (DO) is a reasonable proxy for TDG; and (2) unsaturated reaeration is sufficiently similar to supersaturated dissipation such that the same rate coefficients may be applied to either process. To test the validity of these assumptions and motivate future research, laboratory experiments were conducted to estimate the first-order dissipation rate coefficients for supersaturated DO and TDG and the reaeration rate coefficients for DO under identical turbulence conditions. The results indicate the dissipation process is quantitatively different from the reaeration process, and TDG behavior is quantitatively different from DO. Comparison of laboratory results with prior field research leads to speculation that increasing river turbulence and/or decreasing in water depth may be possible methods for promoting the TDG dissipation rate and reducing the length of a river affected by supersaturation. [ABSTRACT FROM AUTHOR]

Diagenesis of particulate organic matter in sediments can have important effects on the dissolved oxygen (DO) resources of lakes and delay potential benefits from reductions in phosphorus (P) loading. A sediment diagenesis model of intermediate complexity, , is developed and tested for Onondaga Lake, New York, and linked to a validated one-dimensional mechanistic hydrodynamic/stratification model (UFILS4). Modeling was supported by detailed characterization of the pelagic sediments and long-term monitoring of the lake's water column that included transformation from hypereutrophy to upper mesotrophy and coupled improvements in hypolimnetic DO. performed well in representing the observed decreasing trajectory of the downward flux of particulate organic carbon and in predicting the decreases in sediment effluxes of dissolved inorganic carbon and sediment oxygen demand over a 30-year period. and the linked overall water-quality model, , were applied to project the extent and timing of improvements in sediment fluxes and hypolimnetic DO resources for an array of P management alternatives. Substantial delays in response associated with the effects of diagenesis were predicted that extended to reach 95% of steady state. Long-term projections indicate that improvements from the most aggressive P control alternatives may be sufficient to maintain certain coldwater fish. [ABSTRACT FROM AUTHOR]

The City of Atlanta plans to convert a recently acquired urban quarry into Bellwood Quarry Reservoir (BQR), an off-stream supplemental water supply. CE-QUAL-W2, Version 3.5, was used to perform five-year water quality simulations of the reservoir. Nine inflow/outflow design and operation scenarios were evaluated for thermal stratification, dissolved oxygen (DO), algae, and iron (Fe) and manganese (Mn). For a perpetual storage scenario with no inflow and outflow and an operating scenario with monthly pump exercising flows, the model predicts ample reservoir DO to maintain an oxidizing environment in the hypolimnion and low dissolved Fe and Mn. A single portal near the bottom of the reservoir is recommended for alternating pump exercise inflows and outflows. Continuous flow-through operating scenarios do not improve predicted water quality. However, if selected, a mid-depth inflow and bottom outflow design configuration is recommended. A natural monomictic cycle is essential to maintain hypolimnetic DO. BQR's relatively large mean depth (60 m) and low sediment surface area to volume ratio is consistent with other mesotrophic lakes of similar water quality. [ABSTRACT FROM AUTHOR]

The aim of this study is to examine the accuracy of two different artificial neural network (ANN) techniques, the multilayer perceptron (MLP) and radial basis neural network (RBNN), to estimate dissolved oxygen (DO) concentration. The ANN results are compared with multilinear regression (MLR) model. The neural network model is developed using experimental data collected from the upstream (USGS Station No: 07105530) and downstream (USGS Station No: 07106000) stations on Foundation Creek, CO. The input variables used for the ANN models are water pH, temperature, electrical conductivity, and discharge. The determination coefficient (R2), mean absolute error (MAE), and root mean square error (RMSE) statistics are used for the evaluation of the applied models. The MLP and RBNN models are also compared with MLR model in estimating the DO of the downstream station by using the input parameters of the upstream station. Comparison results indicate that the RBNN model performs better than the MLP and MLR models. [ABSTRACT FROM AUTHOR]

Dissolved oxygen (DO) is essential to maintaining flora and fauna in aquatic ecosystems. The DO replenishment through the surface reaeration mechanism is commonly described using the reaeration coefficient (K2). This paper presents a new approach to modeling instream DO and estimating K2. The new approach includes an extension of the Variable Residence Time (VART) model and optimization algorithms for inverse modeling. The VART model is modified in this paper to incorporate the DO reaeration mechanism across the air-water interface, forming the VART-DO model. A major advantage of the VART-DO model is that it is capable of simulating DO exchange across the water-sediment interface through the hyporheic exchange mechanism in addition to the air-water exchange. A sensitivity analysis is conducted to investigate the relative importance of key model parameters to DO modeling. It is found that the K2 value increases markedly with the dispersion coefficient. The simplex-simulated annealing and the genetic algorithm are used to optimize the estimation of the parameter K2. A comparison of the optimized K2 value from the VART-DO model with those from existing empirical equations reveals that K2 value could increase 1.5 to 6 times when the effects of the dispersion and hyporheic exchange mechanisms on DO are taken into account. The results from the VART-DO model provide new insights into the DO variation in streams and rivers. The VART-DO model also provides a general and effective tool for total maximum daily load (TMDL) calculations of DO. [ABSTRACT FROM AUTHOR]

The majority of variable estimation studies in water resources investigate the temporal variation of the variable. In this study, we examined the depth-dependent estimation of a lake's dissolved oxygen (DO) using two artificial neural network (ANN) methods: (1) the radial basis functions (RBFs) and the feed forward back-propagation (FFBP), and (2) the multilinear regression (MLR). We tested two different input layer configurations. In the first case, we employed all other available lake parameters-total dissolved solids (TDS), pH, conductivity, lake depth, and lake temperature-to estimate DO. In the second case, we considered only depth and temperature to estimate DO. The performance evaluation criteria of these two cases were close. ANN estimation performances were noticeably superior to those of MLR, as reflected in the performance evaluation criteria and DO lake depth plots. We saw that the spatial variation of the lake's DO can be captured by ANNs satisfactorily, even if available measurements are quite limited. [ABSTRACT FROM AUTHOR]

A numerical model was developed to calculate the rate of dissolved-oxygen (DO) diffusion across a sediment surface taking into account the surface roughness and biochemical reactions of the sediment. Estimates of DO transfer rate from the model were compared with results from laboratory experiments conducted in a rectangular flume using roughness elements. In experiments, there was maximum value for the nondimensionalized DO transfer rate (Stanton number, St) in the transitional region of surface roughness, in which the mass flux was two to five times larger than that of the smooth surface. The reproducibility of the experimental results by numerical analysis was significantly improved by including terms for flushing frequency of water in cavities between the roughness elements and for nonsteady variations in the diffusion rate due to step changes in DO concentration in the flushed region. A simple method to estimate enhancement effect for St caused by nonsteady variations was also presented. [ABSTRACT FROM AUTHOR]

Catalytic decomposition of dichlorodifluoromethane (CFC-12) in the presence of water vapor and oxygen was studied over a series of solid super acids Mo2O3/ZrO2 that had different ZrO2 content by using a fixed-bed reactor. CO2 and CClF3 were the main products and no CO was detected as by-product. The decomposition activity of solid super acid largely depended on the content of ZrO2 and the calcination temperatures of Mo2O3/ZrO2. The optimal calcination temperature and content of ZrO2 for preparing Mo2O3/ZrO2 with the highest activity for catalytic decomposition of CFC-12 were 450°C and 20-40 wt %, respectively. Adopting a low concentration of oxygen and CFC-12 together with a high concentration of water vapor was preferable for having the high conversion efficiency of CFC-12 and the selectivity for CO2. The catalytic activity of Mo2O3/ZrO2 remained steady for 100 h in continuous operation. [ABSTRACT FROM AUTHOR]

Catalytic decomposition of dichlorodifluoromethane (CFC-12) in the presence of water vapor and oxygen was studied over a series of solid super acids Mo2O3/ZrO2 that had different ZrO2 content by using a fixed-bed reactor. CO2 and CClF3 were the main products and no CO was detected as by-product. The decomposition activity of solid super acid largely depended on the content of ZrO2 and the calcination temperatures of Mo2O3/ZrO2. The optimal calcination temperature and content of ZrO2 for preparing Mo2O3/ZrO2 with the highest activity for catalytic decomposition of CFC-12 were 450°C and 20-40 wt %, respectively. Adopting a low concentration of oxygen and CFC-12 together with a high concentration of water vapor was preferable for having the high conversion efficiency of CFC-12 and the selectivity for CO2. The catalytic activity of Mo2O3/ZrO2 remained steady for 100 h in continuous operation. [ABSTRACT FROM AUTHOR]

Environmental Fluid Dynamics Code, a numerical estuarine and coastal ocean circulation hydrodynamic and eutrophication model, was used to simulate the distributions of dissolved oxygen (DO), salinity, water temperature, and nutrients in the Caloosahatchee River Estuary. Modeled DO, salinity, and water temperature were in good agreement with field observational data from the Florida Department of Environmental Protection and South Florida Water Management District. Sensitivity analyses identified the effects of river discharge, atmospheric winds, and tidal forcing on the spatial and temporal distributions of DO. Simulation results indicated that vertical mixing due to wind forcing increased the bottom DO concentration. River discharge enhanced stratification in deep locations but propagated vertical mixing in the shallow upper estuary. Finally, tidal forcing heavily influenced bottom layer DO concentrations throughout the whole river estuary. [ABSTRACT FROM AUTHOR]

A modeling framework that combines both two-dimensional (2D) and one-dimensional (1D) numerical models for the evaluation of organic-matter transport across the bed-water interface is presented. Emphasis is placed on capturing oxygen demand in the water column associated with the resuspension of organic sediments from the bottom. The proposed numerical approach solves the hydrodynamics coupled with sediment transport and water quality dynamics and represents a substantial improvement to the state of the art of water quality modeling methodologies available in the literature. A biochemical oxygen demand (BOD)–dissolved oxygen (DO) water quality module is incorporated into the 2D depth-averaged numerical model STREMR-HySedWq. The model is applied to the South Fork of the South Branch of the Chicago River, known as Bubbly Creek, with the goal of modeling combined sewer overflow (CSO) events and their impact on DO levels in the short-term (hours or days). Given the intermittent nature of this kind of events, STREMR-HySedWq was used for modeling the overflow phase, while the period following a given CSO event, when water is essentially quiescent in the creek, was modeled with a simpler 1D, area-averaged, dispersion-reaction BOD-DO model. The proposed conceptual and numerical approach is capable of capturing the key processes, thus providing both useful preliminary results as well as important guidance to assess potential water quality improvement and sediment management solutions in Bubbly Creek, Illinois. [ABSTRACT FROM AUTHOR]

Storage tanks are commonly installed in a combined sewer system to control the discharge of combined sewer overflows that have been identified as a leading source for receiving water pollution. The traditional approach to determine the distribution of storage tank volume in the sewer system is confined to the use of objectives within the system itself due to the limits of separate modeling of urban wastewater systems, consisting of the sewer system, wastewater-treatment plant, and receiving water. The aim of this study is to investigate the optimal distribution and control of storage tanks with an objective to mitigate the impact of new residential development on receiving water quality from an integrated modeling perspective. An integrated urban wastewater model has been used to test three optimization scenarios: optimal flow rate control, storage distribution, and a combination of these two. In addition to the cost of storage tank construction, two receiving water quality indicators, dissolved oxygen and ammonium concentration, are used as optimization objectives. Results show the benefits of direct evaluation of receiving water quality impact in the context of storage distribution optimization. Results indicate that storage allocation should be considered in conjunction with optimal flow rate control to achieve the maximum effectiveness in water pollution mitigation. [ABSTRACT FROM AUTHOR]

Spatial interpolation methods are frequently used to estimate values of physical or chemical constituents in locations where they are not measured. Very little research has been conducted, however, to investigate the relative performance of different interpolation methods in surface waters. The study reported here uses archived water quality data from the Chesapeake Bay to compare three spatial interpolation methods: inverse distance weighting, ordinary kriging, and a universal kriging method that incorporates output from a process-based water quality model. Interpolations were performed on salinity, water temperature, and dissolved oxygen “snap shots” (cruise-based data sets) taken between 1985 and 1994 at 21 different depths for multiple locations in the mainstem Bay, using data compiled by the prototypical Chesapeake Bay Environmental Observatory. The kriging methods generally outperform inverse distance weighting for all parameters and depths. Incorporating output from the water quality model through universal kriging appears to improve some of the interpolations by specifically accounting for some physical and biogeochemical features of the estuary. Such integration of process-based information with statistical interpolation warrants further study. [ABSTRACT FROM AUTHOR]

Diel dissolved oxygen (DO) concentrations and temperature were sensed at high-frequency and modeled in an eastern Iowan stream, Clear Creek, in an agricultural setting. The magnitude of the diel changes in DO and temperature were largest at the upstream (headwater) station. Inclusion of temperature change factors increased the accuracy of modeling results and yielded estimates of the reaeration rate constant, primary production rate, and respiration rate. The DO modeling of the high-frequency measurements (15-min intervals) revealed a temperature-driven nonlinear reaeration process that led to increases in nighttime DO concentrations. The DO modeling results from three sensing stations in the watershed revealed decreasing trends in primary productivity, respiration, and the reaeration rate constant with increasing drainage area. Light extinction from suspended solids was the main factor limiting net primary production. As a result, the P/R ratio also decreased with increasing drainage area. High-frequency sensor data and DO modeling revealed the effects of temperature and watershed scale on the primary factors that dictate diel DO dynamics in a stream setting. [ABSTRACT FROM AUTHOR]

The quantitative effects of hydrodynamic conditions on sediment oxygen demand (SOD) under smooth surface conditions were investigated using the following three practical and compact experimental systems: (1) a continuous flow system containing sediment core samples; (2) a rectangular flume system; and (3) a system combining the first two. Experimental results demonstrated that SOD showed a monotonically increasing tendency as the flow velocity increased with reduction of the thickness of the diffusive boundary layer. The experimental results were compared with numerical models theoretically relating SOD and flow velocity under smooth surface conditions. The features of each experimental system are discussed. The continuous flow system is advantageous because it simultaneously produces a steady state and different dissolved oxygen (DO) conditions. The rectangular flume system is suitable for fundamental studies of hydrodynamic effects on SOD because it makes controlling hydrodynamic conditions easy, while the combined system is appropriate for studying the effect of microscopic phenomena on exchange rates, as it can reproduce natural microscopic physicochemical processes. [ABSTRACT FROM AUTHOR]

While constructed treatment wetlands are very efficient at polishing nitrate from secondary effluent, they are much less effective at removing ammonia. A key factor that limits ammonia oxidation via biological nitrification in vegetated wetlands is low levels of dissolved oxygen. This study evaluated the effectiveness of side-stream oxygenation to enhance ammonia removal in replicate surface-flow experimental mesocosms containing wetland sediment and plants (Typha spp.). Mesocosms had a water volume of 29.5 L, a hydraulic retention time of 5 days, and a hydraulic loading rate of 4.3 cm/d, and were loaded with synthetic secondary effluent contain 10 mg-N/L of ammonia. Relative to nonoxygenated controls, oxygenation increased ammonia removal rates by an order of magnitude. Areal removal rates increased from 40 mg-N/m2/d to 450 mg-N/m2/d, concentration removal efficiency increased from 10 to 95%, and area-based first-order removal rates increased from <2 m/year to 50–75 m/year. Ammonia removal rates in oxygenated mesocosms were 2- to 4-fold higher than rates reported for full-scale constructed wetlands treating secondary effluent. Results show that oxygen-activated nitrification wetlands, a hybrid of conventional oxygenation technology and wetland ecotechnology, hold promise in economically enhancing rates of ammonia removal and shrinking the wetland area needed to polish ammonia-dominated secondary effluent. Further study is needed to confirm that oxygenation can promote high rates of ammonia removal at the field scale. [ABSTRACT FROM AUTHOR]

A mechanistic oxygen transfer model was developed and applied to a flow-through hollow-fiber membrane-aerated biofilm reactor. Model results are compared to conventional clean water test results as well as performance data obtained when an actively nitrifying biofilm was present on the fibers. With the biofilm present, oxygen transfer efficiencies between 30 and 55% were calculated from the measured data including the outlet gas oxygen concentration, ammonia consumption stoichiometry, and oxidized nitrogen production stoichiometry, all of which were in reasonable agreement. The mechanistic model overpredicted the oxygen transfer by a factor of 1.3 relative to the result calculated from the outlet gas oxygen concentration, which was considered the most accurate of the measured benchmarks. A mass transfer coefficient derived from the clean water testing with oxygen sensors at the membrane-liquid interface was the most accurate of the predictive models (overpredicted by a factor of 1.1) while a coefficient determined by measuring bulk liquid dissolved oxygen underpredicted the oxygen transfer by a factor of 3. The mechanistic model was found to be an adequate tool for design because it used the published diffusion and partition coefficients rather than requiring small-scale testing to determine the system-specific mass transfer coefficients. [ABSTRACT FROM AUTHOR]

Nutrient enrichment of the South Umpqua River, Oregon was linked to periphyton growth and large diel fluctuations in dissolved oxygen and hydrogen ion (pH) concentrations using the water quality model QUAL2Kw. The available data provide a good case study for the relatively new water quality model. QUAL2Kw simulates a dynamic diel heat budget and water quality kinetics for a one-dimensional, steady-flow system and is part of a family of models meant to serve as an update to the widely used QUAL2E. The model was used to quantify nonpoint source loading, determine the pollutant of concern, estimate natural conditions, and calculate a phosphorus total maximum daily load during summer, low-flow conditions. Control of both nonpoint and point sources is required to achieve the low instream phosphorus concentrations necessary to meet water quality criteria. To our knowledge, this is the first paper that reports on the application of a model for computing the maximum allowable load necessary to manage the diel variation in pH. [ABSTRACT FROM AUTHOR]

A computer model was created as a scientific and management tool for understanding the effects of macrophytes on hydrodynamics and water quality. A model was required that could simulate macrophytes in a complex water body and could be coupled to a multicompartment water quality model of phytoplankton, dissolved oxygen, nutrients, pH, and organic matter. This would permit the investigation of water resource issues where macrophyte growth, phytoplankton growth, nutrient loadings, and flood control were all contributing factors. The model was added as a compartment to the U.S. Army Corps of Engineers two-dimensional, laterally averaged, dynamic water quality model, CE-QUAL-W2 (Corps of Engineers, water quality, width averaged, two dimensional) and applied to the Columbia Slough, Ore. Features of the macrophyte model include the capability to simulate multiple submerged macrophyte species; transport of nutrient fluxes between plant biomass and the water column and/or sediments; growth limitation due to nutrient, light and temperature; simulation of the spatial distribution of macrophytes vertically and horizontally; the modeling of light attenuation in the water column caused by macrophyte concentration; and the modeling of open channel flow with channel friction due to macrophytes. The macrophyte model was tested through mass balances and sensitivity analyses. The modeling of channel friction was evaluated by comparing predicted water levels with data from tests conducted in a laboratory flume. Use of the model in the Columbia Slough showed reasonable predictive capability regarding estimated biomass and water level dynamics. [ABSTRACT FROM AUTHOR]

Most commonly used river water quality models for biochemical oxygen demand (BOD) and dissolved oxygen (DO) simulations are mainly based on advection, decay, settling, and loading functions. Using these concepts, refined river water quality models for BOD and DO simulations are developed in the present work considering a large number of physically based parameters and input variables. The refined models developed can be transformed to some of the commonly used river water quality models, if physically based parameters and input variables are omitted or removed. To test the applicability of the refined models developed and commonly used models, a total of 732 water quality and flow data sets are collected during March 1999–February 2000 from 22 sampling stations of the River Kali in India. River Kali is a highly polluted river in India and receives continuous inflow of untreated point source pollution from municipal and industrial wastes and nonpoint source pollution from agricultural areas. Newton–Raphson technique is used to optimize the model parameters during calibration and the performance of different models are evaluated using error estimation, viz. standard error and mean multiplicative error, and correlation statistics (r2). The results indicate that the BOD–DO models proposed by Camp in 1963 provide better results in comparison to other commonly used models. Moreover, the refined models developed for BOD and DO simulations minimize error estimates and improve correlation between observed and computed BOD and DO values of River Kali. [ABSTRACT FROM AUTHOR]

A common difficulty in stream health assessments is the scarcity of real-time dissolved oxygen (DO) data. Discrete DO measurements, collected at times often imposed by sampling constraints, are difficult to use in assessments because of diurnal variations. An empirical model is developed here to adjust these discrete measurements to a common time-reference value using an extended stochastic harmonic analysis (ESHA) algorithm, which was originally formulated with a fraction of DO saturation model by the authors. The model was calibrated and validated for different stream sites across Minnesota, incorporating effects of different ecoregions and variable drainage areas. Data were normalized to increase the general applicability of the fitted parameters. Model calibration for five long record stations accurately represented observed diurnal variations in DO. The root-mean-square error (RMSE) for predicting hourly DO ranged from 0.53 to 0.80 mg/L and for predicting DO at a standard time ranged from 0.44 to 0.91 mg/L. Estimated model parameters were robust in terms of both spatial and temporal variations. Analytical as well as numerical analyses of parameter uncertainties were performed using sensitivity coefficients. Model validation with independent data for eight different Minnesota streams was performed using three different approaches for estimating parameters. The best approach considered both ecoregional location and watershed size to select representative model parameters. The RMSE for predicting hourly DO and standard DO respectively ranged from 0.53 to 1.65 mg/L and 0.00 to 1.83 mg/L. The developed model is a useful tool for total maximum daily load assessment of aquatic ecosystem health across a range of temporal and spatial scales. It is more elegant and simpler than the application of the ESHA algorithm for the fraction of DO saturation model. [ABSTRACT FROM AUTHOR]

Anaerobic pretreatment followed by aerobic posttreatment of municipal wastewater is being used more frequently. Recent investigations in this field using an anaerobic fluidized bed reactor/aerobic solids contact combination demonstrated the technical feasibility of this process. The investigation presented herein describes the use of a combined upflow anaerobic sludge bed (UASB)/aerobic solids contact system for the treatment of municipal wastewater and attempts to demonstrate the technical feasibility of using the UASB process as both a pretreatment unit and a waste activated sludge digestion system. The results indicate that the UASB reactor has a total chemical oxygen demand removal efficiency of 34%, and a total suspended solids removal efficiency of about 36%. Of the solids removed by the unit, 33% were degraded by the action of microorganisms, and 4.6% accumulated in the reactor. This low solids accumulation rate allowed operating the UASB reactor for three months without sludge wasting. The long solids retention time in this unit is comparable to the one normally used in conventional sludge digestion units, thus allowing the stabilization of the waste activated sludge returned to the UASB reactor. Particle flocculation was very poor in the UASB reactor, and therefore, it required postaeration periods of at least 100 min to proceed successfully in the aerobic unit. Polymer generation, which is necessary for efficient biological flocculation, was practically nonexistent in the anaerobic unit; therefore, it was necessary to maintain dissolved oxygen levels greater than 1.5 mg/L in the aerobic solids contact chamber for polymer generation to proceed at optimum levels. Once these conditions were attained, the quality of the settled solids contact chamber effluent always met the 30 mg BOD/L, 30 mg SS/L secondary effluent guidelines. [ABSTRACT FROM AUTHOR]

This paper describes the influence of water outside a bay on water quality within the bay for a typical enclosed bay with a sill at its mouth. The influence of water outside Ohfunato Bay, Japan was expressed in a one-dimensional vertical dissolved oxygen (DO) model as the rate of seawater exchange caused by tides and the density difference between water inside and outside the bay. A nonhydrostatic, three-dimensional model was used to confirm the occurrence of density intrusions. A one-dimensional vertical DO model was used to examine long-term changes in DO concentrations in the bottom layer of the bay. Results indicated that seawater exchange caused by density differences had a significant effect on the formation and disappearance of anoxic water in the bay’s bottom layer. [ABSTRACT FROM AUTHOR]

The development of water quality models, and also the nature of water quality impairment, is uniquely presented in the point source dissolved oxygen (DO) modeling completed in the Jackson River (Virginia) over the past 50 years. Various water quality modeling studies have been completed in the Jackson River over the years starting with the earliest of modeling frameworks, the Streeter–Phelps equation (1950s and 1960s); progressing to a biochemical oxygen demand–DO model (1970s and 1990s) including diurnal photosynthetic effects (DIURNAL); a Monte Carlo DO analysis using the DIURNAL model (1990s); to the most recent modeling that is currently developing a periphyton model to assess the impact of nutrient loadings on the periphyton community and ultimately DO levels (2000). These early modeling studies were completed by such modeling forefathers as Clarence J. Velz and Donald J. O'Connor, both completing their work at academic institutions (Manhattan College and the University of Michigan) and private consulting firms (Hydroscience and HydroQual, Inc.). Interesting to note is that Earle B. Phelps taught Clarence J. Velz, Donald J. O’Connor’s eventual professor at Manhattan College. Other work completed on the river by early environmental engineers included reaeration studies by Ernest C. Tsivoglou (1966) and the first activated sludge wastewater treatment design for a pulp and paper mill by Wesley Eckenfelder (1950s). The studies investigated: how to improve existing DO conditions in the river; the effects of color reductions on diurnal DO swings; proposed upstream flow regulation effects on water quality and river temperature; and the impact of instream oxygen addition. [ABSTRACT FROM AUTHOR]

In order to understand the redox chemistry of chromium at low concentrations (100 μg/L) under conditions typically found in drinking water systems, three reductants and four oxidants were tested in three different waters at pH 5, 7, and 9. In the absence of any oxidant or reductant, Cr(VI) was stable at all three pHs, while Cr(III) precipitated out of solution at pH 9 and greatly impacted the reduction reactions. Stannous chloride was more effective than sodium sulfite or sodium sulfide for reducing Cr(VI) to Cr(III). Sulfide is not likely to be used as a reductant due to the long reaction time (120 h) to achieve the same reduction as SnCl2, while sulfite may be effective at higher doses. The oxidation of Cr(III) by dissolved oxygen and chloramine was very slow, while Cl2 and KMnO4 were effective oxidants under many conditions. A Cl2 residual in a drinking water distribution system may oxidize any soluble Cr(III) to Cr(VI) because of the long contact time, so Cr treatment strategies will need to remove both Cr(III) and Cr(VI). [ABSTRACT FROM AUTHOR]

A one-dimensional reservoir-bubbler model has been developed to examine the mixing and the change in dissolved oxygen pattern induced by bubbler operation in a stratified reservoir. The reservoir-bubbler model is applied to a tropical reservoir, the Upper Peirce Reservoir, Singapore. For this tropical reservoir with low wind speeds, it is found that bubbler operation dominates oxygen transfer into the reservoir water rather than oxygen transfer from all other sources, including surface reaeration. It is illustrated that selection of airflow rate per diffuser, air bubble radius, and total number of diffusers are important criteria in bubbler designs. Higher dissolved oxygen levels in reservoirs are obtained by increasing the bubbler airflow rate that is associated with lower mechanical efficiency (ηmech) than optimal ηmech of the bubbler. Determining an appropriate airflow rate is shown to be a tradeoff between increased dissolved oxygen levels and increased operating costs as airflow rate increases. When the reservoir is close to well mixed, the water quality is usually reasonably good but the bubbler operates at a very low ηmech—thus the bubbler should be turned off. [ABSTRACT FROM AUTHOR]

The dilution method has been accepted widely as the standard procedure for determining biochemical oxygen demand (BOD). A mass-balance approach was used to analyze four equations for computing BOD: two involving external seed correction methods (Standard Methods) and two involving internal correction methods (Sawyer and McCarty in 1978 and Kline and Gibbs in 1979). Five testing conditions were identified as being pertinent to the underlying assumptions: seed addition method, uniform BOD bottle volume, uniform initial dissolved oxygen (DO), seed uptake linearity, and data handling method. The results of this study showed that internal methods may provide more reproducible results, primarily due to the inherent correction for dilution water demand and better data handling (regression) technique to cope with the variation in BOD bottle volume and initial DO. However, internal methods may violate the requirements imposed by Standard Methods for minimum final DO (1 mg/L) for the high spike runs and minimum 5 day DO depletion (2 mg/L) for the low spike runs. Recommendations are also made to improve the reliability of these methods. [ABSTRACT FROM AUTHOR]

Comments the article "Oxygen Sag Models for Multiorder Biochemical Oxygen Demand Reactions." Citations of fundamental error in the measurement of biochemical oxygen demand (BOD) rates in BOD tests; Basis in the determination of dissolved oxygen sag equation; Difference between the bottle BOD rate and the loss rate in the river.

Stepped waterways are commonly used as river training, debris dam structures, storm water systems, and aeration cascades. The present study was focused on analysis of basic air–water flow properties on a low gradient stepped chute, combined with dissolved oxygen measurements. The oxygen aeration efficiency was found to be about 30% for 12 steps with a total drop in invert elevation of 1.4 m, nearly independently of the inflow conditions. Detailed air–water flow measurements, including void fraction, velocity, bubble count rate, and interface area, were used to integrate the mass transfer equation and to estimate the aeration potential of the waterway. Direct comparisons with dissolved oxygen measurements showed good agreement between the two methods. [ABSTRACT FROM AUTHOR]

A hybrid model which incorporated a biofilm model into the general dynamic model was developed to predict the effluent quality of a combined activated sludge and biofilm process—Taiwan National Central University Process 1. The system was performed under three different dissolved oxygen (DO) conditions in the oxic tank, including 2.0, 1.0, and 0.5 mg/L. When the DO increased from 0.5 to 2.0 mg/L, the soluble biodegradable substrate (Se) and soluble phosphate (PO4) in the effluent were not significantly influenced. Their removal efficiencies were above 92 and 94%. Ammonia-nitrogen (NH3) removal efficiency increased from 36 to 83% and nitrate-nitrogen (NO3) increased from 1.7 to 2.9 mg/L. In biofilm, when the DO was 2.0 mg/L, the active autotrophic biomass (ZA) fraction was 15.7% (surface) to 12.9% (substratum). But when the DO was 0.5 mg/L, the ZA fraction became lower and the fraction was 6.2% (surface) to 3.5% (substratum). The fraction of active nonpoly-P heterotrophic biomass (ZH) in the biofilm did not vary significantly, the values were about 28-35%. ZI decreased as the DO increased. SS in the biofilm did not vary significantly and was maintained at about 2.0 mg/L. When DO increased, NO3 also increased, NH3 decreased from 13.1 to 1.8 mg/L in biofilm. [ABSTRACT FROM AUTHOR]

A method is presented to compute sensitivities of in-stream dissolved oxygen (DO) with respect to perturbations in the load vector and the reaction coefficients that make up the eutrophication cycle. It is shown that the direct sensitivity method, i.e., the repetitive solution of the direct problem, produces the desired information, however at a large computational cost. The utilization of the adjoint sensitivity method proves to be a much more efficient way to compute these sensitivities as large subsets of the sensitivity information domain can be easily extracted with just a few runs. It is found that for the given problem setup, in-stream DO is most sensitive to ammonia, effluent DO, algae, and carbonaceous biochemical oxygen demand (CBOD) loads. Additionally, the computed sensitivities vary considerably in their general trend over the simulation time. Sensitivities also are computed with respect to the reaction coefficients (26 total) that govern the interdependency of all constituents. Sediment oxygen demand proves to be the coefficient with the highest influence that is three orders of magnitude higher than the next set of coefficients comprised of reaeration, CBOD degradation, nitrification, and denitrification. All other coefficients have a negligible influence on DO concentrations. Computations are carried out using a two-dimensional model formulation applied to a long rectangular channel with varying width and slope and periodic but unsteady flow conditions. [ABSTRACT FROM AUTHOR]

This paper presents the development of an efficient strategy for achieving in-stream dissolved oxygen (DO) water quality standards (WQSs) via optimized point-load control strategies using the adjoint method. To this end, a least-squares-type objective function is formulated that measures the difference between desired (WQSs) and current DO concentrations at strategically selected monitoring points in the domain. The goal is to minimize the difference between actual DO concentration and the WQS, hence allowing time-variant loadings to utilize the assimilative capacity of the receiving water body at an optimal level. Time-variant discharge rates for a number of discharge locations are considered as control parameters, while different zone-specific critical DO levels are imposed as constraints. The selection of the control is kept flexible and a number of different scenarios are tested. First, only carbonaceous biochemical oxygen demand is used, which allows for a reduction of the number of equations that need to be solved. In the other tests, all constituents are switched on and different variables at each load node are selected as a control by first varying the concentrations individually, and then linking them through control of the volumetric flow rate. Optimization is achieved using a conjugate gradient search method, for which the gradients are computed through the solution of both the direct and adjoint problems. It is shown that the large amount of gradient information (parameter space has a dimension of several thousands) can be computed very efficiently using the adjoint, and that optimized results are achieved after only a few iterations irrespective of the initial guess. Computations are carried out using both two-dimensional model formulation applied to a long rectangular channel with varying width and slope and a model for the upper Potomac River estuary. [ABSTRACT FROM AUTHOR]

Oxidation processes can oxidize biorecalcitrant compounds into biodegradable intermediates, which in turn can be treated less expensively by a subsequent biological process. To design such a two-step (chemical+biological) process to treat poorly characterized wastewaters, it is useful to model the time evolution of characteristic global variables, chemical oxygen demand (COD) and biochemical oxygen demand (BOD), in order to develop a useful treatment strategy based upon these classical variables. We consider two simple model reaction networks, requiring three- and five-rate constants, respectively. The first model, proposed recently, involves conversion of a nonbiodegradable species, C, into a single biodegradable intermediate S. Here, biodegradable compounds are immediate kinetic products of oxidation. In general, it is not probable that a single recalcitrant compound undergoes a single-step reaction to CO2. However, when working with complex undefined wastewaters streams, single-step reactions may be used to simplify. The second new model corresponds to a lag time in BOD formation due to the necessity of multiple partial oxidations to reach a first biodegradable intermediate. The second network includes two intermediates, I and S, which are, respectively, nonbiodegradable and biodegradable. We then compare model behavior with an unfortunately sparse literature on BOD and COD values versus time in chemical reactors, and demonstrate the convenience of the first model, and the occasional necessity of the second, which reflects the presence of early intermediates which are nonbiodegradable. [ABSTRACT FROM AUTHOR]

The empirical biochemical oxygen demand (BOD) equation is expressed as a multiorder reaction equation of order n, then is combined with the dissolved oxygen mass balance equation to give the differential form of an oxygen sag equation for small rivers and streams for which dispersion can be neglected. The value of n in the BOD reaction is restricted to values that are larger than one (first order). The dissolved oxygen sag equation is verified with two published dissolved oxygen sag models by setting n equal to 3/2 (three-halves order BOD reaction), and n equal to 2 (second order BOD reaction). The proposed dissolved oxygen sag equation may be applied to test the BOD and dissolved oxygen models in large, complex numerical models, such as models used in developing total maximum daily load recommendations. Examples show how the BOD reaction order affects the dissolved oxygen sag characteristics of a river. [ABSTRACT FROM AUTHOR]

Field measurements were conducted to investigate the onset and growth of hypoxia in the Tone River Estuary. Vertical profiles of dissolved oxygen (DO), salinity, and temperature were measured along the deepest line of the estuary. Rates of oxygen consumption by water and sediment in a salt wedge were obtained using laboratory tests. The measurements showed that hypoxia frequently occurred in the front part of the salt wedge and expanded its area toward the river mouth during the summer when the river flow rate was small. The data also suggested that the onset of hypoxia was delayed by the estuarine circulation which supplied oxygen-rich seawater to the salt wedge. To simulate this phenomenon, a two-dimensional flow model in the vertical-longitudinal plane was constructed by transversely integrating the k-ε model equations. The results of model simulation for three months in the summer of 1997 closely matched the field data. The model simulation proved that DO degradation is highly correlated with the residence time of salt water in the estuary. [ABSTRACT FROM AUTHOR]

Reaeration in combined wind/stream driven flows was experimentally studied in a laboratory wind-water tunnel. Wind velocities, stream flow properties, and oxygen transfer rates were measured under a variety of cocurrent and countercurrent flow conditions. For pure open channel flows and pure wind-driven flows, respectively, the results of oxygen transfer measurement can be scaled by the simple forcing parameters and show good agreement with other laboratory studies. For wind/stream combined flows, the transfer velocity depends on the stream flow condition and wind shear velocity. Based on the magnitude of the bottom-shear induced transfer and the wind-shear induced transfer velocity, a criterion was proposed to determine the dominant mechanism for gas transfer in combined wind/stream driven flows. A simple additive model is developed for the reaeration coefficient in combined wind and stream flow-influenced environments. Also, the effects of tidal flow on reaeration in tidal estuaries are briefly discussed. [ABSTRACT FROM AUTHOR]