Dissolved Oxygen Demand at the Sediment-Water Interface of a Stream: Near-Bed Turbulence and Pore Water Flow Effects

A microbial dissolved oxygen (DO) uptake model was developed for a stream bed, including the effect of turbulence in the flow over the bed and pore water flow in the porous bed. The fine-grained sediment bed has hydraulic conductivities 0.01 ≤ k ≤ 1??cm/s, i.e., sediment particle diameter 0.006 ≤ ds ≤ 0.06??cm. The pore water flow is driven by pressure fluctuations at the sediment-water interface, mostly attributable to near-bed coherent motions in the turbulent boundary layer above the sediment bed. An effective mass transfer coefficient (De) coupled to a pore water flow model was used in the DO transport and DO uptake model. DO flux across the sediment-water interface and into the sediment, i.e., sedimentary oxygen demand (SOD), was related to hydraulic conductivity and microbial oxygen uptake rate in the sediment and shear velocity at the sediment-water interface. Simulated SOD values were validated against experimental data. For hydraulic conductivities of the sediment bed up to k ≈ 0.01??cm/s, the pore water flow effect on SOD was found negligible. Above this threshold, the effective mass (DO) transfer coefficient in the sediment bed (De) becomes larger as the hydraulic conductivity (k) becomes larger as the interstitial flow velocities increase; consequently, DO penetration depth increases with larger hydraulic conductivity of the sediment bed (k), and SOD increases as well. The enhancement of vertical DO transport into the sediment bed is strongest near the sediment-water interface, and rapidly diminishes with depth into the sediment layer. An increase in shear velocity at the sediment-water interface also enhances DO transfer. Shear velocity increases at the sediment-water interface will raise SOD regardless of the maximum oxidation rate if the hydraulic conductivity is above the threshold of k ≈ 1??cm/s. The relationship is nearly linear when U*<0.8??cm/s. At shear velocity U* = 1.6??cm/s, SOD for oxidation rates μ = 1000 and 2000??mg?l-1?d-1 are almost five times larger than those with no pore water flow. When pore water transport of DO is not limiting, SOD is a linear function of oxygen demand rate μ in the sediment when 0 ≤ μ ≤ 200??mg?l-1?d-1.     

Water-Circulating Aerator: Optimizing Structure and Predicting Water Flow Rate and Oxygen Transfer

Thermal stratification is a common phenomenon in deep lakes and reservoirs, which often results in water-quality deterioration, including such problems as hypolimnetic anoxia, the release of pollutants from sediments, and algal blooms. Hypolimnetic oxygenation and destratification are the two commonly used methods for resolving these water-quality problems. A new water-quality improvement device, the water-circulating aerator, was designed to destratify lakes and reservoirs, by circulation and oxygenation of upper and lower layers of water. The design of the structure of the water-circulating aerator is detailed. Three mathematical models were built to optimize this structure, estimate the rate of water flow in the aerator, and calculate the rate of oxygen transfer from air bubbles to water in the aerator. These models were verified by experiments. The water-circulating aerator system has been successfully applied in a stratified reservoir to increase dissolved oxygen to reduce the releasing of ammonia-nitrogen from sediments under anoxic conditions.     

Sediment Oxygen Demand and Nutrient Fluxes for a Tropical Reservoir in Singapore

Sediment oxygen demand (SOD) and nutrient flux studies were conducted for a tropical reservoir in Singapore in order to determine the approximate SOD and nutrient release rates from the sediments. SOD values obtained from laboratory experiments ranged from 1.4 to 3.3?g?O2/m2-day. Similar results were also obtained by calculating SOD values from in situ DO measurements taken in the field. The nutrient flux study was performed in the laboratory at a constant temperature of 25°C in oxic and anoxic columns. Except for nitrate+nitrite, higher nutrient release rates were generally observed under anoxic conditions. The ammonium release rate was 0.06?g?O2/m2-day under oxic conditions and 0.117?g?O2/m2-day under anoxic conditions. The nitrate flux rate was 0.17?g?O2/m2-day under oxic conditions but was negligible under anoxic conditions. Orthophosphate flux results were negative throughout the oxic incubation implying that sediments acted as a sink. The release rate of orthophosphate was 0.007?6?g?O2/m2-day under anoxic conditions.     

Evidence of High Vertical Wave-Number Behavior in a Continuously Stratified Reservoir: Boadella, Spain

High vertical wave-number modes clearly dominate the internal wave field during the stratification period in Boadella reservoir in northeast Spain. In this period, the extraction of hypolimnetic water, due to summer irrigation, brought the surface level down by 6 m in one month and the epilimnetic water progressively occupied the whole water column. The temperature profile, with the exception of a few meters at the surface layer, presented an almost constant temperature gradient of about 0.7°C/m. The period of the main vertical mode is 24 h with an amplitude of around 1 m. Thermistor chain records and meteorological data allow us to deduce that this mode is, at least, a third vertical mode forced by the wind, which normally has a typical periodicity of 24 h. However, when the wind changes direction from south to north, the circulation cells developed due to this forced nonstationary oscillation are destroyed. When this occurs, the Bulk Richardson number is Rib ～ 1. Similar vertical structures as a response to wind forcing should be expected in similar systems, although this has not been reported in the literature.     

Simultaneous Removal of Carbon and Nitrogen from Swine Wastewater Using an Immobilized-Cell Reactor

A single-stage phosphorylated polyvinyl alcohol immobilized-cell reactor with three operation modes was employed to investigate the efficiency of simultaneous carbon/nitrogen removal from raw swine wastewater. In continuous aeration mode, the removal efficiency of chemical oxygen demand (COD) and total nitrogen (T-N) exceeded 70 and 8%, respectively, at hydraulic retention time of 10?days. In intermittent aeration (IA) mode, the removal efficiency of COD and T-N was more than 85 and 46%, respectively, when the reactor was set at 50% aeration duration to cycle time to operate at three aerobic-anoxic cycles per day. When oxidation-reduction-potential control was adopted to control the duration of the anoxic period in the real-time controlled (RTC) IA mode for a 4?h aeration period, the total cycle time was reduced by about 20% with a slight increase in removal efficiency of COD (87%) and T-N (47%). The system with no extra chambers required is efficient in simultaneous carbon/nitrogen removal.     

Effect of Long Internal Waves on the Quality of Water Withdrawn from a Stratified Reservoir

The properties of water withdrawn from a stratified reservoir are investigated in a field study conducted in Lake Burragorang, Australia. It is shown that temperature and turbidity fluctuations of the extracted water are directly correlated to the vertical displacement of the thermal structure of the reservoir immediately in front of the offtake and the thickness of the selective withdrawal layer. Scaling of the unsteady withdrawal revealed that the timescale associated with the formation of selective withdrawal is an order of magnitude smaller than the typical period of the internal wave. This means the withdrawal layer is acting as a filter, extracting water of a particular quality as it is swept past the outlet by the internal seiches; the steady-state theory of the selective withdrawal can be used to predict outflow temperature fluctuations in reservoirs where long internal waves are present. To correctly interpret other outflow water parameters, such as turbidity or dissolved oxygen, it is important not only to know the stratification conditions in front of the offtake, but also to understand the local flow dynamics in the lower reaches of the reservoir.     

Case Study: Impact of Reservoir Stratification on Interflow Travel Time

A case study is presented on the relation between interflow travel time and reservoir stratification. A simulation model is calibrated and validated for the Wachusett Reservoir in Massachusetts. The Reservoir has a major controlled inflow which traverses the reservoir as an interflow. The model is used with a range of alternate inflow schedules and the resulting travel time of the interflow is examined. The inflow density is within the range of densities found in the reservoir thermocline and the inflow rate is sufficient to maintain a continuous interflow. Under these conditions it is found that a linear relation exists between the average interflow travel time, as measured by the arrival of a specified fraction of interflow water at the outlet, and the degree of stratification, as measured by the maximum difference in reservoir thermocline temperature, at the initiation of the inflow. The results may be useful for operation of the reservoir under study subject to continued validation of the simulation model used.     

Oxygen Demand by a Sediment Bed of Finite Length

A model of sedimentary oxygen demand (SOD) for a sediment bed of finite length is presented. The responses of diffusive oxygen transfer in turbulent flow above the sediment surface and of microbial activity inside the sediment to a developing diffusive boundary layer are modeled numerically. The developing diffusive boundary layer above the sediment/water interface is modeled based on shear velocity and turbulent boundary layer concepts, and dissolved oxygen (DO) uptake inside the sediment is modeled as a function of the microbial growth rate. The model predicts that the diffusive boundary layer above the sediment/water interface thickens in flow direction, and that DO penetration depth into the sediment is practically constant over the length of the sediment bed. The effect of the developing diffusive boundary layer on SOD is minor, except at very low shear/flow velocities (shear velocity U*<0.01?cm/s) and/or high microbial density inside the sediment. The average SOD over the sediment bed therefore varies only slightly with its length. SOD varies somewhat in flow direction, i.e., SOD is largest near the leading edge (x = 0), decreases with distance, and finally, approaches a nearly constant value for fully developed boundary layer. Including microbial activity in the sediment makes the change of SOD in flow direction much smaller than is predicted by a pure vertical diffusive flux model. The diffusive boundary layer is nearly fully developed at a dimensionless distance x+ = 10,000, regardless of microbial activity inside the sediment. Longer sediment beds are required to eliminate the small leading edge effect on any measured average SOD value. SOD depends strongly on the diffusion coefficient of DO inside the sediment bed. This effect becomes more significant as shear/flow velocity is increased. Overall, SOD is found to be controlled principally by shear velocity of the water flowing above the sediment/water interface, microbial activity inside the sediment, and diffusion of DO inside the sediment. The length of the sediment bed is of lesser influence.     

In Situ Characterization of Resuspended-Sediment Oxygen Demand in Bubbly Creek, Chicago, Illinois

Sediment oxygen demand (SOD) can be a significant oxygen sink in various types of water bodies, particularly slow-moving waters with substantial organic sediment accumulation. In most settings in which SOD is a concern, the prevailing hydraulic conditions are such that the impact of sediment resuspension on SOD is not considered. However, in the case of Bubbly Creek in Chicago, the prevailing slack water conditions are interrupted by infrequent intervals of very high flow rates associated with pumped combined sewer overflow (CSO) during intense hydrologic events. These events can cause resuspension of the highly organic, nutrient-rich bottom sediments, resulting in precipitous drawdown of dissolved oxygen (DO) in the water column. To address this issue, a new in situ experimental apparatus designed to achieve high flow velocities was implemented to characterize SOD, both with and without sediment resuspension. In the case of resuspension, the suspended sediment concentration was analyzed as a function of bed shear stress, and a formulation was developed to characterize resuspended-sediment oxygen demand (SODR) as a function of suspended sediment concentration in a form similar to first-order biochemical oxygen demand (BOD) kinetics with the DO term in the form of Monod kinetics. The results obtained can be implemented into a model containing hydrodynamic, sediment transport, and water-quality components to yield oxygen demand varying in both space and time for specific flow events. The results are used to evaluate water quality improvement alternatives that take into account the impact of SOD under various flow conditions.     

Response of a Tropical Reservoir to Bubbler Destratification

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.     

Influence of Stratification and Shoreline Erosion on Reservoir Sedimentation Patterns

Sedimentation in the main pool of a deep (maximum depth: 50?m), 227?km2 hydropower reservoir was modeled using a three-dimensional numerical model of hydrodynamics and sedimentation for different wind, inflow, and outflow conditions. Short-term velocity measurements made in the reservoir were used to validate some aspects of the hydrodynamic model. The effects of thermal stratification on sedimentation patterns were investigated, since the reservoir is periodically strongly stratified. Stratification alters velocity profiles and thus affects sedimentation in the reservoir. Sedimentation of reservoirs is often modeled considering only the deposition of sediments delivered by tributaries. However, the sediments eroding from the shorelines can contribute significantly to sedimentation if the shorelines of the reservoir erode at sufficiently high rates or if sediment delivery via tributary inflow is small. Thus, shoreline erosion rates for a reservoir were quantified based on measured fetch, parameterized beach profile shape, and measured wind vectors, and the eroded sediments treated as a source within the sedimentation modeling scheme. The methodology for the prediction of shoreline erosion was calibrated and validated using digital aerial photos of the reservoir taken in different years and indicated approximately 1?m/year of shoreline retreat for several locations. This study revealed likely zones of sediment deposition in a thermally stratified reservoir and presented a methodology for integration of shoreline erosion into sedimentation studies that can be used in any reservoir.     

Effect of Aeration on the Quality of Effluent from UASB Reactor Treating Sewage

The treatment of effluent of pilot- and full-scale upflow anaerobic sludge blanket (UASB) reactors operating at steady state was studied in an aeration-settling system. The fine pore submerged diffusers were used to aerate the effluent of UASB reactors under different operating conditions. Forty to 55% of the biochemical oxygen demand (BOD) and the chemical oxygen demand (COD) removal efficiencies were achieved by the direct aeration of the UASB effluent in the laboratory. The maximum removal efficiencies were achieved at 30?min hydraulic retention time (HRT) and a dissolved oxygen (DO) of 5–6??mg/L or high KLa (vigorous aeration). Batch experiments on nitrogen purging and the aeration of sulfides, volatile organic compounds (VOCs), and nonpurgeable organic carbons (NPOCs) were performed to ascertain the mechanism of BOD/COD removal. During aeration, BOD and COD were reduced by the stripping of H2S and VOCs and by the chemical oxidation of total sulfides and organic carbon. The stripping and chemical oxidation depended on the HRT and DO. The performance of a full-scale surface aeration system was compared to the performance of a pilot-scale diffused aeration system. Final sedimentation was effective only in removing the solids from the effluent of the aeration system. The results were confirmed by organic mass balance.     

Air–Water Mass Transfer on a Stepped Waterway

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.     

Long-Term Operation of Irrigation Dams Considering Variable Demands: Case Study of Zayandeh-rud Reservoir, Iran

Dependency of water demands on the climate variation occurs especially in regions where agricultural demand has a significant share of the total water demands. The variability between demands that are based on annual climate conditions may be larger than the uncertainty associated with other explanatory variables in long-term operation of an irrigation dam. This paper illustrates certain benefits of using variable demands for long-term reservoir operation to help manage water resources system in Zayandeh-rud river basin in Iran. A regional optimal allocation of water among different crops and irrigation units is developed. The optimal allocation model is coupled with a reservoir operating model, which is developed based on the certain hedgings that deals with the available water and the water demands mutually. This coupled model is able to activate restrictions on allocating water to agricultural demands considering variation of inflow to the reservoir, variation of demands, and the economic value of allocating water among different crops and irrigation units. Using this model, long-term operation of Zayandeh-rud dam is evaluated considering different scenarios of inflow to the reservoir as well as agricultural demands. The results indicate that the use of operating rules which consider variable demands could significantly improve the efficiency of a water resources system in long-term operation, as it improves the benefit of Zayandeh-rud reservoir operation in comparison with conventional water supply approaches.     

Anaerobic Treatment of High Sulfate Wastewater with Oxygenation to Control Sulfide Toxicity

In this study, oxidation-reduction potential (ORP) was employed to regulate oxygen dosing for online sulfide toxicity control during anaerobic treatment of high sulfate wastewater. The experiment was conducted in an upflow anaerobic filter, which was operated at a constant influent total organic carbon of 6,740 mg/L [equivalent to a chemical oxygen demand (COD) of 18,000 mg/L], but with different influent sulfates of 1,000, 3,000, and 6,000 mg/L. The reactor was initially run at natural ORP (the system’s ORP without oxygenation) of about ?290 to ?300?mV and then was followed by oxygenation to raise ORP by +25?mV above the natural level for each influent sulfate level. At 6,000 mg/L sulfate under the natural ORP, methanogenesis was severely inhibited due to sulfide toxicity, and the anaerobic process was almost totally upset. Upon oxygenation by raising ORP to ?265?mV, the dissolved sulfide was quickly reduced to 12.2 mg S/L with a concomitant improvement in methane yield by 45.9%. If oxygen was not totally used up by sulfide oxidation, the excess oxygen was consumed by facultative bacteria which had been found to stabilize about 13.5% of the influent COD. Both sulfide oxidation and facultative activity acted as a shield to protect the anaerobes from an excessive oxygen exposure. This study showed that direct oxygenation of the recirculated biogas was effective to oxidize sulfide, and the use of ORP to regulate the oxygen dosing was practical and reliable during anaerobic treatment of high sulfate wastewater.     

Diffusional Mass Transfer at Sediment–Water Interface of Cylindrical Sediment Oxygen Demand Chamber

Diffusional mass transfer of dissolved substances across the sediment–water interface in coastal waters is an important factor for realistic determination of sediment oxygen demand (SOD) and nutrient recycle. The benthic diffusive boundary layer inside a cylindrical chamber commonly deployed for in situ measurements of sediment oxygen demand is studied. In a series of laboratory experiments, the SOD is measured with the chamber operated in both continuous flow and batch modes, and a microelectrode is employed to measure the near bed dissolved oxygen (DO) profile for different chamber flows and sediment types. The dependence of the diffusive boundary layer thickness and the sediment–water mass transfer coefficient on the hydraulic parameters are quantified. Using the derived mass transfer coefficient, it is shown that for a given sediment type, the SOD is a function of the bulk DO concentration and chamber flowrate. The theoretical predictions are validated by both laboratory and field SOD data.     

Experimental Study on Selective Withdrawal in a Two-Layer Reservoir Using a Temperature-Control Curtain

An experimental study was conducted to investigate the use of a temperature-control curtain in selective withdrawal from a two-layer stratified reservoir. This study focused on the case where cool water at a depth was forced to flow under the curtain. The evolution of the mean flow, the withdrawal water quality, and the mean velocity field were studied using particle image velocimetry and laser-induced fluorescence. Practical relationships were developed for predicting the withdrawal water quality and the interface height as a function of time. The structures of the flow field in both the upper and lower layers are discussed in detail. The flow in the lower layer was dominated by the recirculation eddy induced by the jet flow under the curtain and a relation between the eddy length and the interface height was obtained. Close to the intake, within about 3d (where d = intake diameter), the velocity field can be well described by the potential flow theory. Beyond 3d, however, the flow field considerably deviated from the potential flow theory due to the jet expansion and stratification. A general discussion of the results and engineering applications are also provided.     

Gas Transfer During Bubbler Destratification of Reservoirs

In this paper, dimensional analysis has been carried out to derive general equations that predict: the total gas transferred to the ambient reservoir water from an air bubbler, total volume entrained, and total energy consumed for a known or equivalent linear stratification. The equations are tested by comparison with a one-dimensional bubbler model developed by the authors. It is shown that the oxygen transfer to the water column can be significant if small bubbles are used. The mechanical destratification efficiency ηmech (%), destratification time per unit surface area Γ?(s/m2), oxygen dissolution efficiency Ω (%), and oxygen transferred per unit input energy are examined as functions of bubble size. It is concluded that an average bubble radius of 1?mm should be considered for design purposes. However, if oxygen transfer from the bubbler is not considered important, then a bubble size of up to 4?mm is acceptable for destratification purposes.     

Modeling Artificial Aeration Kinetics in Ice-Covered Lakes

A lake hydrodynamic model has been enhanced to simulate ice cover and artificial aeration during ice cover periods. Artificial aeration using mechanical surface aerators (“splashers”) and point-source bubblers (“bubblers”) is examined. Applying the model to two lakes in Alberta, Canada, indicate the model's capacity to handle a range of lake conditions and aeration operations. The sediment bed is found to be an important source of both heat and biochemical oxygen demand to the water column, during both natural conditions and artificial mixing periods. The ice cover thickness is shown to be a function of snow weight and insulation effects. The effects of an opening in the ice cover are a net gain in dissolved oxygen and a net loss of heat. The design and placement of aerators in the lake, as well as their operation schedules, are shown to determine the volume of mixed water and aeration effectiveness. This model is suitable for designing lake aeration systems to prevent winterkill in subarctic lakes.     

Dissolved Oxygen Downstream of an Effluent Outfall in an Ice-Covered River: Natural and Artificial Aeration

In ice-covered rivers, dissolved oxygen (DO) might fall below critical levels for aquatic biota in the absence of surface aeration, combined with low winter flow conditions and reduced photosynthesis rates. Open-water zones, however, can be created downstream of a diffuser by warm effluent discharges, resulting in an increase in surface aeration. In this study, we modeled the behavior of the effluent plume and the resulting open-water lead development in the Athabasca River, Alberta, Canada downstream of a pulp mill diffuser. The DO was found to increase by 0.26?mg/L due to surface aeration of an open-water lead of 6.07?km. We also evaluated oxygen injection into the effluent pipeline to increase the DO in the river. At an injection rate of 3,500 and 5,000?lb/day of liquid oxygen, the DO was increased by 0.16 and 0.21?mg/L, which corresponded to an absorption efficiency of about 50%. The artificial aeration technique evaluated here appears to be an effective alternative to increase DO levels in ice-covered rivers. The results of this study are important in developing accurate DO models for ice-covered rivers and in evaluating oxygen injection systems.