Controlling soluble iron and manganese in a water-supply reservoir using hypolimnetic oxygenation

Soluble metals such as iron (Fe) and manganese (Mn) often reach problematic levels in water-supply reservoirs during summer stratification following the onset of hypolimnetic hypoxia. The behavior of soluble and particulate Fe and Mn was studied following the installation of a hypolimnetic oxygenation system in Carvins Cove Reservoir, a water-supply impoundment managed by the Western Virginia Water Authority. During oxygenation, manganese concentrations were very low in the bulk hypolimnion (<0.05 mg l⁻¹), but high concentrations (>2.0 mg l⁻¹) were still observed in the benthic region close to the sediment, despite near-sediment dissolved oxygen concentrations in excess of 5.0 mg l⁻¹. Oxygenation appears to affect the location of the oxic/anoxic boundary sufficiently to restrict substantial transport of soluble Mn to the bulk water of the hypolimnion. However, the position of the oxic/anoxic boundary was not uniformly affected along the reservoir bottom, allowing horizontal transport of soluble Mn from higher elevations in contact with hypoxic sediments. During one summer, when the oxygen system was turned off for a month, the soluble Mn in the bulk hypolimnion increased substantially. Oxygen concentrations were quickly restored after the system was turned back on, but elevated levels of soluble Mn persisted until the sedimentation rate of detritus through the hypolimnion increased. When operated without interruption, the oxygenation system was able to reduce the bulk average hypolimnion soluble Mn concentration by up to 97%, indicating that source water control of soluble Mn and Fe can be accomplished with hypolimnetic oxygenation in water-supply reservoirs.     

Gas transfer from air diffusers

The bubble and surface volumetric mass transfer coefficients for oxygen, k(L)a(b) and k(L)a(s), are separately determined for 179 aeration tests, with diffuser depths ranging from 2.25 to 32 m, using the DeMoyer et al. 12003. Impact of bubble and free surface oxygen transfer on diffused aeration systems. Water Res 37, 1890-1904] mass transfer model. Two empirical characterization equations are developed for k(L)a(b) and k(L)a(s), correlating the coefficients to air flow, Qa, diffuser depth, hd, cross-sectional area, Acs, and volume, V. The characterization equations indicate that the bubble transfer coefficient, k(L)a(b), increases with increasing gas flow rate and depth, and decreases with increasing water volume. For fine bubble diffusers, k(L)a(b) is approximately six times greater than k(L)a(b) for coarse bubble diffusers. The surface transfer coefficient, k(L)A(s), increases with increasing gas flow rate and diffuser depth. The characterization equations make it possible to predict the gas transfer that will occur across bubble interfaces and across the free surface with a bubble plume at depths up to 32 m and with variable air discharge in deep tanks and reservoirs.     

Solving the problem at the source: Controlling Mn release at the sediment-water interface via hypolimnetic oxygenation

One of the primary goals of hypolimnetic oxygenation systems (HOx) from a drinking water perspective is to suppress sediment-water fluxes of reduced chemical species (e.g., manganese and iron) by replenishing dissolved oxygen (O₂) in the hypolimnion. Manganese (Mn) in particular is becoming a serious problem for water treatment on a global scale. While it has been established that HOx can increase sediment O₂ uptake rates and subsequently enhance the sediment oxic zone via elevated near-sediment O₂ and mixing, the influence of HOx on sediment-water fluxes of chemical species with more complicated redox kinetics like Mn has not been comprehensively evaluated.This study was based on Mn and O₂ data collected primarily in-situ to characterize both the sediment and water column in a drinking-water-supply reservoir equipped with an HOx. While diffusive Mn flux out of the sediment was enhanced by HOx operation due to an increased concentration driving force across the sediment-water interface, oxygenation maintained elevated near-sediment and porewater O₂ levels that facilitated biogeochemical cycling and subsequent retention of released Mn within the benthic region. Results show that soluble Mn levels in the lower hypolimnion increased substantially when the HOx was turned off for as little as ∼48 h and the upper sediment became anoxic. Turning off the HOx for longer periods (i.e., several weeks) significantly impaired water quality due to sediment Mn release. Continual oxygenation maintained an oxic benthic region sufficient to prevent Mn release to the overlying source water.     

Factors influencing oxygen transfer in fine pore diffused aeration

A bench-scale experiment was conducted in a 701. tank of tap water to examine the effect of four design variables on oxygen transfer in a fine pore diffused aeration system. The experiment used non-steady state gas transfer methodology to examine the effect of air flow rate, air flow rate per diffuser, orifice diameter and reduced tank surface area on the overall oxygen transfer coefficient (K_La₂₀, h⁻¹); standard oxygen transfer rate (OT₂, g O₂ h⁻¹); energy efficiency (E_p, g O₂ kWh⁻¹) and oxygen transfer efficiency (E_o, %). The experiments demonstrated that K_La₂₀ and OT_s increased with air flow rate (9.4–18.8 1 min⁻¹) in the 40 and 140 μ diameter orifice range; however, E_p and E₀ were not affected. Reducing the air flow rate per fine pore diffuser (40 and 140 μ diameter pore size) significantly increased K_La₂₀, OT_s, E_p and E₀. A decrease in orifice diameter from 140 to 40 μ had no effect on K_La₂₀, OT_s, E_p and E₀. A reduction in tank surface area had a marginally significant inverse effect on K_La₂₀ and OT_s, and no effect on E_p and E_o. The mean bubble size produced by the 40 and 140 μ diffusers was 4.0 and 4.2 mm, respectively. There was no consistent effect of air flow rate on bubble size within the range of air flow rates used in this experiment. In clean water aeration applications, the optimum system efficiency will be obtained using the largest number of fine pore diffusers operated at low air flow rates per diffuser. In wastewater treatment plants, higher air flow rates per diffuser should be used to prevent diffuser biofouling and keep biological solids in suspension. Wastewater systems are purposely operated at less than optimum transfer efficiencies in exchange for reduced diffuser maintenance and improved mixing. In either situation, changes in tank surface area and diffuser pore size (provided that pore diameter remains between 40 and 140 μ) are unlikely to have any significant effect on aeration system efficiency.     

Passive mixing control for innovative air diffusion terminal devices for buildings

This paper presents an innovative concept for optimized air diffusion in buildings. The method uses passive control of air jet through lobed diffusers. An analysis is done experimentally at different scales for a lobed shaped geometry. A cross-shaped jet is characterized first through an isolated orifice and then at the scale of one perforated panel. An intermediary analysis of two coalescent and a row of cross-shaped jets is also proposed. All the results lead to the same conclusion. The lobed diffuser favors the self-induction compared to a reference conventional circular perforated diffuser. The air flow induced in the case of the lobed perforated panel is in average twice as the one of the circular perforated panel. Despite the consequent gain in air induction for the lobed perforated panel flow, the streamwise maximum velocities display comparable values in the far field which signifies comparable throws for the two flows. Consequently, the presented lobed perforated panel concept can be generalized to different type of diffusers to improve mixing ventilation in buildings.     

Increased sediment oxygen uptake caused by oxygenation-induced hypolimnetic mixing

Hypolimnetic oxygenation systems (HOx) are increasingly used in lakes and reservoirs to elevate dissolved oxygen (O₂) while preserving stratification, thereby decreasing concentrations of reduced chemical species in the hypolimnion. By maintaining an oxic zone in the upper sediment, HOx suppress fluxes of reduced soluble species from the sediment into the overlying water. However, diminished HOx performance has been observed due to HOx-induced increases in sediment O₂ uptake. Based on a series of in situ O₂ microprofile and current velocity measurements, this study evaluates the vertical O₂ distribution at the sediment-water interface as a function of HOx operation. These data were used to determine how sediment O₂ uptake rate (JO₂) and sediment oxic-zone depth (z_max) were affected by applied oxygen-gas flow rate, changes in near-sediment mixing and O₂ concentration, and proximity to the HOx. The vertical sediment-water O₂ distribution was found to be strongly influenced by oxygenation on a reservoir-wide basis. Elevated JO₂ and an oxic sediment zone were maintained during continuous HOx operation, with z_max increasing linearly with HOx flow rate. In contrast, JO₂ decreased to zero and the sediment became anoxic as the vertical O₂ distribution at the sediment-water interface collapsed during periods when the HOx was turned off and near-sediment mixing and O₂ concentrations decreased. JO₂ and z_max throughout the reservoir were found to be largely governed by HOx-induced mixing rather than O₂ levels in the water column. By quantifying how JO₂ and z_max vary in response to HOx operations, this work (1) characterizes how hypolimnetic oxygenation affects sediment O₂ dynamics, (2) contributes to the optimization of water quality and management of HOx-equipped lakes and reservoirs, and (3) enhances understanding of the effect of mixing and O₂ concentrations in other systems.     

Predicting diffused-bubble oxygen transfer rate using the discrete-bubble model

A discrete-bubble model that predicts the rate of oxygen transfer in diffused-bubble systems is evaluated. Key inputs are the applied gas flow rate and the initial bubble size distribution. The model accounts for changes in the volume of individual bubbles due to transfer of oxygen and nitrogen (and hence changing partial pressure), variation in hydrostatic pressure, and changes in temperature. The bubble-rise velocity and mass-transfer coefficient, both known functions of the bubble diameter, are continually adjusted. The model is applied to predict the results of diffused-bubble oxygen transfer tests conducted in a 14-m deep tank at three air flow rates. All of the test data are predicted to within 15%. The range of bubble diameters (0.2-2 mm) spans the region of greatest variation in rise velocity and mass-transfer coefficient. For simplicity, the Sauter-mean diameter is used rather than the full bubble size distribution without loss of accuracy. The model should prove useful in the design and optimization of hypolimnetic oxygenation systems, as well as other diffused-bubble applications.     

Air flow characteristics of a room with square cone diffusers

A computational fluid dynamics model with radiant exchange between surfaces is developed to examine the air flow characteristics of a room with square cone diffusers. As an input to the full-scale 3-D room model, a 2-D diffuser model that supplies direction and magnitude of air flow into the room is developed and evaluated using infrared visualization. The room air flow model is assessed using several previously documented problems with various geometries and boundary conditions. Simulations are conducted for heating and cooling of the room with one or two supply air diffusers. The results show that the offset and lips of the diffuser work together to determine the discharge air angles, which play an important role in determining the room air flow patterns. For a certain discharge angle in the heating case, the air flows along the ceiling. The results indicate that, for the same supply air flow, operating only a single diffuser initiates more mixing of the room air flow, which results in enhanced temperature uniformity compared to that for two diffusers. Radiant exchange between the exterior window and interior surfaces causes a significant change in the window temperature.     

A model for the design of hypolimnetic aerators

Diffusor depth, air flow rates, rise velocity and cross-sectional area of the riser tube are the major variables considered in the simple empirical model presented for use in the design of hypolimnetic aerators. Water flow values predicted by the model were correlated with those observed in twenty published field experiments (r = 0.893). The model determined that some aerators are inefficient and their design could be improved. A discussion of required oxygen input during aeration is presented in which the problems of hypolimnetic oxygen depletion rates and oxygen transfer efficiencies (observed and absolute) are considered.     

Hypolimnetic aeration: practical design and application

Hypolimnetic aeration is becoming increasingly important as a fisheries management and water quality improvement technique, however its application has been restricted by a paucity of practical reference material. Hypolimnetic aeration includes partial and full lift designs and several air/oxygen injection systems. Positive displacement compressors flanged to three phase electric motors are the preferred air supply and power for most lake aeration projects. Internal combustion power is adequate for short term use and wind power is in the developmental stage. Rubber compressed air hose is recommended for lake aeration applications. Free air delivery is the air volume taken into the compressor at standard temperature and pressure however actual output volume is regulated by discharge pressure. Performance specifications of full lift hypolimnetic aerators are based on water:air ratios, oxygen increase, transfer efficiencies and oxygenation capacity. An empirical sizing method is proposed using hypolimnetic volume, hypolimnetic oxygen consumption, water flow, air flow and inflow tube radius. Outflow tube radius should equal or exceed inflow tube radius to achieve high flow rates and allow efficient removal of residual bubbles. Floatation requirements are calculated from the total weight of the separator box, inflow and outflow tubes and the theoretical water head.     

Predicting oxygen transfer of fine bubble diffused aeration systems--model issued from dimensional analysis

The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 12 cylindrical tanks (water depth from 2.4 to 6.1m), were analysed using dimensional analysis. A relationship was established to estimate the scale-up factor for oxygen transfer, the transfer number (N(T)) The transfer number, which is written as a function of the oxygen transfer coefficient (k(L)a(20)), the gas superficial velocity (U(G)), the kinematic viscosity of water (nu) and the acceleration due to gravity (g), has the same physical meaning as the specific oxygen transfer efficiency. N(T) only depends on the geometry of the tank/aeration system [the total surface of the perforated membrane (S(p)), the surface of the tank (S) or its diameter (D), the total surface of the zones covered by the diffusers ("aerated area", S(a)) and the submergence of the diffusers (h)]. This analysis allowed to better describe the mass transfer in cylindrical tanks. Within the range of the parameters considered, the oxygen transfer coefficient (k(L)a(20)) is an increasing linear function of the air flow rate. For a given air flow rate and a given tank surface area, k(L)a(20) decreases with the water depth (submergence of the diffusers). For a given water depth, k(L)a(20) increases with the number of diffusers, and, for an equal number of diffusers, with the total area of the zones covered by the diffusers. The latter result evidences the superiority of the total floor coverage over an arrangement whereby the diffusers are placed on separate grids. The specific standard oxygen transfer efficiency is independent of the air flow rate and the water depth, the drop in the k(L)a(20) being offset by the increase of the saturation concentration. For a given tank area, the impact of the total surface of the perforated membrane (S(p)) and of the aerated area (S(a)) is the same as on the oxygen transfer coefficient.     

Performance of three air distribution systems in VOC removal from an area source

Building-related health complaints and symptoms represent a significant occupational health problem. Elevated concentrations of various types of indoor pollutants, frequently associated with inadequate ventilation, have been implicated as a potential cause. The objective of this research is to model and evaluate the performance of several ventilation methods in pollutant removal from indoor environments. Pollutant sources are assumed to be at the floor level, one with a constant emission rate and the other a fast decaying source (volatile organic compound emissions from a wood stain). Three ventilation methods, namely displacement ventilation and two mixing systems using a side grille and ceiling square diffuser respectively are studied. A computer model has been applied to simulate the distributions and the time history of the pollutant concentrations in a mockup office. Experimental data of velocity, temperature, and tracer gas concentration distributions in the chamber with the displacement diffuser are obtained to validate the airflow model. Simulation results show that different ventilation methods affect the pollutant distributions within the room. When the pollutant sources are distributed on the floor and not associated with a heat source or initial momentum, displacement ventilation behaves no worse than perfect mixing ventilation at the breathing zone. Conventional “mixing” diffusers, on the other hand, could perform better or worse than a perfect mixing system. The computer model could be used for selecting appropriate ventilation systems to maximize indoor air quality for occupants.     

Energy saving in activated sludge plants by the use of more efficient fine bubble diffusers

Many activated sludge plants (ASP) use fine bubble diffused air as their source of oxygen. Blowers are attached to air pipework, which distributes air to a network of diffusers installed on the floor of the ASP tank. Modern diffusers are made from a rubber membrane which flexes to allow fine bubbles of air to pass through holes in the diffusers which then pass into the mixed liquors in the tank. The diffusers come as circular discs, tubes and more recently mats or panels. Yorkshire Water is in the process of building new ASP at some of their biggest sewage treatment works to meet new final effluent consent standards associated with the freshwater fisheries directive (FFD). These new works will treat sewage from a combined population of over two million people in the Yorkshire area. Black & Veatch is involved in some of the first works to have a new type of fine bubble diffuser installed in the ASP basins. These diffusers resemble a mat or panel and are fixed to the floor of the tank as opposed to circular and tubular diffusers which as fixed above the floor. Oxygen transfer testing has been carried out to show the efficiency of these aeration systems, which may offer significant savings in operating costs to water operators. This paper examines the results from the tests and compares them with other tests carried out in the United States and tests that have been carried out on other types of diffusers. The paper will discuss the results of the oxygen transfer tests and present capital and net present costs (NPC) for various diffuser installations.     

Effects of whole lake mixing on water quality and phytoplankton

This paper describes the effect of artificial mixing of two Oklahoma lakes with a downflow pump on water quality and algal biomass. Artificial pumping in Arbuckle Lake (951 ha), advanced autumnal turnover, but never destratified the lake completely. Ammonia decreased in the epilimnion, while sulfide (H₂S) declined and dissolved oxygen (DO) increased in the hypolimnion. Other water quality parameters did not change. Near-bottom concentrations of manganese (Mn²⁺) increased, indicating pumping did not affect water chemistry near deep sediments (> 16 m). Pumping did not change significantly the depth of the Secchi disc or algal biomass as measured by chlorophyll a. The algal flora was dominated by diatoms at all times, and the density of blue-green algae was always low. Pumping kept Ham's Lake (41 ha) destratified, but seldom produced completely isothermal conditions or isochemical concentrations of DO. There was no drastic change in other water quality parameters. However, artificial mixing decreased water clarity and increased algal biomass by a factor of about 2.5, probably by reducing sinking rates of the phytoplankton. Artificial mixing apparently eliminated a fall pulse of Microcystis.     

CFD modelling of an industrial air diffuser—predicting velocity and temperature in the near zone

This article describes experimental and modelling results from CFD simulation of an air diffuser for industrial spaces. The main objective of this paper is to validate a manufacturer model of the diffuser. In the air diffuser, the low velocity part is placed on top of a multi-cone diffuser in order to increase airflow rates and maximize the cooling capacity of a single diffuser unit. This kind of configuration should ensure appropriate performance of industrial air diffusers, which is discussed briefly at the end of the article. The paper illustrates the importance of a simulation model jointly with the manufacturer's product model and the grid layout near the ventilation device to achieve accurate results. Parameters for diffuser modelling were adapted from literature and manufacturer's product data. Correct specification of diffuser geometry and numerical boundary conditions for CFD simulations are critical for prediction. The standard k–ε model was chosen to model turbulence because it represents the best-known model utilized and validated for air diffuser performance. CFD simulations were compared systematically with data from laboratory measurements; air velocity was measured by ultrasonic sensors. Results show that CFD simulation with a standard k–ε model accurately predicts non-isothermal airflow around the diffuser. Additionally, smoke tests revealed that the flow around the diffuser is not completely symmetrical as predicted by CFD. The cause of the observed asymmetry was not identified. This was the main reason why some simulation results deviate from the measured values.     

Membrane properties change in fine-pore aeration diffusers: full-scale variations of transfer efficiency and headloss

Fine-pore diffusers are the most common aeration system in municipal wastewater treatment. Punched polymeric membranes are often used in fine-pore aeration due to their advantageous initial performance. These membranes are subject to fouling and scaling, resulting in increased headloss and reduced oxygen transfer efficiency, both contributing to increased plant energy costs. This paper describes and discusses the change in material properties for polymeric fine-pore diffusers, comparing new and used membranes. Three different diffuser technologies were tested and sample diffusers from two wastewater treatment facilities were analysed. The polymeric membranes analysed in this paper were composed of ethylene-propylene-diene monomer (EPDM), polyurethane, and silicon. Transfer efficiency is usually lower with longer times in operation, as older, dilated orifices produce larger bubbles, which are unfavourable to mass transfer. At the same time, headloss increases with time in operation, since membranes increase in rigidity and hardness, and fouling and scaling phenomena occur at the orifice opening. Change in polymer properties and laboratory test results correlate with the decrease in oxygen transfer efficiency.     

Predicting oxygen transfer and water flow rate in airlift aerators

Water flow rate, gas-phase holdup, and dissolved oxygen (DO) profiles are measured in a full-scale airlift aerator as a function of applied air flow rate. A model that predicts oxygen transfer based on discrete-bubble principles is applied. The riser DO profiles are used to calculate the initial bubble size. The range of calculated bubble diameters obtained using the model is 2.3-3.1 mm. The Sauter-mean diameter of bubbles measured in the laboratory ranged from 2.7 to 3.9 mm. The riser and downcomer DO profiles and gas holdups predicted by the model are in close agreement with the experimental results. A model that predicts water flow rate based on an energy balance is used to calculate Kt, the frictional loss coefficient for the air-water separator. Excluding the data at the very lowest air flow rate, the range of calculated values for Kt (3-8) is close to a literature value of 5.5 proposed for hydrodynamically similar external airlift bioreactors. The models should prove useful in the design and optimization of airlift aerators.     

高诱导比低温风口在工程中的应用

简要介绍了用于低温空调工程中的高诱导比低温风口的工作原理，对实际工程中应用的这种低温风口的送风性能作了实测，分析了测试数据，认为这种风口可获得满意的气流组织和均匀的温度场，而且在低温工况下送风口未出现结露现象。     

Water quality control in the river Arno

In this work, we analyzed pollution in the river Arno using a non-steady advection-dispersion-reaction equation (ADRE) calibrated on experimental data. We examined the influence different pollution control strategies have on dissolved oxygen (DO). We considered (i) flow rate variation; (ii) local oxygenation at critical points; (iii) dynamic modification of wastewater load. Results indicate first, that reservoir management is effective in reducing pollution; second, that local oxygenation is necessary to ensure that DO does not fall below safety levels; and finally, that tuning wastewater loads appears to be impractical to manage the river quality given the stringent limitations it would impose on the industrial effluents.     

Sedimentary microbial oxygen demand for laminar flow over a sediment bed of finite length

Dead organic material accumulated on the bed of a lake, reservoir or wetland often provides the substrate for substantial microbial activity as well as chemical processes that withdraw dissolved oxygen (DO) from the water column. A model to estimate the actual DO profile and the "sedimentary oxygen demand (SOD)" must specify the rate of microbial or chemical activity in the sediment as well as the diffusive supply of DO from the water column through the diffusive boundary layer into the sediment. Most previous experimental and field studies have considered this problem with the assumptions that the diffusive boundary layer is (a) turbulent and (b) fully developed. These assumptions require that (a) the flow velocity above the sediment bed is fast enough to produce turbulent mixing in the boundary layer, and (b) the sediment bed is long. In this paper a model for laminar flow and SOD over a sediment bed of finite length is presented and the results are compared with those for turbulent flow. Laminar flow near a sediment bed is encountered in quiescent water bodies such as lakes, reservoirs, river backwaters, wetlands and ponds under calm wind conditions. The diffusive oxygen transfer through the laminar diffusive boundary layer above the sediment surface can restrict the microbial or chemical oxygen uptake inside the sediment significantly. The developing laminar diffusive boundary layer above the sediment/water interface is modeled based on the analogy with heat transfer, and DO uptake inside the sediment is modeled by Michaelis-Menten microbial growth kinetics. The model predicts that the rate of SOD at the beginning of the reactive sediment bed is solely dependent on microbial density in the sediment regardless of flow velocity and type. The rate of SOD, and the DO penetration depth into the sediment decrease in stream-wise direction over the length of the sediment bed, as the diffusive boundary layer above the sediment/water interface thickens. With increasing length of the sediment bed both SOD rate and DO penetration depth into the sediment tend towards zero if the flow is laminar, but tend towards a finite value if the flow is turbulent. That value can be determined as a function of both flow velocity and microbial density. The effect of the developing laminar boundary layer on SOD is strongest at the very lowest flow velocity and/or highest microbial density inside the sediment. Under quiescent conditions, the effective SOD exerted by a reactive sediment bed of a lake or wetland approaches zero, i.e. no or very little oxygen demand is exerted on the overlying water column, except at the leading edge.