Diffused Aeration Systems Research Articles

Bench-scale study of oxygen transfer in coarse bubble diffused aeration

A bench-scale experiment was conducted in a 701, tank of tap water to examine the effect of three design variables on oxygen transfer in a coarse bubble diffused aeration system. The experiment used non-steady state gas transfer methodology to examine the effect of orifice diameter, air flow rate and reduced tank surface area on the overall oxygen transfer coefficient ( K L a 20, h −1); standard oxygen transfer rate (OT s, g O 2 h −1); energy efficiency ( E p, g O 2 kW h −1) and oxygen transfer efficiency ( E o, %). The experiments demonstrated that K L a 20 and OT s increased 122% when air flow rate was doubled (9.4–18.81 min −1), while E o and E P increased approx. 12%. A reduction in orifice diameter from 3175 μm (3.2 mm) to 1588 μm (1.6 mm) to 794 μm (0.8 mm) to 397 μm (0.4 mm) significantly increased K L a 20, OT s, E P and E o. The presence of a floating surface cover had no effect on K L a 20 and OT g, and a marginal decrease in E P and E o. The mean bubble sizes produced by the 397, 794, 1588 and 3175 μm diffusers were 4.7, 6.6, 7.7 and 7.2 mm dia, respectively. There was no significant effect of air flow rate on bubble size within the range of air flow rates used in this experiment. A comparison of this study's coarse bubble results with a concurrent study on fine pore (fine bubble) aeration reveals that K L a, OT s, E P and E o continue to increase as orifice diameter is decreased to the 40–140 μm (i.e. fine pore) diameter range. The most efficient coarse bubble diffuser (i.e. 397 μm) was approx. 68% as efficient as the 140 μm fine pore diffuser. A classification system using existing EPA bubble size criterion for coarse and fine bubble diffusers, and the aeration systems' operational response of E P and E o to increased air flow rates is recommended for differentiating coarse and fine bubble diffused aeration systems.

Evaluation of oxygen transfer efficiency and alpha‐factor on a variety of diffused aeration systems

ABSTRACT: Because the aeration system in an activated sludge wastewater treatment plant typically represents more than 50% of total plant energy requirements, designers and operators can substantially reduce overall plant energy costs by using accurate oxygen transfer information to make the aeration system as energy efficient as possible. This paper presents data from 65 off‐gas analysis tests performed at 21 wastewater treatment plants. The effect of various factors on oxygen transfer was evaluated by comparing pairs of tests in which all factors but one were held relatively constant. Factors evaluated were diffuser type, diffuser layout, diffuser age, solids retention time, and level of nitrification. Flexible membrane diffusers had more than a 30% higher oxygen transfer efficiency than coarse bubble diffusers. Grid layouts had higher oxygen transfer efficiencies than spiral roll layouts. Flexible membrane diffusers that were 3.5 years old had a 20% lower oxygen transfer efficiency than new flexible membrane diffusers. Systems at low solids retention times that were not nitrifying had lower oxygen transfer efficiencies than nitrifying systems at high solids retention times. For some wastewater systems, use of this information on solids retention time in plant design and operation can decrease overall energy costs despite the additional oxygen required to meet the nitrogenous oxygen demand.

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 L a 20, h −1); standard oxygen transfer rate (OT 2, g O 2 h −1); energy efficiency ( E p, g O 2 kWh −1) and oxygen transfer efficiency ( E o, %). The experiments demonstrated that K L a 20 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 E 0 were not affected. Reducing the air flow rate per fine pore diffuser (40 and 140 μ diameter pore size) significantly increased K L a 20, OT s, E p and E 0. A decrease in orifice diameter from 140 to 40 μ had no effect on K L a 20, OT s, E p and E 0. A reduction in tank surface area had a marginally significant inverse effect on K L a 20 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.

Radon in Homes Following Its Reduction in a Community Water Supply

Radon in home air and domestic water supplies was measured throughout a rural community before and after the installation of a diffused aeration system that reduced water radon at the source from approximately 20,000 to <1,000 pCi/L. Continuous monitoring of air radon indicated short‐term contributions from water use in some homes. Following reduction of radon in the water source, no consistent changes in the air radon levels in the homes were observed. Environmental and analytical factors that obscured the detection of the contribution of water to the air radon are discussed, and recommendations are made for the design of future studies.

Effects of orifice size and surface conditions on oxygen transfer in a bench scale diffused aeration system

Abstract Non‐steady state gas transfer methodology was used to examine the effect of orifice size and surface conditions on the rate of oxygen transfer in a 239 L bench scale tank. Each orifice size tested (40 μ, 397 μ and 1588 μ) increased the overall oxygen transfer coefficient (KLa20, hr−1) ; standard oxygen transfer rate (OTs, g O2/hr) ; transfer efficiency (Eo, %) and energy efficiency (Ep/ g O2/kW‐hr) as orifice diameter decreased. The three surface conditions examined exerted a minor effect on oxygen transfer.

Improved oxygen mass transfer modeling for diffused/subsurface aeration systems

AbstractThe mass transfer analysis of the oxygen transfer performance of diffused air or subsurface mechanical aeration systems has progressed very little over the past 20 years. The recently‐developed ASCE Standard method for determination of the oxygen mass transfer performance of diffused or subsurface aeration systems is based on a greatly over‐simplified mass transfer model. Although the ASCE Standard can be used to empirically evaluate point performance conditions, it does not provide a meaningful representation of the actual mass transfer process and is not capable of accurately assessing or predicting performance under changing operating or environmental conditions. A new oxygen mass transfer model has been developed which is a fundamentally more rigorous description of the actual mass transfer process in diffused aeration systems. This model can be confidently used to predict aerator performance under changing operating and environmental conditions. The model is easily adapted to numerical computer solution for routine aeration systems performance evaluation as well as process design.

Low-Cost Wastewater Treatment with the Use of an Intrachannel Clarifier in Oxidation Ditches

The oxidation ditches developed by Pasveer of Holland and Muskat in Germany have been used in small communities for many years as a cost effective secondary treatment. Generally designed and operated as part of an extended aeration system, these plants provide from 18 to 30 hours of hydraulic detention time, followed by a 3 to 6 hours settling period in a secondary clari-fier, with 100 per cent of settled solids return to the aeration tank. Because of these design criteria, the oxidation ditches were not widely used in large populations as the aeration system normally used required a relatively low water depth to be efficient -a fact that meant extensive land areas. Recent innovations in the clarification and aeration systems have rendered this treatment attractive again for communities of all sizes. The use of an upflow intrachannel clarifier, in particular, eliminates the need for a separate secondary clarifier, and return sludge pumping and piping. In addition, an innovative diffused aeration system permits the use of channels with a greater water depth, which not only optimizes process as an oxidation ditch but also makes it remarkably competitive in respect to conventional biological treatment systems. This paper deals with this innovative oxidation ditch system, analyzing its conceptual design considerations and economic aspects, and commenting upon its first application in Argentina using low-cost construction technology.

Diffused aeration systems from theory to design

Aeration has been used successfully in northern Canada and other cold regions for treating domestic and industrial waste waters by aerated lagoons, extended aeration package units and conventional activated sludge plants. Many of these installations used diffused aeration because this system has shown several advantages over mechanical surface aeration. The advantages of using diffused aeration in cold regions for oxygenation as well as for mixing are discussed in this paper.The parameters affecting the efficiency of oxygen transfer and the mixing of the tank contents are identified and the influence of varying these operating parameters on the performance of diffused aeration systems under cold climatic conditions is discussed. Equations showing the influence of temperature, airflow rates, submergence and tank geometry on the overall oxygen transfer coefficient are presented.A process of aeration, employing counter-current flow of air bubbles and waste water, is shown to result in increased contact time and therefore higher oxygenation efficiencies. A typical example for designing a diffused aeration system in cold regions has been worked out.

Mass Transfer of Oxygen in Diffused Aeration Systems

Starting from the fundamental concepts of mass transfer of gases into liquid, a comprehensive mathematical equation, relating the mass oxygen-transfer rate to the various parameters in diffused aeration systems, is developed.The important variables involved are the liquid film coefficient, temperature, waste water characteristics, bubble size, diffuser submergence, airflow rate, and the contact time. The contact time between the air bubbles and the oxygen absorbing liquid can be varied by changing the operating conditions of the aeration system in terms of the direction of air and liquid movement. Investigations on four such combinations are presented.The influence of the above-mentioned variables, individually or in combination, on the mass oxygen-transfer rate and overall oxygen transfer coefficient are discussed. The presentation is supported with laboratory data collected from aeration studies over a period of 3 years. With proper understanding of the influence of operating parameters on mass transfer coefficients, it should be possible to modify the design and operation of aeration basins in order to obtain increased oxygen transfer.

Oxygen Transfer Efficiency in Counter-and Co-Current Air-Water Flow

Abstract This study was undertaken to determine the efficiency of oxygenation in different air-entrained water systems. Both co-current and counter-current air-water flow conditions were used in a closed-loop diffused aeration system, consisting of 4 in. diameter plexiglass U-Tube of variable height and a 4 1/2 × 2 × 1 1/2 ft water re-circulating reservoir at the top. Four separate systems were investigated: System I was a simple water column; System II used only diffused air for water circulation and for creating an upward co-current air-water flow; System III employed pumping for water circulation, with a control on water velocity such that counter-current air-water flow was maintained; System IV used pumping to circulate water with the water velocity made large enough to create a downward co-current air-water flow. The aeration devices were located at the bottom of one side of the U-Tube in Systems I, II and III and near the top in System IV. The operating conditions were varied by changing airflow rates, water depth, water flow rates, and diffuser location. The efficiency of different systems is compared in terms of the rate of oxygen transferred/unit power input, given in lb/KWhr. It is found that, within the limitations of this study. System III is the most efficient.

Diffuser design for discharge to a stratified recipient : Poul Harremoës, Denmark

The standard oxygenation performances of fine bubble diffused aeration systems in clean water, measured in 12 cylindrical tanks (water depth from 2.4 to 6.1 m), were analysed using dimensional analysis. A relationship was established to estimate the scale-up factor for oxygen transfer, the transfer number (NT) The transfer number, which is written as a function of the oxygen transfer coefficient (kLa20), the gas superficial velocity (UG), the kinematic viscosity of water (ν) and the acceleration due to gravity (g), has the same physical meaning as the specific oxygen transfer efficiency. NT only depends on the geometry of the tank/aeration system [the total surface of the perforated membrane (Sp), the surface of the tank (S) or its diameter (D), the total surface of the zones covered by the diffusers (“aerated area”, Sa) 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 (kLa20) is an increasing linear function of the air flow rate. For a given air flow rate and a given tank surface area, kLa20 decreases with the water depth (submergence of the diffusers). For a given water depth, kLa20 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 kLa20 being offset by the increase of the saturation concentration. For a given tank area, the impact of the total surface of the perforated membrane (Sp) and of the aerated area (Sa) is the same as on the oxygen transfer coefficient.