MODELING DISSOLVED OXYGEN CONCENTRATION FOR OPTIMIZING AERATION SYSTEMS AND REDUCING OXYGEN CONSUMPTION IN ACTIVATED SLUDGE PROCESSES: A REVIEW

Aeration accounts for 30% to 75% of the total energy consumption in activated sludge processes (ASPs). This percentage can be significantly reduced since most aeration systems are not optimized for unsteady influent flow rates and oxygen requirements. Reconfiguration, replacement, and the application of optimal dissolved oxygen (DO) control strategies for current aeration systems within the facility and model-based optimization of DO in wastewater treatment plants can lead to impressive increased energy efficiency and savings and improved stability of the system. These measures increase the operational lifetime of the aeration equipment and improve effluent and activated sludge quality. This article provides a review of two critical nonlinear time-varying parameters that characterize the DO concentration dynamics in an ASP: the oxygen uptake rate (OUR), related to microorganism activity, and the volumetric oxygen mass transfer function, represented by the oxygen transfer rate (OTR). Second, the physico-chemical, geometric, and dynamic factors and aerator type affecting the oxygen mass transfer coefficient (K _L a) are thoroughly discussed. The article concludes with model-based optimization, explaining the usefulness of accurate DO models in wastewater treatment, and provides examples for plant-wide or water chain cycle–focused optimizations.     

A DETAILED CHEMICAL ANALYSIS OF CHANGES TO BITUMEN PRODUCED BY THE IN SITU COMBUSTION PROCESS AT THE OXYGEN WOLF LAKE PROJECT,ALBERTA PART 1: WHOLE OIL SAMPLES.

ABSTRACT

The detailed chemical changes in bitumen brought about over a one year period by an in situ combustion process in an oil sands reservoir have been investigated. Relative to a core sample, the fireflood-produced oils exhibited a significant reduction in density and viscosity which began early in the production cycle. This behaviour was correlated with a marked increase in material boiling in the naphtha and middle distillate ranges and a concomitant decrease in the residue cut. The sulfur and nitrogen contents in the produced oils decreased relative to the core sample. A reduction in the acid number of the produced oil samples was coupled with an increase in the oxygen content as the firefiood proceeded. The relationship between these changes and the dynamics of the in situ combustion process are discussed.     

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.     

Effects of interfacial surfactant contamination on bubble gas transfer

Surface active agents depress gas transfer at gas-liquid interfaces. They are present as measurable trace contaminants at all environmental and at most industrial interfaces. An experimental apparatus to concurrently measure dynamic surface tension and mass transfer was constructed and tested for single-bubble and multi-bubble experiments. In this work, the parameters describing time-dependent bubble surface contamination were characterized. The application of a Ward-Tordai transient model and of a Langmuir saturation model showed that for fine-bubbles in low molecular weight surfactant solutions the interfacial surfactant accumulation equilibrates before bubble detachment. This is reflected in the bubbles behaving as solid-spheres, which is shown in our dimensionless results.For a given contamination, interfaces with higher renewal rates have higher mass transfer. At higher renewal rates, the variation due to different contamination is smaller than the variation at lower renewal rates, concluding that higher interfacial flow regimes can offset contamination. Our experimental evidence shows a gas transfer reduction of 30-70% of pure water values in surfactant solutions, which confirms full-scale field measurements. Results are consistent with expectations and correct previous Frössling-like dimensionless correlations for pure water systems. Our results offer a tool for mass transfer prediction from flow regime and surfactant properties.     

Gas-liquid dispersions in structured packing with high-viscosity liquids

Gas holdup in bubble columns containing structured packing was determined for varying liquid phase viscosity and different construction materials of the packing. Three columns, containing packing made from smooth nickel plate, perforated nickel plate and plastic sheets, respectively, were tested. The gas holdup in a column with no internals was also measured for comparison purposes. The effect of viscosity on gas holdup was studied by using water, aqueous glycerol and aqueous CMC solutions as the liquid phase; some of the solutions showed non-Newtonian behavior.The experimental results of gas void fraction were correlated in terms of superficial gas velocity and liquid viscosity through a simple homographic expression. The geometric characteristics of the structured packings were found to influence gas holdup, thus yielding slightly different equations for each structured packing. This effect is discussed in detail. Nevertheless, for engineering purposes a single equation representing all the data is also proposed, which permits the prediction of gas holdup in structured packing with sufficient accuracy.     

大岗山水电站掺气坎数值模拟分析

对大岗山水电站1#掺气坎采用常规挑坎体型和新掺气坎体型进行数值模拟计算,分析水位工况下的水流流态、水流的压力分布、水流速度分布、空腔特性等。结果表明新掺气坎体型具有较好的掺气效果,对类似项目有一定的借鉴作用。     

Temperature Effects On Pig Manure Under Low Level Batch Aeration

This project investigated the temperature effect on some chemical and biological characteristics of liquid swine manure under low level batch aeration conditions. An airflow rate of 0.129 L/min/L manure was used to aerate manure contained in reactors made of plexiglas tubes. Five temperatures, i.e. 5°C, 10°C, 15°C, 20°C, and 25°C, were examined. Data showed that manure pH would decrease with an increase in temperature. When temperature increased from 5°C to 25°C, the average oxidation-reduction potential decreased from +40 mV to m 60mV, resulting in a decrease in aerobic counts from 5.5 2 10 6 to 2.2 2 10 6 . This was accompanied by an increase in anaerobic bacterial counts (from 3.8 2 10 6 to 5.1 2 10 6 ). The increased anaerobic population produced more volatile fatty acids, leading to the decrease in pH. An inversely linear correlation was observed between pH and volatile fatty acids with a correlation coefficient of 0.8742. To prevent aerobic bacteria from decreasing and anaerobic bacteria from increasing, manure temperature should be kept under 15°C during aeration. To reduce odor generation potential (based on reduced VFA production), the oxidation-reduction potential should be maintained at +35 mV or higher.     

Design considerations and economics of different shaped surface aeration tanks

This paper deals with the design considerations of surface aeration tanks on two basic issues of oxygen transfer coefficient and power requirements for the surface aeration system. Earlier developed simulation equations for simulating the oxygen transfer coefficient with theoretical power per unit volume have been verified by conducting experiments in geometrically similar but differently shaped and sized square tanks, rectangular tanks of length to width ratio (L/W) of 1.5 and 2 as well as circular tanks. Based on the experimental investigations, new simulation criteria to simulate actual power per unit volume have been proposed. Based on such design considerations, it has been demonstrated that it is economical (in terms of energy saving) to use smaller tanks rather than using a bigger tank to aerate the same volume of water for any shape of tanks. Among the various shapes studied, it has been found that circular tanks are more energy efficient than any other shape.     

In situ triphasic rheological characterisation of activated sludge, in an aerated bioreactor

This study is devoted to the triphasic in situ rheological characterisation of an activated sludge, with total suspended solid (TSS) ranging from 10 to 35 g/L, and operated in a bioreactor under different stirring and aeration rates. The originality of this work is that flow properties are directly measured inside the bioreactor. Under low mechanical shear rates (below 0.1 s⁻¹) the internal structure of the suspension (configuration of structural units) is driven by the air plume. Due to the shearing of air bubbles, apparent viscosities are strongly lowered by the injection of air but almost independent of the quantity of air (in the range 2–6 L/min). Under high mechanical shear rates (above 100 s⁻¹), the configuration of structural units (i.e. flocs in the case of activated sludge) is only dependent on the mechanical shearing and totally independent on the presence or absence of air. The viscosity of the broth is constant whatever the air flow rate is (0–6 L/min). We also observe a decrease in shear-thinning properties of aerated suspensions compared to non-aerated one, with a plateau above 2 L/min. The effect of TSS at constant air flow rate shows that an increase in TSS induces an increase not only in apparent viscosities but also in shear-thinning properties.     

Determination of agitation and aeration conditions for scale-up of cellulolytic enzymes production by <Emphasis Type="Italic">Trichoderma inhamatum</Emphasis> KSJ1

10 to 35 L jar fermentation scale-up cultures were performed to determine the optimum agitation and aeration rates in the cellulolytic enzymes production culture by Trichoderma inhamatum KSJ1. The optimum agitation rate in the 35 L jar fermenter was provisionally determined to be 150 rpm by using a geometrically resembled scale up method from the 10 L jar fermenter. The optimum aeration rate was determined to be 0.5 vvm by applying the mean values of superficial velocity and vvm. The DO (Dissolved Oxygen) concentration of the culture liquid was maintained below the critical DO concentration (2.336 mg/L) at 150 rpm in the 35 L jar fermenter. To increase the DO above the critical DO concentration, the agitation rate was increased from 150 to 200 rpm, with the aeration rate maintained at 0.5 vvm. As a result, the DO was maintained above critical DO concentration. The OUR (Oxygen Uptake Rate) and k _L a values were 0.91 mg-DO/L·min and 11.1 hr⁻¹, respectively. The amylase and FPase (filter paper activity) activities were 4.48 and 0.74 U/mL, respectively, in the 35 L jar fermenter, which was comparable to that in the 10 L fermenter (4.2 and 0.5 U/mL, respectively). Therefore, the scale-up conditions, 0.5 vvm and 200 rpm, were concluded to be the optimum aeration and agitation rates in the 35 L jar fermenter.