Tariq Ahmed

Oxygen transfer characteristics of hollow-fiber, composite membranes

The oxygen transfer characteristics of a composite membrane bubbleless aerator are evaluated. The hollow fiber composite membrane is hydrophobic microporous polyethylene with a 1 μm thick non-porous polyurethane layer in the middle of the membrane wall that allows high gas pressure to be maintained inside the fiber without producing bubbles. When in contact with water, the membrane pores do not wet; they remain dry and gas-filled. Oxygen diffuses through both the gas-filled pores and the polyurethane, and is transferred to the liquid phase. The hollow fiber membranes provide a high specific surface area and as a result a high gas transfer rate can be achieved. During module operation, pure oxygen under high pressure is maintained on the lumen side of a bundle of individually sealed hollow fibers. Water to be oxygenated is pumped over the outside of the fibers. The high gas pressure can be used in the fibers to produce interfacial oxygen saturation concentrations that are more than an order of magnitude higher than typical oxygen saturation concentrations. In this study, the mass transfer characteristics of laboratory scale composite fiber modules are evaluated as a function of module design parameters and operating conditions. The following empirical design correlation can be used to estimate the mass transfer coefficients of composite fiber modules: Sh=0.056P0.735Re0.645P−0.22Sc0.33 where P=operating pressure in psi. The correlation is developed using the data between the Reynolds number ranges from 2500 to 5500.

Modeling the influence of nonionic surfactants on biodegradation of phenanthrene

Surfactant mediated solubilization and simultaneous microbial degradation of phenanthrene in a completely mixed batch system has been studied. A mathematical model is presented based on the rates of solids dissolution, substrate biodegradation and oxygen uptake in terms of five coupled differential equations. The model accounts for the concurrent utilization of surfactants for cell growth. The system of differential equations has been solved by numerical integration to calculate the oxygen utilization, cell mass production and substrate concentration as a function of time. Sensitivity analysis of the model indicates that the maximum specific growth rate, the oxygen consumption coefficient, cell yield coefficient and dissolution coefficient are the most significant parameters that control the process. Four commercial nonionic surfactants at a concentration of 25 mg/L were tested to evaluate their effect on biodegradation rates of phenanthrene. The model could adequately predict the oxygen uptake, cell growth and substrate disappearance data observed in the experimental studies. The presence of surfactants enhanced the biodegradation rate for phenanthrene. The results also indicated that the most significant effect of surfactant addition was the increase in the dissolution rate of phenanthrene to the aqueous phase.

GAS TRANSFER FROM SMALL SPHERICAL BUBBLES IN NATURAL AND INDUSTRIAL SYSTEMS

The bubble terminal velocity and the mass transfer behavior of a small (diameter 0.2 cm) spherical bubble rising through stagnant water are reviewed. Equations relating the bubble diameter and gas composition as a function of depth are presented. The gas-liquid mass transfer co-efficient was estimated from observed bubble diameter versus time data reported in the literature. The system of equations has been solved by numerical integration to predict the behavior of a bubble as it rises through the water column. The model can predict the rate of dissolution and the change in composition of the bubble as a function of the depth of release, initial gas composition in the bubble, liquid phase gas composition, and gas temperature. The mass transfer correlation was found to compare favorably with the theoretical predictions. Initial bubble diameter and basin depths are the most significant parameters that control the gas transfer efficiency of the process. The results can assist in improving the design of experiments to evaluate the bubble terminal velocity and the mass transfer coefficient more accurately. *The Department of Civil Engineering, University of Minnesota, Minneapolis, provided partial funding for this work.

Energy loss characteristics of parallel flow bubbleless hollow fiber membrane aerators

Abstract The energy loss characteristics of sealed-end parallel flow hollow fiber membrane aerators were evaluated in this study. The membrane aerators were used successfully to oxygenate water without producing bubbles. Typically, a membrane aerator consists of a large number of sealed-end fibers in an external shell. Pressurized pure oxygen is maintained inside the hydrophobic gas permeable fibers at a pressure below the bubble point, and the water to be aerated is pumped over the outside of the fiber. Due to the high concentration gradient, oxygen diffuses through the gas filled membrane pores and dissolves directly into the water. Friction between the fibers and the water is the major energy loss mechanism in this type of modules. The gas supply manifold and other fittings are responsible for minor losses. Pilot scale modules with two different gas supply manifold designs were operated and the energy requirements of the module were measured over a practical range of design and operating conditions. The major and minor energy loss characteristics of the module were correlated with the physical module parameters and the flow conditions in the pipe. The friction factor for the fibers was found to be about a factor of 2 higher than the corresponding friction factor for pipe flow under similar flow conditions.

Phenanthrene mineralization in soil in the presence of nonionic surfactants

This research addresses the effect of surfactant addition on the microbial degradation of slightly soluble organic compounds in soil. Biodegradation of phenanthrene, coated on sand with a low f oc, was studied in the presence of nonionic surfactants. Phenanthrene coated sand was designed to simulate soil contaminated with excess phenanthrene which remains after evaporation of the lighter hydrocarbon solvents. A mixed culture acclimated to phenanthrene was used as the inoculum. Four nonionic surfactants were used in this study: Triton X‐114, Brij 35, Tween 40 and Corexit 0600. Continuous flow columns were employed to simulate groundwater flow through aquifers. The addition of Corexit 0600 and Tween 40 surfactants enhanced the biodegradation rate of phenanthrene while there was no enhancement by the other two surfactants. No appreciable lag period for mineralization was observed in these experiments. Additional tests are required to assess surfactant bacteria interactions to determine why certain surfactant...

Autotrophic denitrification using hydrogen oxidizing bacteria in continuous flow biofilm reactor

Autotrophic denitrification was investigated in five bench‐scale upflow attached growth reactors using hydrogen oxidizing bacteria under anoxic conditions. The performance of sand, granular activated carbon (GAC), crushed pumice, crushed volcanic rock, and plastic media were evaluated as the support material. The reactors were inoculated with acclimated cultures obtained from domestic sewage treatment plant. A synthetic solution containing nitrate was used as the influent. The reactor performance was evaluated by measuring influent and effluent nitrate concentration. The design parameters demonstrated that the effectiveness of autotrophic denitrification is comparable to that of the heterotrophic process and may be utilized economically for drinking water treatment either as the main process or as a supplemental process for ion exchange regenerant treatment.

Performance of a transverse flow bubbleless membrane aerator

A novel transverse flow hollow fiber membrane aerator has been proposed for the oxygenation of water. The aerator can achieve high rates of gas transfer at reasonable power input. In the transverse flow modules studied, individually sealed fibers were mounted vertically at the bottom of a channel with water flow horizontally past the fibers. The gas‐filled fibers tend to rise vertically due to buoyancy, while bending downstream due to the drag force exerted by the flowing water. Mathematical models that predict oxygen transfer rates and gas transfer performance data are presented. With this module configuration, a high mass transfer coefficient can be achieved at low liquid flow rates resulting in an energy efficient process.