Previous studies have suggested that the transfer of VOCs to the atmosphere by air stripping may be a convenient and potentially cost-effective method to remove these compounds from the water. A Rotating Disk Contactor (RDC), similarly to other aeration devices, can provide intimate contact between the VOC-bearing water and air. It is believed that under carefully selected operating conditions, the RDC system are more competitive with packed tower than other aeration methods. The objectives of this study were to evaluate the feasibility of using the RDC to strip VOCs from the contaminated water and to develop a mathematical model to predict the stripping rates. The information obtained from this study will be helpful in assessing the emission of VOCs to the atmospherefor decision makers who have to determine the appropriateness of the design and operation of a RBC system.
A laboratory-scale RDC pilot unit was used in this study to treat a synthetic wastewater containing 0.5 to 5 ppm of TCE, PCE, EB, CLF, and TCEN. Batch and continuous flow systems were applied to study the desorption of VOCs and absorption of oxygen in the RDC pilot!plant. Both systems produced very similar results. The experimental data shows that the RDC pilot plant behaves like a completely mixed reactor. The mass transfer rates of the studied components were found to be a function of disk rotational speed, depth of immersion, disk radius, specific contact area of the bulk surface area and the exposed disk surface area. A semiempirical model, combining several oxygen transfer models proposed in the literature, were developed in this study. It provides a more comprehensive correlation between KL of different compounds and the operational parameters. As indicated by the data, direct transfer from the bulk water surface dominates at rotational speeds less than 0.5 rpm. However, mass transfer through the water film on the disk surface dominates when rotational speeds are above 4 rpm.
The rate of mass transfer of VOCs is shown to be dependent on the solute's diffusivity in water to a power approximately equal to 0.5. This exponent coefficient is indepedent of the rotational speed and other operating conditions. The gas-phase resistance is negligible at high rotational speeds. The significance of the gas-phase is greatest for the less volatile components at low rotational speeds. Although this study was not designed to provide a test of the two-resistance model, the experimental data has shown that the two-resistance approach results in a significant improvement over the simplistic assumption of liquid-phase control.
To achieve the same degree of treatment, the RDC system entails a greater volume and area than the packed tower. Since the cost per unit volume of media is lower for the RDC, preliminary economical analysis shows that the capital cost is lower for the RDC system than for the packed tower; if cheap land is available. The energy consumption is somewhat higher for the RDC system. However, the operating and maintenance labor cost is expected to be lower for the RDC system. When there is the risk of inorganic precipitation, growth of biomass, clogging by solids and particles to limit the application of packed tower, the RDC can be a good alternative for air stripping.
It has been shown that a great potential exists for the RDC system to enhance biodegrading of the VOCs with the aid of air stripping. Under carefully selected operating conditions, it is possible to decrease the VOC concentration to a level that will not violate the air pollution permit and at the same time, allow biodegradation to occur. The combination of the air stripping process with the biological process, in a RDC system to remove VOCs deserves further study and evaluation for the control of VOCs.
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