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Water treatment by microflotation and backpulsed microfiltration.

Two effective techniques are described for the removal of suspended colloidal matter from water.

The removal of fine suspended matter from aqueous streams is a common problem encountered in the water and wastewater treatment industries. For example, membrane-based industrial and municipal water treatment and desalination facilities (based on reverse osmosis or nanofiltration membranes) are faced with the problem of minimizing the total suspended solids content of incoming streams in order to preserve the useful life of the finer membrane unit downstream. Another example is the cleaning of wastewaters contaminated with dispersed hydrocarbons, of particular concern to industrial operations such as metallurgical and manufacturing plants, marine transportation, and petroleum and natural gas operations.

Current treatment technologies consist primarily of flocculation, multimedia and cartridge filtering, sedimentation, froth flotation, hydrocycloning and evaporation; however, these techniques are sometimes found to be ineffective or costly when dealing with waters containing dilute concentrations of fine, suspended colloidal matter. Furthermore, these techniques are not always sufficient to meet current and projected standards. Our laboratory is studying two new methods to remove fine suspended matter from water.

Microflotation: Predicting the Flotation Rate

An effective technique in treating waters containing colloidal particulates and/or droplets is based on flotation with very small bubbles (less than 100 [[micro]meter] in diameter), herein referred to as microflotation [1]. Such small bubbles are needed because the very small colloidal particles in these applications are not effectively collected by large, millimetre-sized bubbles typical of conventional flotation. Recent studies have shown that microflotation is best carried out by dissolved-air or electrolytic methods, which make it possible to generate small bubbles with diameters in the range 20 to 100 [[micro]meter] for large-scale applications [2]. A surfactant is often used to render the particles sufficiently hydrophobic to facilitate flotation.

Recently, with support from the U.S. National Science Foundation, our group has developed a model to calculate the microflotation rate in terms of key known physical parameters in the presence of a surfactant. Relative motion and collisions between bubbles and particles are induced by gravity, Brownian motion, and attractive van der Waals forces. Marangoni stresses arising from the uneven distribution of surfactant along the surface of the bubble are taken into account. The Marangoni stresses result in a retarded velocity profile around the moving bubble, which can be characterized by the product of the dimensionless Marangoni (Ma) and surface Peclet ([Pe.sub.s]) numbers, and they reduce the flotation rate.

By solving the convective-diffusion equation for particle transport to the bubbles, and taking into account detailed hydrodynamic interactions, we calculate a dimensionless flotation rate. This collection efficiency is then related to an overall kinetic constant for use in column design or evaluation.

Figure 1 illustrates the calculated dimensionless flotation rates for 25 [[micro]meter] diameter bubbles as a function of the floated particle diameter and different degrees of bubble interface retardation. The collection efficiency diminishes with increasing surface retardation, since the reduced mobility of the bubble interface provides greater resistance to the fluid being squeezed out from the region separating the approaching particles and bubble. The collection efficiency decreases as the relative particle size is reduced, since smaller particles tend to follow the streamlines and be swept around the rising bubbles. A minimum in the collection efficiency then occurs, because Brownian motion of the particles becomes important for sufficiently small particles and increases the collision rate with further reductions in particle size.

Rapid-backpulsed Crossflow Microfiltration

Crossflow microfiltration can also be used as a water treatment method. Its main drawback is the dramatic permeate flux decline which occurs due to the formation of a fording layer at the surface of the membrane. The resulting low values of the long-term permeate flux result in high capital and operating costs. A promising strategy to overcome the detrimental effects of membrane fouling is rapid backpulsing [3]. In rapid backpulsing, the transmembrane pressure is reversed once every five seconds for a short pulse of less than one second, with the purpose of lifting the deposited foulant off the surface of the membrane. This in-situ cleaning technique permits operation at a much higher average permeate flux, thus reducing the total requirements of membrane area and cost.

With support from the U.S. Bureau of Reclamation, we have undertaken extensive studies of rapid backpulsing applied to water treatment. Figure 2 shows the results of experiments with two different concentrations of bentonite clay in water. Each data point represents the measured long-term permeate flux for a given experiment where periodic backpulses of 0.5 sec duration were applied at the specified time interval. Tire rapid backpulsing is quite effective, increasing the net permeate flux by up to 10-fold, with operation possible at permeate fluxes very close to the clean water flux for dilute suspensions. An optimal forward filtration time (or backpulsing frequency) exists, reflective of the balance between the flux restoration due to the cleaning action of each backpulse, the decline in flux due to fouling during forward filtration between backpulses, and the loss of permeate which occurs during each backpulse. The resulting permeate for these experiments was very clear, containing less than 2 ppm suspended matter.

Concluding Remarks

By using a microphysics-based model, we are able to quantify the effects of particle size, bubble size, and surfactants on microflotation rates. We can now determine the most difficult suspended particle size to float, and the appropriate bubble size and surfactant concentration to achieve the highest possible particle removal rate. This information will help in evaluating the use of microscopic bubbles for removing suspended colloid matter from water, as well as in the preliminary design of microflotation columns. Rapid backpulsing is another effective method for water treatment, especially for surface waters and wastewater which contain dilute concentrations of nonadhesive suspended matter. The resulting improvement in the water processing rate may be used for proper sizing of membrane treatment plants. Economic analysis may then be used for comparison of these two new technologies with each other and with more conventional methods [4].

References

1. Rubin, A.J., Cassel, E.A., Henderson, O., Johnson, J.D. and J.C. Lamb, 'Microflotation: New Low-gas Flow Rate Foam Separation Technique for Bacteria and Algae', Biotechnology and Bioengineering, 8:135-151, 1996.

2. Zabel, T., 'Flotation in Water Treatment', The Scientific Basis of Flotation, Ives, K.J. (ed.), NATO ASI Series, Series E: Applied Sciences - No. 75, Martinus Nijhoff Publishers, The Hague, NL, pp. 349-370, 1984.

3. Redkar, S.G. and R.H. Davis, 'Crossflow Microfiltration with High Frequency Reverse Filtration', AIChE J., 41:501-508, 1995.

4. Ramirez, J.A. and R.H. Davis, 'Application of Cross-flow Microfiltration with Rapid Backpulsing to Wastewater Treatment', J. Haz. Mat., 63:179-197, 1998.

Robert H. Davis is Patten Professor and Chair of the Department of Chemical Engineering at the University of Colorado at Boulder, CO, where Jose Ramirez is a doctoral candidate under his advisory.
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Author:Ramirez, Jose; Davis, Robert H.
Publication:Canadian Chemical News
Geographic Code:1USA
Date:Jan 1, 1999
Words:1142
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