Capacitive electrostatic process for inhibiting the formation of biofilm deposits in membrane-separation systems

Abstract

A high-voltage capacitive electrostatic device is used to improve the performance of membrane-separation processes, particularly reverse-osmosis units, wherein species of a given ionic polarity are separated from water. The device is immersed in the water flowing across the membranes and is operated continuously at very high voltages, preferably greater than 30,000 volts DC. The charge on the wetted surface of the suspended particles is altered by the electrostatic field so generated and is found to reduce biofilm formation, thereby materially enhancing the performance of the membrane-separation system. The application of high-voltage electrostatic fields is also found to reduce corrosion of metallic surfaces in the system.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS [0001] This application is a continuation application of copending U.S. Ser. No. 10 / 047,493, filed Jan. 14, 2002, which was a CIP application of U.S. Ser. No. 09 / 416,255, filed Oct. 12, 1999, abandoned.BACKGROUND OF THE INVENTION [0002] 1. Field of the Invention [0003] This invention generally relates to methods for the electrostatic treatment of water streams. In particular, the invention describes a method for preventing the formation of deposits, especially biofilm deposits, on conventional membrane-separation systems by the application of a capacitive electrostatic device. [0004] 2. Description of the Related Art [0005] Water treatment systems are-commonly subject-to-reduced efficiency and failure as a result of scaling and clogging by solid particles suspended in the aqueous medium, as well as by the formation of biofilms that adhere to exposed surfaces in the equipment. Corrosion caused by biofilms, biosludge and scale deposits is also a...

AI Technical Summary

Benefits of technology

[0017] Another object of the invention is an improved rate of recovery in conventional membrane-separation processes.

[0020] Therefore, according to these and other objectives, the present invention consists of utilizing a high-voltage capacitive electrostatic device operated at more than 10,000 volts, preferably at 30,000 and higher voltages, to prevent the formation of biofilms and correspondingly improve the performance of membrane-separation processes. The device is used in connection with conventional membrane-separation processes, particularly reverse-osmosis units, wherein species of a given ionic polarity are separated from water. The device is immersed in the water flowing across the membranes and is operated continuously at very high voltages, preferably greater than 30,000 volts DC. The charge on the wetted surface of the suspended particles is altered by the electrostatic field so generated and is found to prevent biofilm formation, thereby materially enhancing the performance of the membrane-separation system. The application of high-voltage capacitive electrostatic fields is also found to reduce corrosion of metallic surfaces in the system. The preferred embodiment of a capacitive electrostatic device for the invention is a vitrified ceramic tube of unibody construction having a single open end adapted to receive a high-voltage power cable through an insulated cap. The interior surface of the ceramic tube is lined with a layer of conductive material electrically connected to the power cable, thereby providing a relatively-large conductive surface in intimate contact with the dielectric surface of the ceramic tube.

Problems solved by technology

Water treatment systems are-commonly subject-to-reduced efficiency and failure as a result of scaling and clogging by solid particles suspended in the aqueous medium, as well as by the formation of biofilms that adhere to exposed surfaces in the equipment.

Corrosion caused by biofilms, biosludge and scale deposits is also a source of equipment failure and maintenance challenges.

To the extent that prior-art system have been successful in facilitating the removal of suspended material, they still have not prevented the formation of mineral and organic deposits, especially biofilms, on filtration membranes and wetted surfaces of water treatment plants.

Such intimate contact is very important because any air space left between the metal and the dielectric, such as produced by blisters or bubbles in the dielectric layer, causes electrical arcing that eventually perforates the Teflon® layer, shorts the electrode to the water body, and greatly reduces the electrostatic efficiency of the device.

Moreover, a large air space would form yet another dielectric layer within the system, which is undesirable because of the very low capacitance of air which would further reduce the overall capacitance of the system.

The devices of the prior art are limited in their application by twofold problems.

Because of its well-known physical properties, PTFE material such as Teflon® is not suitable for adherence to the surface of metal conductors other than by heat-shrink processes.

Any attempt to cover an electrode with Teflon® by a process other than heat-shrinking, such as would be required for an electrode having a non-cylindrical shape, would necessarily compel the formation of seams and connections which would be very difficult to achieve and prone to breakdown during use.

In addition, due to the non-stick properties of the material, it would be very difficult to avoid the formation of air spaces between the metal and the dielectric surfaces.

Under normal stresses of operation, the connection between the tube and these end bushings has been the source of leaks which allow the water medium to come into contact with the high-voltage metallic tube and cause a complete-system breakdown.

Another problem relates to the thickness of the dielectric material utilized in the prior art.

In order to optimize capacitance, the layer of Teflon® used to coat the positive electrode is kept to a minimum, thereby causing the dielectric layer to be more vulnerable to imperfections of construction which can cause arcing or other operating stresses that could result in interruption of insulation.

As a result of these constraints, the devices of the prior art are not suitable for efficient and dependable operation at voltages higher than approximately 10,000 volts, beyond which they quickly experience breakdowns.

The major problem encountered in membrane separation plants (reverse osmosis, nanofiltration, ultrafiltration) is the fouling of membranes caused by mineral, organic and biological deposits.

These deposits affect system performance because clean-water recovery and quality necessarily decline as membrane fouling increases.

In addition, the bacteria and other microorganisms entrapped on the membrane's surface cause them to become plugged, which in turn requires increased operating pressures and energy consumption, and greater frequency of chemical washing, all of which results in irreversible equipment damage.

The effect of chemicals on the formation of biofilms, though, has been very limited, both in scope and duration, providing little relief to the pervasive and enduring problem of biofilm formation and related fouling of membrane-separation units.

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