Measuring and testing – With fluid pressure – Porosity or permeability
Reexamination Certificate
1999-12-21
2001-12-04
Williams, Hezron (Department: 2856)
Measuring and testing
With fluid pressure
Porosity or permeability
C210S090000
Reexamination Certificate
active
06324898
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method and apparatus for testing the integrity of filtering membranes.
BACKGROUND OF THE INVENTION
Filtering membranes are used to permeate a relatively particle free liquid from a liquid rich in particles. Reverse osmosis and nanofiltration membranes, for example, are used to produce very high quality water for drinking or industrial applications. Ultrafiltration and microfiltration membranes are used at lower pressure to filter water for drinking or industrial applications and to treat waste water.
One reason for using membranes to filter water is that membranes are able to remove very small particles including pathogenic microorganisms and colloids. Thus, strong chemicals may not be required as a primary disinfectant in drinking water applications and a nearly complete lack of colloids in water produced for industrial purposes improves the performance of many industrial processes. To ensure that undesired particles are removed, however, the integrity of a membrane unit must be monitored and tested regularly. In particular, although membranes are usually tested after they are manufactured, leaks can develop when the membranes are installed in a filtering system and during the subsequent operation of the system. For example, leaks may result from fatigue, from over-pressurization, or from cleaning and maintenance activities.
Membrane integrity can be monitored using continuous or discontinuous methods. Continuous integrity testing techniques, which include particle counting and acoustic analysis, do not evaluate the membrane itself but instead monitor and assess a surrogate parameter to diagnose the membrane condition. For instance, a batch or on-line particle counter generally includes a light scattering sensor, typically laser-based, interfaced with a computer running particle enumeration software that assesses the number of particles in one or more particle size ranges: see generally Panglish et al., “Monitoring the Integrity of Capillary Membranes by Particle Counters”,
Desalination
, vol. 119, p. 65-72 (1998). Similarly, a particle monitor that measures the fluctuation in intensity in a narrow light beam transmitted through a permeate sample is also known. Through subsequent computer analysis, the observed fluctuations can be converted into an index of water quality. Particle counting and particle monitoring techniques require elaborate and expensive measurement equipment that is subject to measurement drift, noise, and periodic maintenance such as calibration. In addition, these methods generally do not differentiate between undesirable particles and other signals that have no relation to membrane integrity, particularly air bubbles produced on the permeate side of the membrane and associated with backwashing operations. Moreover, the number of membrane units or modules that can be simultaneously monitored using these integrity testing methods is limited by dilution effects.
In acoustic membrane analysis methods, as described in Glucina et al., “Acoustic Sensor; a Novel Technique for Low Pressure Membrane Integrity Monitoring”, AWWA Membrane Conference, Long Beach, Calif. (Feb. 28 to Mar. 3, 1999), one or more sound wave sensors or transducers are placed on a membrane unit to detect anomalies in the acoustic response of the membrane, namely noise originating from broken fibres. These acoustic techniques, however, detect only broken fibres and do not detect more subtle defects in, or the general deterioration of, a membrane. Furthermore, these methods are susceptible to interference from surrounding noise and are very expensive, since they require at least one acoustic sensor per membrane unit and each of these sensors must be electrically connected to a central computer for appropriate signal analysis.
In another class of integrity testing techniques, membrane integrity is assessed directly while permeation is temporarily stopped. Typically, air (or another suitable gas) is applied to a first side of a wet membrane at a pressure higher than the pressure of water or air on a second side of the membrane to create a trans-membrane pressure but at a pressure lower than the bubble point of a membrane without defects. A rapid flow of air from the first side of the membrane to the second side indicates a leak in the membrane. Such integrity testing methods are often referred to as air leak tests and examples are discussed in U.S. Pat. No. 5,353,630 to Soda et al. and in International Patent Application No. PCT/FR97/00930 (corresponding to International Publication No. WO 97/45193) assigned to OTV Omnium de Traitements et de Valorisation of France. In U.S. Pat. No. 5,353,630, the water on the feed side of a shelled membrane module is replaced with pressurized air. In International Patent Application No. PCT/FR97/00930, the feed side of an immersed, unshelled membrane module is exposed to air at atmospheric pressure by emptying a tank in which the module is immersed and then a partial vacuum is applied to the filtered water on the permeate side of the module.
In air leak tests, the trans-membrane pressure used is selected to exceed the bubble point corresponding to defects or holes whose size is of interest, i.e. whose undesirable passage requires monitoring. The bubble point is the air pressure which exceeds the surface tension of a liquid in a hole of the relevant size. The bubble point is described theoretically by the Young Laplace equation which provides the pressure difference required across a curved interface in terms of the surface or interfacial tension and the principal radii of curvature. For example, pressures of 0.3 to 1.0 bar are used to detect holes in the range of 0.5 to 2.3 &mgr;m.
In different air leak test methods, the trans-membrane pressure is controlled over time according to alternate strategies to provide an indication of the size or number of leaks. For example, in a pressure hold test (“PHT”), the flow rate of air required to maintain a certain trans-membrane test pressure is measured. In a pressure decay test (“PDT”), the rate of trans-membrane pressure change (decay) from an initial value is measured. With both tests, measured values are compared to membranes known to be free from defects. Both tests require precise air flow or air pressure sensors or both and are accordingly expensive to install.
Another problem with the PHT and PDT is that the accuracy of both tests is limited by the fact that air crosses the membrane by diffusion through water filled pores in addition to flowing through defects in the membrane. Such diffusive air flow is related to the surface area of the membrane unit being tested. In a large membrane unit (ie. with a flow capacity in the range of a thousand or more cubic metres per day), the diffusive air flow may be similar in magnitude to the air flow expected from a defect of the size being tested for. This problem makes detecting a single broken fiber difficult in a membrane unit of this size and generally limits the size of membrane units that can be properly tested with such tests. Thus, in a large municipal or industrial installation with several large membrane units connected together in a filter train, several distinct sets of membrane integrity testing apparatus are required. Thus, there is a need for an improved method and system for accurately measuring the integrity of filtering membranes.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a method and apparatus for providing an integrity test for filtering membranes.
The present invention is a method and system for testing the integrity of membranes using a gas such as air subjected to a trans-membrane pressure. The air that crosses a membrane locally (i.e. on a specific unit of membranes) is collected. The volume of air collected for each membrane unit tested provides a quantified indication of the integrity of the membrane unit, since that volume is directly related to the amount and quality of leaks in the membrane unit.
In one aspect, the invention is directed at an improvement t
Adams Nicholas
Cote Pierre
Janson Arnold
Bereskin & Parr
Cygan Michael
Williams Hezron
Zenon Environmental Inc.
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