Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
2000-09-01
2002-07-09
Drodge, Joseph W. (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S639000, C210S641000, C210S651000, C210S721000, C210S722000, C210S724000, C210S760000, C210S806000
Reexamination Certificate
active
06416668
ABSTRACT:
BACKGROUND OF THE INVENTION
One of the most important phenomenae undermining the performance of membrane processes used to purify potable water and industrial wastewater is membrane fouling and scale formation which reduces the membrane permeate flowrate and, if not mitigated, shortens the useful life of the membrane. The deposition of fouling material or scale compounds on the surface and/or inside the pores of the membrane will take place as a result of the increase in the concentration of these compounds at the membrane surface since they are rejected (i.e. separated) by the membrane.
Typical “membrane foulants” include colloidal suspended solids such as clays and silt, metal hydroxides such as iron hydroxide originating from corrosion of steel piping and tanks, naturally occurring organic matter (NOM) including humic substances, soluble organic compounds and insoluble “oil and grease” which are typically present in industrial effluents and “bio-foulants”. The “bio-foulants” can be “aerobic” or “anaerobic” bio-mass which form due to the bio-degradation of organic compounds in the water in the presence, or absence, of oxygen, respectively. While “membrane foulants” affect all types of membranes, including reverse osmosis membranes (RO), nano-filtration membranes (NF), ultra-filtration membranes (UF) and micro-filtration membranes (MF), scale formation typically occurs in RO and NF membranes only as a consequence of “concentration” of “sparingly or partially soluble” inorganic scale compounds including calcium carbonate, silica and calcium sulphate when these compounds are rejected by these “tight” membranes. Before membranes can be used to purify water from various sources, the water must be pre-treated and conditioned in order to separate suspended solids, colloids, oil and grease and NOM's and provide a feedwater that is free from these membrane fouling compounds and is chemically compatible with the membranes.
PRIOR ART
A large number of pretreatment processes have been reported in the membrane and patent literature. These processes include the separation of colloidal suspended solids by coagulation and flocculation using inorganic multivalent metal hydroxides (e.g. aluminum sulphate or ferric chloride) and/or the highly effective flocculating cationic polymers (e.g. high molecular weight quaternary ammonium compound), respectively. This treatment is typically followed by clarification, depending on the suspended solids loading, and/or filtration using sand filters, dual-media filters or multi-media filters followed by a 5-micron cartridge filter. The multi-media filters contain a support layer of garnet or fine gravel, one or two layers of fine sand and anthracite.
Depending on the application, micro-filtration membranes may be used as an ultimate polishing step in addition to these chemical conditioning and filtration steps, or it may be used exclusively in lieu of these filtration steps. When the water contains high hardness due to calcium or magnesium, water softening resins may be used in the pre-treatment train. Activated carbon media may also be used to pre-treat industrial effluents containing organic compounds, as well as to remove free chlorine which chemically attacks the polyamide membrane film typically used in NF and RO membranes.
For example, Ebara et. al. Disclose in U.S. Pat. No. 4,080,289 a process for the treatment of industrial effluents using RO membranes in which said water is pre-treated by the addition of aluminum and/or iron salts in order to solubilize fluoride ions and prevent formation of calcium fluoride scale on the RO membranes as this compound is concentrated in the RO retentate. This patent also discloses a mechanical cleaning procedure of tubular RO membranes having surface deposits of scale and/or fouling material by using and recycling “sponge balls” to physically remove said scale and fouling material.
Smith discloses in U.S. Pat. No. 5,174,901 a membrane process for purifying wastewater which includes pre-treatment using a sand filter to remove particulate matter, an activated carbon filter to remove free chlorine and tri-halomethanes and a water softener to remove calcium, magnesium, iron and manganese compounds which would otherwise form scale on the membrane surface. The pre-treated water is then purified further with RO and ion exchange to separate soluble ions, organic and biological compounds and finally calcium salt and carbonic acid are added to the treated water to give a better tasting calcium-enriched water.
In another example of Prior Art, Arnaud discloses in U.S. Pat. No. 5,647,977 a system for purifying industrial effluents from laundry and vehicle washing operations. The system includes a coarse solids filtration device, aeration to fluidize and separate oil and other organic compounds, flocculation of colloidal solids, filtration of flocculated solids, activated carbon or polymeric resin to separate chlorine and organic compounds, high purity granular copper and zinc beds to separate heavy metals, two-stage anion exchange resins and two-stage cation exchange resins to remove organic compounds, prevent bio-growth and demineralize the water.
Furthermore, Boyce et. al. disclose in U.S. Pat. No. 5,651,894 a double-pass reverse osmosis process for producing ultrapure water in which the water is pre-treated by the addition of dithiocarbamate which produces a reducing environment to prevent bacterial growth and scavenge metals. The water is then purified with the 2-pass RO membranes and the RO-
2
concentrate is recycled to the RO-
1
feed at a reduced pressure to prevent generation of oxidants.
In U.S. Pat. No. 4,414,113, La Terra discloses a method for pre-treatment of water to be purified by hollow fiber reverse osmosis membranes. The raw water is directed into a pressure vessel containing a number of filter elements, which in turn have hollow fiber RO membranes wound around center cores such that the water flows from the outside of the filter elements towards the center cores. The pure permeate passes into the center bores of the fibers while the concentrate passes into the center cores of the elements.
Pohl et. al. disclose an RO pre-treatment method in U.S. Pat. No. 4,261,833 in which they teach precipitation and flocculation of metal hydroxides by using a combination of the acid salt and the acid having the same anion as the salt in order to produce the metal hydroxide at optimum pH. The resulting concentrated hydroxide flakes and flocculated colloids are separated in a 2-stage centrifuge, followed by acidification of the hydroxide precipitate and recycle of the acid/salt pair for further water treatment.
Henz et. al. disclose in U.S. Pat. No. 4,758,347 a process for purifying dyeing wastewaters by adjusting the pH with alkali or acid to the range 4 to 9 followed by coarse filtration to remove the suspended solids and adjusting the pH to the range 40-60° C. The pre-treated water is then subjected to a 2-stage RO membrane system in which the second stage permeate is recycled to the pre-treatment step and the first stage permeate is sent to a wastewater treatment plant for final treatment before discharge. The second stage concentrate is sent to a wet-air oxidation or combustion system to destroy the organic compounds.
In another related prior art, Comstock et. al. disclose in U.S. Pat. No. 5,374,357 a process for removing colloidal matter from raw water by passing the water through a finely divided filter media which is impregnated with a suitable coagulant. The water-borne colloidal matter is captured and deposited on the large surface of the filter media and is subsequently removed by back-washing of the filter media.
One of the most common problems encountered in the prior art above when using inorganic coagulants and cationic (polymeric) flocculating chemicals for the pre-treatment of water before subjecting the water to membrane purification is the relatively strong positive charge of these coagulating and/or flocculating compounds. This charge is required in order to neutralize the negative surface charge of
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