Tertiary treatment of municipal sewage is a common wastewater application for ultrafiltration and microfiltration membranes; however, such systems need to be capable of operating on high suspended solids feed waters while having a long life with a minimum of chemical cleanings. Suspended solids that need to be removed may be materials that cause turbidity, such as bacteria, cysts and oocysts, viruses, colloidal material, such as iron oxides, clay, silt, sand and other insoluble impurities. Municipal sewage secondary treatment effluent typically has turbidity levels of 5 to 10 NTU with a suspended solids count of 10 to 20 parts per million (ppm). For membrane technology to be economically competitive in a tertiary treatment process, it should operate at sustained permeate flux rates of 15 to 30 gallons per square foot per day (gfd) with minimal energy consumption, while requiring chemical cleaning at a frequency of not more than once per month.
Perhaps the biggest dilemma facing ultrafiltration and microfiltration membrane technology is fouling, where suspended solids and natural organic matter are adsorbed onto the membrane surface and result in a decrease in permeate flux. Decreased permeate flux rates result in higher overall capital and operating costs due to the lower output over a given period of time. The development of new membrane technologies has focused on reducing or eliminating the adverse affects of membrane fouling. A membrane element or cartridge that can produce high flux rates while minimizing fouling can have an extreme economical advantage over competing technologies.
Historically, such difficult applications as treating feed solutions high in organic and suspended solids have employed spiral wound, hollow fiber, capillary, or tubular element designs in a multitude of configurations. The most common of configurations is a submerged membrane system where the membranes, whether it be spiral wound or hollow fiber, are placed vertically in an open tank and filled with a feed solution to be filtered.
Very generally, a spiral wound membrane cartridge contains a permeate carrier sheet, a membrane filter sheet that is adhesively bonded to the permeate carrier sheet (usually to both surfaces thereof to create an envelope about it), and a feed spacer sheet which separates two facing membrane filter layer sheets. Microfiltration (MF) and ultrafiltration (UF) membranes are typically formed of either polyethersulfone (PES), polysulfone (PSF), polyvinylidene fluoride (PVDF), or polyacrylonitrile (PAN) because these polymers are generally recognized in the industry to make membranes having high flux rates, good chemical resistance and good physical durability, which can be produced using conventional casting techniques. Other polymers such as polypropylene, polyethylene, and chlorinated polyethylene may also be used to construct UF and MF membranes. Spiral wound membrane cartridges exhibit good fouling resistance and offer economically attractive filtration with greater mechanical durability.
A traditional submerged membrane system is shown in FIG. 6 of WO 00/78436 patent application (28 Dec. 2000) wherein a spirally wound membrane element is immersed in a tank that is filled with a body of water to be filtered and air is bubbled up through the membrane for the purpose of maintaining a clean membrane surface. The required transmembrane pressure (TMP) is supplied by a pump that creates a vacuum in addition to any contribution from the static head. Alternatively, static liquid heads alone have been used to generate feed pressures for submerged filtration, see U.S. Pat. No. 5,916,441. These systems will typically also employ support frames and manifolds. Materials of construction are typically limited to 316 stainless steel for these type of systems, which is a significant capital expenditure. The use of 316 stainless steel also limits the amount of exposure to certain corrosive water sources, such as cleaning solutions and aggressive feedwaters such as seawater.
Factors which contribute to membrane fouling include water chemistry, suspended solids concentration (TSS), membrane chemistry and transmembrane pressure (TMP), are used to combat same. Membrane chemistries vary between polysulfone (PSF), polyethersulfone (PES), and polyvinylidene fluoride (PVDF); however, all are generally similar in their fouling tendencies. The more hydrophilic a membrane chemistry is, the more resistant it is to fouling, as fouling constituents are generally hydrophobic.
Membrane fouling typically occurs through two different mechanisms; pore impregnation and cake-layer formation. Pore impregnation occurs when organic and suspended solids penetrate and impregnate the pores of a membrane, thus narrowing the effective diameter of the pores and increasing the required net drive pressure for permeate production. Cake-layer formation involves large colloidal compounds and suspended solids accumulating on the membrane surface and forming a discrete layer of organic and suspended solids. Both fouling mechanisms result in decreased permeate flux.
Common methods to offset the effects of fouling for submerged membrane systems include air bubbling, backwashing, and chemical cleaning. Air bubbling, whether it be for a spiral wound or hollow fiber membrane, is quite effective at removing particulate matter from the membrane surface; however, it is an energy intensive process that often comprises 50% or more of the total energy costs associated with a submerged membrane system. The economical advantages of a membrane system that can eliminate or greatly reduce the use of air bubbling without increasing chemical cleaning frequency are obvious.
When the effects of fouling become irreversible, membrane cartridges require extensive cleaning with a variety of chemical cleaners. Existing submerged membrane systems often rely on extensive static soaking with an oxidant-based, caustic-based, and/or acid-based cleaner. Air bubbling is discouraged during cleaning and is often discontinued during the cleaning process so as to avoid excessive foaming and/or release of harmful vapors to the atmosphere. However, static cleaning has been found to be much less effective than dynamic cleaning wherein the added shear force enhances the removal of organic and suspended material from membrane surfaces.
Another problem associated with submerged membrane systems is the accumulation of solids in the process tank and the inefficient removal thereof. With submerged membrane systems, the suspended solids level inside the tank increases over time, due to accumulation, until a steady state value is reached. This steady state suspended solids level is directly proportional to the recovery of which the system operates at. For example, a system that operates at 90% recovery will have a steady state suspended solids value about ten times greater than that of the influent level. Since membrane fouling is directly proportional to suspended solids concentration, it is obvious that membrane fouling will be greater when the suspended solids level has reached its steady state concentration value; however, it is highly undesirable to lower the system percentage recovery for the sake of reducing fouling as such would drastically lower the overall efficiency of the system, potentially rendering it impractical and economically unattractive. Moreover, after every backwash, solids removed from the membrane surface are mixed directly back into the bulk tank solution, further complicating the solids build-up problem. Because large, submerged membrane systems utilize process tanks having very large holding volumes, it is impractical to drain said tanks on a frequent basis in order to purge the accumulated solids from the system.
With the foregoing in mind, work on improvements was undertaken.