Liquid purification or separation – Processes – Liquid/liquid solvent or colloidal extraction or diffusing...
Reexamination Certificate
1999-10-25
2001-10-16
Fortuna, Ana (Department: 1723)
Liquid purification or separation
Processes
Liquid/liquid solvent or colloidal extraction or diffusing...
C210S650000, C210S321690, C134S003000, C134S010000, C134S103100, C134S198000
Reexamination Certificate
active
06303035
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to filtering water using immersed ultrafiltration and microfiltration membranes and in particular to a process for filtering water in a batch mode with a suitable chemical cleaning regimen.
BACKGROUND OF THE INVENTION
Immersed membranes are used for separating a permeate lean in solids from tank water rich in solids. Typically, filtered permeate passes through the walls of the membranes under the influence of a transmembrane pressure differential between a retentate side of the membranes and a permeate side of the membranes. Solids in the tank water are rejected by the membranes and remain on the retentate side of the membranes. Despite the apparent simplicity of this process, however, two related issues have proven to be consistently difficult to address. The first issue relates to controlling the concentration of solids in the tank water. The second issue relates to the need to clean the membranes to prevent their rapid and sometimes irreversible loss of permeability.
Controlling the Concentration of Solids in the Tank Water
Feed water flowing into a tank containing immersed membranes has an initial concentration of solids. As filtered water is permeated through the membranes and removed from the system, the solids are rejected and accumulate in the tank. These solids must be removed from the tank in order (a) to preserve a mass balance and (b) to prevent rapid fouling of the membranes which occurs when the membranes are operated in water containing a high concentration of solids.
One method of removing solids from the tank is to have a continuous bleed of tank water, which may be called retentate. Unfortunately, while this process preserves a mass balance, the tank water must contain a high concentration of pollutants or the process will generate large volumes of retentate.
For example, if a continuous bleed process is operated at a recovery rate of 95% (ie. 95% of the feed water becomes filtered permeate), only 5% of the feed water leaves the tank as retentate. To preserve a mass balance of solids, the retentate must have a concentration of pollutants 20 times that of the feed water. The concentration of solids in the retentate, however, is the same as the concentration of solids in the tank since the retentate is drawn from the tank water. Accordingly, the tank water has a high concentration of pollutants at all times. Operating at a lower recovery rate, 80% for example, results in tank water having a lower concentration of solids but the cost of transporting excess feedwater and then disposing of excess retentate also increases.
Another process involves filtering in a batch mode. For example, PCT Publication No. WO 98/28066 describes a process in which retentate is not withdrawn continuously. Instead, the tank water is drained to remove the accumulated solids at discrete intervals. The tank is then refilled with fresh feed water and operation continues. While regular operation is interrupted in this method, there is a period directly after the tank is refilled in which the membranes are operated in relatively solids lean tank water. For feed water with low suspended solids, the intervals between drainings may be long enough that the benefit gained by emptying the tank offsets the loss in production time. Unfortunately, however, the potential gain in output during the period directly after the tank is re-filled is often not realized because, regardless of the concentration of solids in the tank water generally, the tank water adjacent the membranes can quickly become rich in solids as permeate is withdrawn through the membranes.
Cleaning the Membranes
As filtered water is permeated through the membranes, solids foul the surface of the membranes. The rate of fouling is related to the concentration of solids in the tank water and can be reduced but not eliminated. Further, the solids may be present in the feed water in solution, in suspension or as precipitates and may further include a variety of substances, some not actually solid, including colloids, microorganisms, exopolymeric substances excreted by microorganisms, suspended solids, and poorly dissolved organic or inorganic compounds such as salts, emulsions, proteins, humic acids, and others. All of these solids can contribute to fouling but the fouling may occur in different ways. Fouling can also occur at the membrane surface or inside of the pores of the membrane. To counter the different types of fouling, many different types of cleaning regimens may be required. Such cleaning usually includes both physical cleaning and chemical cleaning.
The most frequently used methods of physical cleaning are backwashing and aeration. In backwashing, permeation through the membranes is stopped momentarily. Air or water are flowed through the membranes in a reverse direction to physically push solids off of the membranes. In aeration, bubbles are produced in the tank water below the membranes. As the bubbles rise, they agitate or scrub the membranes and thereby remove some solids. These two methods may also be combined. For example, PCT Publication No. WO 98/28066, mentioned above, describes a process in which permeation continues for 15 minutes and then stops while the membranes are aerated for 2 minutes and 15 seconds. After the first minute of aeration, the membranes are backwashed for 15 seconds. After the aeration and backwashing, permeation resumes. Accordingly, a process cycle of 17 minutes and 15 seconds yields 15 minutes (or 87% of the cycle time) of permeation. These cycles are repeated several times between tank drainings.
Such back washing and agitation is partially effective in removing solids from the surface of the membranes, but is not very effective for removing solids deposited inside the membrane pores and is almost ineffective for removing any type of solid chemically or biologically attached to the membranes. Accordingly, fouling continues despite regular physical cleaning. This continued fouling is countered by cleaning with a chemical cleaner. For example, the membranes may be soaked in one or more cleaning solutions either in the process tank (after it has been drained and filled with chemical cleaner) or in a special cleaning tank. These methods, however, require either large volumes of chemical cleaner (to fill the process tank) or the expense of providing special cleaning tanks and means to move the membranes to the cleaning tank. These methods also disrupt permeation for extended periods of time.
Other methods involve backwashing the membranes with a chemical cleaner. Examples of such methods are described in U.S. Pat. No. 5,403,479 and Japanese Patent Application No. 2-248,836 in which chemical cleaning is performed without draining the tank or removing the membranes from the tank. Permeation is stopped and the membranes are cleaned by flowing a chemical cleaner in a reverse direction through the membranes while the membranes are simultaneously agitated. Although effective, these methods leave residual chemicals in the tank. In wastewater applications, the chemicals interfere with useful biological processes in the tank water. In drinking water applications, the chemicals pass through the membranes when permeation is resumed resulting in unwanted concentrations of chemicals in the permeate. Further, some chemical cleaner disperses in the tank water during the cleaning event thus increasing the amount of chemical cleaner required.
French Patent No. 2,741,280 describes another method of backwashing membranes with a chemical cleaner. In this method, the tank water is drained before the chemical backwash begins. When the chemical backwash is over, the cleaner is drained from the tank and the tank is refilled. In this way, the chemical cleaner does not contaminate the tank water or permeate. The time required for the backwash, however, is at least 30 minutes which is a significant disruption to the process. Further, in a typical municipal installation the tank may range from 1 m to 10 m in depth. The chemical cleaner inside the lower membranes or the lower po
Adams Nicholas
Cote Pierre
Pederson Steven
Rabie Hamid
Bereskin & Parr
Fortuna Ana
Zenon Environmental Inc.
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