1. Technical Field of the Invention
The present invention relates to a process for enhancing the removal of total organic carbon from a water supply while optimizing the performance of a membrane filtration system.
2. Description of Related Art
Increasingly, municipal drinking water and wastewater filtration facilities utilize membrane microfiltration or ultrafiltration systems as a means of filtering ground water, surface water and wastewater sources (“source water”). Such filtration systems typically utilize a semi-permeable membrane device to filter or reject organic, inorganic and microscopic particulates as water is passed through them. As source water is passed through the membrane filter barrier under hydrostatic pressure, particulate debris (i.e., suspended solids and solutes of high molecular weight) accumulates on the membrane surface and is retained or filtered from the water, while water and low molecular weight solutes pass through the membrane. Typically, insoluble particulates sized larger than 0.04-0.1 microns are filtered or rejected while soluble contaminants or insoluble particulates and ions less than 0.04-0.1 microns pass through the membrane filter.
To function efficiently, it is important that the membrane filter is kept clean. The membrane is, therefore, periodically backwashed to remove the particulate buildup. Accumulated particulates that are not readily removed during backwashing must be removed by chemical cleaning techniques. Such techniques, commonly known as clean in place (CIP) and/or chemical maintenance wash procedures, involve exposing the membrane to chemicals such as caustic soda, sodium hypochlorite (chlorine), various acids and other chemical products to remove the build-up of organic and inorganic compounds. However, chemical cleaning techniques are much more time consuming when compared to backwashing alone. Moreover, the harsh chemicals used in chemical cleaning techniques tend to degrade and deteriorate the membrane filter element over time.
The performance of a filtration membrane is dictated by the fouling rate of the influent contaminants. Fouling is the buildup of organic and inorganic particulates on the membrane surface that are not readily removed during periodic backwashing. As the fouling rate of a membrane filter element increases, chemical maintenance wash procedures are more frequently required. Most membrane filtration systems operate without any additional cleaning processes or chemical treatment past the backwashing and chemical cleaning as described in the above paragraph. Thus, in order to optimize the performance of a filtration membrane it is desirable to minimize the fouling rate of the filtration membrane.
One type of contaminant in water supplies that increases the fouling rate of filtration membranes is total organic carbon (“TOC”). TOC levels vary in water supplies from very pristine (i.e., low levels of TOC) to very contaminated (i.e., high levels of TOC). Higher levels of TOC contribute to taste and odor problems and the formation of disinfection byproducts (“DBP”) such as halo-acetic acid (“HAA”) and total tri-halo methane (“TTHM”). HAAs and TTHMs are created when chlorine reacts with soluble organics and are typically formed when chlorine in the water reacts with soluble TOC in the water distribution system (e.g., a collection of pipes that delivers filtered water to homes and businesses). Reducing the level of TOC, particularly the soluble TOC, in the raw water alleviates the taste and odor problems and minimizes the formation of DBPs.
TOC consists of both soluble and insoluble compounds. However, membrane filtration alone only removes the insoluble component of TOC. A common method for removing the soluble component of TOC includes introducing a chemical coagulant into the water stream and providing adequate mixing and detention prior to membrane filtration. Metal salt-based coagulants react with soluble organics via a process known as “charge neutralization,” which causes a portion of the soluble organic compounds to precipitate out of solution thereby allowing them to be filtered out of the water. The metal base of these coagulants is generally aluminum or iron. Several chemical coagulants can provide this chemical reaction, such as aluminum sulfate, ferric chloride, ferric sulfate, poly-aluminum chloride and aluminum chlorhydrate.
The removal of TOC by coagulants can be further increased by adjusting the pH of the water. Generally, the lower the pH, the greater the TOC removal. For example, when a coagulant is dosed into a water stream, and the pH is depressed chemically to a desired level and maintained (example: pH≈5.5), a higher level of TOC removal can be achieved.
Prior art methods have previously assumed that the pH set point for optimal organic removal is the same for optimum membrane performance, but this is not necessarily true. Optimum membrane performance can be defined as continuous filtration with 1) the lowest pressure rise across the membrane, measured as trans-membrane pressure (TMP); and 2) the lowest chemical cleaning requirement.
The trans-membrane pressure (TMP) is generally a function of the force which drives liquid flow through a cross flow membrane (TMP={(feed pressure+retentate pressure)/2}−permeate pressure), whereas the lowest chemical cleaning requirement is a simply a function of minimizing the fouling rate of the membrane. During filtration, the feed side of the membrane is under higher pressure than the permeate side. This pressure difference forces liquid through the membrane. Consequently, rising TMP is an indicator of membrane fouling.
Prior art methods to remove TOC include 1) installing a process after membrane filtration such as granular activated carbon (GAC) adsorption, 2) dosing a chemical coagulant prior to membrane filtration without controlling pH, 3) only controlling pH ahead of a liquids-solids separator prior to membrane filtration, 4) only controlling pH for organic removal, 5) installing an ion exchange process before membrane filtration, or 6) installing a nano filtration or reverse osmosis process after membrane filtration. Thus, an improved process for water filtration, which enhances TOC removal while optimizing membrane filter performance, is desirable.