Description

FILTRATION: Normal operation refers to the routine operating sequence of a system using the Ultrafiltration module and includes the operating and backwash steps. At initial start up the modules are flushed using a Forward Flush to remove any residual chemicals or trapped air from the module. The forward flush occurs on the outside of the fibers and does not filter the feed water to make permeate. After the forward flush is discontinued the modules can be placed in the operating mode of filtration. An operating cycle varies, based on feedwater conditions and cleaning program. While in normal filtration mode, 100% of the feed water is converted to permeate. As contaminants are removed during filtration, the transmembrane pressure will rise. At the end of the preset operating cycle time, a backwash sequence is triggered. AIR SCOURING: The backwash mode occurs automatically and may include an air scour, but always includes draining, backwash through the top drain, backwash through the bottom drain, and a forward flush. The Air Scour step, when included, is used to loosen particulates deposited on the outside of the membrane surface. Air is introduced on the outside of the fibers and displaced feed flow/concentrate is allowed to discharge through the top of the module for disposal. After 20 to 30 seconds of air scour the module is drained by gravity to remove dislodged particulates. If the air scour step is not included, the backwash sequence is started with simply draining the module by gravity to remove the concentrated feed water before the start of the backwash. After draining, the first backwash step is performed, Backwash (Top). Permeate flow is reversed from the inside of the fiber to the outside and backwash flow is removed from the module housing through the top drain on the module. A top draining backwash is performed first to remove contaminants in the area of greatest concentration. The second backwash step, Backwash (Bottom), is performed to remove contaminants through the bottom of the module housing. Permeate flow is reversed from the inside of the fiber to the outside and backwash flow is removed from the module housing through the bottom drain on the module for efficient removal of heavier materials. The two steps of backwash can be repeated numerous times depending on the degree of fouling. After backwash is complete, a Forward Flush is performed to remove any remaining contaminants and remove any air trapped on the outside of the fibers. After a backwash, the modules are returned to Filtration mode. FORWARD FLUSH: CHEMICALLY ENHANCED BACKWASH (CEB) CEB operation refers to a chemically enhanced backwash. The frequency of a CEB is dependent on the feed water quality. On high quality feed waters a CEB may not be required. However a CEB can be performed as frequently as a backwash. The CEB process occurs automatically but the frequency can be adjusted after gaining operating experience. The CEB is performed using UF permeate but with either an acid or base chemical to effectively clean difficult contaminants from the membrane surface. The CEB follows the steps of a normal backwash, except a CEB chemical is dosed into the backwash water and a soak step is added after the second backwash step, Backwash (Bottom). The soak is performed for 5 to 10 minutes and allows greater time for the oxidant to react with contaminants that have attached to the membrane surface, or penetrated the fiber wall. After the soak a forward flush is performed to remove any remaining contaminants on the outside of the fibers. After a CEB and at the start of the operating step, permeate may be wasted to remove residual chemicals held in the fiber or module. CLEAN IN PLACE (CIP) CIP: A clean-in-place (CIP) operation includes backwash and chemical recycle to clean the fibers. The CIP is an on demand operation. The frequency of a CIP is dependent on the feed water quality but can range from 1 to 3 months. Prior to a CIP, the routine backwash steps including air scour, draining, backwash through the top drain, and backwash through the bottom drain are performed. The backwash steps are repeated 3 to 8 times to remove contaminants or foulants not requiring chemical removal. After completing the backwash steps, the module is drained by gravity to remove excess water and prevent dilution of the CIP chemicals. CIP chemicals are recycled on the outside of the module for 30 minutes through the chemical cleaning tank. A small chemical permeate stream will also be collected and recycled to the chemical cleaning tank. Note that the CIP solution can be heated up to 40ºC to improve its effectiveness at removing contaminants from the membrane. A soak follows the initial recycle step for 60 minutes or longer depending on the degree of fouling that has occurred. After the soak step, CIP chemicals are again recycled on the outside of the module for 30 minutes. When the recycle is completed, an air scour is performed and then the module is drained to remove concentrated chemicals. The two steps of backwash and forward flush are performed to remove any remaining contaminants on the outside of the fibers. After a CIP and at the start of the operating step, permeate may be used to remove residual chemicals held in the fiber or module. The CIP steps described above are for a single alkali or acid chemical solution. If an acid and alkali cleaning are required, the CIP steps would be repeated for each chemical solution.