Over time, the particles that don’t pass through the walls of the hollow fiber begin to build up. The particles reduce the ability of clean water to pass through the walls causing the Trans Membrane Pressure (TMP) to increase. After a certain time (approximately 30 minutes) the hollow fiber membranes will require backwashing.
The Hydraulic Cleaning or backwash cycle consists of three main steps – Ra wWater flush, filtered Water Backwash and a follow up raw water Flush.
During the first hydraulic cleaning step, raw water enters the module from one end and flows though the fibers to the opposite end where it is sent to the backwash outlet header. This step is designed to take away any particles that are on the surface of the membrane and can be flushed away.
During the second hydraulic cleaning step, filtered water is added to the hydraulic flow. The filtered water flow is reversed through the pressure vessel and the membranes. A fresh supply of filtered water will be delivered to the outside of the tubes, passing through the walls of the hollow fibers and washing away the accumulated particles on the inside of the hollow fiber assisted by the raw water.
During the third hydraulic cleaning step, the filtered water is turned off but raw water continues to flow through the module. This step washes away any particles that have been lifted up from the membrane fiber by the filtered water backwash step.
Consistent, periodic backwashing will extend the life of the hollow fibers and prevent pre-mature fouling.
It has also been fund beneficial to perform an enhanced backwash using 10 – 25 ppm of chlorine on a periodic basis will greatly increase the amount of time between chemically-enhanced backwashes. The operator can select the frequency and dosage requirements.
Despite periodic back flushing, organic matter from some surface water sources will cling to the walls of the hollow fibers and begin to grow and become difficult to remove through regular backwashing alone. If these organisms are not removed, they will “foul” the membranes. Most systems address this problem on a monthly basis. The UltraFLEX UF & RO Membrane System provides for chemical cleaning on a daily basis.
Because of the excellent chemical resistance of the X-FlowTM SXL-225 membrane, chemicals can be added more frequently. Because the microorganisms have not been given time to grow and work their way into the membrane, they are easier to remove. In the UltraFLEX UF & RO Membrane Systems, backwashing is enhanced with the use of chemicals. This Chemically Enhanced Backwash alternates between two cycles, CEB1 (low pH) and CEB2 (high pH).
During CEB1 caustic soda and/or chlorine is added through the backwash water and the pH is raised to approximately 12. During CEB2 hydrochloric or sulfuric acid is added through the backwash water and the pH is lowered to approximately 2.
During either of the chemically-enhanced backwash cycles, the pressure vessels and membranes are saturated with the caustic or acidic solution and soaked for 10 minutes. Alternately, CEB1 (chlorine and/or caustic) will dissolve mineral deposits and scale and CEB2 (acid) will hydrolyze organics, microorganisms and parasites. For hard water sources that have a tendency to form scale, CEB2 will immediately be followed by CEB1.
After the membranes have been given time to soak during chemically enhanced backwash, they are backwashed and rinsed to remove the chemically treated waters. A pH meter on the backwash effluent line monitors the pH to ensure that the filters are not placed back into service before all of the chemicals are rinsed out and the pH returns to normal.
Because the microorganisms are killed daily (instead of monthly or less frequently as in other systems) they do not have time to grow until they become problematic. Membrane fouling is prevented. In addition, incorporating the chemical cleaning with the regular backwash cycles eliminates lengthy downtime required for monthly cleaning. The membranes remain in operation constantly.
Because of the periodic, chemically enhanced backwashing, the filtration skid will be out of service for short periods of time. During this time, the clearwell or finished water storage tank provides filtered water to the system.
The backwash supply flow-rate setpoint for both regular and chemically enhanced backwash will be selectable by the operator and controlled by a VFD on the backwash pump.