BackpulsingBackpulse or backwash: 1) Backwash is a procedure in which periodically the flow direction is reversed through the membrane for a short period of time in order to remove particulates accumulated at the membrane surface. 2) Backwash also refers to the waste water produced as a result of the backwash procedure. by itself will not remove all the particulates and foulants on the membrane and so the membrane will gradually see an increase in TMPTransmembrane pressure (TMP): The transmembrane pressure is the difference in pressure between the feed water side and filtrate side of a membrane.. Chemical cleaningClean-in-place (CIP) or in-situ chemical cleaning: Clean-in-place is a procedure performed periodically to clean a membrane more thoroughly than backwashing can achieve in order to restore the permeability of the membrane towards baseline levels. The process uses chemicals such as citric acid and chlorine or others to remove accumulated foulants on the membrane. is performed periodically to remove additional foulants, such as particulates, inorganic scale, organics and biological material, in order to restore the membrane to the best practical condition. It must be remembered that even with backpulsing and chemical cleaning all membranes will see a gradual irreversible fouling that ultimately requires membrane replacement. However, with good design and operation the time to membrane replacement will be acceptably long. Table X.3 shows the variety of chemicals that can be used for cleaning, again depending on compatibility. Other proprietary chemicals such as enzymes have been used in cases where these chemicals are not effective. Some characteristics of chemical cleaning are:
• Cleaning is generally conducted only as needed, e.g., when backpulsing does not restore the membrane to an acceptable TMP; a 30 day interval between cleanings is not unusual although longer times may be seen for systems treating good quality water,
• When the TMP reaches a certain level, some manufacturers recommend using "chemical washes" or "maintenance cleans" consisting of a short duration cleaning at a frequency of several times per day or once every several days depending on conditions in order to prolong the time between full chemical cleaning,
• Citric acid is commonly used to dissolve inorganic scales although other acids can be used,
• A combination of chemicals listed in Table X.3 may be needed because of different types of foulants but these chemicals are typically used one at a time rather than together,
• The cleaning is done on site with the membranes in the membrane units, i.e., clean-in-place (CIP), by recirculating the chemical through the membrane at high flow rates to generate scouring and at higher temperatures to increase the solubility of the foulants (note that softened or demineralized water may be needed for some chemicals),
• The cleaning is followed by a soak cycle and then rinsed,
• The chemical cleaning solutions must be isolated from the treated water and the membrane flushed before putting it back in service with the flushing water wasted until, e.g., turbidityTurbidity: Turbidity is a measure of the cloudiness of a water as well as a gross measure of the amount of suspended solids in a water. returns to acceptable levels (note that the use of surfactantsSurfactant: Surfactants are essentially soaps or detergents that lower the surface tension of water to allow particulates to be more easily removed from surfaces. may generate high volumes of waste water),
• As much as 90% of the cleaning chemicals are recycled in MFMicrofiltration (MF) membranes: Microfiltration membranes are typically hollow-fibers with a pore size range of approximately 0.1 – 0.2 µm (nominally 0.1 µm)./UFUltrafiltration (UF) membranes: Ultrafiltration membranes are typically hollow-fibers with a pore size range of approximately 0.01 – 0.05 µm (nominally 0.01 µm). systems in order to reduce the waste to be disposed of and to reduce chemical costs.
Disposal of wastes is an important consideration and is discussed elsewhere in this educational module.
Table X.3 Chemical Cleaning Agents (USEPA, 2005)
