Cleaning fouled membranes

Kimura et al. (2004) cleaned fouled membranes with different individual chemicals and also consecutively used two different chemicals. For the individual chemicals, they found NaClO (500 ppm) as the best cleaner followed by NaOH (pH 12). Oxalic acid (pH 2)/NaC10 (500 ppm) was found to be a better cleaning combination than HCl (pH 2)/NaC10 (500 ppm) and NaOH (pH 12)/NaC10 (500 ppm). The combinations were better cleaners than the individual chemicals—see Figure 6.16. 

Fouling of the pH sensor may occur in solutions containing surface-active constituents that coat the electrode surface and may result in sluggish response and drift of the pH reading. Prolonged measurements in blood, sludges, and various industrial process materials and wastes can cause such drift. Therefore, it is necessary to clean the membrane mechanically or chemically at intervals that are consistent with the magnitude of the effect and the precision of the results requited. 

Pretreatment For most membrane applications, particularly for RO and NF, pretreatment of the feed is essential. If pretreatment is inadequate, success will be transient. For most applications, pretreatment is location specific. Well water is easier to treat than surface water and that is particularly true for sea wells. A reducing (anaerobic) environment is preferred. If heavy metals are present in the feed even in small amounts, they may catalyze membrane degradation. If surface sources are treated, chlorination followed by thorough dechlorination is required for high-performance membranes [Riley in Baker et al., op. cit., p. 5-29]. It is normal to adjust pH and add antisealants to prevent deposition of carbonates and siillates on the membrane. Iron can be a major problem, and equipment selection to avoid iron contamination is required. Freshly precipitated iron oxide fouls membranes and reqiiires an expensive cleaning procedure to remove. Humic acid is another foulant, and if it is present, conventional flocculation and filtration are normally used to remove it. The same treatment is appropriate for other colloidal materials. Ultrafiltration or microfiltration are excellent pretreatments, but in general they are... 

Second, most membrane materials adsorb proteins. Worse, the adsorption is membrane-material specific and is dependent on concentration, pH, ionic strength, temperature, and so on. Adsorption has two consequences it changes the membrane pore size because solutes are adsorbed near and in membrane pores and it removes protein from the permeate by adsorption in addition to that removed by sieving. Porter (op. cit., p. 160) gives an illustrative table for adsorption of Cytochrome C on materials used for UF membranes, with values ranging from 1 to 25 percent. Because of the adsorption effects, membranes are characterized only when clean. Fouling has a dramatic effect on membrane retention, as is explained in its own section below. 

Backflushing is another way of cleaning heavily fouled membranes. During back flushing a slight overpressure is applied to the permeate side of the membrane forcing solution from the permeate side to the feed side of the membrane. The flow of solution lifts deposited materials from the surface. Typical back flushing pressures are 5-15 psi [48]. 

Lamminen MO, Walker HW, Weavers LK (2006) Cleaning of particle-fouled membranes during cross-flow filtration using an embedded ultrasonic transducer system. J Membrane Sci 283 225-232... 

Cleaning. Fouling films are removed from die membrane surface by chemical and mechanical methods. Dissolved fouling material may pass into the membrane pores. Reprecipitation upon rinsing must be avoided. Membrane-swelling agents, such as hypochlorites, flushout material which may be lodged in the pores. 

Backflushing is another way of cleaning heavily fouled membranes. The method is widely used to clean capillary and ceramic membrane modules that can withstand a flow of solution from permeate to feed without damaging the... 

Figure 6.3 (a) CP for clean membrane (b) CECP for fouled membrane. 

There is a tendency to want to increase throughput shortly after start up or after a successful membrane cleaning, when membranes are performing their best. However, if changes are made without regard to consideration of the other variables in the system that depend on flow and recovery, that will hasten fouling and scaling as a result. 

Figure 8.1 Shows the projected performance of an RO membrane system with ideal, marginal and inadequate pretreatment.1 After an initial period over which time new membranes stabilize performance, a system with ideal performance will show only a slight decline in performance with time due to compaction and the inevitable fouling and scaling that will occur despite good pretreatment and system hydraulics. Marginal pretreatment exhibits more rapid decline in performance than the system with ideal pretreatment. Initial cleaning may be able to revive most of the performance, but after time, foulants and scale that were not removed become irreversibly attached to the membrane and cannot be cleaned away. The RO system with inadequate pretreatment will show very rapid decline in performance that typically cannot be recovered by cleaning the membranes. An RO system with less than ideal pretreatment faces frequent cleaning intervals and short membrane life. Frequent cleaning and membrane replacement costs money, time, and the environment.

Given that the operators were able to maintain a constant product flow rate, albeit by increasing the operating pressure, one might ask why we should even care about the NPF When systems are allowed to operate in fouling or scaling mode for extended periods of time, the foulant or scale can become resistant to removal via cleaning. Thus, NPF is used to determine when it is time to clean the membranes before the surface contamination becomes permanent. 

Ultrafiltration of whey is a major membrane-based process in the dairy industry however, the commercial availability of this application has been limited by membrane fouling, which has a concomitant influence on the permeation rate. Ultrasound cleaning of these fouled membranes has revealed that the effect of US energy is more significant in the absence of a surfactant, but is less markedly influenced by temperature and transmembrane pressure. The results suggest that US acts primarily by Increasing turbulence within the cleaning solution [91]. 

The effect of different cleaning agents on the recovery of the fouled membrane was studied by Mohammadi et al. [78]. Results showed that a combination of sodium dodecyl sulfate and sodium hydroxide can be used as a cleaning material to reach the optimum recovery of the polysulfone membranes used in milk concentration industries. Also a mixture of sodium hypocholorite and sodium hydroxide showed acceptable results, where washing with acidic solutions was not effective. 

Rabiller-Baudry M., Le Manx M., Chaufer B., and Begoin L., Characterisation of cleaned and fouled membranes by ATR-FTIR and EDX analysis coupled with SEM Application to UF of skimmed milk with a PES membrane. Desalination 146 2002 123-128. 

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