Membrane Cleaning Strategies

To reduce the two negative influences of concentration polarization and decreasing flux rates which result from fouling, a cleaning strategy has to be developed together with the membrane suppliers which is adapted to the wastewater characteristics, the membrane material and the configuration of the membrane filtration

Fig. 12.9 Fouling mechanisms, adapted from Melin and Rautenbach (2004).

process. In general, process integrated back-flushing during the filtration process is used to maintain a constant flux. Periodically in situ cleaning with chemicals and intensive washing with chemicals ex situ are necessary to remove irreversible deposits.

MBR processes in submerged or cross-flow mode often use fixed-interval back-washing or back-flushing with permeate, whereby reverse flow is used to wash-out reversible fouling. Usually, a filtration process cycle of about 10 min is followed by a back-washing time of 1 min, resulting in a water recovery of at least 90% as shown in Fig. 12.8. Experiments are necessary to optimize two factors: (a) permeate loss by back-washing and (b) the slight decrease in permeate flux which requires maintenance cleaning to recover 100% flux. In submerged systems, intensified aeration in a range of 0.21-3.0 m3 m-2 h-1 is also used to prevent fouling (Benedek and Côté 2003; Cornel et al. 2001).

Maintenance cleaning is done at longer intervals with various chemicals. The chemicals used and the interval chosen must be specifically adapted to the range of application to avoid membrane damage or the formation of harmful substances (ATV-DVWK 2002b). Chemical cleaning usually restores the permeate flux but produces a certain amount of contaminated water.

In situ chemically enhanced cleaning procedures (intermediate cleaning) are used in MBR processes if the flux decreases by about 20% down to 100 L m-2 h-1 bar-1, or as dictated by a cleaning management time table.

Acidic solutions are used particularly for the elimination of inorganic fouling causing substances and alkaline solutions for the oxidation of organic foulants. Disinfectants may be used to eliminate microorganisms. Commonly used commercial purifiers contain active chlorine concentrations between 300 mg L-1 (Wehrle Umwelt 2005) and 2000 mg L-1 (MUNLV 2003), or may contain special mixtures of acids, leaching agents, surfactants, active enzymes (Berghof 2005) or hydrogen peroxide (AV Aggerwasser 2005). Tests are also often performed with citric acid, NaOH, HCL, oxalic acid (Wozniak 2003) and nitro-hydrochloric acid. Sodium hypochlorite is known to recover total permeability, but its use is decreasing because of the damage caused to microorganisms at high concentrations (MUNLV 2003). Heating of the purifiers, sometimes in a separate tank, may be necessary to reach the required cleaning temperature of 35-40 °C.

An intensive ex situ cleaning (main cleaning) is necessary at least twice a year. The typical cleaning management program combines alternating chemical cleaning and clean water rinsing for neutralization and a reaction procedure enhanced with chemicals.

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