Retention of organic matter by cellulose acetate membranes cleaned with hypochlorite

Cleaning of ultrafiltration (UF) membranes is usually performed with addition of powerful oxidizing agents such as aqueous chlorine. The results of cleaning are assessed by flux levels, and complete restoration of initial flux indicates that the membrane is usable for further operation. The cleaning is performed in short time periods using high concentrations of cleaning agent, and that operation often results in partial loss of membrane integrity. Pristine and chlorine-cleaned 20 kDa cellulose acetate membranes were compared to find how the cleaning influences the retention of organic matter. The experiments were performed on secondary effluents of Ashkelon municipal wastewater treatment plant (Southern Israel) with average total organic carbon level of 14 mg/L. The effluents were pretreated by 130 mg/L FeCl3 . 6H2O and 0.6 g/L powdered activated carbon. The results of current study indicate that despite partial disintegration, flux drop and loss of mechanical strength, proper pretreatment is effective for retention of organic matter by UF membranes. Minor difference in organic content of permeates of pristine and chlorine-treated membranes were observed. The possible pathways of gradual degradation of cellulose surface, with formation of carboxyl, aldehyde and ketone end groups were described. Enlargement of membrane’s pore size was followed with streaming potential measurements.

Chlorine cleaning of CA membranes affects their mechanical characteristics such as mechanical strength at break and elasticity. The negative influence of excessive chemical cleaning on flux was observed for C ⋅ t dosages of 20 and 50 g/L-h where the membrane flux after CIP was lower than before the cleaning. Gradual degradation of membrane surface, with formation of carboxyl, aldehydes and ketones end groups was observed. Enlargement of membrane’s pore size was observed and followed with streaming potential measurements. The above changes have little effect on membrane performance. Even after loss of integrity and despite a significant flux drop after excessive chemical cleaning, organic matter removal A CH2OH O COOH OH O OH HO O O O OH O HOCl HO O O HOOC COOH H+ Cl – CH2OH OH O O CH2OH B CH2OH O O HO OH HO OH O O O OH O O HOCl O OH CHO OH H+ Cl – CH2OH CH2OH OH C CH2OH CH2OH OH O O O HOCl HO HO OH O O O OH OH O O H+ Cl – CH2OH O O CH2OH Fig. 6. Pathway of chlorination of cellulose with formation of various carbonyl groups such as carboxyl COOH (a), aldehyde –CHO (b) and ketone C=O (c). 1.10E+06 O (540 eV) 35.3 1.00E+06 C (290 eV) Strength, MPa Intensity 9.00E+05 Membrane treated by NaOCl (2 g/L h) 8.00E+05 7.00E+05 6.00E+05 Pristine membrane 5.00E+05 4.00E+05 Binding energy, eV 15.9 9.6 Fig. 7. XPS survey spectra of virgin and aqueous chlorine-treated CA membrane.