Investigation of pre-ozonation on the performance of membrane filtration (Oxeram 1 - D 4.2)

Low-pressure membrane filtration of secondary effluents allows disinfection and, combined with chemical coagulation, advanced phosphorus removal. However, the loss of filtration performance due to membrane fouling is still a fundamental problem and has a strong impact on the costs of the process. Biopolymers as well as colloids in the range of 50 to 350 nm were identified as main foulants during ultrafiltration (UF). In this project the impact of a pre-treatment by ozonation (2-10 mg O3/L) and subsequent coagulation (FeCl3: 2-6 mg Fe3+/L) on the performance of a polymer UF membrane was studied. No free dissolved ozone was in contact with the membrane. Lab tests were performed using Amicon test cells in dead-end mode fed with 500 mL secondary effluent of the WWTP Ruhleben (Berlin) and the flux decline during filtration was measured. The effect of the two pre-treatment steps on the character of DOC, especially the biopolymer fraction, was investigated using size exclusion chromatography. The pre-treatment enables phosphorus removal of 75 up to 95 % with permeate concentrations of 30 to 50 µg P/L. In filtration tests pre-ozonation without flocculation leads to a less distinct flux decline (1-7 %). Coagulation without pre-ozonation increases the flux by 5 to 14 % compared to filtration of effluent without pre-tretament. The combination of both pretreatment steps improves the filtration performance up to 30 % and reduces the filtration time for 500 mL by 50 %. Different mechanisms are considered as reasons for the improved performance. It is known that coagulation partially removes the fouling-active biopolymers and humic substances. The pre-treatment with ozone, even at low dosages (2 mg/L), leads to a significant decrease of UV254 absorbance, pointing on a shift to more polar molecules. Higher ozone dosages (> 6 mg/L) additionally induce disintegration of biopolymers and a shift to smaller organic compounds. The interaction between ozonation and coagulation leads to a partial complexation of iron in solution. As a consequence, less iron is provided for the coagulation process. As the percentage of complexation of iron decreases with increasing coagulant dosage, the synergetic effect of pre-ozonation and coagulation on the filtration performance increases with increasing iron dosage. These results suggest that combining pre-ozonation and coagulation can be a promising pretreatment process to reduce the fouling of organic membranes, without the necessity of applying free dissolved ozone on the membrane surface.

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