Filtration

Martin Schulz (2012) Submicron particle analysis to characterize fouling in tertiary membrane filtration, Diplomarbeit, Technische Universitat Berlin, Institut fur Technischen Umweltschutz Berlin, April 2012

Abstract
Within the research project OXERAM a combination of preozonation, coagulation and subsequent lowpressure membrane filtration is tested as an option for tertiary wastewater treatment. The main drawback of lowpressure membrane processes is fouling which reduces membrane life time as it increases chemical cleaning frequency and thus affects operational costs. Various fractions of organic matter as well as colloidal/particulate substances were identified as major foulants. However, fouling studies are still challenging, mostly due to the lack of adequate analysis techniques for measuring specific foulants or the overall fouling potential of a water. The goal of the present study is to proof the potential of assessing the impact of a pretreatment by ozonation and/or coagulation on the colloidal water ingredients and to predict the according filtration behavior of a treated domestic wastewater by submicron particle analyses. It is shown that by Nanoparticle Tracking Analysis (NTA) a reliable and reproducible detection of the colloid content in treated domestic wastewaters is possible. The effects of the pretreatments on submicron particle size distribution and on the absolute concentration can be detected. Ozonation and coagulation were found to reduce the content of small colloids < 200 nm by forming larger agglomerates, resulting in a better filterability of the water. A combination of both treatments shows synergetic effects and a further reduction of the particle content as well as of the total fouling resistance was observed. The best results regarding colloid removal were obtained at a specific ozone dosage of 0.5 – 0.6 mg O3/mg DOC0 in combination with the largest applied coagulant concentration (8 mg Fe3+/L). Higher specific ozone dosages (> 0.8 mg O3/mg DOC0) resulted in an impairment of this effect and were found to limit the efficacy of coagulation, resulting in a worse colloid removal. In labscale tests a direct correlation was determined between the amount of colloids < 200 nm and filtration behavior of the water. Lower colloid concentrations in this size range resulted in a lower total fouling resistance. This relationship was confirmed by online measurements at a UF pilotplant. A correlation between the irreversible fouling resistance and the colloid content could not be found, especially when ozone was applied, because the detection limit of the instrument for that type of water excess the nominal pore size of the applied UFmembrane. An increase of irreversible fouling was observed by samples which were previously treated with ozone.

Ozoneinduced modifications on the scale of molecular dissolved substances below 50 nm are not detectable by NTA. Water constituents at this size are believed to cause irreversible fouling via pore constriction and adsorption onto and into the membrane matrix. Further tests are required to assess the effect of ozonation in long term trials, with other membrane materials and varying poresizes.