Water. Desalination + reuse
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TECHNOLOGY | 28 | Desalination & Water Reuse | November-December 2015 To remove biofouling and eliminate biological slime S-152 should be used with non-oxidant biocides such as Adiquimica's Adiclean 141 or 128. Evaluation Laboratory performance tests on the cleaning performance of S-152 deployed flat sheet cells. The tests were carried out with concentrate and permeate total recycling to the feed vessel to maintain a constant composition. Assays were conducted under the pressure and salinity conditions established by the membrane manufacturer. The cleaning tests were carried out on membrane samples of a Filmtech BW30LE-440 element fouled organic fouling, biofouling and alumino-silicates. The element autopsy determined that the density of deposit on the membrane was 4.6 mg/m 2 . The dried deposit analysis identified organic matter (46.1%) and alumino-silicates (44.1%). Biofilm level – colony forming units (CFU) on the membrane surface – was 3.5x103 CFU/cm 2 of aerobic bacteria. To evaluate the effectiveness of S-152, a two-phase cleaning regime was used: Phase 1: sanitization with non-oxidizing biocide Adiclean 128, and Phase 2: alkaline cleaning with S-152. The normalized permeate flow and the normalized salt rejection results before and after cleaning process were compared to design values provided by membrane manufacturer (table 1). Fouling had reduced permeate flow by about two-thirds of the design value. Cleaning increased the normalized flow to return to the design value of the element. Fouling composition analyses were performed before and after the cleaning procedure to evaluate the effectiveness of the cleaning using: • scanning electron microscopy and energy dispersive X-ray spectroscopy analysis (SEM- EDX); and • attenuated total reflectance Fourier- transform infrared spectroscopy (ATR- FTIR). SEM-EDX analysis SEM-EDX analysis was used to study the elemental composition, crystal structure and morphology of deposits and scales deposited on the membrane surface before and after applying the cleaning procedure. SEM provided high-resolution images revealing topographical features of the foulants and particles on the membrane surfaces. The captured images were analyzed for different chemical elements while EDX was used to map the distribution of elements in a selected area. Elemental mapping allows analysis simultaneously of the elements present and of the relative amounts of each element as the density and intensity of mapping dots is proportional to the amount of each element. During the treatment of filtered samples for SEM analysis, samples were coated with graphite to make them conductive so a carbon distribution map was not recorded. Elemental mapping of SEM images at 150-times magnification of the membrane surface before cleaning showed that it was covered with a colloidal layer comprising mainly clays. There were different types of clay present with the most common one composed of alumino-silicates. Other clays included aluminum, iron, magnesium, potassium, calcium and sodium. All the elements in the clay deposits were present in the same location, indicating that they were part of the same deposit. Insoluble phosphorous compounds and sodium chloride were detected at much lower concentrations. The elemental mapping results of the SEM image of the membrane surface after the cleaning showed that the main element was sulphur which corresponds to the polysulphone support layer of the membrane. The high density and intensity of sulphur dots demonstrated that the cleaning protocol removed the fouling layer to allow the electron beam to reach the polysulphone layer. Trace levels of deposit of clay (silica, aluminum, iron, calcium, magnesium and potassium) were detected. Partially quantitative indicators of the Normalized permeate flux (l/h·m 2 ) Normalized salt rejection (%) Before After Change % Before After Change % 15.47 43.12 +178.73 98.34 98.79 +0.45 Table 1. Results of Adiclean 128 and Adiclean S-152 cleaning procedure. Filmtec element design values were: permeate flux, 44.72 l/hm 2 and salt rejection, 99%. Parameter Element Si Al Fe Ca Mg K P Weight % -99.14 -99.07 -98.84 -100.00 -98.31 -97.95 -100.00 Table 3. Percentage change in the weight of each element, between before and after cleaning. Table 2. Weight percent (W) and atomic percent (A) of elements for the area of the membrane surface before and after cleaning. Parameter Element C O S Si Al Fe Ca Mg K P Na Cl W% before 16.18 44.06 99.14 19.79 8.61 5.17 0.67 1.78 2.44 0.20 0.09 0.03 A% before 24.89 50.89 0.57 13.02 5.90 1.71 0.31 1.35 1.15 0.12 0.08 0.01 W% after 75.78 15.67 6.98 0.17 0.08 0.06 0.00 0.03 0.05 0.00 1.01 0.20 A% after 83.37 12.94 2.88 0.08 0.04 0.01 0.00 0.02 0.02 0.00 0.58 0.06 elemental composition of the membrane surface before and after cleaning – expressed by weight and atomic percentage – demonstrate the effectiveness of the cleaning process (tables 2 and 3). Clay constituents were reduced by 99% after cleaning and 100% of phosphorus compounds were removed. The increase of sodium and chloride was caused by addition of sodium chloride to feedwater to obtain the membrane characterization salinity. ATR-FTIR analysis ATR-FTIR reveals the molecular composition and structure of foulant deposits. An area of membrane fouled mainly by organic matter and microorganisms was analyzed before and after cleaning. The findings showed that cleaning reduced the protein amide bands at 1650 cm -1 and 1550 cm -1 that are related to the presence of microorganisms. The carbon- oxygen bond-stretching band at 1040 cm -1 related to polysaccharides was also suppressed after cleaning. These results confirmed the removal of the organic matter and biofouling from the membrane surface by S-152. Final analysis Microscopic and chemical analysis of the RO membrane surface showed S-152 to be effective in the removal of the chief organic and inorganic foulant components in biofouling and colloidal fouling. It is effective at low concentrations so it can reduce operating costs and minimize chemical discharge to the environment.