Membrane fouling is an inherent phenomenon in UF membrane processes, making it necessary to periodically perform backwashes (BW) and chemical "cleanings in place" (CIP) to restore the initial permeability of the membrane. The objective of this study was 1) to explore systematically the effect of distinct BW-related variables (BW transmembrane pressure, duration, frequency, composition) on the reversibility of UF membrane fouling and on the permeate quality (in terms of total organic carbon, turbidity and UV absorbance) over successive filtration/BW cycles; and 2) to identify which organic fractions were most removed by the membrane and, of these, which were most detached after BW, alkaline and oxidant CIP and acid CIP episodes. For this purpose, a bench-scale outside-in hollow fibre module operated under dead-end filtration mode at constant transmembrane pressure and treating settled water from a drinking water treatment plant was employed. Dissolved organic carbon fractionation was performed by high performance size exclusion chromatography (HPSEC). Results showed that in general the more intensive a BW was (in terms of high transmembrane pressure, shortened frequency and prolonged duration) the more effective it was in removing fouling from the membrane. Concerning the composition of the water used for the BW, the addition of NaClO led to maximum fouling reversibility, closely followed by the combination of NaOH+NaClO, while citric acid and NaOH contributed little compared to water alone.However, results also showed that irreversible fouling was never completely avoided whatever the BW regime applied, leading to a gradual increase of the total resistance over time. Larger
The primary problem for the application of microfiltration (MF) and ultrafiltration (UF) membrane technology is membrane fouling. Such is the case that understanding membrane fouling has become one of the major factors driving MF and UF membrane technology for- ward. Nevertheless, identifying the constituents that most contribute to membrane fouling 20 and quantifying how they are detached when backwashing (BW) and cleaning-in-place (CIP) are applied still remains a challenging task. The aim of the present study was to quan- tify membrane fouling development during filtration and membrane fouling detachment during BW and CIP in terms of membrane permeability changes and masses of inorganic and organic constituents accumulated on the membrane. The study was conducted using 25 bench-scale MF and UF modules fed with coagulated and settled water coming from a drinking water treatment plant and operated under dead-end and cross-flow operation modes. The experiments consisted inconsecutive permeation (20 min) alternated with BW with permeate water (1.0 min) (periodically chemically assisted with NaClO and NaOH) and followed by a two-stage CIP consisting first in an oxidising and basic step (NaClO and 30 NaOH) and second in an acidic step (citric acid). Feed, permeate, retentate (when present) and cleaning discharge streams were monitored for turbidity, total and dissolved organic carbon (TOC and DOC, respectively), UV 254 and inorganic ions (Al, Fe, P). DOC was frac- tionated by high-performance size exclusion chromatography to gain insight into the beha- viour of the different organic fractions. Results showed that both MF and UF membranes 35 successfully removed turbidity, Al and Fe, whereas UV 254 was moderately removed and TOC and DOC poorly removed, with removal percentages higher for UF than for MF. With regard to the organic fractions, the largest molecular weight compounds were moderately removed while the smallest organic fractions seemed to totally permeate through both membranes. The results also showed that foulants were poorly washed out from thePeer ReviewedPostprint (author's final draft
A B S T R AC TMembrane biofouling represents an important drawback in full-scale water reclamation plants as it affects energy consumption, permeate productivity and even quality. Due to the high number of water-/membrane-/site-specifi c variables affecting membrane fouling and lifetime, cleaning and maintenance procedures need to be specifi cally designed for each application. Cleaning activities cannot directly be optimized onsite due to the necessity of ensuring a constant and safe product water fl ow and quality. For this reason, cleaning optimization needs to be transposed from plant to lab. In this work, a Spanish wastewater reclamation plant was selected to optimize cleaning procedures. For this, a reverse osmosis element was sampled and submitted to several cleaning protocols at lab-scale. The infl uence of the basic cleaning agent nature, pH and presence of additives on cleaning effi ciency was studied. The optimal membrane cleaning conditions were achieved with a NaOH solution at pH 12 containing 0.03% SDS. At these conditions, permeate fl ux after cleaning was two fold greater than fouled membrane permeate fl ux. In this work, lab-scale membrane cleaning tests have shown to be a successful tool to optimize cleaning activities for full-scale plants and understand the infl uence of cleaning variables on membranes restoring effi ciency.
Wastewater reclamation contributes to the preservation of conventional water resources and thus helps to ensure appropriate human development for future generations. Wastewater reclamation can be achieved through several technologies. One of the most common technologies is the tertiary treatment of urban municipal wastewater, which is often based on membrane technologies. Reverse osmosis is an effective separation technology for removing dissolved salts and low molecular weight organic compounds. However, membranes suffer from fouling, which directly reduces technical, environmental and economic feasibility of the process and hence of the reclamation plant. One of the strategies helpful to reduce fouling is the optimisation of the membranes' cleaning and maintenance. The aim of this work is to test the impact of the membrane cleaning protocol design on the recovery of the original properties of a reverse osmosis membrane used for several years in a wastewater reclamation plant in Spain. Furthermore, the work is focused on the validation of the adequacy of the most-common indicators used for assessing membranes' cleaning efficiency.
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