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A major drawback to ultrafiltration membrane operation for drinking water treatment is fouling, which results in lower water production and increased maintenance costs. The impact of different fouling mitigation strategies including, sparging conditions (duration and air flow rate) as well as phased in-line coagulation were investigated at pilot-scale. Unexpectedly, sparging during permeation and backwash resulted in a significantly higher rate of increase in irreversible resistance compared to Sparging only during backwash. Lower irreversible fouling observed without sparging during permeation was attributed to the formation of a protective layer on the membrane in the absence of sparging. Sparging at reduced air flow rates and intermittently, during the permeation cycle did not improve the irreversible resistance rate when compared to sparging only during backwash. The application of phased in-line coagulation to pre-coat membranes was also investigated. Coagulating for only the first half of the permeation cycle (phased coagulation) did not negatively impact membrane performance in terms of irreversible resistance and organics removal. Phased coagulation, which would lead to reduced coagulant and sludge disposal costs, appears to be a promising fouling control strategy.

The natural organic matter (NOM) removal efficiency and regeneration behavior of ion-exchange filters with promoted biological activity (BIEX) was compared to operation where biological activity was suppressed (i.e. abiotic conditions). The impact of BIEX pre-treatment on fouling in subsequent ultrafiltration was also investigated. Biological operation enhanced NOM removal by approximately 50% due to an additional degradation of smaller humic substances, building blocks and low molecular weight acids. Promotion of biological activity significantly increased the time to breakthrough of the filters and, therefore, is expected to lower the regeneration frequency as well as the amount of regenerate of which to dispose. Pre-treatment using BIEX filters resulted in a significant decrease in total and irreversible fouling during subsequent ultrafiltration. The decrease was attributed to the effective removal of medium and low molecular weight NOM fractions. The results indicate that BIEX filtration is a robust, affordable and easy-to-operate pre-treatment approach to minimize fouling in ultrafiltration systems and enhance the quality of the produced permeate.

A pilot-scale study was performed to evaluate a coagulant dose which had been optimized for biopolymer (i.e., foulant) removal on subsequent ultrafiltration (UF) fouling, as well as disinfection by-product (DBP) precursor removal. Polyaluminum chloride (PACl) dosages were selected based on a point of diminishing returns for biopolymer removal (0.5 mg/L) and directly compared to that applied at full-scale (6 mg/L). Membrane fouling (reversible and irreversible) was measured as resistance increase over a 48 hour filtration period. DBP formation potential (total trihalomethanes (TTHMs), haloacetic acids (HAA9) and total adsorbable organic halides (AOX)) were measured in both raw and treated waters. Results of the study indicate that application of a PACl dose optimized for biopolymer reduction (0.5 mg/L) resulted in 65% less irreversible UF fouling when compared to 6 mg/L. The addition of PACl prior to the membrane resulted in up to a 14% reduction in DBP precursors relative to the UF membrane alone. A similar level of DBP precursor reduction was achieved for both 0.5 and 6 mg/L dosages. The results have implications for cost savings, which may be realized due to decreased chemical use, as well as increased membrane life associated with lower irreversible fouling rates.

Surface shear stress induced by different air sparging regimes on a submerged hollow fiber ultrafiltration module with horizontally-oriented, densely packed fibers was characterized. Continuous and intermittent (cycling on and off) coarse bubbles (0.75–2.5 mL), as well as large pulse bubble (150 and 500 mL) sparging were considered for a range of air flow rates. The power required to induce surface shear stress on the surface of the hollow fibers was substantially lower when using large pulse bubble sparging compared to both continuous and intermittent coarse bubble sparging. Results indicated that the air flow required for pulse bubble sparging was more than 80% lower than that required for coarse bubble sparging to induce comparable surface shear stress (and corresponding fouling control). This study demonstrates the potential value and efficiency of pulse bubble air sparging as a fouling control option in densely packed hollow fiber membrane systems.

A bench-scale study was performed to optimize backwash frequency and air sparging conditions during ultrafiltration (UF) for drinking water treatment in order to minimize hydraulically irreversible fouling as well as operating and maintenance costs. Surface shear stress representing different air sparging conditions (continuous coarse bubble, intermittent coarse bubble, and large pulse bubble) was applied in combination with various backwash frequencies (0.5, 2, and 6 hours) during UF of two natural surface waters. Results indicated that air sparging during permeation with intermittent coarse or large pulse bubbles significantly reduced the rate of irreversible fouling. This allowed for longer permeation times (up to 6 hours) between backwashing, when compared to a baseline condition which assumed a 0.5 h-backwash frequency with no air sparging during permeation. As a result, operation and maintenance cost savings estimated at > $350,000/year for a 29 MLD membrane train could be realized. This study demon...

Coagulation as pre-treatment to ultrafiltration (UF) was optimized for the removal of biopolymers, i.e., a primary UF foulant, for three different natural water matrices. The impact of pre-coagulation on membrane reversible and irreversible fouling, as well as the retention of organic micropollutants, was investigated at bench scale. Jar test experiments indicated that the optimum alum dosage for removal of biopolymers, based on a point of diminishing returns analysis, was relatively low (0.5 mg/L as coagulant; 0.05 mg/L as Al 3+ ). This dose was effective at reducing membrane reversible and irreversible fouling (up to 48%) for waters with higher concentrations of organics (>4 mg/L as DOC) over 24 h of permeation and backwash cycles. Biopolymers were identified as contributing to both reversible and irreversible fouling. The retention of organic micropollutants was relatively low for UF alone ( K ow > 2). This study demonstrates that a low dose of coagulant, optimized for biopolymer removal, may reduce membrane fouling and may provide value added for the retention of some organic micropollutants; however dosages depend on the specific water being treated, as well as treatment-related water quality targets.

This study investigated the impact of surface shear stress, to represent air sparging employed for fouling control, on the retention of organic micropollutants during ultrafiltration for drinking water treatment. The retention of 16 different pharmaceutically-active and endocrine disrupting compounds was examined during ultrafiltration of three natural surface waters (two lake, one river) under four different surface shear stress regimes: no shear stress, low peak shear stress (representative of continuous coarse bubble sparging), sustained peak shear stress (representative of intermittent coarse bubble sparging), and high peak shear stress (representative of large pulse bubble sparging). Results indicate that surface shear stress does impact the retention of emerging contaminants; however, it is dependent on water matrix and compound properties. The greatest retention of micropollutants was observed in waters with a higher concentrations of organic matter, and for conditions where no surface shear stress was applied (average 32% retention), and under conditions representative of large pulse bubble sparging (average 34% retention). The observed retention under conditions of no shear stress was likely due to a heavy fouling layer that altered the membrane selectivity and was able to entrap organic micropollutants of larger molecular weight. Under conditions that mimicked air sparging, increasing the shear stress (quantified as the root mean square applied shear) resulted in increased retention of organic micropollutants, particularly those that are neutral and hydrophobic in nature. This may be related to solute–solute complexes, which are kept in solution when shear stress is applied, or related to modification of the fouling layer by the shear stress induced onto the membrane surface. The results suggest that there may be value added with respect to removal of organic micropollutants, such as pharmaceuticals, when employing air sparging as a fouling control strategy during ultrafiltration.

The effect of surface shear stress on membrane fouling during submerged hollow fiber ultrafiltration of three different surface waters (two lakes, one river) was investigated. Surface shear stresses that mimicked those induced when applying continuous and intermittent coarse bubble air sparging, large pulse bubble air sparging, as well as no air sparging were considered. The results suggest that fouling was mainly due to the accumulation of the biopolymer fraction of the natural organic matter present in the raw water. Inducing shear stresses onto the membrane surface significantly decreased the rate of membrane fouling (relative to no shear stress applied) in all waters tested. Of the shear stress conditions studied, that which mimicked large pulse bubble sparging had the greatest effect, reducing fouling by up to 80% when compared to conditions with no sparging applied. Conditions that mimicked intermittent and continuous coarse bubble fouling reduced the rate of fouling by up to 77 and 49%, respectively. These results suggest that the shear stresses induced by sparging can promote back transport of soluble organic material from the membrane surface. A semi-empirical relationship was developed to estimate the effect of raw water characteristics and applied sparging conditions on membrane fouling.