Heated Aluminum Oxide Research Articles

Efficacy of hybrid adsorption/membrane pretreatment for low pressure membrane

Fouling by natural organic matter (NOM) is a major obstacle when water from natural sources is treated using low-pressure membranes. Prior research by our group has demonstrated that passing natural water through a thin, pre-deposited layer of heated aluminum oxide particles (HAOPs) can remove substantial amounts of NOM from the feed and thereby reduce the fouling rate of downstream membranes. The work reported here explored the technical efficacy of such a pretreatment process under more challenging (and therefore realistic) conditions than reported earlier. Several analytical techniques were applied to the feed and permeate in an attempt to identify the key fouling components. The results demonstrate that a HAOPs layer can be pre-deposited on a stainless steel mesh and then be readily washed off at the end of a filtration cycle with very little irreversible fouling due to residual NOM or HAOPs left on the mesh. In addition, the pretreatment step removes enough foulant to allow a downstream UF membrane to operate at significantly higher fluxes than when conventional pretreatment is applied. HAOPs pretreatment also reduced the formation of chlorinated and brominated trihalomethanes (THM4) by more than 67% and of haloacetic acids (HAA9) by 64%–88% in simulated distribution system (SDS) tests.

A multi-spectral approach to differentiate the effects of adsorbent pretreatments on the characteristics of NOM and membrane fouling

Pretreatment of feed water is widely applied to mitigate NOM-induced fouling of low-pressure membranes. This research investigated the effectiveness of two pretreatment modes for NOM removal by heated aluminum oxide particles (HAOPs) and the associated reductions in membrane fouling and trihalomethane (THM) formation potential. One mode, referred to here as pre-adsorption, is the conventional process in which adsorbent particles are added to and thoroughly mixed with the feed, after which the particles are separated from the water either upstream of or by the membrane. By contrast, in the pre-deposition mode, a thin layer of adsorbent particles is deposited on a support media (which could be the membrane) prior to passing feed through the layer and the membrane.Although both pretreatment methods remove similar amounts of DOC at the same adsorbent dose, pre-deposition is superior with respect to mitigating membrane fouling and reducing DBP formation. UV and fluorescence spectroscopy and HPSEC analysis indicate that a pre-deposited adsorbent layer removes more chromophores and low apparent molecular weight (AMW) material than pre-adsorption does. Based on absorbance ratios at selected wavelengths, a pre-deposited HAOPs layer removes more aromatic moieties than aliphatic carboxyls, especially at higher HAOPs doses. In addition, pre-deposition is more effective than pre-adsorption at reducing the THM formation potential. The results provide new insights into the interactions between HAOPs and NOM molecules and shed light on the significantly different effects of different adsorbent contacting modes on the fouling potential of the pretreated water.

Virtual elimination of MF and UF fouling by adsorptive pre-coat filtration

Fouling by natural organic matter (NOM) is a major obstacle when water from natural sources is treated using low-pressure membranes. Prior research by our group has demonstrated that pre-deposition of a thin layer of heated aluminum oxide particles (HAOPs) on pristine microfiltration and ultrafiltration membranes can remove substantial amounts of NOM from the feed before the water reaches the membrane and thereby reduces the membrane fouling rate. The work reported here demonstrates that the pre-deposited layer can be readily washed off the membrane, and a new layer deposited, with very little evidence of irreversible fouling due to residual NOM or HAOPs left on the membrane. Furthermore, in direct comparisons of systems receiving equal doses of Al (i.e., mg Al per liter of water treated), the system in which the HAOPs were pre-deposited removed more NOM and mitigated fouling more effectively than systems where either alum or HAOPs were added to the bulk feed, even though the system with pre-deposition was backwashed only one-tenth as frequently as the other systems.The benefits of passing the feed through pre-deposited HAOPs are even greater if that step is separated from and upstream of the membrane filtration step. When this operating mode is used and the upstream unit with HAOPs is backwashed at a frequency of ~1/d, downstream microfiltration and ultrafiltration membranes can be operated at aggressive fluxes for long periods (two weeks or more in our experiments) without any hydraulic or chemical cleaning whatsoever. The process is effective not only with high quality source water, but also with water that is high in TOC and/or turbidity.

Fouling mechanisms in low-pressure membrane filtration in the presence of an adsorbent cake layer

Fouling of low-pressure membranes treating natural waters can be substantially mitigated by pre-depositing a thin layer of micron-size adsorbent particles on the membrane, a process we refer to as microgranular adsorptive filtration (μGAF). The role of adsorbent particle size, adsorbent surface loading, and membrane pore size in fouling of μGAF systems by natural surface water has been investigated. μGAF tests using heated aluminum oxide particles (HAOPs) and powdered activated carbon (PAC) reveal that fouling in such systems occurs both on the membrane and in the cake layer. Fouling on the membrane is primarily caused by soluble NOM and is exacerbated by the use of larger adsorbent particles and smaller-pore membranes. Such fouling is mitigated by removal of foulants in the pre-deposited layers, so the extent of mitigation is proportional to the adsorbent surface loading (i.e., the thickness of the cake layer). By contrast, fouling in the cake layer is caused by larger foulants such as colloids and particulate matter. Such fouling is insensitive to the layer's thickness. Use of smaller adsorbent particles improves the capture of colloids and particles but also exacerbates such fouling. In these cases, increasing the membrane pore size decreases the rate of fouling on the membrane, but does not affect the cake layer fouling.

NOM Fractionation and Fouling of Low-Pressure Membranes in Microgranular Adsorptive Filtration

Membrane fouling by natural organic matter (NOM) was investigated in microgranular adsorptive filtration (μGAF) systems, in which a thin layer of adsorbent is predeposited on low-pressure membranes. The adsorbents tested included heated aluminum oxide particles (HAOPs), ion exchange (IX) resin, and powdered activated carbon (PAC). Size exclusion chromatography (SEC) separated the NOM into four apparent MW fractions with significant UV₂₅₄. HAOPs and the IX resin performed almost identically with respect to removal of these fractions, and differently from PAC. However, while HAOPs and PAC reduced fouling substantially, IX resin did not, indicating that fouling could not be attributed to the NOM fractions detected by SEC. Rather, the key foulants appear to comprise a very small fraction of the NOM with almost no UV₂₅₄ absorbance. Alginate, a strongly fouling surrogate for natural polysaccharides, is adsorbed effectively by HAOPs, but not by IX resin or PAC, suggesting that polysaccharides sometimes play a key role in membrane fouling by NOM.

NOM fouling mechanisms in a hybrid adsorption/membrane system

This study investigated the distribution of NOM, and its foulant fraction, in hybrid adsorbent/membrane systems. The adsorbent used in the work was heated aluminum oxide particles (HAOPs). Color changes, SEM images, and analysis of the carbon content in the HAOPs layer support the view that NOM molecules can form a gel layer at the surface of the HAOPs layer, but that this gel layer does not contain the majority of the removed species or the foulant. Rather, the HAOPs layer acts as a short, packed bed, with steadily less adsorbate removed in each successive layer of particles. In constant flux systems, the amounts of NOM and foulant that can be removed are approximately proportional to the mass of HAOPs in the layer. Fouling is attributed to the small, but steadily increasing, amount of NOM that is rejected by the membrane after passing through the HAOPs layer. This NOM appears to be relatively easy to remove by disruption of the HAOPs or rinsing of the membrane surface. The system functions well when either fresh or saline water is the feed.

Membrane interactions with NOM and an adsorbent in a vibratory shear enhanced filtration process (VSEP) system

The effects of torsional membrane vibration and the addition of heated aluminum oxide particles (HAOPs) on the interactions of natural organic matter (NOM) with UF membranes were investigated using a vibratory shear enhanced filtration process (VSEP) laboratory module. Torsional vibration reduced fouling in both dead-end and crossflow modes. Incorporating crossflow into a system with vibration increased NOM removal efficiency. In systems with crossflow, increasing the vibration amplitude decreased fouling up to a point, and provided negligible benefit thereafter. On the other hand, it had a negligible effect on NOM rejection up to the same point, and improved permeate quality thereafter. Although a mass balance suggested that a negligible portion of the NOM remained on the membrane surface at the end of the crossflow tests, foulant aggregates were observed on the membrane both visually and in SEM images. The SEM images indicated that vibration changed the morphology of the foulants from a continuous layer into scattered clumps, and that higher vibration amplitude facilitated this transformation. Deposition of a HAOPs layer on the membrane surface relieved fouling only initially, after which the HAOPs/UF system fouled almost as fast as a bare UF membrane. However, intermittent vibration to re-mix HAOPs particles during the HAOPs/UF operation kept permeate flux relatively high for much longer, thereby significantly improving the performance.

Simultaneous removal of phosphorus and foulants in a hybrid coagulation/membrane filtration system

At many wastewater treatment plants, chemical addition is required to meet the requirements for discharge of phosphorus. Membrane filtration is an attractive option for removing the chemically precipitated phosphorus, but that process can be impeded by fouling of the membrane by effluent organic matter (EfOM). In the current study, membrane fouling and ortho-P removal were explored in hybrid adsorbent/membrane systems dosed with alum and/or a new aluminum-based adsorbent (heated aluminum oxide particles, HAOPs). HAOPs prevented fouling more effectively than alum did, even though the two adsorbents removed similar amounts of EfOM. The HAOPs were particularly effective when they were pre-deposited on the membrane (as opposed to being mixed into the feed upstream). On the other hand, alum removed ortho-P more efficiently than HAOPs did. Combining both adsorbents in a single membrane system takes advantage of the most attractive features of each, accomplishing good ortho-P removal while simultaneously minimizing fouling. In addition to being potentially applicable directly, the results demonstrate the potential benefits of using multiple coagulants and/or adsorbents in a single hybrid membrane system.

NOM Removal by Adsorption and Membrane Filtration Using Heated Aluminum Oxide Particles

Heated aluminum oxide particles (HAOPs) are a newly synthesized adsorbent with attractive properties for use in hybrid adsorption/membrane filtration systems. This study compared removal of natural organic matter (NOM) from water by adsorption onto HAOPs with that by adsorption onto powdered activated carbon (PAC) or coagulation with alum or ferric chloride (FeCl3); explored the overlap between the NOM molecules that preferentially adsorb to HAOPs and those that are removed by the more conventional approaches; and evaluated NOM removal and fouling in hybrid adsorbent/membrane systems. For equivalent molar doses of the trivalent metals, HAOPs remove more NOM, and NOM with higher SUVA254, than alum or FeCl3. Most of the HAOPs-nonadsorbable fraction of the NOM can be adsorbed by PAC; in fact, that fraction appears to be preferentially adsorbed compared to the average NOM in untreated water. Predeposition of the adsorbents on a microfiltration membrane improves system performance. For the water tested, at a flux of 100 L/m2-hr, predeposition of 11 mg/L PAC and 5 mg/L HAOPs (as Al3+) allowed the system to operate 5 times as long before the transmembrane pressure increased by 1 psi and to remove 10-20 times as much NOM as when no adsorbents were added.