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

• In a novel pretreatment process, raw water passes through a thin layer of adsorbent.
• The adsorbent removes NOM better than conventional alum treatment.
• The adsorbent is washed out of the system approximately once per day.
• The pretreated water can be treated by MF or UF with essentially zero fouling.

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.