Use of adsorbent mixtures to mitigate membrane fouling and remove NOM with microgranular adsorptive filtration

This study investigated the removal of natural organic matter (NOM) from water, and in particular the NOM responsible for membrane fouling, by treatment with a mixture of heated aluminum oxide particles (HAOPs) and powdered activated carbon (PAC) in both batch and microgranular adsorptive filtration (µGAF) reactors. In batch reactors, increasing the portion of HAOPs in the adsorbent mixture led to a steady increase in fouling when the pretreated water was applied to a UF membrane. By contrast, when the water was pretreated with the same adsorbent mixtures in a µGAF reactor, increasing the portion of HAOPs led to a steady and dramatic decline in membrane fouling. The opposing trends in fouling in the two reactor types tracked the trends for removal of the biopolymer fraction of the NOM. When the feed water is pretreated in batch reactors, fouling is mitigated by adsorption of some of the foulants on each adsorbent, with PAC collecting the foulants more effectively than HAOPs do. However, when pretreatment is carried out in µGAF reactors, foulant removal is enhanced by formation of the gel on the tightly packed adsorbent particles in the µGAF layer. That removal mechanism is apparently much more effective on HAOPs than on PAC, so that a HAOPs-rich µGAF layer removes more foulant (and a larger fraction of the disinfection byproduct formation potential [DBPFP]) than a PAC-rich layer does. Generalizing this result, it appears that using mixtures of adsorbents in µGAF reactors could have significant advantages over more conventional pretreatment approaches in which a single adsorbent is applied in a well-mixed reactor. Sheet 1 (labeled as Figure 2) includes data for removal of NOM from Lake Union water by batch or µGAF treatment with mixtures of HAOPs and PAC at a fixed total (effective) adsorbent dose of 20mg/l. Sheet 2 (labeled as Figure 3) presents TMP profiles for ultrafiltration membranes fed LU water batch-pretreated with mixtures of HAOPs and PAC. Total adsorbent dose: 20 mg/l, Flux=100 LMH. Sheet 3 (labeled as Figure 4a) presents data for pressure increase across μGAF units with different proportions of HAOPs and PAC in the pre-deposited layer; Flux = 150 LMH. Sheet 4 (labeled as Figure 4b) presents TMP increase across UF membranes fed composite permeate from various upstream μGAF units; Flux = 100 LMH Sheet 5 (labeled as Figure 6) presents SEC chromatogram data of raw and pretreated LU water samples.