Temporal Behavior of Post-Filter GAC Adsorbers During THM Formation and Precursor Removal

摘要

The main purpose of this work was to evaluate the temporal behavior of post filteradsorbers employing different types of granular activated carbon (GAC) media in theremoval of precursors of chlorinated organics and in the formation of disinfectionbyproducts. The experimental setup was four pilot scale adsorbers – two units withmineral GAC media (CALGON Filtrasorb 300) and another two units with vegetalGAC media (CARBOMAFRA CAG 119). The four adsorbers (F1 through F4) wereoperated in parallel, with the same empty bed contact time (EBCT) of about 20 minutes.Two of them (F1-CALGON and F2-CARBOMAFRA) were fed with filtered waterfrom a Alto da Boa Vista Water Treatment Plant (ABVWTP) which was ozonated priorto feeding, in order to observe any effects of ozonation on the different adsorbers. Theother two adsorbers (F3-CALGON and F4-CARBOMAFRA) were fed with filteredwater only. The ozonation system was composed by four column shaped contactors inseries. Each column had a hydraulic detention time of 4 minutes. Ozone dosages wereapplied in the first and second columns of the series. The total ozone dosage was in therange 0.2 through 1.0 mg O_3/L and was equally split between columns 1 and 2.Evaluations of precursor removal and disinfection byproduct formation were by meansof COT, UV-254 nm determinations and THM formation potential, respectively. Thefiltered water was from the conventional ABV WTP, whose raw water source is from ahighly eutrophic reservoir with a long detention time. COT and UV-254 values for theozonation system feed water and for adsorbers F3 and F4 were in the ranges 2.0 to 2.5mg C/L and 0.02 to 0.04 cm-1, respectively. It was observed that ozonation did notaffect significantly the COT and UV-254 values. During the first thirty days of postfilter adsorber operation, COT removal efficiencies were around 80 % for F1 and F3(with CALGON GAC media) and around 40 % for F2 and F4 (with CARBOMAFRAGAC media). With sixty days of operation, COT removal efficiencies decreased to 40% and 20%, respectively and stayed that way during eight months of operation. Themineral CAG media performed better regarding COT and UV-254 removals than thevegetal CAG media. On the other hand, no significant difference in effluent quality wasobserved regarding adsorbers with same GAC media type receiving ozonated and non-