BAC & GAC in Tandem for Removing Organics in Boiler Makeup Water: Performance Evaluation from a ZLD Facility

摘要

The results of a case study evaluating organic removal options for a zero liquid discharge power plant have been presented previously [1,2]. At this site, a granular activated carbon (GAC) filter was best suited and it was later shown that the mechanism of organic removal was mainly via adsorption at the early stages of operation, but was dominated by biodegradation of organics as the GAC bed(s) aged [3]. It was hence proposed that by operating an aged and biologically active carbon (BAC) bed (i.e. BAC filter) in series with a freshly replaced GAC filter bed, an advantage can be gained for organic removal via biological degradation with the upstream bed and adsorption by the downstream bed. And with such a process, it is possible to reduce excessive microbial growth on the downstream bed, to extend the life of the downstream bed to enable organic removal principally by adsorption, and to minimize carryover of organic and biofouling potential to downstream demineralizing beds. The current paper evaluates the performance of a BAC filter (aged -4 years) in series with a freshly replaced downstream bed (i.e. a BAC-GAC process) for organic removal from distillate from a brine concentrator with a tendency towards biofouling in a boiler make-up treatment train. Changes in the concentration of various physico-chemical and biological parameters (including total organic carbon (TOC), pH, conductivity, turbidity, dissolved oxygen (DO), total inorganic carbon (TIC), silica, and heterotrophic plate counts) across each of the two beds were monitored. An overall average of 81 % TOC removal was observed across the two beds in series: 43.7 % by the BAC bed and an additional 37.2 % by the GAC bed (i.e. 66.2 % removal from upstream bed effluent). Correlation studies during the 8th and 9th weeks of operation showed a consistent increase in TIC, and a decrease in DO and pH concentrations across each of the filters, and each correlated significantly with ATOC (or % TOC removed). -25-29 % and -3-7 % of TOC removed by the BAC and GAC beds, respectively, was related to changes in all three parameters. These portions of removed TOC were therefore attributed to organic bio-mineralization into CO2. Changes in concentrations of silica and other inorganic species concentrations across the beds were more complex: consistent increases for most parameters across the GAC bed occurred in the early stages of operation, but more erratic changes (i.e. removal and/or releases) occurred on the BAC bed and on the GAC bed later in operation when bioactivity was a factor. Magnesium and calcium, however, were exceptional: these ions were generally removed by the BAC filter throughout the study period and by the GAC bed from the 18th day of operation. Isolated cases of silica removal seem to relate to influent TOC: particularly with the downstream bed, when bioactivity was a factor, low influent TOC concentration seemed to favour silica removal, but more work is required to confirm this.