Scale-Up of Rapid Small-Scale Column Test Data for Micropollutant Removal from Surface Water
- Allison Reinert - Hazen and Sawyer
The presence of micropollutants (MPs) such as pesticides, endocrine disrupting chemicals, and pharmaceuticals in effluent-impacted water has caused public concern about drinking water quality. Granular activated carbon (GAC) adsorption is an effective option for the control of many MPs. Conducting pilot tests to determine GAC life or identify the most cost-effective GAC is expensive and time consuming. Simulating GAC adsorber performance with rapid small-scale column tests (RSSCTs) is desirable, but little is known about the validity of scale-up approaches when simulating MP adsorption in the presence of natural organic matter. The principal objective of this research was to develop a scale-up approach to predict field-scale GAC adsorber performance from RSSCT data.
A pilot column containing bituminous coal-based GAC (12×40 US mesh) was operated at an empty bed contact time of 7 minutes. Corresponding RSSCTs were conducted with GAC from the same batch. Two RSSCT design approaches were used: the proportional diffusivity design (PD-RSSCT) and the constant diffusivity design (CD-RSSCT). For the PD-RSSCT, it is assumed that adsorbate diffusivity varies linearly with GAC particle size while adsorbate diffusivity is assumed to be independent of GAC particle size for the CD-RSSCT. The influent to the pilot column and the RSSCTs was coagulated, settled surface water with a total organic carbon concentration of 2 mg/L. After cartridge filtration, influent was spiked with 34 MPs (pesticides, endocrine disrupting chemicals, pharmaceuticals, personal care products) at levels ranging from 10-500 ng/L. MP breakthrough curves were determined from paired influent and effluent samples. MP concentrations were measured by LC-MS/MS and GC-MS/MS analysis.
The pilot test was operated for 1.6 years, and 121,000 bed volumes (BVs) were treated. Only 10 of the tested MPs exhibited breakthrough at levels >10% of the influent concentration at the completion of the pilot study. The PD-RSSCT was operated for 8 months (350,000 BVs), at which point 26 MPs had broken through. The CD-RSSCT was operated for 2 months (250,000 BVs), and 31 MPs had broken through. Both PD- and CD-RSSCT designs overestimated MP adsorption capacity, presumably because a larger number of adsorption sites become unavailable to MPs in larger particles when NOM blocks pore entrances. A fouling factor that addresses particle size-dependent differences in MP adsorption capacity was calculated to match PD- and CD-RSSCT adsorption capacities to those obtained in the pilot study. A principal component analysis was conducted to relate the fouling factor to Abraham descriptors for each MP. For the CD-RSSCT, a strong relationship was developed between the fouling factor and the Abraham descriptors. The resulting linear free energy relationship can be used to predict the fouling factor for other MPs and allow water treatment professionals to scale up CD-RSSCT data to predict field scale performance.
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