Evaluation of Available Scale-Up Approaches for the Design of GAC

Evaluation of Available Scale-Up Approaches for the Design of GAC
Contactors [Project #4235]
ORDER NUMBER: 4235
DATE AVAILABLE: May 2014
PRINCIPAL INVESTIGATORS:
R. Scott Summers, Anthony M. Kennedy, Detlef R.U. Knappe, Allison M. Reinert,
Meredith E. Fotta, Angela J. Mastropole, Christopher J. Corwin, and Joseph Roccaro
PROJECT GOAL AND OBJECTIVES
The overall goal of this project was to develop a methodology that can be utilized
to scale up bench-scale results to simulate field-scale granular activated carbon (GAC)
adsorber performance for the control of organic contaminants at low concentrations. This
methodology utilizes the rapid small-scale column test (RSSCT) and will facilitate the
optimal design of GAC adsorbers with savings in time, effort, and cost over the use of
pilot-scale adsorbers. The overall objectives were to evaluate the appropriateness of the
following scale-up approaches: (1) direct use of either the constant diffusivity or
proportional diffusivity designed RSSCT data, (2) RSSCT data combined with adsorption
capacity predictions and numerical models, and (3) adsorbate chemical and physical
properties for capacity predictions and numerical models.
BACKGROUND
Many utilities are concerned about the presence of trace organic micropollutants
(MPs) in their source waters. MPs include regulated contaminants, such as pesticides and
volatile organic compounds (VOCs), as well as unregulated contaminants such as
pharmaceuticals, endocrine disrupting chemicals, and personal care products. One
effective treatment option for the control of many MPs, even at their low levels of
occurrence, is the use of GAC adsorbers. The cost of GAC treatment is made up of the
capital cost of the GAC reactors and their ancillary facilities as well as the operation and
maintenance costs, which are driven by the GAC replacement frequency or GAC use
rate. To accurately assess the GAC use rate, the operation time to the target effluent
concentration, or “breakthrough,” is needed. Because many MPs are strongly adsorbing,
GAC may last for months to years before breakthrough. Since pilot GAC adsorbers
utilize the same GAC particle size, breakthrough occurs at the same time as the full-scale
GAC adsorber; months to years. The cost of running a pilot system for this time frame is
excessive. Bench-scale testing with the GAC ground to a smaller size was developed to
©2014 Water Research Foundation. ALL RIGHTS RESERVED.
decrease run time by a significant factor; 10 to 100. The most common bench scale test is
the RSSCT.
While the RSSCT design approach based on proportional diffusivity (PD) has
been well-established for the GAC adsorption of dissolved organic matter (DOM), the
approach for RSSCT design is not established for specific target contaminants, especially
at the very low concentrations at which MPs are being detected and carcinogenic VOCs
may be regulated. The constant diffusivity (CD) RSSCT design approach has been the
default practice for specific contaminants at high concentrations (ppm). However, some
studies that investigated MP removal at lower concentrations (ppb to ppt) indicate the
PD-RSSCT may be more appropriate. The advantage of the CD-RSSCT is that it can be
run more quickly than the PD-RSST. For both approaches, the RSSCT results often fail
to adequately simulate full-scale adsorption capacity, and thus GAC use rate, for MPs
when DOM is present. Consequently, the RSSCT utility is often limited to the selection
of GAC type and empty bed contact time (EBCT).
PROJECT APPROACH
Four general tasks were completed to address the project objectives: (1) current
and historic scale-up approaches for GAC adsorber design were reviewed, (2) MP
removal in pilot-scale GAC adsorbers was assessed at five geographic locations, (3) MP
adsorbability was determined in PD- and CD-RSSCTs designed to simulate the pilot
tests, and (4) scale-up methodologies for GAC adsorber design were developed and
verified.
RESULTS AND CONCLUSIONS
Volatile Organic Compounds
Specific objectives of the VOC research were to (1) determine the effects of GAC
type, GAC reactivation, adsorber backwashing, and EBCT on VOC removal efficiency
from (2) development of a scale-up approach for data obtained with PD-RSSCTs, and (3)
assessment on whether the scale-up approach is affected by GAC type. Pilot columns at
Suffolk County Water Authority, NY (SCWA) were operated at an EBCT of 11 min and
GACs prepared from bituminous coal, lignite, and coconut shells were evaluated.
Bituminous coal-based GACs produced by both direct activation and re-agglomeration
were tested in both virgin and reactivated forms. PD-RSSCTs were conducted with
lignite, coconut shell, and bituminous coal (re-agglomerated) based GACs.
VOC breakthrough to an effluent concentration that was 30% of the influent
concentration, termed 30% breakthrough, was used as the basis for comparison. 1,1dichloroethane was the least adsorbable VOC, while 1,2,3-trichloropropane,
tetrachloroethene, and trichloroethene were the most adsorbable, with no detectable
breakthrough observed after 30,000 bed volumes. In terms of bed volumes treated or
carbon use rate (CUR), coconut shell-based GAC was the most effective followed by reagglomerated bituminous coal-based GAC and lignite based GAC, while direct activated
bituminous coal-based GAC was the least effective. The performance of reactivated
carbons relative to virgin carbons varied among the VOCs. Only small performance
©2014 Water Research Foundation. ALL RIGHTS RESERVED.
improvements in terms of CUR were seen when doubling the EBCT. Backwashing the
columns had little impact on GAC performance.
For all VOCs, the RSSCTs over-predicted pilot-scale GAC adsorption capacity
and therefore bed life. The difference in time to breakthrough can be explained with
particle size-dependent GAC fouling resulting from the adsorption of background organic
matter. A fouling factor (Y), which addresses particle size-dependent DOM fouling, was
determined for VOCs that had broken through in both pilot columns and RSSCTs. The Y
value was independent of GAC type, but dependent on the target compound
hydrophobicity. For the VOCs studied in this research, a relationship between Y and the
log octanol-water partition coefficient (log Kow) of the target compound was developed.
Once the Y value is determined, a particle size-dependent fouling index (FI) can be
calculated. The RSSCT throughput data can then be divided by the FI to predict the pilot
scale or full scale throughput. A second scale-up method was developed that required
describing the breakthrough data with the pore-surface diffusion model (PSDM).
Micropollutants
Specific objectives of the MP research were to (1) assess the impact of EBCT at 7
and 15 min, (2) compare the CD-RSSCT and the PD-RSSCT designs for predicting fieldscale MP removal water, and (3) develop scale-up procedures for determining GAC
adsorber life and performance and validate the approach. Thirty-three MPs, including
pesticides, pharmaceuticals, and personal care products were studied, and influent
concentrations were <1 ug/L. The surface waters used were conventionally treated water
from the City of Longmont, CO (LM) and Orange Water and Sewer Authority, NC
(OWASA) and conventionally treated water from Big Elk Meadows, CO (Coag BEM)
and preozonated coagulated Big Elk Meadows (O3 BEM).
Micropollutant breakthrough at 10% of the influent concentration was used as the
basis for GAC performance comparison. The worst case scenario evaluated, highest
influent DOC (DOC0) concentration, 3.9 mg/L, and lowest EBCT, 7 min (Coag BEM),
was used as a baseline. For this condition, the largest number of MPs broke through
(n=22 for pilot run and n=24 for the PD-RSSCT). As a reference point for this water, the
throughput to 50% DOC breakthrough (DOC concentration of about 2 mg/L), was about
8,000 bed volumes and if only 20% DOC removal was needed then the system could be
run to 12,000 bed volumes.
Pilot column throughput values in bed volumes to 10% breakthrough (BV10%) for
the Coag BEM are shown in Table ES.1. For this worst-case scenario, only two MPs
broke through in same time frame as DOC; iopromide and MIB. Seven compounds broke
through in the 12,000 to 39,000 bed volume range, 12 broke through in the 40,000 to
100,000 bed volume range, and eight compounds either broke through after 100,000 bed
volumes or were not detected in the effluent. Four additional compounds, not monitored
in the Coag BEM pilot system, broke through after 100,000 bed volumes or were not
detected in the effluent of the OSAWA pilot columns. Several of these compounds may
also be removed by biodegradation in the pilot columns, in particular acetaminophen,
ibuprofen, trimethoprim, and 2-methylisoborneol (MIB). Sucralose was not one of the
spiked compounds, but was found in the GCWW adsorber influent, and broke through at
the full-scale at about 15,000 bed volumes.
©2014 Water Research Foundation. ALL RIGHTS RESERVED.
Table ES.1
Pilot column throughput in bed volumes to 10% breakthrough and CURs under
worst case scenario evaluated, Coag BEM at DOC0 = 3.9 mg/L at an EBCT of 7 min
Less than 12,000
(> 38 mg/L)
DOC
MIB
Iopromide
12,000 to 39,000
( 38 to 11 mg/L)
2,4–D
Clofibric Acid,
Cotinine
Erythromycin
Prometon
Tributyl phosphate
Warfarin
**Sucralose
40,000 to 100,000
(11 to 4.5 mg/L)
Acetochlor, Aldicarb
Atrazine, Caffeine
Carbamazepine,
Diazinon,
Dimethoate, Malaoxon
Methomyl, Metolachlor
Molinate, Simazine
Sulfamethoxazole
Greater than 100,000
( < 4.5 mg/L)
Acetaminophen
Carbaryl, Diclofenac
Diuron, Gemfibrozil
Ibuprofen, Naproxen
Trimethoprim
*Bisphenol A
*Triclosan
*Ethinyl estradiol
*Only monitored in the OWASA water
**GCWW water at 15 min EBCT
In terms of CUR, these results translated into 56 mg/L (0.47 lb/1000 gal) to reach
50% DOC removal, which was the same for 90% removal of iopromide and MIB. The
CURs for the 12,000, 40,000, and 100,000 bed volume cutoffs used in Table ES.1 are 38
mg/L (0.31 lb/1000 gal), 11 mg/L (0.094 lb/1000 gal), and 4.5 mg/L (0.038 lb/1000 gal),
respectively. Thus, for 25 of the MP compounds studied, CURs would be less than 11
mg/L (0.094 lb/1000 gal), suggesting that GAC adsorption is a cost-effective technology
for the removal of many MPs. Pilot-scale results with the OWASA and LM waters
showed that as the influent DOC concentration decreased to 2.1 and 1.7 mg/L,
respectively, BV10% values were on average 39% and 61% larger, respectively. A longer
EBCT of 15 min for Coag BEM water yielded on average 52% longer throughput to 10%
breakthrough compared to the results at 7 min EBCT.
For all MPs, breakthrough in the PD-RSSCT occurred much later than that in the
pilot column, indicating that the RSCCT over-predicts the adsorption capacity. A
regression analysis showed that the BV10% values for the PD-RSSCT were 3 to 4 times
longer than those for the pilot scale. Results from the RSSCT and the pilot tests were
described with the PSDM, and the PSDM analysis supported the use of the PD-RSSCT
design as the intraparticle diffusive flux decreased approximately linearly with decreasing
particle size. The fouling index (FI) values, the ratio between the PD-RSSCT adsorption
capacity and the pilot adsorption capacity, ranged from 2.6 to 3.4, with an average of
2.8±1.0 (n=75) for the five cases studied. A significant finding is the apparent lack of
dependence of average FI values on water quality (DOC0 concentration or preozonation)
or EBCT. Thus, using a FI value of about 3 for MPs may be a good estimation when pilot
plant or full-scale GAC data are not available.
Using the OWASA water, both the CD-RSSCT and the PD-RSSCT were
evaluated. Both RSSCTs over-predicted the field-scale MP adsorption capacity of the
GAC. To scale differences in MP adsorption capacity, fouling factor (Y) values were
determined for the 11 MPs, for which measurable breakthrough occurred in the pilot
column. An analysis of the resulting Y-values showed that the magnitude of Y was
compound specific. For the CD-RSSCT, a linear free energy relationship (LFER) was
developed that relates Y to five Abraham descriptors for each MP using principal
©2014 Water Research Foundation. ALL RIGHTS RESERVED.
component analysis. The variability in the Y-values obtained from the PD-RSSCT, on the
other hand, was not well described by the LFER.
A unified relationship for predicting Y was sought for complete MP data sets
from Chapters 5 and 6, as well as from the literature. Y values from 30 compounds in this
study and the literature were compared to many relevant physical parameters. Y values
were also compared to the operational parameters BV10%,PD-RSSCT and the ratio of influent
MP concentration (C0) to the DOC0 concentration.
APPLICATIONS AND RECOMMENDATIONS
For utilities considering GAC treatment of low levels of organic MPs, bench-scale
testing with the RSSCT affords a considerable savings in time, water, and costs relative
to pilot column testing. For the 44 compounds evaluated, the pilot scale data can be
directly used by utilities to estimate the GAC performance. GAC type was found to
dramatically impact the VOC breakthrough, indicating that careful selection of GAC type
is warranted. On average, decreasing the influent DOC concentration by a factor of 2,
from 4 to 2 mg/L, increased the run time to 10% breakthrough by 50%. Thus, DOC
removal prior to GAC treatment should be optimized. Increasing the EBCT by a factor of
2 from 7 min to 15 min, increased the bed volumes to 10% breakthrough by 50%. Thus,
an analysis of the capital and operation costs needs to be conducted.
By examining the results from the pilot columns and the RSSCTs, methodologies
have been developed that allow the RSSCT results to be scaled up to full-scale GAC
behavior. All of the methodologies are useful, but none are without problems. The PDRSSCT design explicitly addresses the linear particle size-dependence of MP adsorption
kinetics, but calculating a unique Y-value for a given compound based on compound
properties was not successful. Using a FI value of 3 for MPs may be a good estimation.
In contrast, for the CD-RSSCT the linear dependence of the intraparticle diffusive flux on
particle size must be addressed with the PSDM in order to predict full-scale adsorber
performance from data, but calculating a unique Y-value from adsorbate properties was
possible.
Additional work needs to be carried out to verify the results of this study:


Assessing the role of GAC type, influent DOC concentration, and EBCT at the
pilot and RSSCT scales, especially for the VOCs as only one water was
evaluated.
Further validation of the developed scale-up methodologies including those that
use numerical models, as well as the direct scale up approaches.
RESEARCH PARTNER:
U.S. Environmental Protection Agency
©2014 Water Research Foundation. ALL RIGHTS RESERVED.

Download Report