Power August 2023 - 32

WATER & WASTEWATER TREATMENT
tion entering the RO units was 595 mg/L as CaCO3
with four
of the sample days exceeding 700 mg/L TH. The membranes
recovered within 94% of the initial flux values after a 10-gallon
permeate flush.
■ Regardless of pH or residence time, the EC unit removed
between 67% and 98% of silica.
■ In all tests, EC depressed the ORP from > 220 millivolts
(mV) to between -90 mV and -140 mV indicating a reducing
condition and the absence of oxidizers.
■ A one-minute EC residence time with ambient pH resulted
in one of the best performing RO runs where the system
was operated between 600 psi and 640 psi, and fluxes between
15 gfd and 17 gfd, while not irreversibly fouling the
membranes. Combined permeate conductivity remained <
20 microsiemens/centimeter (µS/cm), about 13 mg/L TDS.
Table 1. Water quality data. Courtesy: Garver USA
The Results
Throughout the field testing, parameters were adjusted in the
EC process train such as hydraulic residence time (HRT), electrode
material, electrode jumping configuration, amperage density,
pre-treatment chemistry for pH, and downstream filtration
micron removal size. RO permeate flux and permeability were
challenged based on the various EC configurations and the fact
that the three-stage RO only had one 29-ft2
element in each stage
due to budget constraints. Typically, a three-stage RO has multiple
elements in the first stage and decreases in number as you
increase the number of stages. Electrode configuration, amperage
density, re-seeding of salt slurry, and condensation energy
adjustments were made to the VAED system throughout testing.
Shown in Table 1, the main treatment train consistently met
water quality goals and resulted in high TDS removal efficacy
without fouling the RO membranes. The main treatment train
was operated three to five hours a day for three days a week
between August 2021 and December 2022. After each RO run,
the membranes were forward flushed with eight to 10 gallons
(< 1% of processed flow) of 10 mg/L TDS permeate until the
concentrate conductivity was within 20% of the permeate
feed flush conductivity.
Since VAED utilizes ohmic heating to heat the brine, the
higher the conductivity, the quicker it gets hot. For the donated
VAED system to operate with the existing electrode configuration
on the lower-TDS concentrate (as opposed to the higherstrength
water softener spent brine it was originally designed
for), it needed the addition of rock salt to bring the TDS from
what was leaving the third stage as concentrate to about 56,000
mg/L TDS. Otherwise, it would take much longer to upcycle the
salt to allow the electro-distillation process to occur efficiently.
Notable Trends
The following findings were obtained through the testing:
■ Due to the high hardness, polarity reversal frequency of the
EC unit needed to be reduced from once per minute to four
times per minute to extend runtime between clean-in-place
procedures and prevent scaling.
■ Increasing the pH with sodium hydroxide from 8.6 to between
9.7 and 10 resulted in total hardness (TH) removal
efficacy between 72% and 95% after the EC in the presence
of anti-scalants. Only 40% to 64% hardness removal was
realized at ambient pH of 8.6.
■ For three weeks in November 2021, average TH concentra32
Unanticipated
Positive Consequences of Failure Mode
Analysis
After the last test was performed on the P2P trailer in late November
2021, the system was put into failure mode analysis
(FMA). The system was left static from December 2022 to
early March 2022 without performing the standard RO pickling
routines. After performing only a 20-gallon permeate rinse, the
system's specific flux recovered to its initial value under the
standard configuration without any chemical pretreatment, just
EC + CLAR + MF.
Subsequent FMA testing further challenged the process
train by operating in a batch mode where the concentrate was
recycled in a proprietary configuration. In this mode, recovery
went from approximately 54% to 97% without fouling, the
need for chemical pre-treatment, or the need for permeate
rinses. Later during the FMA, the trailer was only operated
an average of once a month, without suggested downtime
maintenance implementation to the EC, MF, and RO system.
This high of a recovery allowed the process train to close the
ZLD loop with the addition of the VAED system.
Conclusions
Proof of the EC/CLAR/MF/RO with VAED concentrate treatment
train was achieved. EC was a successful alternative
mechanism for foulant precipitation and created ideal water
quality for RO membranes without the use of anti-scalants or
pH depression. Throughout piloting and FMA, RO fouling was
not noticed since the specific flux was consistently recovered.
Automation of the pilot trailer should be considered in
future testing to allow for continuous operations. Although
the EC and VAED performed well, more research behind the
removal mechanisms, and optimization of full-scale design
constraints, capital costs, and operational costs would be
beneficial to the advancement of this technology as a commercialized
solution to manage salt loading. ■
-Eric Dole, PE is the Water and Energy Practice Lead
for Garver USA's Water Team.
www.powermag.com
POWER | August 2023
■ The VAED system removed 98% to 99% of the TDS for three
lab analysis dates where the raw TDS was between 100,000
mg/L and 186,060 mg/L at an energy intensity of 1,750 kWh/
kgal and 620 kWh/kgal, respectively.
■ The higher the conductivity, the more aggressive the boil in
the VAED flash chamber due to ohmic heating and the lower
the energy intensity value.
■ Significant improvements can be made to the VAED cooling
system to increase the throughput and decrease the energy
intensity.
http://www.powermag.com
Power August 2023

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