field
report
BY KENNY HUTCHINS & JAMES SCHAEFER
Schaefer
Hutchins
Abilene Solves Supply Problem With Microfiltration
LOCATED IN TAYLOR COUNTY NEAR THE HEART OF WEST TEXAS, THE CITY OF ABILENE IS DIVERSE, DYNAMIC, AND PROGRESSIVE.
ombining a flavor of the Old West
with a contemporary, forwardmoving city, Abilene serves more
than 115,000 community residents,
businesses, and visitors each day and is
also home to Dyess Air Force Base along
with its fleet of B1 Bombers and C130
transport planes.
In 1999, a heat wave hit the city, leading to the most extensive drought in
Abilene’s recorded history. This followed
just a year after another heat wave caused
more than 140 heat-related deaths in the
state. At the time, Abilene relied on a two
conventional filtration plants for its drinking water. As the drought lingered, the primary water supply reservoir became too
low to use, and the secondary source reser-
C
taxed. To augment the supply, the city
accelerated its plans to develop its share of
water rights to a reservoir constructed a
distance south of Abilene in the late 1980s.
The pipeline and pumping station plans also
included a new 8 mgd (32 ML/d) water
treatment plant (WTP) that would be
expandable to 24 mgd (96 ML/d).
BRACKISH WATER SOURCE REQUIRES
ADVANCED TREATMENT
The source of supply for the proposed
WTP is O.H. Ivie Reservoir, which combines the waters of the Concho and Colorado rivers. Located about 52 mi (84 km)
from the proposed WTP site, reservoir
water quality can best be described as
brackish, with high sodium, chloride,
AS THE DROUGHT LINGERED, THE PRIMARY WATER SUPPLY RESERVOIR BECAME TOO LOW TO USE,
AND THE SECONDARY SOURCE RESERVOIR AND PUMPING FACILITIES WERE JUST ABLE TO MEET THE CITY’S NEEDS.
voir and pumping facilities were just able
to meet the city’s needs. As a result, the
drinking water supply was being heavily
TABLE 1
Abilene microfiltration finished water quality goals
Constituent
Value
Iron—mg/L
Manganese—mg/L
Color—mg/L
Dissolved organic carbon—mg/L
Total organic carbon—mg/L
Turbidity—mg/L
Silt density index (15 min)—mg/L
Particles (2 microns or larger)—per mL
Giardia—log removal
Cryptosporidium—log removal
52 DECEMBER 2003 | JOURNAL AWWA
<0.3
<0.05
<2
<2.2
<2.2
<0.1
<3.0
<10
6
6
2003 © American Water Works Association
hardness, dissolved color, and organic content. With this surface water supply also
having a total dissolved solids (TDS) level
of ~1,200 mg/L and high barium sulfate
scaling potential, Abilene required an
advanced treatment process to achieve
acceptable finished water TDS levels.
The conveyance system consists of a raw
water pump station, a 36 in. (900 mm)
pipeline, a 6 mil gal (24 ML) holding tank
and booster pump station at the halfway
point, and a 10 mil gal (40 ML) holding
tank a few miles from the plant site that
offered the option of gravity feed to the
proposed WTP. Because of the detention
time in the raw water conveyance system, it
was anticipated that an oxidant would be
field
report
Pall microfiltration membranes
and chemical cleaning equipment
in the new 8 mgd (30 ML/d) water
treatment plant provide high quality
water for Abilene, Texas.
added to the raw water for biofilm control and manganese oxidation. Abilene
was working with consulting engineers
CH2M Hill, who designed the first
phase of the project, the pipeline from
Lake Ivie to the WTP site.
FINISHED WATER QUALITY GOALS/
DEMANDS BASED ON CURRENT REGS
AND COMPATIBILITY WITH CURRENT
SOURCES
Abilene’s finished water quality goals
were based on meeting current and
anticipated future drinking water standards and being chemically “compatible” with Abilene’s existing water
(Table 1). The existing water quality is
variable, depending on the time of the
year and the proportion of raw water
taken from the different sources and
treated at Abilene’s existing WTPs.
The available raw water supply for
the first phase of the proposed O.H.
Ivie Reservoir WTP is approximately 8
mgd (32 ML). In the future, Abilene
will be able to withdraw up to 24 mgd
(96 ML/d) from the reservoir. Thus the
proposed WTP was designed to be
expandable to approximately three
times its initial capacity.
REMOTE, BRACKISH SUPPLY
POSES A CHALLENGE
more, in an effort to reduce
color and dissolved organics,
which would improve performance of the RO units, Abilene concluded that coagulation upstream of MF was the
best approach for meeting its
quality goals.
Planned treatment
included upstream chlorination (biofilm control in the 50
mi [80 km] raw water transmission pipeline), potassium
permanganate or chlorine dioxide
addition (iron and manganese oxidation), ferric sulfate coagulant addition
(for color and dissolved organic carbon
[DOC] reduction), and dual membrane
processes (MF for suspended solids
and pathogen removal followed by RO
for dissolved solids removal).
Final treatment steps include disinfection (free chlorine and chloramines)
and corrosion control (sodium hydrox-
In light of the remote, brackish surface water supply, water treatment
challenges included biofilm control,
iron/manganese/color removal, salinity
reduction, turbidity control, and
pathogen removal and/or inactivation
from the surface water supply sources.
Based on the raw water quality of the
reservoir, Abilene concluded that
reverse osmosis (RO)
was the most feasible
way to achieve the
required finished water
quality. Because RO
processes require feedActivity
water low in turbidity
and suspended solids,
Pilot equipment installation
microfiltration/ultrafilPilot testing
tration (MF/UF) was
investigated as the preInterim pilot test reports to engineer
treatment process.
Because the memMicrofiltration/ultrafiltration
brane-treated product
procurement bids, recommendation
water will be blended, it
to Abilene, and award
had to be compatible
Full-scale membrane equipment
with water from existavailable for installation
ing supply and treatPlant startup
ment facilities. Further-
PROJECT SCHEDULE
2003 © American Water Works Association
Completion Date
October 2000
November 2000
to April 2001
November 2000
December 2000
January 2003
September 2003
JOURNAL AWWA | DECEMBER 2003
53
field
report
ide and corrosion inhibitor). A significant portion of the MF product water
will bypass the RO treatment and
blend with RO permeate, which helps
ments were made in reverse filtration
and air-scrub parameters to meet
recovery requirements. Filtered water
turbidity was less than 0.07 ntu.
also maintained integrity throughout
the test period. Overall, the Pall MF
system demonstrated its ability to work
efficiently and cost-effectively on
A B I L E N E ’ S F I N I S H E D WAT E R Q UA L I T Y G OA L S W E R E BA S E D O N M E E T I N G C U R R E N T A N D A N T I C I PAT E D
F U T U R E D R I N K I N G WAT E R S TA N DA R D S A N D B E I N G C H E M I C A L LY “ C O M PAT I B L E ” W I T H A B I L E N E ’ S E X I S T I N G WAT E R .
reduce the capacity of the RO part of
the plant and lower overall costs.
PILOT TESTING CONDUCTED
IN FOUR PHASES
Because of time and resource constraints for pilot-testing and the plan to
use coagulation upstream, the number
of membrane products pilot-tested was
limited. Three vendors were invited to
participate in the pilot. Pall and one
competitor accepted.
Pall installed an automated MF
pilot system that pumps water under
pressure through the membrane module. The pilot was equipped with one
hollow-fiber MF membrane module. A
competitor provided a vacuum-driven
unit immersed in process tanks. Coagulant addition and mixing were provided upstream of the MF units. Pall
conducted a four-phase testing program. The first phase evaluated a range
of fluxes and alternatives for process
optimization. The initial coagulation
dose of 50 mg/L produced high solids
and very rapid increase in transmembrane pressure (TMP). The dose was
then reduced to 21 mg/L, and demonstrated an immediate drop in feed turbidity and subsequent reduction in
slope of the TMP curve. Minor adjust-
During the second phase, the system
operated at a constant flux rate of 11.2
gpm (0.707 L/s) per module. A chemical cleaning (CIP) was performed after
34 days of run time. Following CIP, an
integrity test was performed. The
resulting decay of <0.1 psi (0.7 kPa)
per minute verified membrane integrity.
The third phase of operation was performed with brackish water under the
same operational conditions as the second phase.
The fourth phase of operation was a
15-day cycle without operational
changes. A CIP was performed, followed by an integrity test. The end of
this run concluded the pilot study.
Based on the test results, the Texas
Commission on Environmental Quality
approved a design flux rate of 11.2
gpm (0.707 L/s) per module with coagulation and flocculation (but no settling) upstream of the MF.
CONCLUSION
The Pall MF pilot system was
shown to be a viable technology for the
treatment of brackish water, with effluent turbidity averaging less than 0.07
ntu during design runs. The system was
able to clean the membrane completely
using a standard CIP procedure and
2003 © American Water Works Association
54
DECEMBER 2003 | JOURNAL AWWA
removal of particles, Cryptosporidium
and Giardia, and microbial fouling.
The Pall MF system also proved well
suited to maintaining performance
with the direct coagulation included in
Abilene’s design.
In addition to improving water
quality and RO performance, direct
coagulation brought Abilene several
other benefits. Because the existing
pipeline could be used for coagulation
and flocculation, the flow of water
did not have to be disrupted by
installing traditional flocculation and
mixing tanks. Eliminating the need for
these tanks meant less initial investment, and future cost benefits in the
form of lower labor and maintenance
costs. On the basis of an evaluation
that considered all relevant capital
and operating costs, Abilene selected
the Pall MF system.
—Kenny Hutchins is water treatment
operations leader for the City of Abilene,
555 Walnut St., Suite 203, POB 60, Abilene, Texas 79604-0060. Hutchins can
be reached at (325) 795-1774. James
Schaefer, is technical director for Pall
Corporation, 25 Harbor Park Dr., Port
Wasington, NY 11050. Schaefer can be
reached at (516) 801-9872.