Jim Passanisi is with the City of Port Hueneme, Calif., Janet Persechino is with Ionics, Inc., Watertown, Mass., and Todd K. Reynolds, is with Kennedy/Jenks Consultants.
undefinedIn Port Hueneme, California, a state-of-the-art desalination facility uses three brackish water desalination technologies: reverse osmosis (RO), nanofiltration (NF) and electrodialysis reversal (EDR), operated side-by-side to produce over three million gallons per day (mgd) of high quality drinking water. The Brackish Water Reclamation Demonstration Facility (BWRDF) is the cornerstone of the Port Hueneme Water Agency’s (PHWA) Water Quality Improvement Program. In addition to providing desalted water for local use, the BWRDF also serves as a full-scale research and demonstration facility.
It is usually a difficult task to compare the long-term
performance and operating costs of three technologies due to variables in
source water quality, plant capacities, labor, power and chemical costs.
Operating three full-scale desalination technologies in parallel at the same
site has made a direct comparison possible. During the course of the
plant’s operation, the PHWA will collect data on long-term cost and
performance characteristics of the three membrane systems.
Project Purpose and Goals
Prior to the implementation of the PHWA’s Water
Quality Improvement Program, PHWA’s retail customers (City of Port
Hueneme [COPH], Channel Islands Beach Community Services District [CIBCSD],
Naval Construction Battalion Center—Port Hueneme [NCBC], and Naval Air Weapons Station—Point Mugu [NAS]) were concerned with the long-term reliability and water quality of their existing water supplies. Each of these water purveyors utilized brackish groundwater from the Oxnard Plain Groundwater
Basin, a critically over-drafted basin that is under active basin management.
The groundwater management plan called for reductions in
groundwater extractions by 25 percent over 20 years. Groundwater was extracted
from local deep aquifer wells along the Coast that were increasingly subject to
seawater intrusion, or was delivered from inland upper aquifer wells by the
United Water Conservation District. The total dissolved solids level of these
water sources is normally greater than 1,000 mg/L. Although the groundwater met
the primary drinking water standards of the California Department of Health
Services, it was highly mineralized and was aesthetically undesirable.
Furthermore, customers were burdened with added costs for water softeners and
bottled water, as well as the indirect costs including shortened plumbing and
appliance life and staining of glassware and laundry.
In response to increasing overdraft of the local groundwater
basin, seawater intrusion and poor groundwater quality (especially during
drought years), PHWA implemented its Water Quality Improvement Program. The
program involves demineralization of the local groundwater that is used in
conjunction with imported California State Water Project (CSWP) water.
Implementation of this innovative program has provided
significant benefits to more than 55,000 people within Ventura County, Calif.
Benefits include improved water quality, joint use of facilities and obtaining
a long-term, safe, reliable and environmentally sustainable high quality water
supply that meets current and proposed water quality drinking standards under
the Safe Drinking Water Act. Shared use of facilities by PHWAs customers has
eliminated the need for individual agency projects. Long-term water supply
reliability for PHWA’s customers has been improved by access to both
demineralized groundwater from local sources and imported CSWP water. The
delivery of imported State Water Project water has allowed PHWAs customers to
reduce their groundwater extractions from coastal wells threatened by seawater
intrusion and minimize the capacity of the demonstration facility. Relocating
groundwater extractions from the coastal area to inland recharge areas also is
minimizing seawater intrusion. In addition, by demineralizing the water prior
to distribution, it reduces the need for home water softening and reverse
osmosis units. It also improves the potential to desalinate wastewater for
future reclamation.
Timeline and Cost
The PHWA’s Water Quality Improvement Program was
implemented over a six-year period starting in 1993 and culminating with the
startup of the BWRDF, a joint powers wholesale water agency, in 1999. The first
several years involved establishing the contractual agreements to form the PHWA
and to annex the agency to the CSWP to facilitate importing surface water.
Design of the BWRDF was completed in late 1996 and facility construction was
completed in late 1998. During the period of the BWRDF construction, the PHWA
also constructed several major pipelines to deliver raw and treated surface
water to the facility and to deliver treated and blended water to the
customers. The cost of the BWRDF was $5.7 million. The cost of PHWA Water
Quality Improvement Program also included approximately $7 million for raw and
treated water pipelines and additional legal and annexation costs. Since the
BWRDP also serves as a full-scale brackish membrane research and demonstration
facility, the United States Bureau of Reclamation (USBR) funded approximately
25 percent of the cost of the facility.
Desalination Demonstration
The three membrane treatment processes (RO, NF and EDR)
operate side-by-side to produce a total of 3 mgd of treated and blended water
as shown in Table 1.
Pretreatment Equipment
The source water for the BWRDF is chlorinated groundwater
from inland, upper aquifer wells that are under the influence of source water
and operated by the United Water Conservation District (UWCD). These wells are
recharged with surface water from the Santa Clara River through spreading
basins. Typical source water characteristics are presented in Table 2.
Pretreatment of the source water is required to remove relatively large
particulate matter and free chlorine ahead of the membranes and to adjust the
water chemistry to preclude chemical scaling of the membranes.
The source water is drinking water quality (except for the
salts) and has very low turbidity. The average Silt Density Index (SDI), a
measure of the water’s likelihood to cause particulate fouling of the
membrane, is very low at less than 0.5. However, because the source water comes
from a well field where different wells are started and stopped in the system,
the BWRDF could see periodic episodes of relatively large particles in the
source water. The starting of a well pump can cause rust flakes and particles that
have settled out in the pipeline to become resuspended. Therefore, pretreatment
filtration of the source water is required to protect the membranes from these
periodic episodes.
The membranes are protected from damage from relatively
large particles by an automatic backwashing, bag filtration system with a 5- to
10-micron nominal removal. The automatic backwashing filter system was selected
to permit continuous plant operation and minimize the labor and maintenance
time devoted to the pretreatment filter. There was concern that a standard 5-
to 10-micron cartridge type filtration system could require excessive cartridge
replacement or even become blinded with particles due to the well pump
operations.
While the EDR membranes can tolerate low levels of free
chlorine, the source water must be dechlorinated to protect the RO and NF
membranes from oxidant damage by the free chlorine. Sodium bisulfite, a
reducing agent, is added to the source water after the pretreatment filter
system to remove the chlorine. An oxidation-reduction potential (ORP) analyzer
is used to monitor the water. Another option is to add ammonia to convert the
free chlorine to chloramines. The EDR, RO and NF membranes can tolerate the
chloramines and have been shown to help minimize biofouling of the membranes.
If a salt’s concentration exceeds solubility limits,
precipitates can form mineral scale on the membranes. Acid and antiscalant
typically are added to prevent mineral scale accumulation on the RO, NF and EDR
membranes. In the RO and NF systems, the source water flows along the length of
the membrane and the TDS concentration increases as product water passes
through the membranes. Acid and/or antiscalant are added to the source water.
In the EDR system, the TDS on the concentrate side of the membrane increases as
cations and anions pass through the EDR membranes. In this case, acid and/or
antiscalant are added to the concentrate recycle stream.
Hydrochloric acid is required ahead of the EDR system.
Currently, the RO and NF systems do not require acid addition based on a source
water pH less than 7.5 and TDS levels. Hydrochloric acid feed systems are
provided for the RO and NF systems should they require acid addition in the
future. Each membrane system requires a small amount of antiscalant addition.
Different antiscalants have been tested and are fed to each system based on
recommendations from the manufacturers and operational experience. Each
membrane treatment system has a dedicated acid and antiscalant chemical
metering pump to permit different chemical feed rates and to accurately monitor
the chemicals used by each of the three membrane systems.
Reverse Osmosis System
The RO membranes remove total dissolved solids (TDS) from
the source water. Osmosis is a natural process in which water passes through a
semipermeable membrane from the side with a low TDS concentration to the side
with a high TDS concentration. Reverse osmosis is a pressure driven process
that raises the water pressure on the high TDS (source water) side of the
membrane to well above the osmotic pressure and forces the water to flow
through the membrane to the low TDS (product water) side of the membrane. The
RO membrane permits the passage of water molecules but is a barrier to most of
the ions in the water. As the source water flows along the membrane, the TDS is
further concentrated and finally discharged as a reject stream from the
process.
The BWRDF RO system is a two-stage process with 14 first
stage vessels and seven second stage vessels, each with six elements per
vessel. The concentrated reject stream from the first stage membranes is the
feed water to the second stage membranes. The RO membrane elements are thin
film composite, Filmtec BW40LE-440 elements. The product recovery for the RO
system, defined as the product water out of the system divided by the source
water entering the system, is approximately 75 percent. The RO pressure
required to desalt source water of approximately 1,000 mg/L TDS is about 160
psi. The RO product water has a TDS of about 15 mg/L; however, this is much
lower than the treated water objectives of 370 mg/L TDS and 150 mg/L of
hardness. In order to produce the desired treated water quality, source water
is bypassed around the RO system and blended with the low TDS RO membrane
product water to produce one mgd of desalted water.
Because of project capital cost limitations, the RO and NF
feed pumps are fixed speed pumps with a modulating pressure control valve, and
the RO and NF systems do not have any energy recovery. The operational
efficiency of the RO and NF systems would be improved with variable frequency
drives (VFDs) on the feed pumps and an energy recovery system on the moderately
high-pressure reject stream.
The TDS concentration in the reject water from the RO, NF
and EDR systems is about three to four times the TDS concentration in the
source water. The reject water for all three membrane systems at the BWRDF is
discharged to the headworks of an adjacent wastewater treatment plant and
discharged to the ocean through an existing outfall.
Data collection at the BWDRF is fully automated. The plant
Supervisory Control and Data Acquisition (SCADA) system monitors system flows,
pressures, water quality, chemical use and power consumption for each membrane
system. Plant staff keeps track of operation and maintenance time and costs for
each system.
Nanofiltration System
The NF membrane system operates just like the RO system to
remove TDS from the source water. However, the NF system allows larger
particles through the membrane than the RO system does (0.001 to 0.01 microns
for the NF membranes, 0.0001 to 0.001 microns for the RO membranes). As a
result, the NF system does not require as high a pressure to produce the same
volume of product water as the RO. However, the larger pores also permit more
monovalent salts to pass through the NF membrane.
The BWRDF NP system is a two-stage process with 15 first
stage vessels and seven second stage vessels, each with six elements per
vessel. The concentrated reject stream from the first stage membranes is the
feed water to the second stage membranes. The NF membrane elements are thin
film composite, Filmtec NF90-400 elements. The product recovery for the NF
system is approximately 73 percent. The NF pressure required to desalt source
water of approximately 1,000 mg/L TDS is about 140 psi. The NF product water
has a TDS of about 20 mg/L; however, this is still lower than the treated water
objectives. As with the RO, source water is bypassed around the NF system and
blended with the low TDS NF membrane product water to produce one mgd of
desalted water.
Electrodialysis Reversal System
Electrodialysis is an electrically driven process that uses
a voltage potential to drive charged ions through a semipermeable membrane,
reducing the TDS in the source water. The process uses alternating,
semipermeable cation (positively charged ion) and anion (negatively charged
ion) transfer membranes in a direct-current (DC) voltage potential field. As
the source water flows between the cation and anion membranes, the DC voltage
potential induces the cations to migrate toward the anode through the cation
membrane, and the anions to migrate toward the cathode through the anion
membrane. The cations and anions accumulate in the reject water side of the
membranes and low TDS product water is produced. The electrodialysis reversal
system periodically reverses the polarity of the electric field, and consequently
the dilute and concentrate compartments, to help flush scale forming ions off
the membrane surface and minimize membrane cleaning.
The product water from an EDR system does not pass through
the desalting membrane as it does in an RO or NF system. This reduces the
potential for particulate fouling of the EDR system and is not a regulatory
issue for groundwater desalting. However, for desalting applications that also
require treatment to meet the Surface Water Treatment Rule, the EDR system
would require an additional filtration process for microbiological removal.
The BWRDF EDR membrane treatment system produces one mgd of
desalted water using 15 membrane stacks. Each membrane stack contains 600 cell
pairs of ion-exchange membranes and flow spacers.
The product recovery for the EDR system is approximately 85
percent (some source water is added to the reject water loop to keep the
dissolved ion concentrations low enough to prevent mineral scale formation).
The EDR system, unlike the RO and NF systems, uses no filtered raw water to
blend with the product water. The EDR system adjusts the voltage field
potential to meet the treated water quality objectives. The result is water
with a TDS concentration that just meets the treated water quality criteria and
optimizes the EDR treatment system’s electrical efficiency.
Post Treatment
The pH in the RO and NF systems product water typically is
decreased due to the removal of some of the dissolved alkalinity in the water.
The BWRDF RO and NF system product water is between 5.8 and 6.0 pH units. The
EDR system does not affect pH as much and the product water pH typically is
much closer to source water pH. Sodium hydroxide (caustic soda) is added to
raise the pH of the desalted water to approximately 8.0 pH units for corrosion
control. Sodium hypoclorite and ammonia are added to provide a chloramine
disinfection residual in the PHWA’s distribution system. The BWRDF also
has a 600,000-gallon treated water storage tank and a booster pump station to
deliver the desalted groundwater and low TDS imported surface water to their
customers. Typical treated water quality is shown in Table 3.
Part two of this article compare the treatment methods after
one year of use at the site..
