Appendix A TMs - scwd2 Desalination Program

254 Pages·2010·12.84 MB·English

Appendix A
TMs
TM1: Pretreatment Comparison Investigation
TM1: Pretreatment Comparison Investigation

Introduction
This technical memorandum (TM) summarizes the results from Investigation 1 of the scwd2 seawater reverse
osmosis (SWRO) desalination pilot-scale study: a comparison of four different pretreatment system alternatives.
The four systems included slow sand filters (SSF), two different types of hollow-fiber ultrafiltration (UF)
membranes, and granular media filters (GMF). The slow sand filters operated without any chemical addition.
The GMF filters were operated downstream of chemical coagulation, rapid mixing, 3-stage tapered flocculation,
and clarification (rectangular settling basin with plates). The UF systems were operated in several different
modes including without chemical addition, with upstream coagulation and mixing, and with upstream
coagulation, mixing, and settling.
Conclusions
The conclusions from the operational and water quality data are as follows:
Water Quality Goals:
 All pretreatment systems achieved the target water quality goals with the exception of the TOC
concentration goal of less than 2.0 mg/L. In general, the TOC goal was not achieved by any of the
pretreatment systems when source water TOC exceeded 3.0 mg/L.
 The UF membranes produced the lowest levels of turbidity and particle counts.
 SSF produced the lowest SDI values, and the highest turbidity levels.
 Conventional treatment produced water with the most variability in SDI results and the highest SDI
and particle counts.
Operational Goals:
 All pretreatment systems achieved the target operational goals with the exception of the RO system
cleaning interval, which was only achieved by the SSF.
 Lower filtration rates, deeper media beds, and smaller media sizes improved the performance of
conventional treatment, while achieving the target operational goals. The tri-media configuration was
able to reduce, but not eliminate iron breakthrough with conventional treatment using an iron-based
coagulant.
 The slow sand filter achieved the water quality and operational goals without coagulant chemical
addition.

A TM1-1
City of Santa Cruz & Soquel Creek Water District TM1: Pretreatment Comparison Investigation
 The Zenon submerged UF system also achieved the water quality and operational goals without
coagulant chemical addition except the RO system cleaning interval goal. However, coagulant
addition prior to UF filtration provided increased removal of dissolved organics, reduced fouling rates
of the UF membranes (sustaining lower differential pressures), and appeared to reduce RO fouling
during the red tide simulation event.
 The Norit UF achieved the water quality and operational goals except the RO system cleaning interval
goal. Coagulant addition was required to achieve the target operational goals even at reduced flux and
recovery rates.
 Higher levels of polysaccharides and algal cell breakage were observed with higher levels of differential
pressure across the UF filters, especially when coagulant was not added.
 All filtrations systems removed turbidity, suspended solids, and particles to levels that indicate that
algal cells will be removed readily by each of the four pretreatment filters. However, the data suggests
that dissolved organics released by algae contribute to RO fouling and that algal cell breakage (a.k.a.,
lyse) correlated with high differential pressures during pretreatment system filtration and
backwashing. Related observations are as follows:
 Coagulation, flocculation, and clarification upstream of UF or GMF filtration decreased fouling
during algal blooms. Futhermore, it is anticipated that utilizing a clarification system that enhances
algae removal prior to GMF or UF filtration may further reduce RO fouling. One option is dissolved
air flotation which uses air to float the floc and buoyant material (e.g., algae) out of the water
stream instead of using gravity to settle out the material. Although this process was not tested at
the pilot facility, it is anticipated to improve algae removal based on industry experience1 and
recommendations from Dr. Raphael Kudela, a marine algae researcher at UC Santa Cruz.
 SSF utilizes a low filtration rate and differential pressure which is expected to reduce algal cell
breakage during filtration. Furthermore, the biological activity in the filter is expected to
metabolize some of the dissolved organics released by algae. This is one hypothesis why fouling
was not observed downstream of the SSF during algal blooms.
Cartridge Filter:
The cartridge filters were replaced more frequently on average downstream of the SSF and GMF than
downstream of the UF systems. The differential pressure increase across the cartridge filters downstream of the
UF systems averaged approximately 1 psi per month over the 13-months of testing, which indicates infrequent
cartridge replacement. The differential pressure buildup downstream of the GMF systems averaged
approximately 2 to 3 psi per month during optimized coagulation, filtration, and backwashing and
approximately 5 psi per month when iron breakthrough was observed. The differential pressure buildup
downstream of the SSF systems averaged approximately 2 to 4 psi per month over the 13-months of testing.

1 Dissolved Air Flotation and Me. Dr. James Ezwald. Water Research, Article in Press, 2010. Retrieved at
http://dx.doi.org/10.1016/j.watres.2009.12.040

TM1-2 A
TM1: Pretreatment Comparison Investigation City of Santa Cruz & Soquel Creek Water District
RO Fouling:
 The RO system downstream of conventional treatment had the most frequent cleaning interval. This
was primarily due to iron-particulate fouling caused by particle and iron breakthrough through the
GMF filters after iron-based coagulant addition. Moderate levels of fouling were also observed during
dense algal blooms.
 The RO system downstream of the SSF did not require cleaning during the study, had the lowest
amount of flux decline and had the lowest amount of foulant observed on the membrane surface.
 The RO systems downstream of the UF systems had moderate levels of biofouling during dense algal
blooms. The most rapid flux decline was observed when a coagulant was not being added prior to
filtration and differential pressures typically exceeded 6 psi (and at times increased to levels greater
than 10 psi) during the fall prorocentrum bloom event. It is speculated that the high differential
pressures increased shear and algal cell breakage within the pretreatment system, which led to higher
rates of RO fouling.
Additional Observations
Additional observations from the pilot program and from the desalination industry are as follows:
 Adding a pre-oxidant, increasing media depth, and decreasing media size will improve the water
quality of conventional treatment for a full-scale plant. A well-operated and well-designed
conventional pretreatment system is adequate for municipal-scale desalination plants. Note that the
planned pretreatment system for the proposed 50 mgd SWRO plant in Carlsbad, CA was switched from
UF to conventional treatment with low-rate, deep bed, tri-media, gravity filters because of similar
concerns over algal cell breakage and to reduce costs.
 UF systems provide more reliable filtered water quality when compared to conventional treatment
during changes in source water quality and plant operations, and will reduce O&M optimization to
prevent fouling. UF membranes also provide the greatest amount of microbial removal credits from
DPH. The disadvantage of UF systems is the potential to rupture algal cells during filtration and
backwashing, which may be mitigated by improving algae removal prior to filtration.
 Slow sand filters had excellent water quality and operational performance during this pilot test, mimic
the biological process of beach wells, require no chemicals or pumping power, but require more land
than either conventional pretreatment or UF pretreatment.
 RO membrane fouling, cleaning and replacement are unavoidable with any pretreatment system, so
the pretreatment decision is ultimately based on a balance of costs and reliability of water production.
Project specific design criteria, site plans and capital and costs for the pretreatment alternatives will be
presented in Technical Memorandum No. 12.

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City of Santa Cruz & Soquel Creek Water District TM1: Pretreatment Comparison Investigation
Background
The reverse osmosis (RO) process requires little maintenance and downtime when the source water is very
clean. However, RO desalination plants with ineffective pretreatment require frequent cleanings and
membrane replacement, resulting in excessive downtime and operation and maintenance costs2.
Open intake seawater desalination plants have been operated around the world for decades utilizing chemical
coagulation, clarification and granular media filtration as pretreatment. However, many plants experience
fouling: biological, organic, particulate or scaling, which requires shutdowns and cleanings every 1 to 6 months
and RO membrane replacement every 3 to 6 years. Some plants in the Persian Gulf shut down during red tide
events due to increased rates of fouling3,4. Biological fouling is of particular concern as it increases the power
required for desalination and is often difficult to remove without harsh cleaning solutions5 that increase
membrane replacement frequency to achieve water quality objectives6.
More recently, desalination plants are being constructed with membrane pretreatment systems such as
microfiltration and ultrafiltration (UF). The pretreatment systems typically reduce colloidal and particulate
fouling; however, biological fouling is encountered at some installations7.
The purpose of Investigation 1 was to test four different pretreatment filters side-by-side to determine the
optimum pretreatment system for the source water in Santa Cruz to minimize fouling.
Pilot-scale Equipment Description
The process schematic is presented in Figure 1. A brief description and design criteria tables are presented in
Appendix B for the equipment used during the pilot test program following the data charts presented in
Appendix A.
The pretreatment systems that were tested during the program are as follows:
1. Conventional treatment: defined as chemical coagulation, rapid mixing, 3-stage tapered flocculation,
and clarification (rectangular settling basin with plates) followed by pressure granular media filters
(GMF). After initial testing of filtration rates between 3 to 6 gpm/sf, it was determined that the GMF
would be operated at a conservative loading rate of 3 gallons per minute per square foot (gpm/sf) as
pressurized, constant rate filters because this rate provided the best results in terms of SDI. Backwashes
were performed with air and water to clean the media. The following three GMF media configurations
were evaluated:

2 Reverse Osmosis and Nanofiltration. AWWA Manual M46. Second Edition. American Water Works Association, 2007.
3 'Red tide' forces desalination plant closure. Andy Sambidge Arabian Business.com.. November 2008; Retrieved at
http://www.arabianbusiness.com/538468-red-tide-forces-desalination-plant-closure.
4 Tech focus: Dissolved air flotation technology. Peter Ward Arabian Oil and Gas.com.. May 2009; Retrieved at
http://www.arabianoilandgas.com/article-5488-tech-focus-dissolved-air-flotation-technology/
5 Cleaning solutions with high pH levels are typically required to remove biofouling; however, the high pH levels are also
damaging to the membrane surface and may decrease the effective salt rejection of the membranes.
6 Seawater Desalination Membrane Biofouling Project Scoping Meeting. Information presented by Nikolay Voutchkov at
the University of California, Irvine December 4th, 2008.
7 Reversible and Irreversible SWRO Membrane Fouling Owing to Algae Blooming. Dr. Ahmed Hashim and Professor Kenneth
Persson. International Desalination Association World Congress. October 2007.

TM1-4 A
Flocculation/
Sedimentation
Submerged UF
Ocean
Intake
Strainer Holding Rapid Holding RO Feed Cartridge RO
Tank Mix Tank Tank Filters
Optional Feed Line when
Operating without a Coagulant Pressurized UF
or without Sedimentation
Optional Feed Line when
Operating without a Coagulant
RO Feed Cartridge RO
Tank Filters
To Blending
Flocculation/
Sedimentation Permeate Tank*
Tank
Rapid Holding Granular Media RO Feed Cartridge RO
Mix Tank Filter No. 1 Tank Filters
Optional Feed Line
when Operating
without Sedimentation
Granular Media
Filter No. 2
Slow Sand Filter No. 1
RO Feed Cartridge RO
Tank Filters
Slow Sand Filter No. 2
* RO Permeate and RO Concentrate are mixed in the blending tank and then discharged to the LML Seawater system
W:\REPORTS\Santa Cruz City of\Desal Pilot_Final Report_09\Graphics\Fig1_Basic Pilot Plant Flow Schematic.ai 06/10/09 JJT
Figure 1
Pilot Plant Flow Schematic

TM1: Pretreatment Comparison Investigation City of Santa Cruz & Soquel Creek Water District
 Mono-medium (40 inches of 1.0 millimeter [mm] anthracite) with an approximate L/d ratio of
1,000,
 Dual-media (20 inches of 1.0 mm anthracite over 10 inches of 0.5 mm sand) with an approximate
L/d ratio of 1,000 and
 Tri-media (20 inches of 1.0 mm anthracite over 8 inches of 0.5 mm sand over 6 inches of 0.25 mm
garnet) with an approximate L/d ratio of 1,500.
2. Slow sand filtration (SSF): defined as very low rate filtration (0.1 to 0.2 gpm/sf), deep bed sand filters
with no chemical addition, clarification or backwashing. The filter beds were cleaned by harrowing,
which consisted of scouring the top of the media bed with a rake and discharging the water column to
waste. The following two media configurations were evaluated (note: the sand layers were measured
at 30 inches during installation and 24 inches at the end of testing due to compaction and sand
removal during cleaning):
 SSF1 (24 inches of 0.35 mm sand over 10 inches of gravel), and
 SSF2 (24 inches of 0.80 mm sand over 10 inches of gravel).
3 & 4. Ultrafiltration (UF) Membrane Filtration: hollow-fiber membranes with pore sizes of 0.04 micron or less
(Zenon UF submerged membranes with nominal pore size 0.04 micron and Norit UF pressurized
membranes with nominal pore size 0.01 micron). The UF membranes were operated in a “dead end”
filtration mode with and without coagulant chemical and clarification. The UF membranes were cleaned
with a combination of the following:
 1-minute backwashes every 40 to 60 minutes (air and water for the Zenon UF and water only for
the Norit UF).
 15-minute chemically enhanced backwashes (CEB) multiple times per week (sodium hypochlorite
and citric acid for both the Zenon UF and Norit UF).
 4- to 8-hour intensive cleanings known as clean-in-place (CIP) multiple times per year (sodium
hypochlorite and citric acid were used for both the Zenon UF and Norit UF).
Pilot Plant Test Period
Testing at the pilot plant occurred over a period from March 20,, 2008 to April 15, 2009.
Source Water Quality
The ocean off the coast of Santa Cruz can be characterized as having three distinct water quality conditions:
typical, algal bloom events, and winter storm events, which are described for the purpose of this memorandum
as follows:
 During typical water conditions, the turbidity and total organic carbon (TOC) concentrations are
relatively low (turbidity less than 5 Nephelometric Turbidity Units (NTU) and TOC less than 1.3
milligrams per liter [mg/L]).

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City of Santa Cruz & Soquel Creek Water District TM1: Pretreatment Comparison Investigation
 Algal bloom events occur year-round; however, large algal blooms such as red-tide events occur
between the months of September through December. Large algal bloom events are marked by high
algae counts, moderate turbidity values, and TOC concentrations that can exceed 15 mg/L. A red tide
event was artificially created on April 13th and 15th by spiking concentrated algal cells to a level of 30
micrograms per liter (μg/L) of chlorophyll; the event is referred to as the red tide simulation in this
memorandum.
 Storm events typically occur between December and March. Rainfall and runoff from local streams and
creeks combine with wintertime ocean currents and upwelling to significantly increase the turbidity to
levels that may exceed 50 NTU. The data presented herein was collected during the most significant
storm event which occurred during the week of February 16th.
Source water quality conditions during the 12 months of pilot testing are summarized in Table 1.
Table 1. Observed Source Water Quality Data Summary
Fall & Winter Red tide
Observed Water Quality Spring and Fall Algal Winter Storm Spring Algal
(Non-storm simulation
Periods & Events Summer Bloom Event Bloom
conditions) Event
April – September
Water Quality November February 16, Late March – Mid-April
Units August 2008 – March
Parameter 2008 2009 Early April 09 2009
2008 2009
pH
pH Units 8.0 8.0 8.0 7.9 7.9 7.9
(mean)
Temperature
oC 12.0-18.1 9.5-15.8 13.4-15.6 12.8-14.1 11.9-14.2 11.1-13.4
(range)
Turbidity
NTU 1.5-4.2 2.0-3.5 1.1-2.0 8-40 1.8-2.8 8-15
(range)
Particles
No. per
(> 2 µm) 10,530 9,860 12,340 14,110 9,690 12,790
100 mL
(mean)
TOC
mg/L 1.0-1.2 1.1-6.0 3.2 2.5 3.4-13.0 7.2
(range)
DOC
mg/L 0.9-1.1 1.3-3.8 2.9 2.0 3.1-12.0 4.3
(range)
2.3-21.2
Chlorophyll
µg/L (at SC 1.0 2.7 0.7 9.2 30
(typical)
Wharf)
Algal Cell Count Cells per Not 50,000 to 500,000 to
15,000 28,000 <10,000
(typical) Liter counted 160,000 600,000

Figures A-1, A-2 and A-3 in Appendix A present source water turbidity, particle counts, chlorophyll
concentration, and algal cell loading data observed during the study.
Pretreatment Goals
The objective of the pretreatment testing was to determine which pretreatment system is the optimum in a
reverse osmosis seawater desalination process for the source water off the coast of Santa Cruz. Optimum takes
into account such factors as response to variations in source water quality, sustainable operation, energy usage,

TM1-6 A
TM1: Pretreatment Comparison Investigation City of Santa Cruz & Soquel Creek Water District
chemical consumption, washwater and sludge production, performance of the RO system with respect to
fouling control, and life-cycle costs.
The pretreatment water quality goals are presented in Table 2.
Table 2. Summary of Pretreatment Goals
Water Quality and UF Membrane
Conventional Treatment Slow Sand Filtration
Operational Goals Pretreatment
SDI(1) ≤4.0(2) (99% of the time)
(SDI 15 Units) ≤ 3.0 (90% of the time)
TOC
≤ 2.0(2)
(mg/L)
Turbidity
≤ 0.1(2)
(NTU)
Removal of Particles
≥ 99.0% ≥ 99.0% ≥ 99.99%
>2 microns
> 40 minutes at
≥ 2 weeks at 0.1
Filter Run Time ≥ 24 hours 3 at gpm/sf 25 gfd for Zenon UF
gpm/sf
50 gfd for Norit UF
Chemically Enhanced
n/a n/a ≥ 24 hours
Backwash Interval
UF Membrane Clean in
n/a n/a ≥ 2 months
Place Interval
RO Membrane Clean in
≥ 5 months ≥ 5 months ≥ 5 months
Place Interval(3)
(1) SDI calculated for 500 mL of sample at fifteen minutes.
(2) Typical pretreatment goals listed by RO membrane manufacturers for open intake seawater desalination.
(3) The interval assumes that a cleaning is required when an increase in normalized differential pressure is greater than 15% and/or
a decrease in normalized specific flux or permeate flow of greater than 10% is observed based on Hydranautics Technical Service
Bulletin 107 retrieved at http://membranes.com/docs/tsb/tsb107.pdf.

Pretreatment Testing Results
Table 3 presents a summary of the pretreatment water quality and operational results from the testing. Note
that all water quality and operational goals were achieved except the following:
 TOC concentration goal - the four pretreatment systems were able to achieve the TOC concentration
goal when source water concentrations were less than 3.0 mg/L as expected. No pretreatment system
was able to achieve the goal when the source water concentration was greater than 3.0 mg/L. Thus,
TOC removal percentage will be used in addition to TOC concentration to compare the pretreatment
systems.
 RO membrane cleaning interval goal downstream of conventional treatment - The goal was not
achieved downstream of GMF pretreatment due to observed fouling during algal blooms and
increases in differential pressure caused by iron breakthrough. The goal was not achieved downstream
of UF pretreatment due to observed fouling during algal blooms.

A TM1-7