OPTIMIZATION OF SWRO PRETREATMENT - PART I1 SD1 ...

OPTIMIZATION OF SWRO PRETREATMENT - PART I 1
SD1 MEASUREMENT
ABSTRACT
Ata.M. Hassan, A. Abanmy, A.M Farooque, A. T. M. Jamaluddin,
A. Al-Amoudi and T. Mani
Research & Development Center
Saline Water Conversion Corporation (SWCC)
P.O. Box 8328, Al-Jubai1 31951
Kingdom of Saudi Arabia
An exhaustive, systematic, parametric investigation of the effects of various variables,
i.e., type of coagulant, coagulant-aid, other chemicals and their concentration; filter
media type, layer thickness and particle size; disinfection method and disinfectant
concentration; seawater feed pH; flow rate and ultrafiltration (UF) pretreatment on the
quality of pretreated water supplied to the plant from an open seawater intake (surface
seawater), Al-Jubail, was investigated over the last two years, using a specially designed
coagulation-filtration SWRO pilot plant. The pretreated feed was supplied to four
different SWRO pilot plants, some of which utilize 2.5 x 40 inch or 5 x 22 inch
permeators, and others allow for flat sheet membranes testing. Pretreated feed water
quality was determined: (1) by SDI measurements, (2) by use of membranes and
measuring the feed effect on their performance & fouling, and (3) from bacteria count
determined at the various stages of the SWRO process. In this report (Part I) SDI is
used in pretreatment evaluation, while evaluation by the latter two methods is described
separately in parts II and III of this investigation.
Gulf seawater at Al-Jubail site has an average SDIav = 5.9 + 0.25 and drops to an SDIav
= 4.4 + 0.35 by simple dual media and sand filtration without any chemical addition. It
drops further to SDIav = 4.1 + 0.28 after cartridge filtration using 5u cartridge filter.
Addition of ferric chloride coagulant at the rate of 1 ppm Fe +3 and at a non-chlorinated
feed flow rate of 20 gpm reduces the filtrate SDIav to 3.5 as compared to SDIav 2.6 +
0.2 when the flow rate is reduced to 14 gpm and the fine sand (particle size 0.3 to 0.5
mm) thickness was increased from 279 mm to 533 mm. At the latter feed flow rate and
fine sand thickness the plant was operated using non-chlorinated feed for a period of 21
days on the residual Fe +3 in the filter without coagulant addition or without filter
backwash for the first seven days where the filtrate SDIav remained the same but it
rose to a SDIav = 3.5 when the dual media filter was backwashed several times during
1 Issued as Technical Report No. SWCC(RDC) - 37 - Partly Presented at IDA World
Conference “Desalination and Water Sciences”, Nov. , 18-24, 1995, Abu-Dhabi.
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the last 14 days. The residual Fe +3 remaining within the dual media filter and sand filter
layers are sufficiently large to provide the coagulation without the addition of new
coagulant. This suggest that at this rate of Fe +3 dosing the filter back washing is not
removing all the Fe +3 ions. Furthermore, the results suggest the possibility that the
pretreatment may be operated at lower coagulant dosing rates which is being tried, or
alternatively by a programmed gradual reduction in Fe +3 dosing ratio. This should
further reduce the Fe +3 ions in the feed and minimize their precipitation on the
membrane surface.
Dosing of both coagulant (Fe +3 = 1 ppm) and coagulant-aid (polyelectrolyte = 0.8 ppm)
reduces the sand filtrate to an SDIav = 1.9 but it leads to a noticeable increases in
bacterial count if the residual chlorine in the feed is low, less than about 1.0 ppm. A
major reduction in SDI to about 1 or less was obtained by passing the seawater or the
filtrate feed through ultrafiltration (UF) unit with a molecular weight (MW) cut-off of
15000. However, frequent backwashing is required in the former case. Due to chlorine
degradation of micro-organisms and other organics, the chlorinated feed tends to have
higher SDI value, at least by one unit, than the similarly treated but non-chlorinated
feed. Dosing of acid in seawater ahead of the filtration step has no major effect on
filtrate SDI when compared to the SDI of nonacidified filtrates.
INTRODUCTION
The membrane is the heart of the RO plant and is made of special semipermeable thin
polymeric film deposited on a relatively thick strong support material or made of a thin
polymeric film of the same composition as its strong support material. Membranes are
characterized by their unique properties of high water permeation (flow), very low salt
passage and dimensional and chemical stability. In spite of their small sizes and high
solubilities in water, salts, even those with small molecular weights, do not pass
through the membrane at a significant rate. Their passage is held to a very low level,
e.g., less than 1% using thin film composite (TFC) seawater membranes. Passage of
the larger size colloidal and other suspended solid particles is not permitted at all
through the membrane closed structure. This is also true of microorganism such as
bacteria which, when present in the feed, will be trapped on the membrane surface
causing it to foul Membrane fouling is also introduced by the presence in the feed of
scaling and corrosion products. The presence of any of the above matters in the feed
could cause membrane fouling and the lowering of the RO plant efficiency, hence feed
pretreatment is required.
In water desalination by the RO process feed water pretreatment is essential in order to
remove all the potential membrane foulants from the feed. The degree of pretreatment,
however, is dependent on the raw water quality, particularly its content of suspended
and biological matter as well as on the membrane configuration. Water quality tends to
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e site dependent. For example, the average SDI values of raw, untreated feed to
SWCC SWRO plants located on the shore of the Red Sea are:
SDI = 4.6 Haql, Duba, Umm Lujj,
SDI = 4.75 Jeddah - I, and
SDI = 6.5 Al-Birk [l&2]
By comparison the average SDI values for the untreated Gulf seawater are:
SDI = 5.9 at SWCC Jubail, and
SDI = 6.5 or more at Doha, Kuwait [3]
High SDI values are also reported for untreated seawater at Al-Dur, Bahrain. (The SDI
(4) is a measure of water clarity, the lower the SDI value the greater is its clarity and
the lesser suspended matter it contains.
Membrane configuration has a great influence on the degree of seawater pretreatment.
Hollow line fiber (HFF) membrane configuration with tight fiber packing in the module
requires maximum pretreatment as compared to an intermediate and modest degrees of
pretreatment for the spiral wound (SW) and plate & frame (PF) membrane
configurations, respectively. The manufacturers recommend different treatment for
each of the above configurations. The pretreatment should be able to yield a feed with
the following SDI values:
SDI < 3 for HFF with high fiber packing density,
SDI < 4 for SW, and HFF with moderate fiber packing
density,
SDI < 5 for PF membranes.
An ideal pretreatment is designed to cause a minimum or no membrane fouling at the
lowest possible cost. This not only results in longer membrane life but also improves
the overall plant efficiency and reduces the cost of fresh water production. Seawater
feed derived from a well-designed well requires minimum of pretreatment by passing it
only through cartridge filter, size 5 to 15m. The feed tends to be a high quality feed
with an SDI as low as 1 or less. In this case natural filtration cleans the water from
nearly all foulants. The ultrafiltration and microfiltration membrane processes are
being tried instead of the classical SWRO pretreatment of coagulation-filtration process.
Experiments done at our laboratory with ultrafiltration pretreatment gave pretreated
feed with SDI of less than 1. Although the well-designed beachwell and the filtration
membrane processes yield pretreated feed with very low SDI, the treatment does not
prevent membrane scaling which arises from the concentration to their solubility limits
of sparingly soluble salts, e.g., CaSO 4 , CaCO 3 , etc. To attain high water recovery,
antiscalant needs to be added to feed containing salts with scaling tendency.
Pretreatment of surface seawater, however, is rather more demanding than that of well
water or membrane treated feed and depends on the feed water quality. The feed
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treatment may consist of disinfection, followed by coagulation-flocculation filtration
and dosing of antiscalant. Feed chlorination and dechlorination is also required when
the feed is intended for chlorine sensitive membranes while antiscalant agents are added
as needed.
High quality feed with SDI value less than 3 and in some cases as low as 1 is obtained
with a reasonable pretreatment (coagulation-filtration) at SWCC Haql SWRO plant.
This tends to be true of SWCC SWRO plants located on the Red Sea at Duba and
Jeddah where for both cases SD1 tends to be less than 3 when ferric coagulant is added
to the feed ahead of the dual media filter. Higher SD1 values of 3 to 4 are obtained for
filtrate without coagulant addition at SWCC Umm Lujj SWRO plant [I]. In all above
cases no membrane fouling has been reported. By comparison much higher SDI values
greater than 3 are reported for Gulf seawater even when the Fe +3 = 1 ppm coagulant is
added to the feed [3].
Earlier work utilizing an experimental RO pretreatment plant to treat Gulf seawater at
Doha and Shuwaikh, Kuwait (SDI ~ 6 to 6.5) showed that good quality feed with SDI ~
3 to 4 was obtained by in-line coagulation, dosing 1 ppm Fe +3 coagulant followed by
dual media pressure filtration. Both ferric chloride (FeCI 3 ) and FeClSO 4 serve equally
well as coagulant sources. Addition of 1 ppm of the coagulant-aid, polyelectrolyte,
reduced the gradual build-up in pressure across the dual media filter, leading to a
lowering in the frequency of filter backwash (3].
The feed water SDI was reduced to less than 2 using as low as 0.5 mg/L Fe +3 and 20
mg/L acid by passing the filtrate from the dual media pressure filter having 20 cm thick
sand layer of 0.6 to 0.8 mm particle size through a second filter with a finer sand layer,
thickness 60 cm, particle size 0.3 to 0.5 mm [5].
For more turbid Gulf Seawater with SDI = 6.5, feed water with SDI = 3.5 to 4 was
obtained by dosing of 2 to 3 ppm Fe +3 and 0.3 to 0.5 ppm polyelectrolyte followed by
coagulation - flocculation and filtration through dual media gravity filters. The filter is
made of sand layer, thickness 100 cm, particle size 0.7 to 1.2 mm topped with a second
hydroanthracite layer, thickness 70 cm and particle size 1.4 to 2.5 mm [3, 6 & 7]. The
above pretreatment in which energy was used to control the destabilization step was
found to be sufficient for the treatment of feed to the 1000 m 3 /d seawater Doha (Kuwait)
RO line utilizing Fluid System UOP thin film composite polyamide/polyurea membrane
and to a second line 1000 m 3 /d PF RO, at the same pilot plant 2 utilizing thin film
composite Filmtec polyamide flat membrane in about one third of the modules and
cellulose acetate type flat membranes in the remaining modules. The feed to a third
2 Actually this pilot plant because of its large capacity can be considered as a
commercial production plant.
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line of 1000 m 3 /d HFF RO line within the same pilot plant, utilizing DuPont B-10
polyamide membrane, however, required additional in-line coagulation and sand
filtration to reduce the SDI < 3. Passage of the pretreated, chlorinated feed through
active carbon filter after dosing sodium bisulfite was satisfactory in the removal of
chlorine from the feed to the highly chlorine sensitive the older UOP membrane (UOP
TFC polyamide-polyurea 1501 PA) [8]. We were told that this is the only UOP plant of
its type which is operated with chlorinated/dechlorinated feed [9] while in other plants
using TFC 1501 PA, CuSO 4 is generally used for disinfection. Unlike the old TFC
UOP membrane the new UOP TFCL type membranes can be operated with a
chlorination/dechlorination treatment. The above pretreatment at Doha SWRO pilot
plant yielded good quality feed water in accordance with membrane manufacturers
recommendation which in turn resulted in reduction in the frequency of membranes
cleaning, maintaining design recovery and in an increase in RO plants availability [10
to [3].
Contrary to the above success at Doha Seawater RO plant, Kuwait, a steady decline in
the performance of Al-Birk, 2275 m 3 /d seawater RO plant due to membrane fouling was
observed by SWCC [I, 14]. Similar fouling was also reported at Key West (Florida,
USA), Malta and Ras Tanjib (Saudi Arabia). On the other hand no biological
membrane fouling was observed at the 4400 m 3 /d Umm Lujj RO plant where 0.5 ppm
copper sulfate was added to the surface seawater feed [1, 14]. Moreover, no biofouling
was observed at the 15 mgd Jeddah SWRO plant nor at the Haql and Duba plants which
all utilize Toyobo CTA Hollosep HFF membranes and are fed chlorinated, Fe +3 -
coagulated and dual media filtered feed (SDI = 1 to 3) [I]. Polymer degradation, i.e.,
chain scission (cutting the chain into parts), however, was reported at Jeddah I SWRO
plant due to excess of chlorine in the feed of more than 0.2 ppm in presence of heavy
metals, e.g., copper ions [15]. To avoid CTA polymer degradation by chlorine,
intermittent chlorine dosing in the feed is being tried at Jeddah I SWRO test plant [16].
After one year of successful operation, no decline was observed in the performance of
the 46000 m 3 /d RO plant at Ra’s Abu Jarjur, Bahrain [17]. The product water quality
remained higher than design values. Early problem arising from the growth of algae
and bacteria in the filter was remedied by proper disinfection and backwashing of the
filters and also by painting of the GRP pipes to mask their transparency and to protect
them from sun light. The success at this plant which treats a high salinity underground
water, TDS ~13000 ppm, is attributed to high quality feed, SD1 < 1.5, derived from 15
wells. The pretreatment consists only of simple filtration through dual media filter
followed by active carbon filter to remove trace oil < 0.1 mg/L from the feed. In-line
coagulation which was tried at the start of the plant was found unnecessary [17].
The conclusion that can be drawn from the above review is that pretreatment is an
essential step in the SWRO process in order to remove all potential membrane foulants
and prevent membrane degradation and damage by oxidizing agents, e.g., CI 2 present in
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the feed. The pretreatment itself is dependent on the feed water quality, membrane type
and configuration and is very much site dependent. Optimization of the pretreatment
for a given type of membrane is very essential for its optimum performance and the
membrane life. An exhaustive, systematic study was carried out to optimize the
coagulation-filtration SWRO pretreatment for Gulf seawater taken from an open intake.
Al-Jubail, Saudi Arabia, by investigating the effects of the following pretreatment
variables on feed water quality:
• Feed Flow rate.
• Type of coagulant and coagulant-aid and their concentration,
• Other chemicals added to the pretreatment feed, their dosing rate and dosing
location in the pretreatment plant, e.g., Acid, NaHSO 3 , etc.
• Filtration (sand layer thickness and particle size),
• Disinfectant (type and concentration) and
• Ultrafiltration.
The study was conducted over the last two years utilizing a specially designed pilot
plant that allows for a parametric evaluation of the above variables on the quality of
pretreated seawater derived from Gulf sea, Al-Jubail. The quality of the pretreated
water and the degree of influence of those various factors on it was evaluated by
the SDI measurements and also by the effect of those various factors on the
performance of several SWRO membranes, their biofouling as well as on biolife
and biological activity at different stages of the SWRO process. In this report only
the SDI is used to evaluate the pretreated water quality. Pretreatment evaluation
by the latter two methods under the influence of the above factors is described
elsewhere in separate reports [18&19].
GULF SEAWATER
Table 1 gives an average chemical composition of Gulf seawater. at Jubail and for
comparison it also gives the chemical composition of normal seawater. Gulf seawater
has much higher TDS of over 43800 ppm vs 35146 ppm for normal seawater. The
average ionic ratio of Gulf seawater Jubail : Normal seawater is in the ratio of 1.253 : 1
and consequently the concentration of all ionic species are proportionally higher in the
Gulf seawater than they are in the normal seawater.
The average SDI of untreated Gulf seawater at Al-Jubail SWCC site is 5.9 + 0.26 with a
range of 5.4 to 6.4. All SDI measurements were made at the standard 15 minutes
values which places the maximum SDI value at 6.67. The seasonal variation of SDI
and the effect of pretreatment on reducing the SDI value are discussed in detail in latter
sections.
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The average total suspended solids (TSS) which are responsible for the observed rise in
SDI of untreated seawater is 20 mg/L with a range of about 14 to 25 mg/L and reduces
to only 1 mg/liter or less after seawater feed Fe +3 coagulation/sand filtration
pretreatment. Material composition of suspended solids in the untreated and pretreated
seawater feed after coagulation-filtration is given in Figure 1. The energy dispersive
X-ray (EDX) shows that the TSS components consist mainly of Ca, Si, Mg, O and
Carbon. Other elements found are Fe, and Na but at much lower concentration. The
elements EDX line intensity is drastically reduced and the C line intensity becomes
pronounced in the pretreated seawater where the pretreatment reduces a good part of the
suspended solids. The carbon could arise from organic and inorganic source.
Bacteria count which is determined by the Biology Department as number of Colony
Forming Units (CFU/ml) ranges between 8.0x10 1 to 2.7x10 4 CFU/mL and tends to vary
from one season to another. Bacteria count is minimum in the winter months and rises
significantly during the warm and hot season reaching maximum value during the
month of June. This is discussed in detail in a separate Joint RO/Biology Departments
study [19].
EXPERIMENTAL WORK AND EQUIPMENT
All experiments which dealt with the effect of various operational parameters on the
SDI of pretreated SWRO feed were carried out utilizing the SWRO pilot plant
described below. The variables examined were as follows:
Feed flow rate (14 gpm to 25 gpm),
Sand filter layer thickness,
Ferric chloride coagulant (Fe +3 = 0.0, 0.6, 1.0, 1.2 ppm),
The coagulant-aid polyelectrolyte (0.0, 0.25, 0.5 and 0.8 ppm) with ferric chloride
coagulant (1.0 ppm),
pH of seawater feed to the pretreatment plant (8.1 and 6.5)
Chlorinated (0.2 ppm residual Cl 2 ) and non-chlorinated (0.0 CI 2 ) seawater feed
and
Ultrafiltration.
Pretreatment Pilot Plant
A schematic flow diagram of the pilot plant is given in Figure 2. Basic components of
the unit are: Seawater feed line, destabilization-agglomeration tanks, feed pumps, dual
media and fine sand filters, followed by 5 um cartridge filter and a pretreated feed
holding tank. The unit is equipped with a filter backwash system to wash the filter as
needed and a booster feed pump to boost feed pressure (up to 50 psi) supplied to the
SWRO high pressure pump.
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Chlorinated or non-chlorinated seawater is supplied to the unit by the feed pipeline at
an adjustable flow rate up to 25 gpm. After its dosing with the coagulant agents, feed
flows to the destabilization-agglomeration tanks where floes formation takes place.
Under low pressure (40-60 psi) the feed water pump supplies the feed to the dual media
(sand/anthracite) filter from which the filtrate flows into a second fine sand filter
followed by the cartridge filter. Antiscalant acid and sodium bisulphite are introduced
after the 5 mm cartridge filter to condition the feed and to remove the chlorine from it
prior to the feed entry into the SWRO feed tank.
The pretreatment which was designed at SWCC RDC (20) allows for the testing of a
variety of variables that affect SWRO pretreatment. Flexibility is built in the system to
allow for the by-passing of the destabilization-agglomeration and flocculation tanks and
to evaluate the effects of each of dual media and fine sand filters separately on the
quality of seawater feed.
The unit is equipped with several dosing points which allows for the study of the effect
of dosing location on the pretreated water quality. Furthermore, the unit is
instrumented to record : feed flow, temperature, pH, conductivity, pressure, and
chlorine level by an oxidation reduction potential (ORP) unit. A data logger transfers
the data to an IBM 386 computer which allows for the storage, retrieval and treatment
of the data. The SDI is measured manually. The SDI values plus all other data which
are measured manually are stored in the computer along with the recorded data for
further data treatment.
Sand Filters
Table 2 gives the various media used in the two pressure dual media and sand filters,
their media thickness and particle size before and after modification. Modification was
introduced only in fine sand filter where the active carbon was removed and replaced by
increasing the very tine sand (particle size 0.3 to 0.5 mm) thickness from 279 mm to
533 mm. The dual media filter consists mainly of two layers: a sand layer made of
coarse sand, thickness 152 mm, particle size 0.7 to 1.3 mm, and fine sand, thickness
406 mm, particle size 0.6 to 0.8 mm topped with anthracite layer, thickness 203 mm
and particle size 1.2 - 3 mm. After modification the tine sand filter has also a coarse
sand layer with the same specification as that of the dual media sand filter and a very
fine sand layer, thickness 533 mm, and particle size 0.3 - 0.5 mm.
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RESULTS AND DISCUSSION
Effects of Filtration without coagulant on quality of seawater filtrate
The SWRO pretreatment pilot plant was operated without any chemical addition. No
disinfectant, nor coagulant, nor acid was added to the seawater feed which received only
filtration by passing it through the pilot plant dual media filter (DMF), fine sand filter
(FSF) and the 5u cartridge filter (CF). Media layer thickness and particle size was as
shown in Table 2 prior to the modification of FSF. Feed flow rate was maintained at 20
gpm. Figure 3 shows the obtained SD1 results for raw seawater and filtered feed. The
SD1 of the non-chlorinated filter feed was as follows:
Feed Average SD1 Reduction in SD1 SD1 Range
Raw seawater 5.9 + 0.26 6.4 to 5.4
Filtrate after DMF 5 + 0.33 - 0.9 5.8 to 4.3
Filtrate after FSF 4.4 + 0.35 - 1.5 4.9 to 3.5
Filtrate after CF 4.1 +0.28 -1.8 4.6 to 3.8
All SDI measurements were made at the standard 15 minutes values [3]. Filtration
reduces the feed SD1 by 0.9, 1.5 and 1.8 SDI units when the feed is passed through
DMF followed by FSF and CF, respectively. Main reduction in SD1 is done through
DMF and FSF and with the CF causing an additional reduction of 0.3 SDI units. The
relatively low seawater SDI average of 5.9 indicates that the seawater at Jubail is
relatively clean when compared to seawater at other Gulf sea sites such as Doha,
Kuwait, where the average SDI is over 6.3, and in many cases exceeds the limits of SDI
(15 minutes) measurements, i.e., maximum SDI 6.67.
Effect of Coagulant & Flow Rate on SD1 of Pretreated Nonchlorinated
Seawater
In the above experiment no coagulant was added to the feed and the average SDI of FSF
filtrate was 4.4 + 0.35 with an SD1 range of 3.5 to 4.9. Addition of ferric chloride
coagulant at a concentration of 1 .0 ppm Fe 3 at feed flow rate of 20 gpm reduced the FSF
filtrate SDI to 3.8. These measurements were made prior to the FSF modification by
passing the coagulated feed seawater through Dual media and fine sand filters whose
media composition, thickness and particle size are given in Table 2.
To reduce the filtrate SDI below 3 which is recommended by certain membrane
manufacturers and since the seawater feed contains nearly no organic substances the
active carbon layer, particle size 1.2 to 3.0 mm, was removed and replaced by adding of
254 mm layer of very fine sand to FSF with particle size of 0.3 to 0.5 mm. The media
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thickness and particle size in DMF, however, remained the same without any
modification, The only modification was introduced in FSF by increasing the thickness
of the very tine sand to 533 mm from 279 mm before modification and the complete
removal of active carbon layer from it Table 2, section B.
Figure 4 shows the effects of coagulant and feed flow rate on SDI of seawater and FSF
filtrate, while Table 3 summarizes the SDI values obtained for seawater feed and FSF
filtrate as well as the reduction in SDI with treatment at different flow rates and ferric
ion concentrations. Seawater has an average SDI of 5.8, with a range of 5.5 to 6.3. At
the feed flow of 20 gpm without addition of coagulant the average SDI of filtrate was
4.4 and varied from 3.9 to 5.3. The SDI, however, dropped to 3.2 (range 2.6 to 3.9) at
ferric flocculant concentration in the feed of Fe +3 = 1.2 ppm and feed flow rate of 21
gpm. A further reduction to SDI 2.6 + 0.2 and SDI range of 2.3 to 3 was obtained
when the feed flow rate was reduced further to 14 gpm and Fe +3 of 1.1 ppm in the feed.
The SDI continued to be less than 3 (SDIav 2.4) for a period of one week without sand
backwash even when the ferric chloride dosing was stopped ( Figure 4 and Table 3).
After the seven day period the SDI started to rise but continues to fluctuate between 2.3
to 4 over an additional period of 14 days, during which no coagulant was added, but the
dual media and sand filters have been backwashed several times. Repeat of the
experiment showed that the filtrate SDI remained less than 4 after 70 hours of operation
without coagulant addition. The combination of low feed flow rate and residual ferric
captured within the dual media and sand filter media are presumably responsible for the
reduction in SDI. Also decreasing the flow rate allows for longer residence time in the
destabilization - agglomeration tank and should allow for formation of larger floes.
These results indicate, as expected, that feed flow rate in the coagulation-filtration
step has a great influence on the quality of the filtrate as measured by SDI
Furthermore, the results suggest that ferric dosing may be stopped for a certain
operation period provided the SDI does not change significantly beyond the limits
of membrane SDI requirement. Alternatively, the ferric ion concentration in the
feed may be reduced or gradually reduced while maintaining acceptable SDI
values. Reduction of Fe +3 in the feed by either method should minimize its capture
on the membrane surface.
Effect of Coagulant-aid on Quality of Pretreated Non-chlorinated
Water
Figure 5 shows the SDI of non-chlorinated filtrate collected after line sand filter at an
average flow rate of 19 gpm which is the amount of feed required to feed the SWRO
membranes.. The ferric chloride coagulant was added ahead of filters at the rate of 1
ppm Fe +3 in the feed. The filtrate average SDI is about 2.5 + 0.5 over the test period of
over 12000 hours as compared to an average SDI = 6 to 6.4 for the untreated seawater.
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Figure 6 shows the effects of varying the concentration of coagulant-aid on SDI of
pretreated seawater at fixed coagulant concentration of Fe +3 = 1 .0 ppm. Both coagulant
and coagulant-aid were added to the feed ahead of the filters. For comparison the figure
shows also the SDI of untreated Gulf seawater and that of filtrate without the addition
of coagulant agents.
By comparison to the untreated seawater feed, filtration without coagulation reduces the
filtrate SDI by about 2 SDI units from an average SDI value of 6.3 for the untreated
seawater feed to an average SDI value of 4.3 for the filtrate of the non-coagulated feed.
Addition of the coagulant (Fe +3 = 1 ppm) to the raw seawater feed reduces the filtrate
SDI to an average SDI of 2.9. The filtrate SDI is reduced further by the addition of both
coagulant (1 .0 ppm of Fe +3 ) and coagulant-aid polyelectrolyte (0.25 ppm to 0.8 ppm) to
the feed ahead of the filter. The filtrate SDI drops to an average value of 1.9 at the
coagulant-aid level of 0.8 ppm at the fixed coagulant concentration in the feed of 1.0
ppm of Fe +3 .
In conclusion significant reduction in filtrate SDI is obtained when both coagulant
and coagulant-aid are added to feed. Moreover, the addition of coagulant-aid led
to a decrease in frequency of filter backwashing. However, addition of the
coagulant-aid with the ferric coagulant leads to a noticeable increase in bacteria
count (CFU/mL) in the filtrate, which makes the use of this type of coagulant-aid
questionable. However, it was shown in a separate work at our laboratory that no
rise in bacteria count was observed in presence of residual chlorine > 1 ppm (21).
Other coagulants and coagulant-aids are being tried at different concentration
ratio for both.
Effect of Chlorination on SDI
Figure 7 shows the SDI of FSF filtrate with and without feed chlorination. The
pretreated feed was filtered through the dual media and fine sand filters after the
addition of the coagulant agents. For comparison the SDI of untreated Gulf seawater
and the SDI of filtrate without feed coagulation are shown in the same figure. In all
cases the filtrate from the chlorinated feed has higher SDI than that of similarly
treated feed. For example, the SDI of filtrate from the chlorinated, non-coagulated
feed is 5.1 as compared to an SDI of 4.3 for the non-chlorinated, non-coagulated
filtrate. Similarly, the chlorinated filtrate of feed coagulated with 1 ppm Fe +3 and 0.8
ppm polyelectrolyte has an SDI of about 3 which is higher by over 1 SDI unit by
comparison to the filtrate from similarly treated but not chlorinated feed. The reason
for this behavior, i.e., increase in SDI with feed chlorination, may be explained by
the high oxidation capacity of chlorine which leads to an increase in the number of
suspended particles by breaking the larger particles mainly of high molecular
weight humic/fulvic substance and microorganisms into smaller particles. These
particles pass through the filter without being trapped within the media layers but are
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caught on the top of the 0.45 pm SDI filter paper slowing down water flow through the
SDI filter paper.
Effect of pH on SDI
All previous experiments were made without dosing acid in the feed ahead of the dual
media filter. Basically the filtrate has the same pH as that of seawater with only small
changes in pH due to the addition of ferric chloride to the feed. The pH of the filtrate
was reduced to 6.5 by dosing acid in the feed ahead of the dual media filter.
The effect of pH on filtrate SDI is shown in Figure 8 which compares the SDI at pH = 8
and pH = 6.5 for the following cases:
Seawater,
Filtrate with and without chlorine but without the addition of the
coagulant(Fe +3 ) to the feed in both cases,
Filtrate with and without the addition of chlorine but with the addition of Fe +3
(1 ppm) to the feed, and
Filtrate with the addition of both Fe +3 (1 ppm) and polyelectrolyte (0.8 ppm)
without the addition of chlorine.
With the exception of filtrate derived from feed coagulated with coagulant (Fe +3 = 1
ppm) and coagulant aid (polyelectrolyte = 0.8 ppm), where the acidified feed
appears to have slightly lower SDI values than the nonacidified feed, the SDI in all
other cases were generally higher at lower pH, though the increases in these cases
were quite insignificant. As was mentioned and explained earlier, however,
chlorination introduces significant change in the filtrate SDI.
SUMMARY AND CONCLUSION
The effect of the various variables, i.e., type of coagulant, coagulant aid, other
chemicals and their concentration, filter layer thickness and particle size,
disinfection method, feed flow rates, feed pH and ultrafiltration (UF), on quality of
pretreated water was investigated using a specially designed SWRO pilot plant.
Water quality was determined from SDI measurement, bacteria count at various
stages of the SWRO process and by the effect of the pretreatment on membrane
performance using several different types of membranes. Only SDI results are
reported here. Results of latter two measurements are reported separately as Parts
II and III of this investigation.
Gulf seawater at Al-Jubail site has an average SDI = 5.9 + 0.25 (SDI range 5.4 to
6.4) and drops to and average SDI = 4.4 + 0.35 (SDI range 3.5 to 4.9) by simple
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dual media and sand filtration without chemical addition and it drops further to an
average SDI = 4.1 + 0.28 after cartridge filtration using 5 µ m cartridge filter.
Addition of ferric chloride coagulant at the rate of 1 ppm Fe +3 at a feed flow rate of
20 gpm reduces the filtrate SDI to about 3.5 as compared to an average SDI = 2.6
+ 0.2 (SDI range 2.3 to 3.0) when the flow rate is reduced to 14 gpm and fine sand
(particle size 0.3 to 0.5 mm) thickness was increased from 279 mm to 533 mm.
At the latter feed flow rate while maintaining the fine sand thickness at 533 mm,
the plant was operated for a period of 21 days on the residual Fe +3 in the filter
without coagulant addition and without filter backwash for the first seven days and
with several filter back wash during the last 14 days. The sand filtrate average SDI
remained less than 3 (SDI range 2.1 to 4.0). Repeat of the experiment showed that
the liltrate SDI remained less than 4 after 70 hours of test without coagulant
dosing. The results suggest that the pretreatment may be operated with a
programmed gradual decrease in coagulant dosing and with the coagulation being
induced by Fe +3 residue trapped within the filter layers and which are not removed
by the filter backwashing. A programmed gradual decrease in coagulant dosing is
being tried at the pilot plant along with the reduction in Fe +3 level in the feed.
Dosing of both coagulant FeC1 3 , (Fe +3 = 1 ppm) and coagulant-aid (polyelectrolyte
= 0.8 ppm) reduces the sand filtrate to an average SDI = 1.9 but it leads to a
noticeable increase in bacterial count if the chlorine residue in the feed is less than
about 1 ppm.
A major reduction in SDI to about 1 or less was obtained by passing the seawater
or the filtrate feed through ultrafiltration unit. Frequent backwashing is required
in the former case when passing untreated seawater
Due to chlorine degradation of organics and micro-organisms. chlorinated feed
tends to have higher SDI value, at least by one unit, than similarly treated but
non-chlorinated feed.
Addition of acid to seawater ahead of the filtration step has only a minor effect on
filtrate SDI when compared to the SDI of nonacidified filtrates.
RECOMMENDATION AND PROPOSED FUTURE WORK
1. Pretreatment is to be evaluated with filtration alone without any chemical addition
followed by examining the effect of the filtrate on the various types of SWRO
membranes.
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2. Study the effect of copper sulfate on bacteria count and SDI at 0.0, 24 and 48
hours aftergrowth at various stages of the SWRO process, i.e., on untreated
seawater, filtrate and brine in view of lack of biofouling at Umm Lujj SWRO
which uses this disinfectant,
3. Investigate the possibility of reducing coagulant and other chemical addition on
plant operation and membrane performance along with the optimization of Fe +3
rate.
4. Investigative the effect of residual Cl 2 (0, 0.2, 0.5, 0.8 ppm) on SDI.
5. Furthermore, investigate the effect of acid added ahead of filter on SDI.
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