Press Clipping Desalination (Filtration + Separation) - Inge AG

Press Clipping
Desalination (Filtration + Separation)
Volume 4 Issue 1
Desalination pre-treatment: Efficient ultrafiltration for seawater
Ralf Krüger from inge watertechnologies describes the growing popularity of
ultrafiltration as pre-treatment in seawater desalination, and looks at its successful
application at an Italian power plant that required an efficient and low-maintenance
ultrafiltration solution.
Ultrafiltration (UF), as pre-treatment technology, has become the preferred choice
over reverse osmosis (RO) for problematic feed water such as surface or wastewater. UF
can provide superior water quality compared with conventional treatment and delivers a
continuously good filtrate quality independent of feedwater characteristics caused by, for
instance, seasonal changes.
In seawater applications UF is becoming more popular as well. This is due to the
advancements in UF membrane technology and the scale of economies resulting from an
increasing demand. Recent integral cost comparisons have shown that, when all cost
components and impacts on the treatment steps downstrearn are taken into account, UF
can compete with conventional pre-treatment technologies.
One of the largest seawater desalination plants in Europe where UF is installed is located
in Italy. This UF system features two innovative technologies in the field of membrane and
module design which particularly address the need for highest operational safety and low
operating cost. In the following article, the project background and the overall plant
design will be described in detail, and operational data from the pilots and full-scale plant
will be discussed.
Focussing on environmental concerns
The new water treatment plant (WTP) at the Torrevaldaliga Nord Power Station, some 50
km north of Rome, is equipped with one of Europe's largest seawater UF systems. The plant
is part of a complete revamp of the existing power plant station. The power plant owner,
Italian's largest power and utility company ENEL, decided to replace four old oil-fired
systems with three new coal-fired units with a total capacity of 660 MW. The plant
concept, which focused on environmental, quality and innovative aspects, was rewarded
with the Power-Gen Award for Technical Innovation 2005.
The water treatment plant (WTP) was designed and built by Termomeccanica Ecologia
based in La Spezia, Italy. A UF pilot plant was operated in November/December 2006 to
verify and optimise the process parameters for the large scale plant. Construction work
started beginning of 2007, and the WTP was commissioned in June 2008.
Process design of the plant
The feed water is seawater from an open intake in the Tyrrhenian Sea. To minimise the
risk of oil being present in the feed water, the inlet point is located upstream of the
prevailing current, at a distance to the harbour where oil and coal is unloaded for the
power plant. Despite this, some oil and grease may be present in the feed water
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so a dissolved air floatation unit (DAF) is installed. A welcome side effect of this is the
overall improvement of the feed water quality it entails. The unit is designed for operation
without any coagulant and operated in this way at present - however, ferric chloride can
be used in principle.
After the DAF, the water is purified by UF, and the UF filtrate is further treated by UV
before it enters the two-pass RO plant. Some 5,600 m 3 /d RO permeate from the second
pass is then treated by a mixed ion bed unit for use as boiler feed water, while another
3,100 m3 /h RO permeate from the first pass is used for other technical services. A process
overview is given in Figure 1.
Benefits of UF modules
The 372 dizzer® 5000 modules in the UF system are manufactured by ingle
watertechnologies, a German company specialising in UF. The decision to install inge
UF dizzer modules was due to a large extent on the additional operational safety along
with the commercial benefits for the OEM and the enduser by implementing inge UF
technology. The technology provided by the company includes two unique and innovative
features designed to meet the specific challenges of the UF process - fibre integrity and
Hydro dynamically optimised module design.
Fibre integrity
inge's dizzer modules are equipped with patented Multibore® UF hollow fibres operating in
in-to-out mode. Multibore fibres are made from modified polyether sulphone with a high
pH tolerance from 1 - 13 which allows efficient cleanings even under extreme conditions.
The inner diameter of each individual capillary is 0.9 mm, which is larger than the common
0.7 - 0.8 mm size, which increases the suspended particles limit. They have a larger
diameter which can reduce the pressure drop and produce an even flow distribution along
the fibre. This in turn helps distribute contaminants more evenly on the membrane surface
and increases the backwash efficiency. The nominal pore size of the membranes is 0.020
mm, which provides for an effective rejection of bacteria and viruses.
The Multibore fibre combines seven capillaries into one fibre (see Figure 2) which results
in an exceptionally high mechanical fibre strength. This can overcome the major problem
facing UF hollow fibre technology, which is that there can be a rather large number of
fibre breakages during operation. In municipal applications, fibre breaks have to be
repaired immediately because of the bacteria and viruses that can enter the filtrate water.
In industrial applications, the RO membranes will be fouled quicker and UF will not fulfil
its capabilities. No no fibre breakages have been reported in more than six years of
operation in hundreds of plants that use Multibore fibres. The benefits for the plant and
the enduser are:
• reduced maintenance and repair cost;
• increased protection and performance of treatment steps downstream;
• increased operating safety.
Hydrodynamically optimised module design
The internal design of the dizzer module has an annular gap instead of the conventional
central core tube which is commonly used to collect the filtrate and for backwashing. This
design improves backwash efficiency by providing an even flow distribution along the
module cross section and ensuring that all fibres are backwashed with the same intensity.
During backwash, the water flows from the annular gap towards the middle of the module.
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On its way, the total backwash water flow is reduced because apart of the water enters
the fibres for the actual backwash. Since the number of fibres also decreases towarcls the
middle of the module, the backwash flux will remain at a similar level over the complete
cross section of the module. This results in a more efficient cleaning and subsequently in
higher permeability and less chemical consumption.
The modules are mounted vertically for proper de-aeration to avoid water hammers. This
arrangement makes it easier to access each module in case of maintenance.
How the system works
The UF system, designed for a total net output of 23,500 m 3 /d, consists of six trains, each
equipped with 62 dizzer 5000 UF-modules. The modules are mounted in a vertical position
with two rows of modules on either side of the central headers (Figure 3). The UF system
was originally designed for five trains in operation plus one train on stand-by, but to avoid
fouling in the stand-by train and to avoid increased wear from starting-up and shuttingdown
it was decided to operate all six trains permanently at lower than design flux. The
design flux can still be reachedif one train is shut-down for maintenance.
Each train is equipped with one feed pump which is controlled by a flow transmitter in
the feed line for individual flow control. Next to the feed pump is a self cleaning filter
with a nominal filtration rate of 300 micron. The purpose of the filter is to protect the UF
fibres from potentially harmful objects - such as swarfs. For the backwash three 50%
pumps are installed, and both feed and backwash pumps are equipped with a variable
frequency controlled motor.
The UF process
UF is operated in dead end mode with a design flux of 79 l/m 2 h with five trains in
operation. This figure originates from the tender, where a maximum net flux of 60 I/m 2 h
with all trains in operation was specified. During filtration mode, the feed water enters
the modules from either from top or from bottom, and the feed directions are swapped
after each backwash. With these alternating flow directions. The total fibre length can be
used more efficiently when compared to a design in which the feed water enters the
module continuously from the same end. In this way, build-up of an irreversible fouling
layer on the membrane surface can be avoided or delayed.
The operating philosophy of the UF system is to keep the transmembrane pressure (TMP),
the main indicator of membrane fouling, at a continuously low level close to the initial 0.1
to 0.2 bar of a new module. In this way, irreversible fouling and extensive CIP cleanings
can be avoided to a large extent. To clean the membranes a backwash is carried out every
45 minutes, and to control organic fouling, a chemical enhanced backwash using sodium
hypochlorite with approximately 20 ppm residual chlorine and a soak time of 10 minutes
takes place once a day. An additional chemical enhanced backwash using hydrochloric acid
is carried out depending on membrane condition to remove any inorganic material carried
over from the DAF.
For additional operational safety the plant is equipped with connections for optional
inline coagulation upstream of the UF to further reduce organic material and improve
the backwash efficiency. Currently no inline coagulation is applied.
Running (operating) a pilot plant
A containerised pilot plant with one dizzer 5000 module was set up after the order award
to demonstrate the functionality and operability of the modules and verify the operational
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data proposed in inge's bid. The company also wanted to optimise its UF operating
parameters for the large scale plant.
The pilot tests started in November 2006 and took four weeks. The water used, taken
from the collecting basin for the cooling tower make-up water was actually worse than
that conceived in the design basis, because it came from near the coast and from the
surface. Construction work in the harbour basin had also caused increased turbidity
levels. Since the power plant was shut down, water in the collecting basin was not running
and had developed a poor microbiological quality. Upstream of the UF system, a small
flotation system had been installed to which no coagulants were added. Considering all the
above, therefore, it could be said that the UF feed water quality was significantly worse
than that conceived of in the design.
During the first three days, the pilot plant operated at different fluxes of 60, 75, 90 and
100 l/m 2 h to get a feeling for how the plant worked with the given feed water. After this
time, the flux was set to 80 l/m 2 h for the rest of the pilot period. The pilot plant could be
operated at a stable flux of 80 l/m 2 h. Regular backwashes were sufficient to fully recover
the membrane permeability. The two daily chemical enhanced backwashes with sodium
hypochlorite did not have any noticeable cleaning effect. However, in the long run the
chlorine backwashes certainly enhance the cleaning efficiency and, therefore, are applied
in the large scale plant as well. Only evaluation over a long term period will help to
optimise plant operation.
Throughout the pilot period all SDI15 measurements showed values of below 2.
Installation of the full scale plant
Commissioning of the WTP started in June 2008, while the UF system started up at end of
July and went on stream at beginning in the first week of August 2008. Because of the
ongoing construction work at the power plant, there had been a reduced demand for
process water and so only half of the WTP is now in operation. Currently, three out of six
UF lines are operating and delivering purified water to two out of four RO lines. To prevent
biological growth in the three UF lines not in operation, the racks are filled with water
containing 0.1 % sodium bi-sulphite which is being replaced on regular intervals.
From start-up, the three operating UF lines have been run at design settings. Flux,
filtration cycle time, backwashes and chemical enhanced backwashes are performed in
accordance with the original design values.
The graphs in Figure 4 showing the main operating parameters of UF line A since start-up
are representative for the other two operating UF lines as well. The system has been
continuously operated at a flux of 79 1/m 2 h with the exception of a short period where the
capacity had to be reduced due to maintenance at the power plant. The transmembrane
pressure stayed very stable at around 0.3 bar in spite of reduced water temperatures in
the winter months - probably because of reduced biological activity causing less fouling.
The normalised permeability has been very stable at around 250 l/m 2 hbar recently rising
up to 360 l/m 2 h, with recovery higher than 93%.
Above operating parameters are particularly interesting against the background of an
average SDI 5 value of around 15 in the feed water which is beyond the design parameters.
On the other hand, the filtrate water shows a continuously good water quality with an
SDI15 below two almost all the time delivering excellent water quality to the reverse
osmosis system. This very stable operation of the UF system could potentially lead to the
extension of filtration cycles and reducing chemical consumption to reduce OPEX costs.
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Since commissioning, the UF system has been operated under very stable conditions
meeting all projected parameters in spite of challenging feed water quality.
Contact
Ralf Krüger
inge watertechnologies
Tel: +498192997721
rkrueger@inge.ag
www.inge.ag
Acknowledgements
The author gratefully acknowledges the support of Andrea Pagliari, Andrea Solito and
Roberto Landi from Termomeccanica Ecologia in making available the information and data
of the plant.
Coarse
Filter
DAF Buffer
Tank
NaOCl Coagulant Coagulant
Optional
Antiscalant
Figure 1: Simplified flow diagram of the Torrevaldaliga Nord Water treatment plant.
Figure 2: Cross section of a Multibore ® fibre.
Strainer UF Filtrate /
Backwash Tank
NaOCl Hcl
UV RO 1st Pass Decarbonation RO 2nd Pass
Mixed Bed IX
Technical
Services
Sodium
Bisulfite
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Figure 3:The first ultrafiltration skid.
Figure 4: Average data of full scale plant since start-up.
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