4
4.1
TECHNICAL ASPECTS OF DESALINATION PLANT
DESALINATION TECHNOLOGIES
Desalination technologies can be classified according to the process involved, for example:
a)
Those involving phase change in water, as in distillation, freeze separation, and hydrate
separation;
b)
Those utilizing surface properties of membranes, as in electrodialysis and reverse osmosis
c)
Those utilizing ion-selective properties of solids and liquids, as in ion exchange and solvent
extraction.
Another common classification of desalination plant is the one which involves the various forms
of energy required namely heat, mechanical, electrical or chemical.
A further classification is one which distinguishes between the withdrawal of water from
solution, as is exemplified by the comparison of any distillation process with electrodialysis.
Within the scope of this project, the promoter intends to install a reverse osmosis process
desalinator.
4.2
REVERSE OSMOSIS PROCESS
In the reverse osmosis process, water is made to pass from the more concentrated solution to a
less concentrated one, which is the reverse of the principle of osmosis. The force necessary to
accomplish this is the application of pressure greater than the osmotic pressure of the saline
solution.
If a saline solution is in contact with a semi-permeable membrane which is placed under
pressure being in excess of its osmotic pressure, water from the solution will flow through the
membrane. Water flow will continue till the pressure created by the osmotic head equals the
osmotic pressure of the salt solution.
Acetate membranes have proved most successful to be used for this purpose. Membranes are
not perfectly semi-permeable as they allow certain quantities of ions to cross through the
membranes. The salt content in the water produced can be controlled by reducing the pressure
or increasing the number of filtrations.
4.2.1 Advantages of Reverse Osmosis Process
The main attraction of this process is its low energy consumption. The energy required for
operating the process increases with feed water salinity. The technical difficulties include
fabrication, degree of semi-permeability, fouling, membrane supports and energy recovery
associated with the membranes.
4.2.2 Characteristics of Reverse Osmosis Process
The characteristics of the reverse osmosis desalination process are summarized in Table 4-1
below.
Table 4-1: Characteristics of Reverse Osmosis Process
Advantages
Disadvantages
 Suitable for both sea and brackish water
 Low quality (250-500ppm)
 Flexibility in water quantity and quality
 Requires high quality feed water
 Low power requirements compared with
other desalination processes
 Relatively high capital and operating
costs
 Flexibility in site location
 High pressure requirements
 Flexibility in operation start-up and shut off
 Long construction time for large plants
 Simple operation
Despite the disadvantages, nevertheless reverse osmosis process has been ranked as the best
desalination technology when performance is evaluated in terms of economic, technical and
environmental aspects.
4.3
COMPONENTS OF DESALINATION PLANT
The proposed desalination plant will have a nominal capacity of 300m3/day to cater for the
hotel’s daily domestic requirements and will include:
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2 No sand filters, upstream of the R.O apparatus
1 No. Cartridge filter
1 No desalinator of capacity 300m3/day – model Pro 2700 Series
1 No high pressure pump
1 No energy recovery unit with associated booster pump and accessories
1 No re-mineralisation filter
1No Inlet borehole at 200m from the HWM, and 43metres from the stream which cuts
across the site. The borehole is adjacent to the RO plant room.
1No Rejection soakaway at 32m from the HWM, at the western boundary of the hotel
site.
1 No CIP system
1 No antiscalant dosing
1 No NaOCl dosing
The desalination plant will be located as shown on the Massing Plan enclosed at Annex 4A at
the end of Section 4.
The Technical sheet for the proposed desalination plant is enclosed at Annex 4B at the end of
Section 4.
The basic simplified flowchart of the desalination plant is shown below in Figure 4-1.
High Pressure
Pump
Brackish Feed Water
Membrane
Assembly
Pre-Treatment
Post-Treatment
By-product Brine
Stabilized Water
Figure 4-1: Basic Process Flow Diagram
4.3.1 Production Capacity of Desalination Plant
The plant will be rated to produce a treated water volume of the order of 300m 3/day. This
production is roughly equivalent to 1day demand of the hotel.
The water produced will feed the existing domestic water tanks which are adjacent to the
proposed desalination plant.
Disinfection of the stored water will be given consideration as part of the pipework exiting the
tank. UV disinfection is a clean and safe alternative to chlorine, and is being explored.
The RO plant will be demand-based. The service reservoir will be fitted with float switches.
When the level in the service reservoir drops to a pre-determined level, the RO unit will start
automatically and begin producing potable water. It will keep running until the reservoir upper
level switch is reached, when it will automatically switch off again. The tank will be equipped
with an outlet valve and associated piping for draining and cleaning the reservoir, as required.
4.4
FEATURES OF DESALINATION PLANT
As already mentioned, the desalination plant will be of the reverse osmosis type and will make
use of latest technology.
The desalination plant will be made-up of high quality materials (such as high quality stainless
steel resistant to corrosion). The model to be installed at the Maritim Hotel will be PERMAQ Pro
2700 manufactured by Best Water Technology (BWT) HOH Water Technology Group of Greve,
Denmark.
At this point in the project’s procurement cycle, it is envisaged that the supplier will be BWT
HOH Water Technology Group of Denmark, a Danish company with over 39 years experience in
the design, manufacture, production, installation and commissioning of desalination plant in
numerous countries worldwide including Maldives and Seychelles in the Indian Ocean. The
firm has been established in Seychelles for over 12 years and specializes in high-tech solutions
for tourist industry applications, and produces ready-to-use Reverse Osmosis desalination
plants.
A typical desalination kit is shown in the photographs below.
Photograph 4-1: Ready-To-Use R.O Plant
Photograph 4-2: Typical Containerised RO Plant
The desalination plant system will be equipped with all the appropriate sensors and security
devices to stop the production of desalinated water in case of any failure; hence, discharge from
the RO plant will automatically stop if salinity level of the outgoing water was sensed to be
above maximum acceptable level.
To ensure removal of brine solution from membranes and also avoid any standstill corrosion of
piping material, high pressure pump and other components, a “clean-in-place (CIP) system” is
integrated in RO-container and it will enable flushing with fresh water at any plant stop.
In case of necessity of cleaning the membranes with chemicals, the respective solution is
normally prepared in a dedicated tank and re-circulated through the membranes to restore
their desalination capacity.
4.5
DESCRIPTION OF PREFILTRATION FILTER UNITS
The proposed prefiltration units will consist of two components:
(i)
2No sand filters, followed by
(ii)
1No Cartridge Filter
Sand Filters
Multimedia pressure filters – type HS will be used as sand filters. The Multimedia pressure
filters can be as single or as twin units. The twin units will be installed as part of the prefiltration system. The filter bed consists mainly of the following layers of quartz:
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High density fine quartz sand
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Low density coarse anthracite
The sand filter will filter the bulk of impurities from the raw water coming from the intake
borehole through the upper course anthracite layer and the fine particles/impurities through
the underlying fine quartz sand.
Full technical details on the multimedia pressure filters are enclosed on the technical sheet
enclosed at Annex 4C at the end of Section 4.
4.6
REVERSE OSMOSIS – PERMAQ PRO 2700
4.6.1 Introduction
The reverse osmosis which will be installed at Maritim Hotel is a compact and containerised
model. The RO model PERMAQ Pro 2700 is of very high performance and is equipped with
energy and water savings devices through the energy recovery system.
The RO system is equipped with a PLC (programmable logic controller) control unit with LCD
touchscreen and graphical display for easy viewing of important measurements and detailed
status information for the benefit of the maintenance and operation staff. A USB mode can also
be installed to enable simple and easy transfer of data an operation history to and Excel Sheet.
4.6.2 Features and Functions of RO
The main features and functions of the RO – PERMAQ Pro 2700 are listed below:
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Robust and compact construction of stainless steel tripod
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4.7
PLC control system with touch-screen
Control information via the LCD touch-screen graphical
Display of plant operations
Flow transmitter to permeate
Flow transmitter for concentrate
Alarm panel
Time and quality rinse, ensuring improved water quality and protection of membranes
Low noise
Recovery system
CIP (Clean-In-Place) unit
Anti-scaling device
CONTROL PANEL
A control panel will be delivered with the plant so as to control and monitor all the operating
and security functions of the plant. The control panel includes the following components:
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4.8
PLC-system (programmable logic controller) for data processing
Process control system
Main interruptor
Central visual alarm
All necessary contactors, fuses, status lights/displays
A busbar for power cable connection
CLEAN-IN-PLACE UNIT
The CIP-system (“Clean-In-Place”) is integrated in the RO-container and allows flushing with
fresh water at any plant stop. This ensures removal of brine solution from the membranes and
also avoids any stand still corrosion of piping material, high pressure pump and other
components.
4.9
INTAKE BOREHOLE
The feeding seawater will be pumped from an intake borehole located within the hotel
premises adjacent to the desalination plant. The intake borehole is at about 200 metres from
the High Water Mark as shown on the Massing Plan enclosed at Annex 4A at the end of Section
4.
The borehole has been drilled by Messrs Water Research Co. Ltd, who possesses considerable
years of experience in the field. The borehole has been drilled to a depth of 60 m below ground
level and in 225mm diameter. The borehole is lined with a PVC lining of a diameter of 200mm.
The PVC lining is plain from ground level (0m) to 6m below ground level and slotted between
6m to 48m below ground level.
After drilling, an air lift has been carried out during 1 hour in order to clean and develop the
borehole, following which a pumping step - test was carried out at different pumping rates and
a long term pumping test over 24 hours, to determine the maximum aquifer recharge rate and
associated draw-down. Finally, a physical and chemical test of the seawater has been
undertaken to determine the water quality during the pumping test.
The borehole will be equipped with a submersible pump placed at a depth of approximately 59
metres within the 60m deep borehole. The submersible pump will directly the RO plant via a
150mm HDPE pipeline connected to it.
Brackish water will be pumped directly from the intake borehole at a rate of 31.25m3/hour (i.e.
750m3/day) at a pressure of about 3 bar for the desalination process. A raw water storage tank
is not recommended, in order to inhibit possible biological activity and water contamination.
A copy of the report for the borehole drilling and associated pumping tests carried out at the
locus site is enclosed at Annex 4D at the end of this Section.
4.10
BRINE PRODUCTION
The output of the desalinator which can also be referred to as the by-product of the
desalination plant process will be a concentrated salt solution having a salinity level of about
5100ppm as determined by the mass balance calculation detailed below. This concentrated
brine solution which will be discharged from the desalination plant at a rate of 21m3/hour
(5.8litres/second) during the operation of the plant will be pumped at the required pressure and
flow rate to the rejection soakaway located at about 32metres from the HWM at the western
boundary of the hotel’s compound.
Mass Balance Diagram
Feed Intake
Borehole
Desalinator
3
31.25 m /hr at 3.2 ppt
Storage reservoir
3
12.5m /h at 0.4 ppt
3
18.75 m /hr Brine at Salinity S2
Calculations
31.25 m 3/hr x 3.2 ppt = 18.75 m 3/hr x S2 (ppt) + 12.5 m 3/hr x 0.4ppt
31.25 x 3.2
= 18.75S2 + 12.5 x 0.4
112
= 18.75S2 + 5
S2
= 5.1 ppt (i.e. 5100 ppm)
Hence from above calculation, it can be seen that the brine salinity will be 5100 ppm.
Consequently, the salinity of the rejected brine (5100 ppm), being below the ambient salinity of
the seawater in the lagoon (33,325ppm during low tide and 33,575ppm at high tide), there is no
need for any dilution of the rejected brine.
4.10.1 Brine Disposal
The brine with a salinity level of 5100ppm will be discharged into a rejection soakaway/tank
situated at about 32metres from the High Water Mark as shown on the Massing Plan enclosed
at Annex 4A at the end of Section 4.
As mentioned in sub-section 3.12 “Lagoon Water Quality”, the average salinity of the lagoonal
water at low tide and high tide is 33,325ppm and 33,575ppm respectively. Accordingly, the
brine with a salinity level of 5100ppm emanating from the desalination process need not be
diluted prior to discharge to the sea through the rejection soakaway.
Further, the marine flora and fauna will NOT therefore be adversely impacted by the rejection
of brine in the lagoon.
4.10.2 Dimensioning of Rejection Soakaway/ Dispersion Tank
The dimensioning of the rejection/dispersion tank has been based on the following calculations:
 Volume/discharge of diluted brine from dilution tank…........................18.75m3/hr
 Percolation rate of sandy soil matrix (use lowest conservative figure)……..500mm/hr
 Area of application (L x B) + D (2L + 2B) where:
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4.11
L is Length of tank
B is breadth of tank
D is depth of tank
Adopting L: 3m, B: 3m and D: 3m
Percolation capacity: [(3x3) + 3((2x3) + (2x3))] x 0.50m/hr = 45 x 0.50 = 22.5m3/hr
Hence greater than in-coming volume of diluted brine of 18.75 m3/hr.
Hence, conclusion that the design of the rejection soakaway is adequate.
SUMMARY OF TECHNICAL FEATURES
The technical features of the desalination plant are summarized as below:
Feeding seawater temperature…………………..............
25 0C
Feeding silt density index………….……………………
Max. 3
Feeding brackish water pH……………………………………
Approx.7.5
Feeding seawater Total Dissolved Solids (TDS)………
4,088 TDS
Pumping rate seawater from beach intake borehole……
31.25m3/hr, 3 bar
Hourly production of desalinated water………………..
12.5m3/hr
Daily production of desalinated water……….…………
300m3/d
Expected salinity of desalinated water……..…………
400ppm
Rate of brine effluent discharge………………….….......
18.75m3/hr
Expected brine salinity………………………………….
5066ppm
Expected brine temperature………………….…….……
Slightly higher than inlet
Expected brine pH………………………………………
≤7
Power required…………………………………………
11kWh/m3
Supply voltage………………………………………….
3 x 400V/ 50Hz
Overall dimensions of housing container (metres) ……
5.9 x 2.3 x 2.4
Expected salinity of final effluent ………………………
400ppm after post
treatment
4.12
DESALINATION PLANTROOM
The desalination plant room will in actual practice be a 20-foot container.
The 20-foot container will house the desalination plant components such as RO membrane kit,
the sand filters, the chlorine tank, the dosing pump and the inter-connecting pipeworks, the
PLC, etc.
The container will be provided with a sump from which any collected water on the floor will be
pumped out.
The various piping arrangements for the connection of the intake borehole to the desalination
unit will be designed by the project team during the Detailed Design Stage, namely:
1. The raw water supply pipeline from the beach borehole
2. The pure water pipeline from the desalinator to the existing potable water tank of the
hotel.
3. The CIP system pipework
4. Support plinths (concrete) for RO skid.
4.13
CLEANING PROCESSES
Flushing will take place using clean water at each stop of the RO equipment/membrane, using
the permeate collected in the Clean-In-Process (CIP) tank. The RO membranes will periodically
be cleaned with cleaning solutions to prevent salt accumulation and fouling. The duration and
frequency of such process will be determined through the monitoring of the pressure drop
across the membrane in-coming and out-going points.
Furthermore, in order to prevent bacterial growth on the membrane surface, the membranes
will periodically be regenerated using a diluted acid solution, and via the CIP system. Organic
acids have proven to be attacked by microbes and can contribute to degradation of the
membranes; for that reason this process has been selected.
Manual cleaning of the tank is required in order to mix the required chemical solution for the
CIP. From the tank, the solution will be pumped through the entire membrane system.
Circulation takes place for a few minutes, then the CIP tank and RO system will be flushed and
the water drained out of the system into a pre-fabricated holding tank – from which it will be
disposed of under controlled conditions.
If the plant has to stop for a prolonged period of more than 1 month, prior to stoppage of
operation, a regeneration process will be carried out after which the membranes will be soaked
and preserved in a more concentrated solution of acid in a proportion of 1.5kg in 50 litres of
water which will be drained in the same way as described above when the plant will be put back
in operation.
4.14
LIFESPAN AND COMPONENTS OF DESALINATION PLANT
The desalination plant will consist of components which will be made-up of non-corrosive
materials having a minimum estimated life span of 10 years.
The different components comprising the desalination network system are as follows:
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4.15
1No. set of fine cartridge filters – immediately upstream of the R.O apparatus
1No desalinator – model PERMAQ Pro 2700
1No re-mineralisation filter
SOLID WASTES
Brackish water pumped from the intake borehole will be pre-filtered through the geo-textile
cloth provided around the 200mm PVC lining in the intake borehole. Consequently the majority
of solid particles will be stopped at the well level while only fine silt or sludge particles are
expected to reach the sand filters positioned upstream of the RO.
Backwashing to clean the sand filters will then be done for not more than 5 to 10 minutes. The
backwash water will afterwards be discharged into the sewage treatment plant – which is
situated close to the RO plant.
4.16
DOCUMENTATION, TESTING AND COMMISSIONING
All the technical documentation including erection drawings and wiring diagrams will be
provided by the plant supplier.
The desalination equipment frame will be delivered entirely assembled in a 20-foot container,
ready to operate. The supplier will be responsible for the final testing and commissioning of the
entire desalination plant, at the site under the supervision of the Project’s M&E Engineer.
4.17
CHLORINATOR
It is imperative that the RO permeate be treated prior to sending it to the service storage
reservoir of the hotel via the connecting pipeline at the connection point shown on the
desalination plant pipe layout plan enclosed at Annex 4E at the end of Section 4.
A hypochlorite dosing station will be installed in the 20-foot Container, whose purpose will be to
disinfect the permeate water from the desalination plant in order to produce a chlorine residual
dosage – which will keep the water potable during its transit in the hotel’s main storage
reservoir.
4.18
INTAKE BOREHOLE
For the purpose of extracting seawater for the desalination process, a borehole has been drilled
at 200 metres from the HWM at a position indicated on the Massing Plan enclosed at Annex 4A
at the end of Section 4. The borehole has the following characteristics
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Depth: 60metres
Diameter: 200mm
Diameter of rising main: 150mm
Surrounding geo-textile to prevent ingress of sand particles
Partly solid and partly perforated PVC casing.
Submersible pump at the bottom of the borehole.
A typical arrangement of the borehole installation is reproduced at Annex 4F at the end of
Section 4.