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CHAPTER I
INTRODUCTION
1.1 GENERAL
Reverse Osmosis (RO) was invented in 1959 by Prof Reid of the University
of Florida, and was put into practical use by Sidney Loeb and Srinivasa
Sourirajan.
Reverse osmosis is a separation process that uses pressure to force a
solvent through a membrane that retains the solute on one side and
allows the pure solvent to pass to the other side. More formally, it is the
process of forcing a solvent from a region of high solute concentration
through a membrane to a region of low solute concentration by applying a
pressure in excess of the osmotic pressure. This is the reverse of the normal
osmosis process, which is the natural movement of solvent from an area of
low solute concentration, through a membrane, to an area of high solute
concentration when no external pressure is applied.
Figure 1.1 Reverse Osmosis Process
The membrane here is semi permeable, meaning it allows the passage of
solvent but not of solute. The membranes used for reverse osmosis have a
1
dense barrier layer in the polymer matrix where most separation occurs. In most
cases the membrane is designed to allow only water to pass through this dense
layer while preventing the passage of solutes (such as salt ions). This process
requires that a high pressure be exerted on the high concentration side of the
membrane.
Reverse Osmosis is the phenomenon of water flow through a semi
permeable membrane that blocks the transport of salts or other solutes through it.
It removes both dissolved organics and salts.
Reverse osmosis is used to reject bacteria, salts, sugars, proteins,
particles, dyes, and other constituents. Separation of ions with reverse osmosis is
aided by charged particles. This means that dissolved ions that carry a charge,
such as salts, are more likely to be rejected by the membrane .The larger the
charge and the particle, the more likely it will be rejected.
Figure 1.2 Reverse Osmosis Principle
Reverse Osmosis is for de-salination and purification of
brackish(other than sea water) and sea water for drinking and industrial use. This
can also be used for a variety of specialized membrane applications for chemical
recovery and waste water reclamation projects. The process achieves rejections
2
of 99.9% of viruses, bacteria and pyrogens. So now a days this reverse
osmosis is widely used in waste water treatment.
Figure 1.3 MOLECULE SIZE COMPARISON
1.2. CLEAN-IN-PLACE
CIP (Clean-in-Place) is a method of cleaning the interior surfaces of
pipes, vessels, process equipment, and associated fittings, without
disassembly . The closed systems were disassembled and cleaned manually.
The advent of CIP was a boon to industries that needed frequent internal
cleaning of their processes. Industries that rely heavily on CIP are those
requiring high levels of hygiene, and include: dairy, beverage, brewing, processed
3
foods, pharmaceutical, and cosmetics. The benefit to industries that use CIP is
that the cleaning is faster, less labour intensive and more repeatable, and poses
less of a chemical exposure risk to people. CIP started as a manual practice
involving a balance tank, centrifugal pump, and connection to the system being
cleaned. Since the 1950s, CIP has evolved to include fully automated systems
with programmable logic controllers, multiple balance tanks, sensors, valves, heat
exchangers, data acquisition and specially designed spray nozzle systems.
Simple, manually operated CIP systems can still be found in use today.
4
CHAPTER 2
LITERATURE REVIEW
1) Brackish water RO desalination plants installed by CSMCRI.
Base on the indigenous TFC membranes, CSMCRI has designed, fabricated and
installed several brackish water RO desalination plants having product water
capacities in the range 1000-5000 LPH to cater to the needs of rural community in
TN., Gujarat, W.B., and Rajasthan in the last few years. In this regard,
Department of Science and Technology, New Delhi has provided the financial
assistance while state Science & Technology Councils of different states have
provided the logistic support like site selection and infrastructure. The success of
RO desalination units has made considerable impact on decision making
authorities who are now more amenable to the idea of RO desalination of
providing water in problem villages. Before RO the water is sent to the cartridge
filter where 10 and 5 micron pore size is used. CSMCRI has also fabricated a
mobile desalination unit by mounting a small RO unit (500 LPH product water) on
a mobile van.
2) Characterization of foulants by autopsy of RO desalination membranes
F. H. Butt, F. Rahman and U. Baduruthamal
Research Institute, King Fahd University of Petroleum and Minerals, Box 1891,
Dhahran 31261, Saudi Arabia
Received 16 October 1996; accepted 21 July 1997.Available online 14 April 1998
A study was undertaken to identify various types of scales that were
responsible for shortening the useful life span of the membrane permeators in a 5
commercial reverse osmosis (RO) desalination plant. Compositions of the raw
and treated feed water and of the reject brine were determined using the
inductively coupled plasma (ICP) spectrometry and ion chromatography (IC).
Various scaling index calculations showed that the feed and brine were non-scale
forming with respect to CaCO3 (calcite), SrSO4, CaSO4.2H2O (gypsum), and silica
(SiO2). Two completely fouled membrane permeators, retired from stage 1 and
stage 2 of a commercial plant, were subjected to membrane autopsy using
scanning electron microscopy (SEM), X-ray diffractometry (XRD), optical
microscopy (OM), and energy dispersive x-ray florescence (XRF). The deposits
were predominantly amorphous in nature. The membrane autopsy showed that
CaCO3, SrSO4, and CaSO4.2H2O (gypsum) scales did not constitute a serious
problem in the plant. The advanced phosphonate+polyacrylate based scale
inhibitor had itself formed Ca phosphonate sludge, but the amount was quite
small. Though below saturation, silica is believed to have been precipitated due to
the catalyzing effect of trivalent Al3+ and Fe3+ ions. Iron fouling was the major
cause of reduced life span of the membranes and, to a lesser extent, calcium-
alumino-silicates
3) Fouling Development on Reverse Osmosis Membranes
Fouling is a major obstacle that prevents efficient
operation of reverse osmosis (RO) systems, causing deterioration of both the
quantity and quality of treated water, and consequently resulting in higher
treatment cost.
4) Chemical cleaning of reverse osmosis membranes
Fouling is the most important problem associated with the application of
membranes. A strategy for membrane regeneration is chemical cleaning of the
fouled membranes. One of the major applications of reverse osmosis
6
membranes is processing of water from different resources or for various
applications. This includes desalination or ion removal for makeup water for
boilers. In all cases fouling restricts membrane performance. In this work reverse
osmosis membranes were fouled with water. Chemical cleaning of the RO
membranes using acid, alkaline, surfactant and detergent solutions has been
discussed. Cleaning efficiency depends on the type of the cleaning agent and
its concentration. It has been shown that the efficiency increases with
increasing the concentration of the cleaning agent. Operating conditions such as
crossflow velocity, turbulence in the vicinity of the membrane surface,
temperature, pH and cleaning time also play a role in the cleaning process.
Optimum membrane cleaning requires in depth understanding of the
interactions between the foulants and the membrane as well as the
effect of the cleaning procedure on deposit removal and membrane
performance.
5) Membrane autopsy — a case study
L. Y. Dudley and E. G. Darton
Houseman Desalination Products, The Priory, Burnham, Slough SL1 7LS, UK
Received 31 May 1995; accepted 30 June 1995. Available online 11 December
1998.
This paper describes the authors' experiences of membrane autopsy
procedures to identify the cause of poor membrane performance at a European
power plant and the subsequent proposals made for improvement to the
operating, pretreatment and cleaning programmes. The reverse osmosis (RO)
system is a 2,500 m3/d unit producing water for cooling purposes. The 3-year-old
plant experienced continual output loss, which required the membranes to be
cleaned on a twice weekly basis, which increased to every 4 days in warm
weather. The autopsy objective was to carry out a destructive analysis on a fouled
membrane to identify the major causes of the fouling. These were identified as
7
being biological in nature with the significant presence of iron, which together
formed a biomass on the membrane surface.
CHAPTER 3
CONFIGUREURATIONS OF REVERSE OSMOSIS
3.1 GENERAL CONFIGUREURATIONS
3.1.1 FEED PRESSURE
The feed pressure is the most important parameter, which helps the
membrane to provide good efficiency. The pressure varies from 10kg/cm 2 to 28
kg/ cm 2. Usually 2–17 bar (30–250 psi) for fresh and brackish water, and 40–70
bar (600–1000 psi) for seawater, which has around 24 bar (350 psi) natural
osmotic pressure which must be overcome.
3.1.2 PERMEATE PRESSURE
The permeate pressure is the output pressure of the membrane, it will be
lower than the feed pressure.
But the pressure difference between feed pressure and permeate
pressure should be less than 4.
P = feed pressure – permeate pressure = less than 4
3.1.3. FLOW RATE
The waste water which should be treated through the membrane is let in to
the membrane input. That is known as flow rate. This flow depends upon the
production of the industry. It may vary.
8
3.1.4. PERMEATE FLOW
The out let flow from the membrane is called permeate flow. This will be
lesser than the feed flow because of the filtration process done inside the
membrane and some amount is rejected.
3. 1.5 DIRECTION OF FLOW OF WATER:
The direction of water inside the membrane is in two types. They are
1) Cross - flow membrane filtration
In cross flow filtration, the feed is passed across the filter membrane
(tangentially to the filter membrane) at some pressure difference. Material which
is smaller than the membrane pore size passes through the membrane as
permeate or filtrate, and everything else is retained on the feed side of the
membrane as retentive.
In this the feed water direction and reject water flow will be in same
direction. The permeate water direction will be perpendicular to that.
Most probably spiral wound membrane has this filtration.
9
Figure 3.1 CROSS-FLOW MEMBRANE FILTRATION
2) Dead – end membrane filtration
This is a filtration technique in which all the fluid passes through the
membrane, and all particles larger than the pore size of the membrane are
retained at its surface. This means that the trapped particles start to build
up a "filter cake" on the surface of the membrane, which has an impact on
the efficiency of the filtration process.
Figure 3.2 DEAD-END MEBRANE FILTRATION
3. 1.6 TYPES OF MEMBRANE BASED ON SOURCE OF WATER
1) Brackish water membrane
2) Sea water membrane
The sea water membrane is used for treating the sea water only
having high TDS vale(more than 3500 mg/lt).
10
So the brackish membrane is used for treating the effluents of
municipal and industrial waste water.
3. 1.7 MEMBRANE MATERIALS
1. CTA (cellulose triacetate or CA),
2. Polyamides (TFC – Thin Film Composite)
Figure 3.3 TFC- POLYAMIDE WITH CARBOXYLATE GROUPS
The membrane material refers to the substance from which the
membrane itself is made. Normally, the membrane material is manufactured
from a synthetic polymer, although other forms, including ceramic
and metallic “membranes,” may be available. Currently, almost all
membranes manufactured for drinking water production are made of
polymeric material, since they are significantly less expensive than
membranes constructed of other materials. The material properties of the
membrane may significantly impact the design and operation of the
filtration system. For example, membranes constructed of polymers that
react with oxidants commonly used in drinking water treatment should not
be used with chlorinated feed water. Mechanical strength is another
consideration, since a membrane with greater strength can withstand
11
larger transmembrane pressure (TMP) levels allowing for greater operational
flexibility and the use of higher pressures with pressure-based direct
integrity testing Similarly, a membrane with bi-directional strength may
allow cleaning operations or integrity testing to be performed from either
the feed or the filtrate side of the membrane. Material properties influence
the exclusion characteristic of a membrane as well. A membrane with a
particular surface charge may achieve enhanced removal of particulate
or microbial contaminants of the opposite surface charge due to electrostatic
attraction. In addition, a membrane can be characterized as being
hydrophilic (i.e., water attracting) or hydrophobic (i.e., water repelling). These
terms describe the ease with which membranes can be wetted, as well as
the propensity of the material to resist fouling to some degree. MF membranes
may be constructed from a wide variety of materials, including cellulose
acetate (CA), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN),
polypropylene (PP), polysulfone (PS), polyethersulfone (PES), or other polymers.
Each of these materials has different properties with respect to surface
charge, degree of hydrophobicity, pH and oxidant tolerance, strength, and
flexibility.
RO membranes are generally manufactured from cellulose acetate or
polyamide materials (and their respective derivatives), and there are various
advantages and disadvantages associated with each. While cellulose
membranes are susceptible to biodegradation and must be operated within
a relatively narrow pH range of about 4 to 8, they do have some resistance to
continuous low-level oxidant exposure. In general, for example, chlorine doses
of 0.5 mg/L or less may control biodegradation as well as biological fouling
without damaging the membrane. Polyamide (PA) membranes, by contrast, can
be used under a wide range of pH conditions and are not subject to
biodegradation. Although PA membranes have very limited tolerance for the
presence of strong oxidants, they are compatible with weaker oxidants such
as chloramines. PA membranes require significantly less pressure to operate and
have become the predominant material used for RO applications. A
characteristic that influences the performance of all membranes is the trans-
wall symmetry, a quality that describes the level of uniformity throughout the
12
cross-section of the membrane. There are three types of construction that are
commonly used in the production of membranes: symmetric, asymmetric
(including both skinned and graded density variations), and composite. Cross-
sectional diagrams of membranes with different trans-wall symmetry are material,
while composite membranes use different (i.e., heterogeneous) materials.
Symmetric membranes may be either homogeneous or heterogeneous. In a
symmetric membrane, the membrane is uniform in density or pore structure
throughout the cross-section, while in an asymmetric membrane there is a
change in the density of the membrane material across the cross sectional area.
Some asymmetric membranes have a graded construction, in which the porous
structure gradually decreases in density from the feed to the filtrate side of the
membrane. In other asymmetric membranes, there may be a distinct transition
between the dense filtration in layer (i.e., the skin) and the support structure. The
denser skinned layer is exposed to the feed water and acts as the primary
filtration barrier, while the thicker and more porous under structure serves
primarily as mechanical support. Some hollow fibres may be manufactured as
single- or double- skinned membranes, with the double skin providing filtration at
both the outer and inner walls of the fibres. Like the asymmetric skinned
membranes, composite membranes also have a thin, dense layer that serves as
the filtration barrier. However, in composite membranes the skin is a different
material than the porous substructure onto which it is cast. This surface layer is
designed to be thin so as to limit the resistance of the membrane to the flow of
water, which passes more freely through the porous substructure. NF and RO
membrane construction is typically either asymmetric or composite, while most
MF, UF, and MCF membranes are either symmetric or asymmetric
3. 1.8 TYPES OF MEMBRANES MODULE:
Membranes are never applied as one flat plate, because this large
surface often results in high investing costs. That is why systems are built densely
to enable a large membrane surface to be put in the smallest possible volume.
13
3. 1.8.1 TUBULAR MEMBRANES
Tubular membranes are not self-supporting membranes. They are located
on the inside of a tube, made of a special kind of material. This material is the
supporting layer for the membrane. Because the location of tubular membranes is
inside a tube, the flow in a tubular membrane is usually inside out. The main
cause for this is that the attachment of the membrane to the supporting layer is
very weak.
Tubular membranes have a diameter of about 5 to 15 mm. Because of the
size of the membrane surface, plugging of tubular membranes is not likely to
occur. A drawback of tubular membranes is that the packing density is low, which
results in high prises per module
.
Figure 3.4 Tubular membranes
14
3. 1.8.2 CAPILLARY MEMBRANES
With capillary membranes the membrane serves as a selective barrier,
which is sufficiently strong to resist filtration pressures. Because of this, the flow
through capillary membranes can be both inside out and outside in.
The diameter of capillary membranes is much smaller than that of tubular
membranes, namely 0.5 to 5 mm. Because of the smaller diameter the chances
plugging are much higher with a capillary membrane. A benefit is that the packing
density is much greater.
Figure 3.5 Capillary membranes
3. 1.8.3 HOLLOW FIBRE MEMBRANES
Hollow fibre membranes are membranes with a diameter of below 0.1 µm.
consequentially, the chances of plugging of a hollow fibre membrane are very
high. The membranes can only be used for the treatment of water with a low
15
suspended solids content. The packing density of a hollow fibre membrane is very
high. Hollow fibre membranes are nearly always used merely for nanofiltration
and Reverse Osmosis (RO). These membranes are mostly used for gas
separation and filtration.
Outside diameter: 0.5 – 2.0 mm Inside diameter: 0.3 – 1.0 mm Fiber wall thickness: 0.1 – 0.6 mm Fiber length: 1 – 2 meters
Figure 3.6 HOLLOW FIBRE MEMBRANE
3. 1.8.4 SPIRAL WOUND MEMBRANES
The design of spiral wound elements contains two layers of membrane
glued back to back onto a permeate collector fabric(permeate channel spacer).16
This membrane envelope is wrapped around a permeate empties from the
permeate channel spacer.
A plastic netting into the device and maintains the feed stream channel
spacing. It also promotes mixing of the feed stream to minimize concentration
polarization.
Spiral membranes consist of two layers of membrane, placed onto a
permeate collector fabric. This membrane envelope is wrapped around a centrally
placed permeate drain (see picture below). This causes the packing density of the
membranes to be higher. The feed channel is placed at moderate height, to
prevent plugging of the membrane unit.Spiral membranes are only used for nano
filtration and Reverse Osmosis (RO) applications.
Figure 3.7 Spiral wound membrane module
17
3. 1.8.5 DISK MEMBRANE MODULE
Figure 3.8 DISK MEMBRANE MODULE
3. 1.8.6 PLATE MEMBRANE MODULE
These type of membrane will be in flat.
Figure 3.9 PLATE MEMBRANE MODULE
3. 1.8.7 PILLOW-SHAPED MEMBRANES
Membranes that consist of flat plates are called pillow-shaped membranes.
The name pillow-shaped membrane comes from the pillow-like shape that two
18
membranes have when they are packed together in a membrane unit. Inside the
‘pillow’ is a supporting plate, which attends solidity. Within a module, multiple
pillows are placed with a certain distance between them, which depends on the
dissolved solids content of the wastewater. The water flows through the
membranes inside out. When treatment is done, the permeate is collected in the
space between the membranes, where it is carried away through drains.
3. 1.9 MEMBRANE SYSTEMS
The choice for a certain kind of membrane filtration system is determined
by a great number of aspects, such as the benefits of membrane filtration , costs,
risks of plugging of the membranes, packing density and cleaning opportunities.
The arrangement of the RO membranes in the vessels are termed as membrane
system arrangement.
Generally system of RO is many types .They are
1) Single module system:
Single module system retreats its own reject water along with feed water.
Figure 3.10 SINGLE MODULE SYSTEM
19
2) Single array system:
In this system the permeate and reject are collected separately and
the reject is not again taken as feed and treated.
Figure 3.11. SINGLE ARRAY SYSTEM
2) Multi array system
This system is twice of single array system, which takes first
system’s concentrate as feed for second system
20
Figure 3.12 MULTI ARRAY SYSTEM
4) Permeate staged system:
This system is differed from others, here first membrane’s permeate
is used as feed water to second membrane module and second membrane’s
concentrate is mixed with first membrane’s feed water.
Figure 3.13 PERMEATE STAGED SYSTEM
3. 1.10 SYSTEM COMPONENTS:
21
1) High pressure pump.
2) Pressure vessel
3) Shut down switches
4) D.C.S. system(control room)
5) Control instruments.
6) Tanks
7) Pressure gauges.
3. 1.11 SAMPLING METHODS
The two most common approaches for sampling are the grab and
composite methods.
1) Grab sampling involves the collection of one or more aliquots from the feed or
filtrate stream,
2) composite sampling involves collection of the entire process stream for
processing and subsequent analysis.
3. 1.12 FACTORS INFLUENCING R.O. PERFORMANCE
1) Flow rate
2) TDS value
3) Decrease in permeate flow and pressure
4) Scaling
5) Fouling (major problem caused)
6) Recovery of permeate
22
Table 3.1 FACTORS INFLUENCING MEMBRANE PERFORMANCE
INCREASING PERMEATE FLOW SALT PASSAGE
Effective pressure Increase Decrease
Temperature Increase Increase
Recovery Decrease Increase
Feed salt Decrease Increase
FOULING
Foreign materials which may be present in the feed water such as
hydrates of metal oxides, calcium precipitates, organic and biological mater. This
includes all kind of build–up of layers on the membrane surfaces. Inner surface of
the feed line tubing and the feed end scroll of the membrane element if it is
reddish brown, fouling by iron material may be considered.
Three kinds of fouling that reduces membrane performance
A membrane treatment system can be fouled by virtually anything present
in the water being fed to the unit. However, in common treatment systems such
as reverse osmosis, the fouling materials may be generally categorized as
inorganic, organic, or biological.
Inorganic compounds that cause fouling of membrane modules include
inorganic salts with low solubility. These may enter the treatment system in
particle form, or they may precipitate inside the system as a result of
concentration changes occurring in the feed water as permeate is recovered
through the membrane. The highest concentration of dissolved solids happens to
23
occur immediately adjacent to the surface of the membrane in the treatment
module.
If the feed water contains salts of low solubility, it is likely that these salts
will precipitate on the surface of the membrane to form scale. Salts such as
calcium carbonate (CaCO3) and calcium sulfate (CaSO4) are common in most
feed waters. Other salts such as barium sulfate (BaSO4), strontium sulfate
(SrSO4), and calcium fluoride (CaF2) also may be in solution. In many feed water
sources these salts are present at or near their solubility limits and will precipitate
as the concentration of the feed water increases in the system. Although this
precipitation can be controlled with proper pretreatment, fouling due to these salts
does occur frequently because of operator error or unknown changes in feed
water quality.
Metal hydroxides are other inorganic compounds that cause fouling. The
most common culprits are iron hydroxide, (Fe(OH3)) and aluminum hydroxide,
(Al(OH3)). As in the case of inorganic salts, these hydroxides may enter the
system as suspended particles or they may form inside the system. Unlike the
inorganic salts however, metal hydroxides do not deposit a hard crystalline scale
but rather a soft, gelatinous layer.
Clay, silt and other silica-based materials can cause fouling if the particles
are not removed in the pretreatment equipment located in the process train ahead
of the membrane treatment system. In some feed water sources clay occurs as
very finely divided (1 to 5 micron) particles. These small colloidal particles can be
very difficult to remove with conventional equipment. Silica may also enter the
membrane system in the dissolved or reactive form. This low molecular form of
silica will polymerize as the feed water concentration increases at the surface of
the membrane. The resulting solid silica deposit on the membrane can be
extremely difficult, if not impossible, to remove.
Organic compounds make up the second category of fouling materials. Surface
water sources like rivers and lakes may contain naturally occurring organics, for
example humic acids. Clarified water may contain residual polymers, and
24
wastewater influents may contain any number of organic compounds. The
mechanism behind organic fouling depends upon the size and chemical nature of
the specific substance causing fouling. High molecular weight compounds may
act more as particles and can plug the feed spacer in the membrane element.
This plugging may be worsened if inorganic particles, such as clays and metal
hydroxides, also are present.
Figure 3.14 FOULED MEMBRANE
Low molecular weight organics may foul the surface of the membrane
through chemical interaction. As an example, chlorinated phenols will adhere to
the surface of an RO membrane by means of hydrogen bonding. In this situation,
a small concentration of the chlorinated phenol in the feed water can cause a
large loss of flux in the treatment system.
Biological organisms also are troublesome because of their tendency to foul
membrane surfaces. Although they are technically organic, biological organisms
demand special consideration. In terms of fouling, the concern is primarily with
single cell organisms. These include bacteria, algae and fungi. Of these, bacteria
25
cause the majority of problems in membrane water treatment systems for a
variety of reasons.
First, many types of bacteria can adapt to the environment inside the
membrane modules. Unfortunately, a great number of these species are found in
typical feed waters, particularly water from a surface source, such as a river or
lake.
Second, since the bacteria are rejected by the membrane, they end up on
its surface. While their presence there is bad enough, their food, consisting of
organic matter, also is being concentrated at the membrane surface. When
bacteria are placed in a liveable environment with sufficient food, they multiply
rapidly. This means that even more bacteria end up on the membrane surface.
Finally, bacteria have a number of defence mechanisms. Several have
small hair like appendages, called fimbriae, which stick out from all sides of the
cell. These allow the bacteria to attach themselves, and remain attached, to the
surface of the membrane or to the feed spacers. In addition, bacteria secrete a
mucous capsule, or slime, which coats the cell and protects them from any harsh
elements entering their environment.
TABLE 3.2. THICKNESS OF FOULANT VS EFFICIENCY LOSS
Thickness of foulant materials on
membrane surface
Percentage loss of efficiency
0.4 mm 4%
0.8mm 8%
3. 1.13 BENEFITS OF MEMBRANE FILTRATION
• Osmosis offers a unique advantage that it is a process that can take place while
26
temperatures are low. Therefore, this enables the treatment of heat-sensitive
matter. That is why these applications are widely used for food production.
• It is a process that does not require much energy and thus, energy costs are
low. The process just requires energy to pump liquids through the membrane.
This is far too low when compared to the total amount of energy required for a
technique such as evaporation.
• The process can easily be expanded.
• Process management of membrane filtration systems is simple.
Membrane filtration systems can be managed in both dead-end flow as
well as cross-flow. The purpose of the optimization of the membrane techniques
is the achievement of the highest possible production for a long period of time,
with acceptable pollution levels.
27
CHAPTER 4
CONFIGURATIONS OF MFL RO PLANT
4.1 GENERAL
The project work was done at Madras Fertilizers Limited in Manali, Chennai.
M.F.L. was incorporated on dec. 8, 1966 as a joint venture between GOI
AND AMOCO India incorporated of U.S.A. (AMOCO) in accordance with the
fertilizer formation agreement executed on 14.5.1966 with equity contribution of
51% and 49% respectively.
About 1000 employees are at present working in the company.
The company has a very good relationship with public and farmers. So the
feed back given by the formers are updated at every moment
The company produces the following products:
1. UREA
2. NPK-complex
3. NK-mixture
4. MOP-IMPORTED
5. DAP-IMPORTED
28
4.2 WATER NEEDS OF MFL
Water is vital material for the production of fertilizers. Approximately
630 m3/hr of water is processed every day at R.O. Plant. The potable water is
supplied by Chennai Metro Water Supply and Sewage Board(CMWSSB).
The boiler and feed water for fertilizer production is taken from the treated
sewage water from MFL’s treatment plants.
CMWSSB supplies water for fertilizer production. Due to increase in
population and scarcity at summer seasons the supply of raw water was
restricted.
In order to eliminate raw water shortage and to reduce the cost of raw
water the Tertiary treatment plant and R.O. plants were installed in the year
1993/94.
The sewage water is taken from Kodungaiyur sewage treatment plant.
Owned by CMWSSB and supplied to M.F.L. Tertiary treatment plant and R.O.
plant. Which reduces the raw water cost/consumption.
After 1995 the full need of water is being supplied by R.O. Plant.
The primary treated sewage water is being bought for Rs. 9/m3
(approximately) and taken to tertiary treatment plant. After tertiary treatment of the
sewage the cost is Rs. 56/m3 (approximately).Then the T.T.P. water is supplied
to R.O. plant.
The R.O. plant production is then taken to the Demineralisation plant and
stored and then to the boilers as feed water.
29
Figure 4.1 REVERSE OSMOSIS PLANT DIAGRAM
1.Pilot plant 2. R.F. sump 3. Roughing filters 4. Filtered water storage tank
5. Chlorine retention tank 6. Dual media filters 7. Cartridge filters 8. Hp
pumps 9. Membrane stack 10. Blended storage tank 11.Settlement tank
12.Backwash tank13.Chlorine storage tank &Injection tank 14.Chemical
dosing tank 5.Digital control system room 16.Tertiary treated water storage
pond
30
4.3 DETAILS OF RO PLANT IN MFL
4.3.1 MULTI ARRAY SYSTEM.
In MFL multiple array system is being used. Three trains operates in
parallel, each designed to produce 120 m3/hr of permeate. Each of the three
trains will be a three staged system with the reject from each stage forming the
feed of the following stage.
The product water from the three stages are combined to form the train
product(permeate) stream.
1) 3 trains
2) Each have 4 banks
3) Each banks are named as A, B,C,D.
4) A,B,C banks have 10 vessels each and D bank has 7 vessels.
5) Each vessel houses 6 membranes connected in series (8”
diameter and 1m long, RO membranes).
Table 4.1 TRAIN AND ITS COMPONENTS
BANK No. OF VESSELS No. OF MEMBRANES
A1& A2 5+5=10 60
B1& B2 5+5=10 60
C1& C2 5+5=10 60
D 7 42
Total No. Of membranes 222
31
Figure 4.2 MULTIPLE ARRAY SYSTEM AND BANK ARRANGEMENT
Each trains are designed to operate with a permeate recovery of 75% and at this
recovery it is anticipated to give the efficiency of 95% at ambient temperature.
32
4.3.2. SPECIFICATIONS OF R.O. PLANT
Feed water supply : 160 m3/hr/train.
Permeate water flow : 120 m3/hr/train.
Reject water flow : 40 m3/hr/train.
Feed water pressure : 16 kg/cm2 g.
P : less than 4.
R.O. membrane material type: T.F.C Spiral wounded brackish water membrane.
Size of a membrane : 8” dia.
: 40” long.
Feed temperature : not more than 40 o c
Percentage recovery : 75 % (Permeate)
Percentage recovery = permeate flow x 100/ feed flow
Dimensions of pressure : 200 mm dia.
vessels : 6 m long.
MOC of pressure vessels : GRP.
Thin film composite R.O. membrane give excellent performance for a wide
variety of application including low pressure tap water use, single – pass sea
water & brackish water desalination, chemical processing and waste
treatment. This membrane exhibits excellent performance in terms of flux, salt
rejection and microbial resistance.
R.O. element can operate over a pH range of 2 to 11, are resistant to
compaction and are suitable for temperature upto 45 o c.
Salt rejection 99.5% and flux – 24 l/m2hr
33
4.3.3 PROBLEMS FACED BY R.O. PLANT
The main problem is caused by fouling and scaling. Because the sewage
water contains more organic matter, microbes and dissolved organic solids.
These will accumulate on the surface of the semi permeable membrane.
The foreign materials which may be present in the feed water such as
hydrates of metal oxides, calcium precipitates, organic and biological matters.The
term includes the build up of all kinds of layers on the membrane surfaces,
including scaling.
Inner surface of the feed line tubing and the feed end scroll of the
membrane element, if it is reddish brown fouling by iron content.
Biological fouling (or) organic material is often shinny or gelatinous.
Due to the increase in accumulation of these particles day by day the
efficiency of the membrane will be slowly decreased. After some years the
efficiency will be very much low. The pressure difference ( P ) will be
increased to 4 and above. It leads to failure of membrane by wear and tear of
brine seal and membrane.
Figure 4.3. FOULED MEMBRANE PHOTO(unrolled)
34
4.3.4 . MAINTENANCE OF RO ELEMENT
The R.O. element are frequently washed by chemicals, so as to keep the
membrane surfaces clean and face of deposits.
Table: 4.2 RO WASH PROCEDURES
Chemical
concentration
Quantity of
permeate
Solution
concentration for
circulation
Wash
sequence Duration
HCl 33% 4000 lts 0.5% ACD 30 min.
HCl 33% 4000 lts 0.5% BCD 30 min.
HCl 33% 4000 lts 0.5% A 4 hrs
HCl 33% 4000 lts 0.5% B 4 hrs
HCl 33% 4000 lts 0.5% C 4 hrs
HCl 33% 4000 lts 0.5% D 4 hrs
HCHO 37% 1700 lts 1.0%(60 kgs) ACD 8 hrs
HCHO 37% 1700 lts 1.0%(60 kgs) BCD 8 hrs
All banks are flushed with permeate
SLS(85 %)+
EDTA(98%)
1700 lts 0.2%(3.5 kgs+
3.5kgs)
ACD/BCD30 min
each
do do do A 2hrs
do do do B 2hrs
do do do C 2hrs
do do do D 2hrs
SLS + EDTA Soaking 12 hrs
35
Chemical
concentration
Quantity of
permeate
Solution
concentration for
circulation
Wash
sequenceDuration
NaOH (10-11 pH ) 500 lts A 30 min
NaOH (10-11 pH ) 500 lts B 30 min
NaOH (10-11 pH ) 500 lts C 30 min
NaOH (10-11 pH ) 500 lts D 30 min
Total wash hours 80 hrs
.
The R.O. elements are washed when any one of the following conditions arrives:
1) Permeate flow reduction
2) P change
3) Increase in Salt passage
Acid wash:
0.5% HCl is being used to wash the membrane for the acid wash.
Alkaline wash:
NaOH is used for alkaline wash.
Acid wash is desirable for removing organic and inorganic salts like
CaCO3, CaSO4 and BaSO4.
Alkaline wash is desirable for removing silica, biofilms and organic matters.
These leads to foul the membrane. These results with low efficiency, recovery
change and increase in P, decrease in salt passage.
36
CHAPTER 5
MATERIALS AND METHODS
5.1 SOURCE
After identifying the solution the same membrane was performed to treat
the tertiary treated waste water.
5.2 SAMPLE COLLECTION AND ANALYSIS
The inlet water and out let water of the membrane were taken. These
samples were analysed at Laboratory in Madras Fertilizers Limited, Manali.
5.3 ANALYSIS OF THE VARIUOS PARAMETERS
The following parameters of the samples were analysed.
5.3.1 COLOUR:
After the collection of the sample the colour of the sample was noted.
5.3.2 pH Value:
The pH value was found by pH meter.
5.3.3 CHLORIDES:
Procedure:
100 ml of sample was taken in two flasks, 1 ml of potassium chromate
indicator is added in each flask. The liquid is literate with N 35.5 silver
nitrate solution from burette drop with constant stirring unit there is a
37
permanent change from yellow to brick red. The volume of titrant is recoded as
‘A’ml. The above procedure is repeated for distilled water to obtain ‘B’ ml.
Quantity of chlorides present
in the sample = ((A-B)XNX35450)/Volume of sample mg/l.
N : Normality of AgNO 3
5.3.4 SULPHATES:
Procedure:
20 ml of the sample was taken in a beaker. 10 ml of 2N HCl
acid was added to it and heated till it was boiled. At the time of boiling 30
ml of barium chloride solution was added. The solution is filtered in the
beaker through wattman after No. 42. The barium Chloride react with
sulphate present in the sample in the presence of HCl and barium sulphate
was allowed to settle. The filtered sulphate was taken in the weighed
crucible and was placed in the muffle furnace till got charged. Then it was
weighed (W2). The difference between the weight of empty crucible and weight
W2 gives the amount of sulphate present in the sample .
mg of residue X molecular weight of BaSO4
Amount of sulphate =
Volume of samples
5.3.5 TOTAL DISSOLVED SOLIDS:
Total Dissolved Solids
20 ml of the sample was filtered through No. 42 wattman filter paper and it
was cooled in a weighed crucible. It is heated in the water bath and evaporated to
dryness in the oven for one hour, then the container was weighed and the
38
increased weight was noted down. The increased weight is known as dissolved
solids present in the 20 ml of sample.
mg of residue present in the filter paper
Total Dissolved solids = X1000 in mg/l
ml of sample taken
5.3.6 TOTAL HARDNESS:
Procedure
Standardisation of EDTA with standard Hardwater
20 ml of standard hard water was taken in a clean conical flask. 5 ml of
ammonia buffer solution was added and a pinch of Erichrome Black–T indicator
was added and it was titrated against EDTA solution which was taken in the
burette. At the end point wine red colour changes to blue. The volume was noted
(V1).
Estimation of hardness
20 ml of sample was taken in a clean conical flask. 5 ml of ammonia buffer
solution and a pinch of Erichrome black – T were added and titrated against
EDTA solution. The end point was just the change from wine red to blue colour.
The volume was noted (V2).
1ml of standard hard water = 1 mg of CaCO3
20 ml of standard hard water = V1ml of EDTA
1 ml of EDTA = 20/V1 mg of CaCO3
20 ml of sample water = V2 ml of EDTA
Therefore total hardness of sample = (20 X V2) / (V1X 20) X 1000 mg/l.
39
5.3.7 CALCIUM HARDNESS:
Take 50 ml sample add 1N NaOH solution followed by pattern reading
indicator then titrate vs EDTA solution.
End point : pink to blue colour.
Calcium Hardness = Titrant volume X Normality of EDTA X equivalent weight
volume of sample taken
5.3.8. LOSS OF IGNITION (LOI):
Take 1 gram sample make it to dry at 105o c. After drying keep the
crucible at 500 o c in muffle furnace for half an hour. Weight difference is the
loss of ignition.
LOI = weight difference X 100
weight of the sample
5.3.9. TOTAL IRON :
Take definite volume of sample add conc. HCl followed by Hydroxyl
Ammonium Hydrochloride solution. Reduce the volume to 5 ml by drying over hot
plate. Then cool it add Ammonium Acetate buffer followed by 1,10-
orthophenonthroline indicator. Then take the absorption at 510 nm, run a
standard and blank along with the sample.
Take the absorbance using spectrophotometer.
Total Iron = Sample OD X Std. Conc. X 1000
40
(Std. OD X Volume of sample taken)
5.3.10. PHOSPHATES:
Take the definite volume of sample add Conc. HNO3 and H2 SO4 then
keep it for fuming. After fuming over, cool it then add phenolphthalein followed
by sodium hydroxide solution. Then add Ammonium Molybdenum solution
followed by stannous chloride solution. Take the absorbance at 690 nm, in
spectrophotometer. Run a standard and blank along with the sample.
Phosphate = Sample OD X Std. Conc. X 1000
( Std. OD X Volume of sample taken)
5.3.11. SILICA:
If the sample is turbid filter it through wattman 42 filter paper take
the definite volume of filtered sample add 1:1 HCl and Ammonium Molybdate
solution followed by oralic acid. Take the Absorbance at 410nm using
Spectrophotometer. Run a blank and standard along with the sample.
Silica = Sample OD X Std. Conc. X 1000
(Std. OD X Volume of sample taken)
5.3.12. TURBIDITY:
Calibrate the Nephelo turbidity metre using standard solution. Now
shake the sample well and find out the turbidity using Nepheloturbidity metre.
41
5.3.13. OIL AND GREASE:
Take a definite volume of sample ( in a separate funnel) add small
volume of concentrate HCl followed by Petroleum Ether(W1). Shake well. Discard
the bottom layer. Collect the upper layer and dry it in a weighed beaker (W2).
Weight difference ( W1- W2) is the oil and grease.
42
CHAPTER 6
RESULTS AND DISCUSSION
6.1 MEMBRANE AUTOPSY
6.1.1. MEMBRANE SELECTION
Old used membrane was selected in wet condition. Its last performance
was noted. The efficiency of the old membrane was less than 70%.The same
membrane was opened. Then it was unsealed and the brine seal removed.
Then it was unrolled carefully and one membrane sheet was selected
to cut.
Figure 6.1 MEMBRANE SELECTION
It was marked 0.5 m x0.5 m area in one membrane sheet.
In the same way the other membrane sheets were marked. They
were cut as the mark. They were spreaded over a white sheet and
43
then the membrane was scrapped without damaging the membrane
surface. The sample collection was grab sample.
Membrane specifications:
Pore size of the membrane : 0.01 µm.
Feed rate : 8 m3 / hr
Diameter of the membrane : 8 inches.
Length of the membrane : 40 inches.
Type of membrane : Brackish water membrane(spiral wound).
Feed pressure : 16 kg /cm 2 g.
Percentage recovery : 75% (permeate)
Membrane maintenance washes:
The membrane is washed by the following chemicals, when ever there is
a change in p (feed pressure – permeate pressure), efficiency, flow rate,
recovery and time period.
1) 0.5% HCl wash
2) 1.0% HCHO wash
3) 0.2% (SLS+ EDTA) wash
4) NaOH (10-11 pH ) wash
The efficiency of the membrane will be slowly decreased through out the
operation period due to accumulation of particles over the membrane surface.
After some period there will be no change in efficiency, between pre and post
wash of these chemicals mentioned above. This may occur inbetween the life
time of the membrane mentioned by the manufacturer or after the life time. This
depends on the pre-treatment and solids presented in the feed water of reverse
44
osmosis membrane. So it is considered as the membrane should be discarded
and disposed or removed from the vessel and new membrane shall be installed.
. Figure 6.2 PHOTO OF UNROLLED MEMBRANE
45
Figure 6.3 PHOTO OF FOULED MEMBRANE
The Figure. 6.3 shows the film of the deposited particles. The scrapped materials
were collected from membrane surface in wet condition and they were analysed.
TABLE 6.1 INGREDIENTS OF SAMPLE (SCRAPPED MATERIAL)
Sl.
No.PARAMETERS RESULTS(mg/l)
1 Total hardness as CaCO3 352
2 Calcium hardness as CaCO3 264
3 Loss Of Ignition(LOI) 99.26%
4 Total Iron 10.10
5 Phosphates 7.18
6 Sulphates 795
7 Silica 45.0
8 Chlorides 556
9 Total Dissolved Solids 5300
10 Turbidity(NTU) 30.0
11 Total Organic Compounds 440
46
As per the results, the Loss Of Ignition is 99.26%. So the organic compound
materials are high as foulant materials which were deposited throughout the
period of membrane, which were not washed out by ordinary wash.
6.1.2 METHODOLOGY
The membrane wash was done by 0.5% of HCl. By overcoming this the
deposition and accumulation on the membrane seen. Another used old
membrane was selected which was same batch and same efficiency while
disposed. So now 0.6%, 0.75%, 1.0% HCl solutions were tried over the old
membrane. 0.6 & 0.75% of HCl gives no improvement in removing the
depositions. 1.0% HCl (washed for 5 min) removes the deposition easily without
any stress. The analysis of 1% HCl washed solution is given below.
TABLE 6.2 HCL WASH SAMPLE
Sl. No. Parameters Results(mg/l)
1 Total hardness as CaCO3 220
2 Calcium hardness as CaCO3 120
3 Loss Of Ignition 96%
4 Total Iron 9.77
5 Phosphates 1.77
6 Sulphates 285
7 Silica 40.0
8 Chlorides 920
9 Total Dissolved Solids 1440
10 Sodium EDTA 1983
11 Total Organic Compounds 360
12 SLS(Sodium Laurel Sulphate) 1525
47
13 Oil & grease 10.0
6.2 RESULTS:
After the autopsy and washing by 1 % HCl the same
membrane’s removal efficiency was checked. The 75% recovery of permeate
water was set.
The collected samples were tested to find the amount of
pollutants available in both feed and permeate water of reverse osmosis
membrane. So the efficiency of the membrane could be easily found.
Table 6.3 MEMBRANE PERFORMANCE AT STARTING STAGE.
Sl.
No.Parameters Feed mg/l Permeate mg/l
Efficiency
%
1 Na 420 56 86
2 Ca 64 12.8 83.2
3 Cl 620.3 75.4 86.3
4 TDS 1348.5 210 84.4
5 pH 7.4 7.3 -
6 Total hardness 340 60.7 82.3
7 Sulphates 230 17.5 76.08
48
8 Total iron 13.4 2.7 79.8
The overall efficiency was 82.58 %.
Then the membrane was experimented for more than 100hrs and the performance and efficiency was found out. It is tabulated below.
Table 6.4 MEMBRANE PERFORMANCE AFTER 100hrs
Sl.
No.Parameters Feed mg/l
Permeate
mg/l
Efficiency
%
1 Na 385 60.45 84.3
2 Ca 72.3 11.56 83
3 Cl 630.5 87.0 86.2
4 TDS 1403.5 227.36 83.8
5 pH 7.2 7.1 -
6 Total hardness 365.3 55.8 84.7
7 Sulphates 276 60.16 78.2
8 Total iron 12.7 2.6162 79.4
Now the overall efficiency was 82.9%.
So from the above results, the efficiency was maintained throughout
the experimented time period. So 1 % HCl wash may be adopted followed by high
pH and detergent washes.
49
CHAPTER 7
CONCLUSION
From the above project study it can be seen that the major foulants
are iron and fouling. More over Loss Of Ignition (LOI)of pre and post
cleaning is found to be more than 95% which clearly indicates the presents of
fouling to a greater extent.
Owing to the reason the source of feed water is treated sewage,
hence it is concluded that one of the methods of membrane maintenance
is to adopt membrane cleaning with 1% HCl using Cleaning In Place(CIP)
followed by high pH and detergent washes.
50
BIBILIOGRAPHY
1.N.Manivasakam, “Industrial Effluents Origin, Characteristics, Effects, Analysis
and Teatment”, 1987.
2.Evaluation of Membrane Processes and their Role in Wastewater Reclamation,
Final Report of Contract for US Department of interior OWRT,David Argo and
Martin Rigby, November 30,1981.
3. “Analysis of Water and Waste Water”,BIS Publication (1993),New Delhi
4. Wesley Eckenfelder Jr. W (2000), “Industrial Water Pollution Control” Tata
McGraw Hill Publishing Co.Ltd, New Delhi.
5. Metcalf and Eddy (1979), “Waste Water Engineering Treatment and Disposal”.
McGraw Hill publishing Co.Ltd, New Delhi.
6. Nemerow N.L (1978), “Industrial Water Pollution”, Wesely Publishing Company
Inc., USA.
7. Control of Fouling of Reverse Osmosis Membranes When Operating on
51
polluted Surface Water, J.E.beckman, E.Bevage, J.ECurver, I.Nusbaum, and
S.S.Kremen, Office of Saline Water Report CA-10488, Gulf Environmental
System, February 1971.
8. M.N.Rao and A.K.Dutta (1995), “Waste Water Treatment”. Oxford I.B.H
Publishers.
9. Evaluation of Membrane Processes and their Role in Wastewater
Reclamation, Final Report of Contract for US Department of interior
OWRT,David Argo and Martin Rigby, November 30,1981.
10. Syed R.Qasim, Waste Water Treatment Plants, Planning, Designing and
operation”.
11. S.K.Garg (1986), Volume – I, “Environmental Engineering and Pollution
Control”, Khanna Publisher, New Delhi.
12. Study and Experiments in Waste water Reclamation by Reverse osmosis, I.
Nusbaum, J. H.Sleigh and S.S.Kremen, Water pollution Research Series
17040-05/70 (1970).
13. Design Study of Reverse Osmosis pilot Plant, D.T.Bray and H.F.Menzel,
Office of Saline Water, Research and Development Progress Report No.176
(1966).
14. Reverse Osmosis-Producers for Replacing Elements, Mr. Jacy Choi, Journal
of Environmental Science and Engineering, 2005.
52
15. M.Wilf, “New generation of Low Pressure High Salt Rejection membranes”,
Proceedings of the 1996 Biennial Conference and Exposition, Monterey,
California (August 1996).
53