U.S. patent application number 14/601788 was filed with the patent office on 2015-05-14 for low energy reverse osmosis process.
The applicant listed for this patent is Aquatech International Corporation. Invention is credited to Ravi Chidambaran.
Application Number | 20150129495 14/601788 |
Document ID | / |
Family ID | 49483818 |
Filed Date | 2015-05-14 |
United States Patent
Application |
20150129495 |
Kind Code |
A1 |
Chidambaran; Ravi |
May 14, 2015 |
LOW ENERGY REVERSE OSMOSIS PROCESS
Abstract
We provide a system and method for reverse osmosis treatment of
water, including seawater and brackish water. Methods and systems
of embodiments of the invention may include, for example,
ultrafiltration followed by biofoulant removal, both of which
precede reverse osmosis. In preferred embodiments the system is run
at a low flux. For example, a flux of 6-8 GFD may be used with
seawater. Additional embodiments may provide the above process in
conjunction with a reverse-osmosis membrane cleaning system. The
membrane cleaning system is a "clean in place" system that includes
use of the natural pressure differential in the reverse osmosis
system to remove biofoulants and their precursors.
Inventors: |
Chidambaran; Ravi;
(Canonsburg, PA) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Aquatech International Corporation |
Canonsburg |
PA |
US |
|
|
Family ID: |
49483818 |
Appl. No.: |
14/601788 |
Filed: |
January 21, 2015 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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13989939 |
Nov 22, 2013 |
8980100 |
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PCT/US2013/037744 |
Apr 23, 2013 |
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14601788 |
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61636930 |
Apr 23, 2012 |
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Current U.S.
Class: |
210/636 ;
210/638; 210/650 |
Current CPC
Class: |
B01D 2321/16 20130101;
B01D 61/58 20130101; C02F 1/444 20130101; B01D 65/06 20130101; C02F
2103/08 20130101; Y02A 20/131 20180101; B01D 2321/22 20130101; B01D
2321/06 20130101; C02F 1/48 20130101; B01D 65/02 20130101; B01D
61/025 20130101; C02F 1/24 20130101; C02F 1/42 20130101; C02F
2303/10 20130101; C02F 2303/16 20130101; Y02W 10/30 20150501; C02F
1/46 20130101; B01D 2321/04 20130101; C02F 2303/20 20130101; C02F
1/441 20130101; C02F 9/00 20130101; B01D 61/145 20130101 |
Class at
Publication: |
210/636 ;
210/650; 210/638 |
International
Class: |
C02F 1/44 20060101
C02F001/44; B01D 65/02 20060101 B01D065/02; C02F 1/42 20060101
C02F001/42 |
Claims
1-20. (canceled)
21. A method for removal of biofoulants from water, comprising:
filtering water through an ultrafiltration membrane, thereby
removing turbidity including bacteria and viruses from the water;
filtering the water for biofoulant removal, thereby removing
biofoulants, additional bacteria, and additional viruses from the
water, wherein the step of filtering the water through an
ultrafiltration membrane precedes the step of filtering the water
for biofoulant removal.
22. The method of claim 21, wherein the removal of biofoulants
occurs prior to a membrane purification of the water, and wherein
the removal of biofoulants decreases fouling of at least one
membrane in the membrane purification.
23. The method of claim 21, wherein said ultrafiltration membrane
reduces SDI of the feed stream to less than 3-5 SDI.
24. The method of claim 21, wherein said ultrafiltration membrane
reduces turbidity of the feed stream to less than 0.1.
25. The method of claim 21, wherein said biofoulant removal step
reduces turbidity to less than 0.08.
26. The method of claim 21, wherein said removal of biofoulants is
conducted using a member of the group consisting of ion exchange
materials, positively charged media, electrochemical removal, and
electrode-based removal.
27. The method of claim 21, wherein said cleaning is conducted
based on a preset increase in differential pressure or numbers of
hours of operation of the reverse osmosis membrane.
28. The method of claim 21, wherein said method excludes addition
of chlorine upstream of the UF membrane, and wherein the method may
include addition of chlorine during a backwash to avoid ingress of
chlorine in feed water to be desalinated.
29. A method for removal of impurities from water, comprising:
filtering water through an ultrafiltration membrane, thereby
removing turbidity including bacteria and viruses from said feed
stream; treating the water to remove biofoulants, additional
bacteria, and additional viruses from the water, wherein the step
of filtering the water through an ultrafiltration membrane is
conducted prior to the step of treating the water to remove
biofoulants, additional bacteria, and additional viruses from the
water; and treating the water by reverse osmosis, wherein the
reverse osmosis is conducted at an operating flux, wherein the
operating flux results in an energy consumption, and wherein
conducting the reverse osmosis at a flux lower than the operating
flux does not reduce the energy consumption by more than 10%; and
cleaning at least one reverse osmosis membrane used in treating the
water by reverse osmosis, comprising, for a reverse osmosis
membrane having a feed side and a permeate side: maintaining a
concentration differential by adding reverse osmosis concentrate to
the feed side of the reverse osmosis membrane, thereby allowing the
reverse osmosis concentrate to remain in the feed side and causing
a flow of permeate water from the permeate side to the feed side
due to the concentration differential.
30. The method of claim 29, wherein said removal of biofoulants is
conducted using a member of the group consisting of ion exchange
materials, positively charged media, electrochemical removal, and
electrode-based removal.
31. The method of claim 29, wherein said cleaning is conducted
based on a preset increase in differential pressure or numbers of
hours of operation of the reverse osmosis membrane.
32. The method of claim 29, wherein said method excludes addition
of chlorine upstream of the UF membrane, and wherein the method may
include addition of chlorine during a backwash to avoid ingress of
chlorine in feed water to be desalinated.
33. A method for in situ cleaning of a reverse osmosis membrane for
treating water, comprising, for a reverse osmosis membrane having a
feed side and a permeate side: maintaining a concentration
differential by adding reverse osmosis concentrate to the feed side
of the reverse osmosis membrane; allowing the reverse osmosis
concentrate to remain in the feed side; and causing a flow of
permeate water from the permeate side to the feed side due to the
concentration differential, thereby cleaning the reverse osmosis
membrane.
34. A method for regenerating a biofoulant filter, comprising
treating the biofoulant filter with a cleaning method selected from
the group consisting of chemical cleaning and electrochemical
cleaning.
35. The method of claim 34, wherein the cleaning method is chemical
cleaning.
36. The method of claim 34, wherein the cleaning method is
electrochemical cleaning.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application claims priority to U.S. Provisional Patent
Application No. 61/636,930, filed on Apr. 23, 2012, and
incorporated by reference herein.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] Embodiments of the invention relate to methods, systems, and
processes for desalination using reverse osmosis.
[0004] 2. Background of the Related Art
[0005] Water desalination is growing to meet industrial and
drinking water demand worldwide. Although both thermal desalination
(multi effect distillation or "MED", and multi stage flash
evaporation or "MSF") and membrane based seawater reverse osmosis
("SWRO") processes are used in these plants, SWRO has grown
predominantly over the last 15-20 years. SWRO has become very cost
effective and efficient in terms of energy consumption as compared
to where the technology was few years ago.
[0006] In conjunction with the ascendance of SWRO, there have been
several developments related to low energy membranes and energy
recovery devices designed to reduce energy consumption. At the same
time, energy costs have been increasing more steeply, and there is
a continuous need for reduced energy consumption in SWRO plants to
offset the energy costs and maintain cost of water. This challenge
is mostly experienced with seawater plants due to higher energy
consumption, but there has been elevated attention on brackish
water plants as well, due to significant increases in energy costs.
These energy costs are further exacerbated by escalation of energy
costs due to fouling problems during plant operation.
[0007] One challenge with plant management is that once the plant
has been designed for some energy consumption, the plant's energy
consumption does not remain steady and consistent once the water
production starts. This may be due to several reasons, but
predominantly it is because of fouling, scaling or membrane
compaction. Out of these three, scaling may be the biggest
contributor to energy consumption in brackish water, but fouling is
the biggest cause of energy consumption in seawater and surface
water-based RO plants. Moreover, due to heavy emphasis on recycling
and reuse of water, it has become typical to design RO brackish
water plants at as high as 97-98% recovery, which makes the fouling
and scaling problems much more challenging. Sometimes the water
itself may not be scaling but due to initiation that has already
happened due to some other reasons scaling salts may start
precipitating.
[0008] Another serious problem that RO plants encounter is
bio-fouling, which reduces productivity of water, increases the
differential pressure and increases power consumption. This problem
is compounded in plants where there are open intakes and where
water temperature increases during summer. Chlorine treatment makes
this worse due to formation of oxidized products, which provide
potent feed for the residual bacteria right on the membrane surface
where they are rejected along with the bacteria after the
de-chlorination process. Chlorination typically cannot be
considered as a sustainable process option to control bio fouling,
because the balance bacteria left after chlorination multiply much
faster after de-chlorination with the potent nutrients as food for
bacteria. Therefore it is not prudent to depend on chlorination to
control bio-fouling on membranes. Moreover chlorinated organic
products may be undesirable due to formation of carcinogens.
Alternative techniques to control, minimize or eliminate
bio-fouling are of significant interest.
[0009] Other chemical approaches like biocide treatment have found
limited success and are too expensive. There have been several
approaches which plants have adopted by optimizing chlorination and
de-chlorination dosing, their locations and frequency including
shock chlorination in the pretreatment section. These approaches
have improved productivity and reduced the magnitude of this
problem but have not provided a sustainable solution for plant
productivity and power consumption efficiency. So there is a need
to improve bio-fouling performance of SWRO and surface water and
recycle reuse RO plants. Bio-fouling increases the power
consumption so a low energy membrane design cannot work alone
without a comprehensive approach on bio fouling control.
[0010] In an effort to maintain healthy operational efficiency in
terms of water production and energy consumption, membranes should
be kept in clean condition with minimum differential pressure
across membranes. As the differential pressure increases it becomes
difficult to clean the membranes and regain the original
performance when the membrane was in its clean condition. It is
also known that with higher differential pressure, permeate quality
deteriorates. Beyond a point, cleaning conditions become much more
aggressive and cleaning chemicals must be used for a longer time to
reestablish clean membrane performance. As a matter of fact, some
part of the fouling becomes irreversible and permanent. Many
chemical cleanings are not practical to perform under aggressive
conditions because membranes lose performance. Moreover disposal of
cleaning chemicals need elaborate treatment and neutralization,
which consumes additional chemicals.
BRIEF SUMMARY OF THE INVENTION
[0011] We present a novel RO desalination process that focuses on
achieving low energy consumption, at least in part by reducing
biofouling on the membrane through process design, and integrating
a cleaning methodology that prevents buildup of any residual
biofilm on the membrane surface. To achieve sustainable lower
energy consumption it is important to ensure the membranes do not
foul and the differential pressure does not increase. A cleaning
methodology should be available to clean membranes in a very
initial phase of bio-fouling formation, before it impacts
differential pressure, and before any fouling becomes permanent and
starts impacting plant performance in terms of water production,
power consumption and product quality.
[0012] Our process offers a number of advantages. Where typical
processes encounter substantial biofouling when flux is reduced,
our combination of ultrafiltration and biofoulant removers at low
flux operation reduces both the amount and severity of biofouling
and energy consumption. The combination of these features provides
a unique low energy and low fouling process. Further, by decreasing
the severity of the biofouling, we are able to provide an effective
low-pressure differential osmotic cleaning mechanism that is
low-cost, low-chemical, and effective, and that makes sure
sustained low energy operation can be continued.
BRIEF DESCRIPTION OF THE FIGURES
[0013] FIG. 1 shows a block flow diagram of a low energy SWRO
process of one embodiment of the invention.
[0014] FIG. 2 shows a comparative graph of RO unit pressure drop
performance under various conditions with and without additional
pretreatment and osmotic cleaning.
[0015] FIG. 3 shows an RO Unit performance graph when operated with
ultrafiltration ("UF") only.
[0016] FIG. 4 shows an RO Unit performance graph when operated with
UF and a biofoulant removal unit.
[0017] FIG. 5 shows RO Unit Performance with a new RO membrane with
an embodiment of the devised process and osmotic cleaning.
[0018] FIG. 6 shows a graph of flux vs. feed pressure.
[0019] FIG. 7 shows a graph of flux vs. power.
[0020] FIG. 8 shows RO feed water temperature vs. feed
pressure.
[0021] FIG. 9 shows RO feed water temperature vs. permeate total
dissolved solids.
DETAILED DESCRIPTION OF THE INVENTION
[0022] We present a novel RO desalination process that focuses on
achieving low energy consumption by reducing biofouling by design
on the membrane surface and integrating a cleaning methodology that
prevents buildup of any residual biofilm on the membrane surface.
This is made possible by the following an innovative process
approach that may include one or more of the following aspects.
[0023] Ultrafiltration.
[0024] Typical embodiments include an ultrafiltration pretreatment
step. Ultrafiltration membranes may give more than 6 log reduction
of bacteria and 1-2 log reduction of virus load when used to treat
typical seawater or brackish water.
[0025] In a preferred embodiment the ultrafiltration membrane will
have a molecular weight cutoff of approximately 100,000 and a
membrane pore size less than 0.1 micron. More preferably the
membrane pore size is between 0.02 and 0.05 micron. The
ultrafiltration permeate provides a silt density index ("SDI") of
less than 3, and very often between 1-2.
[0026] UF is able to remove majority of the colloidal particles,
which are positively charged in nature. It also removes some
biofoulants, but it is not able to remove all contaminants that may
cause biofouling on membranes.
[0027] To calibrate the performance of UF, the UF should receive
water of turbidity around 5-8 Nephelometric Turbidity Units
("NTU"), preferably 6-7 NTU. Optional treatment of water upstream
of UF may be designed to achieve these parameters. This may be
accomplished by one of skill in the art with the benefit of this
disclosure and based on water analysis and site conditions. This
level of inlet turbidity will deliver a product quality of UF
permeate to around 0.06-0.08 NTU and SDI values of less than 3. If
the UF performance is not calibrated, there will be excessive load
on the downstream system, and it will not perform at a preferred
level. Use of feed water at this level of turbidity also ensures
the downstream system is not going to experience any colloidal load
of positively charged particles which would soak up capacity that
should be made available to remove leftover charged
biofoulants.
[0028] Biofoulant Removal.
[0029] Further treatment happens through a biofoulant removal step.
This step removes a majority of nutrients that are potential
biofilm formers. This includes, for example, humic acids,
polysaccharides, proteins, amino acids, carbohydrates, bacteria,
viruses, and other potential bio film formers. Although
ultra-filtration membranes provide the filtered water properties
mentioned above, UF does not reduce all types of TOC. Since the
pre-filtered water goes through the ultrafiltration membranes
before going through the biofoulant removal filter, the biofoulant
removal filter will deliver large quantities of treated water with
much more reduced turbidities and SDI while removing a majority of
biofilm formers relative to untreated or conventionally treated
water. The biofoulant filter will further provide 6 log reduction
of bacteria and 1-4 log reduction of virus. Therefore, the
downstream water will be virtually disinfected without use of any
chemicals, and will be without biofoulants to serve as nutrients
for bacteria. This diminishes the chances of any biofilm formation
on a membrane surface.
[0030] The typical SDI value at the outlet of a biofoulant filter
is less than 1 and typically close to 0.6-0.8. The process
highlights the importance of treating biofoulants downstream of UF
treatment, which is critical for eliminating or minimizing biofilm
formation at reduced flux of RO.
[0031] There are multiple options for biofoulant or nutrient
removal. They operate under a wide range of TDS and provide TOC
reduction of at least 40-60%, preferably at least 60-80%, most
preferably at least 80%, on an overall basis, but remove the bulk
of the negatively charged TOC. Suitable biofoulant or nutrient
removal can be accomplished through, for example, ion exchange
materials, positively charged media or electro-chemical or
electrodes based methods. Cleaning, disinfection or regeneration
improves the bio foulant media performance. This is done by
chemical or electro chemical methods. This is an optional feature
of this scheme. It should be herein that total organic carbon, or
"TOC," is used throughout this disclosure as a measure of
biofoulants.
[0032] Reduced Flux.
[0033] System Design and plant operation are done at lower flux
than conventional reverse osmosis systems. Although typical reverse
osmosis is conducted at 10-20 GFD, our process uses flux at an
energy efficient point where reduction of flux does not reduce
energy consumption. In preferred embodiment the flux that is used
(alternatively referred to as the "operating flux") is at a level
where further reduction of flux does not reduce energy consumption
by more than 5% relative to the energy consumption at the prior
flux level.
[0034] This is range of flux is around 6-8 GFD (gallons/square
foot/day) for SWRO and could be around 10-12 for BWRO and 6-8 GFD,
or, in some embodiments, 8-10 GFD for waste water RO. This is based
on feed water quality, permeate quality requirement and temperature
range. This is done through a low flux reverse osmosis (RO)
process. The flux can be marginally increased for lower total
dissolved solids (TDS) or low fouling waters. For example, it might
be increased by 5-10% more than what is stated above.
[0035] Although a flux of less than 6-8 may be used for SWRO,
typically but below this flux there could deterioration in permeate
quality and also does not provide any energy savings. At this level
of 6-8 GFD flux there is reduction in the concentration of bacteria
and nutrients over the membrane surface and this reduces the
buildup of differential pressure to minimum. Moreover at this
reduced flux operating pressures reduce significantly. For example,
operating pressure may be reduced by about 10-20%. So this flux was
determined to be the best for SWRO for providing the low energy
service with minimum bio fouling. Similarly lower level fluxes were
determined for other sources of water also.
[0036] It must be emphasized that the operation at reduced flux is
not merely an example of optimization of flux to reduce energy
consumption. At reduced flux alone without UF and biofoulant or
even with UF and without bio foulant removal filter one may be able
to achieve low energy consumption to start with but can not sustain
low energy operation due to bio fouling problems as evidenced in
the exampled provided below. To the contrary, the reduced flux was
unexpectedly determined to lead to a reduced energy consumption on
a sustained basis by virtue of the inclusion of the ultrafiltration
and biofoulant removal filter. This process of low flux works at
minimum energy consumption on a sustained basis in combination with
ultrafiltration and biofoulant filtration in combination.
[0037] Use of low flux provides additional advantages by providing
a minimum variation in difference in operating pressure with
variation of feed water temperatures. When the design flux is
higher as per the conventional process, there is significant
variation at operating pressures at minimum and maximum pressures.
This requires sophisticated controls to adjust or control the
pressures but this still results in loss of energy when the actual
temperatures are higher than design pressures. Alternatively speed
control devices have to be installed to adjust pump RPM for changes
in water temperatures which still result in some loss of energy but
make the system complex and expensive. Operation at low flux design
avoids this complication and reduces energy consumption by 20%. In
some embodiments one or more of pressure control and speed control
devices for adjustment of pump flow may be excluded, though in many
cases for reasons of safety or flexibility they might still be
present unless specifically excluded by the claims.
[0038] To provide one embodiment of the invention we conducted a
detailed analysis of 35000 ppm TDS water to determine if our novel
method might reduce energy consumption. For example, for 35000 PPM
TDS, if the system is designed at 9-10 GFD, the power consumption
is around 2 KWH/M3 for the RO pump and energy recovery system. If
the same system is designed at 6 GFD the power consumption reduces
to 1.7 KWH/M3 (FIG. 7) and reduces feed pressure from 55 kg/cm2 to
46 kg/cm2 (FIG. 6). At this level variation in pressure due to feed
water temperature within a wide range of 25-40.degree. C. is only
0.5-0.7 kg/cm.sup.2 (FIG. 8) for different types of membranes and
provides TDS within acceptable limits even at the highest possible
temperature (FIG. 9). The energy consumption has been calculated
based on 85-86% efficiency of pumps and more than 96% efficiency of
motors.
[0039] This data is more of less consistent for different membrane
makes available from different membrane manufacturers. The
difference if any is very small. It is evident from these studies
that at these levels of flux the energy consumption is at best
levels, can handle wide range of temperature with minimum variation
in power and also provides acceptable range of permeate TDS. But
the biggest benefit is at this level of flux the biofilm formation
is reduced to insignificant levels, especially when it is
pretreated with UF and with a biofoulants removal device as
mentioned above. This makes sure that energy consumption design is
not only minimum to start with but also remains low on a sustained
basis due to reduced or insignificant bio fouling.
[0040] Over a period of a day operation the increase of
differential pressure less than 0.1 kg/cm.sup.2 and more often less
than any detection limits. Also, due to reduced driving pressure
across the membrane whatever fouling happens is not firmly attached
to membrane surface due to lack of a charge. Therefore it can be
easily removed under mild cleaning conditions. If one fakes certain
precautions in pretreatment as described below, the residual
foulants are not able to adhere to the membrane surface, which is
reflected on the trend of increase of differential pressure.
[0041] Some of these concepts are similar for surface water or
brackish water and waste water recycle based RO plants, including
some low TDS waters, where severe fouling happens on reverse
osmosis and energy consumption creeps and water production
eventually drops. It has been seen that bio fouling alone can
increase the differential pressures across RO stages to more than
4-5 kg/cm2, which results in loss of energy. This may happen even
if we have a pretreatment, which includes UF system. This can be
mitigated by managing flux, calibrating and regulating pretreatment
as described above, and by and stopping buildup of biofouling as
mentioned below. However these sources of water may include
additional pretreatment steps to mitigate hardness and silica
scaling as appropriate.
[0042] Osmotic Cleaning.
[0043] To further augment the process described above with a
cleaning mechanism to overcome any biofouling right before it
initiates, we present a unique methodology of cleaning. The method
is based on natural osmotic pressure differential between reject
and permeate water. When the system is stopped on a manual mode of
operation with a continued regulated flow in the feed side, which
allows the reject water to remain in the feed side, there is a
steady flow of water from the permeate side to the feed side. The
permeate flow continues to the feed side due to concentration
differential. The concentration differential is maintained by
makeup reject water flow to the feed side through a clean in place
system. In automatic mode of operation the system does not stop but
shifts into a cleaning mode but water production may stop from the
complete train or part of the train. After the completion of
osmotic cleaning the system shifts into water production mode. In
manual mode the process may take around 10-15 minutes and automatic
mode the process may take maximum of 5 minutes. This duration can
be adjusted for every site.
[0044] If one allows this process to continue for few minutes for
example 10-15 minutes, any biofilm is dislodged from the membrane
surface. As the plant has been designed at lower flux and also the
feed water has been filtered through UF and passed through a
biofilm filter or device, the buildup of any biofilm pressure drop
is reduced and can be easily cleaned through this cleaning.
[0045] This process should be controlled through regulated flows
and concentration on both feed and permeate side using plant
produced reject and permeate water. The permeate flow under these
conditions is purely a function of concentration gradient and
pressure drop built in the membranes due to fouling, but feed side
flow is maintained by circulation of brine at a minimum flow which
can overcome dilution due to permeate entry and also maintain
dynamic conditions in the feed side. Therefore it possible to
maintain clean membrane pressure drop conditions by using this
cleaning technique and prevent any increase in feed pressure or
membrane differential pressure. The loose debris can be then
flushed out into the reject by at higher velocity pretreated
Seawater rinsing.
[0046] This cleaning methodology is based on a concept that biofilm
formation should be removed as fast as it is formed or prevented
from building up. This can be achieved by shorter cleaning cycles,
typically of 10-15 minutes each on a manual mode, done frequently
or based on predetermined differential pressure increases over
start up conditions or measurement of biofilm formation by biofilm
sensors upstream of the reverse osmosis membranes. Normally the
differential pressure builds up at from 0.1 kg/cm.sup.2 per day to
0.3 kg/cm.sup.2 a day over 24 hours operation depending on site
conditions and plant design. This process will typically not allow
any buildup of differential pressure and the membrane will operate
at clean membrane conditions.
[0047] This cleaning process is not very effective when carried out
on a delayed basis at a higher differential pressure, or it may
require frequent stoppages if the pretreatment has not been done to
remove all bio fouling contaminants. This cleaning process can be
practically and successfully employed only because the biofouling
ability of water is virtually minimized in the pretreatment
mentioned above. Therefore, any residual foulants can cause only
minimum fouling, and their rate of buildup is insignificant and at
this level. Osmotic cleaning process is very effective and
virtually can keep the membranes clean.
[0048] This process has a further advantage that it does not
require use of any cleaning chemicals on a daily basis, but instead
uses brine generated in the reject of SWRO or BWRO plants. A
chemical cleaning option can be exercised to handle any upset
conditions, but typically is used very infrequently. The option of
adjusting brine concentration can be exercised to control the
effectiveness of cleaning process. One more advantage of this
process is the water consumption in cleaning is minimum. The
quantity of water consumption is approximately 0.2-2.0
litres/m.sup.2 of membrane area. Osmotic gradient may be, for
example, 40 to 180. Osmotic gradient is defined as the ratio of the
RO reject and permeate TDS.
[0049] In this process chlorine dosing is almost eliminated or
minimized to upstream of UF for any clarifier or DAF (dissolved air
flotation) etc., and to UF for chemical enhanced backwash based on
local conditions. But frequent or regular use of chlorine can be
avoided. Any chlorine usage is restricted to offline conditions and
chlorine as far as possible is not allowed to become a part of the
system. In some embodiments of the invention the need for chemical
addition for pretreatment of the RO feedwater is eliminated
entirely.
[0050] The combination of steps mentioned above will ensure that
design conditions of low power consumption will remain and the
system will deliver sustained power efficiency and water production
on a continuous basis. The overall process combines membrane
process design for low energy and low fouling with pretreatment and
cleaning processes which will mitigate fouling at the first place
and further clean any fouling before it builds up without using any
chemicals. Of course, those of skill in the art will recognize that
additional treatment steps may be added as desired.
[0051] The flow scheme shown in FIG. 1 includes a preconditioning
of seawater after it is received through the intake system through
an intake pump. Depending on the seawater analysis and seasonal
variations, a clarifier or DAF unit is installed to remove
suspended solids. In case the seawater does not contain high level
of suspended solids, one can take water through a strainer to the
UF membranes. UF membranes are backwashed by the UF permeate water
through a backwash pump. The frequency of backwash can be 10-20
minutes. Chlorine and caustic soda may be used infrequently to
provide chemically enhanced backwash. The backwash outlet is taken
to the clarifier or DAF, or directly to waste water based on the
plant configuration.
[0052] The UF permeate is taken to a biofoulant removal filter
after dechlorination. This filter feeds into the suction of a
high-pressure pump, which further feeds into a low flux RO membrane
unit. The RO permeate is taken into a permeate tank that always
maintains a minimum level to provide low TDS water for cleaning
requirements, and additional water is pumped for beneficial use.
The reject from RO is taken through an energy recovery device. For
example, a pressure exchanger may be used. The energy is
transferred to water coming out of a biofoulants removal filter.
After energy recovery the reject is discharged into waste after
retaining a certain level of water in a clean in place tank, or
"CIP" tank for the purpose of cleaning.
[0053] This configuration generates permeate water as needed at
very low energy depending on the seawater TDS and temperature
conditions. The sustainability of low energy is possible because of
level of pretreatment through UF and biofoulant removal filters and
cleaning methodology ensures that differential pressure does not
build up by removal of any bio film or scaling build up on a
frequent basis. Chemical consumption in pretreatment as well as
later use of cleaning chemical is eliminated or minimized.
[0054] Although reported in the context of seawater recovery, this
methodology can also be used in high recovery BWRO (brackish water
reverse osmosis) where the potential for Bio fouling exists and the
osmotic gradient of the reject water can be used to keep the
membrane clean and reduce energy consumption.
[0055] Those of skill in the art will also recognize, with the
benefit of this disclosure, that the processes described herein may
be particularly suitable for use of reverse osmosis in zero liquid
discharge, or "ZLD" processes. ZLD processes typically have thermal
evaporators downstream of the reverse osmosis unit. These will
benefit from the low energy consumption and sustained plant
operation without loss of water production. These factors help lead
to reliable operation in a ZLD system.
[0056] Certain embodiments of the invention may be better
understood with reference to various examples and comparative
examples as are set forth below.
Experiment-1
[0057] To benchmark base performance, a reverse osmosis (RO) unit
with 2.3 m.sup.3/hr flow rate was operated for 17 months on surface
water having TOC level of 5-10 ppm without any biofoulant removal
unit at the upstream of RO unit. This source of water was selected
due to its history of biofouling for several years. Based on the
original plant design, the surface water was passed through an
ultrafiltration (UF) unit before feeding into an RO Unit. We
maintained the silt density index (SDI) below 5, most of the time
below 3. The RO unit pressure drop was monitored and its results
are shown in FIG. 3.
[0058] During 17 months of RO unit operation, we cleaned the unit
seven times to maintain the pressure drop of the RO Unit. It was
observed that the average service cycle length of the RO Unit was
around 700 hrs, and it required chemical cleaning for maintaining
the pressure drop, product quality and energy consumption.
Operating hours of this RO unit in different service cycles are
shown in Table 1. Progressively the operating hours were adjusted
so that after every chemical cleaning original starting pressure
drop conditions could be regained. During this operation it was
very clear that even with UF pretreatment pressure drop across RO
build up was visible within days and sometimes within hours during
rainy seasons and after a very elaborate cleaning process original
pressure was not regained.
TABLE-US-00001 TABLE 1 RO Unit operating hours vs. service cycle RO
Unit Initial Pressure Final Pressure Operating Drop of RO Drop of
RO RO Unit Service Cycles Hours Unit (kg/cm2) Unit (kg/cm2) 1st
Service Cycle length 1201 3.9 7.4 2nd Service Cycle length 717 5.3
8.1 3rd Service Cycle length 296 5.5 6.5 4th Service Cycle length
650 5.5 7.0 5th Service Cycle length 859 5.2 6.3 6th Service Cycle
length 687 4.7 5.5 7th Service Cycle length 462 3.6 5.0 Average
Service cycle 696 hours length
Experiment-2
[0059] In this experiment, a biofoulant removal unit was installed
in the UF product line, and we monitored the TOC and turbidity
removal across the biofoulant removal unit. The results of TOC
& Turbidity are shown in Table 2. TOC of water was analyzed on
Shimadzo-TOC analyzer, and the turbidity was checked by
HACH-turbidity analyzer. In this experiment the biofilter was made
up of electro positive media material.
TABLE-US-00002 TABLE 2 Bio-Foulant Bio-Foulant Removal unit UF
Product Removal unit UF Product Outlet TOC Turbidity Outlet
Turbidity TOC (ppm) (ppm) (NTU) (NTU) 5.13 3.83 0.069 0.054 5.17
3.92 0.065 0.059 5.16 3.88 0.067 0.064 5.20 2.84 0.069 0.061 5.68
3.98 0.068 0.065 4.38 1.08 0.068 0.06 5.16 3.98 0.066 0.06 5.16
3.27 0.071 0.059 5.28 3.31 0.073 0.063 5.48 3.38 0.066 0.058 5.13
3.05 0.069 0.065 5.26 3.59 0.065 0.058 5.06 3.81 0.067 0.06 4.98
3.02 0.064 0.060 5.13 3.68 0.065 0.061 5.36 3.82 0.066 0.059 5.86
3.02 0.066 0.060 5.02 3.12 0.068 0.060 4.82 3.30 0.070 0.060 5.68
3.82 0.068 0.055 4.58 1.62 0.069 0.059 5.03 2.91 0.068 0.063 4.28
2.03 0.069 0.056 5.21 2.91 0.066 0.059 4.28 2.06 0.065 0.057 4.32
2.12 0.065 0.057
[0060] This indicates that the biofoulant removal unit removed
around 40% to 60% of TOC from the product water of UF unit. Outlet
turbidity of water was always around 0.060 NTU, which directly
helps in maintaining the SDI level below 3 in RO Unit, and
sometimes between 1-2, minimizing the biofouling in the RO
Unit.
Experiment-3
[0061] In another set of experiments, the same RO unit of the
previous examples was operated for nine months with the inclusion
of biofoulant removal unit along with UF at the upstream at the
upstream of RO unit and its effect was clearly observed with
respect to longer service cycle length. RO unit was operated for
around 1425 hours without any cleaning, which is almost twice the
previous average service cycles length of Experiment 1. In this
experiment the RO unit was cleaned only once after the operation of
six months. RO unit performance with bio-foulant removal unit is
shown in FIG. 4. During this operation it was observed that for
more than 3 months there was very insignificant increase in
differential pressure but once it started increasing gradually
subsequent fouling rate started accelerating and progressively
started increasing.
[0062] Even though the biofoulant removal unit minimized the
pressure drop rise & biofouling in RO unit, still the pressure
drop gradually increased over a period of six month. The main
reason for this was the gradual deposition of a fine biofilm on the
RO membrane surface on a day-to-day basis. The intensity of
biofouling was very low, as indicated by longer service length.
[0063] At this stage after the normal chemical cleaning of RO unit,
and bringing back its pressure drop to normal level (3.8
kg/cm.sup.2), osmotic cleaning process as described in one type of
the embodiment was implemented, and every day one natural osmotic
cleaning cycle were performed on the RO Unit by RO Reject water for
10-15 minutes. The impact of natural osmotic cleaning was clearly
observed, and the pressure drop remained unchanged at 3.8
kg/cm.sup.2 for next 500 hrs, of operation as shown in FIG. 2. Due
to the unchanged pressure drop of the RO Unit, its energy
consumption remained the same, and no increase was observed. During
this time no increase of differential pressure was seen.
[0064] It became clear at this stage that with proper feed
conditions of UF and biofoulant removal filter and daily proactive
osmotic cleaning clean membrane conditions can be maintained, which
means no biofouling and no increase in energy. FIG. 2 shows
comparative behavior of pressure drop increase under different
conditions. Based on this data it is very clear that with the
devised process, one can achieve sustained plant operation at lower
energies.
Experiment-4
[0065] In this experiment, the natural osmotic cleaning process was
implemented on fresh RO membranes with low flux design having a UF
and Biofoulant filter at the upstream of RO. The Experiment 3
membranes were not used because they were two and half years old.
In this trial TOC rich surface water was first passed through the
UF unit followed by biofoulant removal unit and then feed to RO
unit. Natural osmotic cleaning was performed at osmotic gradient of
around 40 and above after every 8-16 hours of operation on the RO
Unit by RO reject water for 10-15 minutes. The RO Unit was operated
for 150 hours, and it was observed that the pressure drop of RO
unit remained unchanged as shown in FIG. 5.
[0066] This demonstrates that the RO Unit, when operated with UF
followed by bio-foulant removal unit at upstream with natural
osmotic cleaning at regular interval by RO unit reject water,
allowed the RO unit pressure drop to remain unchanged. Its increase
was minimized significantly, and the energy consumption of the RO
unit was maintained to its original level.
[0067] This further demonstrated that if one constructs a RO plant
for low energy based on low flux RO, as provided herein, and
further if one provides pretreatment and cleaning conditions as
described above, one can achieve low energy consumption in the
beginning of operation. That level of energy consumption may then
be sustained throughout the life of the plant.
[0068] In the above examples, ultrafiltration membranes and
biofoulant removal filters were obtained from Qua Group.
[0069] Although embodiments of the invention have been discussed
herein in the context of certain aspects and advantages, those of
skill in the art will appreciate that various modifications may be
made within the scope and spirit of the claims.
* * * * *