U.S. patent application number 10/134449 was filed with the patent office on 2002-12-12 for biological process for removing phoshporus involving a membrane filter.
This patent application is currently assigned to Zenon Environmental Inc.. Invention is credited to Husain, Hidayat, Koch, Frederic, Phagoo, Deonarine.
Application Number | 20020185435 10/134449 |
Document ID | / |
Family ID | 27758304 |
Filed Date | 2002-12-12 |
United States Patent
Application |
20020185435 |
Kind Code |
A1 |
Husain, Hidayat ; et
al. |
December 12, 2002 |
BIOLOGICAL PROCESS FOR REMOVING PHOSHPORUS INVOLVING A MEMBRANE
FILTER
Abstract
A waste water treatment process for biologically removing
phosphates incorporates a membrane filter. The process includes
three zones, an anaerobic zone, an anoxic zone and an aerobic zone
containing an anaerobic, anoxic and aerobic mixed liquor. Water to
be treated flows first into the anaerobic zone. Anaerobic mixed
liquor flows to the anoxic zone. Anoxic mixed liquor flows both
back to the anaerobic zone and to the aerobic zone. The aerobic
mixed liquor flows to the anoxic zone and also contacts the feed
side of a membrane filter. The membrane filter treats the aerobic
mixed liquor to produce a treated effluent lean in phosphorous,
nitrogen, BOD OR COD, suspended solids and organisms at a permeate
side of the membrane filter and a liquid rich in rejected solids
and organisms. Some or all of the material rejected by the membrane
filter is removed from the process either directly or by returning
the material rejected by the membrane filter to the anoxic or
aerobic zones and wasting aerobic sludge. In a first optional side
stream process, phosphorous is precipitated from a liquid lean in
solids extracted from the anaerobic mixed liquor. In a second
optional side stream process, anaerobic mixed liquor is treated to
form insoluble phosphates which are removed in a hydrocyclone.
Inventors: |
Husain, Hidayat; (Brampton,
CA) ; Koch, Frederic; (Vancouver, CA) ;
Phagoo, Deonarine; (Toronto, CA) |
Correspondence
Address: |
BERESKIN AND PARR
SCOTIA PLAZA
40 KING STREET WEST-SUITE 4000 BOX 401
TORONTO
ON
M5H 3Y2
CA
|
Assignee: |
Zenon Environmental Inc.
3229 Dundas Street West
Oakville
ON
L6J 4Z3
|
Family ID: |
27758304 |
Appl. No.: |
10/134449 |
Filed: |
April 30, 2002 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10134449 |
Apr 30, 2002 |
|
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|
09646115 |
Sep 27, 2000 |
|
|
|
6406629 |
|
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09646115 |
Sep 27, 2000 |
|
|
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PCT/CA00/00854 |
Jul 19, 2000 |
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Current U.S.
Class: |
210/605 |
Current CPC
Class: |
C02F 3/1268 20130101;
C02F 3/308 20130101; C02F 3/30 20130101; Y02W 10/15 20150501; Y02W
10/10 20150501; C02F 3/1273 20130101 |
Class at
Publication: |
210/605 |
International
Class: |
C02F 003/30 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 20, 1999 |
CA |
2278265 |
Claims
We claim:
1. A process for treating water to remove phosphorous and nitrogen
comprising the steps of: (a) providing an anaerobic zone having an
anaerobic mixed liquor having organisms which release phosphorous
into the anaerobic mixed liquor and store volatile fatty acids from
the anaerobic mixed liquor; (b) providing an anoxic zone having an
anoxic mixed liquor having organisms which metabolize stored
volatile fatty acids, uptake phosphorous and denitrify the anoxic
mixed liquor; (c) providing an aerobic zone having an aerobic mixed
liquor having organisms which metabolize stored volatile fatty
acids, uptake phosphorous and nitrify the aerobic mixed liquor; (d)
flowing water to be treated into the anaerobic zone; (e) flowing
anaerobic mixed liquor to the anoxic zone; (f) flowing anoxic mixed
liquor to the anaerobic zone; (g) flowing anoxic mixed liquor to
the aerobic zone; (h) flowing aerobic mixed liquor to the anoxic
zone; (i) contacting aerobic mixed liquor against the feed side of
a membrane filter; (j) producing a treated effluent lean in
phosphorous, nitrogen, BOD OR COD, suspended solids and organisms
from a permeate side of the membrane filter; and, (g) removing some
or all of the material rejected by the membrane filter from the
process, wherein the steps above are performed substantially
continuously and substantially simultaneously and wherein the MLSS
is between 3 and 30 g/L.
2. The process of claim 1 wherein the MLSS is between 5 and 15
g/L.
3. The process of claim 1 wherein material rejected by the membrane
filter is also mixed with the aerobic mixed liquor.
4. The process of claim 3 wherein the step of removing material
rejected by the membrane filter from the process is accomplished by
removing aerobic mixed liquor containing material rejected by the
membrane filter.
5. The process of claim 1 wherein material rejected by the membrane
filter is also mixed with the anoxic mixed liquor.
6. The process of claim 1 wherein the net hydraulic retention time
for the anaerobic, anoxic and aerobic zones combined is between 2
and 12 hours.
7. The process of claim 1 wherein the net hydraulic retention time
for the anaerobic, anoxic and aerobic zones combined is between 2
and 9 hours.
8. The process of claim 1 wherein the net hydraulic retention time
for the anaerobic, anoxic and aerobic zones combined is between 4.5
and 9 hours.
9. The process of claim 1 wherein the sludge retention time is
between 10 and 30 days and a crystalline phosphorous accumulation
occurs in the mixed liquor.
10. The process of claim 1 wherein the recycle ratio between the
aerobic zone and the anoxic zone is between 2 and 4.
11. A process for treating water to remove phosphorous and nitrogen
comprising the steps of: (a) providing an anaerobic zone having an
anaerobic mixed liquor having organisms which release phosphorous
into the anaerobic mixed liquor and store volatile fatty acids from
the anaerobic mixed liquor; (b) providing an anoxic zone having an
anoxic mixed liquor having organisms which metabolize stored
volatile fatty acids, uptake phosphorous and denitrify the anoxic
mixed liquor; (c) providing an aerobic zone having an aerobic mixed
liquor having organisms which metabolize stored volatile fatty
acids, uptake phosphorous and nitrify the aerobic mixed liquor; (d)
flowing water to be treated into the anaerobic zone; (e) flowing
anaerobic mixed liquor to the anoxic zone; (f) flowing anoxic mixed
liquor to the anaerobic zone; (g) flowing anoxic mixed liquor to
the aerobic zone; (h) flowing aerobic mixed liquor to the anoxic
zone; (i) contacting aerobic mixed liquor against the feed side of
a membrane filter; (j) producing a treated effluent lean in
phosphorous, nitrogen, BOD OR COD, suspended solids and organisms
from a permeate side of the membrane filter; and, (g) removing some
or all of the material rejected by the membrane filter from the
process, (h) extracting a liquid containing phosphorous but lean in
solids from the anaerobic mixed liquor; (i) precipitating
phosphorous from the liquid containing phosphorous but lean in
solids; and, (j) producing a phosphorous lean effluent from the
liquid containing phosphorous but lean in solids, wherein the steps
above are performed substantially continuously and substantially
simultaneously.
12. The process of claim 11 wherein material rejected by the
membrane filter is also mixed with the aerobic mixed liquor.
13. The process of claim 12 wherein the step of removing material
rejected by the membrane filter from the process is accomplished by
removing aerobic mixed liquor containing material rejected by the
membrane filter.
14. The process of claim 11 wherein material rejected by the
membrane filter is also mixed with the anoxic mixed liquor.
15. The process of claim 11 wherein the phosphorous is precipitated
by the addition of between 400 to 800 mg/L of alum.
16. A process for treating water to remove phosphorous and nitrogen
comprising the steps of: (a) providing an anaerobic zone having an
anaerobic mixed liquor having organisms which release phosphorous
into the anaerobic mixed liquor and store volatile fatty acids from
the anaerobic mixed liquor; (b) providing an anoxic zone having an
anoxic mixed liquor having organisms which metabolize stored
volatile fatty acids, uptake phosphorous and denitrify the anoxic
mixed liquor; (c) providing an aerobic zone having an aerobic mixed
liquor having organisms which metabolize stored volatile fatty
acids, uptake phosphorous and nitrify the aerobic mixed liquor; (d)
flowing water to be treated into the anaerobic zone; (e) flowing
anaerobic mixed liquor to the anoxic zone; (f) flowing anoxic mixed
liquor to the anaerobic zone; (g) flowing anoxic mixed liquor to
the aerobic zone; (h) flowing aerobic mixed liquor to the anoxic
zone; (i) treating aerobic mixed liquor in a solid-liquid separator
to produce a treated effluent lean in phosphorous, nitrogen
suspended solids and organisms and a liquid rich in solids rejected
by the solid liquid separator; (j) removing some or all of the
liquid rich in solids rejected by the solid-liquid separator; (k)
extracting a phosphorous containing permeate lean in solids from
the anaerobic mixed liquor; (l) precipitating phosphorous from the
phosphorous containing permeate lean in solids; and, (m) producing
a phosphorous lean effluent from the phosphorous containing
permeate lean in solids, wherein the steps above are performed
substantially continuously and substantially simultaneously.
17. The process of claim 16 wherein the phosphorous is precipitated
by the addition of between 400 to 800 mg/L of alum.
Description
[0001] This is a division of U.S. patent application Ser. No.
09/646,115 filed Sep. 27, 2000 which is a National Stage of PCT
application number PCT/CA00/00854 filed on Jul. 19, 2000. The
entire disclosure both of these of applications is incorporated
herein by this reference.
FIELD OF THE INVENTION
[0002] The present invention relates to waste water treatment, and
more particularly to a process for removing phosphorous from waste
water involving biological processes and a membrane filter.
BACKGROUND OF THE INVENTION
[0003] BOD, nitrates and phosphates released into the environment
cause eutrophication and algae blooming resulting in serious
pollution and health problems. Waste water treatment processes
attempt to remove BOD, nitrates and phosphates to produce an
acceptable effluent.
[0004] Conventional processes for removing phosphates from waste
water include chemical precipitation and biological methods. In
chemical precipitation methods, soluble salts, such as
ferrous/ferric chloride or aluminum sulphate, are added to the
waste water to form insoluble phosphate metal salts. The waste
water, however, contains many different ions which create
undesirable side reactions with the precipitants. As a result, and
particularly where very low effluent total phosphorus levels are
required, these processes may require the addition of 5-6 times the
stoichiometric amount of chemicals required to remove the
phosphates. Accordingly, these processes result in high chemical
costs, high sludge production, and a high level of metallic
impurities in the sludge.
[0005] In contrast, biological methods use microorganisms to digest
the phosphates. For example, U.S. Pat. No. 4,867,883 discusses a
process which attempts to create conditions which encourage the
selection and growth of Bio-P organisms, a strain of bacteria which
have the ability to uptake phosphorus in excess of the amount
normally needed for cell growth. The amount of phosphorus removal
that can be achieved is directly proportional to the amount of
Bio-P organisms in the system. Generally, the process consists of
an anaerobic zone, an anoxic zone, an aerobic zone, a clarifier,
and a variety of recycles to interconnect the various zones. In a
preferred embodiment of the process, there is a denitrified recycle
from the anoxic zone to the anaerobic zone, a nitrified recycle
from the aerobic zone to the anoxic zone, and an activated sludge
recycle from the clarifier to the anoxic zone. In the anaerobic
zone, there is BOD assimilation and phosphorus release.
Subsequently, in the anoxic and aerobic zones, there is phosphorus
uptake. In the clarifier, sludge containing phosphates settles out
of the effluent. In some cases, sand filters are employed to try to
further reduce the amount of phosphates in the effluent.
[0006] One problem with the U.S. Pat. No. '883 process is that
there can be a build up of phosphates in the system. At the end of
the process, a portion of the recycled activated sludge is wasted
and is subsequently treated, typically by aerobic or anaerobic
digestion processes. This results in a release of phosphorus taken
up in the process. This phosphorus is then returned back to the
process in the form of digester supernatant. Consequently, this
reduces the efficiency of phosphorus removal in the process and
results in higher levels of phosphorus in the effluent. A partial
solution to this problem is to employ a side stream process called
`Phos-Pho Strip` as described in U.S. Pat. No. 3,654,147. In this
process, the activated sludge, which has a high concentration of
phosphorus, passes from the clarifier to a phosphorus stripper. In
the stripper, phosphorus is released into the filtrate stream by
either: creating anaerobic conditions; adjusting the pH; or
extended aeration. The resulting phosphate-rich filtrate stream
passes to a chemical precipitator. The phosphate-free effluent
stream is added to the main effluent stream, the waste stream from
the precipitator containing the phosphates is discarded, and the
phosphate-depleted activated sludge is returned to the main
process.
[0007] Another disadvantage with the process in U.S. Pat. No. '883
is that significant design limitations are imposed by the settling
characteristics of the sludge in the clarifier. For example, the
process cannot operate at very high process solids levels or high
sludge retention times. As a result, the system is generally
considered to be inefficient and there is a high generation rate of
waste sludge.
[0008] A second type of biological treatment is referred to as a
membrane bioreactor which can be combined with chemical
precipitation techniques. In a simple example, precipitating
chemicals are added to an aerobic tank containing or connected to a
membrane filter. As above, however, dosages of precipitating
chemicals substantially in excess of the stoichiometric amount of
phosphates are required to achieve low levels of phosphates in the
effluent. This results in excessive sludge generation and the
presence of metallic precipitates which increase the rate of
membrane fouling or force the operator to operate the system at an
inefficient low sludge retention time.
[0009] Also relevant to the present invention is U.S. Pat. No.
5,658,458 which discloses a treatment for activated sludge
involving the separation of trash and inerts. Generally, the
process consists of a screen which removes relatively large pieces
of `trash` and a hydrocyclone which uses a centrifugal force to
separate the organics from the inerts. The activated sludge is
recycled back to the system and the trash and inerts are
discarded.
SUMMARY OF THE INVENTION
[0010] It is an object of the present invention to remove
phosphorous from waste water. In some aspects, the invention
provides a process for treating water to remove phosphorous and
nitrogen. The process includes three zones, an anaerobic zone, an
anoxic zone and an aerobic zone. In the anaerobic zone, an
anaerobic mixed liquor has organisms which release phosphorous into
the anaerobic mixed liquor and store volatile fatty acids from the
anaerobic mixed liquor. In the anoxic zone, an anoxic mixed liquor
has organisms which metabolize stored volatile fatty acids, uptake
phosphorous and denitrify the anoxic mixed liquor. In the aerobic
zone, an aerobic mixed liquor has organisms which metabolize stored
volatile fatty acids, uptake phosphorous and nitrify the aerobic
mixed liquor.
[0011] Water to be treated flows first into the anaerobic zone to
join the anaerobic mixed liquor. Anaerobic mixed liquor flows to
the anoxic zone to join the anoxic mixed liquor. Anoxic mixed
liquor flows both back to the anaerobic zone to join the anaerobic
mixed liquor and to the aerobic zone to join the aerobic mixed
liquor. The aerobic mixed liquor flows to the anoxic zone to join
the anoxic mixed liquor and also contacts the feed side of a
membrane filter. The membrane filter treats the aerobic mixed
liquor to produce a treated effluent lean in phosphorous, nitrogen,
BOD, suspended solids and organisms at a permeate side of the
membrane filter and a liquid rich in rejected solids and
organisms.
[0012] Some or all of the material rejected by the membrane filter
is removed from the process. This may be done by locating the
membrane filter outside of the aerobic zone and directly removing
the liquid rich in rejected solids and organisms from the retentate
or feed side of the membrane filter. Alternatively, the membrane
filter may be located in the aerobic zone so that the material
rejected by the membrane filter mixes with the aerobic mixed
liquor. The material rejected by the membrane filter is then
removed by removing aerobic mixed liquor. Further alternatively,
the liquid rich in material rejected by the membrane filter may be
recycled to the anoxic or aerobic zones. The material rejected by
the membrane filter is then removed by removing aerobic mixed
liquor. Combinations of the first and third methods described above
may also be used.
[0013] The steps described above are performed substantially
continuously and substantially simultaneously. In the anaerobic
zone, fermentive bacteria convert BOD into volatile fatty acids.
Bio-P organisms use the volatile fatty acids as a carbon source. In
doing so, they release phosphorus into the liquor, and store
volatile fatty acids for later use. The stored carbon compounds may
come from volatile fatty acids produced in the anaerobic zone or
from materials produced external to the process or both. For
example, upstream waste water fermentation can occur either in
prefermentation units specifically designed for this purpose, or
inadvertently in the sewage system. Subsequently, in the anoxic and
aerobic zones, the Bio-P organisms metabolize the stored volatile
fatty acids and uptake phosphates from the liquor. The recycle
between the anoxic and anaerobic zones allows the process to
operate substantially continuously.
[0014] The stream exiting the aerobic zone passes through the
membrane filter. In the membrane filter, phosphorus-rich activated
sludge, finely suspended colloidal phosphorus, bacteria, and other
cellular debris are rejected by the membrane. A waste activated
sludge containing material rejected by the membrane filter,
optionally combined with aerobic mixed liquor, flows to a sludge
management or processing system. A phosphorous lean effluent is
produced at the permeate side of the membrane filter. The effluent
is also reduced in nitrogen as a result of the anoxic and aerobic
zones and the recycle between them.
[0015] The membrane filter removes colloidal phosphorus and
bacteria which would normally pass through a clarifier. Although
the absolute amount of colloidal solids is relatively small, the
percentage of phosphorus in the colloids is surprisingly high and
its removal results in unexpected low levels of phosphorus in the
effluent. With membrane filters to remove biomass from the effluent
stream, a fine biomass can be maintained in the anaerobic reactor.
This may result in enhanced reaction rates and higher than
anticipated release of phosphorus in the anaerobic reactor, with
resulting higher uptake of phosphorus in the anoxic and aerobic
zones. Further, since the process is not limited by the settling
characteristics of the sludge, the process is able to operate at
very high process solid levels, preferably with an MLSS between 3
and 30 mg/L and short net hydraulic retention times, preferably
between 2 and 12 hours. The short HRT allows increased throughput
of waste water for a given reactor size. In addition, since the
design avoids chemical precipitation of phosphates upstream of the
membrane filters, there is reduced membrane fouling which further
enhances the performance of the process. Moreover, contaminants in
the sludge resulting from precipitating chemicals are reduced
permitting the system to operate at a high sludge age. At high
sludge retention times, preferably between 10 and 30 days, an
unexpected significant crystalline phosphorus accumulation occurs
in the biomass, effectively removing phosphorus from the system. As
well, there is lower net sludge generation.
[0016] The processes described above optionally includes one of two
side stream processes. In a first side stream process, a liquid
lean in solids but containing phosphorous is extracted from the
anaerobic mixed liquor. Phosphorous is precipitated from that
liquid to produce a phosphorous lean liquid which leaves the
process as effluent or is returned to the anoxic or aerobic zones.
In a second side stream process, anaerobic mixed liquor is removed
to a reaction zone and treated to form a liquid rich in insoluble
phosphates. The liquid rich in insoluble phosphates is treated in a
hydrocyclone to separate out insoluble phosphates and create a
liquid lean in insoluble phosphates. The liquid lean in insoluble
phosphates is returned to the anoxic zone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] Embodiments of the present invention will be described below
with reference to the following figures:
[0018] FIG. 1 is a schematic diagram illustrating a first
process.
[0019] FIG. 2 is a schematic diagram illustrating a second
process.
[0020] FIG. 3 is a schematic diagram illustrating a third
process.
[0021] FIG. 4 is a schematic diagram illustrating a first side
stream process.
[0022] FIG. 5 is a schematic illustration of a second side stream
process.
DETAILED DESCRIPTION OF THE INVENTION
[0023] A first process is shown in FIG. 1. An influent 12, which
contains BOD, ammonia and phosphates, enters an anaerobic zone 14
to mix with anaerobic mixed liquor contained there. An anaerobic
exit stream 16 carries anaerobic mixed liquor from the anaerobic
zone 14 to mix with anoxic mixed liquor contained in an anoxic zone
18. An anoxic exit stream 20 carries anoxic mixed liquor from the
anoxic zone 18 to mix with aerobic mixed liquor contained in an
aerobic zone 22. An aerobic exit stream 24 carries aerobic mixed
liquor from the aerobic zone 22 to the retentate or feed side a
membrane filter 26 located outside of the aerobic zone 22 and
preferably having microfiltration or ultrafiltration membranes. A
waste activated sludge stream 32, consisting of components or
material rejected by the membrane filter 26, exits from the
membrane filter 26. An effluent stream 34 exits from the permeate
side of the membrane filter 26 and is substantially phosphate
reduced. A denitrified liquor recycle 28 carries anoxic mixed
liquor from the anoxic zone 18 to mix with the anaerobic mixed
liquor in the anaerobic zone 14. As well, a nitrified liquor
recycle 30 carries aerobic mixed liquor from the aerobic zone 22 to
mix with the anoxic mixed liquor in the anoxic zone 18.
[0024] In the anaerobic zone 14, fermentive bacteria in the
anaerobic mixed liquor convert BOD into volatile fatty acids. Under
anaerobic conditions and in the absence of nitrates, Bio-P
organisms grow which use volatile fatty acids as a carbon source.
In doing so, they release phosphorus into the liquor, and store
volatile fatty acids as a substrate for later use. Anoxic mixed
liquor from the denitrified liquor recycle 28 decreases the
concentration of nitrates in the anaerobic mixed liquor in the
first anaerobic zone 14, should there be any, which enhances the
selection and growth of Bio-P organisms.
[0025] In the anoxic zone 18, Bio-P organisms in the anoxic mixed
liquor metabolize the stored volatile fatty acids, providing energy
for growth, and uptake phosphates from the solution. Thus, the
efficiency of phosphate uptake in the anoxic zone 18 is related to
the uptake of volatile fatty acids in the anaerobic zone 14
particularly given the nitrified liquor recycle 30 from the first
aerobic zone 22 to the anoxic zone 18. In the anoxic zone 18,
denitrifying bacteria also convert nitrates to N2 gas.
[0026] In the aerobic zone 22, Bio-P organisms in the aerobic mixed
liquor further metabolize the stored volatile fatty acids,
providing more energy for growth, and further uptake of phosphates
from the aerobic mixed liquor. In addition NH3 is converted to
nitrates to be recycled to the anoxic zone 18.
[0027] Aerobic mixed liquor flows to the feed or retentate side of
the membrane filter 26. The membrane filter 26 rejects
phosphorus-rich activated sludge, finely suspended colloidal
phosphorus, bacteria, inorganic particles such as grit, trash and
other cellular debris. Liquid containing this rejected material
forms a waste activated sludge stream 32 which may either be
discarded or sent to a secondary sludge processing system, such as
aerobic or anaerobic digestion. Alternatively, some of the liquid
containing material rejected by the membranes can be recycled to
the aerobic zone 22 through a retentate recycle stream 29 or to the
anoxic zone 18 through a second retentate recycle stream 29'. When
liquid containing material rejected by the membranes is recycled to
the anoxic zone 18, its flow and concentration is included in any
calculation of recycle from the aerobic zone 22 to the anoxic zone
18. In particular, where a second recycle stream 29' of high flux
is used, it may not be necessary to provide either a retentate
recycle stream 29 or a nitrified liquor recycle 30.
[0028] Referring now to FIG. 2, a second process is shown. The
second process is similar to the first process but with
modifications as described below. The retentate liquid containing
material rejected by the membranes does not leave the process
directly but are recycled back to process. This material leaves the
process indirectly, combined with sludge from the process in
general, as waste sludge taken from the aerobic zone 22. This waste
sludge flows out of the process in a second waste activated sludge
stream 31 from the aerobic zone 22 and may be periodically
discarded or sent to a secondary sludge management or processing
system, such as aerobic or anaerobic digestion. The liquid
containing material rejected by the membranes can be recycled to
the aerobic zone 22 through a retentate recycle stream 29 or to the
anoxic zone 18 through a second retentate recycle stream 29'. When
liquid containing material rejected by the membranes is recycled to
the anoxic zone 18, its flow and concentration is included in any
calculation of recycle from the aerobic zone 22 to the anoxic zone
18.
[0029] Referring now to FIG. 3, a third process is shown. The third
process is similar to the first process but with modifications as
described below. In the third process, a second membrane filter 33
is immersed in the aerobic zone 22. The second membrane filter 33
is driven by suction on an interior surface (permeate side) of the
membranes, the outside surface (retentate or feed side) of the
membranes is in fluid communication with the aerobic mixed liquor.
Thus material rejected by the second membrane filter 33 mixes with
the aerobic mixed liquor without requiring a retentate recycle
stream. As in the second process, material rejected by the
membranes leaves the process indirectly, combined with sludge from
the process in general, as waste sludge taken from the aerobic zone
22. This waste sludge flows out of the process in a second waste
activated sludge stream 31 from the aerobic zone 22 and may be
periodically discarded or sent to a secondary sludge management or
processing system, such as aerobic or anaerobic digestion.
[0030] Although there are differences between the processes
described above, they are similar in many respects and are operated
under similar process parameters. Net hydraulic retention times
(HRT) for all three zones combined (ie. sum of the volume of all
three zones divided by the feed rate) ranges from 1 to 24 hours, is
preferably between 2 and 12 hours, more preferably between 4.5 and
9 hours. In general, an increase in HRT often increases effluent
quality, but increase in effluent quality is less for each
additional hour of HRT. Similarly, reducing the HRT increases the
output of the process for a given plant size, but effluent quality
decreases more rapidly for each hour less of HRT. Sludge retention
time ranges from 5 to 40 days and is preferably between 10 and 25
days. MLSS concentration typically ranges between 3 and 30 g/L and
is preferably between 5 and 15 g/L. Recycle ratio (recycle to feed)
of the nitrified liquor recycle 30 typically ranges between 0.5 and
5 and is preferably between 1 and 2 where the process is used
primarily to remove phosphorous. Recycle ratio of the nitrified
liquor recycle 30 typically ranges between 1 and 8 and is
preferably between 2 and 4 where the process is used primarily to
remove phosphorous but nitrogen reduction is also important.
[0031] The recycle ratio (recycle to feed) of the denitrified
liquor recycle 28 typically ranges from 0.5 to 3, preferably
between 1 and 2. Less stringent phosphorous effluent requirements
may be met efficiently with a recycle ratio between 0.5 and 1, but
typical effluent requirements require a recycle ratio of over 1.
Recycle ratios over 2 may result in increased phosphorous removal
but only where residual nitrogen levels in the anoxic zone 18 are
very low. In typical processes, nitrogen levels in the anaerobic
zone 14 become detrimentally high with recycle ratios over 2.
[0032] FIG. 4 shows a first side stream process. Although the first
side stream process is shown in use with the first process of FIG.
1, it may also be used with the second and third processes of FIGS.
2 and 3. In general, a portion of the anaerobic mixed liquor is
treated in a solid-liquid separation device 42. A return stream 40
rich in suspended solids, including activated sludge and organic
impurities, is returned to the anoxic zone 18 and a solids lean
stream (a first phosphate-rich supernatant or filtrate 44) which is
rich in phosphorus is fed to a crystallizer or precipitator 50,
where insoluble crystalline phosphates are removed. With this
method, phosphorus is removed from the waste water treatment cycle
with near stoichiometric amounts of precipitating chemicals.
Phosphorous removal is enhanced because less phosphorous needs to
be taken up by the Bio-P organisms in the main process. The ability
to control phosphorus removal in the crystallizer or precipitator
50 through pH adjustments helps ensure that adequate phosphorus is
available in the process for microbial growth to occur. Finally, a
useful by-product, high purity struvite, may be recovered which can
be used as a fertilizer. Alternatively, phosphates may be
precipitated as a metal salt.
[0033] In greater detail, a first side stream process is shown
generally at 37 and draws anaerobic mixed liquor from the anaerobic
zone 14. The first side stream process 37 removes phosphorous from
the main process thereby assisting to reduce the build-up of
phosphorous in the system. In the anaerobic zone 14, activated
sludge releases phosphorous into the liquor. As such, the anaerobic
zone 14 contains liquor with the highest phosphorous concentration.
A first phosphate-rich flow stream 38 is taken from the anaerobic
zone 14 and sent to a separator 42. The separator 42 can use a
membrane or other filter media such as a sand filter, a cloth
filter, or fibre braids. The separator 42 can also be a clarifier
as the inventors' experience with this process has shown the
anaerobic sludge to be surprisingly settleable. A solids rich
return stream 40, comprising the phosphate-depleted sludge and
insoluble organics, exits from the separator 42 and recycles back
to the anoxic zone 18. A first phosphate-rich supernatant or
filtrate 44 exits the separator 42, is mixed with precipitating
chemicals 46, such as calcium or magnesium, and a combined stream
48 is fed into a crystallizer or precipitator 50. Since the first
phosphate-rich supernatant or filtrate 44 is substantially free of
organic impurities, the number of undesirable side reactions with
the precipitating chemicals 46 is reduced. As such, the
precipitating chemicals 46 can be added in near stoichiometric
amounts to precipitate out the insoluble phosphates.
[0034] A preferred method of crystallization involves using
granular seed materials, preferably high density coral sands with
grain size between 0.25 and 2.0 mm, to initiate and aid
crystallization. Preferably, the addition of magnesium, ammonium
and possibly additional phosphates allow high purity struvite
(MgNH3PO4*6H20) to form and collect at the bottom of the
crystallizer or precipitator 50. A bottoms flow stream 54
containing the insoluble phosphates is removed from the system and
collected. A crystallizer or precipitator exit stream 52, which is
both phosphate and nitrate lean may be returned to the anoxic zone
18 (52a), the aerobic zone 22 (52b) or be combined with the
effluent stream 34 (52c) depending on whether it needs further
treatment. For example, crystallizer or precipitator exit stream 52
high in COD is returned to the aerobic zone to decrease its COD
concentration before it is discharged from the process.
[0035] A preferred method of precipitation involves using alum as
the precipitating chemical 46. Surprisingly, there appears to be an
optimum dosage ranging between 400 and 800 mg/L at which maximum
phosphorous is removed. Within this range, phosphorous removal is
over 50% and may be as high as 93%. Phosphorous removal between 75%
to 85% was reliably achieved in testing using a dosage of 600 mg/L
of alum.
[0036] FIG. 5 shows a second side stream process. Although the
second side stream process is shown in use with the first process
of FIG. 1, it may also be used with the second and third processes
of FIGS. 2 and 3. In the second side stream process, sludge is
optionally filtered through a screen to remove any large objects,
hair or other trash that could interfere with the other operations
in the side stream process. Subsequently, chemicals are added to a
reaction zone to create insoluble phosphates. The stream is then
passed to a hydrocyclone which separates the organics from the
inorganics, grit, and inerts, which include the insoluble
phosphates. The phosphates are disposed of as inorganic waste, and
the phosphorous-depleted activated sludge is recycled back to the
anoxic zone.
[0037] The second side stream process is shown generally at 57 to
draw anaerobic sludge from the anaerobic zone 14. The second side
stream process 57 removes phosphates from the main process thereby
assisting to reduce the build-up of phosphates in the system. As
discussed above, the anaerobic zone 14 contains liquor with the
highest phosphate concentration. A second phosphate-rich flow
stream 58 exits from the anaerobic zone 14 and flows to a screen 60
to separate trash etc which leaves the process to be treated
further or discarded. A screen exit stream 62 from the screen 60 is
mixed with a precipitating chemicals flow stream 64, which contains
chemicals such as ferrous chloride and aluminum sulphate, and a
second combined stream 66 is sent to a reaction zone 70, or
alternately to a precipitation tank. A reaction zone exit stream 72
from the reaction zone 70 flows to a hydrocyclone 74. The
hydrocyclone 74 separates the organic material from the inorganics,
including insoluble phosphates, grit, and other inerts, due to the
differences in densities. Hydrocyclone bottoms 76, including the
insoluble phosphates, grit, and other inerts, are landfilled,
applied to the land or otherwise processed or wasted. A sixth waste
activated sludge stream 78 which exits the hydrocyclone 74 is sent
back to the anoxic zone 18.
EXAMPLES
[0038] An experimental reactor was set up as shown in FIG. 3. The
membrane filter consisted of four ZEEWEEDTM ZW-10TM modules
produced by Zenon Environmental Inc. having a total of 40 square
feet of membrane surface area. A control reactor was set up as
shown in FIG. 2 but (a) using a clarifier instead of the membrane
filter 26, (b) recycling the clarifier bottoms to the anoxic zone
18 and (c) not using a retentate recycle stream 29 or nitrified
liquor recycle 30. Both reactors had a volume of 1265 L, the volume
of the clarifier not being counted as reactor volume. Sludge
retention time (SRT) was kept constant at 25 days.
[0039] Three experimental runs were conducted with the experimental
reactor at hydraulic retention times (HRTs) of 9 hours, 6 hours and
4.5 hours produced by varying the feed flow rate. The control
reactor was run successfully at a hydraulic retention time of 9
hours using the same operating parameters as for the run of the
experimental reactor with a 9 hour HRT. Running the control reactor
at a hydraulic retention time of 6 hours was attempted, but
adequate operation could not be achieved (because the clarifier
failed), most conventional processes running at an HRT of about 12.
The sizes of the zones and the HRTs of each zone are summarized in
Table 1 below.
1TABLE 1 Process Anaerobic Aerobic Overall Zone Sizing Zone Anoxic
Zone Zone Bioreactor Volume 1/11 4/11 6/11 11/11 Fraction Working
115 460 690 1265 Volume Operating [hr] [hr] [hr] [hr] HRTs Run #1
0.82 3.27 4.91 9.0 Run #2 0.55 2.18 3.27 6.0 Run #3 0.41 1.64 2.45
4.5
[0040] During the first run, the experimental and control reactors
were operated at a 9 hour HRT for 16 weeks. The MLSS concentration
varied between 3-5 g/L during this period. A summary of the average
P and N concentrations for both reactors is shown in Table 2 below.
As shown in that table, the experimental reactor achieved a greater
reduction of o-PO4. Effluent P was generally below 0.3 mg/L for the
experimental process while effluent P for the control process
varied from 0.2-0.7 mg/L. Both processes had similar reduction of
NH3. The experimental reactor had not been optimized for nitrogen
removal. The nitrified recycle was set nominally at a 1:1 (recycle
to feed) to be the same as the control process. Other experiments,
to be described below, revealed that a recycle ratio of 3:1
produced better nitrogen removal in the experimental process.
Nevertheless, the experimental process removed greater than 80% of
total nitrogen at the 1:1 recycle ratio.
2TABLE 2 Anaero- Aero- Para- In- bic Anoxic bic Reduc- Process
meter fluent Zone Zone Zone Effluent tion Experi- o-PO.sub.4 3.04
8.66 2.61 0.17 0.11 96.4% mental [mg/l] NH3-N 22.0 12.7 5.7 0.1
0.09 99.6% [mg/L] NO3-N -- 0.13 0.28 5.68 5.72 [mg/L] Control
o-PO.sub.4 3.04 6.09 5.07 0.27 0.50 83.6% [mg/L] NH3-N 22.0 11.3
5.4 0.04 0.11 99.5% [mg/L] NO3-N -- 0.15 0.10 5.79 2.60 [mg/L]
[0041] During the second run, the experimental reactor was operated
at a 6 hour HRT for about 14 weeks. The MLSS concentration
increased from about 4 mg/L at the start to about 8 mg/L at the end
of the run. By the end of the run, the experimental process had
stabilized in terns of VFA uptake and phosphorous release in the
anaerobic section. There was a slow and steady improvement in
performance as the experimental run progressed, the monthly average
effluent P dropping from 0.178 mg/L to 0.144 mg/L to 0.085 mg/L
over the approximately three months of the test. The inventors
believe that at least part of this improvement can be attributed to
the increase in MLSS over the duration of the test. Effluent NH3
was less than 0.5 mg/L and total nitrogen removal was greater than
80%.
[0042] As mentioned above, the control reactor could not be operate
adequately at this HRT. During periods when the control reactor was
operated, effluent P varied between 0.1-0.9 mg/L.
[0043] During the third run, the experimental reactor only was run
at an HRT of 4.5 hours. MLSS concentration increased to 15 g/L.
Effluent P concentrations were generally below 0.5 mg/L over a
three month period, still better than the P removal of the control
reactor operated at a 9 hour HRT.
[0044] In other experiments, the experimental process was operated
at an HRT of 6 hours but the recycle from the aerobic zone to the
anoxic zone was modified from a recycle ratio (recycle to feed) of
1:1 to 3:1. N and P removal were measured at each recycle ratio and
the results included in Table 3 below. Nitrogen removal increased
with the increased recycle ratios while P removal was generally
unaffected.
3TABLE 3 Influent Effluent Effluent Total N Total N PO.sub.4
Recycle (mg/L as (mg/L as N-Removal (mg/L as P-Removal Ratio N) N)
Efficiency P) Efficiency 1:1 49.6 11.8 76.4% 0.07 98.9% 1.5:1 49.8
9.0 81.6% 0.15 97.7% 2:1 40.7 5.8 85.8% 0.15 97.3% 3:1 42.1 5.3
87.5% 0.12 97.8%
[0045] It is to be understood that what has been described are
preferred embodiments of the invention. The invention nonetheless
is susceptible to certain changes and alternative embodiments fully
comprehended by the spirit of the invention as defined by the
claims below.
* * * * *