U.S. patent application number 11/262750 was filed with the patent office on 2006-03-16 for membrane bioreactor, process and aerator.
Invention is credited to Pierre Cote, Hidayat Husain, Minggang Liu, Ian Pottinger.
Application Number | 20060054552 11/262750 |
Document ID | / |
Family ID | 32467926 |
Filed Date | 2006-03-16 |
United States Patent
Application |
20060054552 |
Kind Code |
A1 |
Liu; Minggang ; et
al. |
March 16, 2006 |
Membrane bioreactor, process and aerator
Abstract
A reactor has an aerobic tank, an anoxic tank and a sealed
membrane tank with conduits for circulating mixed liquor between
them. Permeation starts when the mixed liquor reaches a high level
and stops when the mixed liquor reaches a low level. A sensor, for
detecting the mixed liquor level, may stop and start permeation.
Pressure builds in the membrane tank when membrane air is on.
Transmembrane pressure is also provided by gravity flow or siphon.
Membrane air generates an air lift which drives the mixed liquor
circulation. The total amount of air provided by an air source is
divided and varied in time between the membrane aerator and the
process aerator. The process aerator acts as a screening inlet to
the conduit to the membrane tank. Chemical maintenance cleaning is
provided by gravity flow.
Inventors: |
Liu; Minggang; (Burlington,
CA) ; Husain; Hidayat; (Oakville, CA) ; Cote;
Pierre; (Dundas, CA) ; Pottinger; Ian;
(Burlington, CA) |
Correspondence
Address: |
BERESKIN AND PARR
40 KING STREET WEST
BOX 401
TORONTO
ON
M5H 3Y2
CA
|
Family ID: |
32467926 |
Appl. No.: |
11/262750 |
Filed: |
November 1, 2005 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10950460 |
Sep 28, 2004 |
|
|
|
11262750 |
Nov 1, 2005 |
|
|
|
10309819 |
Dec 5, 2002 |
6863817 |
|
|
10950460 |
Sep 28, 2004 |
|
|
|
Current U.S.
Class: |
210/605 ;
210/198.1; 210/221.2; 210/614; 210/622 |
Current CPC
Class: |
B01D 2315/06 20130101;
B01D 2321/185 20130101; B01D 65/08 20130101; Y02W 10/10 20150501;
C02F 3/1215 20130101; C02F 2209/42 20130101; B01D 2313/26 20130101;
B01D 2321/40 20130101; C02F 3/302 20130101; C02F 3/1273 20130101;
B01D 65/02 20130101; C02F 2103/002 20130101; B01D 61/22 20130101;
C02F 3/1268 20130101; B01D 2321/16 20130101; B01D 2311/2642
20130101; C02F 2103/005 20130101; Y02W 10/15 20150501; B01D 2321/04
20130101; Y10S 210/903 20130101 |
Class at
Publication: |
210/605 ;
210/614; 210/622; 210/198.1; 210/221.2 |
International
Class: |
C02F 3/30 20060101
C02F003/30; C02F 1/74 20060101 C02F001/74 |
Claims
1. An apparatus comprising, a) one or more process tanks or zones
for one or more of aerobic digestion, nitrification or
denitrification; b) an immersed membrane tank or zone; c) a conduit
or passage for mixed liquor to flow from the membrane tank or zone
to the one or more process tanks or zones; d) a conduit or passage
for mixed liquor to flow from the one or more process tanks or
zones to the membrane tank or zone; e) an inlet for feed to enter
into one or more of the tanks or zones; f) an outlet for permeate
to exit the membrane tank or zone; and, g) a level sensor for
detecting a selected maximum mixed liquor level and a selected
minimum mixed liquor level in one of the one or more process tanks
or zones.
2. The apparatus of claim 1 wherein the level sensor is operatively
connected to devices for one or more of (a) stopping and starting
permeation, (b) increasing and decreasing the total air supply
provided by an air supply or (c) altering a regime of cyclic
aeration.
3. An aeration system comprising, a) one or more process aerators;
b) one or more membrane aerators; c) an air supply; d) a process
air line which provides a path for air from the air supply to the
one or more process aerators; e) a membrane air line which provides
a path for air from the air supply to the one or more membrane
aerators; wherein the air supply is adapted to provide at least a
higher flow rate of air and a lower flow rate of air.
4. The aeration system of claim 3 combined with the apparatus of
claim 1 wherein the level sensor is operatively connected to the
air supply to control when air is provided at the higher rate of
flow and the lower rate of flow.
5. An apparatus comprising, a) one or more process tanks or zones
for one or more of aerobic digestion, nitrification or
denitrification; b) an immersed membrane tank or zone; c) a conduit
or passage for mixed liquor to flow from the membrane tank or zone
to the one or more process tanks or zones; d) a conduit or passage
for mixed liquor to flow from the one or more process tanks or
zones to the membrane tank or zone; e) an inlet for feed to enter
into one or more of the tanks or zones; f) an outlet for permeate
to exit the membrane tank or zone; g) a level sensor for detecting
a selected maximum mixed liquor level and a selected minimum mixed
liquor level in one of the one or more process tanks or zones; and,
h) an aeration system, wherein the aeration system comprises, i)
one or more aerators in the process tanks or zones and one or more
aerators in the membrane tank or zone; ii) an air supply, wherein
the air supply is adapted to provide at least a higher flow rate of
air and a lower flow rate of air; and, iii) air lines which provide
a path for air from the air supply to the one or more aerators in
both the process tanks or zones and the membrane tank or zone;
wherein the level sensor is operatively connected to the air supply
to control when air is provided at the higher rate of flow and the
lower rate of flow.
6. The aeration system of claim 5 wherein air is provided at the
higher rate of flow whenever the level of mixed liquor is above a
high level, the high level related to the need for the higher rate
of flow to meet process requirements.
7. The aeration system of claim 5 wherein air is provided at the
higher rate of flow from the time that the mixed liquor reaches the
maximum mixed liquor level to the time when the mixed liquor
reaches the minimum mixed liquor level.
8. The aeration system of claim 5 wherein the aerator is a fine
bubble aerator.
Description
[0001] This application is a continuation of U.S. Ser. No.
10/950,460, filed Sep. 28, 2004 which is a division of U.S. Ser.
No. 10/309,819, filed Dec. 5, 2002. U.S. Ser. Nos. 10/309,819 and
10/950,460 are incorporated herein, in their entirety, by this
reference to them.
FIELD OF THE INVENTION
[0002] This invention relates to a wastewater treatment process or
apparatus, to a membrane bioreactor, to an aerator and to a method
and system for treating home, multi-residential, commercial,
institutional or industrial wastewater such as black or gray
water.
BACKGROUND OF THE INVENTION
[0003] Currently, many small wastewater treatment systems use a
septic tank, followed by a septic field for final purification and
discharge. Increasingly, this method is becoming unacceptable
because of the low level of treatment achieved, frequent failures
and high cost of reconstruction, contamination of streams and
groundwater, and the requirement for large land area to establish
septic fields.
[0004] Japanese publication 2000-028675, German application DE 198
07 890 A1 and PCT publication No. WO 00/37369 describe wastewater
treatment systems using membranes.
[0005] PCT Publication No. WO 00/21890 describes a cyclic aeration
system.
[0006] Japanese publication JP 2002-066261 describes a device to
catch fibrous foreign substances.
SUMMARY OF THE INVENTION
[0007] It is an object of the present invention or inventions to
improve on the prior art. Other objects of the invention include
providing a wastewater treatment process or apparatus, providing a
membrane bioreactor, providing an aerator and providing a method
and system for treating home, multi-residential, commercial,
institutional or industrial wastewater such as black or gray water.
The one or more inventions consist of combinations of one or more
of the elements or steps described in this document. The summary
below discusses various features of the one or more inventions that
may help the reader understand the one or more inventions, but is
not intended to define any invention.
[0008] In some aspects, the invention provides a reactor having an
aerobic tank or zone, an anoxic tank or zone and an immersed
membrane tank or zone with conduits for circulating tank water or
mixed liquor from the membrane tank or zone to the anoxic tank or
zone to the aerobic tank or zone and back to the membrane tank or
zone. Feed enters the reactor through an inlet, into the anoxic
tank or zone for example, and permeate exits the reactor from the
membrane tank or zone. The permeate has lower concentrations of
organic carbon, ammonia and total nitrogen than the feed. The
membrane tank or zone may be a sealed or substantially sealed area
while the aerobic and anoxic tanks or zones are open to atmospheric
pressure. An optional quiescent zone may be provided in an anoxic
tank or zone to settle and collect a potion of suspended matter.
The anoxic tank or zone or the quiescent zone may have sufficient
capacity such that settled solids needs to be removed only once a
year or less from the system. A coagulant may be added for chemical
precipitation of phosphorus.
[0009] In other aspects, the invention provides a reactor having
one or more process tanks or zones for one or more of aerobic
digestion, nitrification or denitrification and a membrane tank or
zone. Mixed liquor circulates between the membrane tank or zone and
the process tanks or zones. Permeate is removed from the membrane
tank or zone when the level of mixed liquor in the one or more
tanks or zones reaches a selected maximum mixed liquor level and
stops when the level of mixed liquor in the one or more tanks or
zones reaches a selected minimum mixed liquor level. A level sensor
may be provided for detecting the selected maximum mixed liquor
level and the selected minimum mixed liquor level in one of the one
or more process tanks or zones. The level sensor may be operatively
connected to devices for one or more of (a) stopping and starting
permeation, (b) increasing and decreasing the total air supply
provided by an air supply or (c) altering a regime of cyclic
aeration. Isolation valves may be provided in the conduits or
passages to permit the membrane tank to be removed from the one or
more process tanks or zones.
[0010] In other aspects, the invention provides one or more
membrane modules located in a membrane tank or zone and an aerator
for aerating the one or more membranes or membrane modules at least
from time to time. The membrane tank or zone is sealed or
substantially sealed such that pressure in the membrane zone or
tank increases during at least a portion of a time during which the
aerator is operated even though the mixed liquor in the membrane
tank or zone becomes less dense. Permeate flow rate increases when
pressure in the membrane tank or zone increases. Transmembrane
pressure for withdrawing permeate may also be provided by a gravity
flow outlet or siphon connected to a permeate side of the one or
more membrane modules. The membrane aerator may also be used to
generate an air lift effect which facilitates mixed liquor flowing
into the membrane tank or zone through an inlet and flowing out of
the membrane tank or zone through an outlet. The inlet and outlet
may be connected to one or more process tanks or zones.
[0011] In other aspects, the invention provides a reactor having
one or more process tanks or zones, a membrane tank or zone, one or
more membrane aerators for producing bubbles in the membrane tank
or zone, one or more process aerators for producing bubbles in one
of the one or more process tanks or zones and an air source
connected to the one or more membrane aerators. The air source is
connected to the one or more process aerators and the one or more
membrane aerators and the total amount of air provided by the air
source is divided between the membrane aerator(s) and the process
aerator(s). The amount of air provided by the air source to the one
or more process aerators may vary in time and the amount of air
provided by the air source to the one or more membrane aerators may
vary in time.
[0012] In other aspects, the invention provides an aerator having
an aerator body with holes for releasing bubbles from the aerator
body or for allowing mixed liquor surrounding the aerator to enter
the aerator body. During at least some first periods of time, air
enters the aerator body through an inlet to produce bubbles. During
at least some time, which may be different than the first periods
of time, mixed liquor flows into the aerator body through the holes
and exits the aerator body through an outlet. The outlet may be
connected to a mixed liquor conduit for removing mixed liquor from
a tank or zone containing the aerator. The inlet may be connected
to an aeration system which provides an airflow sufficient to
create bubbles from at least some of the holes at some times and
which allows mixed liquor to flow into the holes during at least
some times.
[0013] In other aspects, the invention is related to an aeration
system having one or more process aerators, one or more membrane
aerators and an air supply. A process air line provides a path for
air from the air supply to the one or more process aerators and a
membrane air line provides a path for air from the air supply to
the one or more membrane aerators. A valve is provided in one of
the process air line or the membrane air line, the valve having a
first position in which it is fully or partially open and a second
position in which it is fully or partially closed. When the valve
is in the first position, at least a major portion of the air
provided by the air supply flows to the one or more aerators
serviced by the air line having the valve, and when the valve is
closed, at least a major portion of the air provided by the air
supply flows to the other one or more aerators serviced by the
other air line. For example, the one or more aerators serviced by
the air line having the valve may be located to a lesser depth in
water compared to the one or more aerators serviced by the other
air line. The valve may be controlled by a valve controller which
changes the valve from the first position to the second position
and back to the first position in repeated cycles, for example,
repeated cycles between 10 and 40 seconds long. The air supply may
also be adapted to provide at least a higher flow rate of air and a
lower flow rate of air. Air from the air supply may be provided at
the higher rate of flow whenever the level of mixed liquor is above
a high level, the high level related to the need for the higher
rate of flow to meet process air requirements, or during
permeation. The distribution of air in quantity or time between
process air and membrane air may be altered from time to time to
maintain acceptable process oxygen concentrations in the one or
more process tanks or zones.
[0014] In other aspects of the invention, air provided to scour
membranes in a membrane tank or zone also removes solids from the
membrane tank or zone, or also provides biomass recirculation, for
example from the membrane tank or zone to an anoxic tank or zone,
to an aerobic tank or zone and back to the back to the membrane
tank or zone to facilitate nitrification and denitrification.
[0015] In other aspects of the invention, a wastewater level sensor
and/or a timer are used to control aspects of a process, for
example by changing how air from a blower is split between process
aerators and membrane aerators.
[0016] In other aspects of the invention, a plurality of blowers
are provided and one or more but not all of the blowers are turned
off, for example by a timer, during periods of low influent
flow.
[0017] In other aspects of the invention, chemical cleaning is
provided by a backwashing flow of a chemical cleaning solution
provided, for example, by gravity, followed by a period in which
permeate is not withdrawn. The frequency of chemical cleaning may
depend on the specific application and is determined by the rate of
fouling.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] Exemplary embodiments or one or more inventions will be
described below with reference to the following Figures.
[0019] FIG. 1 is a schematic representation of a system.
[0020] FIG. 2 is an isometric view of a membrane tank of the system
of FIG. 1.
[0021] FIG. 3 is a schematic representation of an
aerator/inlet.
[0022] FIG. 4 is a photograph of an aerator/inlet after a period of
use.
DETAILED DESCRIPTION OF EMBODIMENTS
Basic Configuration
[0023] FIG. 1 shows a reactor 10 for treating wastewater. The
reactor 10 has an anoxic tank 12, an aerobic tank 14 and a membrane
tank 16 containing a membrane module 18 or multiple membrane
modules. The membrane tank 16 is sealed such that the only openings
to it are through the pipes to be described below. The anoxic tank
12 and aerobic tank 14 are open to atmospheric pressure although
they may be covered to prevent unwanted matter from falling into
them. A feed line 20 transports influent or feed water to the
reactor 10, for example to the anoxic tank 12. During regular
operation, the reactor 10 is filled with mixed liquor to a level
between a minimum mixed liquor level 22 and a maximum mixed liquor
level 24. The minimum mixed liquor level 22 is above the tops of
the membrane module 18 and, typically, also above the top of the
membrane tank 16.
[0024] An anoxic line 26 provides a conduit below the minimum mixed
liquor level 22 for anoxic mixed liquor and influent to flow from
the anoxic tank 12 to the aerobic tank 14. An aerobic line 28
provides a conduit below the minimum mixed liquor level 22 for
aerobic mixed liquor to flow from the aerobic tank 14 to the
membrane tank 16. A permeate line 30 provides a conduit for
permeate to exit the reactor 10 from the permeate side of the
membrane module 18. A retentate return line 32 provides a path for
mixed liquor or mixed liquor and air bubbles to flow from the
membrane tank 18 to the anoxic tank 12. During normal operation,
mixed liquor circulates repeatedly through the anoxic tank 12,
aerobic tank 14 and membrane tank 16 to provide alternating aerobic
and anoxic digestion. This facilitates alternating nitrification
and denitrification of the mixed liquor. The membrane module 18
provides filtration. The resulting permeate has reduced
concentrations of solids, organic carbon, ammonia and total
nitrogen.
[0025] If substantial reduction of nitrate and total nitrogen is
not required, the anoxic tank 12 may be omitted with corresponding
alterations. For example, the feed line 20 is connected to the
aerobic tank 14, the anoxic line 26 is omitted and the retentate
return line 32 is connected to the aerobic tank 14. Other
alterations relating to further features described below may also
be required. Without an anoxic tank, mixed liquor circulates
between the aerobic tank 14 and the membrane tank 16 to provide
aerobic digestion and filtration. A modified or retrofitted septic
tank may be used as the aerobic tank 14. Similarly, if phosphorous
removal is desired, additional tanks or other devices may be
provided as described, for example, in U.S. Pat. No. 6,406,629
issued Jun. 18, 2002 to Husain et al. The entire disclosure of U.S.
Pat. No. 6,406,629 is incorporated into this document by this
reference to it.
[0026] Although distinct tanks are shown in FIG. 1 and have been
discussed above, the one or more inventions may be practiced with a
single tank divided into one or more aerobic, anoxic or membrane
zones, or with other numbers of tanks or zones. For example, the
membrane tank 16 and aerobic tank 14 may be merged by placing the
membrane module 18 in a part or zone of the aerobic tank 14
separated by appropriate baffles, walls or shrouding and provided
with a modified air lift configuration. The anoxic tank 12 and
aerobic tank 14 may similarly be merged into a single tank having
anoxic and aerobic zones.
Removable Membrane Tank
[0027] The retentate return line 32 and aerobic line 28 may be
fitted with isolation valves 34. With the isolation valves 34
closed, the membrane tank 16 becomes a completely sealed unit. The
membrane tank 16 may be removed from the reactor 10 for
maintenance, repair or replacement without disturbing the anoxic
tank 12 or aerobic tank 14. The membrane module 18 can also be
removed to a remote location for maintenance, repair or replacement
without removing it from the mixed liquor at the site of the
reactor 10 to reduce the odors and risk of spills at the site of
the reactor 10.
[0028] For example, the membrane module 18 will have a longer life
if it is intensively recovery cleaned from time to time, for
example once a year. A service provider may maintain a number of
membrane modules 18 in membrane tanks 16 in stock or inventory.
When a first user's membrane module 18 requires recovery cleaning
or other maintenance, repair or replacement, the service provider
removes the customer's existing membrane tank 16 and membrane
module 18 and replaces them with clean replacements. The removed
units are taken back to the service provider's facilities, cleaned
and returned to inventory for later delivery to the first or
another user.
Aeration
[0029] Membrane air, meaning air for the purpose of providing
bubbles to inhibit fouling of the membranes, enters the membrane
tank 16 through a membrane air line 36. The membrane air line 36
carries the air (or other gases used for the same purpose) to one
or more membrane aerators 38. Bubbles are formed at the membrane
aerator 38 and rise past or through the membrane module 18 to
inhibit fouling or clean the membranes. Holes in the membrane
aerator 12 may be made to produce large bubbles that are effective
at reducing fouling of the membranes.
[0030] Process air, meaning air to keep the mixed liquor in the
aerobic tank 14 under aerobic conditions, is provided to the
aerobic tank 14 through a process air line 40. The process air line
40 brings air (or other suitable gases) to a process aerator 42.
The process aerator 42 may have smaller holes to produce smaller
bubbles than the membrane aerator 38 does for efficient oxygen
transfer to the aerobic mixed liquor. In the exemplary embodiment,
however, the process aerator 42 has the same hole size as the
membrane aerator 38 and is a coarse bubble aerator.
[0031] Both the membrane air line 36 and the process air line 40
may be connected to the same blower 44 or other source of
pressurized air or other suitable gas. The supply of air from the
blower may further be cycled between the membrane aerator 38 and
the process aerator 42, for example, by using a cyclic aeration
system as described in U.S. Pat. No. 6,245,239 issued on Jun. 12,
2001 to Cote et al. The entire contents of U.S. Pat. No. 6,245,239
are incorporated into this document by this reference to it.
[0032] The exemplary embodiment of FIG. 1 uses other means to
transfer all, substantially all or a major portion of the air
provided by the blower 44 first to one of the aerators 38, 42 and
then to the other of the aerators 38, 42 in repeated cycles. One of
the aerators 38, 42 is located at an elevation sufficiently above
the other aerator 38, 42 such that, with the membrane air line 36
and process air line 40 both open, a major portion, substantially
all or all of the air provided by the blower 44 flows through the
higher of the aerators 38, 42. Differences in factors that create
head losses associated with flow to and through either of the
aerators 38, 42 may also be used to achieve the same or a similar
effect. An air valve 46 is provided in the air line 36, 40 leading
to the higher of the aerators 38, 42 or the one which has higher
air flow if both air lines 36, 40 are open. The air valve 46 may be
closed partially or fully such that a major portion, substantially
all or all of the air provided by the blower 44 then flows through
the other of the aerators 38, 42. For example, the process aerator
42 may be located above the elevation of the membrane aerator 38
and the air valve 46 provided in the process air line 40. The
relative elevations of the aerators 38, 42 and the head losses
involved in flowing air to and through the aerators 38, 42 are such
that, with the air valve 46 fully open, 90% or more of the air
provided by the blower 44 flows through the process aerator 42.
From time to time, the air valve 46 is fully, or substantially
fully, closed and all, or substantially all, for example 90% or
more, of the air from the blower 44 then flows to the membrane
aerator 38.
[0033] The air valve 46 may be operated automatically by an air
valve controller 48. For example, the air valve controller 48 may
use a solenoid or hydraulic or pneumatic piston connected to the
air valve 46. The air valve controller 48 may also incorporate a
timer or programmable logic controller to provide a repeated
aeration cycle. In the exemplary embodiment, a timer is used in the
air valve controller 48 and reverses the position of the air valve
46 (i.e. opens the air valve 46 if it is closed, and closes the air
valve 46 if it is open) at the end of every time period of a preset
duration. The time period may be, for example, between 5 and 20
seconds for a total cycle time between 10 and 40 seconds. In the
exemplary embodiment, the time period ranged between 7 seconds and
10 seconds and the total cycle time ranged between 14 seconds and
20 seconds. The air valve 46 is closed abruptly to produce a
pressure spike in the membrane aerator 38 and an initial rush of
large bubbles. Such an initial rush of large bubbles appears to
effectively inhibit fouling of the membrane module 18. The time
varying supply of membrane air also creates transience in the
membrane tank 16 that further inhibits fouling of the membrane
module 18.
[0034] Optionally, other distributions of air, in time and flow
rate, can be provided. For example, a larger portion of the air may
be provided to either of the aerators 38, 42. With sufficiently
small holes in the process aerator 42, oxygen transfer efficiency
may be sufficient to allow air to be provided to the membrane
aerator 38 for more than half of every aeration cycle. If a fine
bubble aerator is used in the aerobic tank, a regulating or manual
valve may be added to the membrane air line 36, or the air valve 46
may be moved to the membrane air line 36 and the membrane aerator
38 located below the process aerator 42, to ensure that a larger
portion of the air flow can be provided to the process aerator 42
during part of the cycle without requiring an excessive difference
in elevation between the aerators 38, 42. In contrast, particularly
during periods of high feed flow, the cycle may also be changed to
increase the duration or amount of airflow to the process aerator
42 relative to the membrane aerator 38 if required to maintain an
appropriate dissolved oxygen concentration in the aerobic tank 14.
During periods of low feed flow, the cycle may be changed again to
increase the duration or amount of airflow to the membrane aerator
38 relative to the process aerator 42 to provide membrane cleaning
while keeping the dissolved oxygen concentrations in the anoxic
tank 12 low enough for adequate denitrification.
[0035] In the exemplary embodiment, a single blower 44 is used and
it is run at a substantially constant speed over extended periods
of time. Optionally, a two speed or variable speed blower may also
be provided and operated so that a higher total supply of air is
increased during permeation and a lower total supply of air is
provided when permeate is not produced. While providing a higher
total amount of air, the aeration cycle may also be modified so
that the bulk of the increased air is provided to the membrane
aerator 38 or so that the dissolved oxygen content in the aerobic
tank 14 is not increased to an undesirable or unnecessary level.
The higher blower speed may also be used when the level of mixed
liquor in the reactor 10 is high. In this case, the aeration cycle
may be modified to favor the process aerator 42 to provide
sufficient air to maintain aerobic conditions in the aerobic tank
14. Further optionally, a second blower 50 may be used, the two
blowers 44, 50 being sized so that they together have enough output
to meet the expected maximum air requirements of the reactor 10. At
high levels of mixed liquor in the reactor 10 or while permeating,
both blowers 44, 50 are run. While the level of mixed liquor is low
or while not permeating, only one of the blowers 44, 50 is used to
reduce energy consumption and help keep the anoxic zone at an
acceptable oxygen concentration. The aeration cycles may also be
suitably modified depending on which of the blowers 44, 50 is
running and whether permeate is being produced or not. If it is
desired at some point to have a low supply of membrane air while
permeating, membrane permeability may be recovered by providing
additional membrane air after permeating. If only one single speed
blower 44 or another invariable source or air is used, the aeration
cycle may be biased in time or flow rate towards process air when
the mixed liquor level is high to meet the elevated oxygen demand.
The aeration cycle may also be biased in time or flow rate towards
membrane air when the mixed liquor level is low to reduce process
aeration as required to maintain anoxic conditions in the anoxic
tank 12 despite a lower organic loading and to recover membrane
permeability prior to the next significant permeation cycle.
Aerator/Inlet
[0036] As shown in FIG. 1, mixed liquor enters the aerobic line 28
through the process aerator 42. The process aerator 42 of the
exemplary embodiment is also shown in greater detail in FIG. 3. The
process aerator 42 has an aerator body 52 pierced with a number of
aerator holes 54. The holes 54 may be of a various sizes, for
example 0.25 inches in the exemplary embodiment. When process air
is provided, air enters the process aerator 42 from the process air
line 40 through an air inlet 56, flows through the aerator body 52
and bubbles are produced at the aerator holes 54. When process air
is not provided at a sufficient flow rate to produce bubbles at all
of the aerator holes 54, mixed liquor flows through some or all of
the aerator holes 54 into the aerator body 52. The mixed liquor
then flows through a mixed liquor outlet 58 and through the aerobic
line 28 to the membrane tank 16.
[0037] As the air flow to the process aerator 42 is varied as was
described further above, the process aerator alternately acts as an
aerator and as a screening inlet for the aerobic line 28. As a
screening inlet, the process aerator 42 reduces the amount of
solids, particularly trash, hair and other stringy elements; that
flow to the membrane tank 16. Such trash and stringy elements can
cause the membrane module 18 to foul quickly. When air flow to the
process aerator 42 is high, the bubbles created blow the trash and
other elements away from the aerator holes 54. When air flow to the
process aerator 42 is low, liquid mixed liquor flows through the
aerator holes 54 which inhibits the growth of dried sludge deposits
in the holes. Accordingly, cleaning is provided for the process
aerator 42 both as an aerator and as a screened inlet.
Mixed Liquor Circulation
[0038] Circulation of the mixed liquor is provided by the membrane
air which creates an air lift effect in the membrane tank 16. With
membrane air on, bubbles of air travel from the top of the membrane
tank 16 into and through the retentate return line 32 carrying
entrained mixed liquor into the anoxic tank 12. The density of the
mixed liquor in the membrane tank 16 is also reduced, causing mixed
liquor from the aerobic tank 14 to be drawn into the membrane tank
16 while mixed liquor from the membrane tank 16 travels to the
anoxic tank 12. The level of mixed liquor in the anoxic tank 12 is
made slightly higher than the level of mixed liquor in the aerobic
tank 14 and so mixed liquor flows by gravity from the anoxic tank
12 to the aerobic tank 14 to complete the recirculation loop. In
this way, the membrane air is used to circulate mixed liquor
through the reactor 10. However, because the membrane tank 16 is
sealed, or at least substantially sealed, pressure in the membrane
tank 16 is higher than would be expected considering the reduced
density of the mixed liquor in the membrane tank 16 alone. The
added pressure caused by the membrane air works against the
circulation of mixed liquor. This is acceptable, or even desirable,
since the membrane air might otherwise cause too much circulation
and the amount of pressure created due to membrane air can be
controlled by altering the amount of membrane air relative to the
size, position and length of the retentate return line 32. In this
way, an acceptable circulation flow rate and desired membrane air
flow rate can both be achieved.
[0039] With cyclic aeration as described above, the air lift is
alternately created and dissipated and the comments in the
paragraph above refer to average conditions over time. In the
exemplary embodiment, for example, a new air lift is begun every 14
to 20 seconds and lasts for about 7 to 10 seconds. Accordingly, the
rate of mixed liquor circulation fluctuates. In particular, when
the membrane air is first turned on, or the flow rate of membrane
air is suddenly increased, there is a rush of large bubbles that
cause a burst of mixed liquor flow into the anoxic tank 12. The
flow rate of the mixed liquor to the anoxic tank 12 decreases after
this initial burst. When membrane air is turned off, or its flow
rate significantly decreased, the mixed liquor flow rate in the
anoxic tank 12 again decreases. Towards the end of a period of low
or no membrane air, the mixed liquor flow may cease or temporarily
reverse with some mixed liquor flowing back from the anoxic tank
12. The net flow, however, remains positive towards the anoxic tank
12. The bubbles entering the anoxic tank 12 cause some oxygen
transfer to the anoxic mixed liquor but, since they are large
scrubbing bubbles and enter part way up the anoxic tank 12,
conditions in the anoxic tank 12 remain anoxic. Flows from the
aerobic tank 14 to the membrane tank 16 also vary. It is possible
for a slight reverse flow to occur back to the aerobic tank 14 at
the end of a period when membrane air is provided, but net flow
remains towards the membrane tank 16. As mentioned above, an excess
of flow back towards the aerobic tank 14 can be cured by altering
the retentate return line 32 so that less pressure is created in
the membrane tank 16 by the membrane air. In the exemplary
embodiment, the retentate return line 32 was configured so that
there was no back flow from the membrane tank 16 to the aerobic
tank 14 at any time during the aeration cycle.
[0040] The membrane tank 16 and the retentate return line 32 are
sized and configured so that the mixed liquor flow will be adequate
for the process and feed loading considering the aeration regime
that will be used. For example, FIG. 2 shows the membrane tank 16
of the exemplary embodiment in greater detail. The interior of the
membrane tank 16 is about 1300 mm high, 200 mm deep and 760 mm
wide. The retentate return line 32 is made of three 1'' diameter
pipes, each with its own isolation valve 34, projecting upwards
through the top of the membrane tank 16. The membrane tank 16 is
located adjacent the anoxic tank 12 so that only a short retentate
return line 32 is required and the retentate return line 32 lies
close to the top of the membrane tank 16. A mixed liquor inlet 60
is located near the bottom of the membrane tank 16. A single
membrane module 18 is mounted inside of the membrane tank 16. The
membrane module is about 730 mm long, 50 mm deep and 900 mm high
and contains hollow fibres with a total surface area of 9.8 square
metres suspended between top and bottom headers. The membrane
aerator 38 consists of an array of pipes mounted below the membrane
module 18 to provide lines of air holes distributed across the
footprint of the membrane module 18. Other dimensions or
configurations may be appropriate for other reactors.
Permeation
[0041] Transmembrane pressure for the membrane module 18 is created
by pressure in the membrane tank 16 resulting from the head of
mixed liquor, as modified by the membrane air, or suction applied
to the permeate side of the membrane module 18 or both. Suction may
be provided by a pump, by gravity induced flow or by a siphon. For
a siphon, the permeate line 30 or alternate permeate line 30'' are
used. For gravity induced flow, the second alternate permeate line
30' is used. A permeate valve 62 is provided in the permeate line
30, 30', 30'' and is opened to begin permeation. A permeate outlet
64, 64', 64'' and all points in the permeate line 30, 30', 30'' may
be located below the minimum mixed liquor level 22, such that
gravity will induce the flow of permeate through the permeate line
30, 30', 30''. Comparing the three configurations shown in FIG. 1,
permeate lines 30 and 30' create the largest transmembrane pressure
but have permeate outlets 64, 64' near or below the bottom of the
reactor 10 which may not be possible in all locations unless the
permeate lines 30, 30' can discharge into a hole serviced by a sump
pump. To prevent exposing the membranes to air in the event of a
temporary power failure, the valve 62 may be a normally closed
valve, for example a valve that is spring loaded to close when
power is not supplied to it. In the exemplary embodiment, 3/4''
diameter permeate line 30 is used and the highest point in the
permeate line 30 is above the top of the membrane tank 16 but below
the minimum mixed liquor level 22.
[0042] The permeate valve 62 is connected to a permeate valve
controller 66, such as a solenoid, which opens the permeate valve
62 to start permeation and closes it to stop permeation. In the
exemplary embodiment, the permeate valve controller 66 is linked to
a level sensor 68 which senses when the mixed liquor level is at
the maximum mixed liquor level 24 or minimum mixed liquor level 22.
The level sensor 68 communicates with the permeate valve controller
66 such that the permeate valve 62 is opened when the mixed liquor
reaches the maximum mixed liquor level 24 and stops when the mixed
liquor reaches the minimum mixed liquor level 22. The level sensor
68 may also communicate with the blower 44, second blower 50 or air
valve 46 and may be made capable of sensing other levels of mixed
liquor to provide the various cyclic aeration regimes described
further above.
[0043] Membrane air reduces the density of the mixed liquor in the
membrane tank 16 but, because the membrane tank 16 is sealed or
substantially so, pressure in the membrane tank 16 and
transmembrane pressure are higher than would be predicted based on
the density of the mixed liquor alone which boosts the permeate
flow rate. This boost may be provided even when a cyclic aeration
regime is used as described above although the size of the boost
varies in time and may at times be negligible or non-existent. To
enhance the pressure created in the membrane tank 16, the retentate
return line 32 is made small enough to partially restrict the flow
air and mixed liquor flow through it. In the exemplary embodiment,
the retentate return line 32 also projects downwards from the top
of the membrane tank 16 by about 2'' which causes a layer of
compressed air to form at the top of the membrane tank towards the
end of a period during which membrane air is provided. In the
exemplary embodiment, a pressure of about 15.3 kPag develops at the
bottom of the membrane tank 16 during approximately the last half
of the period when membrane air is provided, i.e. for about 5 or 6
seconds out of every 20 second aeration cycle. This approaches, but
does not exceed, the pressure at the bottom of aerobic tank 14
which is about 15.8 kPag. During the part of the cycle when
membrane air is off, the air and mixed liquor separate to some
extent causing the mixed liquor level to drop and leaving an air
space at the top of the membrane tank 16. Mixed liquor flows into
the membrane tank 16, primarily from the aerobic tank 14. The size,
length and location of the aerobic line 28 relative to the length
of time that membrane air is off, and other aspects of the system,
prevent the membrane tank 16 and aerobic tank 14 from equilibrating
in pressure. As a result, in the exemplary embodiment, the pressure
in the membrane tank 16 when membrane air is off is about 11.9
kPag. The elevation of the permeate outlet 64 was located so that,
even at this minimum pressure in the membrane tank 16, permeate
flow was still appreciable at about 0.37 gpm. However, permeate
flow rate with membrane air on was significantly higher at about
0.52 gpm.
[0044] If pressure build up in the membrane tank 16 is used to
boost permeation, its use must be balanced with the role of the air
in circulating mixed liquor. In particular, an elevated pressure in
the membrane tank 16 inhibits mixed liquor flow from the aerobic
tank 14. As mentioned above, when a cyclic aeration regime is used,
there may even be momentary reverse flows of mixed liquor back into
the aerobic tank 14 but the net circulation is still made positive
in the direction from the aerobic tank 14 to the membrane tank 16
to the anoxic tank 12.
Process
[0045] In the exemplary embodiment, the feed or influent is
introduced to the anoxic tank 12. The influent may be residential,
commercial or industrial black or gray water and may be taken
directly from the buildings where it is produced. Alternately, the
feed line 20 may be connected to the outlet of a septic tank or
other tank which may reduce the time rate of change of the influent
flow rate, provide solids settling and storage or some preliminary
digestion. Alternately, to meet the requirements of variable feed
flows, for example from a home, sufficient storage may be provided
in the anoxic tank 12 and aerobic tank 14 to provide capacity for
peak flows, for example morning and evening peak flows in a typical
home. Permeation may occur only during or directly after high flow
periods, and may begin when the mixed liquor level in the reactor
10 reaches the maximum mixed liquor level 24.
[0046] The anoxic tank 12 may have a total residence time of
between 30 minutes and 4 hours. Large solids in the influent settle
to a quiescent zone 70 at the bottom of the anoxic tank 12.
Optional baffles 72 may be used to encourage settling in the
quiescent zone 70. In the anoxic tank 12, the feed mixes with the
recycled mixed liquor from the membrane tank 16. Biological
denitrification occurs in this tank, converting excess nitrates in
the mixed liquor, using the organic matter in the raw feed as the
electron receptor. Nitrogen gas thus formed is volatilized and
removed with the off-gases. The denitrified feed and mixed liquor
mixture then flows to the aerobic bioreactor, where biodegradable
organic matter is digested, and ammonia is oxidized to nitrates.
From the aerobic tank 14, the mixed liquor is recycled through the
membrane tank 16 and then to the anoxic tank 12. About a 5:1
recirculation to feed ratio may be provided for effective
denitrification. The concentration of dissolved oxygen in the
anoxic tank 12 is kept generally low, for example at 0.5 mg/l or
less, at most times and some or all of the mixed liquor in the
anoxic tank 12 may be essentially anaerobic at times. The
concentration of dissolved oxygen in the aerobic tank 14 is kept
above about 1 mg/L at most times.
[0047] Optionally, a coagulant, such as alum, may be added to the
anoxic tank 12 to precipitate soluble phosphates to reduce
phosphorus discharge. The colloidal precipitated phosphorus is
filtered out by the membrane which results in high removal
efficiency. Excess precipitate is settled out in the anoxic tank 12
quiescent zone 70 and removed with the sludge which is pumped out
from time to time in any event. Coagulant dose may be added in
batch to the anoxic tank when the maximum mixed liquor level 24 is
reached and permeation starts. Alternatively, coagulant can be
added at a constant rate throughout permeation. In either process,
the level sensor 68 may be linked to a coagulant dosing device to
provide the desired coagulant dosing.
[0048] Further optionally, the reactor 10 may be run at high mixed
liquor solids concentration up to 30 000 mg/L. High mixed liquor
concentration may be built up by not wasting sludge. This requires
a membrane module 18 with high solids tolerance such as a module of
coarse hollow fibres with low packing density. A high mixed liquor
solids concentration permits better nutrient removal and increases
the interval between pumping out of waste sludge from the reactor
10. This is desirable in many applications, for example home
treatment systems, since sludge disposal is an expense and
inconvenient. High mixed liquor solids concentration also
facilitates rapid establishment of anoxic conditions for improved
denitrification.
Chemical Cleaning
[0049] Chemical cleaning is provided by feeding a cleaning
solution, such as sodium hypochlorite, from a chemical tank 74. The
chemical tank may be located so that the cleaning solution can flow
to the membrane module 18 by gravity. For example, in the exemplary
embodiment the chemical tank 74 is located on top of the anoxic
tank 12. A chemical valve 76 starts and stops the flow of cleaning
solution. Aeration is stopped while the chemical valve 76 is open
to allow reduce dissipation of the cleaning solution in the
membrane tank. In the exemplary embodiment, the chemical cleaning
process is performed manually by opening the chemical valve 76 and
turning off the air blower 44 for the duration of the cleaning
event, followed by 10-60 minutes of membrane relaxation wherein no
permeate is withdrawn. Chemical cleaning may be required from once
a year to twice a month with a sodium hypochlorite solution of
200-500 mg/L. Optionally, chemical cleaning can be controlled
automatically by a controller, for example a timer, linked to one
or more of the permeate valve 62, chemical valve 76, blower 44,
optional second blower 50 or air valve controller 48.
EXAMPLES
Example 1
Steady-State Operation at 1.2 m.sup.3/d
[0050] A pilot system was built and operated as described for the
exemplary embodiment. The pilot system was run for two weeks with a
steady state feed of 1.2 cubic metres per day of municipal sewage.
The pilot system had an anoxic tank and an aerobic tank, each of
800 L volume at depth of 1.8 m in those tanks, and a 200 L membrane
tank. The bottom of each of the anoxic tank and aerobic tank were
0.5 metres above the bottom of the membrane tank. The system was
operated at a generally constant mixed liquor depth of about 1.5
metres above the bottom of the membrane tank. At this depth, the
system had a total liquid volume of about 1080 L. Flow rate in the
membrane air line was 8.5 cubic metres at standard conditions per
hour when air flows to the membranes. A single blower provided a
continuous air flow and the entire air flow was switched between
the aerobic tank and membrane tank every 7 seconds so that the rate
of air flow in each of the aerobic tanks and membrane tanks was
turned on for 7 seconds and off for 7 seconds in repeated cycles.
The membrane module had 135 square feet of surface area. The
aerobic line 28 was 1.5'' in diameter.
[0051] The raw sewage was characterized as: COD of 560 mg/L, TSS of
330 mg/L, NH3-N of 23.3 mg/L and TP of 16.3 mg/L, in average. The
system was operated at a temperature of 20.5.degree. C. and a MLSS
concentration of 12 g/L.
[0052] Below are the concentrations of various chemicals in the
permeate, demonstrating a very high level of treatment:
TABLE-US-00001 Permeate COD 7.7 mg/L (98.6% removal) Permeate NH3-N
0.3 mg/L (98.7% removal) Permeate NO3-N: 7.3 mg/L Permeate TP 0.6
mg/L (96.3% removal)
[0053] Membrane performance and oxygen transfer results were as
follows, demonstrating that the aeration system was able to
maintain adequate oxygen for treatment in each tank, and that the
membrane module maintained acceptable permeability and
productivity. TABLE-US-00002 Permeate flux 13.7 L/m.sup.2/h
Permeate turbidity: 0.35 NTU Membrane permeability: 137
L/m.sup.2/h/bar DO in aerobic tank: 2.68 mg/L DO in anoxic tank:
<0.3 mg/L Recirculation flow rate: 0.44 m.sup.3/h
Example 2
Development of Pressure in the Membrane Tank and its Effect On
Permeation During Membrane Aeration Cycle.
[0054] The system described in example 1 was used for a membrane
tank pressurization test. A pressure sensor was installed onto the
bottom of the membrane tank 16. The sensor read 15.3 kPag when the
membrane air was on and 11.9 kPag when the membrane air was off.
The permeate flow rate when the air was on was 127 L/h, and 86.3
L/h when the air was off.
Example 3
Demonstration of Recycle of Mixed Liquor from Aerobic to Anoxic
Tank Through Membrane Tank.
[0055] For a period of about 10 days during the test of example 1,
the recycle flow rates were measured and average values calculated.
With the mixed liquid level in the anoxic tank 12 set at 1.5 metres
above the bottom of the membrane tank, recycle of mixed liquor from
the aerobic tank to the anoxic tank via the membrane tank was
visually confirmed. A magnetic flow meter installed on the aerobic
line 28 recorded an average flow rate of 0.44 m.sup.3/h, at a total
air flow rate of 8.5 m.sup.3/h. This recirculation flow rate was
8.8 times the average permeate flow rate.
Example 4
Aerator Cleaning Using Process Air.
[0056] FIG. 4 shows the process aerator after running in a reactor
for five weeks of continuous operation in sludge at 12-15 g/L Mixed
Liquor Suspended Solids (MLSS) concentration. The process
aerator/inlet screen was maintained in a sludge free state. All
holes are fully open and there is no sign of accumulation of sludge
inside the aerator.
* * * * *