Note: Descriptions are shown in the official language in which they were submitted.
MN 293167 [2018.005]
IMPROVED METHOD FOR PROCESSING WASTE WATER
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RELATIONSHIP TO OTHER APPLICATIONS AND PATENTS
This application is a Continuation-In-Part of a pending US
patent application, Serial Number 14/985,994, filed December 31,
2015, which is a Continuation-In-Part of a pending US patent
application, Serial Number 14/825,314, filed August 13, 2015 and
now abandoned, which is a Continuation-In-Part of a pending US
patent application, Serial Number 14/674,163, filed March 31,
2015 and now abandoned.
TECHNICAL FIELD
The present invention relates to systems for processing
waste water; more particularly, to such systems for handling
biologically digestible materials in waste water generated
typically in manufacturing and serving foods and potables, e.g.,
bakeries, breweries, dairies, restaurants, wineries, and the
like; and most particularly, to a method for operating a simple,
small volume system for settling solids and adjusting pH in food
process waste water to meet waste water quality standards for
discharge into a municipal sewage system, and to further treat
such food process waste water to meet higher quality standards
for environmental discharge, process recycle, and/or potable
water. Such further treatment can be exceedingly valuable for
foods .and potables manufacturers in, e.g., rural areas having no
municipal sewage system, or arid regions where fresh water
availability is limited and/or expensive.
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As used herein, the term "food materials" should be taken
to mean any and all biologically digestible organic materials,
without limit; the term "food process waste water" should be
taken to mean excess water and by-products, components beyond
just water itself, used in the manufacture and/or use of food
materials, which water must be treated to remove a portion of
the dissolved and/or suspended food materials before being
either sent to a waste water treatment facility or otherwise
discharged to the environment; and "potable water" should be
taken to mean water sufficiently pure to meet EPA standards for
drinking water for humans.
BACKGROUND OF THE INVENTION
Foods and potables manufacturing and handling typically
require large volumes of input process water and generate
substantial levels of biologically digestible materials
dissolved and suspended in their waste process water.
Additionally, the pH of such waste water may be substantially
acidic or alkaline. When directed without pre-treatment to
municipal waste water treatment facilities, such waste water can
place a heavy and costly load on municipal waste treatment
facilities. As a result, many communities impose a substantial
cost on companies that generate such waste waters in the course
of their operations. It is known to monitor the level of food
materials in waste water output of companies and to levy a sewer
surcharge on the companies accordingly. Many of these
companies, for example, "microbreweries", are relatively small
in capitalization and output and thus are in need of a
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relatively inexpensive method and associated apparatus for pre-
treating of process waste water to remove a substantial
percentage of suspended food materials therefrom before the
process waste water is discharged to a municipal sewer system.
Fortuitously, the total volume of process waste water generated
by many such operations may be relatively small, on the order of
1000 gallons/day or less, and therefore amenable to treatment by
a method and apparatus in accordance with the present invention.
Larger scale operations can also be supported by scaling up with
multiple modules of the present invention.
Note: "Biological Oxygen Demand" (BOD), also known as
Biochemical Oxygen Demand, is the amount of oxygen needed by
aerobic microorganisms to decompose all the organic matter in a
sample of water; it is used in the eco-sciences as a measure of
organic pollution. As used herein, the term "BOD" also means
more generally the unit volume load, both dissolved and
suspended, of such organic material in waste water.
Further, Total Suspended Solids (TSS) is a water quality
measurement which, as used herein, is expressed as the unit
volume load of suspended solids, both organic and inorganic, in
water. It is listed as a conventional pollutant in the U.S.
Clean Water Act.
Example:
The following example is directed to the characteristics
and treatment of waste water generated by breweries. It should
be understood that the disclosed method and apparatus are also
well-suited to similar usage in many other types of food
manufacturing and use as noted above.
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Breweries have unique effluent characteristics and specific
treatment needs. Breweries typically have BOD levels of 2,000-
4,000 mg/1 and TSS levels of 2,500-3,500 mg/l. The solids are
fairly heavy and easy to settle out, and much of the dissolved
organic load can also be precipitated out by dosing the waste
water with coagulants. Brewery effluent can typically have a pH
range of 2 to 12, depending on what process is taking place in
the brewery. The pH may have to be adjusted on occasion to meet
municipal requirements and also be bought into optimum range for
effective chemical treatment. Brewery effluent can have
fluctuating levels of BOD, TSS, and pH. There is also a chance
that occasionally the brewery may have to waste a batch of beer,
discharging the batch and introducing high levels of BOD into a
municipal system.
Brewery waste water comprises several contributors to the
total BOD and TSS load. Most of these are organic in nature and
pose no serious threat to public health.
Yeast, spent grain, and hops are the building blocks of
beer. Most of the wastes from these components typically are
side streamed in the brewery and diverted as feed for farm
animals. Inevitably, some of that waste also will get down the
drain and thereby raise the BOD and TSS levels of the, process
effluent.
Wort is the liquid that will become beer once the yeast is
added. Wort comprises fermentable and unfermentable sugars as
well as starches and proteins. Because wort is rich in
dissolved sugar, it is the primary contributor of BOD and SBOD
(soluble BOD).
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Fermented beer left in tanks after transfers and lost
during packaging also contributes to the BOD and SBOD of the
effluent leaving the brewery.
Beer has a characteristically low pH (typically 4-5.5) that
can reduce the overall pH of the waste water.
For cleaning chemicals, breweries typically rely on caustic
solutions for removing organic deposits from their process
tanks. Acid is used on occasion, as are iodine-based sanitizers
and peracetic acid for sanitizing tanks and equipment. These
are diluted when used but will still affect the pH of the final
effluent.
Most of the water used by breweries leaves in the form of
finished beer, so daily waste water flows are relatively low and
comprise mostly cleaning.water. A typical microbrewery may
generate no more than about 200-300 gallons of process waste
water per day, although naturally that volume will grow as
production volumes grow.
In the prior art, US Patent Application Publication No.
2013/0168318 Al to Levecq et al. discloses a system and method
for treating water via ballasted flocculation which is suitable
for treating all types of water, with the intention of producing
drinkable water. The system includes a tank for continuously
receiving and treating a waste water stream which includes a
stationary non-screen decanter comprising a fixed weir for
drawing off supernatant as an overflow. By disclosing a system
and method requiring continuous ballasted flocculation, Levecq
et al. teaches away from the discontinuous method and apparatus
of the instant invention.
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US Patent No. 882,030 A to Traulsen et al. discloses a
simple floating decanter having means for filtration of waste
water through a depth bed but does not disclose to use a screen
of given pore size.
What is needed is a method for operating an appropriately-
sized but scalable, relatively inexpensive waste water settling
,system for removing solids, especially biologically-digestible
food process solids, from waste water to improve waste water
quality for discharging into a municipal sewage system. Such a
system preferably includes a screen decanter (also referred to
herein as an "SBX" or "screen box") for drawing off the
clarified waste water from the upper reaches of the waste water
in the tank. Preferably the screen pore size of an SBX is
between about 25 micrometers and about 75 micrometers, most
preferably about 50 micrometers.
What is further needed is a method for operating a
filtration and membrane system to further purify such treated
waste water to meet quality standards for environmental
discharge, and optionally for process recycle and/or potable
water, especially in areas where available potable water is
expensive and/or not readily available in large quantities.
Such further purification treatment can be exceedingly valuable
for foods and potables manufacturers in, e.g., rural areas
having no municipal sewage system, or arid regions where fresh
=
water availability is limited and/or expensive.
Filtration and membrane systems are known to be sensitive
to the presence of particles in the influent stream which can
readily and undesirably clog the very fine filters and
membranes. Since the effluent from the upstream waste water
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settling system becomes the influent for the downstream
filtration and membrane system, it is prudent that the influent
be filtered again prior to entry into the membrane system.
Accordingly, an additional fine filter may be provided
downstream of the screen decanter and ahead of the membrane
system. Preferably the filter pore size is between about 25
micrometers and about 75 micrometers, most preferably about 50
micrometers.
SUMMARY OF THE INVENTION
The present invention includes improvements to the SBX
design to enable more uniform flow and thus increased waste
water processing capacity when the Enhanced Primary Treatment
(EPT) unit is coupled to a membrane/filter system. The new
design also includes a 50 micrometer screen filter either at the
entrance to the SBX or ahead of the membrane/filter system to
reduce maintenance requirements (e.g. back flushing) and extend
membrane life for the membrane/filter system. The reduced
maintenance comes from improved uniformity of flow through the
50 micrometer screen, thereby reducing fouling in local areas
otherwise subjected to non-uniform high/peak flow channels.
Preferably, the membrane/filter system includes its own 5
micrometer filter ahead of the membrane elements.
Briefly described, a system in accordance with the present
application comprises a pretreatment ("EPT") system to intercept
and treat a process waste water effluent stream before it enters
the municipal sanitary system, or before it is suitable for
entry to environmental discharge or process recycle or human
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ingestion. Systems in accordance with the present invention can
be scaled up or down to meet the needs and economic price point
of even small operations/companies and can then be readily
scaled up as treatment demand increases.
The present system pumps the effluent stream from a
discharge channel such as trench drains or a sump, either
directly into a holding tank for settling and for pH balancing
or dissolved solids adjustment or these operations can be
accomplished as pre-treatment processes prior to entering the
main tank. A sump pump is responsive to a signal such as a
float switch in a sump or drainage trench. The collected
discharge is transferred to the invention system's tank having a
conical bottom with a manual discharge valve for removal of
settled solids. The system has a chemical dosing mechanism to
permit effluent adjustment. The supernatant is decanted using a
decanter, e.g., a floating or vertically driven decanter,
following a predetermined settling period.
The decanter preferably includes a screen, defining thereby
a screen decanter ("SEX"), preferably an outer screen for
filtering waste water as it enters the decanter. Preferably the
screen pore size is between about 25 micrometers and about 75
micrometers, most preferably about 50 micrometers.
The EPT is equipped with a float switch to automatically
activate it when a certain level in the tank is reached, to
prevent overfilling the tank. The discharge pump is equipped
with a timer that can be set to drain the tank slowly after a
pre-set settling period time to reduce the load on the municipal
sanitary system. Preferably, a solenoid valve also controlled
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by the timer is disposed in the drain line to prevent
inadvertent siphoning of the tank via the floating decanter.
The EPT effluent, although partially clarified via
coagulation and settlement processes, requires further
processing before it is suitable for recycling/re-use. Small
hole-size filters can be used for this purpose ('membranes' are
defined by pore sizes of 20 micrometers and smaller), followed
by increasingly finer membranes. However, membranes used in
this manner are prone to clogging and require frequent
maintenance (e.g. back flushing).
The discharge pump may be directed to a drain to a
municipal sewage system or, preferably for further purification,
to a self-contained waste water purification system comprising a
feed pump, a pre-filter having a screen pore size preferably
about 5 micrometers, a first filtration/membrane feed tank, a
plurality of sequential filters/membranes of decreasing
porosity, a reverse osmosis feed tank, at least one reverse
osmosis membrane, and piping leading alternatively to drain or
to further recycled use in manufacturing or as potable water
This invention comprises filtration design enhancements to
the SBX to improve its clarification and filtration performance
while decreasing its cost and reducing the concentration of
entrained organic particles. These improvements make it
possible to combine EFT, a fine filter screen, and membrane
technology to make water recycling/re-use a practical
alternative for many food and beverage processing applications,
e.g., onsite human waste treatment at food/beverage sites in
addition to treating their process waste.
In operation, many anticipated users of the present
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invention system have manufacturing operations that generate
waste water only during the daytime. Thus, in an anticipated
operating protocol the tank is filled progressively with food
process waste water during the work day. Waste water pH and/or
other characteristic may also be adjusted as needed in real-time
or as a batch treatment once the tank is full. Settling of
solids occurs during the nighttime hours when the waste water is
tranquil, followed by decanting of the cleared supernatant
effluent from the tank before the start of the next work day,
after which the accumulated solids are also drawn off through
the valve in the bottom of the tank for landfill, bio-digestion,
or other disposal.
Further, in areas where there is no municipal waste water
treatment facility, the permissible pollution levels of
discharge from manufacturing processes into the environment via
subterranean drainage field, lagoon, spray field, or natural
watercourse is governed by environmental law. A system
including provision for further purification of process effluent
to meet environmental standards thus is highly desirable,
beneficial, and cost effective for anticipated users of this
invention.
Still further, in arid areas where abundant process water
may be scarce and/or expensive, a purification system for
recycling of process water back into the head end of the
process, rather than discard, is highly desirable to allow
businesses to start-up or existing operators to expand.
Thus, there is a further need for a water purification
system complementary to the process waste water settling system,
which water purification system may be close-coupled to the
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process waste water settling system in a closed loop. In the
present invention, a supplemental filtration and reverse osmosis
system is attached, integral to, and downstream of the
aforementioned processing steps. The supplemental system
comprises a series of membrane filters, each of which is
progressively finer. Filters are easily removed, replaced if
fouled, or rded if finer treatment levels are desired. The
composite stem therefore allows anticipated system users to,
select the 4vel of filtration that best meets their onsite
water usage requirements and meets their objectives for
discharging to offsite waste water treatment operations or
process recycle.
*
BRIEF DESCRI14TION OF THE DRAWINGS
The present invention will how be described, by way of
example, with reference to the accompanying drawings, in which:
FIG. 1,is a schematic drawing of an elevational cross-
sectional view of a first embodiment of a primary treatment
settling tank system in accordance with the present invention;
FIG. 2 is a schematic drawing of a waste water purification
system for flArther treating the output of the primary treatment
settling tanik system shown in FIG. 1 to produce recyclable or
potable water;
FIG. 3 is an isometric view from above of a first
embodiment of a screen decanter in accordance with the present
invention, showing an integral fine entrance filter in the
porosity range of 25 micrometers to 75 micrometers;
FIG. 4 is an isometric view from below of the screen
decanter shown in FIG. 3;
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FIG. 5 is a front elevational view of the decanter shown in
FIG. 3;
FIG. 6 is an elevational cross-sectional view taken along
line 6-6 in FIG. 5;
FIG. 7 is detailed view taken in circle 7 in FIG. 6; and
FIGS. 8 through 11 are elevational views (FIG. 9 being
isometric) of various embodiments of a decanter standpipe.
The exemplifications set out herein illustrate currently
preferred embodiments of the invention, and such
exemplifications are not to be construed as limiting the scope
of the invention in any manner.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to FIG. 1, a system 10 for treatment of food
process waste water is shown. System 10 comprises an elevated
tank 12, e.g., a cylindrical 1000 gallon tank formed, e.g., of
polyethylene or polypropylene or stainless steel or other
material able to tolerate caustic by-product of food processing.
Tank 12 includes hopper bottom 14, preferably conical as shown,
and is mounted on a stand 16 providing access to a solids outlet
valve 18 in hopper bottom 14.
Preferably, tank 12 is sized to hold and dilute an entire
spoiled batch (e.g., of beer or wine) and, additionally, one day
or more of process discharge. This allows the user to treat and
dilute spikes in process discharge constituents, e.g., BOD, TSS,
and/or pH. Untreated food process waste water effluent (tank
influent) 15 from a user's trench drain or sump 11 flows into
tank 12 via gravity or a conventional sump pump 20 and backflow
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preventer check valve 23. System 10 is functionally positioned
in the user's waste water effluent line between user's sump 11
and a municipal sanitary sewer 21. Preferably, the tank
influent connection 22 to tank 12 is, for example, PVC pipe, and
is located in the cylindrical tank wall near the transition to
conical hopper bottom 14 and includes a 90 elbow 24 to turn the
flow within the tank substantially parallel to the tank wall to
cause circular circulation of influent within the tank.
Conical hopper bottom 14 has an included cone angle
selected from the group of cone angles consisting of at least
45 , 60 , and all angles therebetween.
System 10 includes a chemical dosing mechanism 25 that
displays at least one chemical characteristic of interest in the
influent and allows adjustment of that characteristic of the
influent by addition of dosing chemicals, for example, alkali or
acid to bring the pH into the required range before discharging
of treated effluent. The chemical dosing mechanism includes a
dosing pump probe 26 disposed within tank 12, preferably about
five inches below the top of bottom 14. Probe 26 is connected
to a pH controller and dosing pump 28 disposed in a control box
30. Dosing pump 28 is supplied with a dosing chemical via a
first dosing hose 31 from a reservoir 32. The dosing chemical
is injected via a tank valve 33 and second dosing hose 34 into
supernatant influent 38 at location 36, preferably at a point
about two inches above elbow 24.
For further BOD and TSS reduction, chemical coagulants
(e.g., ACH, PAC,) can be dosed to the fluid in the tank
specifically to reduce soluble BOD. Preferably, this is done at
the end of each day of production to allow the maximum number of
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hours for settling of solids 37. Dosing rates are very low
(generally 100-150ppm) and have no adverse effect on the waste
water stream.
During a predetermined settling period, the food process
waste water is gravitationally separated into a settled solids
fraction 37 and a clarified supernatant fraction 38. Supernatant
38 is decanted from the top down using a vertically-mobile
decanter 40 that follows the liquid level in tank 12 rather than
being a fixed opening in the side of tank 12 as in the prior
art. Decanter 40 may be either a simple weir-type floating
decanter, or preferably a screen decanter (SEX) for drawing off
the clarified waste water from the upper reaches of the waste
water in the tank. Preferably the screen pore size of the SEX
is between about 25 micrometers and about 75 micrometers, most
preferably about 50 micrometers, and preferably the screen is
disposed at the entrance to the decanter, as described below.
Tank 12 may be equipped with an upper float switch 42 to
automatically activate floating decanter 40 when a pre-set alarm
level of supernatant 38 in tank 12 is reached. This prevents
accidental overfilling and spilling of the tank. Supernatant 38
thus becomes the process effluent 60 from system 10. Screen
decanter 40 is described in greater detail hereinbelow.
Discharge pump 44 is connected to decanter 40 via drain
pipe or hose 46 and rigid PVC pipe 48. System 10 includes a
multiple-setting timer 50 connected to a normally-closed
solenoid valve 52 and effluent pump 44 that can be set for
intermittent flow from tank 12, to drain the tank slowly over
time to further reduce the instantaneous load on the municipal
waste water treatment plant. The cycles can be determined by
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the operator and the municipality. If tank 12 fills completely,
upper float switch 42 activates floating decanter 40, solenoid
valve 52, and effluent pump 44 to pump just enough effluent from
the tank to bring the level down to a safe operating level.
Optionally, decanter 40 is fitted with a fine filter 41 as
described above; or optionally decanter 40 is a non-screen
decanter and the effluent discharge line 48 is configured with a
fine filter 43 having pore size in the range of 25-75
micrometers, as described in detail below.
In one anticipated mode of operation of system 10, daytime
food processing operations cease between approximately 8:00pm
and 6:00am, giving system 10 enough time to allow settling of
solids and then to empty itself before the start of the next
production day. When the level of supernatant 38 reaches lower
float switch 54, floating decanter 40, solenoid valve 52, and
effluent pump 44 are deactivated. After tank 12 is emptied, an
operator drains the settled solids from the conical bottom 14 of
tank 12 at the start of each day of production.
In many applications equipped in accordance with the
present invention, some solids and other contributors of BOD can
be collected, or "side-streamed", from the various point sources
of discharge throughout the facility, and can be captured in,
for example, nylon filter bags. This can reduce significantly
the amount of solids entering system 10 and can lower the total
BOD level as well.
Referring now to FIG. 2, a currently preferred embodiment
of a filtration and membrane system 110 to further purify
treated food process waste water to meet BOD or other quality
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standards for environmental discharge, and optionally for
process recycle and/or potable water, is shown.
In operation of system 110, wastewater effluent 60 from
system 10 (FIG. 1) is pumped by a first feed pump 112 through a
5-micron cartridge filter 114 for the removal of any larger
suspended solids. Filtrate from filter 114 is discharged into a
first feed tank 116 wherein chemicals to enhance downstream
treatment or prevent scaling may be added or pH may be adjusted
via injection apparatus 115.
The mixed contents of first feed tank 116 are pumped via a
second feed pump 118 through one or more membrane canisters
120,122. Preferably, first membrane canister 120 houses a
microfiltration (MF) or ultrafiltration (UF) membrane to remove
colloidal solids in excess of 0.015 microns in size, which
serves to remove fats and proteins. The reject from first
membrane canister 120 is returned via line 124 to first feed
tank 116 which acts as a concentrator to increase the solids
content in first feed tank 116 until such time as a portion 126
of the contents thereof is discharged to the sludge tank 12 of
system 10.
Permeate 128 from first membrane canister 120 exits under
pressure and passes through second membrane canister 122
containing a nanofiltration (NF) membrane that rejects particles
larger than 0.001 microns, which includes some metal ions,
complex sugars, and synthetic dyes. The nanofiltration membrane
allows simple sugars, alcohol, ammonia, short-chain organics,
most metal ions, and salts to pass. It should be noted that the
actual apertures of the MF, UF, and NF membranes may vary from
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manufacturer to manufacturer, so the contaminants rejected or
passed may also vary.
The reverse osmosis (RO) membranes in third membrane
canister 130 operate at a pressure greater than the operating
pressure of the MF, UF, and NF membranes in first and second
canisters 120,122, so an intermediate pump 132 is required.
Therefore, permeate 134 from second canister 122 discharges
uthder exit pressure into an RO feed tank 136. Here, chemicals
may be added and the treated permeate 134 is pumped into the RO
membrane in third canister 130. The RO membrane rejects metal
ion, salts, sugars, and most short chain organics; however,
alcohol and some ammonia may pass the RO membrane. The RO
reject 138 is returned to RO feed tank 136 or the MF/UF/NF feed
tank 116 for further processing.
The permeate 140 from third canister 130 discharges under
pressure into a media canister 142 where activated carbon or
othr adsorbent may be employed to remove some of the remaining
organics, or an ion-selective resin may be used to remove the
ammonia.
All of the above-described steps may be required to produce
a high quality effluent approaching or meeting drinking water
standards. Alternatively, only selected steps may be necessary
to accomplish a lower degree of treatment or the removal of a
specific contaminant. The process steps can also be altered on
client by client basis based on the nature of the wastewater,
contaminants to be removed, and effluent requirements.
Preferably, system 110 further comprises sample ports
144,146,148,150 to permit gauging the performance of each
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process step, as well as to judge the performance of different
membranes and media.
System effluent 152 may be drawn off and used as purified
process water in any desired manner, and further may be recycled
(not shown) into the manufacturing process (not shown) that
creates the need for systems 10,110.
One enabler to a viable water recycling/reuse system is the
EPT itself which separates out sufficient particulate matter to
make a high efficiency SBX possible. Additional classification
is accomplished by using a fine screen filter in the SBX.
Fine filters, such as in the range of 25 microns to 75
microns, are susceptible to fouling and clogging similar to
membranes. Remedially, the key to performance of the SBX itself
is to create conditions that provide Uniformity of flow across
all regions of the fine screens. Extensive modelling is
required to identify configurations that deliver uniform flow
both in the vertical and horizontal planes. Without flow
uniformity, high flow areas of each screen will clog more
quickly requiring early maintenance or replacement or otherwise
render screens and membranes unusable in many waste treatment
applications.
As described below, a particularly useful SBX arrangement
involves a plurality of cylindrical screens, e.g., three,
mounted on a common platform including a drain manifold. The
standpipe within each cylinder has a graduated series of
openings, larger at the top than at the bottom, to compensate
for the increased hydrostatic pressure in the lower regions of
each screen. The plurality of standpipes is connected to the
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common platform that includes a central manifold drain pipe
connected to an EPT drain pipe or hose.
Cylindrical screens are readily fabricated, and the design
may make use of low cost light weight PVC pipe. Depending on
the characteristics of the wastewater influent, these screens
may have openings as small as 50 micrometers. This is the upper
limit for input to a membrane filter system. However, SBX
screens are easily back flushed, so for situations where the
larger particles are encountered, longer life/less maintenance
may be achieved by using larger screen openings at the SBX and
inserting a 50 micron screen between the SBX and the
filter/membrane system.
Referring now to FIGS. 3 through 11, an exemplary screen
decanter 140 in accordance with the present invention comprises
a platform 142 including a drain manifold 143 having a central
drain opening 144. Three decanter frames 146 are mounted to
platform 142, and each frame 146 includes a perforated central
standpipe 147 connected to drain manifold 143 by a connecting
pipe 148. Each frame 146 is surrounded by a cylindrical screen
150 connected to frame 146 as by screws 152 in such a fashion
that all influent flow entering frames 146 must pass through a
screen 150. Preferably, screens 150 have a pore size in the
range of 25-75 micrometers, and most preferably about 50
micrometers.
In operation, screen decanter 140 is partially submerged in
supernatant 38 (FIG. 1) such that much greater lateral flow can
be achieved into the decanter than over a simple weir. Further,
the three cylindrical screens 150 provide a relatively large
surface area for filtration of supernatant 38 as it enters the
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decanter. However, because decanter 140 is submerged to an
operating depth, the hydrostatic head at the bottom of the
screen is greater than at the surface of the supernatant, which
would cause a non-uniform flow through the screen from top to
bottom. To maximize working life between cleanings of the
screen, it is desirable that lateral flow through the screen be
substantially the same at all points. Therefore, to equalize
lateral flow at all depths of screen immersion, each standpipe
147 is perforated in an aperture pattern contrary to the
hydrostatic head imposed on the standpipe to allow less flow
resistance at lesser heads and greater flow resistance at
greater heads. Exemplary standpipes 147a,b,c comprising
respective exemplary aperture patterns 149a,b,c are shown in
FIGS. 8 through 11.
While the invention has been described by reference to
various specific embodiments, it should be understood that
numerous changes may be made within the spirit and scope of the
inventive concepts described. Accordingly, it is intended that
the invention not be limited to the described embodiments, but
will have full scope defined by the language of the following
claims.
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