Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~4~86
MULTI STAGE FLOCCULATION TREATMENT SYSTEM
The present invention is directed to methods and
apparatus for water treatment, and more particularly, is
directed to methods and apparatus for flocculation pro-
cessing in water purification treatment, such as the
treatment and purification of wastewater.
Chemical coagulation and flocculation processing
is conventionally utilized to facilitate, by formation
of a precipitate, the separation of suspended and col-
loidal matter through sedimentation and/or filtra-tion ~;
in wastewater. In coagulation and flocculation treat-
ment, a chemical coagulant, such as aluminum sulate,
-ferric chloride, ferric sulfate, and/or ~errous sulfate,
in conjunction with pH control, is thoroughly mixed
with the water to be treated. Thorough mixing of the
coagulant chemical with the treated water is desirable
to obtain economy of chemical dosage, but relatively
gentle mixing is generally necessary in order to obtain
a resultant floc, or coagulum, tha~ is large enough to
settle or filter. After the floc is formed, provision
may conventionally be made for a quiescent period dur-
ing which the floc can settle and carry down or agylom- ;
erate with it the suspended and colloidal material in
the water being treated.
Coagulation processes employed in water treat-
ment may conventionally utilize two dis~inct stages
.
:
,, "
having dlfferent operating criteria. In this connec-
tion, an initial mixing stage such as a continuously
stirred tank mi~er may be used to provide intimate
mixing of the influent treatment water with the chemi-
cal coaqulant. The resulting mixture may be then con-
ducted to a flocculation stage for floc development. A
relatively high turbulence intensity coupled wlth a high
specific energy requirement may be used to rapidly dis-
perse the coagulant and p~ control reagents in an influ-
ent, raw water stream in the initial mixing stage. Arelatively low turbulence intensity employed over a
longer period of time may be used in the second, floc-
culation stage to promote aggregation of the colloidal
particulate material present in the raw water. The
methods and apparatus conventionally employed to accom-
plish each stage vary widely. In general, however, the
recommended specific energy requirements, residence
time, and turbulence intensity tend to be similar for
various of the methods, such as a specific energy
requireme~t for mechanical mixing energy input to the
initial mixing stage of about 0~26 hp/MGD with a resi-
dence time of about 30 seconds and a turbulence inten-
sity greater than 50%, and a specific energy requirement
of about .09 hp/MGD for the flocculation stage with a
residence time of about 30 minutes and a turbulence
intensity of less than 5%. The turbulence intensity is
a ratio of the specific turbulent energy dissipation
ratio to the specific total energy dissipa~ion rate.
The overall performance of a coagulation process
3~ may be evaluated in terms of the fraction of the light
scattering colloidal material ~turbidity) entering in
the raw water which is rendered se~tleable or removable
in the product stream leaving the flocculation process.
The system performance may be measured as the ratio of
the turbidity remaining in the supernatent of a settled,
~: - , . . .
: : . : :,
l~t j~8~
or treated, effluent sample from the flocculation pro-
cess, relative to the turbidity in the raw, influent
water stream. Research directed at improving perform-
ance of the coagulation process has been conducted at
the University of California at Berkeley. The follow-
ing table summarizes the phenomenal description of the
coagulation process which has result;ed from these
studies:
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~ o ~n
O ~. ~d
n ~. n
cn
Y ,_ U~
,t. ~.
o
t- G~
o 1~
o ~ ~ ~ ~ o
n ~ ~ ~ o ~
` ~D :r tl z t~
1-- ~ rt ~ ~ H ~ O
n) I' ~ u~ W S~
~4 1~ C~
1' O H C
p n ~..... o ~ :n
; rt o~ ~d ~
H oZH
H
O
rt t~l :
o o ~- o ~ ~ .
O O ~ ~ $
~ ,.
Z
~ ~
1 ~ H O
O
0
l~ + p) ~ 1~ ~
~ tD O
P ~ ~ o~ P~ ~ n ~
ID G C ID '~ :~ O n) ~ P~
0 0 ~d ~ O H ~ I _
o 1,
O r~ ~I G ~ ~ U~
G ~t O ~ It O ~t
o O U~ (D 3 1'~ ~ 1~ O :1
1) ~ Q~ ~ 1
n ~_ ~ ,~
O ~S n
0 ~(D ~r: O ~
~ O~ ~ ~ C r~ ~ ,~_ ~..
n ~ r~ ~o ~1 ~, ~ o
~- o ~ n ~
o ~ + IJ-
~ O
C ~ ~ t W
O l`t Y~ t~
~ tD 3 ~ ::~
+
(D G ~
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As shown in the table, the optimal end result
of an initial mixing stage is a homogeneous mixture of
the alum coagulant throughout the raw water stream, and,
as a result of self-aggregation of the alum hydrolysis
products, primary alum floc particles of average volume
vO. In optimal flvcculation processPs, the primary alum
floc particles are aggregated together with the light
scatteringcolloidal and/or suspensoid particles (i.e.,
clay) responsible for the turbidity (enmeshment mechan-
ism of flocculation). Normally, on a batch basis, suchalum-clay floc aggregates grow to a settleable floc vol-
ume VS within a few minutes.
The flocculation process may be considered to
continue until equilibrium flocculation is reached, in
which floc aggregation is balanced with floc breakup due
to the turbulent eddy forces present in the flocculation
process. At this point, the average floc volume has
approached the maximum obtainable floc volume, v.
However, conventional systems for flocculation
treatment, such as continuously stirred tank reactor sys-
tems, tend to have various disadvantages including
undesirably high energy consumption and mechanical cost
or complexity so that systems with improved energy
efficiency and other attributes and other attributes
would be desirable.
As described in U.S~ Patent No. 4,019,983
methods and apparatus utilizing a motive jet stream have
~een developed for wastewater disinfaction processes,
and have particularly effective operating parameters.
Such apparatus has been proposed for initial mixing
of flocculant chemical with the water to be treated, but
improved flocculation systems of the plug flow type
utilizing motive stream energy would be desirable.
Accordingly, it is an object of the presen~
invention to provide improved systems for water floc-
.,
- ' . , ~ ;
L5~38~;
culation treatment. It is a further object to provide such
systems which utilize motive stream energy in a multi-stage
flocculation treatment system having effective energy
utilization.
The invention relates to multi-stage flocculation
treatment apparatus comprising plug flow means for providing
a turbulent mixing field by means of a fluid motive jet for
mixing chemical flocculant with influent water to be treated,
to produce a turbulently mixed product stream. The plug
flow means provides a turbulence intensity of at least about
50% and a mechanical mixing energy input of at least about
0.2 hp/MGD per unit throughput. Means are provided for
introducing the product stream as a motive stream to a
flocculation zone to provide a predetermined relatively low
level of mechanical mixing and-turbulence intensity to the
flocculation zone for flocculation development.
The features of the invention will be apparent from
the following detailed description and the accompanying
drawings in which:
FIGURE 1 is a top view of an embodiment of a water
flocculation treatment systèm illustrating various features
of the present invention;
FIGURE 2 is a cross sectional view of a portion of
the system of FIGURE 1 taken through line 2-2; and
FIGURE 3 is a nomograph of process parameters in
connection with the apparatus of FIGURE 1.
Generally in accordance with the present invention,
the flocculant chemical and the water to be treated are
continuously passed through a first turbulent mixing zone
mg/`~ - 6 -
.
,. : ' . : : . :.
~ L5~8~
where flocculant chemical concentration gradients are
rapidly reduced within a limited period of time, to provide
a thoroughly mixed product stream having residual hydraulic
energy with respect to a flocculation ~one. The raw
influent water to be treated may be any type of water
requiring flocculation treatment, such as partially
treated wastewater (e.g., aerobic oxidation process
clarifier effluent), natural lake or river water intended
for potable water purposes, etc. Conventional EiLocculant,
or coagulation, chemicals may be used, such as a:Luminum
sulfate, ferric sulfate, ferric chloride and ferrous
chloride. These flocculant materials will generally be
used in the i-orm of aqueous solutions, with the selection
of the particular coagulant chemical being a matter of
design choice in accordance with conventional practice.
At least a minimum amount of energy, such as a specific energy
requirement of at least about 0.2 horsepower per milliPn
gallons per day of treated water (hp/MGD) is expended per
~ ' .
mg/ - 6a -
' ' : ~ ' ', ' . , ; : . ;:
.8~tP
unit throughput of raw influent water conducted through
the first mixing zone, with a turbulence intensity of at
least about 50~ in the mixing zone.
Further in accordance with the present inven-
tion, the thoroughly mixed product stream of influent
water and flocculation chemical is introduced as a
motive stream under particular conditions into a floc-
culation zone such that the residual energy is ut.lized
in providing particular conditions for flocculation
development. In this connection, the product stream
may desirably have a hydrostatic potential in the range
of from about one hal~ to about two and one half feet
of water, with respect to the water in the flocculation
zone, at least a portion of which is provided ~y resi-
lS dual energy from the generation of the first stage tur-
bulent mixing zone..The product stream may be introduced
into the flocculation zone by means of one or more
noz~les (or orifices) to provide in the flocculation
zone particular, low degree of mixing energy for floc
development, such as general.ly corresponding to a mean
specific energy requirement of about .5 _t-lb
(.09 hp/MGD) of product stream introduced l~to -the :Eloc-
fulation zone at a turbulence intensity oE less than
about 5~.
In accordance with preferred embodiments of
the method, an ejector-type mixing system is employed
to produce the initial turbulent mixing zone by intro-
ducing a motive stream of minor portion of the raw
influent water to be treated, into an induction zone
which is supplied with the major portion of the water
li~uid to be treated. The motive stream is generated
by forcing the motive stream and the formation of a
turbulent flow field therein. The flocculant
chemical is introduced into the turbulent flow field
and is best introduced into the turbulent mixing zone
with the motive stream as it is introduced in-to the
.
,
11~i;4~G
induction zone. In this manner, as se-t forth i~ U.S.
Patent 4,019,983, a plug flow mixing system may be pro-
vided having a highly efficient residence time distri-
bution and rapid product inhomogeneity recluction capa-
bility.
As indicated, the product stream of the ini-
tial~ plug llow mixing zone may be provided directly as
a motive stream, under relatively low pressure condi-
tions, for producing a desired degree of mixing in a
reservoir of the product stream fluid in a flocculation
zone, which is substantially less than that provided in
the initial mixiny zone. In this manner, by providing
the appropriately coordinated combination of process
steps and apparatus therefor, energy need only be intro-
duced to the flocculation system at the initial turbu-
lent mixing zone, and the mixing energy will be effi-
ciently proportioned and utilized to provide a reliable
and effective flocculation system.
Turning now to the drawings, the present
invention will now be moxe particularly described with
respect to the specific embodiment 10 of a flocculation
system of reinforced concrete construction, which is
illustrated in FIGURES 1 and ~ of the drawings. The
illustrated flocculation system 10 is adapted to re-
ceive wastewater effluent from the clarifier of anaeration treatment zone of a sewage treatment plant,
such as an activated sludge aeration treatment facility
or aerated lagoon (not shown). The partially treated
effluent from the aeration clarifier will have had
removed therefrom a substantial portion of the aerobic
biomass and other materials by sedimentation and clari-
fication treatment, but nevert~eless will contain vari-
ous suspended and colloidal materials which should be
removed prior to return of the treated water to the
.
,
~ . , .
~ 8~
environment. The aeration zone effluent accordingly is pumped
from outfall sewer 12 into wet well 14 to provide the raw water
influent to be treated by the system 10. The raw water influent
flows over submerged wall 16,19 into flash mix inlet channel 17
of the system 10. In the illustrated embodiment, -the flash mix
inlet channel 17 communicates, respectively, with -two flash mix
channel sump zones 18,20 by means of sluice gates 22,24 provided
in the reinforced concrete wall 26 separating the inlet channel
from the respective sump zones 18,20.
The raw influent water conducted to the sump zones
18,20 is rapidly and intensively mixed wit~l chemical flocculating
agent in a manner which will be more particularly described in
connection with FIGURE 2 of the drawings, and the mixed product streams
are introduced into respective flocculation zones, comprising
tanks 28,30. A flocculated stream having a sui-tably developed
floc is conducted from respective flocculation tanks 28,30 to
algae clarifiers 32,34. From the algae clarifiers 32,34, the
effluent may be conducted to appropriate filter zones, and
disinfaction systems such as those described in U. S. Patent No.
4,019,983 enti-t].ed "Disinfec-tion System and Method". The
clarified and disinfected effluen-t may be returned to the
environment following such disinfection in accordance with
conventional practice.
As shown in more detail in FIGURE 2, the raw influent
water undergoes mixing and flocculation treatment in -the
treatment stages of the system 10, from the flash mix channel
sump zones 18,20 to and including the flocculation tanks 28,30,
by means of four substantially identical jet flash mix and
flocculation assemblies 38,40,42,44. As shown in FIGURE 1, each
flash mlx sump zone 18,20 is provided with two of the assemblies
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bm:?i'
. .
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:
,
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--10--
38,43,42,44, which will now be more specifically
described with respect to the assembly 42 of FIGURE 2.
The assembly 42 comprises a submersible pump
46 which is adapted to receive wastewater from the flash
mix sump zone 20, and to provide a pressurized output to
ejector nozzle 4~. The illustrated motive pump 46 is
suitable for handling the type of solids encountered in a
typical waste effluent and is of submersible, non-clog
design with tandem, oil-filled seals. The pump is driven
by a 2.5 HP, 1750 RPM motor. The pump 46 supplies influ-
ent watex to the internal orifice of nozzle 48, having a
diameter of about 1.4 inches, at a pressure of from about
20 to 22 feet of water, and is adapted to provide a jet
stream from the orifice of ejector nozzle 48 having a
velocity in the range of about 30 to 35 feet per second.
The motive pump 46 is fitted with a suitable pipe connec-
tion, tee, elbows, support, and guide rail brackets, so
that t may be removed, and repositioned, without dewater-
ing the tank 20.
The illustrated ejector nozzle assembly 48 is a
precision machined, compound jet nozzle, which is also
utilized for the metering and introduction of coayulant
chemical solution such as aluminum sulfate solutions or
so-called pickle liquor, from chemical feed -tank 50
(FIGURE 1) via supply conduit 52, into the raw water
being treated. The coagulant (i.e., alum) solution is
supplied to an inner cavity of the nozzle 48 and inducted
therefrom into thé motive jet stream issuing from the
nozzle. The motive jet stream from the internal orifice
of nozzle 48 may serve to induce a vacuum in an inner
cavity of ~he nozzle 48 ~o assist the feed of liquid
alum from an alum pump and metering system (not shown).
The outer nozzle assembly, which is coaxial with the
inner, motive stream nozzle structuret is suppllied with
the coagulant alum solution from conduit 52, as indicated,
. : " .
5~
and as the primary motive raw water fluid from the pump
46 is forced through the inner nozzle at substantial
pressure, it forms a motive jet which comes in contact
with and educts the aqueous alum solution supplied -to
the outer nozzle structure of nozzle element 48. The
rate of flow or induction, of the alum coagulant solu-
tion may be controlled independently of the flow rate of
the raw water motive stream from the inner nozzle, and
will be relatively small in comparison with the motive
stream flow rate or the total treatment rate.
A motive jet stream including the educted
coagulant chemical solution is ejected from the nozzle 48
toward flash mixing reactor tube 50, the longitudinal
axis of which is aligned with the longitudinal axis of
the nozzle ~8.
The illustrated mixing reactor tube 50 is
substantially rotationally symmetrical in cross section,
and has a frustoconical, or funnel-shaped flange portion
53 at its end proximate the nozzle 48, an intermediate
cylindrical portion 54 which projects through wall 56
separating zone 20 and zone 30, and a ~erminal divergincJ
portion 58. The diameter D of the reactor tube (i.e.,
of the cylindrical portion 54) is substantially larger
than the diameter d of the orifice of the nozz]e 48, and
the ratio of the reactor tube diameter D to the nozzle
diameter d is desirably in the range of from about 6.5:1
to about 13:1 or more. The illus~rated reactor tube 50 has
a diameter D of about 14", which with the nozzle diameter
d provide a D/d ratio of about 10.
The jet from nozzle 48 entrains raw water to
form a turbulent, cone-shaped mixing field, expanding in
~he direction of travel, which is directed toward the
reactor tube 50 to provide for appropriate intersection o~
the mixing cone with the reactor tube. The discharge end
of the nozzle 48 may be spaced longitudinally from the
proximate end of the reactor tube 50 a distance which is
. ~
;' ' ' :'
488
-12-
also related to the diameters of the nozz:Le 4~ and the
tube 50 and the expansion rate of the cone. In this
connection, the nozzle 48 may desirably be axially
spaced a distance of from about l to aboul: 2 times D
from the proximate end of the tube 50. S:imilarly, the
length of the reactor tube 50 may be from about 2 to
about 5 or more times its diameter. The :illustrated
nozzle-reactor tube assembly is generally similar to
JD-2 disinfection system apparatus sold b~ the Pentech
Division of Houdaille Industries, Inc., bllt has a some-
what shorter nozzle adapted for use with alum solution.
Further general description of the nozzle~reactor tube
systems and operating parameters for such systems may
be had by reference to U.S. Patent 4,019,'383.
Thus, in operation, a minor por~ion of the raw
influent water to be treated is forced th:rough nozzle 4
to form a jetstream which educts the coagulant at a
predetermined, desired dosage rate. The jetstream and
inducted coagulant chemical form a motive stream
issuing from the nozzle 48 which is directed toward the
mixing reactor tube 50 at relatively high stream veloc-
ities, for example, in the range of from about 17 -to
about 82 feet per second or more. The nozzle 48 accord-
ingly discharges both coayulant and motive fluid in the
form of a relatively high velocity jet in-to the sur-
rounding raw inlet water in zone 20, thus producing a
turbulent field and substantially instantaneous mixing
of the coagulant with raw water.
The relatively high velocity jet mixture
produces a conically shaped, highly turbulent field
which provides thorough mixing (i.e., rapid reduction
of coagulant concentration gradients) of the jetstream
motive fluid. At its expanded end, the turbulent mix-
ing zone intercepts the interior wall of flange 53 or
cylinder 54 to provide a continuous turbulent fluid
,
,,
..
-13-
- ~eal which functions to prevent water which has not
passed through the turbulent maxing zone from entering
the product discharge strPam which is conducted through
the tube 50. In the illustrated embodiment, the ratio
M of the flo~ rate of raw water from the flash mix
channel sump zone 20 which is educted into the jet
issuing from the nozzle 48, to the flow rate of the je~
fro~ the nozzle 48 is about 7, and the residence time
in the mixing cone is about a second, and the mixing
rate T 1 iS about 7-8 sec 1, The volocity of the
thoroughly mixed product stream in the reactor tube is
about 6-7 feet per second.
The coagulant chemical mixing with the raw
water is thus carried out under relatively high specific
energy requirements, relatively low mixing zone resi-
dence time, and relatively high turbulence intensities.
Furthermore, it will be appreciated that the plug flow
mixing column provided in the flash mixing reactor tube
50 is provided with forward kinetic energy through the
tube and a controlled hydrostatic potential (or back
pressure) which is utilized for flocculation purpos0s
as will now be more fully described.
The flash mixing tube 50 extends through
reinforced concrete wall 56 into the flocculation zone
30. However, the mixing parallel does not discharge
directly into the zone 30, but rather is discharged into
the zone by means of flocculation jet cluster assemblies
58,60 which utilize the hydrostatic potential of the
flash mixing tube 50 in the provision of a particular
flocculation turbulence intensity and specific energy
requirement in respect of flocculation tank 30.
:.~; ,:
,. ~ ' ; ~ i
5~8c~t;
In this connection, the thoroughly mixed water~chefll:ica~
Elocculant material mixture from the flash mixiny tube 50 is
utilized as a primary motive stream for the two il.lustratecl
flocculation jet clusters 58,60, which are of -the type disclosec!
in U. S. Patent No. 3,951,344 as jet aerator modules. The
illustrated clusters 58,60 are substantially identical, and are
of the type supplied by the Pentech Division of Houdaille
Industries, Inc., as model 100 JA radial jet aerators. The
clusters 58,60 are supplied by respective clowncomers 62,6~
from conduit 66, which in turn connects with the discharge end
of the reactor tube 50 by means of knife gate valve 69.
similar valve 71 controls flow to downcomer 62 for flocculation
jet cluster 58. Each of the flocculation jet cluster modules
comprises a plurality of radially arranged nozzle elements 68
having inner (not shown) and outer frustoconical, axially aligned
nozzle elements 68. The inner nozzle elements are supplied with
fluid from an inner chamber, while the outer nozzle elements
are supplied with fluid from an outer chamher. Th,e thorougllly
mixed water-flocculant discharge stream from tube 50 havincJ
residual energy available from the pump 46 and nozzle 4~,
together with any energy represent.i.ng hydrostatic head di~erence
between sump zone 20 and the flocculation tank zone 3n i.s
directed as a pressuriæed source to the interior c:hamhers ol
the clusters 58,60 such that it is causecl to discharge as motive
streams from the internal interior jet nozzles of the clusters.
The outer chamber zone of the clusters 58,60 surrounding the
interior jets are provided with inlet openings 70,72 directecl
toward the bottom of the flocculation tank 30 such that the
water-flocculant mixture from the tank 30 ls educted into the
low energy motive stream
14-
bm:u
- : ' , .^~ '
' . ~ ' :' ,'
~ 8~
jets from the internal nozzles of the jet cluster mod-
ules 58,60. The motive jet and educted fluid mixture
is discharged from the outer nozzle elements 68 in
radially symmetrical manner into the tank 30. As
indicated in the drawin~, the flocculation jet clusters
58,60 are positioned toward the bottom of the tank 30,
and the em~ission of the jets into the tank provides
the tank with a desired low level of mixing intensity
for the effectiye development of aggregation of floc-
culation material. In this connection, the clusters58,60 and the other illustrated clusters are operated
at a relatively low hydrostatic potential of less than
about 3 feet of water pressure across the nozzle
assembly (which will vary over a range of about 2 feet
lepending upon high or low flow conditions) such that
the clusters provide a degree of mechanical energy
input to the tanks 28,30 corresponding to an average
specific energy requirement in the range of about
.5 ft-lb of treated water or .09 hpjMGD. The turbu-
lence ~lntensity, the ratio of specific energy dissi-
pation rate to specific total energy dissipation rate,
provided to the tanks 28,30 by the action of the
clusters is less than about 5~ to produce optimal
flocculation conditions, and the illustrated tanks 28,
30 each provide for extended residence time for floc
development.
The radial clusters including clusters 58,60
provide a gently circulating three dimensional fluid
motion to the flocculating fluid of tanks 28~3n~which
in turn provides mixing uniformity to the fluid in the
tanks. The outle~ from the flocculation zones is pro-
vided at the base of the tanks 28,30 for subsequent
clarifying and filtering treatment, which may also be
in accordance with conventional practice.
- " . :
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S4~36
-16-
Organopolym~ric, high molecular weight floc-
culation materials may desirably be introcluced as f~oc-
culation aids, and in this connection, a p:Lurali-ty of
taps 74 may be provided along the reactor tube 50, or
downstream oE the tube 50 for flocculant polymer solu-
tion introduction. Such taps may also be provided in
in the region of the flocculation jets 68 and/or in the
region of the inlets 70,72 for educted flocculation
tank fluid to the radial jet modules 58,60, Various
flocculation polymers may be utilized depe!nding upon
conventional conditions of use, including anionic,
nonionic and cationic organopolymeric flocculation mat-
erials. The metering dosage and point of introduction
of such materials may be modified as desired in order
to achieve the most effective results. However, it is
generally not desirable to introduce the flocculant
polymers in the vicinity of the mixing cones from the
pump nozzles (e.g., nozzle 48 in tank 20) because of
the intensity of mixing in the highly turbulent cones
may degrade the polymers and deleteriously afect their
flocculating effectiveness.
Suitable pH adjustment may be made, if appro-
priate, to the raw water influent in accordance with
conventional practice, and in this connection the use of
aluminum sulfate may not require the use of pH adjust-
ment may be more desirable in respect of iron salt
flocculation use.
The illustrated system l0 oi~ FIGURE l is
adapted to be suitable for treating a dry weather flow
of up to a maximum of 8.0 MGD, with each of the four
illustrated pump, nozzle and reactor tube systems being
adapted to each have a maximum capaci1y of 2.0 MGD
with a head loss through each reactor tube of about l.0
inches of water at its maximum flow rate of 2.0 MGD.
,
~54~6
17-
The pump-nozzle-reactor tube assemblies may be individ-
ually controlled so as to provide the treatment capac
ity appropriate to the ambient flow conditions, and as
may be seen from the drawings, each assembly is located
and supported in such a way that substantially all
influent is intimately mixed with coagulant and con-
ducted through the reactor tubes, and through the
hydraulic seals of the turbulent jets from the nozzle
with the reactor tube walls. The system may readily
provide vanal capacity; for example, by closing the
knife gate valves at the downstream ends of the reactor
tubes of tank 30, and turning of the pumps in tank 20,
the system may be readily operated at a capacity of :~
4.0 MGD. The provision of a plurality of assemblies
also results in economies of ~reduced) scale, as will
be discussed in the following mathematical description
of system operation.
Equilibrium flocculated water residual tur-
bidity may be regarded as proportional to the primary
alum floc particle volume. vO, vO should be reduced to
minimum optimum value.
Minimum optimum vO volume condi-tions may be
considered to correspond to one primary alum 10c
.particle per clay or other particle,
or
min opt vO = ~ (1)
where,
0 = average alum floc volume fraction,
cm3/cm3
No = number concentration of clay parti-
cles, cm
8~
18- .
.
Product stream flocculant inhomogeneity conditions may
be represented as a/A where "a'i is the local concen-
tration ~locculant from average value and "A" is the
local concentration, in gm/cm3. a/A is desirably equal
to or less than 0.1. Similarly, the primaxy alum floc
particle volume batch growth in time T (sec.) may be
represented as follows:
Vo = t/T + M {1 - exp ~-t~T~} (2)
kB T
where kB is the Brownian aggregation rate of primary
alum floc volume growth (cm3/sec), t 1 is the mixing
rate (sec 1), and M is the flow ratio of secondary
educted fluid to primary, motive fluid.
Studies of the relationship between the ini-
tial mixing stage phenomena and the overall flocculation
process performance objective of turbidity removal have
indicated a generally linear relationship between resi~ual tur-
bidity and the primary alum floc volume, vO, and as
indicated, flocculation process per~ormance rnay be
regarded to occur when the initial mixing process pro-
duces a homogeneous product stream con~aining primary
alum floc volumes corresponding to one primary alum
floc per clay particle present in the raw water. For
= 39, a specific mixing process objective may be
regarded to be, for example,
kB = 0 497 sec 1.
vO
Product stream homogeneity and continuous
flow residence time distribution tRTD) expressions for
the primary alum floc volume in the initial mixing
product stream may be provided for mixing for plug flow
systems such as illustrated in the drawings, and for
conventional continuously stirred tank reactor systems
(CSTR) as follows:
.
L5~
~ --19--
o _ ~"
~3 H ~ ' ~ ~:
~ ~ ~ H
C ~ 4 }3 Cn
P ~
O ~ .
O ~ ,.
.~ I O
Il 3 t~
~ ~ .
t~ u~
._ C~ ~
Z
~ ~ .
~l ; .
td~q ~IIPJ ~ ~
~ ~ ~ ~-.
+ ~n ~J
æ _ rt ~3 .
n~ o
.
. .
. .. ~
. ~ .
. .
. . ~ . . .
. .
~54
-20-
The nomograph of FIGURE 3 further illustrates various
of these relationships, and the comparative efficien-
cies of plug flow systems. For example, a continu-
ously stirred tank reactor residence time distribu-
S tion may require a mixing rate approximately 10 timesgreater than that of a plug flow RTD. For a similar
scale this would mean that the specific turbulent
ener~y dissipation rate in a CSTR unit may have to be
on the order of 1000 times that of a plug flow system
since this rate is related to the cube root of the
specific turbulent energy dissipation rate.
The information of FIGURE 3 may be used in
designing plug flow system parameters. The follow-
ing examples, in addition to the larger scale system
previously described, are given as examples of initial
mixing stage parameters, with second flocculation
stage parameters of use of the first stage product
stream as second stage motive s.tream assumed to be
similar to the larger scale system:
' :
,:
~ 548~i
21-
TABLE 4
ALUM COAGULANT INITIAL MIXING
RATE MIXING EJECTOR DESIGN
DESIRED PARAMETER VALUE
PARAMETER UNITS MEDIUM SCALE SMALL SCALE
M l 39 39
a/A l 0.10 0.10
9~T l 5.96 5.96
-l -l 23.8 23.8
T C
e (ft/sec) 45.7 0.205
R l 7.25 x 10 4 7.25 x 10 4
D in 3.74 0.25
ul ft/sec 60.6 4.05
PO - Plpsig in 24.7 l.. 50
H20
d = DRl/ in 0.101 0.0007
~ . . .
-
-22-
Where 9 is the residence time, e is the specific energy
required for mixing, R is the area ratio of the primary
jet to the mixing parallel (reactor tube), D is the
diameter of the mixing parallet (reactor tube), ul is
the velocity of the motive jet, P0 - Pl is the
pressure drop across the jet nozzle, and d is the dia~
r,leter of the jet nozzle.
The parameter determination is substantially
the same for both the small and medium scale examples,
except that for the medium scale example, an upper
limit is placed on the specific energy requirement,
whereas, for the small scale example an upper limit has
been place on the scale of the unit.
For a given flow ratio and product stream
inhomogeneity, the area ratio R, may be dete~mined in
the design of the plug flow ejector system, and in th~
examples given, the jet diameter for the primary motive
stream is approximately 3% of the diameter of the down-
stream reactor tube section.
While the invention has been particularly
described with respect to the embodiment illustrated in
FIGURES 1 and 2, it will be appreciated that various
modifications, variations and adaptations may be pro-
vided based on the present disclosure. For example,although in the specific embodiment, the product stream
from the initial flocculant mixing zone is supplied as
the motive stream to a radial jet cluster array in a
flocculation tank, other nozzle systems such as a linear
array of nozzles directed into a flow-through floccula-
tion zone to.create curculatory fluid motion in a dir-
ection orthogonal to the general direction of mass
transport in the zone, may be used to power the floc-
culation zone mixing requirements. Such variations,
~6
--2~-
modifications and adaptations are intended to be within
the spirit and scope of the present invention.
Various of the features of the invention are
set forth in the following claims:
This is a division of copending Canadian
application Serial No. 319,041, filed January 3, 1979.
.
: .
