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Turbulence factors in chlorine disinfection of wastewater
Wawr Rt'~earch Vol. 12. pp. 813 to X22. © Pergamon Press Lid.. 1978. Prinled in Greal Britain.
{X~43-1354/78:1001-0813502.00/1!
TURBULENCE FACTORS I N CHLORINE DISINFECTION OF WASTEWATER KARL E. LONGLEY Environmental Engineering Branch, Health and Environment Division, Academy of Health Sciences, US Army. Fort Sam Houston, TX, U.S.A. (Received in revised form 23 February 1978)
INTRODUCTION
Most wastewater treatment plants employing chlorination add the chlorine as an aqueous solution through a diffuser at the head of a chlorine contact basin with little or no effective mixing. Before the chlorine stream is mixed throughout the mass of the incoming wastewater under these transport conditions, reactions competing with the disinfection process result in the formation of chloramines, other byproducts, and a rapid depletion of free chlorine. Morris & Weil (1950) have shown that the rate of these reactions is dependent primarily on temperature, the pH values of the chlorine and wastewater streams, and the relative forms and concentrations of the chlorine species, nitrogenous species, and other competing substrate~ Since the formation of chloramines at wastewater pH values of 6-9 is very rapid, essentially complete in a few seconds, "conventional" methods of chlorine addition do not take advantage of the short time free chlorine is available. This study was designed to evaluate the effectiveness of several mixing schemes for inactivating bacteria and virus by chlorination. BACKGROUND
Stenquist & Kaufman (1972) mixed an aqueous chlorine stream at bench-scale into a wastewater stream by means of a multiple source grid placed in a pipe. The purpose of the grid was to achieve rapid mixing of the chlorine solution with the wastewater stream. As a control for laboratory studies, chlorine was introduced through a single inlet in the direction of flow so that the primary source of turbulence generation for the control was wall friction. Other conditions were similar. After 0.32 min of contact time, coliform inactivation was 50-55% for the control (single inlet) and 97.4% for the grid mixer. However, for a given chlorine dose, detention time varying inversely with chlorine residual yielded similar amounts of coliform inactivation for both types of reactors. Subsequent field studies conducted on a plant of Disclaimer--The views of the author are his own and do not purport to reflect the position of the Department of the Army or the Department of Defense.
813
approximately 1.7 mg day- m(6440 m 3 day- t) demonstrated no improvement of a grid chlorine diffuser relative to a diffuser placed in-line. White (1974) reported on a survey of the chlorination facilities of several wastewater treatment plants discharging into San Francisco Bay. Plants introducing chlorine at a point of turbulence demonstrated consistently higher coliform removals. Krus6 et al. (1973) studied the chlorine disinfection of the secondary effluent (trickling filters) from a 1.5mgday -I (5680m3day -s) wastewater treatment plant, a study that was the forerunner of the study presented herein. Under normal plant conditions an aqueous chlorine solution was introduced through a diffuser at the head of the contact basin. Improved mixing was achieved by introducing the aqueous chlorine feed stream at a point of turbulence in the wastewater line upstream from the contact basin. Data are shown in Table 1. Coliform inactivation after 2 and 10 rain contact time showed no significant increased coliform inactivation attributable to improved mixing. However, the inactivation of the seeded f2 virus attributable to improved mixing after 10 rain contact was increased from 6 to 91~/o, significant at the 95% level. The Reynolds number is a dimensionless number relating inertial and viscous forces, and can be expressed as: RE -
lap /z
(1)
where RE = Reynolds number, V = fluid velocity, I = characteristic length (such as pipe diameter), p = mass density, p = dynamic (absolute) viscosity. While the intensity of turbulence for a particular system is directly related to RE, severe limitations exist for using RE as the criterion to classify in different mixing systems the degree of material homogeneity (completeness of mixing) which can be attained as a function of time. A prime limitation is that for a pipe flow system the temporal and mixing relationships are inversely related to the pipe diameter, whereas RE is directly related to the pipe diameter. Brodkey (1966) observed that the statistical theory of turbulent mixing has been developed parallel to turbulent motion theory. The basic linear equation for turbulent mixing is that of mass (or heat) conser-
814
KARL E. LONGLEY Table 1. Coliform and fz virus inactivation for plant scale studies by Krus6 et al.
Test condition*
Contact time, min
Mean inactivation, 5o
Standard deviation, ~o
Number of observations
2 10 2 10
89.7 99.5 52 61
5.2 0.3 5 4
12 12 12 12
2 10 2 10
93.7 99.4 81 91
3.0 0.5 2 5
6 6 6 6
Plant conditions Coliform ./~ Virus Improved mixing Coliform f,. Virus
* Chlorine dose of approximately 8.5 mg I- t. temperature of 24--26:C, arid chlorinated effluent pH of 6.3-6.9. vation, which is the counterpart of the non-linear Navier-Stokes equation for turbulent motion. The problem of turbulent mixing presents all the difficulties that turbulent motion does because of the nonlinearity of the governing physical equations when expressed in terms of averages. Hinze 0959) discusses a phenomenological theory describing the distribution of mean values of a quantity, such as momentum or mass, by the effect of turbulence. One of these. Prandti's theory, has its analogy in the kinetic theory treating the molecular transport processes of gas which descrihe the mean free path of a gas as the average distanee a gas molecule travels before striking another. Davies (t972) reports extensively on the develop merit of the Prandtl mixing length theory and its application to experimental data. Over the core of a pipe mad away from its wall it has been shown experimentally that an empirical approximation of the velocity profile for Reynolds numbers to 105 is:
Rearranging equation (5) yields: 17x (center) = 1.22 V=.
Davis (1972) reports that it has been shown experimentally for fully developed turbulent flow that an approximate relationship exists relating root mean square (r.m.s,) value of the instanta~ous velocity fluctuation to the velocity at the center of the pipe, This relationship may be expressed as I?~ = 0.04 V~ (center)
12)
where 17x = time-average axial velocity of the flowing fluid at any point away from the wall, 17x (center) = time-average axial velocity of the flowing fluid at the pipe center, 3' = distance from the pipe wall, a = pipe radius. The total flow rate Q, through the pipe can be determined by integration of equation (3), utilizing equation (2) and the relationship, r = a - y, where r is the distance measured transversely from the center of the pipe. Q =
(3)
2nrlTx dr _
49
2
Q = 2nV,(center) l - ~ a .
(4)
Using the continuity equation, the following relationship is derived for mean flow velocity, V=: V= = Q = 0.817 17~(center). (2"
(5)
(7)
where ~" = instantaneous transverse velocity fluctuation, V~ = the r.m.s, value of the instantmteous velocity fluctuation. For turbulent flow the Prandtl m i x i n g length, I'.
may be expressed as I' -- f(y)
(8)
which when evaluated experimentally results in the f o l l o w i n g a p p r o x i m a t e rel ati onshi p for y -- a.
1' = 0.15a.
Vx(center) =
(6)
(9)
Prandtl eddy frequency, f, can be defined as: f = r~, r.
(10)
Combining equations (6), (7) and (9) into equation (10) yields f = 0.33 V=.
(11)
t2
The analogy between mass and momentum fluxes is sufficient that the effective mean eddy lenilth may be approximated as being the same for both momentum transfer and mass transfer. Camp & Stein (t943) stated that the concepts concerning the mean velocity gradient, dv/dy, are applicable to all phenomena involving fluid friction loss. The mean velocity gradient may be determined from the expression: G =
P V#
(12)
where G = mean velocity gradient (dv/dy), P = power
Turbulence factors in chlorine disinfection of wastewater
815
A~
input, V = volume of system through which power is dissipated,/~ = dynamic (absolute) viscosity. Glover (1972) has related observations of coliform disinfection by Collins et ai. (1971) to the product of the velocity gradient and time of contact (GT). He credits the G T product as being a good parameter to describe mixing intensity in a chlorine contact system. EXPERIMENTAL FACILITIES
A
The studies were carried out at the Fort Meade Sewage Treatment Plant No. 2. The plant is a conventional trickling filter plant. The chlorine stream was produced by passing tap water and chlorine gas through an ejector. The flow rate varied from an approximate minimum of 0.9 mg day- a (3410 m 3 day- ~) during the early morning hours to a maximum of about 1.6mg day - l (6060m 3 day-l), which was attained during the late morning or early afternoon hours. Waste-water streams investigated during the study were primarily those occurring between the hours of 0900 to 1800 during week days. During these hours BODs of the secondary effluent was 20-25 rag i-1, and the organic nitrogen and ammonia concentrations expressed as nitrogen was 4-6 mg I- ~ and 11-15 mg 1-1 respectively. Optimization of mixing in the pilot plant was accomplished using three pipe mixers with internal diameters of 0.5, 1.0, and 2.5 in. and a venturi mixer with an internal throat diameter of 0.44 in. The mixers are shown schematically in Fig. 1. The mixers were mounted and operated in a trailer located near the chlorine contact chamber. The trailer was equipped for the conduct of all chlorine and pH determinations and all bacterial virus and coliform assay procedures. A schematic drawing of the mixer placement and attendant plumbing in relation to the S.T.P. chlorine contact chamber is shown in Fig. 2.
.~1
Disinfectant
Section A-A
PIPE MIXER
Ois/nfectont
VENTURI MIXER Fig. I. Schematic representation of mixers used for disinfection studies• modification. The host bacterium used for f2 virus assay was the strain Escherichia coil K-13 (American Type Culture Collection No. 15766). Large batches of high titer virus was prepared as described by Krus~ et aL (1971). The virus concentrate was seeded by means of a small volume solution feeder to the effluent of the secondary clarifier to a titer of approximately 1 x 106 plaque forming units ml - ~. Indigenous coliforms having a median density of 350,000 per 100ml prior to disinfection were used as indicator organisms for the bacterial inactivation studies. The multiple tube fermentation technique (American Public Health Association, 1971) was used for determination of total coliform densities. Results were confirmed using brilliant green bile lactose broth. For the plant condition studies, composited samples in replicate were taken in sterile bottles containing sodium thiosulfate. For mixer studies samples of the mixed stream for bacterial and viral analyses were withdrawn immediately downstream from the mixer by means of a Cornwall syringe equipped with a three-way valve• At least two 5-ml portions were withdrawn for each sample and injected directly into a vial containing sodium thiosulfate. The chlorine contact time from chlorine introduction into the mixer until injection of the sample into the vial was about 2-4s. The syringe was flushed several times with the sewage-chlorine mixture between samplings. Samples
MATERIALS AND METHODS The f2 bacterial virus was prepared according to the method described by Loeb & Zinder (1961) with slight
MIXER . . . .
J ~
SEWAGE BY- PASS DISCHARGEI
SAMPLING PORT j¢l
STREAM DISCHARGE
PUMP
I)i
SEWAGE PUM[ l
ONTAC
J:,CHAMBE,RII i~.
ROTAMETER,'
MIXED
6'
I I|ISOLUTION ~II ]TANK
MANHOLE
Fig. 2. Schematic diagram of mixer placement and attendant plumbing for mixer studies.
KARL E. LONGLEY
816
Table 2. 1973-1974 ft Meade data: significance of chlorine dose and increased contact time on coliform and ./2 virus inactivation for full scale, plant condition studies Chlorine dose, mg I - ~
Test organism
4.5-4.6
Coliform:~
8.3-8.8
Coliform:~
16.2-17.4
Coliform~
4.5-4.6
f_, Virus
8.3-8.8
J~, Virus
16.2-17.4
.I2 Virus
Contact time. min
Mean inactivation, o,,
2 I0 2 10 2 I0 2 10 2 10 2 10
Standard deviation. o/,,
85.7 99.06 92.4 99.964 99.54 99.9933 3t.4 46.7 48.2 48.5 7t.I 77.3
Number of observations
3.9 0.36 3.3 0.033 0.21 0.0070 9.7 8.9 13.8 7.3 6.4 4.4
7 7 6 6 7 7 7 7 6 6 7 7
Degrees of freedom
t Statistic
10.9
6.6771.
7.2
5.6581
8.3
4.2621"
11.7
1.814"
7.6
0.019
11.8
0.945
* Significant at 95°° level. 1. Significant at 9o"_.,, level. J; Confirmed coliform test.
for contact periods of about 15 s or greater were collected at the discharge point into the contact basin and were held for the required time period before neutralization of the disinfectant with sodium thiosulfate. Total chlorine and free chlorine residuals were determined, the latter qualitatively, using modifications of the leucocrystal violet procedure of Black & Whittle (American Public Health Association, 1971). The sewage stream was pumped from the secondary emuent stream into the trailer and through a rotameter prior to introduction into the mixer. The chlorine stream likewise passed through a rotameter prior to introduction into the mixer as the disinfectant stream. The rotameters were calibrated by a positive displacement technique.
90 I--Z LU O IZ LU Q..
99
O I-> I-O
1
•
EXPERIMENTAL RESULTS
Plant condition studies Full scale, conventional plant evaluations of the terminal disinf©ction practices were carried out. all coliform data gathered during this period being confirmed. The data showing the effects of contact time and chlorine dose are presented in Table 2 and Fig. 3. As expected, both coliform and f2 virus inactivations varied as a direct function of chlorine dose. The chlorine residual at station C (I0 rain contact time} varied as a direct function of chlorine dose. and there
g
~
\~...
16.2-17.4 mg/I
~&
I
99.9
16.2-17.4 m g / l ~ ~
99.99
!
A
Z m
A A
COLIFORM
I
99.999
STA MIN
A---~B 0----~2
--- C ~
I0
-
t
VIRUS I
......
A---~B
= C
0 --'~
=
2
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CHLORINE DOSE WITHOUT MIXING Fig. 3../2 Virus confirmed coliform inactivation for full scale plant conditions and for selected chlorine doses between 4.5 and 17.4mgl -~. Temperature was 15-28~C and mixed stream pH was 6.4-7.0.
Turbulence factors in chlorine disinfection of wastewater I
O"l
SEWAGE STREAM pH: 6 8 - 6 . 9 CHLORINE STREAM pH: 2,1 MIXED STREAM pH: 6.S-G.6 TEMPERATURE: IS-I6*C
Id
i
l
817
~
l
l
&&
i
l
I
90
I~
O SO
r,l~u~s
COt~loct
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time !A)
~
99.9 ~.)
CNl.OfhN[ OOSE; 4,3 m~ll/I
~-I-
<>
z - - 99
N
1~
99.9 ~ O
~
•
9999
•
99.999
CHLORINE OOSE; 17mill
I I S tO CONTACT T I M E , MINUTES
I
COLIFORM
o
-,
MM•
•
T
t__
I
I .e~I
•
,r
I• I
I
I
I I
iS
Fig. 4. f2 Virus inactivation using venturi mixer for chlorine doses of 4.3 and 17mgl -~ and flow rate of 30gpm.
99.9
•
f2 VIRUS
ss.99
I 0
I
I
]
2
t
4
I
I
I
f)
I
8
10
CHLORINE RESIDUAL, mg/I
were no significant differences relative to sewage temperature range of 15-28°C.
Fig. 6../2 Virus and confirmed coliforms inactivation using vcnturi mixer as a function of total chlorine residual
Mixer studies
./2 Virus and coliform inactivations data in Figs. 4 and 5, showing the effects of chlorine dose and contact time, were obtained using only the venturi mixer. The flow rate for the experiment was 30gpm (ll41min-l), resulting in a velocity through the throat of the mixer of approximately 64fps (20 m s-l). Free chlorine residuals were qualitatively detectable for contact times of about 2-4 s. The total available chlorine residuals were 0.3--0.4 mg 1- t and 9.2-10.4 mg 1-' for chlorine doses of 4.3 mg 1- t and 17 mg 1- t, respectively. Other pertinent experimental data are included on the Figs. Both the f2 and coliform inactivations data shown in Figs. 4 and 5 demonstrate typical "L-shaped" curves, st, Virus inacI
0
I
tivations data for chlorine doses of 4.3 and 17 mg l do not show a significant inactivation increase after the initial phase of chlorine stream and sewage stream mixing After a contact time of 15 rain, thefz inactivation was 96 and 99.9% for chlorine doses of 4.3 mg !- ' and 17 m g l - 1 , respectively. Coliform inactivation for a chlorine dose of 4.3 mg 1- t was 99% after a contact time of 15 min which essentially was inactivation achieved with initial mixing. However, for the chlorine dose of 17 mg !-1, coliform inactivation of 99.92% was achieved in about 2-4s. After a 15-rain contact time the coliform inactivation for a chlorine dose of 17 mg 1-t exceeded the detectable limit of 99.999%. Coliform and f2 virus inactivation data in Fig. 6, showing the effect of chlorine residual for contact times of 2 s and 15 rain, were obtained using the ven-
).Z
I
t~J 0 re 90
I
l
£ 0
SEWAGE STREAM pH: 6.8-6.9 CHLORINE STREAM pH: 2.1 MIXED STREAM pH: S.S'S.6 "TEMPERATURE: IS-I6*C
_z Z
~99.S J
99.99
/
90
C~LOmNEDOSE: 4.S ml/I IZ kd U
CHLORINE DOSE: 17 n~l/I
£ 2
SEWAGE STREAM~1: S.S-T.I CHLOmNE STnEAM ~ : 2.1 MIXED STREAM phi S.S'?.O TEMPERATURE: IS - 2 r G
~
CNLORINE OOSE; 17 mql/I
~
E
• N
99.999
99.9999 L 0
99.9 0
tO
CONTACT T I M E , MINUTES
Fig. 5. Confirmed coliform inactivation using venturi mixer for chlorine doses 4.3 and 17mgl -x and flow rate of 30 gpm.
_
•
90
11
CHLORINE DOSE: 4.3 m l / I
99 S
~
~ 99
I01 MEAN
I
I
I
IOl
tO3
I0 4
GRADIENT,
S E C "1
VELOCITY
IOl
Fig. 7../2 Virus inactivation as a function of mean velocity gradient using four plug flow mixers, chlorine doses of 4.3 and 17 mg I-n and chlorine contact times of 2-15 s.
818
KARL E. LONGLEY I
I
•~ O
I[
I
!
I
I
••e
CHLORINE DOSE 17 mg/I
~•
90
90
I
N
E
DOSE; 17mg/t
Iz ~"
bJ ~.
99 ,~WAG~ STREAM~4:6.8-7.1 CHLORINESTREAM oH: 2.1 MIXED STREAM I~H: 6.5-7.0 TEMPERATURE: 13-Z~'C
Z F-
I'~
I
99
z _o 999
.
I
S~WAGESTREAMaH 6 8- 7 I CHI.OmNE SI~IE/~M~ : 2 I MIXED STREAM ~¢ 6 5-70 TEMPKRATURE I~ -28"C
_z •
~ 99.99
|
99
I
I
I
I0 z
~C3
~04
I
0
o~ u
CHLORINE DOSE: 4 3 mgll
CHLORINE DOSE: 4.3m9/I
turi mixer. The correlation coefficients for the coliform inactivation data corresponding to contact times of 2 s and 15 rain were 0.42 and 0.82, respectively. Correlation coefficients for the f2 virus inactivation data corresponding to contact times of 2s and 15 min were 0.52 and 0.53, respectively, The data presented in Fig. 6 were obtained through a wide range of turbulent mixing conditions. Figures 7-12 show both f2 virus and coliform inactivation data as a function of turbulence descriptors
99
I I0z
I01
•
•
*J I I04
I05
Fi& I0 Confirmed coliform inactivation as a function of mean velocity Ip'aulieat ~ four plu~ flow mixers, ~;hlorine doses of 4.3 and 17 rag !- '. and chlorine contact times of 2-15 s. for chlorine doses of 4.3 mg1-1 and 1 7 m g l - L The turbulence descriptors are mean velocity gradient, Prandtl eddy frequmtcy and Reynolds number, T h e data were analyzed using regression analysis tech•
I'
"~
I
I
•8
W ~J 0¢
~
R
I
N
I
DOSE: 17mg/I
m
CHLORINE DOSE: 17mg/I \ 99
°
Z
9
_> ~
.
MEAN VELOCITY GRADIENT. SEC "1
90
Z" 0
.
I ]0 3
I
1
.
•
Fig. 8. ]'2 Virus inactivation as a function of Prandtl eddy frequency using four plug flow mixers and chlorine doses of 4.3 and 17 rag I-L and chlorine contact times of 2-15 s.
,\.
:1
90 ~05
PRANDTL EDDY, FREQUENCY, HERTZ
0
•
99,9
99.9
~WAGE STREAMpH: 6.8-7; Ct~4.ORIN[ STREAMpH: 2.1 MIXED STR(AIM pH: 6.5-7.0 TE MPERATURE: 13- 28"C
O:
CHLORIN~STRIEAMpH: 2,1 MIXED STREAM gH: 6,5-7.0 TEMPERATURE*.15-28"C
_z o5
~99.99 N 0
99.99
I .
0
,,,[
•
.I
.
u 90
so
CHLORINE DOSE: 4.3mgll
| CHLORINE DOSE:
4.3 mg/I 99
9,J
105
tO t
I
I
I04
105
REYNOLDS
I06
NUMOER
Fig, 9. f2 Virus inactivation as a function o f Reynolds number using four plug flow mixers, chlorine doses of 4.3 and 17 rag I-L and chlorine contact times of 2-15 s.
I I02
I
I°
iO 4 103 PRANOTL EDDY FREQUENCY, HERTZ
•
[~ 105
Fig. 11, Confirmed coliform inactivation as a function of ine doses of 4.3 and 17roB! "t, and chlorine contact times of 2-15 s.
Turbulence factors in chlorine disinfection of wastewater •
I
I
\o*
"\
90
Id ~i
\.
99
L
\
z" o ~
~HLORIN Ir DOSE: i T m g / | SrWAGI= STR~'AM pH: 6.8-?.1 CI~.ORINE STN[AM pHt 2.1 MIXEO STR[AM pH: 6 . 5 - 7 . 0 T EIdPF'RATuR~r: 13-28"C
U
_z
~ 9999 ~
\
99.9
•
°
:HLORIN[ D O S E : 4 . 3 m g / I 90
I 9~h I0 3
. I
I
I0 4
105
REYNOLDS
jO G
NUMBER
Fig. 12. Confirmed coliform inactivation as function of Reynolds number using four plug flow mixers, chlorine doses of 4.3 and 17 mgl -!, and chlorine contact times of 2-15 s.
819
niques and the statistics are shown on Table 3. In general, the log-log transformation resulted in the best data fit, The data are derived from experiments employing the four mixers previously described. Sewage stream velocities ranged from 1.0 fps (0.30 m s - ~) to 64 fps (20 m s- i ). Chlorine stream pH was 2. I, the sewage stream pH values were 6.8--7.1, and the mixed stream pH values were 6.5-7.0. Mixed stream temperatures were 13-28°C, and chlorine contact times were 2--4 and 15 s. Coliform and f2 virus inactivation data from runs where both 2-4 and 15 s contact data were obtained were subjected to paired t testing. For the t statistic significant at the 95% level for both the virus and the bacteria there was no difference in this data between the two populations at 2-4 and 15 s chlorine contact times. Statistical information is shown on Table 4 relating the Prandtl eddy frequency as a function of mean velocity gradient, Reynolds number as a function of mean velocity gradient, and Reynolds number as a function of Prandtl eddy frequency, respectively. A remarkable correlation coefficient of 0.98 was achieved when Prandtl eddy frequency was regressed as a function of mean velocity gradient as shown in Table 4. Poor correlation coefficients of 0.53 and 0.39 resulted from the evaluation of Reynolds number as
Table 3. Regression analysis//of coliform and f, virus inactivation for mixer studies as a function of mixing descriptors Independent variable, X
Dependent dependent, Y
Mean velocity gradient Prandtl eddy frequency Reynolds number Mean velocity gradient Prandtl eddy frequency Reynolds number
f2 Virus reactivation f2 Virus inactivation " f , Virus reactivation Coliform reactivation Coliform inactivation Coliform inactivation
Figure Chlorine Number of number dose, mg 1-1 observations 7
4.3 17 4.3 17 4.3 17 4.3 17 4.3 17 4.3 17
8 9 !0 11 i2
19 17 19 17 19 17 19 17 19 17 19 17
Intercept, ao
Regression coefficient of Y on X, a l
Correlation coefficient
0.34 1.05 0.44 1.21 0.60 4.01 0.57 1.92 0.71 2.26 1.27 6.25
-0.19t - 0.65t -0.24t - 0.74t -0.19 - 1.08t - 0.3 i * - 1.10t -0.37* - 1.27t - 0.37 : 1.68t
0.35 0.80 0.39 0.75 0.09 0.62 0.32 0.85 0.34 0.85 0.11 0.55
* Significant at 95% le~,el. t Significant at 99% level. :[:Log Y = ao + al log X. Table 4. Regression analysis//of mixing descriptors Independent variable X
Dependent variable, Y
Fig. number
Number of observations
Intercept a0
Regression coefficient of Y on X, al
Correlation coefficient
Mean velocity gradient Mean velocity gradient Prandti eddy frequency
Prandtl eddy frequency Reynolds number Reynolds number
13
11
0.29
0.86t
0.98
14
11
3.46
0.36
0.53
15
11
3.53
0.35*
0.39
* Significant at 95% level. t Significant at 99% level. Log Y - - - a o + a l logX.
820
KARL E. LONGLEY
a function of mean velocity gradient and Prandti eddy frequency, respectively.
DISCUSSION
An important fact which can be concluded from data gathered using the mixers is that very little f2 virus inactivation occurs after the initial mixing of the chlorine and sewage streams. Unfortunately, no samples for immediate f2 viral inactivation could be obtained at plant scale. However, the fact that little f2 viral inactivation was observed at plant scale between 2 and 10 rain contact time indicates the significant portion of the f2 viral inactivation occurred during the immediate time following mixing of the sewage and chlorine streams. This is supported by the data shown in Fig. 4 where essentially all f2 viral inactivation was achieved during the time immediately following the very rapid mixing process achieved using the venturi mixer. And, the data shown in Fig. 6 shows little difference between viral inactivation at 2 s and 15 min contact times through a wide range of chlorine residuals. Under plant conditions and a 10 rain contact time, f2 viral inactivations were 46.7 and 77.3% for chlorine doses of about 4.5 and 17 ml 1-1. respectively. However, rapid mixing markedly improved the initial f2 viral inactivations to 94 and 99.9% for chlorine doses of about 4.3 and 17 mg 1- z respectively. This comparison dramatically points out the viral inactivation benefit to be gained through the employment of efficient, rapid mixing. Not only was the amount of viral inactivation at a chlorine dose of 4.3-4.5 mg 1- t increased two-fold through the employment of rapid mixing, but the viral inactivation of 94% using rapid mixing and chlorine dose of 4.3 nag 1- t exceeded the viral inactivation of 77.3% at plant conditions and a chlorine dose of 17 mg 1-'. The data presented in Figs. 7-9 further demonstrates that an increase in turbulent conditions at the point of chlorine mjecnon significantly enhances viral inactivation. Thus, it is obvious that efficient, rapid mixing can markedly contribute towards a saving of chlorine and possibly the production of less chloro-organics. A resolution adopted by Montgomery County, Maryland requires that effluent discharge shall contain no more than one detectable infectious virus unit per 10gal (Cookson & Robson, 1975). At an estimated concentration of 1000 enteroviruses per liter of sewage the inactivation efficiency required is 99.997%. At plant scale conditions the mean f2 viral inactivation achieved after a 10 min contact time and chlorine dose of 16.2-19.4 mg i- ~ was 77.3%. Assuming the validity of the f2 virus model and the estimated 200-7000 virus infection units per liter in raw sewage (Clark et al., 1964), the data suggest a complete inability of conventional wastewater disinfection practice to reduce virus concentrations to the recommended level.
As shown in Fig. 3, appreciable coliform inactivation by conventional disinfection practice at plant scale was observed throughout the 10 min contact time. This suggests a different inactivation mechanism for the f2 virus and coliform whereby either a chloramine form directly or free chlorine resulting from the back reaction is the bactericidal species. Thus, the use of a properly designed contact chamber is apparent if the maximum bactericidal benefit is to be achieved from a given dose. This fact is further supported by data shown in Fig. 5 obtained using the venturi mixer and a 17 mgl-~ chlorine dose. Comparison of plant condition data for coliform inactivation in Fig. 3 with the.data presented in Figs. 10--12 show that a great initial improvement in coliform inactivation can initially be achieved through rapid mixing. However, for a chlorine dose of 4.3 to 5 . 4 m g l - t and an extended contact time no significance can be attributed to the effect of initial rapid mixing of the chlorine on the amount of inactivation achieved, the results being 99.1 and 99.4% for plant conditions and rapid mixing, respectively. The coliform inactivation data shown in Fig. 6 indicates the dependence of coliform inactivation on the magnitudes of both chlorine residual and contact time. The absence of further coliform inactivation using the venturi mixer at a chlorine dose of 4.3 mg 1- t and after the immediate, rapid mixing process may be attributable in part to a rapid chlorine demand whose reaction rate is enhanced by very good material transport and diffusion. Poor correlation coefficients were obtained for inactivation data presented in Fig. 6 as a function of chlorine residual with the exception of the coliform inactivation for 15 rain contact time. As the data were obtained through a wide range of turbulent mixing conditions, the data presented on the figure demonstrates that inactivation achieved may not always be adequately predicted by chlorine residual and contact time information only. The f2 virus, as compared to the coliform organisms, appeared to be much less affected by lengthy contact with a chlorine residual. Under the operational conditions for the venturi mixer resulting in a flow rate of 28.8 gpm, the head loss across the mixer with no energy recovery was 35 ft. Assuming a pump efficiency of 75%, an electrical efficiency of 80%, an electrical cost of $0.03 per kWh. and a chlorine cost of $0.02 per pound, the cost incurred by the 35 ft head loss was equivalent to the chemical cost of 0.83 mg 1- 1 of chlorine dose. If more strigent disinfection requirements can be achieved without using more chlorine (and possibly less) through the application of turbulent mixing, a net economic saving could be realized. In order to describe rapid mixing quantitatively for both design and operational considerations, a good descriptor of the mixing process must be identified and evaluated with inactivation data. Accordingly, both viral and bacterial inactivations data were evaluated as a function of mean gradient velocity, Prandtl
Turbulence factors in chlorine disinfection of wastewater e d d y frequency, a n d Reynolds number. These data
are presented in Figs. 7-12. Generally, log-log transform of the data yielded the best fit. Data fit, as evidenced by the statistics of Table 3 are best for the higher chlorine dose, 17 mgl -t, and for the data evaluated as a function of mean velocity gradient and Prandtl eddy frequency. The non-significant correlation coefficients and unremarkable t statistics for 4.3 mg 1-' chlorine dose data expressed as a function of Reynolds number may be due, in part, to the fact that Reynolds number is a direct function of the mixer diameter, whereas, the time required for chlorine transport across a transverse section of the mixer is inversely related to the mixer diameter. Both mean velocity gradient and Prandtl eddy frequency show promise of being adequate rapid mixing descriptors to be used in conjunction with the design and evaluation of disinfection facilities through considerable additional disinfection data must be evaluated to establish firmly any relationship which may exist. The mean energy gradient is an easily calculable quantity which incorporates the design parameters of power, flow rate, and head loss, the knowledge of which are essential to the designer. However, through the development of the Prandtl eddy frequency theory and related concepts, relationships may be developed which will incorporate material transport factors, rather than momentum transfer, and the decay of the free chlorine species as a function of sewage characteristics. This type of relationship is necessary to describe adequately a rapid mix, disinfection system. Reynolds number appears to have limited value as a descriptor for mixing conditions necessary to achieve a required degree of viral or bacterial inactivation. The high correlation coefficient of 0.98 and t statistic significant at the 99% level achieved when Prandtl eddy frequency was regressed as a function of mean velocity gradient requires further evaluation. With the assumption of a direct relationship between these two turbulence descriptors based on the statistics the following expression can be derived where the subscripts G andfdenote those terms attributable to mean velocity gradient and Prandtl eddy frequency, respectively.
FO~V~.-] rv.~7
- ~ _5- L. JoLoJ,
(13)
Where ~, = density of water, 0 = Moody friction factor, and the other terms have been previously described. The density and viscosity of water can be closely approximated with a constant over the range of water temperatures encountered during the study. Therefore, the above relationship may be further simplified as:
821
Application of the continuity equation shows that for a given plug flow mixing system and constant flow rate, V,, will vary inversely with a'. U n d e r the same condition 0 will decrease slowly with increasing V,,. This relationship is, therefore, expected but significant since the use of phenomenological relationships of mean velocity gradient and Prandtl eddy frequency allow a close correlation to be developed between disinfection efficiency, material transport parameters, and energy input to the system. Reynolds number as a function of mean velocity gradient and Prandtl eddy frequency, respectively, is shown on Table 4. The bivariate, linear regression analysis of the log-log transformed data yielded nonsignificant correlation coefficients, the lowest being 0.39 for the regression of Reynolds number on Prandtl eddy frequency. Though the regression coefficient was significant at the 95% level, the relationship between Reynolds number and Prandtl eddy frequency is not significant as the regression equation does not explain 61% of total variation about the mean. The only practical use to which the Reynolds number may be applied for the evaluation of disinfection data, as discussed herein, is the determination of the friction factor necessary for the derivation of the mean velocity gradient for a given mixing system.
CONCLUSIONS
Analysis of data collected in studies for improving disinfection of sewage effluent from the Fort Meade Sewage Treatment Plant No. 2 justifies the following conclusions: (1) The conventional disinfection treatment given the sewage does not produce substantial viral inactivation at the chlorine doses of 4.3 and 17 mg ! - ' . (2) Rapid and substantial viral and bacterial inactivation may be achieved by chlorination of wastewater under highly turbulent, plug flow conditions. (3) Rapid mixing of chlorine with wastewater may achieve a required degree of disinfection by using less chlorine. Added benefits would be material (chlorine) savings and possible decreased formation of chloroorganics. (4) Mean velocity gradient and Prandti eddy frequency are highly correlated parameters requiring further investigation and development as descriptors for the rapid mixing, disinfection process. (5) Reynolds is not a universal descriptor for the rapid mixing, disinfection process.
Acknowledoements--The research from which the data for this paper is derived was supported by the U.S. Army Medical Research and Development Command, Washington, D.C., under Contract No. DADA 17-72-C-2048. I owe many thanks for encouragement and guidance to Dr. C. W. Krus~ who was both the responsible investigator for the project and my advisor while a doctoral candidate at The ' Johns Hopkins University.
822
KARL E. LONGLEY REFERENCES
American Public Health Association (1971) Standard Methods for the Examination of Water and Wastewater 13th edn. Am. Publ. Hlth Assoc., New York. Brodkey R. S. (1966) Fluid motion and mixing. In Mixing. (Edited by Uhl V. W. & Gray ,i. B.) Vol. I, pp. 7-110. Academic Press, New York. Camp T. R. & Stein P. C. (1943) Velocity gradients and internal work in fluid motion. J. Boston Soc. cir. Engrs 30, 219-237. Clarke N. A. et al. (1964) Human enteric viruses in water-source, survival and removability. In Advances h) Water Pollut. Research. (Edited by Eckenfelder W. W.) Vol. 2. p. 523. Pergamon Press, New York. Collins H. F. et al. (1971) Problems in obtaining adequate sewage disinfection. J. sanit. Engng Die. Proc. Am. Soc. cir. Engrs 97, 549-562. Cookson .i.T. & Robson C. M. (1975) Disinfection of wastewater effluents for virus inactivation. In Disinfection. Water and Wastewater. (Edited by Johnson ,i. D.) pp. 391-417, Ann Arbor Science, MI. Davies ,i. T. (1972) Turbulence Phenomena. pp. 69-72, Academic Press, New York.
Glover G. E. (1972) Discussion of problems in obtaining adequate sewage disinfection. (Editedby Collins H. F. et al.,) J. sanit. Engng Div. Proc. Am. Soc. cir. Engrs. 98, 671-673. Hinze ,i. O. (1959} Turbulence. pp. 277-302. McGraw-Hill. New York. Krus6 C. W. et al. (1971) The enhancement of viral inactivation by halogens, pp. 187-193, Proc. 13th Water Qual. Conf. Univ. IL., Bull 69, Urbana, Illinois. Krus6 C. W. et al. (1973) Improvement in terminal disinfection of sewage effluents. Wat. Sewaoe 144¢s 120, 57-64. Loeb T. & Zinder, N. D. (1961) A bacteriophage containing RNA. Biochemistry 47, 282-288. Morris J. C. & Weil I. (1950) The chlorination of water supplies in the field--reactions between aqueous chlorine and ammonia. Final Report to the Engineer Research and Development Laboratories, Fort Belvoir, Virginia. Stenquist R. J. & Kaufman, W. J. (1972) Initial mixing in coagulation processes. US Environmental Protection Agency Report EPA-72-053. Univ. of California, Berkeley, California. White G. C. (1974) Disinfection practices in the San Francisco Bay area. J. War. Poltut. Control. Fed. 46, 89-101.
{X~43-1354/78:1001-0813502.00/1!
TURBULENCE FACTORS I N CHLORINE DISINFECTION OF WASTEWATER KARL E. LONGLEY Environmental Engineering Branch, Health and Environment Division, Academy of Health Sciences, US Army. Fort Sam Houston, TX, U.S.A. (Received in revised form 23 February 1978)
INTRODUCTION
Most wastewater treatment plants employing chlorination add the chlorine as an aqueous solution through a diffuser at the head of a chlorine contact basin with little or no effective mixing. Before the chlorine stream is mixed throughout the mass of the incoming wastewater under these transport conditions, reactions competing with the disinfection process result in the formation of chloramines, other byproducts, and a rapid depletion of free chlorine. Morris & Weil (1950) have shown that the rate of these reactions is dependent primarily on temperature, the pH values of the chlorine and wastewater streams, and the relative forms and concentrations of the chlorine species, nitrogenous species, and other competing substrate~ Since the formation of chloramines at wastewater pH values of 6-9 is very rapid, essentially complete in a few seconds, "conventional" methods of chlorine addition do not take advantage of the short time free chlorine is available. This study was designed to evaluate the effectiveness of several mixing schemes for inactivating bacteria and virus by chlorination. BACKGROUND
Stenquist & Kaufman (1972) mixed an aqueous chlorine stream at bench-scale into a wastewater stream by means of a multiple source grid placed in a pipe. The purpose of the grid was to achieve rapid mixing of the chlorine solution with the wastewater stream. As a control for laboratory studies, chlorine was introduced through a single inlet in the direction of flow so that the primary source of turbulence generation for the control was wall friction. Other conditions were similar. After 0.32 min of contact time, coliform inactivation was 50-55% for the control (single inlet) and 97.4% for the grid mixer. However, for a given chlorine dose, detention time varying inversely with chlorine residual yielded similar amounts of coliform inactivation for both types of reactors. Subsequent field studies conducted on a plant of Disclaimer--The views of the author are his own and do not purport to reflect the position of the Department of the Army or the Department of Defense.
813
approximately 1.7 mg day- m(6440 m 3 day- t) demonstrated no improvement of a grid chlorine diffuser relative to a diffuser placed in-line. White (1974) reported on a survey of the chlorination facilities of several wastewater treatment plants discharging into San Francisco Bay. Plants introducing chlorine at a point of turbulence demonstrated consistently higher coliform removals. Krus6 et al. (1973) studied the chlorine disinfection of the secondary effluent (trickling filters) from a 1.5mgday -I (5680m3day -s) wastewater treatment plant, a study that was the forerunner of the study presented herein. Under normal plant conditions an aqueous chlorine solution was introduced through a diffuser at the head of the contact basin. Improved mixing was achieved by introducing the aqueous chlorine feed stream at a point of turbulence in the wastewater line upstream from the contact basin. Data are shown in Table 1. Coliform inactivation after 2 and 10 rain contact time showed no significant increased coliform inactivation attributable to improved mixing. However, the inactivation of the seeded f2 virus attributable to improved mixing after 10 rain contact was increased from 6 to 91~/o, significant at the 95% level. The Reynolds number is a dimensionless number relating inertial and viscous forces, and can be expressed as: RE -
lap /z
(1)
where RE = Reynolds number, V = fluid velocity, I = characteristic length (such as pipe diameter), p = mass density, p = dynamic (absolute) viscosity. While the intensity of turbulence for a particular system is directly related to RE, severe limitations exist for using RE as the criterion to classify in different mixing systems the degree of material homogeneity (completeness of mixing) which can be attained as a function of time. A prime limitation is that for a pipe flow system the temporal and mixing relationships are inversely related to the pipe diameter, whereas RE is directly related to the pipe diameter. Brodkey (1966) observed that the statistical theory of turbulent mixing has been developed parallel to turbulent motion theory. The basic linear equation for turbulent mixing is that of mass (or heat) conser-
814
KARL E. LONGLEY Table 1. Coliform and fz virus inactivation for plant scale studies by Krus6 et al.
Test condition*
Contact time, min
Mean inactivation, 5o
Standard deviation, ~o
Number of observations
2 10 2 10
89.7 99.5 52 61
5.2 0.3 5 4
12 12 12 12
2 10 2 10
93.7 99.4 81 91
3.0 0.5 2 5
6 6 6 6
Plant conditions Coliform ./~ Virus Improved mixing Coliform f,. Virus
* Chlorine dose of approximately 8.5 mg I- t. temperature of 24--26:C, arid chlorinated effluent pH of 6.3-6.9. vation, which is the counterpart of the non-linear Navier-Stokes equation for turbulent motion. The problem of turbulent mixing presents all the difficulties that turbulent motion does because of the nonlinearity of the governing physical equations when expressed in terms of averages. Hinze 0959) discusses a phenomenological theory describing the distribution of mean values of a quantity, such as momentum or mass, by the effect of turbulence. One of these. Prandti's theory, has its analogy in the kinetic theory treating the molecular transport processes of gas which descrihe the mean free path of a gas as the average distanee a gas molecule travels before striking another. Davies (t972) reports extensively on the develop merit of the Prandtl mixing length theory and its application to experimental data. Over the core of a pipe mad away from its wall it has been shown experimentally that an empirical approximation of the velocity profile for Reynolds numbers to 105 is:
Rearranging equation (5) yields: 17x (center) = 1.22 V=.
Davis (1972) reports that it has been shown experimentally for fully developed turbulent flow that an approximate relationship exists relating root mean square (r.m.s,) value of the instanta~ous velocity fluctuation to the velocity at the center of the pipe, This relationship may be expressed as I?~ = 0.04 V~ (center)
12)
where 17x = time-average axial velocity of the flowing fluid at any point away from the wall, 17x (center) = time-average axial velocity of the flowing fluid at the pipe center, 3' = distance from the pipe wall, a = pipe radius. The total flow rate Q, through the pipe can be determined by integration of equation (3), utilizing equation (2) and the relationship, r = a - y, where r is the distance measured transversely from the center of the pipe. Q =
(3)
2nrlTx dr _
49
2
Q = 2nV,(center) l - ~ a .
(4)
Using the continuity equation, the following relationship is derived for mean flow velocity, V=: V= = Q = 0.817 17~(center). (2"
(5)
(7)
where ~" = instantaneous transverse velocity fluctuation, V~ = the r.m.s, value of the instantmteous velocity fluctuation. For turbulent flow the Prandtl m i x i n g length, I'.
may be expressed as I' -- f(y)
(8)
which when evaluated experimentally results in the f o l l o w i n g a p p r o x i m a t e rel ati onshi p for y -- a.
1' = 0.15a.
Vx(center) =
(6)
(9)
Prandtl eddy frequency, f, can be defined as: f = r~, r.
(10)
Combining equations (6), (7) and (9) into equation (10) yields f = 0.33 V=.
(11)
t2
The analogy between mass and momentum fluxes is sufficient that the effective mean eddy lenilth may be approximated as being the same for both momentum transfer and mass transfer. Camp & Stein (t943) stated that the concepts concerning the mean velocity gradient, dv/dy, are applicable to all phenomena involving fluid friction loss. The mean velocity gradient may be determined from the expression: G =
P V#
(12)
where G = mean velocity gradient (dv/dy), P = power
Turbulence factors in chlorine disinfection of wastewater
815
A~
input, V = volume of system through which power is dissipated,/~ = dynamic (absolute) viscosity. Glover (1972) has related observations of coliform disinfection by Collins et ai. (1971) to the product of the velocity gradient and time of contact (GT). He credits the G T product as being a good parameter to describe mixing intensity in a chlorine contact system. EXPERIMENTAL FACILITIES
A
The studies were carried out at the Fort Meade Sewage Treatment Plant No. 2. The plant is a conventional trickling filter plant. The chlorine stream was produced by passing tap water and chlorine gas through an ejector. The flow rate varied from an approximate minimum of 0.9 mg day- a (3410 m 3 day- ~) during the early morning hours to a maximum of about 1.6mg day - l (6060m 3 day-l), which was attained during the late morning or early afternoon hours. Waste-water streams investigated during the study were primarily those occurring between the hours of 0900 to 1800 during week days. During these hours BODs of the secondary effluent was 20-25 rag i-1, and the organic nitrogen and ammonia concentrations expressed as nitrogen was 4-6 mg I- ~ and 11-15 mg 1-1 respectively. Optimization of mixing in the pilot plant was accomplished using three pipe mixers with internal diameters of 0.5, 1.0, and 2.5 in. and a venturi mixer with an internal throat diameter of 0.44 in. The mixers are shown schematically in Fig. 1. The mixers were mounted and operated in a trailer located near the chlorine contact chamber. The trailer was equipped for the conduct of all chlorine and pH determinations and all bacterial virus and coliform assay procedures. A schematic drawing of the mixer placement and attendant plumbing in relation to the S.T.P. chlorine contact chamber is shown in Fig. 2.
.~1
Disinfectant
Section A-A
PIPE MIXER
Ois/nfectont
VENTURI MIXER Fig. I. Schematic representation of mixers used for disinfection studies• modification. The host bacterium used for f2 virus assay was the strain Escherichia coil K-13 (American Type Culture Collection No. 15766). Large batches of high titer virus was prepared as described by Krus~ et aL (1971). The virus concentrate was seeded by means of a small volume solution feeder to the effluent of the secondary clarifier to a titer of approximately 1 x 106 plaque forming units ml - ~. Indigenous coliforms having a median density of 350,000 per 100ml prior to disinfection were used as indicator organisms for the bacterial inactivation studies. The multiple tube fermentation technique (American Public Health Association, 1971) was used for determination of total coliform densities. Results were confirmed using brilliant green bile lactose broth. For the plant condition studies, composited samples in replicate were taken in sterile bottles containing sodium thiosulfate. For mixer studies samples of the mixed stream for bacterial and viral analyses were withdrawn immediately downstream from the mixer by means of a Cornwall syringe equipped with a three-way valve• At least two 5-ml portions were withdrawn for each sample and injected directly into a vial containing sodium thiosulfate. The chlorine contact time from chlorine introduction into the mixer until injection of the sample into the vial was about 2-4s. The syringe was flushed several times with the sewage-chlorine mixture between samplings. Samples
MATERIALS AND METHODS The f2 bacterial virus was prepared according to the method described by Loeb & Zinder (1961) with slight
MIXER . . . .
J ~
SEWAGE BY- PASS DISCHARGEI
SAMPLING PORT j¢l
STREAM DISCHARGE
PUMP
I)i
SEWAGE PUM[ l
ONTAC
J:,CHAMBE,RII i~.
ROTAMETER,'
MIXED
6'
I I|ISOLUTION ~II ]TANK
MANHOLE
Fig. 2. Schematic diagram of mixer placement and attendant plumbing for mixer studies.
KARL E. LONGLEY
816
Table 2. 1973-1974 ft Meade data: significance of chlorine dose and increased contact time on coliform and ./2 virus inactivation for full scale, plant condition studies Chlorine dose, mg I - ~
Test organism
4.5-4.6
Coliform:~
8.3-8.8
Coliform:~
16.2-17.4
Coliform~
4.5-4.6
f_, Virus
8.3-8.8
J~, Virus
16.2-17.4
.I2 Virus
Contact time. min
Mean inactivation, o,,
2 I0 2 10 2 I0 2 10 2 10 2 10
Standard deviation. o/,,
85.7 99.06 92.4 99.964 99.54 99.9933 3t.4 46.7 48.2 48.5 7t.I 77.3
Number of observations
3.9 0.36 3.3 0.033 0.21 0.0070 9.7 8.9 13.8 7.3 6.4 4.4
7 7 6 6 7 7 7 7 6 6 7 7
Degrees of freedom
t Statistic
10.9
6.6771.
7.2
5.6581
8.3
4.2621"
11.7
1.814"
7.6
0.019
11.8
0.945
* Significant at 95°° level. 1. Significant at 9o"_.,, level. J; Confirmed coliform test.
for contact periods of about 15 s or greater were collected at the discharge point into the contact basin and were held for the required time period before neutralization of the disinfectant with sodium thiosulfate. Total chlorine and free chlorine residuals were determined, the latter qualitatively, using modifications of the leucocrystal violet procedure of Black & Whittle (American Public Health Association, 1971). The sewage stream was pumped from the secondary emuent stream into the trailer and through a rotameter prior to introduction into the mixer. The chlorine stream likewise passed through a rotameter prior to introduction into the mixer as the disinfectant stream. The rotameters were calibrated by a positive displacement technique.
90 I--Z LU O IZ LU Q..
99
O I-> I-O
1
•
EXPERIMENTAL RESULTS
Plant condition studies Full scale, conventional plant evaluations of the terminal disinf©ction practices were carried out. all coliform data gathered during this period being confirmed. The data showing the effects of contact time and chlorine dose are presented in Table 2 and Fig. 3. As expected, both coliform and f2 virus inactivations varied as a direct function of chlorine dose. The chlorine residual at station C (I0 rain contact time} varied as a direct function of chlorine dose. and there
g
~
\~...
16.2-17.4 mg/I
~&
I
99.9
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99.99
!
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I
99.999
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CHLORINE DOSE WITHOUT MIXING Fig. 3../2 Virus confirmed coliform inactivation for full scale plant conditions and for selected chlorine doses between 4.5 and 17.4mgl -~. Temperature was 15-28~C and mixed stream pH was 6.4-7.0.
Turbulence factors in chlorine disinfection of wastewater I
O"l
SEWAGE STREAM pH: 6 8 - 6 . 9 CHLORINE STREAM pH: 2,1 MIXED STREAM pH: 6.S-G.6 TEMPERATURE: IS-I6*C
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~
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I
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CNl.OfhN[ OOSE; 4,3 m~ll/I
~-I-
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z - - 99
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1~
99.9 ~ O
~
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9999
•
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I I S tO CONTACT T I M E , MINUTES
I
COLIFORM
o
-,
MM•
•
T
t__
I
I .e~I
•
,r
I• I
I
I
I I
iS
Fig. 4. f2 Virus inactivation using venturi mixer for chlorine doses of 4.3 and 17mgl -~ and flow rate of 30gpm.
99.9
•
f2 VIRUS
ss.99
I 0
I
I
]
2
t
4
I
I
I
f)
I
8
10
CHLORINE RESIDUAL, mg/I
were no significant differences relative to sewage temperature range of 15-28°C.
Fig. 6../2 Virus and confirmed coliforms inactivation using vcnturi mixer as a function of total chlorine residual
Mixer studies
./2 Virus and coliform inactivations data in Figs. 4 and 5, showing the effects of chlorine dose and contact time, were obtained using only the venturi mixer. The flow rate for the experiment was 30gpm (ll41min-l), resulting in a velocity through the throat of the mixer of approximately 64fps (20 m s-l). Free chlorine residuals were qualitatively detectable for contact times of about 2-4 s. The total available chlorine residuals were 0.3--0.4 mg 1- t and 9.2-10.4 mg 1-' for chlorine doses of 4.3 mg 1- t and 17 mg 1- t, respectively. Other pertinent experimental data are included on the Figs. Both the f2 and coliform inactivations data shown in Figs. 4 and 5 demonstrate typical "L-shaped" curves, st, Virus inacI
0
I
tivations data for chlorine doses of 4.3 and 17 mg l do not show a significant inactivation increase after the initial phase of chlorine stream and sewage stream mixing After a contact time of 15 rain, thefz inactivation was 96 and 99.9% for chlorine doses of 4.3 mg !- ' and 17 m g l - 1 , respectively. Coliform inactivation for a chlorine dose of 4.3 mg 1- t was 99% after a contact time of 15 min which essentially was inactivation achieved with initial mixing. However, for the chlorine dose of 17 mg !-1, coliform inactivation of 99.92% was achieved in about 2-4s. After a 15-rain contact time the coliform inactivation for a chlorine dose of 17 mg 1-t exceeded the detectable limit of 99.999%. Coliform and f2 virus inactivation data in Fig. 6, showing the effect of chlorine residual for contact times of 2 s and 15 rain, were obtained using the ven-
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I
t~J 0 re 90
I
l
£ 0
SEWAGE STREAM pH: 6.8-6.9 CHLORINE STREAM pH: 2.1 MIXED STREAM pH: S.S'S.6 "TEMPERATURE: IS-I6*C
_z Z
~99.S J
99.99
/
90
C~LOmNEDOSE: 4.S ml/I IZ kd U
CHLORINE DOSE: 17 n~l/I
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~
CNLORINE OOSE; 17 mql/I
~
E
• N
99.999
99.9999 L 0
99.9 0
tO
CONTACT T I M E , MINUTES
Fig. 5. Confirmed coliform inactivation using venturi mixer for chlorine doses 4.3 and 17mgl -x and flow rate of 30 gpm.
_
•
90
11
CHLORINE DOSE: 4.3 m l / I
99 S
~
~ 99
I01 MEAN
I
I
I
IOl
tO3
I0 4
GRADIENT,
S E C "1
VELOCITY
IOl
Fig. 7../2 Virus inactivation as a function of mean velocity gradient using four plug flow mixers, chlorine doses of 4.3 and 17 mg I-n and chlorine contact times of 2-15 s.
818
KARL E. LONGLEY I
I
•~ O
I[
I
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CHLORINE DOSE 17 mg/I
~•
90
90
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Iz ~"
bJ ~.
99 ,~WAG~ STREAM~4:6.8-7.1 CHLORINESTREAM oH: 2.1 MIXED STREAM I~H: 6.5-7.0 TEMPERATURE: 13-Z~'C
Z F-
I'~
I
99
z _o 999
.
I
S~WAGESTREAMaH 6 8- 7 I CHI.OmNE SI~IE/~M~ : 2 I MIXED STREAM ~¢ 6 5-70 TEMPKRATURE I~ -28"C
_z •
~ 99.99
|
99
I
I
I
I0 z
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I
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o~ u
CHLORINE DOSE: 4 3 mgll
CHLORINE DOSE: 4.3m9/I
turi mixer. The correlation coefficients for the coliform inactivation data corresponding to contact times of 2 s and 15 rain were 0.42 and 0.82, respectively. Correlation coefficients for the f2 virus inactivation data corresponding to contact times of 2s and 15 min were 0.52 and 0.53, respectively, The data presented in Fig. 6 were obtained through a wide range of turbulent mixing conditions. Figures 7-12 show both f2 virus and coliform inactivation data as a function of turbulence descriptors
99
I I0z
I01
•
•
*J I I04
I05
Fi& I0 Confirmed coliform inactivation as a function of mean velocity Ip'aulieat ~ four plu~ flow mixers, ~;hlorine doses of 4.3 and 17 rag !- '. and chlorine contact times of 2-15 s. for chlorine doses of 4.3 mg1-1 and 1 7 m g l - L The turbulence descriptors are mean velocity gradient, Prandtl eddy frequmtcy and Reynolds number, T h e data were analyzed using regression analysis tech•
I'
"~
I
I
•8
W ~J 0¢
~
R
I
N
I
DOSE: 17mg/I
m
CHLORINE DOSE: 17mg/I \ 99
°
Z
9
_> ~
.
MEAN VELOCITY GRADIENT. SEC "1
90
Z" 0
.
I ]0 3
I
1
.
•
Fig. 8. ]'2 Virus inactivation as a function of Prandtl eddy frequency using four plug flow mixers and chlorine doses of 4.3 and 17 rag I-L and chlorine contact times of 2-15 s.
,\.
:1
90 ~05
PRANDTL EDDY, FREQUENCY, HERTZ
0
•
99,9
99.9
~WAGE STREAMpH: 6.8-7; Ct~4.ORIN[ STREAMpH: 2.1 MIXED STR(AIM pH: 6.5-7.0 TE MPERATURE: 13- 28"C
O:
CHLORIN~STRIEAMpH: 2,1 MIXED STREAM gH: 6,5-7.0 TEMPERATURE*.15-28"C
_z o5
~99.99 N 0
99.99
I .
0
,,,[
•
.I
.
u 90
so
CHLORINE DOSE: 4.3mgll
| CHLORINE DOSE:
4.3 mg/I 99
9,J
105
tO t
I
I
I04
105
REYNOLDS
I06
NUMOER
Fig, 9. f2 Virus inactivation as a function o f Reynolds number using four plug flow mixers, chlorine doses of 4.3 and 17 rag I-L and chlorine contact times of 2-15 s.
I I02
I
I°
iO 4 103 PRANOTL EDDY FREQUENCY, HERTZ
•
[~ 105
Fig. 11, Confirmed coliform inactivation as a function of ine doses of 4.3 and 17roB! "t, and chlorine contact times of 2-15 s.
Turbulence factors in chlorine disinfection of wastewater •
I
I
\o*
"\
90
Id ~i
\.
99
L
\
z" o ~
~HLORIN Ir DOSE: i T m g / | SrWAGI= STR~'AM pH: 6.8-?.1 CI~.ORINE STN[AM pHt 2.1 MIXEO STR[AM pH: 6 . 5 - 7 . 0 T EIdPF'RATuR~r: 13-28"C
U
_z
~ 9999 ~
\
99.9
•
°
:HLORIN[ D O S E : 4 . 3 m g / I 90
I 9~h I0 3
. I
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105
REYNOLDS
jO G
NUMBER
Fig. 12. Confirmed coliform inactivation as function of Reynolds number using four plug flow mixers, chlorine doses of 4.3 and 17 mgl -!, and chlorine contact times of 2-15 s.
819
niques and the statistics are shown on Table 3. In general, the log-log transformation resulted in the best data fit, The data are derived from experiments employing the four mixers previously described. Sewage stream velocities ranged from 1.0 fps (0.30 m s - ~) to 64 fps (20 m s- i ). Chlorine stream pH was 2. I, the sewage stream pH values were 6.8--7.1, and the mixed stream pH values were 6.5-7.0. Mixed stream temperatures were 13-28°C, and chlorine contact times were 2--4 and 15 s. Coliform and f2 virus inactivation data from runs where both 2-4 and 15 s contact data were obtained were subjected to paired t testing. For the t statistic significant at the 95% level for both the virus and the bacteria there was no difference in this data between the two populations at 2-4 and 15 s chlorine contact times. Statistical information is shown on Table 4 relating the Prandtl eddy frequency as a function of mean velocity gradient, Reynolds number as a function of mean velocity gradient, and Reynolds number as a function of Prandtl eddy frequency, respectively. A remarkable correlation coefficient of 0.98 was achieved when Prandtl eddy frequency was regressed as a function of mean velocity gradient as shown in Table 4. Poor correlation coefficients of 0.53 and 0.39 resulted from the evaluation of Reynolds number as
Table 3. Regression analysis//of coliform and f, virus inactivation for mixer studies as a function of mixing descriptors Independent variable, X
Dependent dependent, Y
Mean velocity gradient Prandtl eddy frequency Reynolds number Mean velocity gradient Prandtl eddy frequency Reynolds number
f2 Virus reactivation f2 Virus inactivation " f , Virus reactivation Coliform reactivation Coliform inactivation Coliform inactivation
Figure Chlorine Number of number dose, mg 1-1 observations 7
4.3 17 4.3 17 4.3 17 4.3 17 4.3 17 4.3 17
8 9 !0 11 i2
19 17 19 17 19 17 19 17 19 17 19 17
Intercept, ao
Regression coefficient of Y on X, a l
Correlation coefficient
0.34 1.05 0.44 1.21 0.60 4.01 0.57 1.92 0.71 2.26 1.27 6.25
-0.19t - 0.65t -0.24t - 0.74t -0.19 - 1.08t - 0.3 i * - 1.10t -0.37* - 1.27t - 0.37 : 1.68t
0.35 0.80 0.39 0.75 0.09 0.62 0.32 0.85 0.34 0.85 0.11 0.55
* Significant at 95% le~,el. t Significant at 99% level. :[:Log Y = ao + al log X. Table 4. Regression analysis//of mixing descriptors Independent variable X
Dependent variable, Y
Fig. number
Number of observations
Intercept a0
Regression coefficient of Y on X, al
Correlation coefficient
Mean velocity gradient Mean velocity gradient Prandti eddy frequency
Prandtl eddy frequency Reynolds number Reynolds number
13
11
0.29
0.86t
0.98
14
11
3.46
0.36
0.53
15
11
3.53
0.35*
0.39
* Significant at 95% level. t Significant at 99% level. Log Y - - - a o + a l logX.
820
KARL E. LONGLEY
a function of mean velocity gradient and Prandti eddy frequency, respectively.
DISCUSSION
An important fact which can be concluded from data gathered using the mixers is that very little f2 virus inactivation occurs after the initial mixing of the chlorine and sewage streams. Unfortunately, no samples for immediate f2 viral inactivation could be obtained at plant scale. However, the fact that little f2 viral inactivation was observed at plant scale between 2 and 10 rain contact time indicates the significant portion of the f2 viral inactivation occurred during the immediate time following mixing of the sewage and chlorine streams. This is supported by the data shown in Fig. 4 where essentially all f2 viral inactivation was achieved during the time immediately following the very rapid mixing process achieved using the venturi mixer. And, the data shown in Fig. 6 shows little difference between viral inactivation at 2 s and 15 min contact times through a wide range of chlorine residuals. Under plant conditions and a 10 rain contact time, f2 viral inactivations were 46.7 and 77.3% for chlorine doses of about 4.5 and 17 ml 1-1. respectively. However, rapid mixing markedly improved the initial f2 viral inactivations to 94 and 99.9% for chlorine doses of about 4.3 and 17 mg 1- z respectively. This comparison dramatically points out the viral inactivation benefit to be gained through the employment of efficient, rapid mixing. Not only was the amount of viral inactivation at a chlorine dose of 4.3-4.5 mg 1- t increased two-fold through the employment of rapid mixing, but the viral inactivation of 94% using rapid mixing and chlorine dose of 4.3 nag 1- t exceeded the viral inactivation of 77.3% at plant conditions and a chlorine dose of 17 mg 1-'. The data presented in Figs. 7-9 further demonstrates that an increase in turbulent conditions at the point of chlorine mjecnon significantly enhances viral inactivation. Thus, it is obvious that efficient, rapid mixing can markedly contribute towards a saving of chlorine and possibly the production of less chloro-organics. A resolution adopted by Montgomery County, Maryland requires that effluent discharge shall contain no more than one detectable infectious virus unit per 10gal (Cookson & Robson, 1975). At an estimated concentration of 1000 enteroviruses per liter of sewage the inactivation efficiency required is 99.997%. At plant scale conditions the mean f2 viral inactivation achieved after a 10 min contact time and chlorine dose of 16.2-19.4 mg i- ~ was 77.3%. Assuming the validity of the f2 virus model and the estimated 200-7000 virus infection units per liter in raw sewage (Clark et al., 1964), the data suggest a complete inability of conventional wastewater disinfection practice to reduce virus concentrations to the recommended level.
As shown in Fig. 3, appreciable coliform inactivation by conventional disinfection practice at plant scale was observed throughout the 10 min contact time. This suggests a different inactivation mechanism for the f2 virus and coliform whereby either a chloramine form directly or free chlorine resulting from the back reaction is the bactericidal species. Thus, the use of a properly designed contact chamber is apparent if the maximum bactericidal benefit is to be achieved from a given dose. This fact is further supported by data shown in Fig. 5 obtained using the venturi mixer and a 17 mgl-~ chlorine dose. Comparison of plant condition data for coliform inactivation in Fig. 3 with the.data presented in Figs. 10--12 show that a great initial improvement in coliform inactivation can initially be achieved through rapid mixing. However, for a chlorine dose of 4.3 to 5 . 4 m g l - t and an extended contact time no significance can be attributed to the effect of initial rapid mixing of the chlorine on the amount of inactivation achieved, the results being 99.1 and 99.4% for plant conditions and rapid mixing, respectively. The coliform inactivation data shown in Fig. 6 indicates the dependence of coliform inactivation on the magnitudes of both chlorine residual and contact time. The absence of further coliform inactivation using the venturi mixer at a chlorine dose of 4.3 mg 1- t and after the immediate, rapid mixing process may be attributable in part to a rapid chlorine demand whose reaction rate is enhanced by very good material transport and diffusion. Poor correlation coefficients were obtained for inactivation data presented in Fig. 6 as a function of chlorine residual with the exception of the coliform inactivation for 15 rain contact time. As the data were obtained through a wide range of turbulent mixing conditions, the data presented on the figure demonstrates that inactivation achieved may not always be adequately predicted by chlorine residual and contact time information only. The f2 virus, as compared to the coliform organisms, appeared to be much less affected by lengthy contact with a chlorine residual. Under the operational conditions for the venturi mixer resulting in a flow rate of 28.8 gpm, the head loss across the mixer with no energy recovery was 35 ft. Assuming a pump efficiency of 75%, an electrical efficiency of 80%, an electrical cost of $0.03 per kWh. and a chlorine cost of $0.02 per pound, the cost incurred by the 35 ft head loss was equivalent to the chemical cost of 0.83 mg 1- 1 of chlorine dose. If more strigent disinfection requirements can be achieved without using more chlorine (and possibly less) through the application of turbulent mixing, a net economic saving could be realized. In order to describe rapid mixing quantitatively for both design and operational considerations, a good descriptor of the mixing process must be identified and evaluated with inactivation data. Accordingly, both viral and bacterial inactivations data were evaluated as a function of mean gradient velocity, Prandtl
Turbulence factors in chlorine disinfection of wastewater e d d y frequency, a n d Reynolds number. These data
are presented in Figs. 7-12. Generally, log-log transform of the data yielded the best fit. Data fit, as evidenced by the statistics of Table 3 are best for the higher chlorine dose, 17 mgl -t, and for the data evaluated as a function of mean velocity gradient and Prandtl eddy frequency. The non-significant correlation coefficients and unremarkable t statistics for 4.3 mg 1-' chlorine dose data expressed as a function of Reynolds number may be due, in part, to the fact that Reynolds number is a direct function of the mixer diameter, whereas, the time required for chlorine transport across a transverse section of the mixer is inversely related to the mixer diameter. Both mean velocity gradient and Prandtl eddy frequency show promise of being adequate rapid mixing descriptors to be used in conjunction with the design and evaluation of disinfection facilities through considerable additional disinfection data must be evaluated to establish firmly any relationship which may exist. The mean energy gradient is an easily calculable quantity which incorporates the design parameters of power, flow rate, and head loss, the knowledge of which are essential to the designer. However, through the development of the Prandtl eddy frequency theory and related concepts, relationships may be developed which will incorporate material transport factors, rather than momentum transfer, and the decay of the free chlorine species as a function of sewage characteristics. This type of relationship is necessary to describe adequately a rapid mix, disinfection system. Reynolds number appears to have limited value as a descriptor for mixing conditions necessary to achieve a required degree of viral or bacterial inactivation. The high correlation coefficient of 0.98 and t statistic significant at the 99% level achieved when Prandtl eddy frequency was regressed as a function of mean velocity gradient requires further evaluation. With the assumption of a direct relationship between these two turbulence descriptors based on the statistics the following expression can be derived where the subscripts G andfdenote those terms attributable to mean velocity gradient and Prandtl eddy frequency, respectively.
FO~V~.-] rv.~7
- ~ _5- L. JoLoJ,
(13)
Where ~, = density of water, 0 = Moody friction factor, and the other terms have been previously described. The density and viscosity of water can be closely approximated with a constant over the range of water temperatures encountered during the study. Therefore, the above relationship may be further simplified as:
821
Application of the continuity equation shows that for a given plug flow mixing system and constant flow rate, V,, will vary inversely with a'. U n d e r the same condition 0 will decrease slowly with increasing V,,. This relationship is, therefore, expected but significant since the use of phenomenological relationships of mean velocity gradient and Prandtl eddy frequency allow a close correlation to be developed between disinfection efficiency, material transport parameters, and energy input to the system. Reynolds number as a function of mean velocity gradient and Prandtl eddy frequency, respectively, is shown on Table 4. The bivariate, linear regression analysis of the log-log transformed data yielded nonsignificant correlation coefficients, the lowest being 0.39 for the regression of Reynolds number on Prandtl eddy frequency. Though the regression coefficient was significant at the 95% level, the relationship between Reynolds number and Prandtl eddy frequency is not significant as the regression equation does not explain 61% of total variation about the mean. The only practical use to which the Reynolds number may be applied for the evaluation of disinfection data, as discussed herein, is the determination of the friction factor necessary for the derivation of the mean velocity gradient for a given mixing system.
CONCLUSIONS
Analysis of data collected in studies for improving disinfection of sewage effluent from the Fort Meade Sewage Treatment Plant No. 2 justifies the following conclusions: (1) The conventional disinfection treatment given the sewage does not produce substantial viral inactivation at the chlorine doses of 4.3 and 17 mg ! - ' . (2) Rapid and substantial viral and bacterial inactivation may be achieved by chlorination of wastewater under highly turbulent, plug flow conditions. (3) Rapid mixing of chlorine with wastewater may achieve a required degree of disinfection by using less chlorine. Added benefits would be material (chlorine) savings and possible decreased formation of chloroorganics. (4) Mean velocity gradient and Prandti eddy frequency are highly correlated parameters requiring further investigation and development as descriptors for the rapid mixing, disinfection process. (5) Reynolds is not a universal descriptor for the rapid mixing, disinfection process.
Acknowledoements--The research from which the data for this paper is derived was supported by the U.S. Army Medical Research and Development Command, Washington, D.C., under Contract No. DADA 17-72-C-2048. I owe many thanks for encouragement and guidance to Dr. C. W. Krus~ who was both the responsible investigator for the project and my advisor while a doctoral candidate at The ' Johns Hopkins University.
822
KARL E. LONGLEY REFERENCES
American Public Health Association (1971) Standard Methods for the Examination of Water and Wastewater 13th edn. Am. Publ. Hlth Assoc., New York. Brodkey R. S. (1966) Fluid motion and mixing. In Mixing. (Edited by Uhl V. W. & Gray ,i. B.) Vol. I, pp. 7-110. Academic Press, New York. Camp T. R. & Stein P. C. (1943) Velocity gradients and internal work in fluid motion. J. Boston Soc. cir. Engrs 30, 219-237. Clarke N. A. et al. (1964) Human enteric viruses in water-source, survival and removability. In Advances h) Water Pollut. Research. (Edited by Eckenfelder W. W.) Vol. 2. p. 523. Pergamon Press, New York. Collins H. F. et al. (1971) Problems in obtaining adequate sewage disinfection. J. sanit. Engng Die. Proc. Am. Soc. cir. Engrs 97, 549-562. Cookson .i.T. & Robson C. M. (1975) Disinfection of wastewater effluents for virus inactivation. In Disinfection. Water and Wastewater. (Edited by Johnson ,i. D.) pp. 391-417, Ann Arbor Science, MI. Davies ,i. T. (1972) Turbulence Phenomena. pp. 69-72, Academic Press, New York.
Glover G. E. (1972) Discussion of problems in obtaining adequate sewage disinfection. (Editedby Collins H. F. et al.,) J. sanit. Engng Div. Proc. Am. Soc. cir. Engrs. 98, 671-673. Hinze ,i. O. (1959} Turbulence. pp. 277-302. McGraw-Hill. New York. Krus6 C. W. et al. (1971) The enhancement of viral inactivation by halogens, pp. 187-193, Proc. 13th Water Qual. Conf. Univ. IL., Bull 69, Urbana, Illinois. Krus6 C. W. et al. (1973) Improvement in terminal disinfection of sewage effluents. Wat. Sewaoe 144¢s 120, 57-64. Loeb T. & Zinder, N. D. (1961) A bacteriophage containing RNA. Biochemistry 47, 282-288. Morris J. C. & Weil I. (1950) The chlorination of water supplies in the field--reactions between aqueous chlorine and ammonia. Final Report to the Engineer Research and Development Laboratories, Fort Belvoir, Virginia. Stenquist R. J. & Kaufman, W. J. (1972) Initial mixing in coagulation processes. US Environmental Protection Agency Report EPA-72-053. Univ. of California, Berkeley, California. White G. C. (1974) Disinfection practices in the San Francisco Bay area. J. War. Poltut. Control. Fed. 46, 89-101.