Prepared by

                 E. Joe Middlebrooks
                Utah State University
                     Logan, Utah
     1979 U.S. EPA Technology Transfer Seminars
Wastewater Treatment Facilities for Small Communities

                      July 1979
               CINCINNATI, OHIO  45268

                                                               r. 9 c ? 7 1
                                                               ••j (. -j £. I I

     4.3.7  DISINFECTION


     Municipal wastewaters contain a variety of infectious microorganisms  such

as salmonellae, shigellae, enteropathogenic Escherichia coli,  Pseudomonas

aeruginosa, and enteric viruses.  Outbreaks of gastroenteritis, typhoid,

shigellosis, salmonellosis, ear infections, and infectious hepatitis  have  been

reported for people drinking or swimming in waters mixed with  municipal waste-

waters.  Outbreaks also have occurred when people eat raw shellfish harvested

from waters contaminated with municipal wastewaters.

     All discharges do not necessarily contain all or part of the pathogens

mentioned  above, and  it may be  possible to escape contact with these  organisms

most of  the time.  However, practical public health practices dictate that

constant  protection be provided,  because it  is impossible to detect the

presence  of pathogens before they are discharged  in a wastewater.  Consequently

disinfection of wastewater discharges must be practiced continuously.

     Experience and judgment have shown that  reducing the fecal coliform

concentration  to 14 per  100 ml  or the total  coliform concentration to 70 per

100 ml will prevent disease Outbreaks caused  by  shellfish  (EPA, 1976).

Limited  epidemiological  data indicates  that  concentrations of  fecal coliform

of approximately 200  per 100 ml in recreational  waters  reduces  the probability

of contact to  an acceptable level (EPA, 1976).   Although  the U.S. Environ-

mental Protection  Agency Secondary Effluent  Standards no  longer contain a

standard for  fecal coliform, the  logic  for  its inclusion  initially was based

on the  limited epidemiological  data referred  to  above.

Many chemicals and physical agents are good disinfectarits. Heat, sun-
light, chlorine, bromine, iodine, potassium permanganate, chlorine dioxide,
ozone, and ultraviolet light are effective disinfectants, but experience with
most of these materials as a wastewater disinfectant is limited. Because
of thr extensive experience with chlorine, it is and will likely continue to
be the most widely used disinfectant. The principal disadvantage of chlorina-
tion is the production of toxic substances and its effect on aquatic life. TOXIC EFFECTS
The production of halogenated organic compounds suspected of being
toxic to man has produced concern by public health officials, and efforts
are being directed toward developing other methods of disinfection. The
implementation of dechlorination to reduce the toxicity of wastewater dis-
charges to natural environments has resulted in concern about the compounds
produced by the reactions between the forms of residual chlorine and de—
chlorinating agents such as sulfur dioxide, sodium bisulfite, sodium sulfite
or activated carbon. This effort will eventually result in a reduction in the
use of chlorine, but immediate changes are not likely to occur.
Many of the proposed substitutes may have the saxne disadvantages associ-
ated with chlorination. Ozone is used extensively in Europe to disinfect
drinking water, but li!tle is known about its interaction with organic, matter
in wastewaters. All of the disinfectants listed abo have disadvantages,
and ix st suffer from high cOstS, inefficiency in wastewater with solids,
toxic side effects, and no residual. The other halogens will probably have
disadvantages similar to those of chlorine.

To understand the effects of chlorinating stabilization pond effluents,
it is necessary to review the basic principles of chlorination. When chlo—
rine gas is used the gas reacts with water to form hypochiorous acid (Hod).
In a pure water system, the reaction is as follows:
C1 2 +H 2 0 HOC1+H++C1 (1)
The hypochiorous acid then disassociates to form 0C1 and F1
}IOC1 H + oci (2)
When Ca(OC1) 2 , for example, is used to chlorinate, oci is formed by the
following reaction:
Ca(OC1) 2 —b-- Ca + 20C1 (3)
The 0C1 is then free to form hypochiorous acid in contact with hydro-
gen ions. Chlorine in the form of Hod or OC1 is referred to as free
chlorine. Both forms of free chlorine are powerful disinfectants and react
quickly to destroy bacteria and most viruses.
In wastewater, such as stabilization pond effluents, various chemical
components react with free chlorine to form compounds which are ineffective as
disinfectants. That is, the rates of reactions between chlorine and these
components are faster than the rate at which chlorine attacks and kills
bacteria and viruses. Fe++, Mn++, N0 2 and S are common reducing
agents which combine readily with chlorine to prevent it from disiofecting. A
typical reaction is as follows:
11 2 S + 4C1 + 4H 2 0 - H SO + SHC1 (4)
Free chlorine also reacts with ammonia fo :nd it’ wastewater to form a
series of compounds known as chioramines. Although chioramines are less
than 5 percent as efficient as free chlorine in destroying bacteria and

Comparison of ideal and waste—
water chlorination curves.
Wostewoter Breakpoint Curve
C ,,
de I Breakpoint Curve
Figure 4.1.

viruses, they do play an important role in disinfection because they are
fairly stable and can continue to provide disinfection for some time after
application. The common forms of chioramines, or combined chlorine, as they
are referred to, are monochioramine, dichioramine, and nitrogen trichioride.
The reactions for their formation are as follows:
NH 3 + HOC1 NH C1 + H 2 0 (5)
NH 2 C1 + HOC1 e - NHC1 2 + H 2 0 (6)
NHC1 2 + HOC1 - HC1 + H 2 0 (7)
In some cases, chlorination is used as a treatment step to drive off
undesirable ammonia. This is known as breakpoint chlorination. Basically,
chlorine is added until all the chlorine has reacted to form chloramjnes. With
the addition of more chlorine, the ammonia is converted to nitrogen gas and
driven off. Any additional chlorine added beyond that point is maintained in
solution as free chlorine residual. The mechanisms involved are complex, but
the overall reaction may be represented as follows:
2NH 3 + 3HOC1 - N 2 1 + 3HC1 + 3H 2 0 (8)
A comparison of ideal breakpoint chlorination and wastewater break-
point chlorination i ’ presented in Figure 4.1. Because the chlorine dose
necessary to reach the breakpoint in wastewater is much higher than the
dose necessary to achieve adequate disinfection, breakpoint chlorination is
seldom used in the treatment of wastewater. EFFECTS OF CHLORINATING LAGOON EFFLUENTS
Since chlorine, at present, is less expensive and offers more flexibility
than other means of disinfection, chlorination, is the most practical method
of reducing bacterial populations. However, there is evidence that chlorina-
tion of wastewater high in organic nitrogen content, such as stabilization
pond effluent, may be accompanied by adverse effects.

White (1973) has suggested that chlorine demand is increased by high
concentrations of algae corau nly found in pond effluents. It was found that
to satisfy chlorine demand and to produce enough residual to effectively dis—
infect algae laden wastewater within 30—45 minutes, a chlorine dose of 20—30
mg/i was required. Kott (1971) also reported increases in chlorine demand
as a result of algae, but found that a chlorine dose of 8 mg/I was sufficient
to produce adequate disinfection within 30 minutes and that if contact times
are kept relatively short, no serious chlorine demand by algae cells is
encountered. Of course, the amount of chlorine demand exerted is highly
variable. Dinges and Rust (1969) found that for pond effluents, a chlorine
demand of only 2.65 to 3.0 mg/i was exerted after 20 minutes of contact.
Brinkhead and O’Brien (1973) found that at low doses of chlorine, very little
increase in chlorine demand is attributable to algae, but at higher doses,
the destruction of algae cells greatly increases demand. This is because
dissolved organic compounds released from destroyed algae cells, as explained
by Echelberger et al. (1971), are oxidized by chlorine and thus increase
chlorine demand.
Another concern regarding the chlorination of pond effluents is the
effects on biochemical oxygen demand (80D 5 ) and chemical oxygen demand (COD).
Brinkhead and O’Brien (1973) and Echelberger et al. (1971) found that for
higher chlorine doses, increases in BOD 5 due to destruction of algae cells
were observed. Echelberger et al. (1971) also reported increases in soluble
COD. Horn (1972) found that when 2.0 mg/i chlorine was applied to pond ef-
fluent, the BOD 5 measured was 20 mg/I. However, when 64 mg/i chlorine was
applied, the BOD 5 increased to 129 mg/I. However, Zaloum and Murphy (1974)
observed a 40 percent reduction of BOD 5 resulting from chlorination. Dinges
and Rust (1969) also reported reductions of 80D 5 . Kott (1971) has suggested

that increases in BOD 5 may he controlled by using low chlorine doses coupled
with long contact periods.
The formation of toxic chioramines is also of concern in chlorinating
pond effluents. These compounds are found in waters high in ammonia concen-
tration and are extremely toxic to aquatic life found in receiving water.
For example, Zillich (1972) has determined that a chioramine concentration of
0.06 mg/i is lethal to trout.
Not all of the side effects of chlorinating pond effluents are detri-
mental. Kott (1973) observed reductions of suspended solids (SS) as a result
of chlorination. Dinges and Rust (1969) reported reductions of vol .itile sus-
pended solids (vSS) by as much as 52.3 percent and improved water clarity
(turbidity) by 31.8 percent following chlorination. Echelberger et al. (1971)
reported that chlorine enhances the flocculation of algae masses by causing
algae cells to clump together.
Four systems of identically designed chlorine mixing and contact tanks,
each capable of treating 50,000 gallons per day, were used by Johnson et al.
(1978) to study the chlorination of lagoon effluents. Three of the four
chlorination systems were used for directly treating pond effluent. The
effluent treated in the fourth system was filtered through an intermittent
sand filter to remove algae prior to chlorination. The filtered effluent was
also used as the solution water for all four chlorination systems.
Following recommendations by Collins, Selleck, and White (1971),
Kothandaraxnan and Evans (1972 and 1974), and Marske and Boyle (1973), the
chlorination systems were consttucted to provide rapid initial mixing follow-
ed by chlorine contact in plug ‘tow reactnrs. A serpentine flow configura-
tion having a length to width ratio of 25:1, ‘cupled with inlet and outlet
baffles, was used to enhance plug flcw hydraulic performance. The maximum

theoretical detention time for each tank was 60 minutes, while the maximum
actual detention time averaged about 50 minutes.
The pond effluent was chlorinated at doses ranging from 0.25 to 30.0 mg/i
under a variety of contact times, temperatures, and seasonal effluent condi—
tions from August 1975 to August 1976. A variety of chemical, physical, and
bacteriological parameters were monitored during this period in evaluating the
chlorination of pond effluents. A series of laboratory experiments were also
conducted to compliment the field study. Some of the major findings of this
study are summarized below.
1. Sulfide, produced as a result of anaerobic conditions in the ponds
during winter months when the ponds are frozen over, exerts a significant
chlorine demand (Figure 4.2). For sulfide concentrations of 1.0 to 1.8 mg/i,
a chlorine dose of 6 to 7 mg/i was required to produce the same residual as a
chlorine dose of about 1 mg/i for conditions of no sulfide.
2. For all concentrations of ammonia encountered, it was found that
adequate disinfection could be obtained with combined chlorine residual
in 50 minutes or less of contact time. Therefore, breakpoint chlorination,
and the subsequent production of free chlorine residual, was found to be
rarely, if ever, necessary in disinfecting pond effluent.
3. It was found that total COD is virtually unaffected by chlorination.
Soluble COD was found to increase with increasing concentrations of free
chlorine only. This increase was attributed to the oxidation of suspended
solids by free chlorine. Increases in soluble COD versus free chlorine
residual are shown in Figure 4.3.
4. Some reduction in suspended solids, due to the break down and oxida-
tion of suspended particulates, and resulting increases in turbidity were
attributed to chlorination. However, this reduction was found to be of

- 15
Figure 4.2. Chlorine dose vs. residual for initial sulfide concentrations
of 1.0 1.8 tng/l.
C /)
Figure 4.3.
Changes in soluble COD vs. free chlorine residual——unfiltered
lagoon effluent.
RESIDUAL 1.552 • 0.346 (DOSE)
R 0.956
0 5 10 15 20
0 I 2 3 4 5

limited importance when compared with reductions of suspended solids result-
ing from settling. Suspended solids were reduced by 10 to 50 percent by
settling in the contact tanks.
5. Filtered pond effluent exerted a lower chlorine demand than unfil—
tered pond effluent, due to the removal of algae (Figure 4.4). The rate of
exertion of chlorine demand was determined to be directly related to chlorine
dose and total chemical oxygen demand.
6. A summary of coliform removal efficiencies as a function of total
chlorine residual for filtered and unfiltered effluent is illustrated in
Figure 4.5. The rate of disinfection was a function of the chlorine dose and
bacterial concentration. Generally, the chlorine demand was found to be about
50 percent of the applied chlorine dose except during periods of sulfide
7. Disinfection efficiency was temperature dependent. At colder tempera-
tures, the reduction in the rate of disinfection was partially offset by
reductions in the exertion of chlorine demand; however, the net effect was a
reduction in the chlorine residual necessary to achieve adequate disinfection
with increasing temperature for a specific contact period.
8. In almost all cases, adequate disinfection was obtained with com-
bined chlorine residuals of between 0.5 and 1.0 mg/l after a contact period
of approximately 50 minutes. This indicated that disinfection can be achieved
without discharging excessive concentrations of toxic chlorine residuals
into receiving waters. Also, it was found that adequate bacterial removal can
be achieved with relatively low doses of applied chlorine during most of the

— 15
(a) Filtered Effluent
(b) Unfiltered Effluent
Figure 4.4. Chlorine dose vs. total residual——
filtered and unfiltered effluent.
6 12 18 24 30
z -2.0
- I .0
(a) Filtered Effluent
Figure 4.5. Coliform removal efficiencies
filtered and unfiltered effluent.
5 10 15 20
0 I 2 3 4 5
D l
2 4 6 8
(b) Unfiltered Effluent

Using the data from the study summarized in section, Johnson
et al. (1978) developed a model to predict the chlorine residual required to
obtain a specified bacterial kill.
The model was used to construct a series of design curves for selecting
chlorine doses and contact times for achieving desired levels of disinfection.
An example may best illustrate how these design curves are applied. Assume
that a particular lagoon effluent is characterized as having a fecal coliform
(FC) concentration of 10,000 per 100 ml, 0 mg/i sulfide, 20 mg/i TCOD, and a
temperature of 50C. If it is necessary to reduce the FC Counts to 100 per 100
ml, a combined chlorine residual (CCL) sufficient to produce a 99 percent
bacterial reduction must be obtained. If an existing chlorine contact chamber
has an average residence time of 30 mm, the required chlorine residual is
obtained from Figure 4.6. A 99 percent bacterial reduction corresponds to
log (N 0 /N) equal to 2.0. For a contact period of 30 mm, a combined chlorine
residual of between 1.0 and 1.5 mg/i is required to produce that level of FC
reduction. Upon interpolation, the actual chlorine residual is determined to
be 1.3 mg/i. This is indicated by Point CD in Figure 4.6.
Going to Figure 4.7, it is determined that if a chlorine dose produces
a residual of 1.30 mg/i at 5°C, the same dose would produce a residual of
0.95 mg/i at 20°C. This is because of the faster rate of reaction between
total chemical oxygen demand (TCOD) and chlorine at the higher temperature.
This is indicated by Point in Figure 4.7. For an equivalent chlorine
residual of 0.95 mg/I at 20°C and 20 mg/i TCOD, it is determined from
Figure 4.8 that the same chlorine dose would produce a residual of 0.80 mg/i
if the TCOD were 60 mg/i. This is because higher concentrations of TCOD

P o FECAL 10 4 /ICO ml
o TOTAL 10 4 /ICO ml
Comb.n,d CP lor,n. Re%.ciuol 1.5
1.5 mg/I
0.5 mg/I
TIME (Minutes)
Figure 4.6. Combined chlorine residual at 5°C for coliform = ml.

E 2
U ’ 20
Figure 4.7. Conversion of combined chlorine residual at Temp 1 to equivalent residual at 20°C.

Ui 2.
3.5 .0
Figure 4.8. Conversion of combined chlorine residual at TCOD1 and 20°C to equivalent residual at
20°C and TCOD = 60 mg/i.

increase the rate of chlorine demand. Point © on Figure 4.8 corresponds
to this residual. The chlorine dose required to produce an equivalent re-
sidual of O. 0 mg/i to 200C and 60 mg/i TCOD is determined from Figure 4.9.
For a chlorine contact period of 30 mm, a chlorine dose of 2.15 mg/I is
necessary to produce the desired combined residual as indicated by Point
on Figure 4.9. This dose will produce a reduction of FC from 10,000 per 100
ml to 100 per 100 ml within 30 mm at 5°C and with 20 mg/I TCOD.
If, in the previous example, the initial sulfide concentration was 1.0
mg/i instead of 0 mg/I, it would be necessary to go directly from Figure 4.6
to Figure 4.iO. Here, chlorine residual of 1.30 mg/I at the TCOD of 20 mg/I
and a temperature of SOC is converted to an equivalent chlorine residual
of 1.10 mg/I for a TCOD of 60 mg/i. This is represented by Point ® on
Figure 4.10. Going to Figure 4.11, which corresponds to an initial sulfide
conc ntrat ion of 1.0 mg/i, it is determined that a chlorine dose of 6.65 mg/i
is necessary to produce an equivalent chlorine residual of 1.1 mg/i after a
contact period of 30 mm. Point ® on Figure 4.11 corresponds to this dose.
The sulfide remaining after chlorination is determined to be 0.44 mg/I from
Figure 4.12 as indicated by Point ( DESIGN OF CHLORINE CONTACT TANKS
Although the degree of bacterial kill is proportional to the concen-
tration of chlorine dose times the contact time, disinfection of wastewater
does not necessarily follow Chick’s Law (Collins et al., 1971). Chick’s
Law states that,
= —kt
N 0

J 3.0 -
C l )
l ii
IL l
o 20—
. —Ch orine Dose 2.0 mg/I
— __ _ Chlor I Dose l D mg / I
_______________________________________ — I
0 I 30 40 50 60
TiME (Minutes)
Figure ! • 9. DeLermination of chlorine dose required for equivalent combined
residuals at TCOD = 60 mg/i and Temp. 20°C.
Chlorine Dose 10.0 mg
Dose 7.0 mgI I

S i.
L i
1,5 2.0 3.0 3.5 4.0
Figure 4.10. Conversion of combined residual chlorine at 5°C and TCOD1 to equivalent residual at 5°C
and TCOD = 60 mg/i.

:: 3.0
IL l
z 2.0—
ChIor ne Dosei.0 mg/I
oLO hboeDose6.0/ 1
r ne Dose = 5.0 mg/I
y—Chlorine Dose 3.0 mg/I
tO 20 30 40 50
TIME (Minutes)
Figure 4.11. Determination of chlorine dose required when S = 1.0 mg/l,
TCOD = 60 mg/i, and Temp. = 5 C.
brine Dose i0 mg/I

4 6 8 tO 12 14 16
Figure 4.12. Sulfide reduction as a function of chlorine dose.
18 20
ZZ 1.6
D l
U i
C) 1.2
U -
(I )
0 2

where N is the number of organisms remaining at time t, N is the initial
number of organisms, and k is a constant. The deviation from Chick’s Law
can largely be attributed to the fact that the disinfectant forms of chlorine
in wastewater are mostly chioramines, rather than free chlorine. Chioramines
not only decrease the disinfectant properties of the chlorine residual,
but may also result in differences in susceptability of organisms exposed
to chloramines. Differences in degree of exposure and increases in resistance
triggered by the exposure of organisms to the disinfectant also affect the
way in which chlorine acts to destroy microorganisms. As a result of devia-
tions from Chick’s L iw, either the time of exposure or the chlorine dose must
be increased to produce the same bacterial kill in wastewater as in water.
Problems associated with the design of contact tanks stein from the fact
that most designs are based on a theoretical detention time determined by
dividing the tank volume by the flowrate. In practice, actual detention
times may vary between 30 and 80 percent of the theoretical detention times
(Deaner). Shorter residence times are caused by dead spaces and short—
circuiting and result in decreases in chlorination efficiencies and in-
creases in solids accumulations (Kothandaram and Evans, 1974). With shorter
contact times and extra chlorine demands exerted by the build—up of solids,
chlorine concentrations must be increased to produce desired degrees of dis-
infection. Increasing the chlorine dose often has serious drawbacks. As
well as being an inefficient way to utilize the disinfectant properties
of chlorine, it also increases operational costs. This approach also
increases the concentration of undesirable compounds discharged into the
environment (Hart et al., 1975). Increasing the chlorine dose is also hard

on equipment, because of corrosion resulting from the contact of equipment
with high chlorine concentrations.
Short—circuiting has another effect on adequate operations of chlorine
contact tanks. With short—circuiting, residence times may be continually
changing. This causes difficulty in maintaining prescribed levels of chlorine
residuals. The frequent attention of an operator is required to alter
chlorine doses in maintaining constant chlorine residuals (Stephenson and
Lauderbaugh, 1971).
To provide adequate disinfection of wastewater, the basic approach to
good contact tank design should include a thorough investigation of hydraulic
characteristics of various designs and then the selection of design features
which will optimize hydraulic performance. Some important design considerations
include optimization of mixing, contact time, and chlorine dose.
Evaluation of hydraulic characteristics . The hydraulic characteristics
of a chlorine contact tank are generally determined by conducting tracer
studies on flow patterns through the tank. Several possible tracers are
available. Salt is a comn n tracer and has been used to determine detention
times in contact tanks (Louie and Fohrman, 1968). However, it is often dif-
ficult to handle the large amounts of salt generally required for such studies.
Radioactive tracers are another possibility; however, these are almost never
used because of the hazard and regulations controlling their release.
Perhaps the most useful tracers are fluorescent dyes. Most of these
are inexpensive and easy to obtain. Two of the dyes commonly used in contact
tank tracer studies are Rhodamine WT (Hart et al., 1975) and Rhodamine B
(Deaner; Kothandaram and Evans, 1974). Other fluorescent dyes are also
available and the choice of which dye to use is a matter of personal judgment.
The Rhodamine dyes, however, offer the advantages of being detectable at very

low concentrations and having low sorption tendencies. Also, turbidity has
very little effect on the response of the dye. Fluorescence of the dyes at
concentrations as low as 0.01 ppb can be detected with a fluororneter (Deaner,
In conducting tracer studies, the dye or other tracer should be injected
into the contact tank at the same point the chlorine solution would enter the
tank. If possible, the tracer should also be injected below the water surface
to avoid scattering of the tracer by wind on the surface. The most desirable
method of conducting tracer studies is to obtain a continuous record of the
tracer concentrations at the tank outlet. If fluorescent dyes are used, this
may be done by using a continuous flow fluorometer connected to a recorder.
This type of approach is more reliable than the collection of grab samples.
The flow characteristics of the contact tank may be determined by
evaluating the data obtained from tracer studies in one of several ways.
The methods include conventional, statistical, and dynamic analyses (Sawyer,
1967). Conventional and statistical analyses are the most commonly used.
The dynamic approach is basicaly a mathematical modeling technique and will
not be discussed.
The conventional method of analysis consists of selecting specific
points from the dispersion flow curve as indices to describe the performance
characteristics of a tank. The points and indices commonly used are described
as follows (Hart et al., 1975; Marske and Boyle, 1973).
T = V/Q (theoretical detention time)
= time for tracer to initially appear at the tank outlet
time for tracer at outlet to reach peak concentration
tj 0 , t50, t90 = time for 10, 50, and 90% of the tracer to pass at the
outlet of the tank

tg time to reach the centroid of the effluent curve
t 1 /T index of short—circuiting
t /T index of modal detention time
t 50 /T index of mean detention time
tg/T index of average detention time
t 90 /t 10 Morrill Dispersion Index — indication of degree of mixing
In constructing dispersion flow curves, it is common practice to use
dimensionless expressions for tracer concentrations and times. Triis is done
to facilitate comparisons of hydraulic performance between tanks where dif-
ferent tracei concentrations and detention times are involved. The dimension-
less dispersion flow curve is obtained by plotting C/C 0 against t/T where
C is the tracer concentration at any time t, C 0 is the initial tracer concen-
tration and T is the theoretical detention time (Q/V). A typical dispersion
flow plot is presented in Figure 4.13.
The parameter which is probably the most useful in accurately describing
hydraulic performance is the Horrill Index (MI) (Marske and Boyle, 1973). As
the MI approaches 1.0, the flow through the tank approaches ideal plug flow.
The larger the MI, the more closely the flow in the tank approaches backmix
(complete mixed) reaction conditions. The two extreme flow conditions are
displayed in Figure 4.14.
There are several different statistical approaches used to evaluate
hydraulic performance. One approach, which has gained widespread acceptance,
describes the flow regime of a basin in terms of plug flow and perfect mixing
(Marske and Boyle, 1973; Wolf and Resnick, 1963). It also uses descriptive
parameters to define effective space and dead space. A variation of this
approach uses the entire tracer curve to describe hydraulic efficiency in

Figure 4.13.
t IT
Typical dispersion flow curve.
t/ T
C / C 0
Figure 4.14. Comparison of plug and backniix flow.

terms of a function of time, F(t) (Rebhun and Argaman, 1965). This function
is calculated from the following equation:
Log [ 1 — F(t)1 = (— Log ef(1 — p)(I — m)] [ t/T — p(1 — in)1
m = dead space fraction
1—rn = effective fraction
p plug flow fraction
i—p perfect mixing fraction
t = any time corresponding to time used to get F(t)
T = theoretical detention time
Probably the most widely used statistical approach is the chemical
engineering dispersion index. It is considered to be reliable, since it is
calculated using the entire dispersion flow curve. The dispersion index, (5,
is calculated from the following equations (Marske and Boyle, 1973).
2 ( 5t 2
o = a =
2 td 2
2 lEt \ (Etc
at = I I — I—
EcJ Ec
In these equations, c is the tracer concentration at any time t, = 2 is
equal to the variance of the flow—through curve.
The dispersion index has the strongest statistical probability of cor-
rectly describing the hydraulic performance because it includes all points on
the dispersion flow curve. Conventional parameters only use one point, or at
the best, only a portion of the curve. In comparing the dispersion index with
conventional parameters, it has been found that the Morrill Index is closely
correlated with the dispersion index and can be considered as the most reliable
conventional parameter in accurately describing the hydraulic performance of

a tank. The least reliable indicators of flow characteristics are considered
to be the percent of effective space, t 5 ofT, and tj/T (Marske and Boyle,
Elements of contact tank design . The primary objective of good chlorine
contact tank design is to design for hydraulic performance which will allow
for a minimum usage of chlorine with a maximum exposure of microorganisms to
the chlorine. An evaluation of a number of wastewater chlorine contact tanks
indicates that mixing, detention time, and chlorine dosage are the critical
factors to consider in providing adequate disinfection. Good design not only
optimizes disinfection efficiency, but should also minimize the concentration
of undesirable compounds being discharged to the environment and reduce the
accumulation of solids in the tank by keeping the flow—through velocity high
enough to prevent solids from settling (Hart et al., 1975).
Initial mixing of the chlorine solution with wastewater is necessary for
providing uniform contact of chlorine with microorganisms and for preventing
chlorine stratification in the contact tank. This is especially important
•because most of the disinfection takes place within the first few minutes of
contact. It is also important to note that most of the chlorine demand is
exerted during this same period. Since the formation of chloramines in waste—
water is extremely rapid, it must be remembered that free chlorine is much
more effective as a disinfectant than chloramines. Chloramines are ineffective
in killing viruses in comparison with free chlorine. Although the reaction
rates involved in the formation of chioramines are more rapid than the rate
at which free chlorine reacts with microorganisms, it is important to provide
as much exposure as possible of free chlorine to the microorganisms for ef-
ficient disinfection. Rapid mixing provides this exposure if there is any

free chlorine remaining in solution by the time the chlorine solution is
mixed with the wastewater.
The rapid formation of chloramines indicates that there is a possible
problem in the manner in which chlorine is put into solution. The common
practice is to use a portion of the wastewater stream for the solution water.
When this is done, most of the chlorine is in the form of chioramines before
the solution line is ever mixed with the mainstream of wastewater. However,
studies indicate that this practice does not appreciably affect the efficiency
of the wastevater chlorination process (Collins et al., 1971).
Mixing can be accomplished by applying the chlorine solution to the waste—
water either in a pressure conduit under turbulent conditions or with a
mechanical mixer. A turbulent reactor is generally considered to be the most
effective in producing maximum bacterial kill in the shortest contact time.
It has been found that a contact time of 0.1 to 0.3 minutes is generally suf—
ficient in a turbulent reactor. Slightly longer might be required when a
mechanical mixer is used (Collins et al., 1971). If a mechanical mixer is
used, the chlorine solution should be added to the wastewater immediately up-
stream from the mixer. Another form of mixing, which has been found to be
effective, is the use of a hydraulic jump in combination with over and under
baffles (Louis and Fohrman, 1968). Both the turbulent reactor and the baffle
system of mixing offer the advantage of reducing operation and maintenance
costs over those for the mechanical mixer.
Rapid mixing is followed by flow of the chlorinated wastewater into the
contact tank. Most approaches to good contact tank design are based on the
idea that plug flow is the most desirable hydraulic performance characteristic
to achieve in producing efficient disinfection. Plug flow decreases short—
circuiting, dead spaces, spiraling, and eddy currents and also closes the gap

between theoretical and actual detention times. However, not all designs
are based on the objective of achieving plug flow. At least one design
suggests the use of a series of backmix reactors to improve chlorination
efficiency (Kokoropoulos, 1973). In this approach, the tank shapes are not
important as long as stratification and short—circuiting are eliminated.
One advantage to series reactors is the ease with which treatment capacity
could be increased by simply adding another reactor. However, high initial
and operational costs could offset this advantage.
For the design of tanks in which plug flow is the objective, tank shape
is an import nt cons*deration. Ideally, plug flow conditions could best be
achieved by using a long, narrow, straight contact chamber. A pipe, for
example, would be a good contact chamber. However, because of cost and space
limitations, this approach is generally not practicable. Circular contact
tanks have been used, but generally they do not perform efficiently with
respect to hydraulic characteristics (Warwick, 1968). Most tanks are based
on a rectangular shape, which is the most practial design.
Conventional design practices can be enhanced by paying particular
attention to inflow and outflow structures. They should be designed in such
a fashion as to distribute wasteflow uniformly across the tank cross —section.
One of the riost effect ive designs is that of a sharp—crested weir covering
the width of the contact tank at the inlet and outlet (Marske and Boyle, 1973).
This design minimizes the weir overflow rate and greatly enhances hydraulic
characteristics through the tank.
A common practice for improving plug flow conditions in a contact tank
involves the use of baffles. Longitudinal baffles are generally more effec-
tive than cross baffles. In a study of seven different types of chlorine
contact tank configurations, it was found that the longitudinally baffled

serpentine flow and flow resulting in an annular ring around a secondary
clarifier were the best configurations for approaching ideal plug flow. Both
have the effect of increasing the ratio of the length to the width (L/W) of
the contact tank. The L/W ratio is often considered to be the most important
design consideration for chlorine contact tanks. It has been recommended
that a minimum L/W ratio of 40:1 be used in order to obtain maximum plug flow
performance (Marske and oy1e, 1973); however, Johnson et al. (1978) obtained
excellent hydraulic characteristics using a L/W ratio of 25:1. Baffles have
also been used effectively across the width of contact tanks. Hydraulic
performance bas been improved by placing baffles near the inlet end of tanks
to suppress the kinetic energy of incoming jets.
Often, baffles by themselves are not sufficient to produce desired
hydraulic characteristics. Hammerhead shapes at baffle tips have been demon-
strated to reduce short—circuiting and flow separation. Corner fillers have
been used to eliminate dead spaces and to decrease the build—up of solids in
corners. These fillers, however, seem to have little effect on flow charac-
teristics. In some cases, directional vanes around the ends of baffles have
been found to produce lower head losses and to produce more uniform flow
through the contact tank (Louie and Fohrman, 1968).
Another approach to improving the effectiveness of chlorine contact
tanks has involved aeration. It has been found that mild agitation with
compressed air improves hydraulic characteristics and may improve bacterial
kill by providing closer contact of microorganisms with residual chlorine
(Kothandaraman and Evans, 1974). This method also reduces solids accumula-
tion and thus decreases the chlorine demand caused by putrefaction of solids.
Using this approach in a field evaluation, it has been found that adequate
bacterial kill can be obtained in secondary sewage with a dose of 2 to 3

mg/i chlorine and a contact time of only 15 minutes. Fifteen minutes should
be considered as the minimum actual hydraulic residence time for chlorine
contact tanks. If the accumulation of solids is not adequately prevented by
aeration, it is recommended that they be removed at least once a day by some
mechanical or other means in order to keep chlorine demand as low as possible.
One final design consideration is that of depth. In shallow contact
tanks, it is possible for wind to cause short—citcuiting. However, this is
generally not a problem in tanks designed with standard design depths.
For existing chlorine contact tanks, it is generally not possible to
completely redesign the tank. However, improvements can be made in flow
characteristics with practical alterations. Gates added to screen arid sludge
notches have been found to reduce short—circuiting. Spiraling flow patterns
have been eliminated by circular baffle plates placed at tank inlets. Addi-
tional improvements can be made by using directional vanes to direct flow in
a more uniform fashion and by using stop baffles with curved vanes to reduce
eddying. In one example, the improvements reduced short—circuiting by 80
percent in an existing contact tank (Hart et al., 1975). Another way to
improve hydraulic performance in existing tanks is to use pre—cast baffles.
These can be installed with minimum down time. Although it is more efficient
to use longitudinal baffles, it may be more economical to use cross baffles.
It has been demonstrated that baffles installed in a maze configuration
improved performance sufficiently to make economical factors more important
in choosing a design than efficiency considerations (Stephenson and Lauderbaugh,
In conclusion, the most important design considerations for efficient
disinfection appear to be rapid and complete initial mixing, an adequate L/W
ratio to produce near plug flow conditions, and a sufficienty long residence

time to produce an optimal amount of disinfection for the chlorine dose
applied with a minimum amount of chlorine residual remaining in the effluent.
Design considerations are summarized in Table 4.1. LITERATURE CITED
Brinkhead, C. E., and W. J. O’Brien. 1973. Lagoons and Oxidation Ponds.
Journal Water pollution Control Federation 45(1O):1054—l059.
Collins, Harvey F., Robert E. Selleck, and George C. White. 1971. Problems
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Deaner, Davia C. Undated. A Procedure for Conducting Dye Tracer Studies
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Table 4.1. Summary of chlorination design criteria.
I. Rapid initial mixing should be accomplished within 5 seconds and
before liquid enters contact tank. Design hydraulic residence
time 30 seconds for tanks using mechanical mixers.
II. Methods available
1. Hydraulic jump in open channels.
2. Mechanical mixers located immediately below point of
chlorine application.
3. Turbulent flow in a restricted reactor.
4. Pipe flowing full. Least efficient and should not be used
in pipes with diameter > 30 inches.
Contact Chamber
I. Hydraulic residence time
1. 60 minutes at average flow rate.
2. 30 minutes at peak hourly flow rate.
II. Hydraulic performance
1. Modal value obtained in dye studies 0.6, t /T 0.6
(Figure 4.13).
2. Efficiency of disinfection increases as t /T increases.
3. Design for maximum economical t /T.
III . Length to width ratio
1. L/W 25:1.
2. Cross—baffles used to eliminate short circuiting caused by
IV. Solids removal
1. Baffles arranged to remove floating solids.
2. Provide drain to remove solids and liquid for maintenance.
3. Provide duplicate contact chambers.
4. Width between channels should be adequate for easy access
to clean and maintain chamber.
V. Storage
1. Provide a minimum of one filled chlorine cylinder for each
one in service.
2. Maintain storage area at a temperature 55°F.
3. Never locate cylinders in direct sunlight or apply direct
heat. -
4. Limit maximum withdrawal rate from 100 and 150 pound
cylinders to 40 pounds per day.
5. Limit maximum withdrawal rate from 2,000 pound cylinders
to 400 pounds per day.
6. Provide scales to weight cylinders.
7. Provide cylinder handling equipment.
8. Install automatic switch—over system.

Table 4.1. Continued.
VI. Piping and va lves
1. Use Chlorine Institute approved piping and valves.
2. Supply piping between cylinder and chlorinator should be
Sc. 80 black seamless steel pipe with 2000 pound forged
steel fitting. Unions should be ammonia type with lead
3. Chlorine solution lines should be Sc. 80 PVC, rubber—
lined st eel, saran—lined steel, or fiber cast pipe approved
fr moist chlorine use. Valves should be PVC, rubber—lined,
or PVC lined.
4. Injector line between chlorinator and injector should be
Sc. 80 PVC or fiber cast approved for moist chlorine use.
VII. Chiorinators
1.. Should be sized to provide dosage 10 mg/i.
2. Maxirnum feed rate should be determined from knowledge of
locEl conditions.
3. Direct feed gas chiorinators should be used only in small
installations. Check state regulations. Prohibited in
certain states.
4. Vacuum feed gas chlorinators are most widely used and are
much safer.
5. Hypochiorite solutions should be considered in small
installations where safety is major concern.
VIII. Safety equipment and training
1. Install an exhaust fan near floor level with switch
actuated when door is opened.
2. Exhaust fan should be capable of one air exchange per
3. Gas mask located outside chlorination room.
4. Emergency chlorine container repair kits.
5. Chlorine leak detector.
6. Alarms should be installed to alert operator when
deficiencies or hazardous conditions exist.
7. Operator should receive detailed hands—on training with
all emergency equipment.
IX. Diffusers
1. Minimum velocity through diffuser holes 10—12 ft per sec.
2. Diffusers should be installed f or convenient removal,
cleaning and replacement.

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