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DISINFECTION CRITERIA
AND
DESIGN GUIDELINES
December 15, 1970
FWQA, Northwest Region


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UNITED STATES
ENVIRONMENTAL PROTECTION AGENCY
FEDi; HAL WAT K [t QUALITY ADMINISTRATION
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TABLE OF CONTENTS
I. SUMMARY
A.	Contact Tank Design
B.	Effluent Criteria
C.	Records
II. INTRODUCTION
III. ADEQUATE DISINFECTION
A.	Total Coliform Criteria
B.	Fecal Coliform Criteria
C.	Primary Effluent Total Coliform
D.	Sampling Frequency-Coliform
E.	Stricter Standards
F.	Oxidation Ponds for Disinfection
G.	Conditions for Adequate Disinfection
IV. DESIGN GUIDELINES
A.
CunLact Time
B.
Mixing and Short Circuiting
C.
Contact Tank Skimming and Cleaning
D.
Sampling and Outfalls
E.
Equipment and Spare Parts
F.
Dechlorination
G.
Oxidation Ponds for Adequate Disinfection
H.
Other Disinfectants
V. OPERATION
A.	Total Available Chlorine Residual
B.	Variations Allowable in Residual
C.	Measurement of Chlorine Residual
D.	Coliform Bacteria Killed vs. Residual
E.	Frequency of Chlorine Residual Measurement
F.	Records and Reporting
G.	Continuous Disinfection
H.	Other Tests Relative to Disinfection
I.	Coliform Testing in the Field

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I. SUMMARY
A.	The contact tank design shall assure:
1.	Thorough mixing of chlorine with the inlet flow of waste
water, surface skimming, sampling access, minimal short
circuiting, and continuous year-around disinfection.
2.	A method of dewatering and settled solids removal.
3.	Minimum contact times of,
(a)	60 minutes at design flow
(b)	20 minutes at peak hourly flow.
B.	Effluent from contact tanks shall meet the following criteria:
1.	Total coliform average less than 1,000/100 ml. A fecal
coliform average less than 200 is recommended for
effluent discharged to recreationally-classified waters.
2.	Total chlorine residual after 60-minute contact time,
not less than 1 mg/1 when conclusive coliform data is
not available.
C.	The following disinfection records and Lest data shall be
maintained:
1.	Total daily waste flow.
2.	Total chlorine residual - daily or daily average.
3.	Pounds of chlorine used per day.
4.	Total or fecal coliform concentration after disinfection
for all plants with an average flow of 500,000 gpd or
more, twice a week. Smaller plants should obtain total
or fecal coliform data.
II. INTRODUCTION
All liquid effluent from waste treatment facilities must be
adequately disinfected to minimize the health hazards from water-
borne diseases and to preserve and enhance the quality of the
receiving waters. Design and operation criteria to ensure
adequate disinfection are set forth in this paper.

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FWQA proposes a national disinfection policy which will limit
grants to projects which have, . . disinfection or other
methods to produce substantially complete reduction of micro-
organisms ."
FWQA, Northwest Region, has summarized the best available design
criteria and operational guidelines available in the Region and
from FWQA Headquarters. The States of Oregon, Washington, Idaho,
Montana, and Alaska have contributed comments and disinfection
criteria. The Northwest Region proposes that the design criteria
which follows be used to obtain adequate disinfection. The
operational guidelines should assist the States in their monitoring
responsibility and ensure adequate disinfection prior to discharge
to surface waters.
Henceforth, these criteria will be used as a guide by the North-
west Region of the FWQA in the review of plans for construction
and operation of new waste treatment facilities and for the
approval and award of construction grants to municipalities.
Some flexibility or interpretation will be allowed whenever
adequate disinfection or the specified coliform levels can be
maintained as given in this paper.
111 ADEQUATE DISINFECTION
A.	Adequately treated effluents shall not exceed an average
MPN or MF count of total coliform organisms of 1,000/100 ml
with no more than one sample per week exceeding 2,400/100 ml.
B.	The fecal coliform level is more important from a public
health standpoint than total coliform. All recreationally-
classified waters should not receive effluents with a high
fecal coliform level. Fecal coliform in adequately treated
effluent should not exceed an average count of 200/100 ml.
If a State has a fecal coliform standard of less than 200,
then it will apply.
C.	Adequate disinfection of primary treatment plant effluent is
usually not attainable. However, on an interim basis, pending
the addition of secondary treatment, FWQA will accept a total
coliform average of 2,400/100 ml or less with no more than one
sample per week exceeding 10,000/100 ml as adequate disinfection
provided the receiving water uses will not be endangered.
D.	No more than one week's data using no more than the seven
most recent samples shall be used to determine the average
count of total or fecal coliform organisms.

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E.	More strict effluent standards may be necessary for receiving
waters where existing or potential uses depend upon maintenance
of unusually high quality water.
F.	Oxidation ponds (lagoons) can be designed and operated to
produce adequate disinfection through long-term retention of
wastes and/or series operation of multiple cells. The coli-
form levels specified in A or B above must be maintained on
a year around basis. Bacteriological studies are required to
determine if adequate disinfection is obtained continuously.
Diurnal and seasonal variations in coliform concentration in
the effluent must be determined. At least one effluent sample
per week for the coliform test should be taken-when the coli-
form concentration is at a peak, approximately three hours
after sunrise. See additional criteria under IV DESIGN.
G.	When conclusive coliform testing is not available, the following
minimum evidence of adequate disinfection will apply:
1.	Effluent suspended solids less than 30 mg/1.
2.	Adequate contact time - see IV DESIGN.
3.	A total chlorine residual after 60-minute contact time of
1 mg/1.
IV. DESIGN
A.	Chlorine contact tanks or basins shall be sized to provide
60-minute contact time at design flow, or 20 minutes contact
time at peak hourly flow or maximum rate of pumping, whichever
is greater.
B.	The. contact tanks shall be constructed to minimize short
circuiting. Thorough mixing of chlorine and waste flow is
necessary to achieve maximum disinfection efficiency. This
may be accomplished hydraulically or by use of mechanical
rapid-mix facilities ahead of the contact tanks or basins.
C.	Provision shall be made for removal of floating and settleable
solids from chlorine contact tanks. Provision shall be made
for removal of settled solids without discharging unchlorinated
effluent. To accomplish continuous disinfection, the chlorine
contact tank should be designed with duplicate compartments to
permit draining and cleaning of individual compartments. A
sump or drain within each compartment with the drainage flowing
to a raw sewage wet well is necessary for dewatering, sludge
removal, and maintenance. Unit drains must not discharge into
the outfall sewer. Baffles should be provided to prevent the
discharge of floating material.

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D.	Access shall be provided at the effluent end of the contact
tank for collecting samples. Outfall pipes are often subject
to infiltration and flooding, resulting in unreliable and
uncertain disinfection. Also the sampling point after the
required contact time is not usually convenient to allow
good process control. Therefore, outfalls and pipes are not
considered adequate contact facilities.
E.	Chlorinators shall have adequate capacity to meet the criteria
described in the following section on operation. Capacity
should be provided for at least 15 to 30 mg/1 chlorine dose at
peak hourly flow. Spare parts as recommended by the manufacturer
shall be on hand and readily available.
In shellfish growing areas, swimming areas, and other recrea-
tional areas, duplicate disinfection facilities will be required.
Also larger plants may be required to have duplicate or standby
disinfection facilities to provide flexibility of operation and
continuous disinfection during equipment maintenance. All plant
discharges and bypasses must be disinfected continuously.
F.	Dechlorination to protect fish may be required when chlorine
residuals over 1.0 mg/1 are necessary and the receiving water
flow is small.
G.	Coliform and virus densities are greatly reduced in a lagoon
system which is properly designed and operated. The three
basic approaches commonly used are:
1.	Four- or five-cell series operation with total holding
during the ice-cover period.
2.	Total holding with a combination of land irrigation,
evaporation, seepage, or ridge and furrow disposal.
3.	Fill and draw utilizing two cells following secondary
treatment which are alternately filled, retained long
enough to produce the required coliform concentration
and then emptied gradually.
Unfortunately, there is no way to predict the effluent coliform
concentration from the four or five cells in series. Therefore,
documentation of effluent coliform densities will be required.
If the specified coliform concentrations cannot be documented,
then chlorination will be required. Experience in Idaho indi-
cates that a total retention time of 130 days or more may be
necessary. To minimize short circuiting it is preferable to
channel the flow through the polishing lagoons using baffles or
long, narrow cells. These methods require considerable land
area and are, therefore, not usually feasible for large flowc
(1 mgd) or in areas where land is costly.

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H. Much of FV/QA's research in disinfection of wastewater deals
with the use of chlorine. Chlorine is not necessarily the
answer to all disinfection problems. However, little informa-
tion is available regarding the use of other disinfectants.
Other disinfectants are, therefore, being investigated to
make available a spectrum of proven disinfection processes.
V. OPERATION
A.	Total available chlorine residual of plant effluents as deter-
mined by accepted test procedures in the latest edition of
"Standard Methods,"!./ should be 1.0 mg/1 for plants meeting
the above design criteria.
B.	Higher residuals may be imposed where plants do not meet the
design criteria, or where a total residual of 1.0 mg/1 after a
60-minute contact time is inadequate for bacteria kill or
protection of high-quality water.
When effluent suspended solids exceed 30 mg/1, a chlorine
residual of 1 mg/1 after one hour contact time will probably
not produce adequate disinfection. Chlorine residuals less
than 1 mg/1 may be satisfactory where bacterial testing confirms
that the required bacterial kill is achieved. Residuals less
than 1.0 mg/1 must be watched closely since a drop in effluent
quality or a sharp increase in flow may quickly eliminate
adequate disinfection.
C.	Measurement of total available chlorine may be accomplished
by the orthotolidine color comparison test or the idometric-
titration method. The modified orthotolodine-arsenlte method
is recommended and requires little apparatus for field and
control testing. Warming the sample to 20C (room temperature)
after the addition of the reagent is essential prior to the
reading which is taken five minutes after adding the reagent.
The idometric-titration method using the amperometric end
point is more accurate, requires greater operator skill, and
is more costly. However, it should be used when possible.
D.	The required chlorine residual for adequate disinfection
will fluctuate over the 24-hour period, depending on changing
waste flow and characteristics. A study to determine the
relationship of bacterial kill to chlorine residual should be
made soon after the new plant is placed into operation. Such
_1/ "Standard Methods for the Examination of Water and Waste Water"
prepared and published jointly by the AWWA, the APHA, and the WPCE.

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a study should be repeated yearly or preferably seasonally
to identify changes in disinfection requirements. A bacterial
analysis should consist of total coliform determination on
at least six samples collected over a 12-hour period which
Includes the peak flow, A single sample should be taken
routinely at least twice a week until the residual is demonstrated
sufficient to produce adequate disinfection. Bacterial testing
should continue twice a week for all plants with the average
flow equal to or exceeding 500,000 gpd. All chlorine residual
determinations should be recorded in the plant log.
E.	Total residual chlorine should be determined hourly until
sufficient data are available to justify reducing the testing
frequency. Individual values rather than averages should be
used in developing the chlorine dose. All chlorine residual
determinations should be recorded in the plant log.
F.	Current records of daily chlorine residuals and failures
shall be available for inspection at the plant. Copies of
these records shall be submitted to the appropriate State
agency(s) at the end of each month and the FWQA on request.
The appropriate State agency shall be notified immediately
by telephone when emergency or other conditions prevent
adequate disinfection of waste effluents.
G.	Seasonal or intermittent chlorination will not be allowed.
All waste treatment plant effluent must be subject to adequate,
continuous disinfection.
H.	Several parameters have an impact on the efficiency of disin-
fection. These include temperature, pH, suspended solids, BOD,
chlorine demand and others. Tests for these parameters are
described in "Standard Methods." Routine analysis of these
parameters should be maintained in the plant laboratory records.
I.	The total coliform test can be performed with a membrane
filter (MF) field kit costing less than $500. The fecal coli-
form test requires an additional investment of at least $200
for the necessary elevated temperature incubation water bath.
The skill required is about the same for both tests.
Training of a few days is essential to obtain consistent
results. Training will be available through the Pacific North-
west Water Laboratory, FWQA, to introduce operators and lab
technicians to coliform testing techniques.

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