EPA 430/9-74-011
SUPPLEMENT TO FEDERAL GUIDELINES: DESIGN, OPERATION,
AND MAINTENANCE OF WASTEWATER TREATMENT FACILITIES
WASTEWATER TREATMENT
MARCH 1974
U.S. ENVIRONMENTAL PROTECTION AGENCY
Office of Water Program Operations
Washington, D.C. 20460
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SUPPLEMENT TO FEDERAL GUIDELINES: DESIGN
OPERATION AND MAINTENANCE OF
WASTEWATER TREATMENT FACILITIES
U.S. ENVIRONMENTAL PROTECTION AGENCY
TECHNICAL BULLETIN
WASTEWATER TREATMENT PONDS
1. PURPOSE:
This Bulletin presents technical information which will be used by
Environmental Protection Agency Regional Administrators to review grant
applications involving wastewater treatment ponds.
2. RELATED PUBLICATIONS:
This Bulletin supplements the Federal Guidelines: Design, Operation,
and Maintenance of Municipal Wastewater Treatment Plants. Additional
process design information is contained in EPA Technology Transfer publi-
cations entitled "Upgrading Lagoons" (1) and "Upgrading Existing Lagoons"
(2), and therefore is not repeated in this Bulletin.
3. TERMINOLOGY:
A wastewater treatment pond is a large, relatively shallow basin
designed for long term detention of wastewater which may or may not
have received prior treatment. While in the basin the wastewater is
biologically treated to reduce biochemical oxygen demand and suspended
solids. There are many different types of lagoons and ponds; however,
the following terminology is used for the wastewater treatment ponds
discussed in this Bulletin
a. Photosynthetic pond - A pond which is designed to rely on
photosynthetic oxygenation (i.e. oxygen from algae) for any portion of
the oxygen needed for waste treatment. This includes oxidation ponds
and facultative lagoons. These ponds may have supplemental aeration
by mechanical means. With regard to hydraulic flow, photosynthetic
ponds are either of the (1) flow-through type, in which the pond
discharges relatively continuously throughout the year; or, (2) con-
trol led-discharge type, in which the pond is designed to retain the
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SUPPLEMENT TO FEDERAL GUIDELINES: DESIGN
OPERATION AND MAINTENANCE OF
WASTEWATER TREATMENT FACILITIES
U.S. ENVIRONMENTAL PROTECTION AGENCY
TECHNICAL BULLETIN
WASTEWATER TREATMENT PONDS
1. PURPOSE:
This Bulletin presents technical information which will be used by
Environmental Protection Agency Regional Administrators to review grant
applications involving wastewater treatment ponds.
2. RELATED PUBLICATIONS:
This Bulletin supplements the Federal Guidelines: Design, Operation,
and Maintenance of Municipal Wastewater Treatment Plants. Additional
process design information is contained in EPA Technology Transfer publi-
cations entitled "Upgrading Lagoons" (1) and "Upgrading Existing Lagoons"
(2), and therefore is not repeated in this Bulletin.
3. TERMINOLOGY:
A wastewater treatment pond is a large, relatively shallow basin
designed for long term detention of wastewater which may or may not
have received prior treatment. While in the basin the wastewater is
biologically treated to reduce biochemical oxygen demand and suspended
solids. There are many different types of lagoons and ponds; however,
the following terminology is used for the wastewater treatment ponds
discussed in this Bulletin
a. Photosynthetic*pond - A pond which is designed to rely on
photosynthetic oxygenation (i.e. oxygen from algae) for any portion of
the oxygen needed for waste treatment. This includes oxidation ponds
and facultative lagoons. These ponds may have supplemental aeration
by mechanical means. With regard to hydraulic flow, photosynthetic
ponds are either of the (1) flow-through type, in which the pond
discharges relatively continuously throughout the year; or, (2) con-
trol led-discharge type, in which the pond is designed to retain the
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wastewater without discharge from six months to one year, followed
by controlled discharge over a short time interval (typically about
one to three weeks).
b. Aerated pond - A pond which is not designed to rely on any
photosynthetic oxygenation to provide oxygen needed for biological
waste treatment. Air is supplied by mechanical means. Aerated
ponds are either (1) complete mix, in which sufficient energy is
imparted to the wastewater to prevent deposition of solids in the
pond, or, (2) partial-mix, in which only sufficient energy is used
to dissolve and mix oxygen in the wastewater. Solid materials
settle in the partial-mix pond and are decomposed anaerobically.
There will be algae in the partial-mix aerated pond, but usually
far fewer than in a photosynthetic pond.
c. Complete retention pond - This type of pond relies on evapora-
tion and percolation exceeding inflow so that there is no discharge of
pollutants. This method is acceptable at some locations with suitable
climatic conditions and where consistent with water rights. Special
attention must be given to protecting ground water and preventing odors.
4. USE OF THE CRITERIA:
Projects involving waste treatment ponds proposed for Federal
financial assistance from EPA will be based on the criteria contained
in this Technical Bulletin. Approval can be given to different designs
if reasonable assurance can be given to the EPA Regional Administrator
that satisfactory performance will be achieved.
There is a wide variation in the types of ponds and the wastewaters
treated by such ponds, as well as the performance of ponds in different
geographical locations. The criteria in this Technical Bulletin are
intended to provide a conservative baseline of engineering practice, and
must be applied with engineering judgement on a case-by-case basis.
The EPA Regional Administrator will review each project to identify
and resolve additional factors important to the design of a specific
project. Responsibility for satisfactory performance, however, remains
with the grant applicant. Additional construction may be necessary if
completed facilities are not in compliance with effluent limitations.
It is the policy of EPA to encourage the use of new technology.
EPA Regional Administrators will continue to give full consideration
to new methods which may not be included in this bulletin.
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5. PERFORMANCE REQUIREMENTS:
The Federal Water Pollution Control Act Amendments of 1972 (the
Act) established the minimum performance requirements for publicly
owned treatment works. In accordance with Section 301(b)(l)(B) of the
Act, publicly owned treatment works must meet at least effluent limita-
tions based on secondary treatment as defined by the EPA Administrator.
EPA has published information on secondary treatment in 40 CFR Part
133(3). The criteria in this Technical Bulletin are intended to
result in wastewater treatment ponds which can achieve effluent limi-
tations based on the secondary treatment information. More stringent
performance requirements may be necessary to meet other requirements
such as water quality standards. In such cases the criteria contained
in this Bulletin will have to be adjusted accordingly.
6. BACKGROUND:
There are more than 4,000 publicly owned ponds in the United States.
Generally these ponds are located in small communities and are designed
for flows less than 1 MGD. Ponds have been used because operation is
simple, operating costs are low, and land is available. The great
majority of the existing ponds are the photosynthetic flow-through type.
There is a wide variation in the design of these systems. Organic
loadings per acre (both in design practice and actual operation) have
increased with time. Comprehensive performance data on these ponds is
generally lacking, particularly for the flow-through, photosynthetic
type. At the typical facility there has been no test program or, at
the most, infrequent grab sampling.
Regarding the ability of flow-through photosynthetic ponds to
meet secondary treatment requirements, the limited data indicates that:
a. The BOD level Js borderline, but probably could be achieved by
conservative design. The BOD level would not be met if the pond continued
to discharge while there is prolonged ice cover over the pond.
b. The suspended solids level is generally not achieved because
of the algae in the effluent.
c. Fecal coliform levels are not achieved without a positive
means of disinfection such as chlorination.
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d. The pH of the effluent varies markedly depending on alkalinity/C02
relationships. The variation is, however, rarely sufficient to require
pH adjustment (4).
Despite these generalizations, it is important to note that there
are reports of flow-through ponds which do achieve secondary treatment
performance. Satisfactory performance appears to be attributable to
either favorable year-round climate as in the Southwestern United States
or conservative design (up to 6 cells).
Controlled discharge ponds have been used in the North, where, if
properly operated, they can meet the BOD level. They are borderline on
the suspended solids, but probably could meet the level with careful
operation. Such ponds may not require positive disinfection to meet
the fecal coliform levels.
Aerated ponds with suspended solids separation and disinfection,
if properly designed, can meet the BOD requirements, but partial-mix
units are borderline on suspended solids. Granular media filtration
may be needed to assure satisfactory year round performance.
7. FLOW-THROUGH PHOTOSYNTHET1C PONDS:
Regional Administrators will make grants for this type of pond
without supplemental treatment only when there is reasonable assurance
that the pond will perform satisfactorily.
The determination could be based on satisfactory performance of a
similar pond in a comparable environment or on pilot plant performance
with conservative scale-up factors.. Data from at least one year's
operation should be sufficient to show satisfactory performance. Data
from shorter periods may not adequately reflect seasonal variations in
performance.
When Regional Administrators make such grants, the Facilities Plan
should include a discussion of actions to be taken if upgrading is
determined to be necessary after the plant is placed in operation.
8. CONTROLLED DISCHARGE PONDS:
The controlled discharge pond is designed to receive and retain
wastewaters for six months to one year. At the end of this long-term
detention, the contents of the pond are discharged during an interval
of one to three weeks. Since experience with this type of pond is
presently limited to Northern States with definite climatic seasons,
it may be necessary to run pilot studies in States with only slight
seasonal climate changes.
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Ponds of this type have operated satisfactorily in Michigan using
the following design criteria:
Overall organic loading: 20-25 pounds BOD5/acre.
Liquid depth: not more than 6 feet for the first
cell. Not more than 8 feet for subsequent cells.
Hydraulic detention: At least 6 months above the
2 foot liquid level (including precipitation), but
not less than the period of ice cover.
Number of cells: At least 3 for reliability, with
piping flexibility for parallel or series operation.
The design of the controlled discharge pond must include an anlysis
showing that receiving stream water quality standards will be maintained
during discharge intervals, and that the receiving watercourses can
accomodate the discharge rate from the pond.
Selecting the optimum day and hour for release of the pond contents
is critical to the success of this method. The operation and maintenance
manual must include instructions on how to correlate pond discharge with
effluent and stream quality. The pond contents and stream must be care-
fully examined, before and during the release of the pond contents. A
Statewide program of controlled releases (keyed to tests of 8005, dis-
solve oxygen, and suspended solids, fecal coliform as well as sunlight,
weather, and streamflow) has been effective.
In the Michigan program, discharge of effluents follows a consistent
pattern for all ponds. The following steps are usually taken:
a. Isolate the cell to be discharged, usually the final one in the
series, by valving-off the inlet line from the preceding cell.
b. Arrange to analyze samples for BOD, suspended solids, volatile
suspended solids, pH, and other parameters which.may be required for a
particular location.
c. Plan work so as to spend full time on control of the discharge
throughout the period.
d. Sample contents of the cell to be discharged for dissolve oxygen,
noting turbidity, color and any unusual conditions.
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e. Note conditions in the stream to receive the effluent.
f. Notify the State regulatory agency of results of these observa-
tions and plans for discharge and obtain approval.
g. If discharge is approved, commence discharge and continue so
long as weather is favorable, dissolved oxygen is near or above satura-
tion values and turbidity is not excessive following the prearranged
discharge flow pattern among the cells. Usually this consists of
drawing down the last two cells in the series (if there are three or
more) to about 18-24 inches after isolation; interrupting the discharge
for a week or more to divert raw waste to a cell which has been drawn
down and resting the initial cell before its discharge. When this
first cell is drawn down to about 24 inches depth, the usual series
flow pattern, without discharge, is resumed. During discharge to the
receiving waters samples are taken at least three times each day near
the discharge pipe for immediate dissolved oxygen analysis. Additional
testing may be required for suspended solids.
9. COMPLETE-MIX AERATED PONDS:
This type of pond can be sesigned to meet secondary treatment
requirements on a similar basis as an activated sludge process, with
or without solids return (5). The criteria in this Bulletin are not
applicable to a complete-mix aerated pond.
10. PARTIAL-MIX AERATED PONDS:
The process design can be based on reactor mixing, flow regime,
biological kinetics, and oxygen transfer rates. As defined in this
Bulletin, the partial-mix aerated pond will not include any allowance
for photosynthetic oxygenation.
At least three cells will be provided with aeration in each cell
(except designated clarifier cells) so that dissolved oxygen is present
throughout the surface layer. It is usually beneficial to recirculate
effluent high in dissolved oxygen to the pond influent. The aeration
should be tapered so that the final portion of the final cell is a
quiescent zone and can function as a clarifier, or a separate clarifier
can be provided.
The pond volume will be sized on the basis of low temperature
reaction rates, with allowance for sedimentation. Aeration equip-
ment will be sized for the warm weather oxygen uptake rate and for
mixing in the pond. Oxygen transfer will include consideration of
pond depth, which, for a new pond, typically is 8 feet or greater.
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In cold climates, surface aerators will be designed to ensure
satisfactory operation during freezing weather, including splash guards,
heated housings, and design to keep floating ice away from the aerator.
See EPA Technical Bulletin 430-99-74-001 (6) for aeration unit
reliability criteria.
Partial-mix ponds may have high suspended solids on an infrequent
basis due to algae. To ensure satisfactory performance, capability
should be provided for algae removal. Because of the relatively low
amounts of algae a granular dual media filter, along with capability
for feeding a polymer filter aid, should be satisfactory.
11. GENERAL REQUIREMENTS:
The following criteria apply to the waste treatment ponds covered
in this Bulletin:
a. Positive Disinfection
In the past, pond designs have relied on natural die-off of pathogens.
Performance data shows that this method is not sufficiently reliable
for a flow-through photosynthetic pond to achieve secondary treatment
fecal coliform levels except with recommended loadings and very well
managed controlled discharge systems. A positive means of disinfection
must be provided except where data from a similar pond in a comparable
environment shows satisfactory performance. In that case the grant
applicant must agree to install positive disinfection if performance
is not achieved following construction.
Chlorination can achieve the required fecal coliform kills; however,
if algae are not removed, excessive chlorination can result in algae
die-off and increased BOD due to algae cell decay. Echelberger, et al. (7)
studied the clorination of algae laden waters and concluded that
apparent algae cell lysing following chlorination to a desirable residual
level significantly increases the soluble organic concentration in the
water. They also concluded that if chlorine is used as the disinfectant,
serious consideration should be given to effective algae removal prior
to disinfection. Horn (8) presents a laboratory method to optimize the
the chlorine residual and reaction time when chlorinating algae laden
waters. These considerations would be important where the effluent
BOD is close to the permitted value and BOD increase due to algae die-
off would result in a permit violation.
The chlorine should be applied to the pond effluent at a concentra-
tion and contact time sufficient to achieve effluent limitations. The
optimum chlorine residual will be determined when the system is opera-
tional. A contact time of 20 minutes at peak hourly flow is recommended.
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b. Prevention of Short Circuiting
Multi-cell ponds, operated in series, perform substantially better
than single-cell or two-cell ponds. Additional cells reduce short
circuiting of untreated wastewaters through the pond. No less than
three cells will be provided with the initial cell sized to avoid
anaerobic conditions (see the information beginning on Page 54 of
Reference 4).
The Missouri Basin Engineering Health Council (4), makes the fol-
lowing recommendations for photosynthetic ponds (there are, however,
no performance reports on ponds using this system):
"The first pond should be designed with a 4 ft. normal depth
to give maximum surface area for photosynthesis. The inlet
should be designed to give a circular, deeper, sludge storage
zone below the bottom of the normal pond. This will allow
maximum wind mixing to occur without stirring up the settled
solids. The sludge storage section should have a maximum
diameter of 100-200 ft. with a center depth of 4 to 6 ft.
The raw waste inlet pipe should be located in the center
of the sludge storage section so that the raw wastes enter
the pond in a radial fashion to distribute the load around
the inlet pipe in the same fashion that inlet structures are
designed for circular clarifiers except that all of the
baffles in the oxidation pond should be submerged. This will
permit the heavy solids to remain around the inlet and under-
go anaerobic decomposition with a minimum oxygen demand. The
outlet from the first cell should have the capacity to change
the depth from 3 to 5 ft in 6 inch increments to give opera-
tional flexibility as well as a drain for the entire pond.
The outlet structure should be designed to minimize fluid
velocities at a single point. In small plants a large pipe
outlet with adjustable sections is adequate. In large plants
an adjustable weir will be required. There should be three
sets of baffles concentrically around the effluent structure.
The first baffle should be designed to extend around the out-
let structure 3-5 ft. with the baffle extending at least 6
inches to onefoot above the highest water level and down to
within one foot of the bottom of the pond. Thus, the effluent
will be drawn from the bottom of the pond. The second
concentric baffle rises from the bottom of the pond to
within 6 inches of the surface at the lowest possible level.
The third concentric baffle is the same as the first,
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rising above the maximum surface and dropping to within one
foot of the bottom of the pond. These baffles are designed
to give an up and over type baffle with a bottom drawoff to
minfmtze removal of algae from the active zone and to allow
the algae to congregate at the surface within a quiescent
ring that ts not affected by wind action. In effect, a
stilling basin is created which encourages the algae to ac-
cumulate at the light surface and minimizes mixing to inter-
fere with sedimentation."
c. Protection of Ground Water From Pond Seepage
Ponds containing wastewater, if allowed to drain freely to aquifers
or bedrock crevices, could cause significant ground water pollution.
To prevent ground water degradation, ponds must be designed to minimize
seepage losses and will either: (1) have sufficient distance through
low permeability soil to ground water to ensure protection of the aquifers,
or (2) have all submerged surfaces of the pond sealed BO as to ensure
protection of the ground water.
In borderlines cases the Regional Administrator may require percola-
tion tests or observation wells and a monitoring plan.
12. SUPPLEMENTAL TREATMENT FOR FLOW-THROUGH PHOTOSYNTHET1C PONDS:
Methods of providing supplemental treatment for flow-through ponds
are being researched. Methods included in this Bulletin are those which
are reported to have been successful at pilot or plant scale. EPA is
aware that other concepts have been proposed and some of these are being
tested. The Bulletin will be revised from time to time as information
on other successful methods becomes available.
Most techniques for upgrading flow-through ponds involve algae
removal. Two comprehensive discussions of algae removal techniques
have been prepared (9, 10). In this Bulletin, as in the EPA research
program, priority has been given-to those methods which retain the
operational simplicity features of flow-through ponds.
Supplemental treatment must be designed for the conditions at a
specific site. Pilot testing may be required, particularly if there
are significant quantities of industrial waste and depending on the
size of the facility.
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13. SUPPLEMENTAL TREATMENT METHODS:
a. Conversion to Controlled Discharge.
An existing flow-through pond can be converted to a controlled
discharge pond if the previously outlined conditions are met. Usually
additional land area will be required to obtain the volume required
for controlled detention.
b. Intermittent Sand Filtration.
Intermittant sand filters were used in the past for flows up to
about 0.25 M6D, but the high cost of labor to clean the filter sand
reduced this useage. Application of pond effluents to intermittant
sand filters has been successful on a pilot scale. Information to date
is limited (11) and designs should be conservative. The upper limit
of hydraulic loading for pond effluents should be 0.4 MGD/acre until
more information is obtained. Design Information is contained in
Chapter 12 of Reference 12. When freezing could occur on the filter
surface, the pond should be sized to retain the wastewater during
freezing weather conditions or there should be an alternative operational
plan to ensure effluent limitations are met
In their laboratory and prototype field studies of intermittant
sand filtration of pond effluents, Marshall and Middlebrooks (n) found:
(1) Viable algae cells passed the entire depth
of all the filter sands studied.
(2) Hydraulic loading rate did not affect the
algae or suspended solids removal efficiency
at the 0.1,0.2, or 0.3 MGD/acre employed in
the laboratory study. The effects of hydraulic
loading rate on suspended solids removals in
the field studies were inconclusive because
of the large quantities of fines washed from
the filters, but volatile suspended solids
removal did indicate a reduction in removal
efficiency as the hydraulic loading rate was
increased.
C3) Smaller effective size sands produced better algae
or suspended and volatile suspended solids removals.
Sand size was not a significant factor in algae
removal at applied algae concentrations of 15 and
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30 mg/1, but was significant when the con-
centration was increased to 45-50 mg/1 in
both the laboratory and field filters. At
the 0.5 MGD/acre hydraulic loading rate,
monthly mean volatile suspended solids were
essentially equal for the 0.17 and 0.72 mm
effective size sands. Efficiences fluctuated
considerably from one sand to the other during
the, study^ period. But in general the 0.17mm
effective size sand produced a Setter quality
effluent.
c. Land Treatment of Pond Effluents.
This method of using pond effluents as a water resource has particu-
lar application in water short areas where land is readily available.
Application rates vary widely depending on method of application, crops
involved, and climate. Seasonal application is usually related to crop
growth and additional pond capacity may be required for storage during
the dormant season. Comprehensive information on land treatment systems
is available (13, 14), including many examples where the wastewater has
been stored in a pond before land application. Additional design in-
formation will be contained in EPA Evaluation Procedures for Land
Application Systems (now in preparation). Technical assistance on
complex projects is available through EPA Regional Offices, the Office
of Water Program Operations, and the Robert S. Kerr Water Research
Center, Ada, Oklahoma.
d. Addition of Supplemental Aeration.
A flow-through photosynthetic pond can be upgraded by the installa-
tion of diffused or mechanical aerators. For optimum efficiency in
oxygen transfer and mixing the pond should be deepened about 5 feet
(to about 10 feet liquid depth). Also, additional electrical power
will be required to operate the aeration system.
e. Chemical Coagulation.
Coagulation followed by sedimentation, and possibly filtration
has been used extensively for the removal of suspended and colloidal
material from water. In the case of the chemical treatment of waste-
water treatment pond effluents the data are not comprehensive (10).
Lime, alum, and ferric salts are the most commonly used coagulating
agents. Because of the many variables a pilot testing program will
usually be necessary to ensure proper operation of the system. There
must be a satisfactory method of ultimate disposition of resultant
sludges.
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Unless designed for constant flow, close control of the process
is required to obtain satisfactory performance. Depending on the
alkalinity of the wastewater, the operating cost of the chemicals
for this method can be relatively high. Additional information is
contained in References 1, 2, 9, and 10.
14. ADDITIONAL FIELD EXPERIENCE:
The information contained in this Bulletin will be modified
as additional field experience becomes available. Those having such
information are encouraged to submit it to the Director, Municipal
Construction Division (AW-447), Office of Water Program Operations,
U.S. Environmental Protection Agency, Washington, D.C. 20460.
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Bibliography
1. Upgrading Lagoons, by D. H. Caldwell, D. S. Parker, and W. R. Uhte.
Prepared for the EPA Technology Transfer Program. August 1973.
2. Upgrading Existing Lagoons, by R. F. Lewis and J. M. Smith. Prepared
for the EPA Technology Transfer Program. October 1973.
3. Secondary Treatment Information, 40 CFR Part 133, Federal Register
Volume 38, No. 159, 22298-22299. August 17, 1973.
4. Waste Treatment Lagoons - State of the Art, by Missouri Basin
Engineering Health Council. EPA Research Report 17090 EHX 07/71.
July, 1971.
5. Wastewater Engineering, by Metcalf and Eddy, Inc. McGraw-Hill
Book Company. 1972.
6. Technical Bulletin: Design Criteria for Mechanical, Electric, and
Fluid System and Component Reliability, Office of Water Program
Operations. EPA Publication 430-99-74-001. 1973.
7. Echelberger, W. F., J. L. Pavoni, P. C. Singer, and M. W. Tenney,
"Disinfection of Algae Laden Waters", Journal of the Sanitary
Engineering Division, ASCE, Vol. 97, No. SA 5. October 1971.
8. Horn, L., "Chlorination of Waste Pond Effluent", 2nd International
Symposium for Waste Treatment Lagoons, edited by Ross E. McKinney
for Missouri Basin Engineering Health Council. 1970.
9. Removal of Algae from Waste Stabilization Pond Effluents - A State
of the Art, by V. Kothandaraman and R. L. Evans. Illinois State
Water Survey Circular 108, Urbana, Illinois. 1972.
10. Evaluation of Techniques for Algae Removal from Wastewater Stabiliza-
tion Ponds by E. J. Middlebrooks, D. B. Poreel la, R. A Gearheart,
G. R. Marshall, J. H. Reynolds, and W. J. Grenny. Utah Water Research
Laboratory, Utah State University, Logan, Utah. January 1974.
11. Intermittant Sand Filtration to Upgrade Existing Wastewater Treat-
ment Facilities, by G. R. Marshall and E. J. Middlebrooks. Utah
Water Research Laboratory, Utah State University, Logan, Utah.
February, 1974.
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12. Sewage Treatment Plant Design, ASCE Manual of Engineering Practice
No. 36/WPCF Manual of Practice No. 8. 1959.
13. Survey of Facilities Using Land Application of Wastewater, by
R. H. Sullivan, M. M. Conn, and S. S. Baxter, Prepared for Office
of Water Program Operations. EPA Publication 430-9-73-006. July 1973.
14. Wastewater Treatment and Reuse by Land Application, by C. E. Pound
and R. W. Crites, EPA Research Report 660/2-73-006a. August, 1973.
Note: Information on EPA publications can be obtained from the EPA
Regtonal Administrator.
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