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&EPA
United States
Environmental Protection
Agency
Municipal Environmental Research EPA-tiOO/2-78-070
Laboratory June 1978
Cincinnati OH 45268
Research and Development
Study of Activated
Sludge Separation
by Dynamic
Straining
REGIONS! LIBRARY
U. S. ENVIRONMENTAL PROTECTION
AGENCY
F445 ROSS AVENUE .»'
DALLAS, TEXAS 7520?
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RESEARCH REPORTING SERIES
Research reports of the Office of Research and Development, U S Environmental
Protection Agency, have been grouped into nine series These nine broad cate-
gories were established to facilitate further development and application of en-
vironmental technology Elimination of traditional grouping was consciously
planned to foster technology transfer and a maximum interface in related fields
The nine series are
1 Environmental Health Effects Research
2 Environmental Protection Technology
3 Ecological Research
4 Environmental Monitoring
5 Socioeconomic Environmental Studies
6 Scientific and Technical Assessment Reports (STAR)
7 Interagency Energy-Environment Research and Development
8 'Special Reports
9 Miscellaneous Reports
This report has been assigned to the ENVIRONMENTAL PROTECTION TECH-
NOLOGY series This series describes research performed to develop and dem-
onstrate instrumentation, equipment, and methodology to repair or prevent en-
vironmental degradation from point and non-point sources of pollution This work
provides the new or improved technology required for the control and treatment
of pollution sources to meet environmental quality standards
This document is available to the public through the National Technical Informa-
tion Service, Springfield, Virginia 22161
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EPA-600/2-78-070
June 1978
STUDY OF ACTIVATED SLUDGE SEPARATION
BY DYNAMIC STRAINING
iEGION VI LIBRARY
U.S. ENVIRONMENTAL PROTECTION
1445 ROSS AVENUE
DALLAS, TEXAS 7520?
by
James Dumanowski
Arvid Strom
PMC Corporation
Environmental Equipment Division
Itasca, Illinois 60143
-1
Contract No. 68-03-0427
Project Officer
Richard C. Brenner
Wastewater Research Division
Municipal Environmental Research Laboratory
Cincinnati, Ohio 45268
MUNICIPAL ENVIRONMENTAL RESEARCH LABORATORY
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DISCLAIMER
This report has been reviewed by the Municipal Environmental Research
Laboratory, U.S. Environmental Protection Agency, and approved for publica-
tion. Approval does not signify that the contents necessarily reflect the
views and policies of the U.S. Environmental Protection Agency, nor does
mention of trade names or commercial products constitute endorsement or
recommendation for use.
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FOREWORD
The Environmental Protection Agency was created because of increasing
public and government concern about the dangers of pollution to the health
and welfare of the American people. Noxious air, foul water, and spoiled
land are tragic testimony to the deterioration of our natural environment.
The complexity of that environment and the interplay between its components
require a concentrated and integrated attack on the problem.
Research and development is that necessary first step in problem
solution and it involves defining the problem, measuring its impact, and
searching for solutions. The Municipal Environmental Research Laboratory
develops new and improved technology and systems for the prevention,
treatment, and management of wastewater and solid and hazardous waste
pollutant discharges from municipal and community sources, for the preser-
vation and treatment of public drinking water supplies, and to minimize
the adverse economic, social, health, and aesthetic effects of pollution.
This publication is one of the products of that research; a most vital
communications link between the researcher and the user community.
As part of these activities, the study described herein presents a
pilot-scale evaluation of the feasibility of replacing or supplementing
conventional secondary gravity clarifiers in the activated sludge process
with dynamic strainers equipped with ultrasonic transducers, in conjunction
with sand filtration.
Francis T. Mayo, Director
Municipal Environmental Research
Laboratory
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ABSTRACT
Pilot plant studies were conducted on domestic wastewater to determine
the feasibility of replacing or augmenting conventional activated sludge
gravitational clarifiers by dynamic straining. This work was a continuation
of the successful program accomplished under EPA Contract No. 68-03-0102.
In the prior program, two dynamic strainers in series were employed. In
this program, less expensive techniques for polishing the primary strainer
effluent were investigated.
Phase I covered pilot operations with the strainer in the aeration tank
of an activated sludge plant producing a nitrified effluent. Phase II
involved a non-nitrified effluent. In both phases, upflow and downflow sand
filtration, settling, and flocculation-settling of the strainer effluent
were investigated.
Phase I flows were low but correlated well with the previous studies of
strainer operating variables. Strainer effluent averaged 35 mg/1 suspended
solids with mixed liquor suspended solids (MLSS) of 6800 mg/1. The upflow
and downflow sand filters, operating on strainer effluent, produced effluents
of 8 and 3 mg/1 suspended solids, respectively.
Phase II, which called for high flows and high BOD loadings, was
plagued with an intense growth of filamentous microorganisms, foaming, and
development of a "syrupy" mixed liquor. Proper strainer throughputs could
not be maintained. Flow rates were not reproducible and did not correlate
with prior work.
A special Phase III program extended the effort and determined that the
equipment was functioning properly. However, maximum achievable flows were
still disappointing and could not be correlated with the prior study. These
findings cast doubt on the ability of the strainer to achieve economically
significant throughputs in a variety of process situations.
This report was submitted in fulfillment of EPA Contract No. 68-03-0427
by the PMC Corporation, Environmental Equipment Division, under the sponsor-
ship of the U.S. Environmental Protection Agency. This report covers
experimental work conducted during the period of May - December 1974.
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CONTENTS
Foreword iii
Abstract iv
Figures vi
Tables vii
Acknowledgements viii
1. Introduction 1
2. Conclusions 3
3. Recommendations 5
4. Description of Equipment 6
5. Description of Operation 15
6. Discussion of Results 19
References 42
Appendix 43
v
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FIGURES
Number
1 Dynamic strainer unit 7
2 Dynamic strainer installation 8
3 Strainer photographs 9
4 Sand filter photographs 10
5 Dynamic straining-filtration flow diagram 13
6 Photograph of settling column test set-up 14
7 Photograph of mixed liquor surface during severe foaming
condition 17
8 Photomicrographs of mixed liquor 21-22
9 BODs performance data - Phase I 23
10 COD performance data - Phase I 24
11 Suspended solids performance data - Phase I 25
12 Photomicrographs of strainer effluent 26
13 Composite settling curve for strainer effluent and discrete
settling solids 30
14 Theoretical strainer effluent suspended solids removal during
subsequent gravity settling of strainer effluent 31
15 Downflow sand filtration pressure drop as a function of
loading and run length 33
16 Specific flow rate as a function of total head - July 22,
1974 37
17 Specific flow rate as a function of total head - August 9,
1974 38
VI
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TABLES
Number Page
1 Sampling and Analysis Summary 16
2 Operational Data Obtained 16
3 Phase I Strainer Effluent Particle Size 27
U Phase I Bench Scale Flocculation Studies 28
5 Composite Settling Curve Data 29
6 Primary Strainer Specific Flow Rates, MLSS = 6465 mg/1 40
7 Primary Strainer Specific Flow Rates as a Function of Method
of Strainer Fabric Attachment 41
Vll
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ACKNOWLEDGEMENTS
The support of the project by the Municipal Environmental Research
Laboratory (MERL), Office of Research and Development, U.S. Environmental
Protection Agency, Cincinnati, Ohio, and the assistance of the Project
Officer, Mr. Richard C. Brenner, are acknowledged with sincere t±tanks.
The efforts of Mr. Robert P.G. Bowker of MERL, who assisted in the review
of this report, are also acknowledged.
Vlll
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SECTION 1
INTRODUCTION
The treatment capacity of an activated sludge plant is most often
limited by the secondary clarifier. As the volumetric BOD loading increases,
the MLSS concentration under aeration must be gradually increased to main-
tain the organic (F/M) loading within the desired operating range. However,
an increase in MLSS concentration leads to a proportional increase in mass
(solids) loading on the secondary clarifier. At some point, the solids
loading on the secondary clarifier will begin to exceed the sludge thickening
capability of that unit, necessitating a reduction in MLSS level and result-
ing in a higher than desired F/M loading. This in turn may result in
decreased removal of soluble organics and poor biomass settling characteris-
tics. Poor sludge settling characteristics can also result from a predomi-
nantly filamentous sludge, even though this type of sludge can yield effic-
ient soluble BOD removal. These problems suggest that a better method of
liquid/solids separation would result in greater treatment plant capacity
and reliability.
Pilot plant studies (1) have indicated that straining, which is not
dependent on the settleability of the solids involved, could be used as an
alternative or supplement to gravitational settling to separate activated
sludge solids. FMC Corporation has been investigating the process of
straining for about 7 yr. The dynamic strainer, which evolved from- this
work about 4 yr ago, has been used for several different applications. For
low suspended solids conditions, it was tested on municipal treatment plant
and aerated lagoon effluents. The strainer has also been used to strain the
mixed liquor resulting from the treatment of cheese whey. This represented
an application of a high suspended solids condition.
Beginning in November, 1972, under sponsorship of the U.S. Environmental
Protection Agency, an initial contract, No. 68-03-0102, entitled "Replacement
of Activated Sludge Secondary Clarifiers by Dynamic Straining" was awarded
to FMC Corporation to investigate the use of straining to separate activated
sludge solids (1). The results of this study indicated that commercially
acceptable specific flow rates of up to 8 1/sec/m2 of filtering surface
(12 gpm/ft2) could be obtained using a nominally rated 10-y stainless steel
micromesh fabric with a MLSS concentration of 6500 mg/1.
During the above study, a secondary strainer was evaluated for reducing
the concentration of suspended solids in the primary strainer effluent. The
primary strainer effluent contained an average of 46 mg/1 suspended solids
over 1 mo of non-steady state operation. These solids were reduced to an
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average of 23 mg/1 by the secondary strainer without additional flocculation
or chemical treatment. Even though the secondary strainer was capable of
producing an effluent of acceptable quality, the relatively small amount of
solids passing through the primary strainer indicated that other less costly
methods of clarification of the primary strainer effluent might be applicable.
This potential improvement in process economics led to further pilot plant
investigations and subsequent generation of this report under EPA Contract
No. 68-03-04-27.
The study consisted of two phases; Phase I - steady state operation of
the strainer in activated sludge which was producing a nitrified effluent,
and Phase II - steady state operation of the strainer in activated sludge
which was not producing a nitrified effluent, i.e., a system with a high BOD
loading. During both phases, the strainer effluent suspended solids were
characterized as to concentration, volatile fraction, particle size,
physical appearance, etc. Several additional means of removing the residual
colloidal and suspended solids were examined. These included upflow sand
filtration, downflow sand filtration (coal over sand), gravitational
settling, and flocculation followed by gravitational settling.
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SECTION 2
CONCLUSIONS
1. Dynamic straining is indicated to be a technically feasible process
to replace or augment conventional activated sludge gravitational
clarifiers. A reasonably firm data base, however, exists only for low
throughput applications, where low specific flow rates of 1.4 - 2.7
1/sec/m2 (2-4 gpm/ft2) require a limited number of strainers (1 or 2
units) with plant flows below 3785 mVday ( 1 mgd). Under these flow
conditions, suspended solids removals of 99 percent can be achieved with
strainers operating in mixed liquor with MLSS levels in the range of
6000-7000 mg/1.
2. Further treatment of the strainer effluent by upflow or downflow
sand filtration or chemical or autoflocculation-settling will achieve
final effluent suspended solids below 10 mg/1.
3. The data are inconclusive on the maximum flow rates achievable with
the strainer and the ability of the equipment to sustain high specific
flow rates. Operating set points from prior equipment characterization
studies (1) are reproducible at low specific flow rates of 1.4 - 2.7
1/sec/m2 (2-4 gpm/ft2) and can be sustained in long term operations.
4. Set point conditions for the more economically significant specific
flow rates of 6-8 1/sec/m2 (9-12 gpm/ft2) have not been determined to be
reproducible or capable of being sustained in long term operations.
5. The above specific flow rate restriction would require about three
times the number of strainers for a particular application than believed
necessary from the prior studies. This more than offsets any economic
advantages gained by replacing secondary strainers for polishing primary
strainer effluent with sand filters or flocculator-clarifiers. Under
these conditions, the strainer cannot economically or operationally
compete with conventional approaches to hydraulic and biological
upgrading. The low-flow applications, where the strainer may be
competitive, probably so seriously limit the market that continued
development expense does not seem justified. Even if higher specific
flow rates could be achieved, the principle market is still only in the
vicinity of 3785 mVday (1 mgd).
6. Although there are many interesting technical applications for the
strainer (control of bulking, upgrading, etc.), the current state-of-
the-art does not warrant attempts to develop comprehensive cost estimates
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for this report. Preliminary cost estimates were presented in the
report of tine initial feasibility study, EPA Contract No. 68-03-0102,
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SECTION 3
RECOMVENDATIONS
1. In view of the conclusions discussed in Section 2, it is
recoirmended that development work on the strainer for direct mixed
liquor straining be terminated.
2. If satisfactory cases can be made for other possible strainer
applications, consideration should be given to further work and the
cost/benefit of that work in light of total development cost. This
would include engineering, prototype construction and testing, and
tooling for production.
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SECTION 4
DESCRIPTION OF EQUIPMENT
The dynamic strainer unit used in this study consists basically of four
parts as shown in Figure 1: the supporting drum basket, the micromesh fabric,
'the ultrasonic cleaning transducer, and the drive unit. A sketch of the
dynamic strainer installation utilized on this project is shown in Figure 2.
Photographs of the strainer and sand filter installations are presented in
Figures 3 and 4, respectively.
The 0.61-m (2-ft) diameter by 0.61-m (2-ft) high inner expanded metal
drum basket supports the strainer fabric while allowing the liquid to pass
through to the inside. The fabric was resistance welded to the expanded
metal backing and sealed against leaks by a silicone rubber based adhesive.
Only one stainless steel fabric was used in this study. The cloth specifica-
tions are: nominal micron rating 10, absolute micron, rating 15-18, nominal
mesh count (openings/inch, warp x shoot) 850 x 155, and wire diameter in
inches (warp x shoot) 0.0012/0.004. The fabric is designated Robusta
Reverse Dutch Weave and is supplied by Kressilk Products, Inc., Monterey
Park, California. The cloth was determined to give superior performance in
the previously performed contract.
The ultrasonic transducer is located inside the basket and mounted on
the stationary axle. The power for this unit is supplied by an ultrasonic
generator located remotely in a control cabinet. The transducer is exposed
to the full vertical length of the cloth. The ultrasonic energy excites the
fabric and sheds the solids back into the liquid reservoir. The cloth is
completely cleaned each revolution as the basket rotates past the stationary
transducer. This cleaning method is very simple, economical, and reliable.
The basket and strainer fabric are rotated by a 2.2-kw (3-hp) electric
motor attached to a variable speed drive unit. This unit provides a maximum
rpm of 180 and a minimum rpm of 30. The rotational effect of the strainer
also helps to keep the fabric free of solids buildup.
The strainer unit is supported by brackets made of angle iron mounted
on top of the activated sludge aeration basin. The strainer is connected
to the effluent piping with the use of a WECO Air-O-Union seal. This allows
the strainer to be installed without draining the tank. The seal is then
inflated to 42,000-56,000 kgf/m2 (60-80 psig) to secure the strainer outlet
pipe. The wastewater flow passes through the fabric and discharges through
the outlet pipe to a standpipe outside the tank. The standpipe maintains
a water level in the strainer sufficient to keep the ultrasonic unit sub-
merged, allowing effective cleaning of the fabric.
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An FMC SL-144 modified, complete-mix, activated sludge package treatment
plant was used for this study. The package plant was supplied with municipal
wastewater obtained in continuous supply from an interceptor sewer. The
wastewater was screened and comminuted prior to entering the primary
clarifier. The primary clarifier had a volume of 9.8 m3 (347 ft3 or 2600
gal) and a surface area of 5.9 m2 (63.6 ft2). The primary clarified effluent
was then pumped into a 15.1-m3 (535-ft3 or 4000-gal) aeration tank. The
primary strainer was placed directly in the mixed liquor at one end of the
aeration tank opposite one of three influent ports. The effluent from the
primary strainer was fed by gravity through a 90-degree, V-notch weir to an
effluent holding tank with a volume of approximately 2.1 m3 (75.4- ft3 or
564 gal). The contents of this tank were continually circulated through an
FMC sampler to obtain a 24-hr composite sample of primary strainer
effluent. Primary strainer effluent from the holding tank was used as a
feed stream for the upflow and downflow sand filters and the gravity settling
column.
The dual media (coal over sand) downflow filter was manufactured by
Neptune Microfloc, Inc. The unit was a plastic column pilot scale filter
with an inner diameter of 0.11 m (4.5 in.), a height of 1.3 m (51 in.), and
a surface area of 0.01 m2 (0.11 ft2).
The downflow filter was supplied by Neptune with 0.23 m (9 in.) of their
MS-6 sand and 0.53 m (21 in.) of their MS-4 anthracite. The MS-6 sand had
an effective size of 0.46 mm, a uniformity coefficient of 1.5, and a specific
gravity of 2.6. The MS-4 anthracite had an effective size of 1.0-1.1 mm, a
uniformity coefficient of 1.7 or less, and a specific gravity of 1.6. No
support media was required below the sand since a screen over the discharge
outlet kept the sand from leaving the filter.
The unit, as shown in Figure 4, included a differential pressure
indicator, an effluent pressure regulator and bypass valve, an effluent
flow meter, and piping to allow for both backwashing and surface washing of
the filter. All washing was done manually as required. The filter flow
rate could be varied from zero flow to 6.8 1/sec/m2 (10 gpm/ft2). The
filter was normally operated at a flow rate of 3.4 1/sec/m2 (5 gpm/ft2) or
less.
The upflow sand filter utilized was an FMC Corporation Model USF-3.
The filter was 0.92 m (3 ft) in diameter and 2.4 m (8 ft) high with a
surface area of 2.2 m2 (7.1 ft2). The filter media consisted of 0.92-m
(3 ft) of filter sand supported by three, 0.1-m (4-in.) layers of gravel.
The upflow filter sand consisted of 98 percent natural silica sand
which was spherical in shape, with all flat and crushed particles removed.
The effective size was 1.15 to 1.25 mm with a uniformity coefficient of
1.50 to 1.60 and a specific gravity of 2.60 to 2.65. By weight, 95 percent
of the particles passed a No. 8 U.S. standard sieve and 100 percent were
retained by a No. 20 U.S. standard sieve. The support gravel consisted
of hard, rounded, natural granite, free from flat or elongated pieces. The
bottom layer consisted of 19 to 38 mm (0.75 to 1.5 in.) gravel followed by
a layer of 6 to 12 mm (0.25 to 0.5 in.) gravel and a layer of 3 to 6 mm
11
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(0.125 to 0.25 in.) gravel.
The upflow filter employed a flow rate of 2.0 1/sec/m2 (3 gpm/ft2) under
normal filtering conditions. The feed pump had a capacity of 80 1/min
(21 gpm) at a 6.3-m (20-ft) head. The feed pump was powered by a 1750-rpm,
1.5-hp motor. The filter was washed at the rate of 12.2 1/sec/m2 (18 gpm/
ft2). The wash pump had a capacity of 485 1/min (128 gpm) at a 11.3-m
(37-ft) head. The wash pump was powered by a 2600-rpm, 2-hp motor. During
the wash cycle, the wash water flow was directed over a weir and carried to
a separate point of discharge. The filter was equipped with proper instru-
mentation so that the wash cycle could be initiated automatically by either
back pressure or by a timer, as well as by manual control. Variable speed
pumps were used for the influent raw sewage, upflow sand filter, and down-
flow sand filter feeds.
Three EMC Corporation samplers were utilized on the project. These
samplers collected daily composite samples of strainer effluent, downflow
sand filter effluent, and upflow sand filter effluent. The samplers were
set to collect a sample every 15 min. Samples of influent sewage were
obtained by manually compositing a sample over the 8-hr work day. No
samples were collected during the weekend. A flow diagram for the experi-
mental setup is shown in Figure 5.
Large-scale settling studies were periodically performed during the
course of the study. These were done using a 3.1-m (10-ft) high by 0.095-m
(7.5-in.) I.D. plastic column as shown in Figure 6. The column was filled
during test runs to a height of 2.4-m (8-ft) which corresponds to a volume
of 68.5 1 (18.1 gal). Sampling outlets were located every 0.3-m (1-ft),
although most samples were taken at a depth of 1.4-m (4-ft).
Laboratory-scale flocculation and settling studies were conducted using
a Phipps 8 Bird 6-place multiple stirrer. The studies were performed in 1-
liter beakers. After flocculant addition to the strainer effluent, the
solution was allowed to react for 15.5 min, followed by a 30 min settling
period. Samples for analysis were then removed by siphoning. The floccu-
lants examined were ferric chloride, alum, and two polyelectrolytes; one
anionic and one cationic. The flocculants were studied at dosage levels of
0.5, 1.0, 1.5, 2.0 and 3.0 mg/1 (as Fe, Al, and polyelectrolyte).
Photomicrographs of the activated sludge were taken daily during most
of the project. Studies of the suspended solids passing through the
strainer were also conducted utilizing a microscope equipped with an ocular
micrometer and a Levy hemacytometer chamber with double Neubauer ruling.
Using these two devices, estimates of particle size were made.
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©
13
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Figure 6. Photograph of settling column test set-i
14
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SECTION 5
DESCRIPTION OF OPERATION
Equipment acquisition and installation was completed by the end of
March 1974. The plant, however, was put in operation at the beginning of
March 1974 while equipment installation was being completed. The aeration
tank was filled with 15.7 m3 (4160 gal) of primary settled sewage, and
aeration was started. On March 13, the MLSS level had reached 1235 mg/1
and flow through the strainer was initiated. The plant was then fed primary
settled sewage on a continuous basis. Samples of mixed liquor and strainer
effluent were taken daily, and several analyses were performed. This pre-
liminary startup work was done using a 20-y fabric on the strainer while
waiting for the 10-u fabric to arrive from the supplier. On May 14, the
10-y fabric was installed on the strainer and process equipment shakedown
continued. On May 28, the MLSS concentration had reached 6560 mg/1 and
Phase I of the study was started. Phase I consisted of continuous steady-
state operation of the primary strainer under activated sludge system condi-
tions necessary to achieve a nitrifying mixed liquor population. During
Phase I, MLSS averaged 6820 mg/1, primary strainer effluent suspended
solids averaged 34 mg/1, strainer effluent ammonia nitrogen averaged 0.5 mg/1,
and influent flow averaged 90.5 mVday (23,900 gpd).
Phase I lasted for 5 wk. During this period of time, plant operation,
system monitoring, and data collection were continuous. Samples were taken
Monday through Friday with no samples being taken on weekends. As previous-
ly mentioned, composite samples were taken of primary clarifier effluent,
primary strainer effluent, downflow sand filter effluent, and upflow sand
filter effluent. In addition to these, grab samples were taken daily of
mixed liquor (strainer influent) and periodically of the sand filter wash
water from both sand filters. A list of the analyses performed on each
sample is given in Table 1. All analytical work was performed in accordance
with the procedures and methods detailed in "Standard Methods of the Examin-
ation of Water and Wastewater", Thirteenth Edition, Washington, D.C. (1971).
Various mechanical and operational data were also recorded daily, as shown
in Table 2.
During Phase I operation, primary strainer effluent was continuously
fed into both filters for the 5 wk test period. Bench scale studies on the
primary strainer effluent solids were conducted periodically.
15
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TABLE 1. SAMPLING AND ANALYSIS SUMMARY
1
2
3
4
5
SAMPLE NUMBER
1
2
3
4
5
LOCATION
Primary Effluent
Mixed Liquor
Primary Strainer Effluent
Downflow Filter Effluent
Upflow Filter Effluent
TYPE
Composite (8 hr)
Grab
Composite (24 hr)
Composite (24 hr)
Composite (24 hr)
SETTLED TUR-
SAMPLE pH TSS VSS TOC SOC COD BODs SOLIDS BIDITY NHs-N NOs-N DO
x
x
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
y y
X
y
y
y
x - Daily, Monday through Friday
y - Monday, Wednesday, Friday
z - Occasionally
y
y
y
X
X
X
z
z
z
TABLE 2. OPERATIONAL DATA OBTAINED
PARAMETER
LOCATION
Liquid Flow
Air Flow
Strainer Speed
Hydraulic Head
Sonic Generator Amps
Sand Filter Head
Sand Filter Wash Cycle
Primary Effluent, Primary Strainer
Effluent, Sand Filter Influents
To Aeration Tank
Primary Strainer
Primary Strainer
Primary Strainer
Downflow Filter, Upflow Filter
Downflow Filter, Upflow Filter
Strainer effluent solids were examined for their settling characteris-
tics using the 0.095-m (7.5-in.) diameter plastic settling column. Effluent
was pumped into the column to a depth of 2.4 m (8 ft) and mixed well.
Samples for suspended solids analysis were withdrawn from the column at a
depth of 1.2 m (4 ft). Samples were taken at 1 min, 3 min, 5 min, and every
16
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5 min thereafter for a period of at least 1 hr. Settling characteristics
after flocculation were determined by flocculating multiple 1-liter samples
of strainer effluent using ferric chloride, alum, Betz 1190 (a cationic
polymer), and Betz 1130 (an anionic polymer). The flocculant was added to
the strainer effluent and mixed for 30 sec at 100 rpm, 15 min at 60 rpm,
and then allowed to settle for 30 min. Samples of the supernatant were then
withdrawn for suspended solids analysis.
Photomicrograph techniques were used during Phase I to evaluate the
type and size of primary strainer effluent suspended solids. The size of
the particles was measured using an ocular micrometer. The length and
width measurements were then used to calculate the mean and median length,
width, and area of the particles.
Upon completion of Phase I, the test equipment was slightly modified
in order to handle the increased loading and hydraulic flow for Phase II.
Phase II was intended to demonstrate continuous steady-state operation of
the primary strainer and sand filters at hydraulic loading, detention time,
and MLSS levels which would preclude development of a nitrifying mixed
liquor. Phase II was started on July 8, 1974.
Shortly after beginning work on Phase II, a noticeable change occurred
in the influent sewage. The sewage on many days had the strong odor of
hydrocarbons and on other days the odor and foaming characteristics of
laundry waste. The apparent effect of the combined wastes on the mixed
liquor was to produce a vast amount of filamentous microorganisms, which in
turn produced a polysaccharide slime. This caused the mixed liquor to be
very stringy and have the consistency of a light syrup. On numerous
occasions, the mixed liquor foamed severely during the day as shown in
Figure 7. The major effect of foaming was to cause a drastic decrease
Photograph of mixed liquor surface
during severe foaming conditions.
17
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in the primary strainer flow rate. Maximum flow rates through the strainer
during this time were 1.1 - 2.2 1/sec/m2 (1.6 - 3.2 gpm/ft2). Various
methods (chlorination, reduced air, dilution) were tried to rid the system
of the filamentous material. None of the methods tried had the desired
effect except complete draining of the aeration tank. During September and
October, the mixed liquor appeared to change, back to more normal character-
istics with a decrease in the amount of filamentous organisms present. Flow
rates through the primary strainer increased; however, the high flow rates
that were achieved during the previous study (1) could not be attained.
Phase II work was suspended and, during November and early December, a
special Phase III study was initiated to try to elucidate the cause of the
poor strainer flow rates. Work on Phase III was completed on December 4,
1974.
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SECTION 6
DISCUSSION OF RESULTS
The operating capability of the dynamic strainer in terms of specific
flow rate and suspended solids removal is dependent upon several parameters.
These parameters include the nominal pore size and weave of the filter
fabric, suspended solids concentration of the liquor being strained, the
rotational speed of the strainer, the hydraulic head across the strainer,
and the morphological characteristics of the biomass. These parameters and
their effect on the performance of the dynamic strainer are detailed in the
previous contract report (1).
The ultimate feasibility of using a dynamic strainer to either replace
the final gravitational clarifier in an activated sludge system, or in
conjunction with the final clarifier to handle periods of peak flows at
possibly higher MLSS, will depend upon the consistent performance of the
strainer in a variety of conditions. The strainer suspended solids removal
efficiency must be high enough so that the effluent will be of sufficient
quality to meet existing discharge requirements or be capable of further
treatment to meet such requirements.
Two conditions under which different biomass characteristics develop
and in which the strainer could potentially be applied to are: (1) activated
sludge mixed liquor developed under low BOD loading rates where nitrifica-
tion would occur, and (2) activated sludge mixed liquor developed under high
BOD loading rates where nitrification would not occur. The suspended solids
passing through the strainer filter fabric for these two conditions would
most likely require different means to remove them from the final plant
effluent. The following Phase I and II studies discussed in this section
were undertaken to evaluate strainer operation with these two types of bio-
masses. As previously mentioned, the Phase III strainer flow rate investi-
gative work was initiated in an attempt to elucidate causitive factors of
the severe sludge bulking conditions encountered in Phase II.
PHASE I - ACTIVATED SLUDGE DEVELOPED UNDER LOW LOADING CONDITIONS
During Phase I (May 28 - July 3, 1974), the activated sludge system was
operated such that a nitrifying mixed liquor was produced. The average
organic (F/M) loading during this phase was 0.22 kg BODs/day/kg MLVSS with
a cell retention time of 10 days, and the average flow rate to the plant was
92 m3/day (24,300 gpd).. Influent and effluent ammonia nitrogen averaged 16
and 0.5 mg/1, respectively. Several tests for nitrate nitrogen performed
during this phase of the study indicated that nitrification was occurring
19
-------
in the aeration tank. MLSS averaged 6800 mg/1, and the strainer effluent
averaged 35 mg/1 suspended solids. Other average values for operational
data were a strainer speed of 73 rpm (458 ft/min), a total hydraulic head of
16.5 cm (6.5 in.), and a total strainer flow rate of 63.9 1/min (16.9 gpm).
Using an effective filtering area of 0.59 m2 (6.4 ft2), this flow corresponds
to a specific flow rate of 1.8 1/sec/m2 (2.6 gpm/ft2). No attempt was made
during this phase to maximize flow rate through the strainer.
The daily operation of the strainer was essentially trouble-free. All
mechanical parts of the strainer system as well as the other parts of the
test set-up operated properly. On several occasions, the activated sludge
mixed liquor developed foaming problems such as shown previously in Figure 7.
This primarily occurred toward the end of Phase I. These minor operational
problems did not interfere with the collection of daily samples and operating
data during this phase. Mixed liquor dissolved oxygen levels were maintained
at an average of 1.5 mg/1, and air flow to the aeration tank was 82 kg/hr
(180 Ib/hr).
Daily microscopic observation of the mixed liquor during this period
showed it to be composed of large, dense sludge particles and many forms of
protozoan life. On most days, amoebae, free-swimming ciliates, flagellates,
stalked ciliates, rotifers, and occasionally nematodes could be found.
Stalked ciliates, primarily Vorticella sp., were the predominant micro-
organism. Filamentous organisms were present in minor amounts during most
of this testing phase; however, they gradually increased in number toward
the end of Phase I (Figure 8).
As evident in Figures 9, 10, and 11, the treatment system operated very
well during Phase I. Secondary system BOD5, COD^and suspended solids
removals averaged 96 percent, 83 percent, and 79 percent, respectively.
The average suspended solids concentration in the strainer effluent was
35 mg/1 with a MLSS of 6800 mg/1 (equivalent to a strainer solids removal
efficiency of 99.5 percent). A complete tabulation of the operational data
is presented in the Appendix.
Microscopic observation of the solids passing through the strainer
indicated many of the particles which passed the filter fabric were protozoan
life forms. The most common type was the free-swimming ciliate Trachelo-
phyllum sp. These protozoa average 40.5 y in length and are approximately
10 y wide. Other types of microorganisms commonly observed were the small
bits and pieces of activated sludge floe as seen in Figure 12. Measurement
of the particles was performed microscopically using an ocular micrometer.
Table 3 lists the results of these measurements. On only one day, June 14,
1974, did the strainer effluent used for particle analysis show a large
discrepancy with other tests during Phase I. The photomicrograph for that
date in Figure 12 indicates the particles had probably post-flocculated
after passing through the strainer. For all seven tests, the average
median area, length and width of the particles was 864 y2, 35 y and 23 y,
respectively. Of the particles passing the strainer, 80 percent of them
measured less than 64 y long and 40 y wide. These data would tend to indi-
cate that the particles passing through the filter fabric fell close to the
absolute size of the openings in the filter fabric, i.e., 15-18 y.
20
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MIXED LIQUOR 5-20-74 100X
MIXED LIQUOR 5-21-74 100X
MIXED LIQUOR 5-22-74 10OX
MIXED LIQUOR 6-20-74 100X
Figure 8. Photomicrographs of mixed liquor.
21
-------
MIXED LIQUOR 7-1-74 100X
MIXED LIQUOR 7-10-74 100X
t* .:
MIXED LIQUOR 7-31-74 100X
fr
MIXED LIQUOR 8-1-74 100X
Figure 8 (continued).
22
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MIXED LIQUOR SS
AVERAGE MLSS = 6800MG/L
PRIMARY STRAINER EFFLUENT SS
AVERAGE EFFLUENT SS = 35 MG/L
SANDFILTER EFFLUENT SS
G USF EFFLUENT SS
DFSF EFFLUENTSS
ill i i i i ii
AVERAGE USFSS=8.4MG/L
AVERAGE DFSFSS'2.8 MG/L
29 311 3 5 7 10 12 14 17 19 21 24 26 28 I 3
MAV | JUNE JULV
-START-UP
Figure 11. Suspended solids performance data - Phase I.
25
-------
STRAINER EFFLUENT 5-29-74
STRAINER EFFLUENT 5-30-74
STRAINER EFFLUENT 6-14-74
STRAINER EFFLUENT 6-20-74
Figure 12. Photomicrographs of strainer effluent.
26
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TABLE 3. PHASE I STRAINER EFFLUENT PARTICLE SIZE ANALYSES
AREA (y2)
DATE
5-29
5-30
6-7
6-11
6-14
6-20
6-28
MEAN
1401
1194
1088
1180
5865
1240
1179
MEDIAN
744
682
527
744
1488
930
930
RANGE
124
62
62
62
62
62
62
- 8060
- 9548
- 9300
- 4340
- 62,000
- 4960
- 11,160
MEAN
42
42
37
39
72
39
39
LENGTH
(y)
WIDTH (y
MEDIAN RANGE
32
39
24
32
39
39
39
8
8
8
8
8
8
8
- 102
- 110
- 150
- 94
- 394
- 87
- 142
MEAN
25
23
21
24
44
28
24
MEDIAN
24
16
16
24
32
24
24
)
RANGE
8-94
8-87
8 -118
8-63
8 -197
8-55
8-80
Laboratory scale flocculation-settling tests performed during this
phase of the study showed that the solids passing through the strainer can
be decreased to 10 mg/1 or less by combining either ferric chloride or alum
addition with separate gravity clarification. Neither of the two polymers
used, one anionic and one cationic, produced suspended solids values that
were consistently 10 mg/1 or less. The cationic polymer on one day did
reduce the solids level to below 10 mg/1. The results of the laboratory
scale flocculation-settling tests are given in Table 4. During these tests,
a control sample of strainer effluent was subjected to the same test pro-
cedures, rapid mixing, slow mixing, and settling, but without a flocculating
agent being added. The effluent remaining after settling only exhibited
similar suspended solids concentrations as were found in the flocculated
samples. This appears to indicate that the suspended solids passing through
the strainer cloth are capable of auto-flocculation.
Samples taken during the large-scale settling tests on the strainer
effluent using the 0.095-m (7.5-in.) I.D. plastic column were analyzed for
suspended solids and turbidity. It was assumed that the strainer effluent
solids would settle as discrete particles, maintaining a relatively constant
size, shape, density, and settling velocity. The particle size analyses and
flocculation-settling studies indicated rather that the particles do not
undergo true discrete settling but a modified form where some of the parti-
cles floe and agglomerate during settling. For each settling test conducted
(without the addition of chemicals) in the large plastic column, a plot was
made of the settling velocity in inches per minute versus the percent of the
particles by weight remaining in the supernatant to determine the percent
settling slower than a given clarifier overflow rate. The detailed proced-
ure is given in reference (2). The average values obtained from six large-
scale settling tests, shown in Table 5, were then plotted to obtain a single
27
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representative settling curve (Figure 13). A theoretical discrete settling
curve for a suspension having 500 mg/1 suspended solids content was also
plotted for reference. Determining the area under the curve from zero
settling velocity to a selected clarifier overflow rate permits calculation
of theoretical solids removal for various clarifier overflow rates. Percent
suspended solids removal versus overflow rate is shown in Figure 14. To
obtain a 90 percent reduction in strainer effluent suspended solids for the
53 mg/1 concentration of the experimental sample, a low clarifier overflow
rate of 14.7 m2/day/m2 (350 gpd/ft2) would be needed under ideal conditions.
Allowing for a 30 percent overdesign in clarifier overflow rate due to
diurnal variation, an overflow rate of 10.3 m2/day/m2 (295 gpd/ft2) would be
indicated. To improve clarifier operation, flocculation with or without
chemicals or mixing of the strainer effluent with mixed liquor are possible
approaches.
TABLE 5. COMPOSITE SETTLING CURVE DATA
INITIAL SETTLING % SS REMAINING
TIME (min) SS (mg/1) VELOCITY (in./min) IN SUPERNATANT
0
1
4
5
10
15
20
25
30
35
40
50
60
53
53
50
50
50
50
51
49
42
41
36
32
28
48
12
9.6
4.8
3.2
2.4
1.9
1.6
1.4
1.2
0.96
0.8
100
94.3
94.3
94.3
94.3
96.2
92.5
79.2
77.4
67.9
60.4
22.8
An alternative to either settling or flocculation followed by settling
for the removal of strainer effluent suspended solids is sand filtration.
Two sand filters were studied during this project: a small commercial FMC
upflow sand filter with a design flow rate of 2.0 1/sec/m (3 gpm/ft ) and
a pilot-scale, dual-media dcwnflow unit capable of accepting flows up to
29
-------
SS = 500M6/L
THEORETICAL CURVE
FOR DISCRETE
PARTICLES
84.2M3/DAY/M2
(2000GPD/FT2)
42.1 M3/DAY/M2"
(IOOOGPD/FT2)
21 M3/DAY/M2
(500GPD/FT2)
1 i
STRAINER EFFLUENT-
SS=53MG/L
O STRAINER EFFLUENT
THEORETICAL EFFLUENT
= 2.54CM
4 6 8 10
SETTLING VELOCITY (IN./MIN )
12
Figure 13. Composite settling curve for strainer effluent and discrete
settling solids.
30
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0 1000 2000
CLARIFIER OVERFLOW RATE (GPD/FT2)
Figure 14. Theoretical strainer effluent suspended solids removal
during subsequent gravity settling of strainer effluent.
31
-------
6.8 1/sec/m2 (10 gpm/ft2). The upflow unit was consistently run at 2.0
1/sec/m2 (3 gpm/ft*), and the downflow unit was most often run at 3.4
1/sec/m2 (5 gpm/ft2). The flow to the downflow unit was held constant during
the 8-hr work day. The downflow unit was backwashed daily at approximately
8:00 A.M. and again at 4:00 P.M. Backwashing was accomplished with clean
water directed to the bottom of the unit for 10 min with surface washing
for 6 min, both at a flow rate of 760 1/min/m2 (18 gpm/ft2). Pressure
drop versus time data were obtained for various influent flow rates. These
data are graphically presented in Figure 15 and indicate that the pressure
loss across the filter increased linearly with time.
The upflow sand filter was operated at the standard design flow rate of
2.0 1/sec/m2 (3 gpm/ft2) on a 24-hr basis throughout Phase I. .The filter
was backwashed at least once a day, which was also a standard procedure.
The frequency and duration of backwash in a 24-hr period were varied to
study the effect on filter performance. During the backwash cycle, the flow
through the filter was increased to 12.2 1/sec/m2 (18 gpm/ft2). The wash
water was wasted to the sewer. Prom May 28 until June 10, the filter was
backflushed once per day for a 20-min cycle. The on-stream filter effluent
suspended solids during that time averaged 12 mg/1. It appeared as though
a certain amount of channeling as well as some compaction of the sand was
occurring using the once-a-day wash cycle as indicated by suspended solids
breakthrough during the filter run. The wash frequency was then increased
to three times per day, with the duration of each wash reduced to 10 min.
This pattern of wash cycles was continued until the end of Phase I. The
filter effluent suspended solids during this time averaged 7 mg/1. During
the change from Phase I to Phase II, the backwash cycle was increased to
four times per day for a duration of 5 min each. During this time, no
improvement in the quality of the filter effluent was observed and the
cycle was then returned to three times per day of 10 min duration each.
Figure 11 illustrates the suspended solids removal efficiencies of
both the upflow and downflow sand filters. The average effluent suspended
solids levels for the upflow and downflow filters were 8 mg/1 and 3 mg/1,
respectively.
PHASE II - ACTIVATED SLUDGE DEVELOPED UNDER HIGH LOADING CONDITIONS
Work on Phase II was started on July 5, 1974, immediately upon
completion of Phase I. Phase II was intended to demonstrate the steady-
state operation of the strainer in mixed liquor resulting from a process
loading of approximately 0.40 kg BODs/day/kg MLSS and a cell retention time
of 4 days. Influent flow to the strainer was set at 117 1/min (31 gpm) and
the system allowed to adjust to the loading for about 1-wk.
During the 1-wk acclimation period, strainer flow rates were near1 the
desired level. However, by the end of the 1-wk period, a definite change
in the character of the mixed liquor was observed with a resultant decrease
in strainer flow rate. The influent sewage began to have the odor of hydro-
carbons, although no substantial increase in composite influent TOG, BOD5,
or COD was detected. At various other times, the influent sewage had the
32
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odor of laundry wastes but again no substantial increase in oxidizable
material in the sewage was observed.
The aeration tank developed severe foaming problems as previously
illustrated in the photograph of Figure 7. The foam and the mixed liquor
were very stringy and had a "slimy" character. The mixed liquor at this
time contained principally filamentous microorganisms. During the previous
contract program, the strainer appeared to operate better with higher
suspended solids removals and higher specific flow rates when the mixed
liquor was filamentous in nature. It was theorized that the 10-y cloth
yielded better flow rates than the more open 20-y cloth because the filaments
could get more entangled in the 20-y openings, partially blocking the area
for flow through the strainer cloth. During the work on Phase II, the
mixed liquor developed filamentous microorganisms to a much greater extent
than had been seen previously. When the strainer, after being removed from
the mixed liquor and washed off, was replaced into the mixed liquor, the
desired flow rate could be maintained for a period of time. However, flow
rates would gradually drop to practically zero after 1 to 8 hr of operation.
The type of filamentous microorganisms that developed during this
study could have been different than those developed during the previous
study. Because of the high concentration of these organisms, a fairly
small fraction, which still comprised a large number of individual particles,
could have been of small enough size to become entangled in the 10-y cloth
openings in much the same way it was theorized that the 20-y cloth was
blinded. However, a more likely explanation for the extreme reduction in
strainer hydraulic capacity is the secretion by the organisms of excess
amounts of extracellular, polysaccharide slime material. This could plug
the cloth openings, cause excessive foaming, and create higher head losses
through the strainer cloth.
During the remainder of July, in an effort to correct the flow problem,
various combinations of strainer peripheral velocity and differential head
did cause increases in flow rates. However, in all cases, the flow rates
deteriorated with time.
On August 2, it was decided to waste most of the aeration tank contents
in order to develop a diluted mixed liquor which was not as "slimy" or
filamentous and would be more amenable to straining. On August 6, tests
were conducted to determine maximum strainer flow rates. Flow rates of up
to 7.4 1/sec/m2 (10.9 gpm/ft2) were obtained at a MLSS concentration of
2390 mg/1. The total hydraulic head was 27.9 cm (11 in.). The strainer
controls were adjusted for operation at these flow rates. However, mechani-
cal problems developed and testing was delayed for several days. By
August 16, the mixed liquor was again extrememly filamentous and "slimy"
with accompanying poor strainer flow rates. On August 19, the mixed liquor
tank was completely drained and washed out and, on August 22, influent flow
to the aeration tank was again started.
After restarting the flow to the strainer, flow rates of 3.8 1/sec/m2
(5.6 gpm/ft2) were obtained for several days at MLSS levels of 1635 to 5515
mg/1. The total hydraulic head ranged from 5.1 cm (2 in.) to 35.6 cm (14 in.)
34
-------
By the beginning of September, the mixed liquor was again beginning to foam
and the amount of filamentous organisms was increasing. Addition of calcium
hypochlorite was initiated to control the amount of filamentous organisms.
After adding chlorine for 2 days, the foaming of the mixed liquor stopped
and the filaments were observed to decrease in length as well as in amount.
The addition of chlorine was accompanied by the addition of ammonium sulfate
to provide an additional nitrogen source. The nitrogen addition was done
to make up for the losses in ammonia nitrogen caused by the chemical reac-
tions of chlorine with ammonia nitrogen. Chlorine and nitrogen addition
were continued until September 13.
Flow rates through the strainer, even after chlorine and nitrogen
addition, did not meet the Phase II requirement for flow. Thus, the spec-
ified 0.40 F/M loading rate on the secondary system could not be maintained
at the prevailing primary effluent BOD5 concentration. The addition of beet
molasses was started in order to increase the F/M ratio. The data collected
during this part of the study are reported in the Appendix for the period
September 17 until October 4.
The daily addition of 18.9 1 (5 gal) of beet molasses to the mixed
liquor added 16.3 kg BODs (36 Ib BOD5) to the average 12.4 kg BOD5 (27.3 Ib
BOD5) supplied daily by the primary effluent. The average influent waste-
water flow was 64.3 m2/day (16,980 gpd). The average organic loading during
molasses addition was 0.38 kg BOD5/day/kg MLVSS. The strainer flow rate,
because of the type of mixed liquor that was present, averaged 1.2 1/sec/m2
(1.8 gpm/ft2) at an average 22.6-cm (8.9-in.) total hydraulic head. The
highest strainer flow rate recorded during this period was 2.2 1/sec/m2
(3.2 gpm/ft2) at a total hydraulic head of 22.9 cm (9 in.). Strainer
suspended solids removals averaged 98 percent.
PHASE III - STRAINER FLOW RATE INVESTIGATIONS
During Phase I of this study, the strainer specific flow rate averaged
1.8 1/sec/m2 (2.6 gpm/ft2) at a total hydraulic head of 16.5 cm (6.5 in.)
and a peripheral drum speed of 2.3 m/sec (7.5 ft/sec). The previous work
(1) indicated this is the specific flow rate that would be expected for
these equipment operating conditions. This specific flow rate was adequate
to meet process requirements for the low-loading, nitrifying portion of the
study based on analyses of settled sewage influent. The strainer operated
consistently at these conditions, with only minor modifications during all
of Phase I. The troublesome, foaming, viscous mixed liquor did not develop
until the end of this portion of the experimental program when it was time
to increase flow for Phase II. This subsection discusses the work that was
done in an attempt to reach Phase II design conditions and to ensure that
the flow problems were associated with the unusual mixed liquor process
conditions and not the equipment.
Various tests conducted included checks of the sonic cleaning apparatus,
tests of various combinations of head and peripheral velocity, experiments
with different methods of attachment of the micro-mesh fabric to the strainer
drum, operations with diluted and fresh mixed liquor, and chemical control.
35
-------
On July 19, total head versus specific flew rate curves were determined
for rotating speeds of 60, 90, and 120 rpm (equivalent to tip speeds of 1.9,
2.9, and 3.8 m/sec, respectively). These were then compared with data
obtained during the previous contract at similar MLSS levels. The previous
data indicated that a maximum specific flow rate of 8.1 1/sec/m2 (12 gpm/ft2)
should have occurred at a speed of 120 rpm and a total head of 30.5 cm
(12 in.). However, a maximum flow rate of 2.5 1/sec/m2 (3.6 gpm/ft2) at
120 rpm and 22.9 cm (9 in.), of total head was all that could be attained.
The same test was repeated after allowing the mixed liquor to aerobically
digest for several days at 3.8 m/sec and 4.8 m/sec (120 rpm and 150 rpm,
respectively). The curves for this test are shown in Figure 16. The
maximum specific flow rate for 120 rpm had increased to 3.1 1/sec/m2 (4.5
gpm/ft2) at a total head of 20.3 cm (8 in.) after the period of aerobic
digestion. The maximum specific flow rate for 150 rpm could not be deter-
mined because the test set-up lacked the capability to pump sewage into the
aeration tank at a rate greater than 145 I/man (38 gpm). However, the
strainer flow rate obtained at 150 rpm was sufficient to operate the Phase II
system at its design flow rate of 117 1/min (31 gpm). Therefore, the
strainer was set to operate at 150 rpm and a total head of 31.8 cm (12.5 in.).
The strainer operated at this flow rate for approximately 24 hr at
which time the fabric was torn loose from one of the panels by the increased
peripheral drum speed. The strainer was repaired and placed back in the
aeration tank. After 1 additional day of operation, the mixed liquor again
began to change to a very filamentous, viscous nature and the strainer flow
rate dropped off substantially.
All but 587 1 (155 gal) were emptied from the aeration tank on August 2
and raw sewage flow was again started. By August 9, MLSS had reached 3000
mg/1 and total head versus specific flow rate curves were again obtained
for 1.9 and 3.8 m/sec (corresponding to strainer tip speeds of 60 and 120
rpm, respectively). These curves are shown in Figure 17. The specific flow
rates obtained at both 60 and 120 rpm met the design requirements for Phase
II work. The strainer was set to operate at 60-rpm for over a week-end
period.
During the period of week-end operation, the strainer interior drum
baffle broke loose and damaged the micro-mesh filter fabric. The baffle
was replaced and a new panel of fabric was installed on the strainer. When
the strainer was placed back into the aeration tank, a maximum specific
flow rate of only 1.6 1/sec/m2 (2.3 gpm/ft2) could be obtained at either
60, 90, or 120 rpm due to the mixed liquor again changing to a very filamen-
tous, viscous character.
On August 20, the entire contents of the aeration tank were wasted and
the tank was cleaned. The strainer was put back in operation, and flow
rates as high as 5.4 1/sec/m2 (8.0 gpm/ft2) were obtained during start-up
operation, August 22 through August 27, at a total head of 27.9 cm (11 in.).
On August 28, strainer specific flow rates again dropped below the design
requirement for Phase II work. The addition of calcium hypochlorite and
ammonium sulfate was then undertaken to try and remedy the problem of
excessive filamentous growth. Although this increased the strainer flow
36
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rates somewhat, the design flow rate of 3.3 1/sec/m2 (4.8 gpm/ft2) could
not be consistently obtained. Experimental work was suspended for the month
of September, but sewage flow and chemical addition to the aeration tank
were continued.
Total hydraulic head versus specific flow data were next obtained on
October 7 and 8 and are listed in Table 6. The data show that the maximum
flow rate attainable was 2.0 1/sec/m2 (2.9 gpm/ft2) at 120 rpm and a total
head of 22.8 cm (9 in.). During the previous study, a flow rate of 3.5
1/sec/m2 (5.1 gpm/ft2) was obtained at the same head and MLSS level. When
the results of the 120-rpm run on October 7 are compared with the 120-rpm
run on October 8, it is evident that a problem existed with reproducibility
of the strainer specific flow rates.
The results differ substantially in terms of maximum flow rate even
though the data were obtained at the same MLSS level and only 1 day apart.
The possibility that the sonic cleaning device was not properly operating,
thus not thoroughly cleaning the strainer fabric, was suggested.
Tests were made to determine the condition and the cleaning ability of
the sonic transducer located on the interior of the strainer drum. A piece
of aluminum foil was placed over the outside of a panel which was located
directly in front of the sonic transducer. This was done both with a piece
of strainer fabric mounted on the panel between the transducer and the
aluminum foil and without the strainer fabric mounted on the panel. In both
cases, the characteristic pitting of the aluminum foil associated with good
sonic transducer performance was observed. The transducer output was tested
with the aluminum foil at currents of 2, 4, 6, and 8 amps. Only at the 2-
amps setting was very little pitting of the aluminum foil seen. This would
tend to indicate that the sonic cleaning device was functioning properly
and, thus, cleaning the fabric adequately.
The strainer flow rate was also tested with the sonic transducer turned
off. In this condition, cloth cleaning would be retarded and the effluent
flow would stop. With a MLSS concentration of 3700 mg/1 and the strainer
operating at 120 rpm, the effluent flow decreased from 1.4 1/sec/m2 (2 gpm/
ft2) to less than 0.07 1/sec/m2 (0.1 gpm/ft2) in approximately 5 min.
The method of strainer fabric attachment to the strainer panels was
also examined. In the work done on the previous contract, the strainer
fabric was wrapped around the outside of the strainer panels and held in
place by an additional expanded metal covering wrapped around the fabric.
All of the flow data reported for the previous contract were obtained using
this form of fabric attachment. However, during the interim between con-
tracts, a modification of the method of fabric attachment was developed to
increase the life of the strainer fabric and allow for greater ease of
attachment. The revised method consists of welding the strainer fabric to
each individual strainer panel. This eliminates the need for the exterior
expanded metal covering as well as allowing for rapid replacement of only
one panel at a time should one become damaged.
Tests were conducted using the same method of fabric attachment as was
39
-------
TABLE 6. PRIMARY STRAINER SPECIFIC FLOW RATES, MLSS = 6465 MG/L
OCTOBER 7, 1974
90 RPM
120 RPM
OCTOBER 8, 1974
120 RPM
TOTAL
HEAD
(in.)
2.0
2.5
3.0
3.5
4.0
4.5
5.0
6.0
7.0
8.0
9.0
10.0
11.0
13.0
15.0
SPECIFIC
FLOW
(gpm/ft2)
0.0
0.2
0.7
1.1
1.5
2.1
2.1
2.4
2.4
2.6
2.6
2.4
2.2
2.1
2.1
TOTAL
HEAD
(in.)
5.0
5.5
6.0
6.5
7.0
7.5
8.0
9.0
9.5
11.0
12.5
14.0
15.0
SPECIFIC
FLOW
(gpm/ft2)
0.4
1.1
1.5
1.8
2.2
2.7
2.9
2.9
2.7
2.4
2.2
2.1
2.0
TOTAL
HEAD
(in.)
10.0
10.5
11.0
11.5
12.5
13.5
14.5
15.5
SPECIFIC
FLOW
(gpm/ft2)
0.1
0.2
0.3
0.3
0.4
0.4
0.4
0.4
used on the previous contract as well as another modification whereby the
fabric was held in place by three 1/2-in. wide bands located at the top,
middle, and bottom of the strainer drum. This latter method of attachment
was used in previous work and yielded satisfactory flow rates. The data for
these tests are tabulated in Table 7. The data indicate that the change
to the welded panels did not adversely affect the specific flow rates and,
in fact, may have improved the flow rates under the mixed liquor conditions
which existed during much of the testing.
All tests conducted indicated that the strainer mechanical equipment was
functioning properly. This indicated that the problem being encountered
with respect to strainer specific flow rates was indeed due to the unusual
mixed liquor process conditions. Since the activated sludge plant continued
to develop a type of mixed liquor in which it was difficult to attain and
hold the Phase II design flow rate of 117.3 1/min. (31 gpm), it was decided,
in conjunction with the Project officer to cease work on the contract at
this point.
40
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REFERENCES
1. Joyce, M., Schultz, W., and Strom, A., "Replacement of Activated Sludge
Secondary Clarifiers by Dynamic Straining," U.S. Environmental Protection
Agency, Environmental Protection Technology Series Report No. EPA-670/2-
75-045, Cincinnati, Ohio (May 1975).
2. Thackston, E.L., Eckenfelder, W.W., Editors, "Process Design in Water
Quality Engineering - New Concepts and Developments," Chapter 3, Jenkins
Publishing Company, New York, N.Y. (1972).
42
-------
APPENDIX
Table Title Page
A-l May 1974 Strainer Performance Data 44
A-2 May 1974 Sand Filter Performance Data 45
A-3 June 1974 Strainer Performance Data 46
A-4 June 1974 Sand Filter Performance Data 47
A-5 July 1974 Strainer Performance Data 48
A-6 July 1974 Sand Filter Performance Data 49
A-7 August 1974 Strainer Performance Data 50
A-8 August 1974 Sand Filter Performance Data 51
A-9 September 1974 Strainer Performance Data 52
A-10 October 1974 Strainer and Sand Filter Performance Data 53
A-11 November-December 1974 Strainer Performance Data 54
43
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44
-------
TABLE A-2. MAY 1974 SAND FILTER PERFORMANCE DATA (PHASE I)
DOWNFLOW COAL/SAND FILTER
SS/VSS TOC/SOC BOD5 TURB.
Date pH (mg/1) (mg/1) (mg/1) (JTU)
UPFLOW SAND FILTER
SS/VSS TOC/SOC BODs TURB.
pH (mg/1) (mg/1) (mg/1) (JTU)
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
8.25
8.10
8.10
8.05
8.10
8.00
8.10
8.15
7.75
16/7
2/1
4/2
5/2
2/1
3/1
4/2
1/1
8/7
9/7
9/9
9/8
13/10
10/8
9/9
7/6
7/4
8/8
7.0 7.40 4/2 11/11
1.3 7.70 2/2 9/9
1.6 7.60 4/2 10/8
2.9 7.60 10/7 19/9
2.3 7.70 32/24 14/8
1.3 7.25 16/13 11/7
1.8 7.80 31/22 40/6
1.5 7.75 8/4 11/7
3.5 7.60 27/21 14/6
7
7
2.2
1.4
1.6
6.8
8.2
6.2
8.7
4.3
8.1
45
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46
-------
TABLE A-4. JUNE 1974 SAND FILTER PERFORMANCE DATA (PHASE I)
DOWNFLOW COAL/ SAND FILTER
Date
1
2
3
4
5
6
7
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
pH
8.12
8.05
8.08
8.25
8.05
8.09
8.10
8.19
8.10
8.30
8.20
8.35
8.09
8.10
8.25
8.18
8.25
8.29
8.25
8.25
8.39
8.01
SS/VSS
(mg/1)
3/1
2/2
4/2
3/2
1/1
1/N.D.
1/N.D.
3/N.D.
1/N.D.
3/1
3/1
2/2
1/N.D.
1/N.D.
1/N.D.
3/3
5/2
5/4
4/3
4/2
3/3
6/4
TOC/SOC
(mg/1)
7/7
6/5
6/5
5/5
4/4
4/4
8/3
6/6
7/6
6/6
5/5
6/5
4/4
4/4
4/4
4/4
4/4
7/7
6/4
8/5
4/4
13/4
BOD5
(mg/1)
2
_
2
-
4
1
1
-
2
_
1
N.D.
1
_
3
2
-
2
-
4
TURB.
(JTU)
1.6
1.8
1.8
1.5
1.3
1.3
1.8
1.4
1.3
1.6
1.9
1.5
1.3
1.4
1.1
1.4
2.1
2.7
1.7
1.8
1.7
2.4
PH
7.69
7.65
7.68
7.50
7.70
7.73
7.80
7.95
7.70
7.90
7.75
8.00
7.75
7.75
7.75
7.75
7.60
7.85
7.78
7.72
7.72
7.70
UPFLOW
SS/VSS
(mg/1)
6/1
8/6
7/4
13/12
6/6
6/6
4/3
9/2
4/2
15/10
12/8
6/5
1/N.D.
1/N.D.
1/N.D.
3/1
3/2
8/7
6/5
7/4
8/6
16/12
SAND FILTER
TOC/SOC
(mg/1)
8/6
10/7
7/7
13/7
8/4
8/7
5/3
7/6
7/7
8/8
9/4
8/6
5/4
4/4
4/4
3/3
4/4
7/4
5/4
6/4
7/3
11/4
BOD5
(mg/1)
-
4
-
1
-
2
_
-
-
5
-
1
N.D.
-
1
-
3
3
-
3
-
7
TURB.
(JTU)
2.8
3.3
3.2
4.5
3.7
3.3
3.0
3.5
2.4
5.0
5.0
3.0
1.2
1.3
1.5
1.4
1.7
4.3
2.4
2.5
3.0
6.2
47
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48
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TABLE A-6. JULY 1974 SAND FILTER PERFORMANCE IATA
DOWNFLOW COAL/SAND FILTER
Date
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
22
23
24
25
26
27
28
29
30
31
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
8
PH
.20
.10
.00
.20
.10
.15
.20
.10
.10
.10
.00
.20
.20
.20
.20
.10
SS/VSS
(mg/1)
4/3
2/1
4/2
3/2
8/6
6/3
7/5
5/2
6/3
4/4
2/2
3/2
5/2
7/5
7/5
7/6
TOC/SOC
(mg/1)
5/3
3/3
6/6
_ TTvii
14/5
10/9
10/5
6/6
5/5
6/6
7/6
4/4
6/6
4/4
3/3
BODs TURB.
(mg/1) (JTU)
3 2.
1.
2 1.
d of Phase
1 2.
3.
2.
3.
3 2.
2.
3.
1.
1.
3.
2.
2.
2.
0
4
7
I -
3
5
6
0
4
7
2
4
2
1
3
3
6
7
7
7
7
7
7
7
7
7
7
7
7
7
7
6
7
PH
.75
.80
.90
.70
.70
.70
.70
.60
.65
.65
.65
.60
.60
.70
.70
.80
UPFLOW
SS/VSS
(mg/1)
4/3
3/2
5/3
2/2
11/8
9/5
11/9
12/10
9/5
7/5
3/3
3/3
10/7
12/10
25/21
5/4
SAND FILTER
TOC/SOC BOD5
(mg/1) (mg/1)
4/3 3
4/4
6/5 2
10/7 2
10/10
6/6
11/7
9/7 ' 6
9/7
8/8
3/3
4/4
10/10
5/1
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(mg/1)
1.7
1.3
2.7
1.5
4.3
4.0
4.5
7.6
4.3
4.3
1.3
1.3
3.7
3.8
7.5
2.4
49
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TABLE A-8. AUGUST 1974 SAND FILTER PERFORMANCE DATA
Date
DOWNFLOW COAL/SAND FILTER
SS/VSS TURB.
pH (rog/1) (JTU)
UPFLOW SAND FILTER
SS/VSS TURB.
pH (mg/1) (JTU)
1 8.05 5/2 1.7 7.90 3/1 1.4
2
3
4
5 - - - 7.55 16/10 8
6 7.65 62/51 22 7.40 59/48 24
7
8
9
10
11
12
13 7.50 15/- 8.5 7.20 24/- 11
14 7.20 5/5 2.4 7.80 6/6 2.6
15
16
17
18
19
20
22
23
24
25
26
27
28
29
30
31
51
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TABLE A-ll. NOVEMBER-DECEMBER 1974 STRAINER PERFORMANCE DATA
Date
11-6
7
8
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
12-2
3
4
SECONDARY INFLUENT STRAINER INFLUENT
Q SS/VSS SS/VSS
(gpd) pH (mg/1) pH (mg/1)
46,100 7.30 128/103 7.40 156/124
40,607 7.30 70/57 7.25 274/110
42,690 7.40 78/57 7.35 242/230
18,270
16,290
46,190 7.65 65/50 7.30 140/105
11,500
41,440
34,940
39,690
28,020
41,520
48,270
<1,000
0
31,060
25,020
37,770
37,020
46,440
39,940
39,190
37,770
43,100
<1,000 7.50 139/121
<1,000
<1,000 7.30 607-
MTXED LIQUOR
pH
7.80
7.25
7.30
7.20
7.30
7.05
7.50
7.40
7.30
7.25
7.40
7.10
7.20
SS/VSS
(mg/1)
3650/3050
5540/4500
5510/4600
6890/5710
5635/4605
4130/3820
3020/2450
2395/1955
3260/2705
13207 -
7760/6540
2000/ -
14707 -
54
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