PHOSPHATE REMOVAL BY ACTIVATED SLUDGE
- Amenability Studies at
Mansfield, Ohio
by
M. R. Scalf, B. L. DePrater, F. M. Pfeffer,
L. D. Lively, J. L. Witherow, and C. P. Priesing*
*The authors are respectively, Research Sanitary Engineer, Research
Chemist, Research Chemist, Research Chemist, Research Sanitary
Engineer, and Acting Chief, Treatment and Control Research Program,
Robert S. Kerr Water Research Center, Federal Water Pollution Control
Administration, U. S. Department of the Interior, Ada, Oklahoma.
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TABLE OF CONTENTS
Item Page
ABSTRACT ii
INTRODUCTION 1
Experimental Procedures 3
RESULTS AND DISCUSSIONS 6
Plant Characteristics 6
Detention Studies 8
Plant Performance 10
Aeration Jug Studies 11
Plant Manipulation 27
Sewage Characterization 29
Microbiological Studies 30
SUMMARY 32
CONCLUSIONS 34
RECOMMENDATIONS 36
APPENDIXES 37
Appendix I - Analytical Procedure 37
Appendix II - Plant Operational Data 42
ACKNOWLEDGMENT 43
TABLES
1 Plant Monitoring Data 12
2 Jug Components — Suspended Solids Variation. . 14
3 Analytical Results — Suspended Solids Variation 14
4 Jug Components--BOD Variation 16
5 Analytical Results—BOD Variation 16
6 Jug Components—Orthophosphate Variation . . 19
7 Analytical Results—Orthophosphate Variation 19
8 Jug Components —Chemical Addition 23
9 Analytical Results—Chemical Addition .... 23
FIGURES
1 Sewage Treatment Plant, Mansfield, Ohio ... 7
2 Dye Detention in Aeration Tanks ... 9
3 Effect of BOD on Phosphate Removal ... 18
4 Effect of Phosphate Concentration on Removal- 21
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ABSTRACT
Biological phosphate removal was investigated in pilot and plant
scale at Mansfield, Ohio, to determine waste and sludge amenability
and the suitability of the activated sludge plant for a full-scale
research or demonstration project. Pilot studies on the effect of
MLSS, BOD, phosphate, and hardness concentrations showed only BOD
exerted a significant change in phosphate removal. The addition of
iron or aluminum salts effectively precipitated the phosphate.
Amenability was established following sludge acclimatization when
high levels of phosphate were removed from the primary effluent
supplemented with a BOD material. Plant studies prior to and
following manipulation of the operating conditions did not reveal
phosphate removal. The plant is not suitable for full-scale research
or demonstration studies without design modifications.
11
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INTRODUCTION
This report describes investigations at the Mansfield, Ohio,
Sewage Treatment Plant concerned with the removal of phosphate
from municipal sewage by the activated sludge process. Field
research conducted at San Antonio, Texas, established that high
removals of orthophosphate from the liquid to the suspended solids
were accomplished in the aeration tank. The efficiency of the
process was found at San Antonio to be controlled by operational
and design parameters including aeration detention times, mixed
liquor suspended solids (MLSS) and dissolved oxygen (DO) concen-
trations, biochemical oxygen demand (BOD) and phosphate loads,
rapid solid-liquid separation, minimum solids detention time in
the final clarifiers, and separate disposal of the phosphate
concentrates.
The purposes of the amenability studies conducted were:
1. To determine if biological phosphate removal is feasible
in municipalities of various geographical locations, populations
served, and sewage characteristics by pilot and plant investi-
gations .
2. To locate activated sludge plants in various geographical
regions to be used for demonstration of biological phosphate removal
by operation in accordance with parameters identified at San Antonio.
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3. To locate one activated sludge plant of suitable design and
operational flexibility for use as a full-scale research plant to
further specify, define, and optimize the biological phosphate
removal process.
4. To verify on pilot scale that biological phosphate removal
can be controlled by the parameters identified at San Antonio
and/or isolate and identify additional controlling parameters.
Aerated jugs of mixed liquor have been previously shown to
simulate an aeration tank. Aerated jugs circumvent many of the
limitations exerted by normal operational characteristics in
activated sludge plants, such as control of aeration rate, sewage
flow, suspended solids concentration, and phosphate and BOD load-
ings. Sludge solids and waste from various points in the secondary
system may be conveniently studied. Detention time of the mixed
liquor in the aerated jug system is rigidly controlled. For these
reasons, the method is selected for testing the amenability of the
waste and activated sludge to phosphate removal.
By deliberate variation of the operational parameters, the
conditions for sludge response and maximum phosphate removal can
be established. These amenability studies indicate those waste
and activated sludges most readily adaptable and, in conjunction
with plant investigations, aid in specifying operational levels,
design changes, or time for sludge adaptability necessary to attain
phosphate removal in the full-scale plant.
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Experimental Procedures
Plant Analysis
The plant was surveyed to determine pertinent design and
operational characteristics. Programs of plant sampling and
tracer studies were carried out in addition to jug studies to
determine plant performance.
Plant records were examined for sewage flow, concentrations
of BOD, suspended solids, and orthophosphate, other chemical and
physical parameters, and performance characteristics. Typical
data are presented in Appendix II. Plant personnel provided
information on sampling practices, analytical procedures,
anomalies of wastes being treated, and peculiarities of oper-
ational control. The schedule for wasting raw and activated
sludges was determined. Disposal methods of waste activated
sludge, primary sludge, digester supernatant, sludge thickener
supernatant, drying bed underflow, and other waste streams were
studied in anticipation of auxiliary, inhibitory, or latent
effects on the phosphate removal process.
Tracer studies were conducted using Rhodamine WT dye on
aeration tanks and final clarifiers to determine hydraulic
characteristics such as detention, short circuiting, and degree
of longitudinal mixing.
Grab samples were collected to determine the phosphate
levels and degree of removal. These samples were analyzed for
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concentration of orthophosphate and, occasionally, total phosphates,
suspended solids, total oxygen demand (TOD), nonvolatile organic
carbon (NVOC), and soluble nonvolatile organic carbon (SNVOC).
Chemically fixed samples were shipped to the Robert S. Kerr Water
Research Center at Ada, Oklahoma for total phosphate analyses.
The sample sources were raw sewage, primary effluent, return sludge,
aeration tank influent, aeration tank effluent, and final effluent.
Occasionally, supernatant samples were taken from digestion tanks
and sludge thickeners. Dissolved oxygen was measured in con-
junction with the sampling program. If phosphate was being removed,
more detailed sampling on a slug-flow basis was undertaken to
specify the magnitude of removal.
Aeration Jug Studies
Return sludge or mixed liquor from various points in the
secondary system was mixed with primary effluent or raw sewage to
obtain the desired test conditions. The mixtures were prepared
in five-gallon polyethylene jugs for subsequent aeration.
A portable air compressor produced the air supply. Air was
delivered through a manifold containing individual needle valves
and rotometers for each jug. Polyethylene tee fittings were used
as diffusers. The air flow was maintained constant throughout
the experiment, except where noted.
The dissolved oxygen content and the temperature of the mixed
liquor were monitored several times during each run.
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Samples were withdrawn from the jugs usually at half-hour or
hourly intervals. Preceding sample collection, approximately
200 ml of mixed liquor was siphoned through a plastic withdrawal
tube for purging and returned to the original jug. A volume of
100 ml was sufficient for orthophosphate analysis, and 250 ml was
taken when additional analyses were planned.
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RESULTS AND DISCUSSIONS
Plant Characteristics
The Mansfield, Ohio, Sewage Treatment Plant shown in Figure 1
is an activated sludge system which treats an average of 8-9
million gallons per day (mgd). The system has three tanks for
preaeration, grit, and grease removal; two primary clarifiers;
six aeration tanks; and two final clarifiers. Approximately 90
and 95 percent of the BOD and suspended solids, respectively, are
removed by these processes.
Four of the six aeration tanks were in operation during these
studies. Compressed air at a rate of about 6.0 million cubic feet
per day (mcfd) was supplied to these tanks by one of four blowers.
(Another 0.5 mcfd of air was delivered to the preaeration tanks.)
The aeration tanks were operated as a conventional activated sludge
system; however, design of the tanks also permits operation of a
step aeration process, reaeration, or combinations of these. Return
sludge rates varied from about 20 to 50 percent of the sewage flow
during these studies.
The primary sludge and excess activated sludge are pumped to
one of two 0.15 million gallon sludge thickeners. Thickened
sludge is pumped to three primary digesters and thence to a single
secondary digester. Sludge from the secondary digester is
dewatered by vacuum filtration for land disposal. The gas produced
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in the digesters is used as fuel for the boilers and gas engines
that drive the blowers. Filtrate from vacuum filters and super-
natant from thickeners and digesters are returned to the primary
system.
Detention Studies
Tracer studies were conducted on two of the aeration tanks on
May 16, 1967. Aeration Tanks No. 5 and 6 use longitudinally placed
air diffusion tubes while Aeration Tanks No. 3 and 4 use transverse
tubes. To determine the difference in the hydraulic efficiencies
of the two systems, Tanks No. 4 and 5 were selected for dye
tracing.
Two liters of 20 percent Rhodamine WT solution were introduced
to the influent end of both aeration tanks, and dye concentrations
were monitored at the effluent ends. Dye dispersion curves are
presented in Figure 2.
Both tanks exhibited appreciable short circuiting with 90
percent of the tracer reaching the effluent within the theoretical
displacement time. Tank No. 4, which has transverse diffusion
tubes, demonstrated a peak tracer concentration at 30 percent of
the theoretical detention time. Tank No. 5 with longitudinal
diffusion tubes showed a peak concentration at 60 percent of the
theoretical detention time. The tanks with longitudinal diffusion
tubes were hydraulically nearer plug flow than the tanks with
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10
transverse tubes, but the dye studies indicate that much of the
waste to both tanks receives less than three hours' detention
time. These conditions are not considered conducive to good
phosphate removal.
Plant Performance
Examination of plant records indicated that raw waste to the
plant had an average BOD of about 190 mg/1. Approximately five
hours' displacement time in the preaeration tanks and primary
clarifiers reduces the BOD to less than 100 mg/1 in the primary
effluent. Further reduction in the aeration tanks and final
clarifiers produces a final effluent with 15 to 20 mg/1 BOD and
suspended solids.
Dissolved oxygen concentrations were, unusually high in the
raw sewage and in the primary clarifiers, often 1 to 2 and 3 to
4 mg/1, respectively; however, about 6 mcfd of compressed air or
0.7 cubic feet per gallon of waste was insufficient to maintain
dissolved oxygen concentrations above 1.0 mg/1 in the aeration tank
effluent. As a result of the high primary effluent concentrations,
dissolved oxygen was often significantly higher in aeration tank
influents than effluents.
The plant was usually operated with mixed liquor suspended
solids between 3,500 and 4,500 mg/1. The return sludge contained
0.5 percent total phosphate as P on a dry weight basis. Soluble
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11
phosphate and BOD loadings were about 0.3 Ib. P/day/100 Ibs. MLSS
and 9 Ibs. BOD/day/100 Ibs. MLSS, respectively.
Mansfield has a significant amount of industry including some
metal plating. The waste to the sewage treatment plant formerly was
subject to heavy concentrations of chromium and, on some occasions,
still exhibits appreciable concentrations. On May 20, 1967, the
raw waste was green, and a grab sample of the primary effluent
contained 50 mg/1 total chromium. Although the toxicity of this
concentration of hexavalent chromium on activated sludge organisms
has not been found significant,1 other metals associated with the
metal plating industry such as copper, cadmium, and zinc could be
present in concentrations detrimental to biological activity.
Samples collected and analyzed during approximately 10 days
of studies at the Mansfield Sewage Treatment Plant indicated that
the plant did not remove significant amounts of phosphate either
in the aeration tanks or through the plant as a whole. Soluble
phosphate concentrations in the raw waste ranged from 2.7 to 4.3
mg/l-P and in the final effluent from 2.1 to 5.8 mg/l-P. Results
of plant sampling are presented in Table 1.
Aeration Jug Studies
A series of experiments was devised to test the amenability
of sewage and sludge to orthophosphate removal. Factors tested
for their effect on the rate and magnitude of orthophosphate
1 U. S. Department of Health, Education, and Welfare, Public Health
Service, "Interaction of Heavy Metals and Biological Sewage Treatment
Processes," Public Health Service Publication No. 999-WP-22. May
1965.
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removal were: suspended solids, BOD, and orthophosphate concen-
trations; metal precipitants; and sludge condition. Results from
these experiments served as a guide to the utilization of plants
for process demonstration.
Suspended Solids Variation
Studies at other activated sludge plants have shown that high
orthophosphate removal occurs within a limited range of mixed liquor
suspended solids concentrations; outside this range percent removal
decreases.
To determine the optimum suspended solids concentration, a
series of jug experiments was performed on May 17, 1967. Seven
jugs were prepared for aeration with MLSS concentrations ranging
from 450 to 4,300 mg/1. Jug components are recorded in Table 2
and results in Table 3.
The results were reasonably consistent with respect to initial
and final orthophosphate concentration. There was a tendency for
percent removal to increase (28 to 42 percent) with increasing
suspended solids concentration; however, significant levels of
removal were not obtained, and an optimum MLSS concentration was
not determined. The remaining jug studies were run at a concen-
tration range considered practical for the Mansfield Plant, i.e.,
3,000 to 3,500 mg/1.
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14
TABLE 2. Jug Components—Suspended Solids Variation
Jug No.-/
1
2
3
4
5
6
7
Primary
Effluent
Final
Effluent
Return
Sludge
(Liters of Component Added)
10 4.6 0.4
10 4.2 0.8
10 3.8 1.2
10 3.4 1.6
10 3.4 2.0
10 1.3 3.7
= ,Aeration rate for all jugs - 15 liters per minute.
-Mixed liquor from aeration tank influent.
Mixed „ ,
Liquor-
15
TABLE 3. Analytical Results—Suspended Solids Variation
Jug No.
Aeration 1
Time-Hrs
0 (11:15 am) 2.5
1 1.8
2 2.0
3 1.8
4 1.8
5 1.8
6 1.8
ML Settled
(30 min) 1.8
% Removal 26.5
MLSS (mg/1) 454.0
Note: During the
2
2.5
1.8
2.0
1.8
1.8
1.8
1.8
1.8
26.5
937.0
aeration
3456
Orthophosphate (mg/1 P)
2.8
2.0
2.0
1.8
2.0
1.8
1.8
2.0
35.2
1369.0
period,
2.6
2.0
2.0
1.8
1.8
1.8
1.8
2.0
31-. 1
1794.0
mixed
2.8
2.0
1.8
1.8
1.8
1.6
1.8
1.8
35.2
2251.0
2.6
1.8
1.6
1.6
1.8
1,6
1.6
1.8
37.5
4300.0
7
3.1
2.0
1.6
1.6
1.6
1.8
1.8
1.8
42.1
3154.0
liquor temperature
increased from 14 to 18°C. Similarly, dissolved oxygen
concentration fluctuated randomly between 8.5 and 10 mg/1.
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15
Jug No. 6, composed of mixed liquor from the influent end of
an aeration tank, served as a control for comparing removal
efficiency between mixed liquor prepared in the plant and in the
jugs. The removal efficiency of this jug was most comparable to
that of Jug No. 7, which also had a comparable suspended solids
concentration.
BOD Variation
Studies at other activated sludge plants have indicated
that orthophosphate removal is also affected by primary effluent
BOD concentration. On May 18, 1967, a set of jugs was prepared to
determine the magnitude of this effect.
The components of the aeration jugs are shown in Table 4 and
analytical results in Table 5. The BOD's of the jug components
were not measured directly but were roughly estimated by adding to
each jug a known quantity of Metrecal, which had a BOD of 290 mg/ml.
Generally, the jug studies were based on a substrate or primary
effluent volume of 10 liters with the remaining 5 liters a mixture
of concentrated return sludge and final effluent to provide the
desired solids concentration. In this experiment, Jug No. 1 was
considered the base with 10 liters of primary effluent estimated to
have a BOD equivalent to the "normal" or annual average, i.e.,
95 mg/1. Metrecal was then added to the primary effluent for Jugs
No. 2 through 5 to increase the BOD concentrations by a factor of
1.8, 2.3, 3.1, and 4.7, respectively.
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16
TABLE 4. Jug Components—BOD Variation
I/
Substrate
Jug No.-7 (Est. BOD)
mg/1
I
2
3
4
5
6
7
95
173
220
295
440
95
190
ml
2.7
4.3
6.9
12.0
Primary
Effluent
Liters
10
10
10
10
10
~"~" 1 /
4 /
10-'
Return
Sludge
Final
Effluent
Mixed
Liquor
of Component Added
2.7
2.7
2.7
2.7
2.7
—
2.7
2.3
2.3
2.3
2.3
2.3
__
2.3
__
—
—
—
—
15
—
•= -Aeration rates for all jugs - 15 liters per minute.
^Metrecal contains 1.1 rag/ml 0-PO, and BOD of 290 mg/ml.
- Raw sewage used in lieu of primary effluent.
TABLE 5. Analytical Results—BOD Variation
Jug No.
Aeration
Time
Hrs.
0 (10:00 am)
1
2
3
4
5
ML Settled
(1 Hour)
% 0-PO,
Removed 17.9 38.3 50.0 59.0 66.7 16.1 32.4
MLSS (mg/1) 3440.0 3214.0 3356.0 3434.0 3244.0 4374.0 3346.0
Note: During the aeration period, mixed liquor temperature increased
from 14 to 17°C. Similarly, dissolved oxygen concentration
increased from 4 to 8 mg/1.
1
2.8
2.5
2.3
2.3
2.3
2.3
2.5
2
3.4
2.5
2.1
2.0
2.0
2.1
2.1
345
Orthophosphate (mg/1 P)
3.6
2.5
2.0
2.0
1.8
1.8
2.0
3.9
2.5
1.8
1.8
1.6
1.6
1.6
3.9
2.5
1.5
1.5
1.3
1.3
1.3
6
3.1
2.8
2.5
2.5
2.5
2.6
2.6
7
3.4
2.5
2.1
2.3
2.1
2.3
— —
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17
Jug No. 6 was composed of mixed liquor from the effluent end of
the aeration tanks. It served as a control for comparison of plant
and synthetic jug mixed liquor. The removal efficiency of this jug
was comparable to Jug No. 1 which contained no supplemental BOD.
The raw sewage, with an annual average BOD of 190 mg/1, was
used in Jug No. 7 in lieu of primary effluent to increase the esti-
mated BOD concentration of the substrate to twice that of the
primary effluent. The orthophosphate removal efficiency in this
jug was most comparable with Jug No. 2 which was supplemented with
Metrecal to produce an estimated BOD 1.8 times that of a "normal"
primary effluent.
Figure 3 presents percent orthophosphate removal versus esti-
mated BOD concentration for the jugs in this experiment plus three
other jugs aerated during a later study on orthophosphate variation.
Orthophosphate removal generally increased with increased BOD with
the greatest effect at estimated BOD concentrations less than about
300 mg/1. Thus, benefit from supplemental BOD would be limited
at concentrations above 300 mg/1. Subsequent jug studies were run
with supplemented BOD loading to obtain maximum phosphate removal.
Orthophosphate Variation
A set of six jugs was prepared on May 19, 1967, to determine
the effect of orthophosphate concentration on its removal at the
Mansfield Plant. Jug components are given in Table 6 and analytical
results in Table 7.
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TABLE 6. Jug Components—Orthophosphate Variation
Ju&
No.
Substrate
(Est. BOD)
mg/1
Metrecal
ml
Primary
Effluent
Return
Sludge
Final
Effluent
Mixed
Liquor
mg-P
Liters of Components Added
1 95
2 790 24 10 2.7
3 480 15 — 5 2.7
4 440 12 50 10 2.7
5 440 12 450 10 2.7
6 440 12 — 10 2.7
-^Aeration rate for all jugs - 15 liters per minute.
•=.City water in lieu of final effluent.
-0.4 liters each of thickener and digester supernatant added.
2.3
7.3-
2.3
15
TABLE 7. Analytical Results—Orthophosphate Variation
Jug No.
Aeration
Time-Hrs
0 (11:00 am)
1
2
3
4
5
6
ML Settled
(30 Min.)
3.8
3.4
3.4
2.6
2.5
2.8
3.0
3.0
0-PO Removed
(mg/1)
234
Orthophosphate (mg/1 P)
3.8
3.4
3.4
2.6
2.5
2.8
3.0
3.0
0.8
4.6
2.8
2.1
1.1
0.8
—
0.7
0.7
3.9
2.3
1.0
0.6
0.3
0.3
—
0.5
0.5
1.5
6.6
5.4
5.2
3.1
3.4
—
3.1
3.4
3.2
37
34
31
26
24
—
23
22
5.0
3.8
2.6
2.1
—
1.3
—
—
1.3
2.5
Removed 21.0 84.8 78.3 53.0 38.8 65.8
MLSS (mg/1) 4494.0 3595.0 3208.0 3464.0 3532.0 3366.0
Note: During the aeration period, mixed liquor temperature increased
from 15 to 23°C. Similarly, dissolved oxygen concentration
increased from 3 to 8 mg/1.
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20
Jug No. 1 contained plant mixed liquor for removal efficiency
comparison with phosphate and BOD supplemented and synthesized mixed
liquor. Tap water was used in Jug No. 3 to reduce the orthophosphate
concentration to approximately 50 percent of that in the control
jug, and additional Metrecal was added to compensate for BOD reduction.
Potassium dihydrogenphosphate was added to Jugs No. 4 and 5 to produce
initial soluble phosphate concentrations of about 7 and 35 mg/1 as P,
respectively. In addition, 0.4 liter each of thickener and digester
supernatant was added to Jug No. 5. The high orthophosphate load
was added to simulate potential levels should thickener and digester
supernatant streams be returned to the activated sludge system.
Jug No. 6 served as a control for the study. Metrecal addition to
Jugs No. 3 through 6 increased BOD loading to the level which gave
highest phosphate removal in the BOD variation study. Jug No. 2
had twice the quantity of Metrecal used in the control jug to
obtain supplemental data on BOD addition. Mixed liquor suspended
solids concentrations ranged from about 3,200 to 4,500 mg/1.
Figure 4 indicates that supplemental orthophosphate (Jugs No,
4 and 5) increased the magnitude of removal but decreased the
percent removal. Jug No. 3 with decreased phosphate showed decreased
magnitude but increased percent removal, and a better quality effluent
was obtained. Thus, supplemental phosphate was detrimental to the
phosphate removal process in terms of both percent removal and
effluent quality.
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234
AERATION TIME (MRS.)
6
FIGURE 4 — EFFECT OF PHOSPHATE CONCENTRATION
ON REMOVAL
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22
Jug No. 2 with an estimated substrate BOD of 790 mg/1 removed
about 20 percent more orthophosphate than the control (Jug No. 6)
which contained an estimated 440 mg/1 substrate BOD. These results
agree with those discussed in the previous experiment and are shown
in Figure 3.
Chemical Addition
Three jugs were set up May 21, 1967, to determine the effect
of iron and aluminum salts on orthophosphate removal. The mixed
liquor was prepared using primary effluent, concentrated return
sludge, and final effluent. Jug components are recorded in Table 8
and results in Table 9.
Jug No. 1 served as a control while Jugs No. 2 and 3 were dosed
with 25 mg/1 of ferric and aluminum ion, respectively. Aluminum
sulfate and ferric chloride were the chemicals used; pH was not
controlled. Orthophosphate concentrations of the primary effluent
plus final effluent components before chemical and return sludge
additions ranged from 4.0 to 4.2 mg/1 as P. After chemical
addition and thorough mixing, orthophosphate concentration was
determined on the contents of Jugs No. 2 and 3 and before the
return sludge was added and on all the jugs just after the addition
of return sludge.
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23
TABLE 8. Jug.Components—Chemical Addition
Jug No. -
Primary
Effluent
Return
Sludge
Final
Effluent
Liters of Components Added
Fe
4-3
Al
+3
mg/1 Added
1
2
3
4
5
10
10
10
103
10-'
2.7
2.7
2.7
2.7
2.7
2.3
2.3
2.3
2.3
2.3
25
25
-Aeration rate for all jugs - 15 liters per minute. 12 milliliters
of Metrecal was added to all jugs to increase the estimated substrate
,BOD to 440 mg/1.
0/8.6 liters of deionized PE plus 1.4 liters straight PE.
-4.2 liters of deionized PE plus 5.8 liters straight PE.
TABLE 9. Analytical Results—Chemical Addition
Aeration
Time-Hrs
1
i /
0 (12:30 pm)i/ —
0—'
1
2.5
3.5
4
ML Settled
(30 Min.)
% 0-PO.
4
Removed
4.5
4.7
3.9
3.9
3.8
4.4
15.5
Jug No.
234
Orthophosphate (mg/1 P)
0.7
1.2
0.3
0>24/
0-2-'
—
0.2
95.6
0.4
0.1,.
0.2-3/
—
—
—
0.2
95.6
—
4.5
5.1
4.8
4.8
4.8
5.7
—
5
—
4.2
5.5
4.6
4.4
4.5
5.1
—
MLSS (mg/1) 2628.0 2836.0 2686.0 2740.0
w,Primary effluent plus chemical additive
•5,After return sludge addition.
T/Aeration stopped at 1 hour and ML settled.
- Aeration stopped at 3.5 hours and ML settled.
2694.0
Note; Temperature of the jug mixed liquor ranged from 14-18 C.
Initial and final dissolved oxygen concentrations of the
jug mixed liquor were about 4 and 8.5 mg/1.
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24
Two additional jugs were prepared to determine the effect of
hardness on orthophosphate removal. Primary effluent was passed
through a cation exchange resin to reduce the hardness. The mixed
liquor was prepared using concentrated return sludge, final effluent,
and appropriate quantities of unaltered and deionized primary
effluent. Jugs No. 4 and 5 were prepared with hardness concentrations
of 75 and 250 mg/1, respectively.
Mixed liquor suspended solids in the five jugs ranged from about
2,500 to 3,000 mg/1. Metrecal was added to increase the BOD concen-
tration to an estimated 300 mg/1.
Addition of iron and aluminum resulted in immediate orthophosphate
removal. Aluminum produced the most dramatic effect in that greater
than 90 percent of the orthophosphate was immediately precipitated.
The jug containing iron removed 83 percent by precipitation and
greater than 95 percent overall after two and one-half hours of
aeration time. Only 16 percent orthophosphate removal occurred in
the control jug compared to about 60 percent in previous jugs with
similar contents.
Phosphate removal was not accomplished in Jugs No. 4 and 5.
Both jugs indicated a greater quantity of orthophosphate at the end
of the experiment than at the beginning. Decreasing the hardness
was not beneficial to phosphate removal. However, the previous
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25
high phosphate removals under conditions similar to the control jug
suggest that the slug of metal waste in the sewage on May 20 may
have altered the phosphate removal characteristics of the sludge.
Sludge Adaptation
On June 19, a study was begun to acclimate sludge microorganisms
to environmental conditions suitable for phosphate removal. The
activated sludge biota was maintained at a concentration of 2,000-
4,000 mg/1 suspended solids in the presence of sufficient supply
of oxygen and food for several days.
Jug No. 6, which was used as a control during the phosphate
concentration experiment on June 19, was used for the acclimatization
procedure. Following the initial six hours of aeration when 65
percent of the soluble phosphate was removed, 400 milliliters of
a Metrecal-phosphate-distilled water solution (174 mg/ml BOD;
0.3 mg/ml P) was fed continuously at a rate of 0.5 ml/min, during
overnight aeration. Decanting the supernatant from the settled
mixed liquor in the jug and the addition of fresh primary effluent
to the sludge were planned for the following day; however, because
the primary effluent contained a high chromium concentration, no
fresh primary was added. Instead, 600 milliliters of Metrecal-
phosphate-distilled water solution was added during aeration
throughout the next day and again overnight.
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26
On May 21, the contents in the jug were allowed to settle, the
supernatant was decanted and replaced with 10 liters of primary
effluent and 12 milliliters of Metrecal. Five hours of aeration at
15 liters of air per minute reduced the soluble phosphate 58 percent
from 5.9 to 2.5 mg/1 as P. A control jug, with identical primary
effluent but containing fresh sludge, was aerated simultaneously
and removed less than 20 percent of the soluble phosphate. The
contents of the test jug were again settled, decanted, and charged
with 10 liters of fresh primary effluent. About 400 milliliters of
the Metrecal feed solution was again added during overnight aeration.
On May 22, the jug was again decanted and fresh primary effluent
with a 24 milliliter slug of Metrecal was added. During the sub-
sequent 8 hours of aeration at 15 liters of air per minute, soluble
phosphate was reduced 86 percent from 9.6 to 1.3 mg/1 as P.
This acclimation test established that the activated sludge
at Mansfield, Ohio, could be highly amenable to phosphate removal
under optimum conditions of operation. Although increased removals
resulted from operation at increased concentrations of suspended
solids, dissolved oxygen, and BOD in addition to an adapted sludge,
none of these parameters have been sufficiently specified for trans-
lation into plant operating criteria at Mansfield.
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27
Plant Manipulation
Following the amenability studies at Mansfield, Ohio, minimal
operational changes were made to induce phosphate removal in the
plant. Although phosphate removal had not been obtained in the
plant during these studies, high removals were accomplished in
the jugs at high dissolved oxygen concentrations by increasing
the BOD concentration. Therefore, operational modifications were
designed to increase the BOD load to the plant and increase dissolved
oxygen in the aeration tank effluents. The changes agreed upon by
the city and plant officials were:
1. Increase rate of return sludge and wasting of sludge to
reduce and maintain mixed liquor suspended solids concen-
trations at 2,000 to 3,000 mg/1.
2. Place all six aeration tanks in service and operate one
blower at full capacity. If DO concentrations in aeration
tank effluents remain below 1.0 mg/1, utilize two blowers
at full capacity.
3. Discharge all digester supernatant and sludge filtrate to
lagoons to prevent recirculation of phosphate.
These conditions were to be maintained for about two weeks
before monitoring the plant again for phosphate removal. If no
removal was occurring, it was agreed that one of the primary
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28
clarifiers would be removed from operation to increase the BOD load
to the secondary system. Following another stabilization period, the
plant was to be monitored again for phosphate removal.
Grab samples collected on June 21, 1967, two weeks after initial
changes were made, indicated that DO concentrations in the aeration
tank effluents were consistently less than 1.0 mg/1. Analyses of
samples collected also indicated that no significant phosphate
removal was occurring in the plant.
Following failure of initial modifications to induce phosphate
removal at Mansfield, two blowers were operated to increase the DO
concentration. In addition, one of the primary clarifiers was
taken out of service to increase the BOD load to the aeration tanks.
After operation for one month under these conditions, the plant was
again monitored on July 19, 1967. The DO levels had increased but
were not consistently above 1.0 mg/1 in the aeration tank effluent.
Grab samples indicated that there was still no phosphate removal
through the aeration tanks.
Following the July 19 visit, both blowers were increased to
their full capacity for a two-week stabilization period. The last
visit on July 30 and 31 showed dissolved oxygen concentrations at
the effluent ends of the aeration tanks from 0.3 to 5.1 mg/1; how-
ever, grab samples collected continued to indicate no phosphate
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29
removal as noted in Table 1. During this last monitoring, two
previous days of rain had diluted the BOD concentration, and the
raw and primary effluent exhibited a yellowish-green color of
chromium waste.
Although dissolved oxygen concentrations in all the aeration
tank effluents were not maintained at satisfactory levels for an
extended period of study, it was concluded that modifications other
than the increased air and BOD would be necessary for phosphate
removal. The aeration tanks would require physical modification
to reduce short circuiting and increase plug-flow conditions and
mixed liquor detention time.
Sewage Characterization
A related purpose of the amenability analytical program was to
characterize samples from the various unit processes of activated
sludge plants with regard to selected chemical parameters.
Additional samples from an aeration jug study were included in the
characterization scheme. These samples were taken from a jug
study whose operations and constituents were those which resulted
in maximum phosphate removal. Primarily, the intention was to
search for trends or correlatable functions within or between
various chemical parameters which would be useful in defining
the phosphate removal process. Frequently, phosphate removal
realized from jug aeration was higher than that occurring in the
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30
aeration tanks. During such instances, the probability of identify-
ing gross differences of possible significance was greatly enhanced.
The samples were analyzed with and without solids to differen-
tiate between the quantity of each respective chemical parameter
associated with the solids and that associated with the liquid.
Solids separation was accomplished by first decanting, then sub-
jecting the resulting supernatant to further solids removal using
a Sharpies Super-Centrifuge. Samples resulting from such treatment
are referred to as centrates of the original sample.
The samples characterized and the results therefrom will not
be discussed in this report since the basic purpose was for com-
parison with similar data from other studies. Data and comparisons
are made and reported under separate cover.2
Microbiological Studies
The microbiological studies conducted at the plants tested for
amenability to phosphate removal were divided into two phases.
Phase I consisted of selection of predominant colonial types, trans-
fer of such to agar slants, and shipment to the Ada laboratory for
identification. The final portion of this phase is under way. The
results from Mansfield and the other amenability studies are a separate
2Lively, L. D., et al., "Phosphate Removal by Activated Sludge,
Waste Characterization," Internal Report, USDI, FWPCA, Robert S.
Kerr Water Research Center, Ada, Oklahoma. 1968.
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31
report.3 Phase II consisted of determining bacterial number by
the total plate count method from various plant and aeration jug
samples. A correlation was sought relating bacterial population
with phosphate removal and suspended solids concentration.
There is no apparent relationship between suspended solids
content and bacterial contents. The bacterial counts remain
essentially constant through the aeration tanks and during the
course of the jug runs. Bacterial counts and phosphate removal,
if related, were not detectable by the methods employed.
3Moyer, J. E., et al., "Survey of Activated Sludge Treatment Plants
for Predominant Bacterial Types," Internal Report, USDI, FWPCA,
Robert S. Kerr Water Research Center, Ada, Oklahoma. 1968.
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32
SUMMARY
Studies conducted at the activated sludge plant of Mansfield
Ohio, from May 16 to May 26, and follow-up investigations on three
other occasions after plant manipulation during June and July 1967,
revealed no phosphate removal by the plant.
Dye tracer studies conducted on the aeration tanks showed
appreciable short-circuiting, especially those tanks containing
transverse air diffusers. Air supplied to these tanks during the
experimental periods was insufficient to maintain dissolved oxygen
concentrations consistently above 1.0 mg/1.
The industry in Mansfield, including metal plating, subjected
the waste influent to occasional high concentrations of metals.
One grab sample had a chromium concentration of 50 mg/1 indicating
the presence of associated metals of the metal plating industry
such as copper, cadmium, and zinc which could be detrimental to
biological activity.
The plant usually operated within a mixed liquor suspended
solids concentration range of 3,500 to 4,500 mg/1, and the return
sludge rate varied from about 20 to 50 percent of the sewage flow.
Air supplied averaged approximately 0.7 cubic foot of air per gallon
of waste treated.
-------
Min.
:.7
1.1
1.9
Max.
4.3
3.8
5.3
Min.
4.4
3.7
4.5
Max.
9.9
13.4
7.3
(.Urtho/Total)
0.5
0.5
0.7
33
Phosphate concentrations in the waste streams during the study
were:
Source Ortho (mg/l-P) Total (mg/l-P) Average Ratio
Mil
Raw Sewage 2.7
Pri. Effl. 2.1
Final Effl. 3.9
The return sludge contained 0.5 percent total phosphate as P
on a dry weight basis. Soluble phosphate and BOD loadings were
0.3 Ib. P/day/100 Ibs. MLSS and 9 Ibs. BOD/day/100 Ibs. MLSS,
respectively.
Pilot studies were conducted in aerated jugs to determine the
effects of varying concentrations of MLSS, BOD, phosphate, hardness,
and iron and aluminum salts. With the exception of iron and
aluminum salts which precipitated the phosphate on contact, only
BOD concentration exerted a significant effect on phosphate removal
within the ranges studied.
Phosphate removal was increased in the aeration jugs from less
than 20 percent where no supplemental BOD was added, to almost 85
percent when about 700 mg/1 of supplemental BOD in the form of
Metrecal was added to the substrate at the start of the aeration
period,
An "acclimation" process produced a sludge which, with the
addition of BOD, removed 86 percent of the soluble phosphate during
an 8-hour aeration period.
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34
CONCLUSIONS
1. The Mansfield, Ohio, activated sludge plant, at the time of
these investigations, was not removing phosphate.
2. The aeration tanks exhibit a high degree of short-circuiting
which is not conducive to good phosphate removal.
3. The aeration tanks with transverse diffuser systems exhibit
greater short-circuiting than those tanks with longitudinal
diffusers.
4. The air supplied and the hydraulic conditions in the aeration
tanks are insufficient to maintain dissolved oxygen concentrations
necessary for phosphorus removal.
5. The detention time in the preaeration and grit tanks and primary
clarifiers is long, and the primary effluent BOD may contain
insufficient substrate for the development and maintenance of
a phosphate removing sludge.
6. The phosphate removal in jug studies is greatly increased by
the addition of substrate in the form of Metrecal.
7. Varying mixed liquor suspended solids between about 1,300 and
4,300 mg/1 has little effect on phosphate removal.
8, Within the ranges studied, increased hardness concentrations
did not significantly increase phosphate removal.
9. Iron and aluminum salts were very effective in precipitating
phosphate with aluminum having the most immediate effect.
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35
10. The failure to induce phosphate removal in the plant by limited
operational modifications was attributed to (a) the poor
hydraulic conditions in the aeration tanks, (b) failure to
maintain sufficient DO in the aeration tank effluent, (c)
insufficient BOD load to the aeration tanks, and (d) the
possibility that slugs of metal wastes were detrimental to
biological activity.
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36
RECOMMENDATIONS
The Mansfield, Ohio Sewage Treatment Plant is not recommended
for demonstration of phosphorus removal without design modifications
and operational changes. The changes necessary prior to such utili-
zation are:
1. Convert the six single-pass aeration tanks into two
three-pass tanks, all with longitudinal diffusers, to
improve hydraulic efficiency.
2. Increase the air supplied to the aeration tanks, if
necessary after tank modification, to maintain aeration
tank effluent dissolved oxygen concentrations at 2 to 4
mg/1.
3. Prevent return of sludge thickener supernatant and
digester supernatant to the system unless phosphate
is chemically precipitated beforehand.
Another amenability study would be desirable before undertaking
a full-scale program.
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37
APPENDIXES
Appendix I
Analytical Procedures
by
B. L. DePrater
Sample Preparation
Samples collected for analyses in the field were processed as
outlined in the respective analytical test procedures.
Samples returned to the Robert S. Kerr Water Research Center
are referred to as whole, centrate, whole fixed, and centrate fixed.
The following table lists the treatment each type received prior
to shipment to Ada.
Treatment
Sample
1. Whole
2. Whole Fixed
3. Centrate
4. Centrate Fixed
Shipped as is with no treatment.
Shipped as is plus 1 ml cone, sulfuric
acid per liter of sample.
The sample was passed through a Sharpies
motor driven laboratory model continuous
centrifuge, equipped with a clarifier
bowl driven at 23,000 rpm. The sample
was delivered to the centrifuge by a
peristaltic pump at a feed rate of 150
ml/min. The bowl was cleaned and rinsed
with distilled water after each sample.
The centrate sample plus 1 ml/1 cone.
sulfuric acid.
All samples were shipped in either 250 ml polyethylene bottles
or 1,000 ml cubetainers.*
*Hedwin Corporation, 1600 Roland Heights Avenue, Baltimore, Maryland
21211.
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38
Chemical Tests
1. Orthophosphate
In the field, initial samples for orthophosphate were
filtered immediately using Schleicher and Schuell No. 588 paper.
Subsequent analysis by the stannous chloride procedure in Standard
Methods5 included the use of a B&L Spectronic 20 at 690my.
A continuous automatic sampling device, built specifically
to support jug-study phosphate analyses, supplied whole samples to a
Technicon AutoAnalyzer platformed according to the method by Gales and
Julian. The Manifold and reagents were modified, however, to more
closely approximate Standard Methods.5 The arrangement, in order of
sequence, was as follows:
a. A six-port peristaltic pump circulating jug mixed liquor
continuously.
b. An open-shut solenoid valve system selectively sampling
the flow from a T-connection in each circulating jug line.
GO A stepping relay alternately activating one of six jug
sample solenoids or the solenoid to a distilled wash water supply.
5Standard Methods for the Examination of Water and Wastewater, 12th
Edition, p. 234-236. 1965.
6M. E. Gales, Jr., and E. C. Julian, "Determination of Inorganic
Phosphate or Total Phosphate in Water by Automatic Analysis."
Presented at the 1966 Technicon Symposium on Automation in
Analytical Chemistry.
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39
d. A master timer regulating the stepping relay at
two-minute intervals.
e. A Technicon proportioning pump providing the flow
of samples and distilled water to a Technicon continuous filter.
This pump also diluted filtered samples with distilled water in
the ratio of 1:40, respectively.
Whole plant samples were run on the Technicon AutoAnalyzer
using a Technicon Sampler II and a continuous filter with samples
reaching the filter within thirty minutes. No significant ortho-
phosphate bleedback occurred in unfiltered samples during this
period.
2. Total Phosphate
Analyses were conducted on fixed whole samples at the
Ada laboratory within 15 days of sample collection. Initially
whole samples were blended for three to five minutes in a Waring
Blendor and then analyzed by the persulfate procedure of Gales
and Julian.6 The procedure was modified to more closely approxi-
mate the Standard Methods5 procedure for orthophosphate in that
the samples were neutralized after digestion. Also the manifold
design and reagents for the AutoAnalyzer were adjusted to deliver
approximately the amount of reagents per sample outlined by
Standard Methods.5
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40
3. Total Carbon and Total Nonvolatile Organic Carbon
Whole samples were run in the field using the methods of
Van Hall, Safranko, and Stenger7 and Van Hall, Earth, and Stenger.?
A Beckman Carbonaceous Analyzer was used. Preliminary homogeni-
zation with a Waring Blendor provided representative syringe
sampling of whole samples. As the whole acidified sample was
further purged with nitrogen gas for five minutes, results were
reported as total nonvolatile organic carbon. Acetic acid stan-
dards were used for instrument calibration.
4. Total Oxygen Demand
Whole samples were run by the method of Stenger and Van
Hall^ using the instrument and techniques described therein. Pre-
liminary homogenization with a Waring Blendor was practiced.
Sodium acetate standards were used.
5. Total Hardness
In the field, hardness was determined by EDTA titrimetry
according to Standard Methods, Method B, p. 147-152.5
7C. E. Van Hall, J. Safranko, and V. A. Stenger, "Rapid Combustion
Method for the Determination of Organic Substances in Aqueous
Solutions," Anal. Chem. 35:3, p. 315-319. 1963.
8C. E. Van Hall, D. Barth, and V. A. Stenger, "Elimination of
Carbonates from Aqueous Solutions Prior to Organic Carbon
Determination," Anal. Chem. 37:6, 769. 1965.
9V. A. Stenger and C. E. Van Hall, "Rapid Method for Determination
of Chemical Oxygen Demand," Anal. Chem. 39:2, 207. 1967.
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41
6. Total Chromium
Chromium was determined in the field using the procedure
of Standard Methods5 (Total Chromium Method B, p. 474-479).
Color was measured photometrically on a Beckman Model B
Spectrophotometer.
Physical Tests
1. Solids
Tests for total suspended and total volatile suspended solids
were conducted according to Standard Methods (Methods C & D, p. 424-
425). Reeve Angel 2.4 cm glass fiber filters grade 934AH were used
in lieu of asbestos mats. Gooch crucibles were fired at 600°C, cooled,
and the mats placed in the crucibles and dried at 103 C for at least
1 hour before initial weighing. At intervals crucibles with filters
were subjected to 600°C furnace temperatures to check for weight loss
due to the filter. Total and total volatile solids were determined
according to Standard Methods5 (Methods A & B, p. 423-424).
2. Dissolved Oxygen
Measurements were made in situ with a YSI Model 51 Oxygen
Meter equipped with a Model 5103 oxygen/temperature probe. The meter
was calibrated against the Azide Modification of the Winkler Method
described in Standard Methods5(Method A, p. 406-410). The meter was
also calibrated against saturated air at the temperature at the test
medium.
NOTE; Mention of products and manufacturers is for identification
only and does not imply endorsement by the Federal Water Pollution
Control Administration or the U. S. Department of the Interior.
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42
r>- CN ro oo
oo *"H oo in
oo CM r-» <• i i
« ., * « i i
o^ ro ro co
**-!
C
vO
I
a
3
00
c
3
4J
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43
ACKNOWLEDGMENT
Grateful acknowledgment is given to many individuals who
significantly contributed to this study. The major portion of
this work was made possible through the cooperation of the City
of Mansfield, Ohio. Particular recognition is given to Mr. David
Conant, Jr., Manager, Sewage Treatment Plant, for invaluable
assistance during the field phase of these studies. Appreciation
is also expressed for the cooperation of Mr. George R. Cunitz,
Engineer-Manager of the City of Mansfield, and Mr. Merle Lutz,
Assistant Manager of the Sewage Treatment Plant. Members of the
Robert S. Kerr Water Research Center staff who participated
extensively were Mr. J. F. McNabb, Microbiologist; Mr. B. D. Newport
and Mr. M. L. Cook during field investigations; and Mr. K. F. Jackson
and Mr. B. Bledsoe during laboratory phases.
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