United States
Environmental Protection
Agency
Water Engineering
Research Laboratory
Cincinnati OH 45268
Research and Development
EPA/600/S2-86/046 May 1986
Project Summary
Trickling Filter/Solids Contact
Process: Full-Scale Studies
Raymond N. Matasci, Arthur H. Benedict, and Denny S. Parker
This report outlines the characteristics
that distinguish the trickling filter/solids
contact (TF/SC) process for municipal
wastewater treatment from other similar
processes. The SC feature provides a
short aerobic contact period between the
TF effluent and the recycled underflow
solids from the secondary clarifier to pro-
mote solids capture and produce a final ef-
fluent with a low suspended solids
concentration.
The report also summarizes the results
of field studies at TF/SC facilities in Ocon-
to Falls, Wisconsin; Tolleson, Arizona;
Medford, Oregon; and Chilton, Wisconsin.
These studies and a review of historical
operating records at these plants and at
Corvallis, Oregon (where the process was
first successfully demonstrated in 1979),
provide additional insight into the perfor-
mance of the TF/SC process under differ-
ent design and loading conditions.
This Project Summary was developed
by EPA's Water Engineering Research
Laboratory, Cincinnati, OH, to announce
key findings of the research project that
is fully documented in a separate report
of the same title (see Project Report order-
ing information at back).
Introduction
In certain circumstances, the trickling
filter/solids contact process (TF/SC) for
municipal wastewater treatment may qual-
ify for funding as innovative technology;
but more information is required about the
design and performance of this process.
The objectives of this study were therefore
as follows:
1. To develop a definition of the TF/SC
process that distinguishes it from
other similar processes
2. To document the design and per-
formance information available from
existing TF/SC facilities, and
3. To conduct special field investiga-
tions to provide additional insight into
the TF/SC process. The project was
not intended to produce a. design
manual, since field investigations and
operating data were limited.
Process Characteristics
TF/SC is a biological and physical pro-
cess that includes (1) a TF, (2) an aerobic
solids contact period, (3) a flocculation
period, and (4) secondary clarification.
Two operating features are also important:
Solids must be maintained in an aerobic
flocculant state, and solids are recycled
from the secondary clarifier to combine
with TF effluent as a mixed liquor.
The primary function of the first element
in the TF/SC process, the TF, is to reduce
the soluble BOD in the wastewater. The
aerobic solids contact period is then used
to provide contact between finely divided
solids in the TF effluent and recycled
biological solids and to provide additional
soluble BOD removal if necessary. The
contact opportunity provides for initial
flocculation of dispersed solids into floe.
The length of the aerobic solids contact
period is governed by the requirements for
particulate and soluble BOD removal. The
third element in the TF/SC process is the
flocculation period. Flocculation is initiated
in the contact tank and continues in the
clarifier, perferably in a mildly stirred en-
vironment of a center well. The floccula-
tion step promotes clear effluent and
growth of large, settleable floe that is
removed during secondary clarification.
The following primary characteristics
distinguish TF/SC from other processes:
1. The main function of the contact
tank and clarifier flocculation fea-
tures is to increase flocculation and
solids capture and reduce particulate
BOD.
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2. The majority of soluble BOD removal
occurs in the TF.
3. Return sludge solids are mixed with
TF effluent rather than with primary
effluent, as with some other
processes.
4. The aerated solids contact tank is
not designed to nitrify, although
nitrification may occur in the TF.
5. The aerated solids contact time is 1
hr or less, based on total flow in-
cluding recycle.
6. The solids retention time (SRT) of
the aerated solids contact tank is
less than approximately 2 days.
Performance of Existing Facilities
Design and performance data were re-
viewed for six facilities located in Chilton,
Wisconsin; Corvallis, Oregon; Medford,
Oregon; Norco, California; Oconto Falls,
Wisconsin; and Tolleson, Arizona. Most of
the information used in the process evalu-
ation comes from the four facilities de-
scribed in Table 1.
The Corvallis plant was the first TF/SC
plant in operation. The Oconto Falls facility
was originally a rock TF plant that was not
meeting its discharge limits; modifications
to the facility included addition of a solids
contact chamber and a new flocculator
clarifier for secondary clarification. Tol-
leson was originally a two-stage rock TF
plant treating industrial and residential
flows. The first-stage rock filter was re-
placed with a 20-ft-deep plastic media
filter with intermediate clarification, and
the remaining rock filter was followed by
aerated SC and a flocculator clarifier. The
Medford plant was originally an activated
sludge (AS) plant that was converted to
a coupled TF/AS plant. This plant presently
operates in the TF/SC mode since its flows
and loads are significantly below design
levels.
Monthly performance data for the four
facilities listed in Table 1 are presented in
Table 2. Clearly, these plants produce a
high-quality effluent in all cases.
Field Investigations
A total of 29 weeks of field investiga-
tions were undertaken at four facilities to
augment the information available from
historical operating records. Special study
objectives were as follows:
1. To assess the influence of cosettling
waste secondary solids with raw
sewage solids on primary sedimen-
tation tank performance.
2. To assess soluble BOD removal
kinetics with TF depth.
3. To assess the effect of TF loading on
TF/SC performance.
4. To assess the effect of media type on
aerated solids contact tank
performance.
5. To assess the effect of aerated solids
contact tank operating parameters
on TF/SC performance.
6. To assess soluble BOD removal in the
aerated solids contact tank.
7. To assess the effect of aeration rate
on TF/SC performance.
8. To assess the effect of secondary
clarifier overflow rate on final effluent
quality.
9. To assess the effect of coagulant ad-
dition for phosphorus removal on
TF/SC performance.
Conclusions
The following conclusions are based on
the special study results and a review of
historical operating records:
1. Cosettling —Primary treatment sus-
pended solids (SS) removal averaged 53
Table 1. Design Data for Operating TF/SC Facilities
Element
Tolleson
Oconto Falls
Corvallis
Medford*
Design flow, m3/s (mgd)
ADWF
PWWF
Design loading, 1,OOO kg/d (1,000 Ib/dayl
BOD
SS
Primary overflow rate, m3/m2 d tgal/day/ft2)
Trickling filter
Media type
BOD loading, g/m3'd (lb/day/1,000 ft3)
Return sludge aeration time,1 min
Aerated solids contact time," min
Flocculator center well
Percent of clarifier area
Detention time,* min
Secondary clarifier
Overflow rate,+ + rrP/rrf-d (gal/day/ft2)
Side water depth, m (ft)
Sludge removal system
Weir Location
0.36 18.3)
0.78 (17.7)
10.9 (24.0)
9.80 (21.61
39.5 (970)
Plastic/rock*
881/146+ (55/9.1)
13
25
17.9 (440)
4.9 (16)
Suction header
Inboard
0.017 (0.38)
0.033 (0.75)
0.30 (0.67)
0.36 (0.79)
15.1 (37O)
561
Rock
(35)
16
38
12.2 (300)
4.6 (15)
Suction tube
Inboard
0.43 (9.7)
1.23 (28.0)
4.94 (10.9)
5.22(11.5)
39.9 (98O)
Rock
384 (24)
9
2
12
25
19.1 (4701
5.5 (18)
Suction tube
Inboard
0.79 (18)
2.63 (60)
15.9 (35)
12.7 (28)
41.8 <1,O25)
Plastic
1,840 (115)
5
5
29.3 (720)"
4.6 (151
Suction header- 1
Suction tube -o
Inboarc
*'Originally designed as coupled TF/AS plant. Design data for TF/AS.
+ First stage/second stage.
1Based on 33 percent return rate.
^Not used at this plant.
"Based on total flow including recycle.
* *Contact time at existing flow of 8.8 mgd plus 33 percent return is 39 min.
+ +Based on total clarifier area.
tfBased on existing number of clarifiers.
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Table 2. Monthly Performance at Operating TF/SC Facilities
Tolleson *
Parameter
Influent flow
Average, m3/s
(mgdt
Influent characteristics
BOD, mg/L
SS, mg/L
Temperature, °C
Primary effluent
BOD, mg/L
SS, mg/L
TF effluent
BOD, mg/L
SS, mg/L
Return sludge SS, g/L
Mixed liquor SS, mg/L
Secondary effluent*"
BOD, mg/L
CBOD, mg/L
SS, mg/L
High
0.29
(6.71
350
300
_ t
373
400
42. 51
45. S1
_#
1.621
15.4
_t
20.2
Low
0.22
<5.0)
222
192
_ t
107
57
T0.41
5.31
_tt
55 1
3.5
_ t
4.0
Average
0.27
16.1)
277
224
_ t
173
121
22. 8"1
23. ff1
_tt
1,042
7.2
t
8.5
High
0.02
10.46)
179
152
19
T
t
_t
_t
_#
_ t
31.7
_ t
22.6
Oconto Falls"
Low
0.01
10.28)
119
100
8
_ t
_ t
_t
_ t
_#
_ t
14.2
_ t
6.7
Average
0.02
10.36)
146
118
13
_ t
t
_t
_t
-#
_ t
20.5
_ t
12.8
High
0.78
117.9)
188
191
22
114
82
39
72
17.2
4,982
9
7
13
Corvallis*
Low
0.25
15.6)
48
112
13
35
56
22
54
5.4
1,557
5
4
7
Average
0.46
(10.5)
108
154
17
70
66
30
59
11.3
3,127
6.8
5.1
9.4
High
0.43
(9.9)
173
159
22
90
38
81
$9
_»
1,868
23
11
9
Medford^
Low
0.36
(8.2)
142
119
16
76
29
51
39
_»
1,475
14
6
6
Average
0.39
(8.9)
157
138
19
81
34
66
71
_#
1,615
19
8
8
"April 1983 through March 1984.
+April 1984 through July 1984.
'Not routinely measured.
"intermediate clarifier effluent.
"Return sludge aeration not used.
* 'Monthly averages.
to 62 percent at three TF/SC plants that
cosettle and 74 percent at Medford, which
does not cosettle. The Meford results are
exceptional. Primary sludge concentra-
tions were 3.7 and 5.3 percent at the two
plants practicing cosettling where samples
could be obtained for analysis.
2. TF Soluble BOD Removal-The Velz
equation successfully modeled soluble
carbonaceous BOD5 removal with TF
depth at Tolleson.
3. TF Loading—In the range of average
TF BOD6 loadings studied under this pro-
ject (5.8 to 29 Ib/day per 1000 ft3), BOD5
loading does not always exert a strong in-
fluence on final effluent SS. Final effluent
SS were always correlated with TF ef-
fluent SS, which are most sensitive to
primary effluent SS concentration. The
results show the need for reliable primary
treatment and consideration of the effect
of primary effluent SS on final effluent
quality.
4. Solids Retention Time (SRT)—Correl-
ations between SRT in the aerated solids
contact tank and final effluent SS were
not statistically significant at Corvallis and
Tolleson. A statistically significant but
weak correlation was observed at
Medford.
5. Mixed Liquor Suspended Solids
(MLSS) —MLSS concentrations of 900 to
2300 mg/L at Medford and Tolleson did
not affect final effluent SS significantly
and only produced an average increase of
about 2 mg/L at Corvallis where the MLSS
concentration varied from 1500 to 7000
mg/L. The insensitivity to mixed liquor
level means simplification of operation,
since less attention can be given to sludge
inventory management.
6. Sludge Volume Index (SVI)-SVI
values varied from 60 to 130 mL/g at Med-
ford, and increasing values were correlated
with reduced final effluent SS. No correla-
tion was observed at Tolleson or Corvallis.
Corvallis and Tolleson have large floc-
culator center wells, whereas those at
Medford are much smaller.
7. Solids Flocculation —Field test re-
sults at Medford suggest the majority of
flocculation in the aerated solids contact
channel occurs within the first 12 min of
aerated solids contact time in a channel
that has a total hydraulic retention time of
39 min. Additional SS removal occurs in
the flocculator center well. The results
agree with observations at Corvallis,
Oregon.
8. Contact Tank Soluble BOD Removal
—Although the primary function of the
contact tank is to flocculate SS and par-
ticulate BOD, a significant fraction of the
filter effluent soluble BOD can be remov-
ed. The Medford contact tank removed an
average of 75 percent ,of the residual
soluble BOD from the filter in 39 min of
contact time.
9. Secondary Clarifier Overflow Rate—
Secondary clarifiers that include inboard
launders, high sidewater depths, and floc-
culator center wells are insensitive to 1300
gpd/ft2 at Corvallis and up to 700 gpd/ft2
at Tolleson. These are the maximum over-
flow rates at these respective plants.
10. Coagulant Addition—Ferric chloride
addition in the aerated solids contact tank
for phosphorus removal at Oconto Falls did
not adversely affect TF/SC operation.
The full report was submitted in partial
fulfillment of Contract No. 68-03-1818 by
Brown and Caldwell Consulting Engineers
under the sponsorship of the U.S. En-
vironmental Protection Agency.
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RaymondN. Matasci, Arthur H. Benedict, and Denny S. Parker are with Brown and
Caldwell Consulting Engineers. Pleasant Hill, CA 94523.
James F. Kreissl is the EPA Project Officer (see below).
The complete report, entitled "Trickling Filter/Solids Contact Process: Full-Scale
Studies," (Order No. PB 86-183 100/A S; Cost: $ 16.95, subject to change} will
be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650
The EPA Project Officer can be contacted at:
Water Engineering Research Laboratory
U.S. Environmental Protection Agency
Cincinnati, OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
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