vvEPA
United States
Environmental Protection
Agency
Municipal Environmental Research
Laboratory
Cincinnati OH 45268
Research and Development
EPA-600/S2-81-061 June 1981
Project Summary
Joint Dry/Wet Weather
Treatment of Municipal
Wastewater at
Clatskanie, Oregon
Arthur H. Benedict and Vernon L Roelfs
The joint dry/wet weather waste-
water treatment facility at the small
community of Clatskanie, Oregon,
underwent a 2-year evaluation. Wet
weather flows (WWF), as much as 6 to
10 times dry weather flows, result
from severe infiltration/inflow (I/I) to
the collection system. Rather than
undergo expensive sewer rehabilita-
tion, the treatment facility was de-
signed to process flows ranging from
0.13 to 1.25 mgd (490 to 4730 m3/
day) where average dry weather flow
is 0.2 mgd. During dry weather, the
plant operates up to 0.5 mgd with
primary clarification and an activated
sludge mode. For higher flows, the
plant can be run in a contact-stabiliza-
tion mode and the primary clarif ier can
be operated in a dissolved air flotation
(DAF) mode. The waste activated
sludge generated during dry weather
operation is held as standby sludge for
the contact-stabilization operation.
Operating criteria and performance
capabilities were developed to assist
in other potential applications. The
capital cost of the DAF-contact stabi-
lization capability was estimated to be
14% over the cost of a standard dry
weather flow plant. The annual opera-
tion and maintenance cost was esti-
mated to be 3% over the operation and
maintenance cost of a standard dry
weather flow plant. The results indicate
that for other communities this treat-
ment scheme may be more cost effec-
tive than extensive sewer rehabilitation
provided the community is in the
process of upgrading its treatment
plant. For this approach to be a long-
term solution to the I/I problem, how-
ever, the community must continue a
sewer rehabilitation program, or an
inflow control program, or both to
keep pace with the growth of the city.
This Project Summary was developed
by EPA's Municipal Environmental
Research Laboratory. Cincinnati, OH,
to announce key findings of the re-
search project that is fully documented
in a separate report of the same title
(see Project Report ordering informa-
tion at back).
Introduction
The City of Clatskanie, Oregon, is
located in rural northwest Oregon,
within 3 miles (4.8 km) of the Columbia
River, and approximately 50 miles (80
km) north-northwest of the Portland
metropolitan region. The city encom-
passes an area of 650 acres (263
hectare) and has a population of 1750.
Sewer loads are primarily domestic; no
major industrial dischargers are con-
nected to the system.
Evolution of joint treatment at
Clatskanie began in 1971 when the
Oregon State Department of Environ-
mental Quality requested the city to
undertake modifications to improve per-
-------
formance of the existing trickling filter
plant. A preliminary investigation, also
made that same year, showed that the
collection system was subjected to
severe I/I. In 1974 when the city con-
ducted an I/I study under Public Law
92-500, several alternatives were iden-
tified to meet the necessary pollution
control objectives. These alternatives
ranged from sewer rehabilitation with
expansion of the existing trickling filter
system to development of a new treat-
ment facility designed to accommodate
the full range of expected wet and dry
weather flows.
A cost effective analysis indicated
that the latter alternative—development
of a joint dry/wet weather facility with-
out major collection system improve-
ments—was the best solution at
Clatskanie. Application of this alterna-
tive centered on developing a treatment
facility able to handle the entire range of
anticipated flows. Design of the facility
was initiated in 1974, and construction
was completed in January 1978.
The joint dry/wet weather treatment
facility at Clatskanie was designed to
process a range of flows between 0.13
and 1.25 mgd (492 and 3731 mVday).
This range represents a flow variation of
10:1 based on the ratio of the design wet
weather storm flow to the current
minimum daily flow of 0.13 mgd (491
mVday), or a flow variation of approxi-
mately 6:1 based on the ratio of the
design storm flow to the current average
daily flow of 0.2 mgd (757 mVday). This
wide variation in anticipated wastewater
flows, coupled with concomitant
changes in raw waste biochemical
oxygen demand (BOD, 5-day) and total
suspended solids (TSS) loads during
WWF led to the conceptual develop-
ment, design, and ultimate construction
of the facility. Flows exceeded 1.25 mgd
(4731 mVday) once each year during
the 2-year demonstration study.
The design concept at Clatskanie em-
ploys a dual-use primary clarifier capable
of DAF operation with high flows in con-
junction with a contact stabilization
activated sludge system having flexible
volumetric capacity to allow processing
of highly variable incoming loads. During
dry weather, the plant operates up to 0.5
mgd (1890 mVday) with primary clarifi-
cation and an activated sludge mode.
For higher flows, the plant can be run in
a contact stabilization mode and the
primary clarifier can be operated in a
dissolved air flotation mode. The standby
sludge for the contact stabilization
operation is acquired by holding waste
activated sludge generated during dry
weather operation.
Later in 1978, after construction was
complete, the research study at Clatskanie
was initiated with five principal objec-
tives in mind:
• to demonstrate effluent control
capabilities of the joint dry/wet
weather flow system;
• to define and evaluate performance
characteristics of individual process
components;
• to identify parameters and condi-
tions affecting process performance;
• to present cost data for joint dry/wet
weather treatment; and
• to develop comparisons for nation-
wide application of the treatment
concept demonstrated.
To meet these objectives, a field
survey and data collection program was
developed to document plant perform-
ance, assess operational procedures
used at the plant, evaluate biotreatment
responses, and provide cost information
for the treatment works.
Results
This plant was evaluated during wet
and dry weather flow (DWF) conditions.
The basic analyses for evaluating the
performance of the plant were BOD and
TSS, although COD, nutrients, fecal
coliform, and heavy metal analyses
were also performed. The operational
parameters monitored included food-to-
microorganism ratio, respiration rate.
contact basin mixed liquor suspended
solids (MLSS), stabilization basin MLSS,
total solids, depth of blanket, sludge
volume index, and dissolved oxygen.
DWF treatment was characterized by
using routine compliance monitoring
data and two different dry weather
surveys performed as part of the dem-
onstration study (Table 1). Also listed in
Table 1 are average operating condi-
tions. To evaluate plant performance
during WWF, a statistical analysis of
eight different storm surveys was made.
The effluents BOD was less than 15
mg/L 98% of the time, TSS concentra-
tion was less than 20 mg/L 80% of the
time, and the flow was below 0.84 mgd
(3179 mVday) 70% of the time. For
storms monitored in the 1979-1980 wet
season, overall WWF removal efficiencies
were 73% for BOD and 71% for TSS.
These values are based on total mass
amounts for storms in that period. Table
2 characterizes WWF and includes a
range of average operating conditions
for WWF treatment.
In general terms, the plant was capable
of controlling the largest organic loads
encountered provided the MLSS con-
centration was more than 2100 mg/L.
The difficult-to-remove pollutants were
the fine inorganic materials usually
present in the influent at peak annual
WWF's. These fine inorganics stayed in
suspension through the primary unit
and were initially removed in the sec-
ondary, although in time they broke
through the secondary plant.
Table 1. Dry Weather Flow Plant Performance and Typical Operating Conditions
Plant Characteristic/
Operating Conditions*
Characteristic:
Flow, mgd
Raw Waste. mg/L
Primary Effluent, mg/L
Secondary Effluent, mg/L
Plant Removal, %
Conditions:*
MLSS, mg/L
Secondary Return Sludge, mg/L
Temperature, °C
Food to Microorganism Ratio (F:M)
Mean Cell Retention Time, days
Sludge Volume Index
Respiration Rate, mg Oz/hr/gm-SS
Primary Overflow Rate, gpd/sq ft**
Secondary Overflow Rate, gpd/sq ft**
Average
Values BOD
0.19
208
111
13
94
5.400
10.900
15
0.12
5.6
150
10
600
120
TSS
166
55
10
94
* Metric conversion: gpd/sq ft X 0.041 = m3/m2/day; mgd X 3785 - m3/day.
* Complete mix activated sludge and conventional primary clarification.
**Based on raw waste flow.
-------
Table 2. Wet Weather Flow Plant Performance and Operating Conditions
Plant Characteristic/
Operating Conditions*
Values
Characteristic:
Peak Flow Range, mgd
Annual BOD Mass Removal. %
Influent BOD Concentration, mg/L
Influent TSS Concentration, mg/L
Effluent BOD Concentration, mg/L
Effluent TSS Concentration, mg/L
Conditions:*
Contact Time q + r, min
Reaeration Time, hr
MLSS, Contact Basin, mg/L
F:M, Contact Basin
F:M, Contact + Reaeration
Secondary Return Sludge, mg/L
Primary Overflow Rate, gpd/sq ft
Secondary Overflow Rate, gpd/sq ft
0.5 to 2.31
73
71
Range of Average Storm
Values
35 to 243
79 to 179
6 to 20
2 to 39
11 to 67
1.6 to 2.2
1528 to 3656
0.38 to 0.35
0.05 to 0.35
10,480 to 15,800
1,590 to 3,981
314 to 1.452
* Metric conversion: gpd/sq ft X 0.041 - mVmVday; mgd X 3785 = m3/day.
^Contact-stabilization secondary, DAF primary.
A cost effectiveness analysis (sum-
marized, Table 3) provided a direct com-
parison of joint wet/dry weather treat-
ment with other I/I control and treatment
methodologies. The alternatives con-
sidered in the cost effectiveness analysis
included:
• joint wet/dry weather treatment,
• complete sewer system rehabilita-
tion and DWF treatment,
• flow equalization storage and DWF
treatment, and
• DWF treatment and bypassing of
WWF.
The last alternative, DWF treatment
and bypassing, was considered as a
basis for comparing the other alterna-
tives.
The means for evaluating the alterna-
tives were percent cost increase above
DWF treatment, percent increase in
annual mass removal efficiency above
DWF treatment, and dollars spent per
pollution unit removed above DWF
treatment. A pollution unit is defined in
this report as an average of BOD and
TSS values.
As indicated by the annual removal
efficiency increases, joint treatment
costs less than half as much as either of
the other alternatives and is nearly as
effective in controlling I/I discharges.
Conclusions
1. Based on mean values obtained in
this study, the DAF-contact stabilization
treatment system configuration used at
Clatskanie is capable of maintaining
effluent TSS concentrations of 2 to 24
mg/L and BOD concentrations of 6 to
11 mg/L for average flows up to 1.07
mgd (4050 mVday) or 5.6 times DWF.
Transient concentrations, however,
exceeding these values can be expected.
2. With the DAF-contact stabilization
mode, WWF up to 1.25 mgd (4731
mVday) or 6.6 times DWF can be
treated to achieve effluent TSS and BOD
concentrations equivalent to those con-
sidered representative of DWF activated
Table 3. Cost Effectiveness Analysis Summary*
Alternatives
Joint Treatment
System Rehabilitation
Flow Equalization
Total Cost
lncrease%
14
40
51
Annual Removal
Efficiency lncrease%
4
5
5
Dollars Spent
per Pollution
Unit Removed
71
149
201
sludge operation. Under the conditions
stated, the effluent BOD concentration
was less than 15 mg/L 98% of the time,
TSS concentration was less than 20
mg/L 80% of the time, and the flow was
below 0.84 mgd (3179 mVday) 70% of
the time.
3. Fine inorganics flushed from the
system during high flows proved dif-
ficult to remove.
4. For this installation, the DAF-
contact stabilization capability required
an additional 14% in capital cost.
5. When compared with sewer sys-
tem rehabilitation and flow equalization
storage at Clatskanie, joint dry/wet
weather treatment, although it does not
offer the most complete control of I/I,
does offer the most I/I control per dollar
spent. Annual mass removals of BOD
and TSS were 93% for joint treatment as
opposed to estimated 94% for rehabili-
taion and flow equalization alternatives.
Joint treatment as an I/I control method
costs less than half as much as either of
the other methods.
6. Dissolved air flotation is capable of
achieving up to 66% TSS removal and
up to 45% BOD removal.
7. Deterioration of TSS and BOD
removal by dissolved airflotation occurs
at overflow rates, based on raw waste
flow, in excess of 5000 gpd/ft2 (200
mVmVday).
8. The DAF-contact stabilization
capability should be considered for
incorporation into treatment systems
for cities with similar I/I problems. The
process modifications are comparatively
inexpensive and secondary effluent
standards can be maintained.
*This comparison is valid forthe City of Clatskanie, Oregon. Results of similar analyses
may vary significantly for other cities.
? US GOVERNMENT PRINTING OFFICE 1981-757-012/7143
-------
Arthur H. Benedict and Vernon L. Roe/fs are with Whiteley-Jacobsen and Asso-
ciates, Portland, OR 97203.
Douglas C. Ammon is the EPA Project Officer (see below)..
The complete report, entitled "Joint Dry/'Wet Weather Treatment of Municipal
Wastewaterat Clatskanie, Oregon,"(OrderNo. PB 81 -187262;Cost:$11.00,
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:
Municipal Environmental Research Laboratory
U.S. Environmental Protection Agency
Cincinnati. OH 45268
United States
Environmental Protection
Agency
Center for Environmental Research
Information
Cincinnati OH 45268
Postage and
Fees Paid
Environmental
Protection
Agency
EPA 335
Official Business
Penalty for Private Use $300
RETURN POSTAGE GUARANTEED
PS 000032*
(J S ENVIK PROTECTION
REGION 5 LIBRARY
230 S DEARBORN STRfiET
CHICAGO IL 60604
------- |