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5369 001R79102
DESIGN EXAMPLE
HIGH RATE SYSTEMS
Prepared by
Dr. Al Wallace
University of Idaho
Moscow, Idaho
Prepared for
Environmental Research Information Center
Seminar on
Land Treatment of Municipal Wastewater Effluents
June 1979
ENVIRONMENTAL RESEARCH INFORMATION CENTER
OFFICE OF RESEARCH AND DEVELOPMENT
U.S. ENVIRONMENTAL PROTECTION AGENCY
CINCINNATI, OHIO 45268
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DESIGN EXAMPLE - HIGH RATE SYSTEMS
Introductory Note
The input data for this example represent a composite of conditions
encountered during the preparation of several facilities plans. Although
that makes the example somewhat artificial, the intent is to touch as
many bases as possible in the demonstration of the process of site selec-
tion and process design. The site selection and design illustrated herein
closely parallels that for an actual system.
General Design Conditions
The City of Jason has an existing trickling filter plant located
near the south city limits. Effluent discharge is to a small stream which
flows south and discharges to the Rush River about one-half mile west of
the village of Belson. The existing plant, constructed in 1930, is grossly
overloaded and structurally, in rather bad shape. In addition, the draft
NPDES permit for continued discharge to the stream will call for ammonia
limitations and dechlorination in addition to a high level of secondary
treatment. As development is continuing in areas south of the plant, it
has been decided to locate a new plant about 3 to 3 1/2 miles south of the
present plant site.
Pertinent design data are listed below.
1978 2000
Population 6,400 12',500
Average sewage flow 1.54 mgd 3.0 mgd
Draft NPDES permit for discharge to the stream [period: May 1 to Oct. 14 -
monthly averages.j BQD _ 2Q mg/L ^^ daW , *>
SS - 20 mg/L \>*. 7_
NH3-N - 6 mg/L 150 Ib/d °
residual C12 - < .02 mg/L, combined
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period: Oct. 15 - April 30. - monthly averages
BOD - 30 mg/L
SS - 30 mg/L
NH3-N - 10 mg/L, 250 Ib/d
residual CK - < .05 mg/L, combined.
Land treatment with eventual recharge of the Rush River is seen as the
most cost-effective way of meeting the effluent limitations while meeting
the highly restrictive environmental requirements of the area and avoiding
heavy commitments of energy and chemicals. In addition, slow-rate land
treatment options are not acceptable for two reasons; the unavailability
of large enough tracts of land and the very cold climate and short growing
season in the area. For example, there are normally only 100 days between
the last and first 32°F temperatures.
There are, however, four sites, shown on figure 1, which are available
from local landowners and which may be adequate for development of a high-
rate land treatment system. Pertinent field data for a preliminary screen-
ing of these sites were gathered during the early stages of the facilities
planning.
Si! te_ Screening Process
Groundwater table elevations are highest in the area during late May
and through June and lowest from September through most of the winter. By
installing 15 observation wells and utilizing 10 shallow private wells and
one public well, enough data were gathered to construct a rough water table
contour map for the period of maximum water table height. The results are
shown on figure 2. The groundwater flow is seen to be generally south,
turning southeast at the point where the valley necks down with a gradient
of from 4 to 5.5 ft. per 1000 ft.
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A backhoe was used to dig several pits in order to inspect the soil
profile on each site. Then a series of double-ring infiltrometer tests
were run to get a preliminary estimate of the intake rates of the soils
on each site. Three depths in the profile at site 1 were tested, two
were tested on site 2 and only the surface soils were tested on sites 3
and 4. The decisions concerning which parts of each profile to test were
determined entirely by the preliminary inspections.
Soils Data
Note: All profiles are to approximately 12 ft.
A. Site 1
0' - I1 Sand topsoil, some silt
I1 - 4' Sand, medium and clean
4' - 12' Gravelly sand, trace of silt, scattered large
cobbles to 10 in.
Infiltration Tests (2 in each profile)
Surface
3'
5'
8.4, 12.1 in/h
15.0, 21.2 in/h
6.6, 7.1 in/h
B. Site 2
0' - 1.5' Sandy top soil, trace of clay
1.5' - 4.5' Gravelly sand, trace of clay
4.5 - 5.5" Gravelly, sandy clay
5.5' - 8.0' Sandy clay, appears dense
8.0 -12.0 Sandy gravel, trace of clay
Infiltration Tests (2 in each profile tested)
Surface : 6.2, 7.4 in/h
6' : 0.2, 0.6 in/h
C. Site 3
1. East end
O1 - 2.5' Silty topsoil
2.5' - 6.0' Silty sand, many 4 in. cobbles
6.0' -12.0' Sand, trace of silt, many 4-10 in. cobbles
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Infiltration Tests
Surface: 1.1, 2.3 in/h
2. West end
0' - 2.0' Sandy silt topsoil
2.0' - 3.5' Silty sand, some 4 in. cobbles
3.5' -12.0' Silty sand, lots of 4-6 in. cobbles
Infiltration Tests
Surface: 2.8, 3.1 in/h
D. Site 4
1. East end
0' - 2.0'
2.0' - 5.0'
5.0' -12.0'
Fine sand topsoil
Sand and gravel, poorly graded
Sandy gravel, many large 4-8 in. cobbles
Infiltration Tests
Surface: 4.7, 4.7 in/h
West end
0' - 2.0' Sandy topsoil
2.0' - 4.5' Sand gravel, some 4 in. cobbles
4.5' -12.0' Fine gravel and sand, poorly graded, large cobbles
Infiltration Tests
Surface: 5.9, 8.1 in/h
A. Log
Surface
Bel son Village Well Data
Elev.
Bottom of grout pack
Top of gravel pack
Perforations
3020-2995
3056
3054
3051
3036
3027
3020
0' - 2' Topsoil
2' - 5' Sand, trace of silt
5' -20' Gravelly fine sand,
some silt
20' -66' Gravel, sand
Water
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Pump suction pt. _ 3000
Bottom of bore hole 2990 Hard limestone
Bottom of casing
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B. Production
Tested at 400 gpm with 5.5 ft. of drawdown. Present pump produces
175 gpm
Based upon these preliminary data, sites 1 and 2 are dropped from
further consideration. Site 1 was dropped because there is no easy way
to ensure that the percolate would not impact the shallow village well which
completely penetrates the same aquifer and is directly down-gradient from
the point of wastewater application. Site 2 was dropped because of the
underlying layer of restricted permeability which would create a perched
water table and a reduction in the available zone of aeration.
Between sites 3 and 4, site 3 is superior in many respects. It has
finer grained surface soils, affording more renovation capacity; more depth
to the water table and is closer to the highway, requiring less access road
construction and decreasing the environmental impact of sewer construction
to the site. On the other hand, its percolate would discharge, at least in
part, to Slater creek which has a far smaller flow than Rush River. This
site would also have a greater visual impact from the highway. Another
factor is price. Site 3 is valued much higher than site 4 by their owner
due to its location near the highway, its fine stand of timber, and its
superior view should he choose to develop it.
Thus, even though site 4 is somewhat inferior to site 3 in some
respects, it is concluded that additional planning should focus on this site.
This decision involved input from many quarters including the Jason and
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Belson city governments, the county commissioners, the state environmental
agency, the EPA, the consulting firm and the landowner (who also owned
site 3).
Preliminary Design: Site 4
A. Preapplication Treatment Considerations
Something approaching, but perhaps not completely equivalent
to, secondary treatment will be necessary to ensure that adequate
levels of nitrification can be achieved. In addition, it is
desired to achieve the permit limitations on fecal coliforms in
the river without having to resort to continuous disinfection and
this result will be easier to achieve as the level of "secondary"
treatment is increased. The two systems of preapplication treat-
ment investigated were a three-cell aerated lagoon system and an
oxidation ditch. The oxidation ditch showed a slightly higher
cost than the aerated lagoon system and would also prove to be
harder to operate and have a more troublesome sludge disposal
problem. It was nevertheless recommended because of its smaller
land requirement and ability to produce a warmer effluent during
winter operation. It was considered that the heat balance across
an infiltration basin might prove critical during the winter months.
B. Rapid Infiltration Basin Sizing
The surface soils which are not to be greatly disturbed during
construction, represent the least permeable materials in the profile.
In addition to the four tests already available, a total of ten
more were run in these surface soils at random locations. The geo-
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metric mean value of all 14 infiltration tests was found as
5.5 in/h. A value of 5% of this figure, equivalent to 0.55
ft/d (200 ft/yr.) was selected as a reasonable design figure.
Thus, based on bottom area, about 17 acres are required. This
area will be provided in 7 roughly equal sized basins, located
at the east end of the site to avoid the high groundwater table
near the southwest corner. With access and flood protection
dikes, the basins will occupy about 25 acres of the site. The
oxidation ditch, clarifiers and a storage pond capable of holding
three days flow will occupy about 8 acres. This leaves roughly
56 acres for future expansion of the rapid infiltration system, all
on the west end of the site. Future development of this set-aside
area will require some attention to interceptor drains around the
northwest perimeter to divert some of the groundwater flow around,
rather than under the site. In addition, one or more lines of re-
lief drains may be required under the site to prevent mounding.
These potential requirements can be addressed much more confidently
after a few years of experience with the system as presently proposed
and shown as figure 3.
Design Check
During the course of the planning and preliminary design, some additional
field work was performed at site 4. In particular, five more shallow wells
were installed to follow the groundwater table during a full season of
fluctuation. Well locations are shown on figure 4 which also shows ground-
water level contours during the period of maximum water table levels. Table 1
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gives the seasonal depths to groundwater for the array of seven wells.
Table 1. Depth (ft.) to Groundwater at 7 Observation
Wells on Site 4.
Well No.
Week
4
4
2
4
2
2
3
Month
5
6
7
7
8
9
10
1
5.0
5.2
5.1
6.4
7.1
7.1
7.4
2
3.7
3.7
3.7
4.7
5.2
5.1
5.5
3
6.4
6.3
7.1
9.0
9.6
9.8
9.7
4
4.6
4.8
5.0
6.4
6.9
7.4
7.8
5
5.1
5.3
5.5
6.4
7.2
7.6
8.3
6
6.2
6.4
6.6
7.4
7.8
8.6
9.2
7
7.1
7.0
7.1
7.8
8.4
8.9
9.4
Using the May water table depths and assuming design load conditions
on the site, some approximate calculations were performed to predict the
water table rise during the May-June period of operation. The following
approximations and assumptions were used in making the mound growth
calculations using equation C-10 in the "Design Manual."
1. The infiltration area can be considered circular with an equiva-
lent radius of 612 ft.
2. The "steady" application rate over the area is 0.34 ft. per day.
3. The saturated depth over the barrier layer (limestone) is 30 ft.
(estimated from stock watering well log).
4. The hydraulic conductivity is 760 ft/day (estimated from Belson
Village well data).
5. The drainable voids average 0.3.
6. Average depth to water during June will be 5.7 ft. (average of
observed levels in wells 4, 5, 6 and 7.)
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13
The results of the calculations are shown on figure 5 and indicate
an almost certain mounding problem. Thus, a system of horizontal drains
is indicated to ensure maintenance of a reasonable zone of aeration.
Drainage Design
To employ equation C-ll of the "Design Manual" it is necessary to decide
how high you wish the maximum water table height to be. Assuming we decide
to control it to a depth of greater than 4.5 ft. and further, that we de-
cide to install the drains in the water table, at a depth of 6 ft., we can
get the drain spacing by trial and error. Trial and error is necessary
only because of the correction which must be made to the depth from the
bottom of the drains to the impermeable layer. See page C-45 of the
"Design Manual" or references on drainage design. In any event, a spacing
of 847 ft. is computed as sufficient to control the water table depth. This
could be accomplished with three lines of drains running N-S and discharging
to a surface ditch which in turn discharges to the Rush River. Of course
this ditch would have to be monitored when the drains were flowing and
a NPDES permit written for this discharge.
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