GUIDANCE
FOR
DESIGN AND CONSTRUCTION
o; A
SUBSURFACE FLOW
CONSTRUCTED WETLAND
U.S. EPA-REGION 6
WATER MANAGEMENT DIVISION
MUNICIPAL FACILITIES BRANCH
TECHNICAL SECTION
AUGUST 1993
-------
Addendum 1 to Document, Guidance for Design and Construction of a
Subsurface Flow Constructed Wetland
In the initial publication of this document, the recommended
size of the wetland media was 2-5 inches (50.8-127 mm) in diameter.
Research conducted since the initial publication has shown that
smaller media, gravel size, 0.5-1 inch (12.7-25.4 mm) supports
better growth of the plant root systems than does the larger media
originally recommended. As a result, the recommendation now is to
use smaller gravel as the media. Since the hydraulic conductivity
is dependent on both the rock and void space size, use of a smaller
media will decrease the hydraulic capacity of a system in
comparison to that of the larger media. To offset the loss of
hydraulic capacity resulting frou the use of the smaller media,
change the surface configuration of the wetland to decrease the
length and increase the width. Using Darcy's equation, determine
the flow that can pa?- through pach configuration change (length
and width dimensions) of the wetland and compare this with the
design flow. If the flow does not equal or exceed the design flow,
adjust the length and width and repeat the process. The optimal
point would be where the wetland flow equals the design flow.
System flows exceeding the design flow could result in systems
being larger than necessary.
Typical media characteristics for fine gravel are as follows:
1. Effective Size - 0.67 inches (16mm)
2. Hydraulic Conductivity - 24,608 ft3/ft2/day (7,500 m3/m2/day)
3. Use the porosity value - n suggested in Chapter III, Page 10.
Use of smaller media will ^.Iso affect the recommended gradation found 01.
Page 17. Based on the new inedii size recommendation, use the following
gradation:
Recommended Gradation for SFCW Media
40%
0%
- 80%
100%
Retained
Retained
Retained
1.0
3/4
1/2
inches
inches
inches
[25
[19
[12
.0
.0
.b
mm]
mm]
mm]
sieve
sieve
sieve |
For surface layer gradation, use the following:
Recommended size of surface media 0.5 to 0.75 inches [12.5 mm
19.0 mm]. Gradation of this size is not as critical as for the media,
however, care should be taken not to have media size less than 1/2 inch.
Smaller media can migrate through the filter media an cause a loss of
void space. It is also critical to make sure that the stone is washed
to remove all fines.
-------
TABLE OP CONTENTS
Foreword i
I Subsurface Flow Constructed Wetlands Technology 1
Overview 1
SFCW 1
Types of Precedent Treatment 3
II Factors Capable of Influencing Performance Expectations . 5
III Design and Construction Considerations for SFCW 9
Calculating Design Requirements 9
Reoommended Design and Construction Considerations 16
IV Operational Considerations 20
-------
FOREWORD
This manual is intended to provide guidance in the selection,
design, construction, and operation of subsurface flow
constructed wetlands. The guidance information contained herein
is considered necessary due to the proliferation of the use of
this technology, particularly by small communities, where there
has been considerable effort to construct and maintain treatment
facilities cost-efficiently, capitalizing on the minimal energy
requirements of this technology.
The technology is also considered an attractive alternative for
use.in unsewered areas such as the colonia developments along the
U.S.-Mexico border. In addition, it may also have applications
in other similar situations internationally.
It is recognized that problems have developed with the use of
this technology in some installations. This manual will discuss
probable causes for the problems ^nd will delineate guidance in
an attempt to avoid the problems that have been experienced.
The guidance material contained in this document is the result of
the operating experience and observation of existing subsurface
flow constructed wetlands in the Region 6 area and the work
performed under contract to the Environmental Protection Agency
by Mr. Sherwood C. Reed, P.E.
The contribution of Mr. Ancil A. Jones' recognition of the
value in the application of this technology is gratefully
acknowledged. Mr. Jones dedicated himself to the use of new
and innovative technologies for their cost and energy saving
characteristics, especially in small community applications.
Mr. Jones recognized the potential for use of the subsurface
flow wetlands technology when he was introduced to experimental
work conducted by the National Aeronautics and Space
Administration for possible use in space stations. This manual
is respectfully dedicated to the memory of Mr. Jones who passed
away during the preparation of this manual in March 1993.
For additional information, you may contact:
Chief, Municipal Facilities Branch (6W-M)
Water Management Division
U.S. Environmental Protection Agency, Region 6
1445 Ross Avenue
Dallas, Texas 75202-2733
-------
CHAPTER I
SUBSURFACE FLOW CONSTRUCTED WETLANDS TECHNOLOGY
OVERVIEW
The casual observer may ask, "What is a subsurface flow
constructed wetland?" The answer to this question is as follows:
A subsurface flow constructed wetland (SFCW), as discussed in
this manual, consists of a bed of graded stone media with an
influent distribution device to introduce wastewater at one end
of the bed of stone and an effluent collection device at the
opposite end of the bed to collect and discharge the treated
effluent from the bed. Vegetation in the form of plants
strategically placed on the surface of the bed of stone may also
be used.
Microbes attach themselves tc the stone media, and, if plants are
used, to the root systems of the plants. These microbes are
useful in synthesizing dissolved organics in the wastewater
thereby providing treatment. The dissolved organics are
synthesized by the microbes to provide cellular growth. Some
oxygen is believed to be provided by the plants through the root
systems but the actual amounts provided are not known at this
time.
The SFCW technology as used in wastewater treatment system
applications is generally credited to the experimental work
accomplished by the National Aeronautics and Space Administration
in efforts to recycle wastewater in space stations.
The use of SFCW technology by small communities and other
relatively small entities seeking a cost-effective, energy
efficient, and relatively unsophisticated method of wastewater
treatment has resulted in a proliferation of the use of the
systems. Many of these systems have experienced problems in
consistent performance with the passage of time. This manual is
intended to identify possible causes of the problems encountered
and to recommend design, const.ruction, and operations
characteristics in an attempt to avoid the problem areas.
If no plants are used in a SFCW, it is commonly referred to in
Region 6 as a microbial rock filter (MRF). If vegetation in the
form of plants are used in a SFCW, it is referred to as a
microbial rock plant filter (MRPF).
SFCW CONFIGURATION
The SFCW with the use of plants is schematically represented in
Figures 1 and 2. ^riginally, it was believed that a large
length-to-width ratio on the order of 10 to 1 or greater was
desirable to prevent the possibility of short circuiting of the
-------
TYPICAL SUBSURFACE FLOW CONSTRUCTED WETLAND
PLAN VIEW
INFLUENT
\
EFFLUENT
— . o o
0 0
. 0 0
FLOW o 0
-T O O
10 0 0
o.cl
— -*-O o
.10 OC;
o
o
o
o
o
0
o
o
o
0
0
0
o
o
o
o
0
NOTE PLANT DENSITY SHOULD
BE MAINTAINED AS SHOWN
ADDITIONAL PLANTS THAT
MIGHT DEVELOP AND REDUCE
THE ORIGINAL PLANT SPACING
SHOULD BE REMOVED
FIGURE I
PLANT
FREE
ZONE
10'
LONGITUDINAL SECTION
y
N/
6' LAYER OF
•3/4'-\ 1/2' ROCK
PLANT
FREE
ZONE
10'
880. jg^gQP pffA3pQ
L J I J-S-'^.y^L
oo
DO.
PERFORATED
INLET PIPE
2' - 5' ROCK
HOLD DOWN
STRAP
PERFORATED
OUTLET
ANCHORS P1PE
VOLUME • MINIMUM OF 24-HR DETENTION
VOID SPACE WITHIN 2'-5' ROCK IS 35% WHEN PLANTS
ARE USED AND 45% WHEN NO PLANTS ARE USED
IT IS SUGGESTED THAT PLANTS BE PLACED ON 10 FT. CENTERS
FIGURE 2
-------
wastewater through the filter. Experience has revealed that a
long length-to-width ratio can adversely impact the hydraulic
gradient of the stone bed. Today, lower length-to-width ratios
are preferred, particularly in the larger systems. Short
circuiting can be prevented by segmenting the SFCW into smaller
parallel components.
The depth of the stone bed has characteristically been 24 to 30
inches. There is some evidence to indicate that the bed depth
can be 18 inches and possibly avo^d anaerobic conditions at or
near the bottom of the bed.
The influent device is typically a perforated or slotted plastic
pipe extending across the width of the bed in order to distribute
the flow across the entire width of the system. It is
recommended that the pipe be located at or near the top of the
stone bed. The design of the influent pipe is very important due
to the need for the flow to be e\enly distributed across the
width of the bed.
f
The effluent collection device is usually a perforated or slotted
plastic pipe extending across the width of the bed to uniformly
collect the effluent. It is recommended that the pipe be located
at or near the bottom of the bed of stone. The outlet pipe at
the effluent end of the SFCW is recommended to be capable of
elevation adjustment to enable raising or lowering the water
level in the bed of stone.
Types of plants that have been used in SFCW systems in Region 6
include the following:
• Southern Bulrush (scirpus californicus)
• Reed (phragmites communis,
• Pickerel Weed (pontederia cordata)
• Arrowhead (sagitaris spp.)
• Soft Rush (juncus effusus)
• Water Iris (iris pseudacorus)
• Duck Potato (sagittaria falcata)
• Canna Lily (canna flaccida)
• Calla Lily (zantedeschia aethiopical)
• Thaylia (dealbata and divericata)
There is no data available on the amount of oxygen that may be
released in the root zone of the plants. There is reason to
believe that some oxygen is released and is an aid in the
maintenance of an aerobic condition in the wastewater in the bed
of stone.
-------
TYPES OF TREATMENT THAT MAY PRECEDE THE SFCW
Lagoon Precedent Treatment
In a lagoon system preceding the SFCV7, the lagoon treatment works
should be designed and constructed to achieve the maximum degree
of treatment feasible consistent with acceptable criteria for
lagoon systems. Effluent works are recommended to be arranged to
minimize the discharge of solid materials from the lagoons to the
SFCW. It is recommended that applicable sources of design
information for lagoon systems be consulted with appropriate
state agencies. The EPA Design Manual, "Municipal Wastewater
Stabilization Ponds", EPA-62511-83-015 is recommended as a
reliable source to use.
Septic Tank Precedent Treatment
Septic tanks may be used as a precedent treatment process. The
tanks should be sized appropriately to maximize the reduction of
settleable solid materials and should be serviced at proper
intervals to maintain the removal efficiency. The appropriate
state agency should be consulted regarding the design and
construction of septic tanks. In general, the concentration of
the organics in the effluents from the septic tanks will be much
greater than the concentration of organics in the effluent from
the lagoons. The higher concentrations in the septic tank
effluents will necessitate longer detention times in the SFCW's
to achieve the desired effluent qualities.
-------
CHAPTER II
FACTORS CAPABLE OP INFLUENCING PERFORMANCE EXPECTATIONS
Based on data gathered from existing facilities as shown in Table
1, the performance of the SFCW, Microbial Rock Plant Filter
(MRPF), appears to be quite attractive. However, it must be
realized that much of the data in Table 1 was obtained within one
to two years after initiation of operation of the facilities.
Since the data was obtained, the performances of some of these
facilities have deteriorated. It is important to note that many
of these earlier systems were possibly designed by means of
empirical information and without reference to rational design
procedures and were constructed with inadequate hydraulic
gradients, materials that did not provide sufficient hydraulic
conductivity, and construction practices that later proved
detrimental to long term performance efficiency. These potential
problem areas were not recognized at- the time of conception or
construction and have only become apparent as the experience base
has increased with time as may be expected with any relatively
new technology.
The causes of some of the impaired performances are believed to
be inadequate control over the gradation and sizing of the stone
media, the improper type of stone media, the failure to remove
fine particles from the stone media by washing prior to
construction, improper construction practices such as running
heavy equipment repeatedly over the stone media after it was in
place, failure to line the media bed with an impervious liner,
failure to protect against erosion of side slopes resulting in
fine material entering the stone media and contributing to
clogging, inadequate hydrauMc gradient to accommodate the volume
of flow, improper placement of thp. influent and effluent works,
improper spacing of plants which may have contributed to clogging
of the stone media, failure to remove plant detritus from the
stone media, and improper sizing of the SFCW to allow for loss of
media void space with time.
It is believed the deterioration of the performance of some of
the systems, with the passage of time, has resulted from a
combination of the above mentioned causes. It should be noted
that some of the facilities exhibiting performance deterioration
are still meeting their effluent requirements.
"Pertinent design features of the facilities contained in Table 1
are shown in Table 2. It is significant that a very flat
hydraulic gradient will exist for those facilities with a filter
bed depth of 2 feet or less and a large length-to-width ratio.
Systems with large length-to-width ratios do not provide
sufficient hydraulic gradient to force the wastewater flow
through the stone ^edia and plant roots, and many have
experienced impaired performance over time.
-------
With proper attention to the hydraulic design features and the
organic removal capabilities, the systems should be able to
provide biochemical oxygen demand (BOD) removals in the range of
85 to 90%. This translates to anticipated effluent qualities as
low as 5 mg/1 BOD dependent upon the BOD of the influent to the
SFCW. With the proper hydraulic design and organic removal
capabilities, proper media selection, sizing and gradation,
proper washing of the media, adequate preparation and lining of
the filter bed, protection against side slope erosion, careful
placing of the media during construction, proper arrangement of
the influent and effluent collection devices, and adequate plant
spacing should result in a system capable of long term
performance with very little deterioration in performance with
the passage of time.
It is also important to recognize that certain management
practices are necessar1 to insure continued long term
performance. Practices such as removal of plant detritus from the
filter media surface and periodic thinning of plant growth may be
necessary. Removal of undesired extraneous vegetative growth may
also be required periodically. Trimming some types of plants may
be in order to encourage continued plant growth. If elevation
adjustment capability is included in the effluent collection
device, periodic adjustment may assist the system performance.
The capability to reeirculate the effluent back to the preceding
treatment or to the influent end of the SFCW is a recommended
feature.
Adequate attention to the above factors influencing the
performance of SFCW's should result in a system with long term
and efficient treatment capabilities.
-------
TABLE 1
MICROBIAL ROCK PLANT FILTER - SYSTEMS DATA
(Extracted from Draft Report Dated February 1993, Prepared by Sherwood C. Reed, P.E.)
MUNICIPALITY
Greenleaves Subdivision
(Mandeville LA)
DeGussa Corporation
(Mobile AL)
Phillips High School
(Bear Creek AL)
Monterey V?
Denham Springs LA
B^nton LA
Haughton LA
Carville LA
Mandeville LA
Benton KY
Hardin KY
(Phragmites Side)
Hardin KY
(Scirpus Side)
Utica MS (North)
Utica MS (South)
INFLU
TOTAL
BOD,
•g/1
36
5
13
39
25
10
12.5
20
41
26
51
51 .
38
31
EFFLU
TOTAL
BOD,
•g/1
12
4
1
15
10
6
2
8
10
9
9
4.1
14
11
INFLU
TSS
mg/1
42
23
60
32
48
57
47
93
59
56
118
118
52
32
EFFLU
TSS
•g/1
10
4
3
7
14
4
14
17
7
4
17
9.4
23
11
INFLU
NHj~ll
•9/1
4.2
10.0
9.3
0.7
0.6
1.1
4.8
1.4
5.1
10.1
10.1
6.7
5.6.
EFFLU
NH,-N
mg/1
2.3
2.0
8.0
10.0
2.8
7.2
5.1
2.1
7.4
9.9
8.3
2.9
3.1
INFLTT
NO,
•g/1
26.0
0
4.4
14.4
0.5
0.5
0.3
0.3
EFFLU
NO,
•g/1
6.0
0
0.8
9.8
0.3
0.3
0.2
0.2
FECAL
COLI
#/ 100 .1
10
3800
700
-------
TABLE 2
MICROBIAL ROCK PLANT FILTER - SYSTEMS DATA
(Extracted from Draft Report Dated February 1993, Prepared by Sherwood C. Reed, P.E.)
MUNICIPALITY
Creenleavea
Subdivision
(Mandeville LA)
DeGuaea Corp
(Mobile .AL)
Phillip* High
School
(Bear Creek AL)
Monterey VA
Denham Sprnga LA
Bent on LA
Haughton LA
Carville LA
Mandeville LA
Benton KY
Hardin KY
(Phragmitea
Side)
Hardin KY
(Scirpua Side)
Utica MS
(North)
Utica MS
(South) t
PLOW
NOD
0.149
1.78
0.0155
0.022
1.73
0.1
0.1
0.1228
1.224
0.1881
0.062
0.0492
0.05
0.11
FILTER
SURFACE
AREA
ACRES
1.1
2.2
0.502
0.056
15.2
1.2
1.5
0.64
4.56
3.6
0.79
0.79
1.5
2.0
FILTUt
union
FEET
457
475
175
74
1050
90C
934
528
470
1092
475
475
280
315
FILTER
WIDTH
FEET
105
28
125
33
630
58
72
G2
207
144
72
72
140
158
LiM
RAXIO
4.4:1
17.0:1
1.4:0
2.2:1
5.0:1
15.5:1
13.0:1
8.5:1
2.0:1
7.6:1
6.6:1
6.6:1
2.0:1
2.0:1
FILTER
DEPTH
FEET
2
2
1
3
2
2
2.5
2.5
2
2
2
2
2.1
2.1
FILTER
CROSS
SECTION
AREA
FT1
210
56
125
99
1260
lib
180
155
414
288
144
144
294
332
FILTER
CROSS
SECTIONAL
BOO LOAD
LB/DAY/PT*
0.213
1.3
1.68
0.07
0.29
0.13
0.058
0.13
1.01
0.14
0.18
0.15
0.05
0.09
FILTER
SURFACE
AREA-RYD
LOADING
OAL/DAY/FT1
3.1
133.8
0.71
9.0
2.62
1.9
1.49
3.75
12.6
1.15
1.81
1.44
1.28
2.21
PRELIM
TREAT
Acnud
Lifooi
Oxid*
Dfeb
Brt.^
AM*
Imboff
Tuk
FKU|
LMOO.
Fieri
L*fooa
F«ul
Ufoo.
A*nud
Lafooa
3C*II
Aenud
F*c«l
UfOOfl
Coaucl
Sub
CoMMt
Sub
Facal
Ijigoem
Ptat
Ufoo.
DESIOM
BRT
DAYS
1.0
1.0
3.9
0.9
1.0
2.1
4.5
1.4
0.7
5.0
3.3
4.2
5.0
3.7
oo
-------
CHAPTER III
DESIGN AND CONSTRUCTION CONSIDERATIONS FOR
SUBSURFACE FLOW CONSTRUCTED WETLANDS
The information in this chapter pertains to suggested procedures
for the design and construction of a SFCW for the reduction of
biochemical oxygen demand (BOD) and total suspended solids (TSS).
While there are no specific design equations relating solely to
TSS removal in this information, historically the TSS
concentration in the effluent from SFCW's has shown a close
correspondence to the concentration of BOD in the effluent.
This chapter does not include suggestions for design procedures
to remove ammonia, achieve denitrification, phosphorus reduction,
or reduction of fecal coliform. Additional research may be
necessary to arrive at criteria for usa in design procedures for
th-jse items.
Hydraulic characteristics play a vital role in the successful
performance of a SFCW system. The importance of uniform
distribution of the influent flow across the cross section of the
bed of stone cannot be overemphasized. The hydraulic gradient
through the system must be sufficient to drive the flow through
the media in a subsurface mode. The gradation of the media must
provide sufficient void spaces to accommodate the flow quantity.
Therefore, adequate provisions must be made to allow for some
loss of void spaces due to root growth if plants are used and for
some solids accumulation.
CALCULATING DESIGN REQUIREMENTS
The process of designing a SFCW involves several steps which are
discussed in the following paragraphs. Final sizing of the
filter may require several iterations of this process.
Suggestion of specific criteria are based on observed results in
existing facilities in Region 6.
1. Determine the existing conditions (influent BOD, TSS,
average winter temperature, average daily influent flow.
2. Determine the desired quality of the effluent (BOD and
TSS) .
3. Select bed depth (sugcrest a maximum of 2 feet [0.62 m) of
filter media), media type, and size (use a hard, insoluble
rock, 2-5 inches in diameter).
-------
4. Select a value for the void space within the rock media.
n = 0.35 if plants are used
n = 0.45 if no plants are used
5. An initial length-to-width ratio of the SFCW must be
selected based on the area calculated to achieve the
desired BOD reduction. It is suggested that a length-to-
width ratio of 2:1 be selected initially. The final
overall length-to-width ratio depends on the hydraulic
considerations.
6. Calculate the surface area required using the first order
BOD removal equation given as Equation 1.
As = (L) (W) = c;in(C0/Ce) ] :- Ktdn (Equation I)1
-2°° C
Kt -
K2Q= 1-104
0 = 1.06
"Where:
As = Surface Area of SFCW (Ft2)[m2]
L = Length (Ft)[m]
W = Width (Ft)[m]
Q = Flow (Ft /day)[m3/day]
Co = Influent BOD (mg/1)
Ce = Effluent BOD (mg/1)
Kt = Rate constant at wastewater temperature T°C
K20= Rate constant at wastewater temperature T = 20°C
d = Average depth of water in filter (Ft)[m]
n = Porosity of filter media (% as a decimal)
7. After determining the surface area required and the
corresponding dimensions based on the initial length-to-
width ratio, use Darcy's Equation to determine the
capability of the design to conduct the flow through the
SFCW.
1 Reed, Sherwood C., P.E., "Subsurface Flow Constructed Wetlands
for Wastewater Treatment," Draft, February 1993
10
-------
Q = KSAS(Equation 2)*
Where:
Q = Flow capable of being passed through SFCW (FT3/day)
[m3/day]
Kg = Hydraulic conductivity of a unit area of the media
(for 2-5 inch [50.8-127 mm] media Ks = 328,100
Ft3/Ft2/day [100,000 m3/m2/day]) (suggest using 0.3 Ks as
a safety factor)
S = Hydraulic gradient of the water surface in the system
(d/L) (suggest using 0.1 of maximum S as a safety factor)
A = Cross-sectional area of SFCW (Ft2) [m2]
If Q in Equation 2 does not equal or exceed the design flow, the
length-to-width ratio must be ad_ usted to decrease the length
while increasing the width to maintain the surface area
determined by Equation 1. This process is repeated until the
design flow is less than or equal to the flow determined by
Equation 2.
NOTE: Anytime the length-to-width ratio is adjusted, the
hydraulic flow capability should be checked by Darcy's Equation
(Equation 2) . „________________„
In. some cases, the larger facilities will require a width larger
than the length. In these cases, it is suggested that the filter
be partitioned into several smaller filter units so that each
individual filter would have tlie length greater than tho width.
This would aid in preventing short circuiting within the filter.
The following are examples using the above design procedures:
Example 1 - Construction of New System with Lagoon Precedent
Treatment
A small unsewered community must build a sewer system to treat
their wastewater. The State Water Pollution Control Agency
requires them to discharge wastewater containing no more than 20
mg/1 BOD and 20 mg/1 TSS. The estimated flow to the treatment
facility is 50,000 gal/day [189.3 m /day] and the average
wintertime temperature is 4.5° C. Further assume that precedent
treatment is by a 2-cell lagoon that is designed to produce an
effluent of 30 mg/1 BOD and 90 mg/1 TSS (required by the State).
What size SFCW would be needed?
2 Ibid.
11
-------
1. Existing conditions
a. Influent quality
BOD - 30 mg/1
TSS - 90 mg/1
Flow - 50,000 gal/day = 6,684 FtJ/day [189.3 nr/day]
b. Effluent quality
BOD - 20 mg/1
TSS - 20 mg/1
2. a. Bed depth - 2 ft [0.61 H]
b. Media size range - 2-5 inches [50.8-127 mm] diameter
granite
c. Media porosity - 35% (with plants)
d. Initial length-to-width <:atio - 2:1
3. Calculate the surface area required for BOD removal
(Equation 1)
As = (L) (W) = Q[ln(C0/Ce)] -5- Ktdn(Equation 1)
Where:
Kt = K2o (0)T~2°°C T = 4.5°C
0 = 1.06
K2Q= 1,104
Kt = 1. 104(1. 06)4*5°~20°
Kt = 0.447
d = average water depth in filter
d = 1 Ft [0.305 m]
n = 0.35
Based on these values, the length and width calculations are as
follows:
L x W = (6,684 Ft3/day)[ln (30/20)] -s- (0.447)(1 Ft) (0.35)
L x W = 17,322 Ft2
12
-------
L = 2W
2W.X W .= 17,322 Ft2
W = (17,322 Ft2 -s- 2)°'5
W = 93 Ft [28.3 m]
L = 2W
L = 2(93 Ft)
L = 186 Ft [56.7 m]
4. Calculate the hydraulic flow using Darcy's Equation
Q = KS AS (Equation 2)
Where:
Ks = 328,100 Ft3/Ft2/day [100,000 m3/m2/day] (Use 0.3 KS as
a safety factor)
Ks = 109,366 Ft3/Ft2/day [33,335 m3/m2/day]
S = 2 Ft -r 186 Ft = 0.011 (Use 0.1 S for safety factor)
S = 0.0011
A = 2 Ft X 93 Ft = 186 Ft2
Q - (109,366 Ft3/Ft2/day)(186 Ft2)(.0011)
Q = 22,376 Ft3/day [S33.6 nr/day]
Notice, in this example, the hydraulic capacity of the designed
filter is 22,376 ft /day [633.6 m /day] whereas the average daily
flow is 6,684 ft /day [189.3 m3/day]. Where a disparity this
large exists between design and need, the designer should revise
the cross-sectional area of the filter to approach optimal
design.
Using equation 2, with a depth of 1.5 ft, calculate the hydraulic
capacity:
Q = KSAS
Ks = 328,100 Ft3/Ft2/day x 0.3
Ks = 109,366 Ft3/Ft2/day
S = 1.5 Ft -H 186 Ft
13
-------
S = 0.008
0.1 S = 0.0008
A = 1.5 Ft X 93 Ft
A = 140 Ft2
Q = (109,366 Ft3/Ft2/day X 140 Ft2) X (0.0008) , '
Q = 12,249 Ft3/day [347 m3/day]
•
The hydraulic capacity of 12,249 is still much larger than the
needed 6,684. Continuing this revision process will yield the
optimal design configuration of 186 ft long, 93 ft wide, and 1/25
ft deep with a hydraulic capacity of 8,881 ft /day.
It is suggested that '-'here possible, the SFCW be partitioned into
multiple cells, separated by berns wide enough, to accommodate
heavy equipment. This would allow for placement of the stone
media and maintenance of the SFCW without having heavy equipment
on the media itself. Such partitioning will also promote uniform
flow within each cell to facilitate treatment and to allow at
least one cell to be taken out of operation for maintenance.
Using this suggestion with the above example, the SFCW could be
divided into three cells, each being 31 ft x 186 ft, with a stone
media depth of 1.25 ft.
In summary, it should be noted that each design will need to
follow the process outlined in the above example. It will take
several steps to determine the optimal sizing for a facility but
careful consideration of these design suggestions and procedures
should minimize the problems that have occurred at existing
facilities.
Detention Time
Calculation of the theoretical detention time is as follows:
Detention time = (Volume x void space) -*• (Flow)
= {(186 Ft)(93 Ft)(2 Ft) x 0.35} + 6,684 Ft3/day
Detention time = 1.81 days
14
-------
Example 2 - Design a SFCW to Receive Septic Tank Effluent
A SFCW is proposed to be added to a septic tank. This on-site
system would be sized for a typical family of four and an
effluent flow per capita of 80 GPD [0.303 m /day]. The quality
of the effluent desired from the SFCW is 20 mg/1 BOD and 20 mg/1
TSS. The average wintertime wastewater temperature is 4.5°C.
1. Existing Conditions
Organic loading/capita =0.17 Ibs BOD/cap/day [77.1 grams
BOD/cap/day]
Loading = 0.17 Ibs/cap/day x 4.0 persons/residence = 0.68
Ibs BOD/day [308.4 grams BOD/day]
Flow = 80 gal/cap/day x 4.0 persons/residence = 320 gal/day or
Flow = 43.0 Ft3/day [1.2k1 i /day]
Concentration of BOD to septic tank = 0.68 Ib BOD/day +
0.00032 MGD x 8.34 Ib/gal x 1 ppm/mg/1
Concentration of BOD to septic tank = 255 mg/1
Removal of BOD in septic tank = 30% (Assumption)
Concentration of BOD to filter = 0.70 x 255 = 180 mg/1
2. Calculate the surface area required for BOD removal.
AS = L x W = Q[ln(C0/Ce)] - Ktdn (Equation 1)
Where:
a.T-20-C
Kt = K20 (
K2Q= 1.104
<=) = 1.06
T = 4.5°C
Kt = (1.104)(1.06)4'5"20 = 0.447
d = 1 Ft [0.305 m]
n = 0.35
L X W = (43 Ft3)[ln(180/20)] {(0.447)(1 Ft)(.35)}
L X W = 604 Ft2 [56.2 m2 ]
15
-------
Assuming a 10:1 length-to-width ratio
L = 10W
10W x W = 600 Ft2
W = (600 H- 10)°'5
W = 7.8 Ft [2.38 m]
Length = 78 Ft [23.8 m)
3. Calculate the hydraulic capacity with Darcy's Equation.
Q = Ks AS (Equation 2)
Where:
A = 2 Ft x 7.8 Ft = 15.6 Ft2 [1.45 m2]
Ks = 328,100 Ft3/Ft2/day (Ura 0.3 Ks as a safety factor)
Ks = 1/3(328,100) = 109,366 Ft3/Ft2/day [33,335 m3/m2/day]
S = d -s- L = 2 Ft -r 78 Ft
S = 0.026 (Use 0.1 as a safety factor)
S = 0.10(0.026) = 0.0026
Q= (109,366)(15.6)(.0026)
Q = 4,436 Ft3/day [125.6 m3/day]
Based on the above calculations, the design should obtain the
necessary BOD removal and provide adequate hydraulic capacity.
By using the same procedure as in Example 1, the depth of the
on-site SFCW can be adjusted to optimize the volume of the SFCW
while maintaining the BOD removal capability.
Detention Time
Calculation of the theoretical detention time is as follows:
Detention time = (Volume x void space) + (flow)
= (7.8 Ft)(78 Ft)(2 Ft) X 0.35 * 43 Ft3/Day
Detention time =9.9 days
16
-------
RECOMMENDED DESIGN AND CONSTRUCTION CONSIDERATIONS
In developing the SFCW, several design and construction
featuresshould be considered to enhance the ability of the
constructed facility to function properly and to improve the
operational capability.
Gradation of Media
The SFCW has two layers of graded stone media: (1) a subsurface
layer to which the wastewater flow should be confined and (2) a
surface layer which is used as a base to hold the plants. The
subsurface layer is a maximum of 2 ft in depth and the surface
layer is a maximum of"6 inches in depth. Each layer should be
graded as follows.
Subsurface Layer
With the relatively flat hydraulic gradient available in the
SFCW, a media with large void spaces enhances the movement of the
wastewater through the SFCW. To achieve adequate void space, it
is recommended that media size be 2-5 inches [50.8-127 mm] in
diameter with a gradation as follows:
Recommended Gradation
0%
10-20%
30-40%
50-80%
100%
Retained
Retained
Retained
Retained
Retained
5.
4.
4.
3.
2.
for SFCW Media
0
5
0
0
C
in
in
in
in
in
[125
[106
[100
[ 75
[ 50
mm]
mm]
mm]
mm]
mm]
sieve
sieve
sieve
sieve
sieve
The gradation should be verified at the origin and again at the
construction site. The media should also be washed to remove any
fine material before being placed in the SFCW.
Surface Layer
The six inch layer of rock on the surface of the SFCW should be
comprised of 3/4 - 1\ inch [19.1-38.1 mm] washed stone to
provide a base for the plants.
Recommended Gradat
0%
40- 75%
85-100%
100%
Retained
Retained
Retained
Retained
ion for Surface
1.5
1.0
3/4
1/2
in
in
in
in
[37.5
[25.0
[19.0
[12.5
Media
mm]
mm]
mm]
mm]
\
sieve
sieve
sieve
sieve
17
-------
Placement of Media
The placement of the media is critical because of the need to
maintain the void spaces within the SFCW. Media should be placed
on a dry subgrade, preferably on top of a synthetic liner. The
liner prevents the upward migration on soil particles from the
subgrade. Heavy equipment should never be allowed on the filter
media. ' This is to protect the void spaces from being reduced by
compaction. Placement of the media should be accomplished by
hand or by clamshell bucket.
Sideslope/Substrate Protection
Prevention of sideslope erosion and subgrade migration are
critical to protecting the integrity of the SFCW. Installation
of a synthetic liner is recommended. The liner should be
installed to prevent seepage beneath the liner and on the
sideslopes.
Recirculation Capabilities
Larger systems designed with conventional types of precedent
treatment (other than septic tanks) should have the capability to
recirculate the effluent from the SFCW back through the preceding
process. Recirculation is required when the effluent does not
meet the permit requirements; it is also important to be able to
recirculate to augment low flow conditions so the filter will not
dry out.
Influent/Effluent Structures
The influent structure should be designed to distribute the flow
evenly across the width of the SFCW and be should placed near the
surface of the stone media. The effluent structure should be
placed near the bottom of the SFCW and the effluent discharge
pipe should be designed to be adjustable to allow for control of
the water level within the SFCW.
Use of Plants
While plants have been recommended for use in the SFCW, several
systems without plants have achieved desired BOD and TSS
reductions. Plants play a role in transferring oxygen to the
wastewater in the filter though the actual amount of this
transfer by type of plant is not known. Plants may also add to
the organic load on the filter if leaves are allowed to drop on
the surface and decay . This should be taken into consideration
when the plant species are selected.
Plants are sometimes used for aesthetic reasons. Some
communities prefer to see the plants as a cover for the SFCW;
some communities believe that the plants provide odor control.
18
-------
The plant species selected for use should be indigenous to the
area so that it is tolerant of the local climate. Consult a
horticulturist, botanist, county agent, or other authority on
type of plants and timing of planting.
Plant Spacing
For comparatively large installations (up to 0.5 MGD) preceded by
lagoon systems, initial research and design for the SFCW
recommended plant spacing of about 2.5 feet on center. It has
been found that such close spacing results in root growth that
can reduce the available void space and contribute to ponding,
i.e., flow that doesn't remain below the surface in the SFCW.
The current recommendation is to place the plants 10 feet on
center and stagger the rows. If more dense patterns are used,
additional volume should be provided to account for loss of void
space to the plant roots. Plant spacing for on-site SFCW
systems can be much closer, on the order of 2.5 ft, because the
hydraulic capability of these systems is generally greater than
for larger systems.
Typical Configurations
Typical arrangements of SFWC systems with lagoon precedent
treatment and septic tank precedent treatment are shown in
Figures 3 and 4 respectively.
19
-------
TYPICAL CONFIGURATION FOR SCFW
PLAN VIEW
INFLUE
C
LAGOON
CELL #1
ZD
LAGOON
CELL #2
LAGOON
EFFLUENT
NT TO LAGOON
EN
SFCW CELL #3
FLOW *
SFCW CELL tfi
FLOW
0 0 ° o SFCW CELL #1
O n ° ,
?0 o 0 FLOW
T TO SFCW 1
FIGURE 3
EFFLUENT
TYPICAL SFCW CONFIGURATION FOR ON-SITE TREATMENT
LONGITUDINAL SECTION
SEPTIC TANKS
T=0
INFLUENT
SAMPLE
EFFLUENT
SAMPLE
18-24 IN
A —i
LARGE
STONE
(2.0 - 5.0 IN.)
6- OF SMALL STONE
(0.75 - 1.5 IN.) 12 MIL
PI PLASTIC LINER
EFFLUENT
DISCHARGE
FIGURE 4
SECTION A-A
20
-------
CHAPTER IV - OPERATIONAL CONSIDERATIONS
Although the SFCW system is very simplistic in design and
operation, it cannot be left to operate by itself. While it is
not a maintenance-free system, the maintenance required is
primarily manual labor. With the exception of a recirculation
pump that might be used only part time, there are few mechanical
parts that must be maintained. At the same time, attention must
also be given to the operation of the facility. The operational
considerations or management practices will have an effect on the
ability of the facility to maintain long term performance
capabilities. The following management practices are necessary
for the system to operate properly:
1. RECIRCULATION - the system should have the capability of
recirculating the effluent for the SFCW back to either the
preceding treatment or to the influent end of the SFCW.
During times when the effluent from the system does not
meet discharge permit effluent requirements or when
additional flow is needed within the SFCW, the effluent
flow should be recirculated.
2. PLANT MANAGEMENT - The SFCW facilities are designed for a
specific root volume and a effort must be made to maintain
this volume. Any increase in the root volume over that far
which the SFCW _was designed will result in a decrease in
the available void space. Thus, as plants grow and
multiply, thinning of the plants is necessary to maintain
the design root volume. Dead and dying material should
also be removed to prevent decaying material from entering
into the void spaces where it could aid in reducing the
void space available within the stone media. Such material
could also add to thr BOD and ammonia in the wastewater.
All undesired extraneous vegetation should be removed
periodically to prevent over growth of the stone media.
Such growth could hamper the efficiency of the SFCW.
In order to be effective in this system, some plants must
be in their growth stage. These types of plants may
require periodic trimming'to encourage growth. Whenever
any trimming is done, all debris should be removed from the
surface of the SFCW to prevent eventual migration into the
stone media.
21
-------
BED MAINTENANCE - Where multiple SFCW cells are used in
parallel, provisions should be designed into the system to
allow for the removal of one or more of the cells from
operation to perform maintenance (cleaning, media
replacement, etc.). To extend the life of the SFCW,
periodic back flooding of the stone media is recommended.
This c^n be done with a high pressure hose inserted in the
effluent collection line. Such a procedure will help in
removing some of the detritus from the surface of the stone
media and some of the solids deposition from the void
spaces within the media.
Control of liquid in the SFCW - If the SFCW is constructed
with an adjustable effluent line as recommended, periodic
adjustments may be necessary to maintain the proper liquid
level in the stone media. For example, during periods of
low flow, the effluent line should be adjusted to raise the
level of the li~"id in the SFCW. At other times, the SFCW
may need to be drained for maintenance. Experience has
shown that if freezing temperatures occur, the SFCW should
be flooded in order to protect the plant roots and to
prevent freezing within the media which will cause the
plants to be pushed upward out of the media.
22
------- |