| (A) Inlets. All treatment units shall have multiple
inlets (a minimum of three) and provide a method to mitigate erosion
of the media.
(B) Outlets. All treatment units shall have multiple
outlets (a minimum of three). FWS outlets shall be submerged and be
able to exclude floating detrital material and scum.
(C) Water levels. The design should allow inlets and
outlets to be raised and lowered, so that water levels within the
basin can similarly be varied and provide the ability to flood the
beds when necessary.
(D) Basin hydraulic design.
(i) Submerged flow systems (SFS). SFS systems should
be designed to prevent surface ponding of wastewater. The hydraulic
loading of these systems should be limited to the effective hydraulic
capacity of the media in place. This effective hydraulic capacity
will be a function of the clean media's hydraulic capacity reduced
by root intrusion, slime layer, detritus, algae, and other blockages.
(ii) Free water surface systems. FWS systems should
be designed to prevent scour, erosion, and plant damage during peak
flow periods. The hydraulic loading of these systems should be limited
to the open channel carrying capacity of the unit at full growth.
(11) Flow equalization. Flow to the units shall provide
for a uniform environment and growth conducive to wetlands.
(12) Initial vegetation spacing. Plants should be placed
no greater than 66 inches apart (center to center). All plants to
be used should be healthy, insect free, and undamaged. A broad diversity
of plant species within any unit is recommended.
(13) Total suspended solids (TSS) removal. The TSS
removal efficiency of the wetland system is dependent on the quiescence
of the system. However, if the facility is unable to meet its permitted
parameters, alternate means of solids removal must be pursued.
(14) Nitrification. Current wetland technology has
not proven the ability to consistently nitrify typical domestic strength
sewage to meet average permit limitations below 5.0 mg/liter. The
design of any wetland proposed for use in this type situation will
incorporate a separate nitrification process.
(15) Harvesting. Harvesting of dead wetland vegetation
and detritus plant matter is recommended.
(c) Submerged flow system design.
(1) Basic design parameters. SFS wetlands are sized
according to primary and/or secondary treatment efficiency preceding
the units, i.e., fraction of remaining five-day biochemical oxygen
demand (BOD5 ), and the permitted 30-day
average effluent discharge concentration of BOD5 .
The following factors shall be considered in the selection of the
design hydraulic and organic loadings: strength of the influent sewage,
effectiveness of primary and/or secondary treatment, type of media,
ambient wastewater temperature for winter conditions, and treatment
efficiency required.
(A) Rock/media design. The following are minimum requirements
for material specifications of the rock media.
(i) Crushed rock, slag, or similar media should not
contain more than 5.0% by weight of pieces whose longest dimension
is three times its least dimension. The rock media should be free
from thin, elongated, and flat pieces and should be free from clay,
sand, organic material, or dirt. The media should have a Morhs hardness
of at least 5.0.
(ii) Rock media, except for the top planting layer,
should conform to the following size distribution and gradation when
mechanically graded over a vibrating screen with square openings:
(I) passing six-inch sieve--100% by weight;
(II) retained on two-inch sieve--90% to100% by weight;
(III) passing one-inch sieve--less than 0.1% by weight.
(B) Installation of the rock media.
(i) Rock media shall be rinsed or washed to remove
sediment. This washing should be sufficient to remove any significant
amounts of dirt or accumulated debris.
(ii) The proper placement and installation of media
is vital to the success of the system. Undue compaction exerted on
the media's surface, as it is installed and after its installation,
can fracture and consolidate the media. The introduction of foreign
fine particles and fracturing can adversely affect the system's hydraulic
conductivity. Therefore, the following guidelines are recommended.
(I) A layer of smaller rock (0.5 - 1.0 inches) may
be used on the top of the unit to ease planting of the vegetation
and aid in vector control.
(II) Media should be gently put in place, avoiding
excessive dropping, jostling, and abusive handling.
(III) Heavy machinery should not be allowed on the
surface of the media after final placement. If machinery is allowed
on the surface, all tire ruts should be smoothed over to prevent ponding
in ruts.
(IV) Provisions should be made prior to planting to
provide water and nutrients to the plants if the system start-up will
be delayed.
(2) Organic loadings. The following tables present
typical ranges for detention time within the system in days. Each
detention time represents combinations of different classes of secondary
and advanced secondary treatment and different effluent parameters.
Design engineers may submit sufficient operating data for similar
installations, and/or actual field conditions to justify their efficiency
calculations. These times represent the theoretical detention time
of wastewater within the basin. Therefore, the amount of detention
volume available is equal to the basin's volume multiplied by the
average porosity of the media. Evapotranspiration and precipitation
should also be considered when calculating detention time. The tables
are based upon an average effective porosity media of 32%, and an
average wastewater treatment plant influent BOD5 of
200 mg/liter.
(A) Secondary and advanced secondary treatment. The
detention times in Table Number 1 are based on the fractional BOD5 remaining in the wetland system's influent
and the permitted effluent limits. For permitted effluent BOD5 concentration and removal efficiencies that
fall between the listed quantities, linear interpolation is permissible.
Table Number 1 is based on the following assumptions:
(i) ambient winter conditions wastewater temperature
of 7.5 degrees Centigrade (45.5 degrees Fahrenheit); and
(ii) an average wastewater treatment plant influent
BOD5 of 200 mg/liter. If the wastewater
winter temperature is lower than that indicated above, detention times
must be modified.
Attached Graphic
(B) Advanced secondary treatment following pond systems
only. The detention time is based on the assumption that the treatment
facility is composed of a facultative lagoon followed by two stabilization
ponds, each sized according to the current state design criteria found
in this chapter. For applications where pond effluent is to be polished
to meet an effluent BOD5 concentration
of 30 mg/liter, a minimum of one-day detention time through the wetland
system will be required.
(3) Oxygen loadings. Since SFS should function in an
aerobic environment, the wastewater dissolved oxygen level is critical.
Surface area needed to maintain sufficient oxygen transfer through
developed plant roots shall be designed based on approved and acceptable
engineering methods.
(d) Free water surface system design.
(1) Basic design parameters. FWS wetlands are sized
according to primary and/or secondary treatment efficiency, i.e.,
fraction of remaining BOD5 , and the permitted
30-day average effluent discharge concentration of BOD5 . The following factors are considered in
the selection of the design hydraulic and organic loadings: strength
of the influent sewage, effectiveness of primary and/or secondary
treatment, type of media, ambient wastewater temperature for winter
conditions, and treatment efficiency required.
(2) Organic loading. The following tables present typical
ranges for detention time within the wetland system in days. Each
detention time represents combinations of different classes of primary
and secondary treatment and the different effluent parameters. Design
engineers may submit sufficient operating data for similar installations,
and/or actual field conditions to justify their efficiency calculations
for the wetland system. The tables are based on the following assumptions:
specific surface area of the media (stems, roots, detritus, etc. 15.7
m2/m3; ambient winter conditions wastewater temperature of 7.5 degrees
Centigrade (45.5 degrees Fahrenheit); and an average wastewater treatment
plant influent BOD5 of 200 mg/liter.
(A) Secondary treatment. These detention times are
based on the type and efficiency of the primary treatment unit which
precedes the FWS wetlands.
(i) Septic tank or facultative pond as primary treatment
method.
Attached Graphic
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