| (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
(ii) Imhoff tank or clarification as primary treatment
method.
Attached Graphic
(B) Advanced secondary treatment. The detention times
given in Table Number 4 are based on the fraction of BOD5 remaining after secondary treatment. Table
Number 4 assumes a wastewater treatment plant influent BOD5 of 200 mg/liter. For percentages that fall
between the listed quantities, linear interpolation is permissible.
Attached Graphic
(C) Vector control. The presence of mosquitos and other
vectors has been associated with open water. Since the FWS systems
will have open water surfaces, vector control must be a priority.
Vector control mechanisms using natural controlling agents such as
introduction of Gambusia spp. (mosquito fish) have been proven effective.
However, if the predatory fish are used to control vectors, provisions
must be made within the basin for designated open water areas so the
fish can surface for oxygen. At least 20% of the basin's surface should
be open to the atmosphere. Other methods of vector control may be
considered. However, the introduction of chemicals (such as pesticides)
should be carefully evaluated so that there are no adverse effects
on vegetation or on effluent water quality.
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