The overland flow process is the application of wastewater
along the upper portion of uniformly sloped and grass-covered land
and allowing it to flow in a thin sheet over the vegetated surface
to runoff collection ditches. The primary objective of this process
is treatment of wastewater. Utilization of this process does result
in a discharge and therefore a waste discharge permit from the Texas
Water Commission is required. This process is best utilized on soils
with low permeability. The performance of the overland flow process
is dependent on the detention time of the wastewater on the vegetated
sloped area. Therefore, in order to meet a specified effluent criteria,
the hydraulic loading rate, the application rate, and the effectiveness
of the distribution system are essential design considerations. For
detailed process design guidance, the latest edition of the Environmental
Protection Agency Technology Transfer Process Design Manual for Land
Treatment of Municipal Wastewater may be used.
(1) Hydraulic loading rate. The hydraulic loading rate
and application rate can vary depending on levels of pretreatment,
quality of effluent, temperature, and other climatic conditions. A
hydraulic loading rate of 1.5 to 2.0 inches per day and an application
rate of six to eight gallons per hour per foot of slope width are
suggested as general guides. The design rates selected and their justification
shall be submitted in the design report.
(2) Wastewater storage. Storage capacity for inclement
weather conditions shall be provided. To minimize the impact of algae
on the treatment performance, this storage shall be designed as an
off-line basin, used only as needed and emptied as soon as possible
by blending with other pretreated wastewater prior to application.
To control odors, provisions for aeration in the storage basin should
be considered.
(3) Soil testing. For the overland flow process, the
soil profile evaluation should extend to a depth of at least three
feet. The soil sampling and testing specified in subsection (b) of
this section shall be representative of the soil to this depth.
(4) Other design considerations.
(A) The overland flow process treatment area shall
be subject to the same buffer zone requirement as a treatment plant.
(B) The minimum slope length for the applied wastewater
shall be 100 feet.
(C) The sloped areas to receive wastewater shall be
uniformly graded to eliminate wastewater ponding and short circuiting
for the length of the flow. Site grading procedures and tolerances
shall be included in the specifications. Minimum slopes shall equal
or exceed 2.0%; maximum slope shall not exceed 8.0%. The application
site shall be protected from flooding.
(D) The application cycle should provide a maximum
of 10 hours for dosing followed by a minimum period of 14 hours of
resting.
(E) The method of application shall provide uniform
coverage of the area.
(F) A vegetative cover shall be provided on the application
site. The plant types selected shall be suitable for overland flow
conditions and shall provide uniform coverage of soil to prevent short
circuiting and channelization of the area.
(G) Wastewater quality and disinfection requirements
for overland flow process discharges will be established by the discharge
permit.
(H) An effluent sampling station shall be provided
prior to discharge to surface waters. The sampling and reporting requirements
will be established by the discharge permit.
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