Current through Register Vol. 41, No. 3, September 23, 2024
A. General design. Basins with surface areas
many times larger than conventional biological reactors, that utilize
relatively low (less than 500 mg/l) levels of biomass, are typically referred
to as stabilization ponds (if unaerated) but are referred to as facultative
lagoons if aerated. This section provides criteria for achieving final effluent
levels of 45 mg/l BOD5 and 45 mg/l, or higher suspended
solids, as permitted limits applicable to the geographic allowance for sections
of Virginia. This level of treatment has been established in accordance with
the federal requirements for secondary treatment equivalency as achievable
through the use of stabilization ponds and facultative lagoons. The design
information contained herein pertaining to features other than biological
treatment performance criteria shall apply to the construction of earthen
basins used in the treatment of sewage.
Stabilization ponds or facultative lagoons may be
designed to achieve a higher degree of treatment or used as a biological
treatment phase in conjunction with other unit processes. Proposed design
parameters to achieve other than 45 mg/l BOD5 effluent
limits shall be thoroughly reviewed with the area engineer during the
preliminary engineering conference. Necessary features for protecting public
health and welfare and preventing potential violations of water quality
standards shall be addressed in the design report.
1. The engineering design report shall
contain pertinent information on location, geology, soil conditions, area for
expansion, and any other factors that may affect the feasibility and
acceptability of waste stabilization ponds or aerated lagoons used for sewage
treatment. Specifically, the report shall contain the following supplementary
field survey data.
a. The location and
direction of all residences, commercial development, recreation areas and
potable water supplies within one-half mile of the proposed pond or lagoon
site. If practicable, ponds and lagoons should be located so that local
prevailing winds will be in the direction of uninhabited areas.
b. Borings or other necessary geophysical
analyses required to determine surface and subsurface characteristics of the
immediate area and their effect on the construction and operation of ponds or
lagoons located on the site.
c.
Data demonstrating anticipated permeability at the elevation of the proposed
pond or lagoon bottom.
d. A
description, including maps showing elevations and contours, of the site and
adjacent areas suitable for expansion.
e. A closure plan shall be submitted to the
department prior to issuance of an operating permit.
2. The proximity of ponds or lagoons to
potable water supplies and other water resources subject to potential
contamination and location in areas of porous soils and fissured rock
formations within the depth directly affected by the ponds or lagoons shall be
reported to avoid area contamination. Monitoring and more stringent
construction requirements may be required after consideration of such factors
as distance from water sources, water uses, installation size, liner design,
and wastewater characteristics. Adequate provisions shall be made to divert
storm water around the ponds or lagoons and otherwise protect pond
embankments.
3. Access control for
the immediate area surrounding the ponds or lagoons shall be addressed by
sufficient means, such as a woven wire fence at least six feet high. Vehicle
access control shall be provided. Any access gate(s) shall be provided with
locks.
a. Appropriate signs shall be provided
along the secured perimeter or fence around the ponds or lagoons to designate
the nature of the facility and advise against trespassing. The size of the sign
and lettering used shall be such that it can be easily read by a person with
normal vision at a distance of 50 feet.
b. Access for maintenance equipment,
transporting chlorine cylinders and inspection shall be provided by an
all-weather entrance road.
B. Loading design. For stabilization pond
design with relatively uniform organic and hydraulic loading, the maximum
loading shall be 30 pounds of BOD5per day per total
surface acreage, measured at the four-foot water depth level. For stabilization
ponds that are not intended to meet federal secondary treatment equivalency
limitations but will be used for pretreatment, higher loading rates may be
acceptable.
1. In no case shall the detention
time be less than 45 days, based on a four-foot operation level. For purposes
of design, evaporation is to be considered equal to rainfall. At a minimum, a
pond system shall consist of two physically separated ponds providing three
separate treatment cells. For treatment works receiving an average design flow
of less than 0.04 mgd, a minimum of one pond with two treatment cells may be
acceptable. Organic loading to the first upstream or primary cells receiving
sewage influent shall be a maximum of twice the total design loading for the
system.
2. The shape of all cells
shall be designed to provide even distribution of flow throughout the system.
Round or square ponds are acceptable; however, rectangular ponds with high
length to width ratios (up to 10:1) are considered most desirable. If round or
square ponds are used, appropriate aeration arrangements and baffling shall be
provided in order to minimize short-circuiting. Earth dikes shall be rounded at
corners to minimize accumulations of floating materials.
3. Multiple sections of pond volume or cells
designed so as to be capable of receiving design loadings under both series and
parallel operation are required for all except small treatment works (one-half
acre of pond surface or less). The minimum freeboard shall be two feet above
the maximum operation depth, except for treatment works receiving less than
40,000 gpd. Operation depth requirements include:
a. The minimum operation depth shall be two
feet, excluding any sludge storage section.
b. The maximum operating depth shall be five
feet, excluding any sludge storage section.
4. For Class I reliability, the treatment
works should provide for operation under winter conditions. The design should
include considerations for, but not limited to, winter storage and supplemental
aeration, to prevent effluent deterioration during cold weather
conditions.
5. Installations
provided for intermittent operation at a higher than normal loading for a
relatively short portion of the year will be individually considered, taking
into account the ability of the volume of the pond system to absorb shock
loads.
6. The pond design shall
include provisions for sludge storage. The volume of sludge storage should be
based on a 20-year design life. The sludge storage section should be located in
the upstream portion of the primary cells of the pond system.
7. Piping should be provided around the first
cell in order to allow for parallel operation of the first two upstream cells
in a pond system.
C.
Features. Embankments and dikes shall be constructed of relatively impervious
materials and compacted sufficiently to form a stable structure. Vegetation
should be removed from the area upon which the embankment is to be placed.
Embankment material shall be free of vegetative material and large rocks (more
than six inches in length). Topsoil relatively free of debris may be used as
outer slope cover material. Construction details including methods of
construction, compaction details, inspection and construction certification
shall be included in the design specifications. Soils used in constructing the
side slopes shall either be compacted within 3.0% of the optimum moisture
content to at least 90% Standard Proctor Density, or compacted in accordance
with the proper site specific geotechnical recommendations.
1. The minimum embankment top width should be
eight feet to permit access of maintenance vehicles. Lesser top widths will be
considered for lagoons designed to serve 200 persons or 0.040 mgd or less. The
top width must be designed to allow adequate maintenance.
2. Outer slopes should not be less than
three-horizontal-to-one-vertical and the inner slope should not be less than
three-horizontal-to-one-vertical nor greater than
four-horizontal-to-one-vertical.
3.
Exposed embankments and excavated areas shall be protected against erosion by
suitable seeding, sodding or other methods. Additional protection for
embankments, such as riprap, may be necessary to protect against wave action
and flood currents. A method shall be specified that will prevent vegetation
growth one foot above and below the operating water levels.
4. The pond shall be as level as possible at
all points. Finished elevations shall not be more than three inches from the
average elevation on the bottom. The bottom shall be cleared of vegetation and
debris. Organic material thus removed shall not be used in the dike core
construction.
D. Liners.
A liner shall be provided for all ponds in order to minimize seepage. Material
shall be of acceptable standard to assure uniform placement and quality.
Standard ASTM procedures or acceptable similar methods shall be used for all
tests. Natural soil and enhanced soil (bentonite, cement, etc.) material used
as liners should be capable of achieving a maximum coefficient of permeability
of one tenth of one millionth of one centimeter each second
(1X10-7cm/sec) or approximately three centimeters
per year or less. Following the specified level of compaction, liner material
used for the pond's side and bottom shall have a coefficient of permeability of
one millionth (1X10-6) cm/sec or less. Bentonite,
asphalt, and other sealant additive materials should be considered to enhance
the impermeability of natural soil liners.
1.
Synthetic liner material shall be selected considering the application and
manufacturer's use recommendations. Minimum requirements for generally used
materials are:
a. Plastic film
(nonreinforced, covered)-thickness equal or greater than 0.020
inches.
b. Plastic film
(nonreinforced, noncovered)-thickness equal or greater than 0.050
inch.
c. Asphalt panels
(covered)-thickness equal to or greater than 0.25 inch.
d. Asphalt panels (noncovered)-thickness
equal to or greater than 0.50 inch.
2. Construction should be planned and
implemented to assure liner integrity throughout the coverage area for the
design life of the liner. The design specifications shall include details of
construction, inspection, and certification. Services of qualified soil
scientists, manufacturer material certification and inspection, and other
qualified means of assuring proper material installation should be used. The
liner substrate should be free of organic material, graded, rolled and be level
and smooth in nature. The preparation of a stable and adequately smooth
substrate is important for liner installation.
3. Natural soil or enhanced soil liners shall
be compacted at or up to 4.0% above optimum moisture content to at least 95%
Standard Proctor Density (or 90% Modified Proctor Density) throughout the
bottom and side coverage area. Soil liners shall not contain rock fragments
greater than two inches in the longest dimension and shall have a compacted
thickness of at least 12 inches. Soil layers shall be applied in multiple
compacted lifts of six inches or less.
4. Soil enhancers (bentonite, cement, hot
asphalt) used to improve soil impermeability can be used to reduce the required
liner thickness. Although thickness may be reduced with improved
impermeability, a minimum thickness of two inches shall be provided. The
enhanced soil liner soil matrix should be screened and free of stones greater
than 3/4-inches in the longest dimension. Reduced thickness enhanced soil
liners should be covered with a six-inch compacted protective soil layer. All
layers should be applied in lifts of six inches or less. Presence of smaller
gravel will assist in erosion protection.
5. Synthetic liners shall be constructed in
accordance with the manufacturer's applicable instructions for liner usage.
Generally, these liners should be covered by a protective layer of soil to
prevent surface damage and deterioration. The liner shall be top anchored with
a minimum berm set back and anchor depth of 18 inches. Unless the manufacturer
specifies otherwise, all seams should be perpendicular to the slope with the
overlap in the down slope direction. The pond should be subsurface drained or
the liner vented to protect against damage due to gas accumulation under the
liner. Special care and design will be required to assure a tight seal around
inlet and outlet structures. Pads will be required in areas of aerator action
and other sources of high velocity flow.
a. If
mechanical equipment may result in damage to liner, then a protective layer of
soil or other material shall be provided.
b. The pond bottom liner shall be located at
least two feet above the seasonal high water table.
E. Hydraulics. The influent line
to the pond system shall conform to acceptable material requirements of this
chapter. A manhole shall be installed at the terminus of the influent sewer
line, preceding the pond system, and shall be located as close to the dike as
topography permits. Its invert shall be at least six inches above the maximum
operating level of the initial upstream pond to provide sufficient hydraulic
head without surcharging the manhole. The influent line to the initial upstream
pond shall slope uniformly to the inner toe of the sloping embankment. A bend
may be used where the influent line changes direction at the inner toe of the
dike embankment and pond bottom.
The sewer upstream from the manhole should not be
surcharged unless the means to routinely flush the influent pipeline is
provided. If sewage is discharged to the pond system through a force main or
mains, an antisiphoning device shall be provided on the force main.
1. Influent and effluent piping shall be
located as far apart as possible along the flow path to minimize
short-circuiting within the pond.
a. The
influent line to each pond should be located approximately at the center of the
influent area provided to uniformly distribute influent flow. Influent lines or
interconnecting piping to downstream or secondary cells of multiple cells in
the pond system, that are operated in series, may consist of pipes through the
separating dikes.
b. Influent
mixing or dispersion shall be provided for ponds having two acres or more of
water surface area. All gravity lines shall discharge horizontally above an
erosion resistant surface. Force mains shall discharge vertically upward and
shall be submerged at least two feet when operating at the three feet depth.
Velocity in the force main at normal pumping rate must be sufficient to prevent
deposit of grit in the force main.
c. A concrete-lined pad with a minimum size
of four feet square or a surface with equivalent resistance should be provided
to prevent erosion at the influent point of discharge to the pond.
2. The outlet structure shall be
placed on the horizontal pond floor adjacent to the inner toe of dike
embankment. A permanent type walkway from top of dike to top of outlet
structure for access shall be provided. The outlet structure shall consist of a
well or box equipped with multiple-valved pond draw-off lines. An adjustable
draw-off device is also acceptable. The outlet structure shall be designed such
that the liquid level of the pond can be varied from a three-foot depth to a
five-foot depth in increments of one-half foot or less. Withdrawal points shall
be spaced so that effluent can be withdrawn from depths of 0.75 feet to 2.0
feet below pond water surface, irrespective of the pond depth. The lowest
draw-off lines shall be 12 inches off the bottom to control eroding velocities
and avoid pick-up of bottom deposits. The overflow from the pond shall be taken
near but below the water surface. The structure shall also have provisions for
draining the pond. A locking device shall be provided to prevent unauthorized
access to level control facilities. An unvalved overflow placed six inches
above the maximum water level shall be provided.
3. Interconnecting piping for multiple pond
installations operated in series should be valved or provided with other
arrangements to regulate flow between structures and permit flexible depth
control. Interconnecting piping and outlets shall be of materials meeting the
requirements of this chapter.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
