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Virginia Administrative Code
Title 9. Environment
Agency 25. State Water Control Board
Chapter 790. Sewage Collection and Treatment Regulations
1/18/2020

9VAC25-790-720. Sewage Stabilization Ponds and Aerated Lagoons.

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.

Historical Notes

Former 12VAC5-581-780 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-720, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

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