Section 670. Attached growth processes

Article 6. Biological Treatment

A. The contactor, or media filled reactor, utilized for attached growth biological processes shall be preceded by primary clarification equipped with scum and grease collecting devices. Other pretreatment facilities equivalent to primary clarification may be proposed for evaluation by the department. The media shall provide sufficient surface area to support the attached biological growth necessary to achieve the desired performance standard. Recirculation of treated wastewater back to the contactor influent should be provided to maintain design loadings.

B. Trickling filters. Biological contactors called trickling filters shall be designed so as to provide either the reduction in biochemical oxygen demand required by the issued certificate or permit, or the treatment necessary to properly condition the sewage for subsequent treatment. This section provides performance criteria to achieve final effluent limits to meet federal secondary equivalency requirements for trickling filters. Such biological contactors may be designed to achieve higher degrees of treatment or used in conjunction with other unit operations. Where the design intent is to achieve other than secondary equivalency levels, the proposed design parameters shall be thoroughly reviewed during the preliminary engineering conference.

1. The hydraulic loading used for design of standard rate trickling filters shall be between two and four million gallons per acre per day with an organic loading between 400 and 800 pounds of BOD₅ per acre foot per day.

2. The hydraulic loading used for design of high-rate filters shall be between 10 and 30 million gallons per acre per day with an organic loading between 1,200 and 3,300 pounds BOD₅ per acre foot per day.

3. Other design loadings that are based on pilot studies and related to design and performance parameters through rational design equations or models will be evaluated by the department.

4. The performance of biological contactors can be detrimentally affected by diurnal loading conditions. The volume of media as determined from either pilot plant studies or from acceptable design equations shall be based upon the design peak hourly organic loading rate rather than the average rate. An alternative for reducing the design peak flow would involve provision of adequate flow equalization prior to the contactor.

5. Consideration should be given to the use of two-stage biological contactors in series operation where single stage reactors may not accomplish the required removals. Expected treatment efficiencies shall be calculated and documented.

C. Features. All hydraulic factors involving proper distribution of sewage on the contactor media shall be carefully calculated. For reaction type distributors, a minimum head of 24 inches between the low water level in siphon chamber and the horizontal elevation of the center of distribution arms shall be required. Surge relief to prevent damage to distributor seals shall be provided where sewage is pumped directly to the distributors. A minimum clearance of six inches between the media surface and the bottom of distributor arms shall be provided.

1. The sewage may be distributed over the contact reactor media surface by rotary distributors or other suitable devices that will permit reasonably uniform distribution to the surface area. At design average flow, the deviation from a calculated uniformly distributed volume per square foot of the filter surface shall not exceed plus or minus 10% at any point.

2. Sewage may be applied to the contactor media by siphons, pumps or by gravity discharge from preceding treatment units when suitable flow characteristics have been developed. Application of sewage should be continuous. In the case of intermittent dosing, the dosing cycles shall normally vary between five to 15 minutes with distribution taking place approximately 50% of the time. The maximum rest should not exceed five minutes based on the design average flow. Consideration shall be given to a piping system that will permit recirculation.

3. Underdrains with semi-circular inverts or equivalent shall be provided and the underdrainage system shall cover the entire floor of the filter. Inlet openings into the underdrains shall have an unsubmerged gross combined area equal to at least 15% of surface area of the filter. The underdrains shall have a minimum slope of 1.0%. Effluent channels shall be designed to produce a minimum velocity of two feet per second at the average daily rate of application to the filter. Provision shall be made for flushing the underdrains. The use of a peripheral head channel with vertical vents is acceptable for flushing purposes. Inspection facilities shall be provided.

4. The underdrainage system, effluent channels and effluent pipe shall be designed to permit free passage of air. The size of drains, channels, and pipe shall be such that not more than 50% of their cross-sectional area will be submerged under the design hydraulic loading. Provision shall be made in the design of the effluent channels to allow the possibility of increased hydraulic loading. Consideration should be given to the use of forced ventilation, particularly for covered trickling filters and deep (10 feet or more) contactors filled with a manufactured media.

5. The design should provide for variable rates of recirculation for various purposes; for example, to prevent drying of a standard rate filter between dosing. Devices shall be provided to permit measurement of flow to the filter process, including recirculated flows. The design should include provisions to flood filter structures where applicable.

6. All distribution devices, underdrains, channels and pipes shall be installed so that they may be properly maintained, flushed or drained. Mercury seals shall not be permitted. Ease of seal replacement shall be considered in the design to ensure continuity of operation.

7. A freeboard of four feet or more should be provided for all deep bed contactors with manufactured media that also utilize fine spray distributors, so as to maximize the containment of windblown spray.

8. Protection such as covers or windbreaks shall be provided to maintain operation and treatment efficiencies when climatic conditions are expected to result in problems due to cold temperatures.

D. Reactor media. Contact reactor media may be crushed rock, stone or specially manufactured material. The media shall be durable, resistant to spalling or flaking and relatively insoluble in sewage. The top 18 inches of rock or stone media shall have a loss by the 20-cycle, sodium sulfate soundness test of not more than 10% (as prescribed by ASCE Manual of Engineering Practice, "Filtering Materials for Sewage Treatment Plants," Manual of Engineering Practice No. 13, ASCE, New York, New York), the balance to pass a 10-cycle test using the same criteria. Stone media shall be free from iron. Manufactured media shall be chemically and biologically inert. The media shall be structurally stable to allow for distributor maintenance or a suitable access walkway shall be provided.

1. Rock or stone filter media shall have a minimum depth of five feet above the underdrains. Manufactured contactor media should have a minimum depth of 10 feet to provide adequate contact time with the wastewater. Rock and stone filter media depth should not exceed 10 feet and manufactured filter media should not exceed 30 feet except where special construction is justified through performance data or pilot plant studies.

2. Rock, stone, and similar media shall not contain more than five percent by weight of pieces whose longest dimension is three times the least dimension. They shall be free from thin elongated and flat pieces, dust, clay, sand or fine material and shall conform to the following size and grading when mechanically graded over vibrating screens with square openings:

a. Passing 4-1/2 inch screen—100% by weight

b. Retained on three-inch screen—95-100% by weight

c. Passing two inch screen—0-2% by weight

d. Passing one inch screen—0-1% by weight

e. Maximum dimensions of stone--five inches

f. Minimum dimensions of stone--three inches

3. Applications of manufactured media such as wood, plastic, etc., will be evaluated on a case-by-case basis. The handling and placement of the media should be specified.

E. Roughing reactors. Roughing contact reactors are used to reduce the organic load applied to subsequent oxidation processes. They are particularly applicable preceding an activated sludge process or a second stage filter in a treatment works receiving high strength wastewater (excessive organic loadings). Roughing filter designs differ from other contactors principally on the basis of the deeper depths and media design utilized for given loadings in comparison to high rate trickling filters. Since it is used to reduce the downstream organic loading rather than to provide a stabilized effluent, it is designed to receive organic loadings exceeding those applied to conventional biological contactors.

F. Granular media filters. Intermittently dosed biological sand filters utilized to process septic tank effluent to meet secondary treatment standards should be limited to schools, day camps and other installations that have part-time usage. These reactors should also be limited to those installations generating a sewage flow of 20,000 gallons per day or less and provide lengthy rest periods for filter operation. Biological sand filters may serve year-round residential dwellings if the design capacity is restricted to 1,000 gallons per day or less.

1. Biological sand filters shall not be used to treat raw wastewater and shall be preceded by a minimum of pretreatment designed to produce a settled sewage with adequate grease management. The use of biological sand filters designed to enhance effluent from other sewage treatment reactors shall be evaluated on a case by case basis.

2. Sand filter media beds shall consist of level areas of sand beneath which there are graded layers of gravel surrounding the underdrains. Each filter bed shall have an impervious bottom. Sewage is discharged onto the beds through rotary distributors or pipes onto splash plates or, in the case of subsurface filters, through lines of drain tile laid with open joints. Open sand beds shall be surrounded by a concrete, brick or cinder block wall extending above the sand and at least one foot above ground level. For subsurface sand filters, the surrounding wall is not necessary except to prevent caving of the earth walls while the sand and gravel are being placed. The underdrainage system shall consist of open joint or perforated pipe tied together into a manifold and vented to the atmosphere. The minimum size for the underdrain shall be four inches in diameter. The underdrain pipes should be placed on a slope of not less than 1.0%.

3. Rock, gravel and sand media components shall be clean and free of organic matter, clay or loam soils and fine limestone material.

a. The media depth shall not be less than 30 inches. Sand media for intermittently dosed and recirculated effluent, shall have an effective size of 0.30 mm to 1.0 mm and 0.8 mm to 1.5 mm, respectively. The uniformity coefficient should not exceed 4.0. No more than 2.0% shall be finer than 0.177 mm (80 mesh sieve) and not more than 1.0% shall be finer than 0.149 mm. No more than 2.0% shall be larger than 4.76 mm (4 mesh sieve). Larger granular media up to 5 mm in effective size may be considered on a case by case basis.

b. The gravel base for sand media shall conform to the Virginia Department of Transportation's Road and Bridge Specifications (1974). The base gravel shall consist of No. 3 sized gravel with at least a three-inch depth above the sloped underdrains. The middle layer shall consist of at least three inches of No. 68 gravel, and the top layer shall consist of at least three inches of No. 8 gravel.

4. Dosing tanks with either siphons or pumps for sand filters shall have the capacities to effect the dosage volumes required. The siphons and the rotary distributor should be supplied by the same manufacturer. The influent line to the rotary distributor shall be equipped with a valved drain.

5. Sand filters designed for intermittent flooding should be divided into at least two beds for small filters and three beds for the larger filters. Distribution boxes must be provided for diverting the sewage onto the filter bed or beds desired, as it is often necessary to take one filter bed out of operation during scheduled rest periods. Providing such rest periods will prevent surface clogging that results in sewage ponding above filter media. When three filters are employed, only two beds are normally used at any one time, the other bed being held out of operation for rest periods or maintenance, if required.

6. In the design of intermittently flooded sand filters the area of the filter beds is normally based upon a rate of application of 2.3 gallons per square foot per day. Also, a sufficient amount of settled sewage should be discharged onto the sand bed surface to cover the sand to a depth of two inches.

7. A rotary distributor will accomplish uniform application of settled sewage over the sand filter surface. A uniform application will maintain the design treatment efficiency of the filter so that a relatively higher dosage rate may be utilized or, for equal sewage flows, the area of sand bed required may be less than other designs. The design of the area of the filter beds equipped with rotary distributors should be based on an application rate of 3.5 gallons per square foot per day. The amount of sewage applied to the sand filter at each discharge of the dosing siphon should be equal to a depth exceeding one-half inch over the entire sand bed area being dosed.

8. The rate of dosage onto a buried sand filter shall not exceed 1.15 gallons per square foot per day of settled sewage. Settled sewage shall be applied to the filter through lines of drain tile laid with open joints, with the tile placed in a 12-inch layer of No. 3 stone. The top of the filter may be finished with a 12-inch layer of stone. Where it is not feasible or desirable to finish the top of the subsurface filter with stone, a 3-inch layer of straw covered with a four to eight inch layer of top soil may be used. Open joint underdrain tiles shall be sloped one inch per 10 feet and shall be installed in the base gravel and connected to the effluent pipe. The ends of the distribution lines should be tied together into a manifold and should be vented to the atmosphere. All open joints shall be covered with collars of asphalt paper or other suitable material.

Distribution boxes must be provided for diverting sewage onto the filter beds through headers, with each header connecting to not more than four distribution lines, where multiple units are used. Each application must completely fill the tile lines in use.

9. Consideration should be given to providing recirculation for granular media filters to improve treatment performance. Recirculating sand filters should be designed using a hydraulic loading rate of 3-1/2 gallons per day per square feet, based on average daily flow, with an organic loading rate not to exceed 0.005 pounds of BOD₅ per day per square foot of surface area. A recirculation ratio greater than 3:1 shall be provided. The use of granular media filters for nutrient removal will be evaluated on a case by case basis based on evaluation of performance data. Granular media filters shall be timer dosed and adjustable from one to 10 minutes of dosing per 30 minutes on time.

Historical Notes

Former 12VAC5-581-730 derived from Volume 18, Issue 10, eff. February 27, 2002; amended and adopted as 9VAC25-790-670, Virginia Register Volume 20, Issue 09, eff. February 12, 2004.

Statutory Authority

§ 62.1-44.19 of the Code of Virginia.