Virginia Law

A. Low intensity aerated basins containing relatively low levels (less than 500 mg/l) of biomass are also known as aerated lagoons. The designed construction details of aerated lagoons are often similar to stabilization ponds. However, the aerated lagoon liquid depth shall be sufficient to provide for uniform distribution of dissolved oxygen in the design range of six feet to 15 feet.

B. Design. Not less than two physically separated basins providing a minimum of three treatment cells shall be used to provide the detention time and basin volume required by the lagoon system design. For treatment works less than 0.04 mgd, one basin with two treatment cells may be acceptable. The basins shall be designed to receive established loadings for both parallel and series operation. The air diffusion equipment shall be capable of maintaining sufficient mixing and oxygen concentration in the aerated volume under maximum seasonal demand conditions. Consideration should be given to fixed or floating-type in-pond baffle walls with carefully placed openings, to minimize short circuiting effects and to maximize flow path length. Deep ponds with depths exceeding 10 feet shall be provided with baffling to ensure adequate flow distribution and proper detention.

1. Detention time is dependent on many variables including type of waste, temperature, effective volume and nutrient balance. For a typical sewage influent strength of 300 mg/l or less of BOD₅ or TSS, the lagoon system design shall require total detention times in the range of 20 days. In addition to adequate volume to achieve the desired detention time, the design for primary lagoons shall include a minimum of 10% additional volume for sludge storage.

2. The initial upstream, primary cell receiving influent flow shall contain a minimum of one third of the total system volume. For small treatment works (design flow of 0.04 mgd or less) the primary cell shall contain at least one half of the total design volume.

3. Design requirements, as with detention time, may be dependent on many variables. Generally, mixing energy to maintain adequate solids suspension will be the limiting factor. All aerated lagoon systems shall be designed to maintain a normal dissolved oxygen concentration of two mg/l throughout the system. Minimum aeration requirements shall be based on established mass transfer models considering the treatment variables involved. Aeration equipment shall be capable of transferring two pounds of oxygen per pound of BOD₅ applied to the basin. Calculations shall be submitted to justify equipment and aeration patterns.

4. The influent to a lagoon shall discharge into a highly turbulent area, if applicable, to facilitate mixing effects. Baffles and pipe diffusers shall be considered for provision of uniform distribution of flow into basins with a surface area of 10 acres or more. All systems shall be designed with piping flexibilities to permit isolation of any cell without affecting the transfer and discharge capabilities of the total system. In addition, the ability to discharge the influent waste load to a minimum of two cells or all primary cells in the system shall be provided. Screening shall be provided on influent lines to prevent damage to mechanical surface aerators.

5. The outlet structure shall be located in a quiescent zone, at such a depth and at the most remote location possible with respect to the basin inlet, so as to minimize suspended solids carryover and maximize basin detention. The outlet structure shall provide for withdrawal at controlled rates for multiple depth levels, such that the liquid level in the basin can be drained and can be varied in an easily accessible manner. A minimum of three incremental withdrawal elevations should be provided, including the minimum and maximum operating depths.

6. Provisions shall be made to allow final solids settling prior to discharge. This provision should be made through the use of either a final settling basin or by providing an adequate quiescent zone toward the end of the final treatment cell. If a final settling basin is used, it shall provide a minimum of 1.5 hours settling time and conform to applicable requirements specified in this chapter.

7. It may be desirable to provide for concrete or soil cement stabilization of bottoms, walls and embankments. However, they will not be required initially unless experience dictates their necessity. Adequate concrete pads shall be provided under mechanical surface aerators to prevent bottom scour. For surface aeration, earthen embankment walls one foot above and one foot below the normal water level must be riprapped or stabilized with other suitable material to prevent erosion by wave action.

C. Mechanical aeration. Not less than two aeration units shall be used to provide the horsepower required. Aerators shall be located such that their circles of influence touch. The circle of influence is that area in which return velocity is greater than 0.15 feet per second as indicated by certified data. Without supporting data the following may be used as a guide:

1. The horsepower shall be sufficient to provide the oxygen required for BOD₅satisfaction and mixing. In no case shall the horsepower be less than 10 horsepower per million gallons of basin volume.

A sufficient number of aerators shall be provided so that a design level of dissolved oxygen within a particular cell shall be maintained with the largest capacity aerator in that cell out of service. Installation of the backup aerator should not be required, provided that it can be placed into service prior to a detrimental decrease in dissolved oxygen levels.

2. Floating surface aerators should be anchored in at least three and preferably four directions. Interconnection of floating aerators is discouraged. Flexible cables are preferred over rigid ones.

3. Surface aerators should be designed to prevent icing. Consideration should be given to the installation of splash plates for control of misting. For platform mounted aerators, the platform legs should be spaced at a sufficient distance from the aerator to minimize the effect of ice build-up caused by splashing.

a. Aerator design should provide for periodic and major maintenance and repairs and shall provide for removal of the aerators for replacement if necessary.

b. Provisions shall be made for independent operation of each aerator by on/off switches, time clocks, etc.

D. Diffused aeration. The design for compressed air volume requirements shall include the basin aeration requirements together with air used in other channels, pumps, or other air-use demands. The air diffusion equipment shall be capable of maintaining sufficient mixing and oxygen concentration in the aerated volume under maximum seasonal demand conditions. Provisions shall be made for removal of deposits for unclogging of air diffuser openings. Consideration should be given to minimizing the points of access necessary for cleaning.

1. The specified capacity of blowers or air compressors, (particularly centrifugal blowers), shall take into account that the air intake temperature may reach 40°C (104°F) or higher and the pressure may be less than normal. Air filters shall be provided in numbers, arrangement, and capacities to furnish at all times an air supply sufficiently free from dust to protect equipment and prevent clogging of the diffuser system used.

2. The blowers shall be provided in multiple units, so arranged and in such capacities as to meet the maximum air demand with the single largest unit out of service. The design shall also provide for varying the volume of air delivered in proportion to the design load for individual cells of the lagoon system.

3. Calculations shall be provided to verify that blower pressure is sufficient to dewater the diffuser lines at saturation conditions under normal operating depths.

4. Diffusers shall be arranged in each basin to provide tapered aeration with maximum intensity near the inlet. The spacing of diffusers shall be in accordance with the oxygenation requirements of the total process, i.e., the organic loading in each cell. Diffuser spacing should be designed to facilitate adjustments without major revision to air header piping. The arrangement of diffusers should also permit their removal for inspection, maintenance, and replacement without completely dewatering the basin and without shutting off the air supply to other diffusers in the basin.

5. Individual assembly units of diffusers shall be equipped with control valves, preferably with indicator markings for throttling or for complete shut-off. Provisions must be made for subsequent air flow or pressure measurements and necessary air flow adjustments. Diffusers in any single assembly shall have substantially uniform pressure loss.