Virginia Administrative Code › Title 9 - ENVIRONMENT › Agency 25 - STATE WATER CONTROL BOARD › Chapter 790 - VIRGINIA POLLUTANT DISCHARGE ELIMINATION SYSTEM (VPDES) PERMIT REGULATION › Part III - Manual of Practice for Sewerage Systems and Treatment Works › Article 6 - Biological Treatment › Section 9VAC25-790-730 - Aerated lagoons

Current through Register Vol. 40, No. 24, July 15, 2024
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.