Current through Register Vol. 41, No. 3, September 23, 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.
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:
| Nameplate Horsepower |
Radius in feet |
| 5 |
35 |
| 10-25 |
50 |
| 40-60 |
50-100 |
| 75 |
60-100 |
| 100 |
100 |
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.
Statutory Authority
§ 62.1-44.19 of the Code of Virginia.
