C. Design. The design of an equalization
basin shall incorporate the evaluation and selections of a number of features
as follows:
a. On-line versus off-line
basins.
b. Basin volume providing
for a total storage detention of one-third or more of the daily design
flow.
c. Degree of
compartmentalization relative to dry weather and wet weather peak
flows.
d. Type of construction:
earthen, concrete or steel.
e.
Aeration and mixing equipment.
f.
Pumping and control in order to uniformly introduce flow into the treatment
process at approximately the daily design flow rate during peak flow
events.
g. Location in treatment
system to provide uniform loadings on downstream unit operations.
The design decisions shall be based on the nature and
extent of the treatment processes used, the benefits desired and local site
conditions and constraints.
1. The
minimum mixing requirements for equalization basins receiving raw or untreated
domestic wastewaters or sewage containing an average suspended solids
concentration exceeding 45 mg/l, shall equal or exceed 0.02 hp/1,000 gallons at
a depth providing at least one-third of the maximum storage volume. Oxygen
shall be supplied at a rate of 15 pounds per hour per gallon. Multiple mixing
and aeration units shall be provided for continuous operability.
2. Flow equalization basins receiving treated
wastewater or sewage with an average suspended solids concentration of 45 mg/l
or less shall be provided with a means of sludge removal or mixing equipment
that shall have a minimum power input of 0.01 hp/1,000 gallons of maximum
storage volume. Aerobic conditions shall be maintained. Multiple mixing and
aeration units shall be provided for continuous operability.
3. Sufficient storage shall be provided to
allow subsequent downstream unit operations that follow equalization to operate
at or less than their ted design capacity.
a.
Storage capacity shall be determined from flow data when available. Basin
volume for equalization shall at a minimum be determined from an inflow mass
hydrograph of the hourly fluctuations for a typical daily wastewater flow,
where typical daily wastewater flow is defined as the desired flow rate out of
the equalization basin. Additional equalization basin volume shall be provided
to accommodate:
(1) Continuous operation of
aeration and mixing equipment.
(2)
Anticipated concentrated treatment works recycle flows.
(3) Unforeseen changes in diurnal
flow.
b. An evaluation
of infiltration/inflow shall be conducted where influent flow data are not
available. The minimum detention time shall be eight hours of the estimated
daily maximum flow as determined by the study.
4. Flow equalization basins with a storage
capacity exceeding 20,000 gallons should be constructed as compartmentalized or
as multiple basins. Single basin installation with a bypass to downstream
treatment units may be used for treatment works with capacities less than
200,000 gpd that are not located in critical water areas. The storage basins
shall be provided with the means to be dewatered.
5. Basins designed for a combination of
storage of wet weather flows and equalization shall be compartmentalized to
allow for utilization of a portion of the basins for dry weather flow
equalization. Floating surface aerators shall have provisions to protect the
units from damage when the tank is dewatered.
6. Multiple pumping units shall be provided
that are capable of delivering flow to an overflow device so that the desired
flow rate can be maintained from the equalization basin with the largest
pumping unit out of service, unless a suitable gravity flow system is provided.
Gravity discharge from equalization shall be regulated by an automatically
controlled flow-regulating device. If a flow-measuring device is provided
downstream of the basin to monitor and control the equalization discharge, then
a raw sewage influent flow meter will not be required in accordance with this
chapter.
7. Equalization shall be
preceded with screening and should be preceded by grit removal. Facilities
shall be provided to flush solids and grease accumulations from the basin
walls. A high-water-level takeoff shall be provided for withdrawing floating
material and foam.
8. An overflow
shall be provided for equalization basins so that such basins are not flooded,
and these overflows are transmitted to downstream treatment units prior to the
disinfection unit operation.
