ENVE 420
Industrial Pollution Control
EQUALIZATION
Dr. Aslıhan Kerç
Objective of Using Equalization Basins
• Minimize or control fluctuations in
wastewater characteristics and flowrate.
• Size and type of equalization basin varies with:
- Quantity of waste
- Variability of the wastewater stream
Purpose of using equalization for
industrial treatment facilities
• Dampening of organic fluctuations  prevent
shock loading of biological systems
• pH control  Minimize chemical
requirements for neutralization
• Minimize flow surges to physical – chemical
treatment. Permit chemical feed rates
compatible with feeding equipment
Purpose of using equalization for
industrial treatment facilities
• Provide continuous feed to biological system
(even when the factory is not operating)
• Controlled discharge of wastes to municipal
system. Distribute waste loads evenly.
• Prevent high concentrations of toxic materials
from entering the biological treatment plant.
Mixing Requirements for Equaliztaion
Tanks
• Mixing is required for:
- Adequate equalization
- Prevent settlement of solids
- Oxidation of reducing compounds
- Reduction of BOD by air stripping
(limited)
Mixing Methods
1.
2.
3.
4.
Distribution of inlet flow and baffling
Turbine Mixing
Diffused air aeration
Mechanical aeration
Mixing Methods
Most common method  Submerged mixers
For readily biodegradable wastes use surface
aerators  0.003 – 0.004 kW /m3
Air requirement for diffused air aeration:
3.5 – 4.0 m3air / m3 waste
Variable volume in the equalization tank 
provide a constant effluent flow
Program effluent pumping rate  Discharge
maximum quantity of waste during periods
of low flow
Organic loading of treatment plant is
maintained constant for 24-h.
Equalization basin equalize
- Flow
or
- Concentration
or
- Both
Flow equalization
Plot cumulative flow versus time
Determine maximum volume with respect to
constant discharge line
Concentration Equalization
Equalization basin may be sized to restrict the
dischatge to a maximum concentration
Equalization volume
• Volume of completely mixed basins used in
treatment can be considered as part of
equalization basin (e.g. Activated sludge)
Example: Total time required for equalization: 16h
Aeration basin: ?
8h
Required equalization: 8 h
Patterson and Menez Method
• Used to calculate equalization volume when both
the flow and the strength vary randomly
Based on material balance
CiQT + CoV = C2QT + C2V
Construction details for
equalization basins
1. Influent should be given to the equalization
basin close to the mixing device to prevent short
circuiting
2. Cleaning system for grease and scum which can
accumulate on the walls should be considered
3. An overflow weir should be installed to
discharge wastewater in case of failure of
pumps
4. Spray nozzles should be installed to break the
foams
Factors effecting need for equalization
1. Cost of construction
and implementing
effective
equalization
2. Cost saving by
reducing the effects
on downstream
treatment systems
Flow Equalization Processes
1. ALTERNATING FLOW:
Collects total flow of the effluent for a given
period of time (normally 24 hours) while a
second basin is discharging. Alternative between
filling-discharging. High degree of equalization
by leveling all discharge parameters.
Disadvantage  high construction cost
Flow Equalization Processes
2. INTERMITTENT FLOW DIVERSION:
Allows any significant variance in stream
parameters to be diverted to an equalization
basin for short durations. Diverted flow is then
bled back into the stream at a controlled rate.
Rate at which the diverted flow is fed back into
main stream depends on:
- volume
- variance of the diverted water  to reduce
downstream effect
Flow Equalization Processes
3. COMPLETELY MIXED COMBINED FLOW:
Complete mixing of multiple flows combined at
the front end of the facility. Reduce variance in
each stream by thorough mixing with other
flows.
Flow Equalization Processes
4. COMPLETELY FIXED FLOW:
A large completely mixed holding basin before
wastewater treatment facility. Levels variations
of influent stream parameters and provides a
constant discharge.