FLOW EQUALIZATION

The influent to an industrial wastewater treatment system (IWTS) varies in flow and concentration

of pollutants because the discharges from the manufacturing and utility processes are not constant.

This variation affects the operation of the IWTS and could adversely affect the effluent quality

from the plant. The objective of equalization is to minimize or control fluctuations in wastewater

characteristics in order to provide optimum conditions for subsequent treatment processes. There

are two common methods of flow equalization: in-line and off-line equalization as shown below:

In-line and off-line equalization systems are both effective ways to equalize flow volumes, but in-

line equalization is more effective for leveling out the variations in influent concentration because

the entire flow is blended with the entire contents of the holding tank. The size and type of

equalization basin provided varies with the quantity of waste and the variability of the wastewater

stream. The basin should be of a sufficient size to adequately absorb waste fluctuations caused by

variations in plant-production scheduling and to dampen the concentrated batches periodically

dumped or spilled to the sewer.

The purpose of equalization for industrial treatment facilities are:

1. To provide adequate dampening of organic fluctuations in order to prevent shock loading of

biological systems.

2. To provide adequate pH control or to minimize the chemical requirements necessary for

neutralization.

3. To minimize flow surges to physical-chemical treatment systems and permit chemical feed

rates compatible with feeding equipment.

4. To provide continuous feed to biological systems over periods when the manufacturing plant

is not operating.

5. To provide capacity for controlled discharge of wastes to municipal systems in order to

distribute waste loads more evenly.

6. To prevent high concentrations of toxic materials from entering the biological treatment plant.

Mixing is usually provided to ensure adequate equalization and to prevent settleable solids from

depositing in the basin. In addition, the oxidation of reduced compounds present in the wastestream

or the reduction of BOD by air stripping may be achieved through mixing and aeration. Methods

that have been used for mixing include distribution of inlet flow and baffling, turbine mixing,

diffused air aeration and mechanical aeration. The most common method is to provide submerged

mixers or, in the case of a readily degradable wastewater such as a brewery, to use surface aerators

employing a power level of approximately (0.003 to 0.004 KW/m3 ). Air requirements for diffused

air aeration are approximately 3.74 m3 air / m waste (Eckenfelder, 1989). Equalization basin types

are shown below:

Equalization basin types (Eckenfelder, 1989)

The equalization basin may be designed with a variable volume to provide, a constant effluent

flow or with a constant volume and an effluent flow which varies with the influent. The variable-

volume basin is particularly applicable to the chemical treatment of wastes having a low daily

volume. This type of basin may also be used for discharge of wastes to municipal sewers. It may

be desirable to program the effluent pumping rate to discharge the maximum quantity of waste

during periods of normally low flow to the municipal treatment facility.

Equalization basins may be designed to equalize flow, concentration, or both. For flow

equalization, the cumulative flow is plotted versus time over the equalization period (that is, 24 h).

The maximum volume with respect to the constant discharge line is the equalization volume

required.