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 side-line equalization as shown if Fig. 1.
Fig. 1 Methods of equalization (I.W.T, 1999)
In-line and side-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 in Fig. 2.
Fig. 2 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.