Preliminary treatment consists of a series of steps which prepare industrial wastewaters for the actual treatment processes that follow it. Typically, preliminary treatment includes equalization, screening and, if necessary, pH adjustment. In certain industries, oil and grease removal may also be necessary
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 side-line equalization as shown in Fig.1
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
To provide adequate dampening of organic fluctuations in order to prevent shock loading of biological systems.
To provide adequate pH control or to minimize the chemical requirements necessary for neutralization.
To minimize flow surges to physical-chemical treatment systems and permit chemical feed rates compatible with feeding equipment.
To provide continuous feed to biological systems over periods when the manufacturing plant is not operating.
.To provide capacity for controlled discharge of wastes to municipal systems in order to distribute waste loads more evenly.
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/m ). Air requirements for diffused air aeration are approximately 3.74 m air / m waste (Eckenfelder, 1989). Equalization basin types are shown in Fig.2
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