Page 531 - Industrial Wastewater Treatment, Recycling and Reuse

Zero Liquid Discharge Solutions 501

Table 13.2 Parameters of lean stream
Parameters Unit Concentration

pH 7–8
TDS ppm 3000–4000
COD ppm 3500–5000
BOD ppm 1500–2000
Ammoniacal nitrogen ppm 50–100

The effluent in the lean stream is collected in the equalization tank
(EQT). The parameters of the lean streams are listed in Table 13.2.
The utility waste that does not have high COD levels is mixed in the equal-
ization tank or it is treated for TSS & Turbidity. Mixing after biological treat-
ment saves the hydraulic flow rate of overall ETP (Effluent Treatment Plant).

13.5.1.3 Primary Treatment
In the plant, primary treatment consists of the removal of TSS and turbidity.
This process is accomplished through the use of the following units.
1. Oil and grease trap
Wastewater from floor washing may contain oil and grease, and hence,
this system includes a trap for removing oil and grease from the raw efflu-
ent. The oil and gas traps comprise top and bottom flow cascades, through
which effluent moves, while oil floats to the top of the medium to be
skimmed off. After oil removal, the effluent flows to the EQT.
2. Equalization tank
As suggested by its name, the EQT retains the effluent until its flow
and characteristics have equalized. A coarse bubble air grid in the tank
gently mixes the contents to encourage the process, and once equalized,
the effluent is pumped to the tube settler for TSS removal.
3. Flash mixer
In the flash mixer, coagulant is introduced into the effluent. The
effluent and the coagulant must be mixed quickly, and an agitator is
employed to assist with this process.
4. Flocculator
The flocculator is the device in which the coagulant-dosed effluent is
mixed with high density polyelectrolyte and mechanically agitated.
5. Tube settler
The suspended solids load of the effluent is reduced in the tube settler.
The solids are agglomerated in the settler, and the clarified overflow is
sent to a clarified water tank. The sludge from the hopper bottom is then
sent to the sludge handling system.Compared with all other clarifiers