Page 532 - Industrial Wastewater Treatment, Recycling and Reuse

502 Industrial Wastewater Treatment, Recycling, and Reuse

available today, the tube settler provides maximum surface area, and as a
result it requires a smaller footprint than competing clarifiers do, while
ensuring maximized TSS removal from the effluent.

13.5.1.4 Secondary Treatment
The objective of secondary treatment is to reduce the organic load in the
form of COD and BOD. It also reduces ammoniacal nitrogen in effluent.
The secondary treatment process relies on the following units:
1. Biological system
Until recently, the conventional activated sludge process was used for
the treatment of BOD, as it was a proven method. As space has become
an issue, however, the size of the aeration tank has been reduced by using
advanced systems such as moving bed bioreactors (MBBRs). A two stage
MBBR is often used to ensure the maximum efficiency of the system.
MBBR biofilm technology is based on specially designed plastic biofilm
carriers or biocarriers that are suspended and continuously moving
within a tank or reactor. An aeration grid located at the bottom of the
reactor supplies oxygen to the biofilm, along with the mixing energy
required to keep the biocarriers suspended and completely mixed within
the reactor.
2. Secondary settling tank
Biomass carried from the MBBR is taken, along with the overflow
effluent, to a secondary settling tank. Unlike the primary settler, the sec-
ondary settling tank does not employ any dosing aid, as effluent from the
secondary tank will eventually be taken to the second stage MBBR.
Thus, the solids are agglomerated in the secondary settler, and the clar-
ified overflow is sent to the filter feed tank. The sludge from the hopper
bottom is sent to the sludge handling system.
3. Filter feed tank
Clarified effluent from tube settler III is collected in a filter feed tank,
which is an reinforced cement concrete (RCC) tank.

13.5.1.5 Membrane Recycling System
The biologically treated effluent from ETP passes through the following
units:
1. Filter feed pump
The effluent from the clarified effluent tank is first pumped through
a pressure sand filter (PSF), a multigrade filter (MGF), and an ultrafil-
tration (UF) system for removal of suspended solids.