Industrial Wastewater Treatment and
Zero Liquid Discharge
How Yong NG
Centre for Water Research
Department of Civil and Environmental Engineering
National University of Singapore
Zero Liquid Discharge (ZLD) – Why?
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Environmental compliance regulations get stricter each year
 Sometimes the only viable means of meeting discharge restrictions
is to eliminate discharge
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Public scrutiny of the industry’s impact on the environment is growing
 ZLD has a real potential for improving public relations with plant
neighbours and government regulatory agencies

Water use needs to be minimized to allow continued industry growth.
 ZLD is mainly about discharge, but a very real effect is a reduction of
inlet water needs, by as much as 20 – 30%
Zero Liquid Discharge – Benefits & Costs
 Benefits (Mickley, 2006):
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ZLD generally does not require a
lengthy or tedious permitting
process.
It may gain quick community
acceptance.
It can be located virtually
anywhere.
It represents a positive extreme
in recycling, by efficiently using
the water source.
Selective salt recovery is a
promising consideration in
approaching the goal of
sustainability.
 Costs:
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Capital and operating costs
associated with ZLD are higher
than any other concentrate
disposal method.
Significant Energy costs
associated with the evaporation
process.
High chemical costs associated
with treatment following a
second membrane process.
High costs associated with final
brine or solids disposal.
How to Achieve ZLD? Example:
Step 1: Effluent after solvent recovery
Step 2: Raw effluent from industry
Step 3: Anaerobically treated effluent
Recovery of solvents by using distillation process
Removal of organic matter by using anaerobic digestion
Removal of balance organic matter by using aerobic digestion
Step 4: Treated effluent from ETP
Clarification by using, tube settler/clariflocculator
Step 5: Effluent after clarification
Further reduction in TSS by using filters
Step 6: Effluent after filters
Step 7: Effluent after UF
Step 8: High TDS reject from RO
Step 9: Solids Extraction & Disposal
Reduction in turbidity and SDI by using ultrafiltration
Recovery of low TDS permeate by using RO
Concentration of RO rejects by using evaporators
Separation of salts from mother liquor using centrifuge/ dryer
Zero Liquid Discharge
- Approaches
 Examples of ZLD technologies:
 Recycle using Cold Lime Softening
 Discharging to the Plant Process
 Evaporation Ponds
 Evaporation/Crystallization of Discharge Stream
Discharge to the plant process
 Enhanced ZLD Process
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Combination Thermal Process with ZLD
Enhanced Membrane and Thermal ZLD
Wind-Aided Intensified Evaporation (WAIV)
Dewvaporation
Salt Solidification and Sequestration
 New Approaches
Evaporation/Crystallization
Process
Evaporation
Cold Limepond
Softening