1. Industrial Wastewater Treatment and Zero Liquid Discharge
How Yong NG
Centre for Water Research
Department of Civil and Environmental Engineering
National University of Singapore

2. Zero Liquid Discharge (ZLD) – Why?
Environmental compliance regulations get stricter each year
Sometimes the only viable means of meeting discharge restrictions is to eliminate discharge
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%

3. Zero Liquid Discharge – Benefits & Costs
Benefits (Mickley, 2006):
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:
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.

4. How to Achieve ZLD? Example:
Step 1: Effluent after solvent recovery
Recovery of solvents by using distillation process
Step 2: Raw effluent from industry
Removal of organic matter by using anaerobic digestion
Step 3: Anaerobically treated effluent
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
Reduction in turbidity and SDI by using ultrafiltration
Step 7: Effluent after UF
Recovery of low TDS permeate by using RO
Step 8: High TDS reject from RO
Concentration of RO rejects by using evaporators
Step 9: Solids Extraction & Disposal
Separation of salts from mother liquor using centrifuge/ dryer

5. 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
Enhanced ZLD Process
Combination Thermal Process with ZLD
Enhanced Membrane and Thermal ZLD
Wind-Aided Intensified Evaporation (WAIV)
Dewvaporation
Salt Solidification and Sequestration
New Approaches
Discharge to the plant process
Evaporation/Crystallization Process
Evaporation pond
Cold Lime Softening