Industrial Wastewater Treatment and

Zero Liquid Discharge

Centre for Water ResearchDepartment of Civil and Environmental Engineering

National University of Singapore

How Yong NG

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%

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.

How to Achieve ZLD? Example:Step 1: Effluent after solvent recovery

Step 5: Effluent after clarification Further reduction in TSS by using filters

Step 8: High TDS reject from RO Concentration of RO rejects by using evaporators

Step 7: Effluent after UF Recovery of low TDS permeate by using RO

Step 6: Effluent after filters Reduction in turbidity and SDI by using ultrafiltration

Step 2: Raw effluent from industry Removal of organic matter by using anaerobic digestion

Step 4: Treated effluent from ETP Clarification by using, tube settler/clariflocculator

Step 3: Anaerobically treated effluent Removal of balance organic matter by using aerobic digestion

Recovery of solvents by using distillation process

Step 9: Solids Extraction & Disposal 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 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

Cold Lime SofteningDischarge to the plant process

Evaporation pondEvaporation/Crystallization Process