RO is the method of cleaning polluted water with a membrane and pressure unit. Also, RO produces high amount of liquid discharge, i.e. saline water. To reduce discharge into streams and make such a system which is self-sustainable and gives zero effluents, Zero Liquid discharge system is employed.
ZERO LIQUID DISCHARGE
RO is currently the best and most efficient desalination technology available. The purpose then is to use RO to recover as much water as possible from evaporation. The most basic type of ZLD system is Evaporation and Crystallisation which aims at 90% water recovery and 100% crystallisation.
HYBRID ZLD TECHNOLOGIES
Due to the high cost of ZLD there is a strong motivation to employ more energy? saving process to minimize the MVC/Crystallization costs in ZLD and use some more energy efficient process or any hybrid of it.
Some examples of Hybrid ZLD are
- 1. Reverse Osmosis (RO) – rejects salt, passes water, 2?4 kWh/m3
- 2. Nano filtration (NF) – like RO, but passes some salt
- 3. Electrodialysis(ED) or ED reversal (EDR) – removes ion, costs intermediate to RO
- 4. Natural Evaporation – slow and large foot prints, not so economical.
Another possible motivation is presence of organics, volatiles, colloids etc., which complicates the treatment and water reuse
- 1. Conventional bioremediation
- 2. MBR/UF pre-treatment
ZLD COMBINED WITH RO
RO is presently the best and most energy?saving available technology for desalination. The purpose is then to use RO to recover as much water as possible before evaporation. The ZLD cost drops as RO recovery increases.
Recovery in RO is limited to some extent by 3 main factors
- High osmotic pressure
- Scaling by soluble salts and
- Fouling (by organic matter, colloids, colloids, biofilms, biofilms, etc.).
RO recovery is enhanced using the simple 2-stage method
The recovery rate of ZLD is enhanced in RO with ZLD using the below mentioned techniques or methods:
- The simple 2-stage method(different membranes and pressures are used in each stage)
- Intermediate softening / precipitation (more chemicals used)
- Biological Pre-treatment
- High Efficiency RO process (HERO)
All of the above-mentioned processes give higher recovery efficiency till 90%.
ALTERNATIVE FOR HYBRID ZLD
The alternative hybrid technologies give even higher efficiency than conventional and above mentioned hybrid ZLD’s.
- a. SPARRO (Seeded RO)
- b. ARROW – pH elevation + IX + RO
- c. VSEP (membranes vibrated)
- d. HEEPM (by EET Corporation) – ED treats the feed to RO
- e. ForwardOsmosis (FO)
- f. Molecular distillation (MD)
- g. Wind?assisted intensified evaporation (WAIV)