Zero Liquid Discharge Wastewater Treatment
Zero-liquid discharge (ZLD) is a water treatment process in which all waste water is purified and recycled; therefore, leaving zero discharge at the end of the treatment cycle. ZLD is an advanced wastewater treatment method that includes ultrafiltration, reverse osmosis, evaporation/crystallization, and fractional electro deionization.
Applications of ZLD:
- Plant Discharge Compliance
- Cooling Tower Blowdown
- Flue Gas Desulphurization (FGD)
- Gasification Wastewater
- Coal to Chemicals (CTX) waste
- IGCC Plant treatment
- Falling Film Brine Concentrators
- Forced Circulation Crystallizer
- Horizontal Spray Film Evaporator
- Hybrid Systems with Membrane Pre-Concentrators
- Biological Treatment
- Solids Waste Handling
Different ZLD systems:
For over 30 years vapor compression evaporation has been the most useful technology to achieve zero liquid discharge. Evaporation recovers about 95 % of a wastewater as distillate for reuse. Waste brine can then be reduced to solids in a crystallizer/dewatering device. However, evaporation alone can be an expensive option when flow rates are considerable.
One way to solve this problem is to integrate membrane processes with evaporation. These technologies are nowadays often combined to provide complete ZLD-systems. The most common membrane processes used so far are reverse osmosis (RO) and electrodialysis reversal (EDR). By combining these technologies with evaporation and crystallization ZLD systems have become less expensive. They are however combined differently depending on the circumstances. Together with these components, a variety of other well-known water treatment technologies are used in ZLD-systems for pre-treatment and polishing treatment. These treatments are:
- pH adjustment
- mixed/separate bed
- oil/water separator
- oxidation (UV, ozone, sodium hypochlorite) 4
- dissolved air flotation (DAF)
- carbon adsorption
- anaerobic or aerobic digestion
The variation of ZLD-systems are endless
Designing a ZLD System:
Characterizing the waste stream is difficult yet essential when designing a ZLD-system. It is important to start off with a realistic estimate of composition, feed chemistry and flow rate. A poorly described waste stream will likely lead to a design which is far from its optimum.
The system will either be too large and expensive or too small to achieve the required separation. The selection of the waste water flow rate typically determines the size and therefore the initial capital cost of the ZLD-system.
But how does one characterize a waste stream? For existing plants, waste stream compositions can be measured directly, preferably on multiple occasions to characterize a range of compositions.
Depending on the process, the feed chemistry may change occasionally, and it is of great importance that one has this in consideration. The most common measurements today include organics, for example, chemical oxygen demand (COD), biochemical oxygen demand (BOD), total organic carbon (TOC) and inorganics (anions, cations, silica).
If the water flow rate is small, not many components are necessary. The following general guidelines are accepted today:
- Below 10 gpm of feed – crystallizers and/or spray dyers can be combined.
- 10 – 50 gpm of feed – use a crystallizer alone.
- 50 – 100 gpm of unsaturated feed – use an RO/EDR/crystallizer combination.
- 50 – 100 gpm of saturated feed – use an evaporator/crystallizer combination.
- 100 – 500 gpm of feed – either an RO/crystallizer or an evaporator/crystallizer combination may be the most economical.
- 500 – 1000 gpm of feed – all three should be used.
Zero liquid discharge technologies help plants meet discharge and water reuse requirements, enabling your business to:
- Meet stringent cooling tower blowdown and flue gas desulfurization (FGD) discharge regulations
- Treat and recover valuable products from waste streams
- Better manage produced water
Panorama offers complete thermal and non-thermal ZLD solutions to manage tough-to-treat wastewaters. Panorama’s solutions can help recover more than 95% of your plant’s wastewater.