The integration of waste reduction, process efficiency and product recovery has become increasingly important in industrial processes as a result of declining global water supplies and rising environmental awareness.
Zero liquid discharge is an effective strategy for overcoming these difficulties.
This extremely efficient technique prevents wastewater discharge, water is recycled and valuable sediments and chemicals are recovered.
What is a Zero liquid discharge?
Zero liquid discharge is a water treatment process whose objectives are to eliminate all liquid waste from the system and purify and recycle all wastewater.
ZLD (Zero Liquid Discharge) focuses on producing appropriate clean water for reuse and minimizing the cost of wastewater treatment.
ZLD is an advanced wastewater treatment method that involves ultrafiltration, fractional electrode ionization, evaporation/crystallization and reverses osmosis.
Traditional ZLD technologies form from brine concentrators and crystallizers, that can use thermal evaporation to convert brine into high-changed water and a solid dry product suitable for waste disposal or salt recovery.
ZLD is required when water is scarce or local water bodies are legally protected due to increasingly stringent government regulations on the discharge of brine due to the environmental impact.
As a result, many manufacturing sites and brine effluent contributors that have previously discharged brine to nearby available surface water or the sea, and contributors to wastewater treatment plants are trying to find new ways to address this problem.
Benefits of zero liquid discharge
Zero liquid discharge plant technology is becoming more common around the world as an important wastewater management strategy for minimizing water contamination and extending water supply.
The primary advantages of ZLD are as follows:
1-Reduces wastewater
2-Increases water
3-Produces a valuable product that can be sold or further processed.
4-Many environmental issues are eliminated.
5-Reduces reliance on local water.
6-Reduces the possibility of penalties for waste emitted in violation of legal requirements.
Applications of zero liquid discharge
Discharge flow streams can come from a variety of sources, involving:
1-Cooling tower blowdowns in heavy industry and power plants
2-Ion exchange regenerative streams, particularly in food and beverage processing
3-Desulfurization of flue gases, the wet wastewater stream
4-Municipal potable water and wastewater
5-Reuse of process water from agricultural, industrial and municipal sources.
6-Various industrial wastewater streams from the textile, coal-to-chemical and pharmaceutical industries as well as industries such as dairy or battery.
7-Plant Discharge Compliance
8-IGCC Plant treatment
Design of zero liquid discharge
in general, the main zero liquid discharge treatment system will involve some form of:
Clarifier and/or reactor
to precipitate metals, hardness and silica, use a clarifier and/or reactor.
Chemical feed
Any metals and suspended solids can be precipitated, flocculated, or coagulated with the help of a chemical feed.
Filter press
A filter press is used to concentrate secondary solid waste after pre-treatment or in conjunction with an evaporator.
Ultrafiltration (UF)
is used to remove all residual suspended solids and prevent fouling, scaling and corrosion down the line of treatment.
Reverse osmosis (RO)
In the primary phases of concentration, reverse osmosis (RO) is used to remove most dissolved solids from the water stream.
Brine concentrators
further concentrate the reject RO stream or electrodialysis reject to reduce waste volume evaporator used to vaporize access water evaporator before the crystallizer, in the final stages of waste concentration.
Crystallizer
to remove any remaining liquid, leaving you with a dry, solid cake for disposal
Steps of zero liquid discharge
Regardless of the various sources of a wastewater stream, a ZLD system usually consists of two steps:
The first step
Pre-concentration of the liquid waste stream is a critical step since it minimizes the volume of trash and reduces the cost of the evaporation/crystallization stage.
Electrodialysis (ED) or membrane techniques like forwarding Osmosis (FO) and Membrane Distillation (MD) are commonly used to accomplish this.
Preconcentrating with a membrane system, depending on the wastewater composition, can drastically reduce the sizing required of the backend evaporation system, and therefore the system capital and operational costs.
It is worth noting that proper pretreatment, such as softening and pH correction, is frequently required to achieve high recoveries in a wastewater membrane system.
The second step
Evaporation/Crystallization: The following step in thermal processes or evaporation is to evaporate all remaining water, collect it, and drive it for reuse.
The remaining waste is sent to a crystallizer, which boils all the water until all the contaminants crystallize and are filtered out as a solid.
Evaporation systems are more expensive in terms of both capital and operating costs than membrane systems, with crystallizers being the most expensive.
As a result, membrane systems can be used to lower the evaporation system’s capital and operating costs wherever possible.
Disadvantages of Zero liquid discharge
Because of the variety of ZLD systems in use and the differences in treatment depending on your industry and individual process, it is difficult to determine exactly what you are seeing, but here are the most common problems with zero liquid discharge and solutions:
Fouling
is the result of an excess of deposits in your filtration membranes caused by Pollutants such as Calcium, Magnesium, Silica, Bacteria, metal oxides and other colloidal and suspended solids.
If left untreated, these pollutants can produce catastrophic deposition that can plug piping and decrease the efficiency of your ZLD system that affecting energy usage and causing membrane damage.
These problems can take place in a matter of hours, and cleaning these membranes is difficult once they have become fouled, so it is best to take steps to prevent this fouling in the first place by ensuring proper pretreatment.
Scaling
It occurs in RO membranes and heat-related treatments (such as evaporation) when small particles pass through pretreatment and filtration for deposits; however, scaling forms much harder deposits than fouling.
As a result, water permeability is reduced, energy consumption increases and membranes and equipment are damaged.
To reduce the possibility of scale formation, these scale-forming minerals must be removed during pretreatment and any other parts of the ZLD process where they may be present.
TSS/TDS
When using ZLD, it is critical to remove minerals to the proper concentration, especially when using evaporation or other thermal-related treatment methods.
Adding heat to solutions that have an incorrect total suspended solids (TSS) to total dissolved solids (TDS) ratio can quickly and easily result in piping corrosion, fouling, and/or scaling.
As water evaporates, the remaining solids concentrate.
These solids and hardness start to create on heat transfers and other internal piping if they are not properly removed during blowdown or avoided in the first position by properly pretreating the wastewater.
This can cause the system to plug up and cause downtime or failure.
Minimal liquid discharge VS ZLD
ZLD has long been recommended as an eco-friendly option for the industry to satisfy ever-stricter discharge limits and for recycling their wastewater streams.
ZLD techniques, however, are technically challenging, very pricey, and not always environmentally benign because of the extra resources and energy they demand.
Minimum liquid discharge (MLD) operations generate small quantities of liquid waste with nearly zero liquid discharge.
When compared to a ZLD solution, which will have greater capital and operational expenditures, using an MLD solution is typically more cost-effective.
By deploying trustworthy filtration-based technologies that may achieve high water recovery at a fraction of ZLD’s costs, end users are increasingly embracing a minimum liquid discharge (MLD) solution to the wastewater treatment problem to reduce their water footprint.
References
[1] Aquatech, zero liquid discharge, online, available at https://www.aquatech.com/solutions/zero-liquid-discharge/
[2] Lenntech, zero liquid discharge, online, available at https://www.lenntech.com/processes/Brine-Treatment/zero-liquid-discharge-zld.htm
[3] Samco, JANUARY 19, 2017, Five Common Problems with Zero Liquid Discharge and How to Solve Them, online, available at https://www.samcotech.com/five-common-problems-zero-liquid-discharge-how-to-solve/
[4] Mega, zero liquid discharge, online, available at https://www.mega.cz/zero-liquid-discharge/
