Industries like textile, paper, pharmaceuticals, food, chemical, electric power plant, iron and steel, nuclear, oil and gas, petroleum, and others produce colossal amounts of wastewater and discharge it through plant outfall to a water body’s surface, an evaporation pond, deep well, streams, and sometimes inject deeply in lands. However, the growing environmental concerns regarding such discharge practices have led to the development and inclusion of Zero Liquid Discharge systems (ZLD).
As a result, the Union Environment Ministry of India, from 2016, has imposed mandates for incorporating zero liquid discharge units for industries like sugar, fertilizer, pulp, paper, thermal power plants, textiles, slaughterhouses, coffee, common effluent treatment plants, sewage treatment plants, etc.
Zero liquid discharge is a wastewater treatment process where wastewater is purified and reused, leaving zero liquid waste. It recovers around 95% of liquid waste and produces dry cake for disposal.
What are the fundamentals of zero liquid discharge system design?
The specific components of a zero liquid discharge system depend on factors like quality and quantity of contaminants in water, the needs of industries and municipalities, and the system’s required flow rate. However, a basic zero liquid discharge system includes;
- Clarifiers reduce metals, hardness, and silica through precipitation
- Chemical feed facilitates the precipitation, coagulation, and flocculation of suspended solids.
- Filter press to concentrate the solid waste after pretreatment.
- Ultrafiltration reduces leftover traces of suspended solids and prevents corrosion in other lines of treatment.
- Reverse osmosis lessens the dissolved solids from the wastewater, the primary concentration phase.
- Brine contractors further concentrate the rejected reverse osmosis stream and reduce waste volume.
- The evaporator vaporizes excess water in the final stage of concentration.
- Crystallizer removes remaining liquid, leaving a dry solid cake for disposal.
How does a zero liquid discharge water treatment technology work?
- The pretreatment stage of wastewater treatment involves clarifiers that remove large matters and suspended solids through precipitation.
- Many times in pretreatment, lime or caustic soda is used to stimulate coagulation, where chemicals are added in wastewater that reacts with suspended solids. During this process, water is mixed continuously to induce a better chemical reaction. Moreover, pH adjustment helps the coagulation process.
- Flocculation follows coagulation, where polymers are added to wastewater and slowly stirred to form flocs.
- After this, the water flows into a sedimentation tank where flocs are settled down and raked out from the water.
- Depending on wastewater quality, additional treatment may be required for an efficient treatment.
Ultrafiltration can be used after clarifiers, and in some cases, pretreatment involves only ultrafiltration because of the successful results of advanced membrane technologies.
The concentration process of wastewater is mainly done through reverse osmosis (RO), brine concentrators, or electrodialysis. Many times these processes are combined to get efficient results and are able to pull out 60-80% of water.
- RO removes the majority of dissolved solids, but water should be pretreated to avoid the fouling of the membranes.
- Brine concentrators also remove dissolved solids and are beneficial for the water having high salt concentrations.
- Electrodialysis involves positively and negatively charged ions that create an electric field to extract charged particles from wastewater and concentrate the brine to saturation level. It is often combined with reverse osmosis systems to gain successful results.
- Evaporation is a thermal process involving water evaporation, its collection, and reuse.
- Falling film evaporation concentrates the water up to the initial crystallization point, also called supersaturation. Moreover, it is also an energy-efficient method.
- Acids are added to neutralize the solution to avoid the scaling of heat exchangers during the heating process.
- Here, deaeration is often used to release dissolved oxygen, carbon dioxide, and other gases.
The solution from evaporators goes to crystallizers, where it is concentrated beyond the solubility of contaminants to form crystals.
The end product is dewatered through a filter press, and the water rereleased goes through the crystallization process.
- Reuse of treated water
The treated water is pumped into a holding tank from where it can be used for required purposes. It can be used for other industrial processes, drinking purposes, and irrigation.
Why is a zero liquid discharge water treatment system important?
- The government tightens the wastewater discharge limits, and thus zero liquid discharge system is advantageous as it leaves zero liquid at the end.
- The zero liquid discharge process can recover valuable resources from the wastewater, which can be reused or sold.
- Potassium sulfate (K2SO4) from a salt mine
- Caustic soda (NaOH)
- Sodium sulfate sodium chloride (NaCl) from a battery manufacturing unit
- Gypsum (CaSO4.2H2O) from mine water and flue gas desalination
- Wastewater management costs are reduced due to decreased wastewater.
- Recycled water is used for many industrial purposes and agricultural purposes.
- It reduces the transportation costs for off-site disposal of treated water and waste material.
- When wastewater is treated on-site, it reduces water acquisition costs and risks.
- It protects natural water resources- ponds, lakes, and rivers.
How to choose a zero liquid discharge system?
ZLD system design largely depends on factors like,
- The amount of dissolved material in wastewater
- Flow rate required by the system
- Type of contaminants present in water
- It typically consists of three stages.
- A wastewater treatability study will determine if the wastewater can be treated and how it can be treated. In addition, it can clearly identify the contaminants present in water.
- Pilot testing can validate and optimize the design.
Though zero liquid discharge systems require high energy and capital investment, there are many reasons to implement a zero liquid discharge system for wastewater treatment. The most significant is the regulatory consideration for water discharge.