The rapid growth of Industrialization over the past few decades has degraded the quality of water resources available in nature, especially freshwater resources. Freshwater scarcity around the globe has posed a major threat to economic growth, water security, and ecosystem health. Numerous industrial processes have threatened the availability and value of freshwater resources.

Most of the polluting industries like Tanneries, Paper-pulp, Pharma, Dyeing, Chemicals, etc. generate wastewater with high TDS, salinity, and pH. Industrial processes today require water as a raw material or an intermediate processing material which over a course of time reduces the availability of water for the environment or other processes. In addition to this these, industrial processes often contaminate and release water that damages the local environment further. Discharge of saline but treated wastewater severely pollutes ground and surface waters.

Due to heavy contamination of numerous important rivers by industrial wastewater, several countries have begun investing in setting up Effluent Treatment Plants (ETPs) to treat the industrial wastewaters before they are ejected into the rivers and streams. Rising water demand over the past two decades has led to recovering and recycling of wastewater becoming a growing trend. However, improper management of funds and resources has led to the failure of ETP facilities in numerous countries like India and Nigeria since such processes involve intensive use of
energy and high cost. Many governments are now creating regulations that require Zero Liquid Discharge compliance, in an attempt to push high-polluting industries towards ZLD.

What is Zero Liquid Discharge?

Zero Liquid Discharge (ZLD) is an engineering approach to water treatment where all water is recovered and contaminants are reduced to solid waste. The process is widely considered by environmental experts to be beneficial to industrial and municipal organizations as well as the environment because no effluent, or discharge, is left over. ZLD systems employ the most advanced wastewater treatment technologies to purify and recycle virtually all of the wastewater produced. This technology also converts wastewater from an industrial process to solids and treats water for reuse.

Zero liquid discharge technologies help plants meet discharge and water reuse requirements, enabling businesses 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.

ZLD enables an industry to reuse wastewater so that it is no longer considered a “pure waste” that potentially harms the environment, but rather an additional resource that can be harnessed to achieve water sustainability

Importance of ZLD technology

Zero Liquid Discharge (ZLD)

In the beginning, the government came out with common effluent treatment plants (CETPs) to collect the polluted discharges from various industries at one point objectively to treat remove pollutants to an extent and then discharge the treated effluent into the rivers, lakes, ponds etc.
Again this noble objective failed for two primary reasons:
  • Industries have passed the responsibility on to CETPs started sending out (discharging) severely contaminated water coupled with escalating flows.
  • Administrative malfunction of CETPs management.
Finally, the apex body (related ministry/government department, pollution control boards, and equivalent body ) foreseeing the future came down with a legal directive for each and every industry to install a ZLD scheme.

ZLD is considered to be the most demanding wastewater/effluent treatment technology of the modern era since the cost and challenges of recovery increase as the wastewater gets more concentrated. Other water treatment processes attempt to only maximize recovery of freshwater and minimize waste. ZLD is today touted as the best approach to eliminate liquid waste and maximize water use efficiency for an industry.

Targeting ZLD for an industrial process or facility holds a number of benefits like:

  • Lowered waste volumes decrease the cost associated with waste management.
  • Recycle water on site, lowering water acquisition costs and risk. Recycling on-site can also result in fewer treatment needs, versus treating to meet stringent environmental discharge standards.
  • Reduce trucks associated with off-site wastewater disposal, and their associated greenhouse gas impact and community road incident risk.
  • Improved environmental performance, and regulatory risk profile for future permitting.
  • Some processes may recover valuable resources, for example, ammonium sulfate fertilizer or sodium chloride salt for ice melting.

Zero liquid discharge technology is also being popularly accepted due to its potential for recovering resources that are present in wastewater. Numerous global organizations have installed ZLD technologies for their waste in addition to ETPs because they can sell the solids that are produced or left over as residues after the ZLD treatment or reuse them further as a part of their industrial process. It ensures that the majority of the water is recovered for reuse.

ZLD technologies obviate the risk of pollution associated with wastewater discharge into rivers, lakes, and streams, thereby striking a balance between exploitation of freshwater resources and preservation of aquatic environments.

Concept of ZLD

Water having the highest dielectric constant is also called universal solvent because it can virtually dissolve everything in its intramolecular space.
During its passage through industrial processes (in manufacturing or utilities) it comes out as wastewater having large amounts of contamination mostly inorganic but also organic referred to as pollutants. This highly contaminated water when discharged into free flowing water bodies like lakes, rivers make the surface water polluted and unfit for human as well as plants and aquatic life.
From a typical water cycle, it is well known that the source of water is limited but consumption is increasing manifold due to increasing human lifespan and the corresponding industrialization. Over the years due to the selfish nature of human beings the industries continued to withdraw fresh water while simultaneously discharging and polluting the rivers, lakes, ponds etc.

How does a ZLD system work?

Zero Liquid Discharge (ZLD)

ZLD technology includes pre-treatment and evaporation of the industrial effluent until the dissolved solids precipitate as salts and residue. These salts are removed and dewatered. The water vapor from evaporation is condensed and returned to the process. It is the most “In demand” water treatment technology that can treat wastewater as the contaminants are concentrated.

In general, most of the ZLD systems in operation these days are based on stand-alone thermal processes, where wastewater was typically evaporated in a brine concentrator followed by a brine crystallizer or an evaporation pond. The condensed distillate water in ZLD systems is collected for reuse, while the produced solids are either sent to a landfill or recovered as valuable salt byproducts.

Normally the evaporation-crystallizing section receives the reject from a Reverse Osmosis (RO) section that concentrates dissolved solids. To prevent fouling during the reverse osmosis process, ultrafiltration is often used to eliminate suspended solids. The basic structure of a properly functioning ZLD system comprises of the following elements:

  • Clarifier and/or Reactor to precipitate out metals, hardness, and silica
  • Chemical feed to help facilitate the precipitation, flocculation, or coagulation of any metals and suspended solids
  • Filter press to concentrate secondary solid waste after the pre-treatment or alongside an evaporator
  • Ultrafiltration (UF) to remove all the leftover trace amounts of suspended solids and prevent fouling, scaling, and/or corrosion down the line of treatment
  • Reverse Osmosis (RO) to remove the bulk of dissolved solids from the water stream in the primary phases of concentration
  • Brine concentrators to further concentrate the reject RO stream or reject from electrodialysis to further reduce waste volume
  • Evaporator for vaporizing access water in the final phases of waste concentration before crystallizer.
  • Crystallizer to boil off any remaining liquid, leaving you with a dry, solid cake for disposal.

The typical chemical constituents a zero liquid discharge system is primarily concerned with are as follows:

Sodium Chloride
Potassium Fluoride
Calcium Sulfate
Magnesium Nitrate
Barium Phosphate
Strontium Ammonia
Silica Boron
TDS/TSS Alkalinity
COD/TOC/BOD Oil & Grease
pH

Because of the broad range of industries and depending on the needs of your plant and process, there might be some features or technologies you will need to add on if your plant requires a system that provides a bit more customization.

It’s a stage-wise scheme of gradually removing suspended impurities depending in size of-of particle using grate filters, sand filters, clarifiers, UF filters etc, removing COD , BOD & Color by chemicals, oxidation & settling, removing dissolved salts by multi-stage Reverse Osmosis units to its threshold and finally Evaporators and crystallizers to separate out all salts. Newer technologies called Forward Osmosis are being tried out for lower cost alternatives.
Here are the steps that a typical ZLD treatment facility process will usually include:
Zero Liquid Discharge (ZLD)
  • Pretreatment and conditioning 

Pretreatment is used to remove simple things from the wastewater stream that can be filtered or precipitated out, conditioning the water and reducing the suspended solids and materials that would otherwise scale and/or foul following treatment steps.

Typically this treatment block consists of some type of clarifier and/or a reactor to precipitate out metals, hardness, and silica. Sometimes this step requires the addition of caustic soda or lime to help with coagulation, a process where various chemicals are added to a reaction tank to remove the bulk suspended solids and other various contaminants. This process starts off with an assortment of mixing reactors, typically one or two reactors that add specific chemicals to take out all the finer particles in the water by combining them into heavier particles that settle out. The most widely used coagulates are aluminum-based such as alum and polyaluminum chloride.

Sometimes a slight pH adjustment will help coagulate the particles, as well.

When coagulation is complete, the water enters a flocculation chamber where the coagulated particles are slowly stirred together with long-chain polymers (charged molecules that grab all the colloidal and coagulated particles and pull them together), creating visible, settleable particles that resemble snowflakes.

The gravity settler of the ZLD treatment process is typically a large circular device where flocculated material and water flow into the chamber and circulate from the center out. In a very slow settling process, the water rises to the top and overflows at the perimeter of the clarifier, allowing the solids to settle down to the bottom of the clarifier into a sludge blanket. The solids are then raked to the center of the clarifier into a cylindrical tube where a slow mixing takes place and the sludge is pumped out of the bottom into a sludge-handling or dewatering operation. The settlers can also be designed using a plate pack for a smaller footprint.

Depending on the material in the feed, additional reactors or chemistry may be required for the reduction of metals or silica. Careful consideration must be given to the pretreatment step for a successful ZLD system.

Ultrafiltration (UF) can also be used after the clarifiers instead of the gravity sand filter, or it can replace entire clarification process altogether. Membranes have become the newest technology for treatment, pumping water directly from the wastewater source through the UF (post-chlorination) and eliminating the entire clarifier/filtration train.

Out of this process comes a liquid that is then filter-pressed into a solid, resulting in a solution much lower in suspended solids and without the ability to scale up concentration treatment.

  • Phase-one concentration 

Concentrating in the earlier stages of ZLD is usually done with membranes like reverse osmosis (RO), brine concentrators, or electrodialysis.

The RO train will capture the majority of dissolved solids that flow through the process, but as mentioned in a prior article about common problems with ZLD, it’s important to flow only pretreated water through the RO system, as allowing untreated water to go through the semipermeable membranes will foul them quickly. Brine concentrators, on the other hand, are also used to remove dissolved solid waste but they are usually able to handle brine with a much higher salt content than RO. They are pretty efficient for turning out a reduced-volume waste.

Electrodialysis can also be used in this part of the ZLD treatment system. It’s a membrane process that uses positively or negatively charged ions to allow charged particles to flow through a semipermeable membrane and can be used in stages to concentrate the brine. It is often used in conjunction with RO to yield extremely high recovery rates.

Combined, these technologies take this stream and concentrate it down to a high salinity while pulling out up to 60–80% of the water.

  • Evaporation/crystallization

After the concentration step is complete, the next step is generating a solid, which is done through thermal processes or evaporation, where you evaporate all the water off, collect it, and reuse it. Adding acid at this point will help to neutralize the solution so, when heating it, you can avoid scaling and harming the heat exchangers. Deaeration is often used at this phase to release dissolved oxygen, carbon dioxide, and other noncondensible gases.

The leftover waste then goes from an evaporator to a crystallizer, which continues to boil off all the water until all the impurities in the water crystallize and are filtered out as a solid.

  • Recycled water distribution/solid waste treatment

If the treated water is being reused in an industrial process, it’s typically pumped into a holding tank where it can be used based on the demands of the facility. The ZLD treatment system should have purified the water enough to be reused safely in your process.

The solid waste, at this point, will enter a dewatering process that takes all the water out of the sludge with filter or belt presses, yielding a solid cake. The sludge is put onto the press and runs between two belts that squeeze the water out, and the sludge is then put into a big hopper that goes to either a landfill or a place that reuses it. The water from this process is also typically reused.