What to know about ZLD/MLD

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Whether motivated by stricter environmental regulations, an increasingly competitive market, or higher costs for water, energy and other resources, industrial facilities are more concerned than ever with finding ways to operate more efficiently. Here, we’ll look at a couple of strategies that are rising in popularity—MLD and ZLD—and explore how they help to cut industrial water use.

What are MLD/ZLD?

Minimal liquid discharge (MLD) and zero liquid discharge (ZLD) are wastewater treatment processes designed to recover as much water as possible. MLD can recover upwards of 80% to 90% of the water in a wastewater or process stream. ZLD goes a step further, recovering more than 95% of the water in a liquid stream, leaving behind nothing but solid residues. MLD and ZLD are typically used together with a water reuse strategy, with the recovered water being recycled for other applications.

How do MLD/ZLD work?

Both MLD and ZLD are complex multi-step processes that require a train of different treatment technologies. Exactly which systems are used will depend upon the composition of the wastewater stream and process conditions. The process consists of a few phases:

Pretreatment

The goal of the pretreatment and phase is to remove excess contaminants or otherwise condition the stream to help downstream equipment perform more efficiently. This includes reduction of Total Suspended Solids (TSS), larger particles, and hardness, and other materials that can be precipitated or filtered out of the stream. Technologies typically used for pretreatment can include physical-chemical treatment, such as coagulation, flocculation, and clarification, as well as coarser types of membrane separation, such as microfiltration and ultrafiltration.

Concentration

In the next phase, the pretreated or conditioned stream undergoes further separation, effectively splitting the stream into a clean water stream that is suitable for use in other applications, and a concentrated liquid brine stream that is high in dissolved solids. Common treatment technologies used for this step include reverse osmosis (RO), brine concentrators, and electrodialysis (EDI). Brine concentration yields a water recovery rate ranging from 60 to 90% of the water in a stream, and most MLD systems conclude with this phase.

Evaporation & Crystallization (ZLD only)

This final phase is exclusive to the ZLD process, and the goal is to drive off all water remaining in the brine so that only solid wastes remain. This is typically achieved through evaporation and crystallization, both of which involve heating the brine to boil off water content. Any water that is removed in this way is captured for recycling or reuse. Evaporation and crystallization carry significant capital and operational costs compared to earlier phases, making ZLD significantly more expensive than MLD.

Why do industrial facilities use MLD/ZLD?

The use of both MLD and ZLD is growing in various industries across the world, with some analysts forecasting that the global ZLD market will quadruple in value by 2032. While ZLD dates back to the late 1960s, it wasn’t until the last decade that its adoption has seen a significant growth trend. The reason? MLD and ZLD solutions are used to virtually eliminate wastewater discharges to the environment, and to reduce overall use and consumption of water. As companies are increasingly challenged by the rising costs of water, tighter environment regulations on wastewater discharges, and concerns around water scarcity, MLD and ZLD are seen as a worthwhile investment by more and more industrial facilities.

Benefits of MLD/ZLD

Minimal liquid discharge and zero liquid discharge share many of the same advantages. Specifically, MLD/ZLD can benefit an industrial facility by:

Reducing water pollution: Using MLD/ZLD allows a facility to recover most of the water it uses, significantly reducing the amount of wastewater that it releases to the environment or to POTWs.
Reducing water consumption: Both MLD and ZLD recover water for reuse in additional applications, allowing facilities to reduce the amount of water drawn from local water sources.
Recovering valuable products: By separating a wastewater stream into its various constituents over a multistep process, MLD/ZLD systems provide opportunities for recovering products or raw materials for sale or reuse. Recovering materials from waste streams can yield indirect environmental benefits, too, such as by reducing the demand for mineral extraction, for example.
Improving corporate reputation: MLD/ZLD can help to enhance a company’s public image by helping facilities to achieve water stewardship goals or other environmental sustainability goals.
Enhancing regulatory risk profile: At minimum, MLD/ZLD help facilities to comply with strict environmental regulations. For many plants, though, MLD and ZLD go well beyond legislative standards, better positioning facilities for future permit applications, and minimizing the risk of fines or other sanctions for non-compliance.

Many of these benefits also generate related cost savings, though the degree to which an MLD/ZLD system will benefit a given facility depends upon the composition and volume of waste streams, among many other factors.

Drawbacks of MLD/ZLD

For all their benefits, MLD/ZLD do have some disadvantages, including:

High cost: Perhaps the biggest drawback of ZLD systems is that they are quite expensive to build and operate. Historically, ZLD’s high costs have kept many industrial facilities from considering it, but rising costs for water use and discharge are making it easier to justify the costs of ZLD. MLD, on the other hand, runs only about 30-40% of the cost of a ZLD system, since it doesn’t require costly evaporators and crystallizers. Thus, MLD can be a good compromise for facilities who want to significantly reduce liquid wastes, but not necessarily eliminate all wastewater discharges.
Energy consumption: MLD and ZLD systems require moderate to large amounts of energy to operate, particularly for the thermal processes used in ZLD. In some cases, the amount of energy needed to run an MLD/ZLD system is high enough that it offsets whatever positive environmental impacts it might have gained through water conservation.
Operational complexity: MLD and ZLD systems are complex treatment trains consisting of multiple types of water treatment technologies. As such, they may require operators with specialized knowledge or expertise, and/or additional training for existing workers.

In some cases, these drawbacks can be mitigated in part through careful solution design and engineering. For example, in some cases, ZLD systems can improve energy efficiency by incorporating hybrid membrane technologies that minimize the volume of wastewater. Less wastewater to treat during the evaporation and crystallization phase means less energy consumption, and lower operational costs.

Likewise, certain minerals and metals in wastewater can collect on equipment in deposits known as scaling and fouling, leading to higher maintenance costs due to more labor for cleaning, and more frequent replacement of parts such as membranes. By incorporating pretreatment systems that are appropriate to the wastewater stream in question, facilities can minimize these kinds of operations costs.

How can SAMCO help?

SAMCO has over 40 years’ experience in the design and manufacture of wastewater treatment systems and process separation solutions across a range of industrial sectors. Our wastewater treatment solutions have helped our clients achieve goals like water conservation, resource recovery, and regulatory compliance.

If you’re interested in learning more about minimal liquid discharge and zero liquid discharge solutions, we invite you to contact us. Our engineers will walk you through the process for developing a wastewater treatment solution to meet your specific objectives. You can also request a quote to help you develop a realistic budget.

For more information, head on over to our blog to learn more about the relationship between wastewater treatment and sustainability issues. Some articles that might be of specific interest to you include: