Zero Liquid Discharge (ZLD) in Industrial Wastewater Treatment

Summary: From ZLD Process Challenges to Business Impact

ZLD Challenge	Ultrasound Impact	Operational Outcome	Business Value
Scaling and fouling in evaporators	Controlled nucleation and cleaner heat transfer surfaces	Improved heat transfer efficiency	Lower energy consumption
Unstable crystallization behavior	More controlled crystal growth	Increased process stability	Reduced downtime
Frequent cleaning cycles	Reduced surface deposition	Longer cleaning intervals	Lower maintenance costs
High chemical consumption	Improved process control without chemicals	Reduced chemical use	Lower OPEX
Variable process performance	Stabilization across ZLD train	More predictable operation	Improved reliability and process stability

Zero Liquid Discharge (ZLD), a water treatment process where all wastewater is treated and recovered, producing zero liquid waste, is becoming a strategic priority across water-intensive industries. ZLD is typically driven by severe water stress, stricter discharge regulations, license to operate and the need for resource efficiency. Therefore more industrial plants are moving toward closed-loop water systems.

ZLD is an industrial water management approach in which the plant aims to eliminate or minimize the liquid effluent discharge by recovering water for reuse and converting dissolved solids into a solid waste stream.

While ZLD promises to recover as much water as possible, it also introduces a new level of operational complexity. The main challenges don’t come from the basic idea, but from how the system works operationally day to day.

ZLD in Practice: From Side Stream to Core Process

In industrial settings where water is scarce, wastewater is no longer just a byproduct; it’s a crucial part of the production process. Across process industries, process water is a valuable side stream that must be treated and reused efficiently. In this type of setup, water is recovered through a ZLD process and freshwater dependency is significantly reduced.

Zero Liquid Discharge is not a single technology, but a process chain combining several treatment stages.

Typically, the process begins with pretreatment, where solids are removed and pH is adjusted to protect downstream equipment. This is followed by reverse osmosis, which removes dissolved solids from the water and produces a concentrated brine stream.The remaining liquid is then further concentrated through evaporation, where water is separated from the brine using heat. Finally, in the crystallization and solids handling stage, the last traces of liquid are converted into dry solid residue, which can be safely disposed or, in some cases, reused or sold as a byproduct.

The goal is straightforward: maximize water recovery and eliminate liquid discharge to the environment. However, achieving this in practice requires stable performance at every stage, and even small inefficiencies can negatively affect the entire process.

Evaporation: The Economic Core of ZLD in Mineral Processing

Within high-salinity industrial environements, such as mineral processing, where water contains high levels of dissolved salts and minerals, evaporation becomes the critical stage of ZLD. Furthermore, evaporation represents the most energy-demanding phase of the process, dominating the total operating cost of the system.

When heat transfer is working well, energy use is kept low, and the system runs efficiently. However, when performance declines, the impact is immediate and costly. In ZLD, the evaporator separates water from brine through a phase change. This process step uses a lot of energy compared to other treatment steps.

Therefore, the economic performance of the evaporation stage is very sensitive to how well heat transfer works. Even small losses in efficiency significanly increase the amount of energy required per unit of water recovered.

At the same time, the evaporation stage is not only a cost driver, it is also crucial for the quality of water produced. The regulation of dissolved substances, especially reducing water hardness, is vital for ensuring stable and effective operation. This is critical for evaporation performance, as compounds such as calcium and magnesium are primary contributors to uncontrolled scaling. This uncontrolled scaling leads directly to rising operaring costs and maintaining process stability becomes more difficult.

Especially, in mineral processing, where water reuse is closely linked to production, these inefficiencies are not limited to the treatment system; they directly affect the overall economic performance of the process.

Scaling: The Hidden Cost Driver in ZLD

One of the main reasons for declining evaporation performance is scaling, a structural challenge in high-concentration ZLD systems.

Dissolved substances such as calcium are only partially removed in upstream treatment stages, such as filtration and reverse osmosis. As the process continues, these compounds concentrate further and enter the evaporation stage, where they contribute to scaling and crystallization. From there, solids are separated through filter presses or centrifuges, but part of the load remains within the system, creating a continuous cycle of concentration, deposition, and removal.

Scaling occurs when dissolved salts exceed their solubility limits, which is affected by increasing concentration and temperature. This leads to supersaturation, where the liquid can no longer hold all dissolved substances. From there, the process typically follows a well-known sequence: supersaturation, nucleation, crystal growth, and surface adhesion.

As crystals form and attach to heat transfer surfaces, they gradually build up a layer that acts as a thermal resistance barrier. This reduces heat transfer efficiency and directly impacts evaporator performance. In ZLD systems, this is not an occasional disturbance. Because brines are intentionally concentrated to very high levels, supersaturation, and therefore scaling, is part of the process.

When scaling is not properly controlled, the consequences are immediate:

Heat transfer efficiency declines

Energy consumption increases, because more energy is required per unit of water recovered.

Scaling becomes a system-wide challenge, affecting not only evaporation performance, but also downstream efficiency and overall process stability.

In practice, scaling leads to operational and economical impacts

More frequent cleaning shutdowns

Increased chemical consumption

Reduced operational stability

Higher risk of unplanned downtime

In water hardness-driven cases, calcium and magnesium compounds — such as calcium carbonate and calcium sulfate — are among the most common contributors to scale formation, often appearing in complex mixtures with other salts.Because ZLD systems are designed to push water streams toward extreme concentration levels, scaling cannot be fully avoided. The key question is not whether scaling occurs, but how effectively it is controlled and managed.

From Scaling Control to Measurable Value

In many industrial ZLD applications, particularly in mineral processing, the plant is operated under a BOOT (Build–Own–Operate–Transfer) model, where the operator is responsible for operating costs, availability, and performance delivery.

This fundamentally changes the role of wastewater treatment. What is often seen as a necessary side stream for the industrial client becomes a direct margin driver for the operator. Improvements in evaporation efficiency, cleaning intervals, and uptime translate directly into financial performance for those responsible for operating the wastewater treatment plant.

In this context, performance optimization is not only a technical question, it is a business-critical one. ZLD value creation is tightly linked to evaporation performance, and ultimately to how effectively scaling and crystallization are controlled.

This is where process intensification methods, such as Altum’s high-power ultrasound, play a key role. By controlling nucleation and influencing crystal growth behavior in evaporators, ultrasound helps maintain cleaner heat transfer surfaces, improve process output quality, and reduce instability across the entire ZLD train. Rather than adding new process steps, the approach focuses on improving the performance of existing infrastructure.

The impact is measurable. When heat transfer efficiency is maintained, energy consumption decreases, cleaning intervals increase, and process stability improves. These effects are reflected in lower steam and power consumption, reduced chemical use, longer operating cycles, and improved overall performance. For operators, this translates directly into lower OPEX and improved EBITDA.

Altum’s high-power ultrasound technology has been used in demanding industrial environments, including black liquor evaporation processes in the pulp and paper industry, as well as in mineral processing wastewater treatment plants.

In these applications, ultrasound has demonstrated measurable improvements in evaporation performance, delivering clear economic benefits for operators.

Contact us to improve evaporation performance, reduce scaling, or enhance the efficiency of your ZLD system, we’re happy to take a closer look at your process.

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