Zinc is widely used in modern industry. It is essential to numerous industrial processes, including protecting steel against corrosion, manufacturing batteries and pigments, and treating metal surfaces.
However, when excessive amounts of zinc are discharged into industrial wastewater, it becomes a problematic contaminant for aquatic environments.
Here is what you need to know.
Zinc in the environment: an essential metal that requires monitoring
Zinc (Zn) is a trace element naturally present in soil, rocks and water. At low concentrations, it is essential to living organisms, including humans.
Problems arise when zinc concentrations exceed natural levels, particularly in industrial discharges.
In water, zinc can be found:
- in dissolved form (Zn²⁺ ions), its most mobile and toxic form;
- bound to suspended particles;
- associated with sediments.
Dissolved zinc is particularly concerning because it is directly bioavailable to aquatic organisms.
Sources of zinc pollution
Zinc contamination in water is predominantly caused by industrial activities.
Industries most affected
Several sectors are particularly concerned:
- Galvanising and surface treatment: direct use of zinc-rich baths to protect metal parts.
- Metallurgy and hydrometallurgy: extraction and refining of zinc from ores.
- Battery manufacturing: production of components containing zinc.
- Chemical and pigment manufacturing.
- Tanneries and the textile industry.
- Urban runoff: corrosion of roofs, gutters and metal infrastructure.
In surface-treatment facilities, for example, rinsing water may contain significant zinc concentrations if it is not properly treated upstream.
Zinc is the most frequently identified pollutant among regulated metal contaminants in industrial wastewater. In the RSDE II study (Recherche de Substances Dangereuses dans l’Eau – Search for Hazardous Substances in Water), zinc accounted for 32% of the exceedances measured for the nine regulated metals investigated.
Although RSDE II examined wastewater discharges from 42 industrial sectors, several stood out for their zinc emissions:
- The chemical industry, with 45 sites exceeding the thresholds for ongoing monitoring.
- The plant-based food-processing industry, with 38 sites.
- Surface treatment, animal-based food processing and slaughterhouses, with 28 sites each.
Effects of zinc on aquatic environments
Contrary to a common misconception, zinc is not harmless when present in excessive concentrations.
Impact on ecosystems
At excessively high concentrations, zinc can:
- damage fish gills;
- disrupt ion exchange in aquatic organisms;
- inhibit the growth of algae and microorganisms;
- affect invertebrate reproduction.
Even at relatively low concentrations—just a few hundred µg/L—ecotoxicological effects may be observed depending on the pH and hardness of the water.
Effects on drinking water quality
In water intended for human consumption, zinc:
- alters the taste at concentrations above 3 to 5 mg/L;
- may cause gastrointestinal disorders at high doses;
- can lead to deposits and turbidity.
Zinc pollution therefore presents environmental, health and regulatory challenges.
What solutions are available for treating zinc-contaminated wastewater?
The choice of treatment solution depends on:
- the zinc concentration;
- the flow rate to be treated;
- the presence of other pollutants;
- regulatory requirements;
- recovery or recycling objectives.
The main technologies currently used are presented below.
1. Chemical precipitation
Chemical precipitation is a conventional method used in industrial wastewater treatment plants.
It involves adjusting the pH—often by adding lime or caustic soda—to precipitate zinc as zinc hydroxide, Zn(OH)₂.
Advantages:
- well-established technology;
- relatively low cost.
Limitations:
- production of large quantities of sludge;
- performance highly dependent on pH;
- difficulty achieving very low residual metal concentrations and therefore complying with increasingly stringent discharge standards.
2. Coagulation–flocculation
Coagulation–flocculation is often used in combination with precipitation.
It improves particle agglomeration, facilitating their separation through settling or filtration.
However, its removal performance also has limitations. A fraction of the dissolved metals generally remains untreated by coagulation–flocculation. This process therefore usually requires an additional polishing treatment downstream.
3. Ion exchange and resins
Specific resins capture dissolved Zn²⁺ ions.
Advantages:
- high removal efficiency;
- possibility of regeneration.
Limitations:
- high costs;
- sensitivity to other ions present in the water;
- operational complexity;
- handling of chemicals during regeneration.
4. Membrane processes (nanofiltration/reverse osmosis)
Membrane processes are used when particularly stringent discharge limits must be achieved.
Advantages:
- very high level of treatment performance.
Limitations:
- high energy consumption;
- management of the resulting concentrate.
5. Adsorption using specialised materials
An increasing number of industrial operators are turning to filtration media capable of selectively capturing heavy metals.
These solutions offer several benefits:
- reduced sludge production;
- continuous operation;
- easy integration into compact treatment units;
- simple operation, with no regeneration step to manage;
- a potentially more sustainable approach, depending on the material used.
Media produced from natural minerals, including certain modified apatites, offer a particularly strong affinity for metal cations such as zinc.
Zinc: an already critical regulatory issue
Treating zinc pollution is no longer simply a matter of anticipating future environmental requirements. In many industrial sectors, complying with metal discharge limits is already a daily challenge.
Discharge agreements impose strict limits on total zinc concentrations, generally below 1 mg/L and sometimes as low as a few µg/L, depending on the receiving environment and the discharge flow rate.
Many operators experience occasional or even recurring exceedances of regulatory limits.
These instances of non-compliance may result in:
- formal notices from the authorities;
- financial penalties;
- increased monitoring;
- or even the suspension of discharge permits.
Moving towards more sustainable metal treatment solutions
Managing zinc-contaminated industrial wastewater can no longer be limited to primary and secondary treatment processes. Polishing treatments are needed to ensure that discharges consistently comply with regulatory requirements.
Advanced adsorption technologies, reactive mineral media and modular treatment solutions offer a promising way to combine performance, sustainability and cost control.
Reducing metals at source and capturing them effectively before discharge are key measures for the long-term protection of water resources.
For a comprehensive comparison of polishing solutions for trace metal pollution, read our article: Comparison of water treatment solutions for trace heavy metals.