What is zinc and how does it affect your industrial process water and wastewater treatment?

Zinc plays an important role in the manufacture of products and materials across a number of industries, including mining and metals, automotive, construction, paints and pigments, cosmetics, paper, and others. While trace amounts of zinc are necessary to support human, plant, and animal life, excess levels of zinc can pose a risk both to the environment and to human health.

This article explores the effects of zinc on your industrial process water or wastewater, and why industrial facilities use treatment or separation technologies to reduce zinc levels in process or wastewater streams.

What is zinc and where does it come from?

Zinc is a naturally-occurring heavy metal that is abundant in the earth’s crust, and present in comparatively smaller quantities in soil and in surface waters. In its pure elemental form, zinc is a bluish-white metal with a lustrous appearance. It is moderately reactive with oxygen and other non-metallic elements, and corrodes very slowly when exposed to air and water. In terms of its physical properties, zinc offers a high heat tolerance and moderate conductivity for heat and electricity. It is brittle at ambient temperatures, but offers excellent malleability and ductility when heated to 212-302˚F (100-150˚C).

The chemical and physical characteristics of zinc make it particularly useful for galvanization, where zinc is applied as a corrosion-resistant coating on other metals, like iron or steel. In fact, approximately half of all mined or recycled zinc is used to produce galvanized metals, while the remainder is used in the manufacture of a wide variety of other products, such as batteries, rubber, pesticides, fertilizers, die casts, inks, paper, plastics, cosmetics, and construction materials, to name just a few.

Most zinc comes from metal ore deposits that are scattered across the world, while a small but increasing share of zinc is sourced from recycled metals. Worldwide demand for zinc has increased significantly over the past several decades, which, along with improper discharge of wastewater from mining and mineral processing, landfill and dumpsite spills, and fossil fuel combustion, has contributed to rising levels of zinc in air, soil, and water. As zinc pollution becomes a greater threat, these and other human activities are facing growing scrutiny and mitigation efforts to prevent further zinc pollution.

Environmental and public health impacts of zinc

Zinc is an essential mineral needed to support good health in a variety of plant and animal life, including humans. Humans need only trace quantities of zinc in food and water, but can generally tolerate exposure levels that exceed nutritional needs many times over, typically with no ill effects. In this way, zinc stands apart from other heavy metals like lead, mercury, or cadmium, where relatively low levels can cause serious health issues. Nonetheless, zinc toxicity is still possible when highly excessive levels are present in air, water, or food.

In humans, zinc toxicity can result in nausea, anemia, skin irritation, disruptions to protein metabolism, and arteriosclerosis. Zinc is also associated with metal fume fever, a flu-like illness resulting from occupational exposure to zinc or other metals. Additionally, elevated zinc levels can harm natural ecosystems and reduce agricultural productivity by inhibiting the uptake of other essential nutrients by plants, and it can also cause damage through its tendency to bio-accumulate in aquatic and terrestrial animal life.

How does zinc affect process water treatment?

Industrial process water has a number of uses within a facility, such as washing, rinsing, heating, cooling, and plating, to name just a few. Different processes and equipment have widely divergent specifications for stream quality, so facilities will typically rely on some form of process water treatment to ensure that the stream quality is appropriate for its intended application. Zinc can affect process water in a number of ways—both good and bad—as some applications are tolerant of high zinc levels, while others are much more sensitive.

In short, the effects of zinc on a facility’s process water treatment strategy will depend on the specific processes and equipment at hand. Below, we’ll take a look at some of the key reasons why a facility may need to monitor or manage zinc content in process water.

Protecting downstream equipment and processes

Depending upon the specific conditions and applications of a process stream, zinc can be either a benefit or a detriment to process water. In some cases, a facility may actively choose to add certain zinc compounds to a process stream in order to fulfill a specific goal or purpose, such as when a facility uses a zinc solution as a protective anti-corrosion treatment for equipment.

Zinc can also enter process streams inadvertently, such as when a facility draws from ground or surface water, or as certain activities are carried out within the facility. High zinc content can complicate downstream processes by contributing to scaling, premature membrane fouling, or interfering with temperature or pressure control in boilers, among other issues. For these reasons, industrial facilities need to monitor process water quality for zinc and other constituents to ensure suitability for a given process, and ensure treatability of any resulting wastewater. 

One common way in which zinc can enter process water is during hot dip galvanization, where water used for rinsing and washing activities can pick up zinc and other contaminants. A facility may choose to adopt some means of process water treatment to allow for reuse or recycling of such rinse or wash water, or it may opt to treat the spent stream in order to ensure successful downstream wastewater treatment performance.

In another example, this one particular to the food and beverage industry, zinc can enter process streams through fertilizer and pesticide residues present on agricultural products. While far less toxic than other heavy metals, zinc and certain zinc compounds can still disrupt production lines by imparting bitterness or otherwise affecting flavor in coffee, fruit juices, and other products. Careful control of ingredient and process water quality is therefore necessary in order to preserve the safety and palatability of products.

Reuse and recycling

Industrial recycling and reclamation strategies seek to reuse water, raw materials, or chemicals in ways that reduce net consumption. Facilities may adopt reclamation strategies to serve any of various goals—such as cutting costs associated with sourcing or disposing of various materials, complying with waste discharge regulations, or even building a reputation around environmental stewardship.

In some cases, zinc may be the target of a reclamation strategy, as is the case for facilities that recycle galvanized steel products, for example, or for facilities that separate zinc from wastewater for sale or reuse. Capturing zinc from process and wastewater streams in this way can offer cost savings benefits by reducing demand for raw material.

Even when zinc reuse isn’t the main goal of a reclamation program, a separation strategy to control zinc content may still be necessary. This is often the case for facilities who choose to reuse process solutions such as those used for pickling, electroplating, or closed-loop cooling systems. In many cases, reusing process streams in this way can generate cost savings by reducing wastewater volume, even when investment in separation or treatment technologies are necessary to ensure that the stream is fit for reuse. In short, if your facility is looking for ways to cut costs and improve material utilization, then zinc removal and reclamation may be worth considering.

How does zinc affect wastewater treatment?

Because of risks posed to environmental and human health, zinc content in industrial waste is generally subject to regulation across much of the world. Additionally, zinc, like other heavy metals, can inhibit the function of a biological wastewater treatment systems. As a result, facilities that use or handle zinc or zinc compounds generally must implement strategies for monitoring and treating wastewater, both for compliance purposes, and to ensure optimal function of any onsite wastewater treatment systems.

Compliance with wastewater discharge regulations

In order to protect the public from health risks and environmental impacts associated with zinc, regulatory bodies have established limits for the maximum concentration of zinc allowable in wastewater. In the United States for example, the Environmental Protection Agency (US EPA) lists zinc as a Priority Pollutant, and industrial facilities must therefore test and manage zinc levels in wastewater. Failure to comply with relevant discharge limitations can leave a facility liable to substantial fines and legal action.

Facilities that release effluents directly to US waterways must obtain a National Pollutant Discharge Elimination System (NPDES) permit, which specifies limits on wastewater volumes and contaminant levels, as well as any testing and reporting requirements. Limits on zinc are defined based on US regulatory standards, which offer some variation based on industry, facility type, available treatment technologies, plant size, and location. Metal finishers, for example, are limited to a monthly average of 1.48 ppm of zinc content in wastewater. Limits for battery anode producers are measured differently, and are defined as a monthly average that can range anywhere between 0.42 and 1946 mg of zinc in wastewater per kilogram of zinc anodes produced. In short, if your facility discharges wastewater to public waterways, then you will likely need to manage and monitor zinc levels to stay in compliance with regulatory limits.

In the US, facilities that route effluents to wastewater treatment facilities are not subject to NPDES permit requirements, however, they must still comply with regulatory requirements enforced by the receiving facility. This can include municipal or publicly owned treatment works (POTWs), each of which enforce their own water quality standards to ensure that they can meet their own effluent limitations under the US EPA. Depending upon a number of factors, a centralized treatment facility can expect that its monthly average limits on zinc will range somewhere between 0.252 and 4.50 ppm. Given these stringent guidelines, a POTW will typically require that industrial facilities pretreat wastewater before discharge. In short, if your facility discharges wastewater to a POTW, then you will likely need to manage and monitor zinc levels to stay in compliance with limits set by your receiving facility.

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