Series – The Removal of Metals from Wastewater

Affected Industries, Environmental Impact and General Removal Methods

This article kicks off a multipart series focusing on the removal of metals from wastewater.  Our first installment will provide a summary of the different types of metals involved, the industries where they are present, their implied toxicity, impact on the environment and the treatment methods used to promote their removal.  Are you ready?  Ok then, here we go…

Metals and metal-based solutions are at the heart of many of the industrial processes that are used to produce the goods and services we rely on each and every day.  They are present or consumed as raw materials, used as an intermediate to accommodate a production process or present as a contaminant that must be removed to meet a specific operational requirement.  Below are some of the more common industries and applications where metals or metal-based materials are actively involved.

• Mining
• Iron Ore / Steel Production
• Surface Finishing
  • Metals Passivation, Etching and Plating Operations
• Pigment/Dye Production
• Landfills
• Metal Recycling
• Environmental Remediation (Soil & Water)
• Drinking and Raw Water Treatment

Many of the aforementioned industrial applications call on large volumes of water to accommodate their operational requirements.  As a consequence, many of the metal-based materials get absorbed and accumulate in concentration to a point where discharge is not possible unless some form of wastewater treatment is put in place.

Even though metals are natural elements, part of the earth’s crust and often present in our ground and surface waters, there is a limit to how much considered safe.  Certain metals in small concentration are considered essential to life, but when their concentrations climb too high, they can quickly become toxic.  Heavy metals are of particular concern because of their tendency to bioaccumulate.  In short, this means living organisms can absorb them faster than they can get rid of them.  If subject to frequent and or continuous exposure, their accumulation over time will eventually be harmful.  This is why regulators keep a close eye on any metals that are present in wastewater.  Especially anything that is being discharged to a sewer system or directly into the environment.

The diagram below provides a break down of the ways heavy metals can get introduced into the environment, how they migrate through our ecosystems and the risks they pose as they start to accumulate.

Figure 1:  Inspired by “Saxena, Gaurav & Purchase, Diane & Mulla, Sikandar & Saratale, Ganesh & Bharagava, Ram. (2019). Phytoremediation of Heavy Metal-Contaminated Sites: Eco-Environmental Concerns, Field Studies, Sustainability Issues and Future Prospects. Reviews of environmental contamination and toxicology. 10.1007/398_2019_24. “

When it comes to discharging water, metals that are considered less toxic—like aluminum and iron—typically have more lenient discharge limits.  As a result, they are measured and controlled on a milligram-per-litre (mg/L) or parts-per-million (ppm) basis.  For those considered more toxic and hazardous to the environment, their limits are much lower.  Heavy metals fall within this category and are therefore limited to a microgram-per-litre (µg/L) or parts-per-billion (ppb) concentration when present in any water that is being discharged.  Because of this reality, wastewater treatment processes must be overtly robust, effective and efficient.  Below is a list of heavy metals that are commonly used in industry and by default, are closely regulated.

Given the critical role metals play within industrial processes and the environmental risks posed by their improper discharge, effective treatment strategies are essential.  More often than not, the removal of a metal(s) from a contaminated stream requires a targeted approach.  In each case, consideration must be given to the form it is present – as a solid or in solution – and its intended destination once removed – recycled or sent for disposal.

The following outlines some of the metal removal treatment methods that are in use today.

Sedimentation and or Filtration – This methodology focuses on the removal of metals that are in solid form.  On their own, these systems can be positioned at the front end of a treatment process and are intended to remove large volumes of metal waste.  Often their resultant accumulation is large enough to promote some form of re-use or recycle.  Coagulant and flocculants chemistries are commonly applied to enhance solids accumulation and removal efficiency.

Ion Exchange – This method preferentially captures metals in their soluble form using ion exchange resins.  These systems are often used to promote the removal and recovery of a high value metal from its high concentration regenerant waste or installed as a polishing medium to achieve ultra-low-level metal concentrations in a water stream.

Membrane Filtration – Quite simply, micro- and ultrafiltration systems are used to remove metals in their solid form while nanofiltration and reverse osmosis are used to remove or reject metals in their soluble form.  Independent of the physical in which they are present, membrane systems are viable tools that can be used to recover electrolytes or metals from their reject flows and yield low-level metal concentrations in their permeate streams.  Depending on the type of membrane being used, many permeate streams can be routed back to a facility to accommodate some form of water re-use.

Chemical Precipitation – In a pH-controlled environment, the addition of alkalis – carbonates and hydroxides – or sulfur-based additives – inorganic or organic sulfides – will promote the transformation of soluble metals into insoluble precipitates.  The decision to use one precipitant over another is largely driven by the cost, the relative solubility of the precipitates being formed, the inability to target an optimal precipitation pH for all the metals that are present and their ability to limit the formation of soluble complexes in the presence of chelating agents.  Quite often, coagulant and flocculant chemistries are used in conjunction with these materials to enhance metals removal as they are being precipitated out.

Chemical precipitation processes are very robust and one of the most common methods to promote heavy metal removal in wastewater treatment plants today.  As such, chemical precipitation methods will be the focus in Part 2 of our series on “The Removal of Metals from Wastewater”.

Independent of the challenges you face, Aquasan has a well-versed team of experts and a product portfolio that is capable of meeting your operational needs.  Whether it’s the sourcing of chemistry or the provision of technical support services, Aquasan is ready to help.  Your success is how we measure ours.