Series part 2– The Removal of Metals from Wastewater

In Part 1 of this series, we talked about the types of metals found in wastewater, their relative toxicity, impacts on human health and the environment and treatment methods that can be used to promote their removal.  For Part 2, we will expand the dialogue and focus in on what is considered the most common treatment method in use today… The Treatment and Removal of Metals from Wastewater via Chemical Precipitation.

Let’s get started!

Chemical precipitation processes apply hydroxides, carbonates or sulfur-based additives to promote the transition of soluble metals to their insoluble form so they can be removed from a waste stream.  The decision to use one or a combination thereof is largely driven by the following.

• The relative solubility of the metal precipitates that are being formed.
• The plant’s ability to get metal concentrations down to a level that will meet environmental discharge targets.
• The precipitant’s ability to perform in the presence of chelating agents.
• Their ability to limit or prevent leaching in dewatered sludges.
• Individual product and overall treatment costs.

When applied properly, chemical precipitation processes are very robust and can readily remove large quantities of metal materials.  These additives are typically added upstream of a sedimentation process, usually in a reaction tank, with enough time to ensure the metals can be precipitated out.  Quite often, coagulant and flocculant chemistries are used to enhance the sedimentation process and are added just upstream of the equipment being used (clarifiers, decanters, etc.).

Hydroxide Precipitation and the Manipulation of pH

Of the precipitants that are available, hydroxide additives are by far the most common.  This is largely driven by the fact that these chemistries are cheap to source and the metal hydroxides that form have a solubility that is low enough to ensure their removal and be compliant with effluent discharge targets.  Of the hydroxide sources that are available, these are the precipitants that are most commonly used to promote the removal of metals from wastewater.

• Calcium Hydroxide/Hydrated Lime (Ca(OH)₂)
• Sodium Hydroxide (NaOH)
• Magnesium Hydroxide (Mg(OH)₂)

As can be seen in the diagrams below, the precipitation of a metal is not solely achieved by the simple addition of a hydroxide additive.  Its resultant formation and solubility are also a function of pH.  Each metal has a sweet spot along their individual curves where its hydroxide precipitate solubility is at a minimum.  As the pH increases or decreases along each curve (a reflection of how much hydroxide is added) so does the solubility of the metal precipitate or complex that is being formed.  For example, if the optimum pH needed to maximize cadmium precipitation is 10.2 to 10.4 and the waste treatment process is only operating at a pH of 9, higher levels of soluble cadmium will be present in the treated water that is being discharged.  Consequently, the same effect can take place if the targeted pH of a system is too high.

Figure: Solubility of Metal Hydroxides in Water by Ajiboye, Timothy & Onwudiwe, Damian & Oyewo, Opeyemi. (2021). Conventional and Current Methods of Toxic Metals Removal from Water Using g‑C3N4‑Based Materials. Journal of Inorganic and Organometallic Polymers and Materials. 31. 10.1007/s10904-020-01803-3.

For waste streams that only have to focus on the removal of one metal, adding hydroxides and targeting a pH that minimizes its solubility is relatively easy.  For systems that need to treat a mixture of metals, many will opt to converge on a pH setting that will maximize the removal of all that are present or focus on a pH setting that will address the most troublesome metal(s) that is present.  If this tactic is not successful in getting all metals to an acceptable discharge concentration, the following treatment strategies can be applied.

• Incorporate multiple precipitation steps into the treatment process that target different pHs for specific or small groups of metals sharing similar solubility characteristics.
• Supplement the addition of sulfur-based chemistries capable of producing less soluble precipitates within the targeted pH range.

Sulfur-based Precipitation Additives

As a rule, sulfur-based product chemistries can promote the formation of metal precipitates that are much less soluble than their hydroxide counterparts.  Because of their reduced solubility, sulfur chemistries are much less dependent on pH when compared to the solids that are formed via hydroxide precipitation.

On the surface, it would appear that using sulfur-based chemistries in lieu of hydroxides to promote metal precipitation would be more beneficial, however, in doing so, there are few notable disadvantages.

• Sulphur additives are typically higher in cost compared to hydroxides.
• The potential release of H₂S gas (highly toxic) in acidic environments
• Residual sulfide ions can add toxicity to the treated water.

Given the disadvantages that have been listed, many sulfur additives are applied on a supplemental basis to enhance removal efficiency or address a deficiency that is present within an existing treatment process.  Some examples of this would include:

• Chelants are present and highly soluble metal complexes have formed and need to be precipitated out.
• The target pH of the treatment process is not optimal to promote the removal of all metals.
• Effluent metal concentrations are a bit high and a polishing agent is needed to meet discharge targets.

Of the sulfur sources that are available, all are categorized as inorganic or organic sulfides.  Many of the disadvantages listed above are tied to the use of inorganics and less so for their organic counterparts.  As such, organically bound sulfur molecules (example; carbamates) are a great choice when looking to improve the performance of a WWT system that is struggling.  Organic sulfides are also easy to apply and fed stoichiometrically in proportion to the concentration metal ions that are still present in the water.

Carbonate Precipitation

Of the precipitants that are available and in use, carbonates are less common.  This is because some metals are not as effectively removed when carbonates are applied.  That being said, there are circumstances where carbonates are very effective and often applied to address a specific need.  For example, both cadmium and lead are metals that are more favourably removed when carbonates are applied.  As such, systems that focus on their removal can be tailored to accommodate some form of carbonate additive.  The only downside to these applications is that their respective reactions tend to be a bit slower and therefore require more retention time within the system when applied.  The different sources of carbonate available for use are typically limited to carbon dioxide, calcium carbonate and sodium carbonate.

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 can ensure the removal of metals from your wastewater is precipitously successful!

At Aquasan, your success is how we measure ours!