As a category, heavy metals encompass 56 naturally occurring elements stratified by atomic weight. Weight and density vary, with the lightest of the heavy metals sometimes considered to be calcium (atomic weight 20), scandium (40), or mercury (200). Many, such as iridium, nickel, selenium and chromium, are commercially mined. An additional 34 heavy metals, all radioactive and more or less unstable, are produced by transmutation in a laboratory. These include radium, polonium and flerovium. Some heavy metals are fairly innocuous to human health, some harmful in high quantities and others are toxic in even trace quantities.

It is therefore important that heavy metals are removed entirely from industrial wastewater, or reduced to a safe level, before it is returned to the mains. Heavy metals are used to manufacture a wide range of common consumer products, including batteries, agricultural products, paint, wood treatment products, PCB boards, golf clubs, solar panels and many types of plastic. The permitted volume of a heavy metal in wastewater will depend on its toxicity.

Please check with your regulatory authority and a water treatment specialist before purchasing a treatment system.

Water Treatment Methods

Water treatments for heavy metals use different technologies to reduce toxicity and/or remove heavy metal particles. These can be divided into four categories:

1) Absorption of heavy metal particles through treatment of water with an absorbent agent
2) Membrane filtration
3) Photo catalysis
4) Electro dialysis

Ion exchange (deionisation) systems have also proved effective at removing heavy metal contaminants, as has chemical precipitation treatment, flotation and deposition.

Each system has advantages and disadvantages. The technology or combination you choose for your facility will depend on the type of heavy metal you need to remove, the concentration of contaminants and the pH value of your wastewater, among other variables.

Absorption & Filtration

Of the four treatment types, the first two are the most commonly used. For low concentrations of contaminants e.g. 1 g/L or less, treatment with a lime-based absorbent has been shown to be one of the most effective ways of reducing toxicity. This has the advantage of being comparatively environmentally friendly compared with other chemical treatments. An absorbent agent can be a mineral, such as zeolite, or it can be organic/biological. Some low-cost absorbents are produced using agricultural and industrial by-products, while some microorganisms actively consume heavy metal particles. This, of course will require a further treatment process to remove the cleansing bacteria. If metal-laden bacteria enter the food chain, metals can build in concentration to reach harmful levels by the time they reach human beings. Other absorbent agents are biopolymers, which bind to the heavy metals and are then filtered out of the wastewater.

Filtration treatment can also remove heavy metals, although this is more expensive than simply treating the water. You may need to run the effluent through a series of filters of varying density to remove all the particles, especially when the water is contaminated with more than one heavy metal. Some businesses use a combination of reverse osmosis, ultra-filtration and nano-filtration to thoroughly cleanse their wastewater. The more filtration stages you require, the more complex your plant setup.

Electro Dialysis & Photo Catalysis

The alternative treatments of electro dialysis and photo catalysis are gaining in popularity in some sectors. Electro dialysis is effective at removing heavy metals and ticks all the boxes for environmental sustainability, although it uses more energy than other treatment methods.

Photo catalysis is a new process with many potential benefits for industry. A photo-catalytic system bombards a waterborne solution of heavy metals with a stream of ultraviolet photons. These break down the water molecules into oxygen and hydroxyl ions. The hydroxyl radicals are powerful oxidising agents, which destroy the contaminants by stripping away their electrons. Studies in 20041 and 20062 demonstrated the effectiveness of the method for degrading cyanide and titanium dioxide contamination respectively.

Our Free Guide To Industrial Water Purification Systems

Find out more about industrial water treatment systems in our Industrial Water Purification Guide, which explains all the main treatment systems and how to choose the best solution for your facility. Click here to access your copy today.