Heavy metal removal
Industry has often favoured sites close to rivers for three reasons:
- Transportation of raw materials,
- Water supply for use in cooling systems,
- Disposal of effluents.
For decades rivers have been used to dispose of industrial effluents as well as other liquid wastes e.g. from the extraction or conversion of raw materials and from the production processes.
Even though the major industrial companies are equipped with specific wastewater treatment plants, most of the effluents are direct effluents sometimes called "natural effluent".
Heavy metal trace elements in particulate form are mainly exposed due to soil erosion. Surface run-off as well as anthropic sources further contribute to the presence of heavy metals.
Once present in water their impact upon the environment are two fold due to chemical reactions within the water (linked to acidity, alkalinity, temperature and oxygenation) and to their effect upon aquatic life through two phenomena: bioaccumulation and biomagnification.
As a result of the latter trace elements gradually become more concentrated when absorbed up the food chain (water plankton, herbivore fish, carnivore fish, and man).
Heavy metals are micro pollutants which are toxic even when they are discharged in very small quantities (their toxicity increases due to bioaccumulation and biomagnification).
Their proportionally small quantities (measured in micrograms per liter) remain important where high volumes of water are concerned.
Industry is responsible for almost all heavy metals discharges in water. The need to reduce these discharges is undeniable.
Techniques for heavy metal removal
There are 5 different process families for heavy metal removal:
1- Chemical Precipitation:
By controlling the pH and by adding to the raw water precipitation reagents (sulphors, carbonates, phosphates), coagulation-flocculation agents (iron chloride, aluminum hydroxide) and polymer feeding, any heavy metals found in the raw water are retained within the resulting flocs which may then be collected during the clarification stage.
2- Filtration :
If the raw water contains a small amount of suspended matter and colloidal particles, the flocs are retained during the filtration stage. This may be either conventional (mono or multi-media) filtration or membrane filtration (microfiltration and ultrafiltration).
3- Fixed BedAdsorption:
On a selected media, for example granular iron hydroxide or manganese dioxide (Filtraflo™ As and Mangaflo™ processes).
4- Fluidized-bed Adsorption:
Uses the principle of crystallization on different supports such as carbonates, phosphates etc...
5- Adsorption on a continuously regenerated active surface:
In this last case, the heavy metals are retained on a fluidized-bed formed from granular iron hydroxide or manganese dioxide (from the Veolia Water Solutions & Technologies Metclean™ process for example).