Heavy metals removal systems
Common heavy metals like Nickel, Chrome, Zinc, Cadmium and Copper are typical by-products of industrial and laboratory processes and can pose considerable health risks to human beings. It is required that their presence in wastewater be reduced to legal levels before being discharged into the environment as mandated by the Environmental Protection Agency (EPA).
Metals can find their way into wastewater generated as a result of various industrial processes including the following:
- Metal working operations
- Spent baths from plating and etching operations
- Rinse water from plating baths
- Casting operations
- Air scrubbers
- Ore process waters
- Dissolution of metal-containing catalysts
- Oil & gas extraction (produced water)
- Incidental corrosion, such as cooling tower blowdown
Wastewater treatment and heavy metal pollution as a result of industrial activities are significant issues faced by many countries, especially developing ones. When heavy metals, such as arsenic, cadmium, chromium, copper, lead, nickel and zinc, contained in wastewater discharge into rivers and trenches without proper treatment, the result is severe pollution, leading to environmental impacts on aquatic life, plants and ecology. Many heavy metals are biologically accumulated and cannot be easily decomposed, jeopardizing human health through the food chain. Sources of industrial wastewater containing heavy metals include complex organic chemicals, electric power plants, electronics manufacturing, electroplating, iron and steel, and mines and quarries. Commonly chemical methods for removing heavy metals from wastewater are used.
Chemical Oxidation and Advanced Oxidation
Chemical oxidation involves the introduction of an oxidizing agent to the wastewater, causing electrons to move from the oxidant to the pollutants, which undergo structural modification and become less destructive. However, chemical oxidation may produce toxic by-products. Advanced oxidation, through processes such as steam stripping, air stripping, or activated carbon adsorption, can help remove any toxic by-products of chemical oxidation. Chemical oxidation and advanced oxidation are always used for the pre-treatment of heavy metal wastewater containing organic compounds.
Chemical precipitation involves adding a precipitation reagent to the wastewater, resulting in a chemical reaction that converts the dissolved metals into solid particles. The particles can then be aggregated by chemical coagulation and removed by filtration or sedimentation. Chemical precipitation is the most common method for removing dissolved heavy metals from wastewater. It is also among the least expensive technologies and is always combined with chemical coagulation for heavy metal wastewater treatment. However, it is not effective for treating wastewater with high acid content, and it produces a large quantity of toxic sludge that needs to be treated with chemical stabilization and disposed of properly.
Chemical coagulation, which is always used following chemical precipitation, destabilizes fine solid wastewater particles so that they aggregate during chemical flocculation. The particles are normally negatively charged, preventing them from forming larger groups and settling. Chemical coagulation introduces positively charged coagulants that reduce the negative particles’ charge, enabling them to form larger groups. An anionic flocculant introduced to the mixture reacts against the positively charged mixture to bind the particles into larger groups, and can be removed by filtration or sedimentation. Along with chemical precipitation, chemical coagulation is among the least expensive technologies for removal of heavy metals from industrial wastewater. It is not acid-resistant, so it is not appropriate for treating wastewater with high acid content. The sediment also contains heavy metals that must be treated by extra processes such as neutralization or dehydration.
Chemical stabilization involves treating sludge, a by-product of chemical precipitation and chemical coagulation, with an oxidant, which decreases the rate of toxic growth within the sludge. The sludge is either incinerated or solidified and then disposed of in landfills.
An ion exchange is a chemical reaction in which heavy metal ions from wastewater are exchanged for a similarly charged ion attached to a solid particle. The solid ion exchange particles are either naturally occurring inorganic zeolites or synthetically produced organic resins. There is a maximum quantity of exchanges due to a distinct number of mobile ion sites per unit of resin. The reusability of the adsorbed metal is high with ion exchange. This method is also limited by the matrix of the wastewater with high acid levels and high concentration of heavy metal ions, thus requiring pre-treatment before the process.