Do you remember the heavy metal removal technique from the water pollution chapter in your science books?
There were images of industries discarding heavy metal waste into river bodies.
And in the “ways to stop water pollution” section, there was a point “Convincing the industries not to dump the heavy metal waste in water bodies without any treatment.” But we all know that it is not that easy.
In this blog, you will get to know about heavy metal removal from wastewater. Heavy metals can be toxic in nature. They can have adverse effects on the ecosystem and billions of people worldwide.Discarded heavy metals lead to water pollution which is a global issue. It contributes towards scarcity of clean drinking water.
If you are looking for information about heavy metal removal from wastewater then this blog is your answer.
According to a research, industries discard 2 million tons of industrial waste into water worldwide everyday. The discharge from industry consists of organic and inorganic pollutants. Among these pollutants are heavy metals which can be toxic in nature. Most heavy metal ions introduced into the environment cannot decompose. This poses a threat for the entire food chain.
Emerging technologies provide solutions for water pollution and its treatment. Some water purification methods claim to have 99% efficiency. This is only true under idealized conditions of pH, and other operating parameters.
Heavy Metal Removal Can Be a Challenge
Some techniques like ion exchange resins can target one contaminant at a time. This makes their use impractical for polluted water where various contaminants occur. This decreases the efficiency of the process.
Most of the technologies for heavy metal removal are expensive. Some are affordable too but they often generate secondary pollution. Thus, heavy metal removal from wastewater remains challenging. These costly and inefficient technologies lead to irreversible environmental damage. This leads to the availability of clean drinking water becoming a challenge.
Water treatment services need to deliver clean drinking water in a sustainable way. Sustainable treatment technology also needs to consider energy saving as an important need.
According to section 3 of article Ranking Efficiency Product, assessing different techniques by accounting for efficiency, flexibility, ease of implementation, operating costs, energy consumption, and other factors related with sustainability can help.
Primary, Secondary and Tertiary Treatment for Heavy Metal Removal
Wastewater treatment is a multi-step process. It includes several methods from the polluted water to the final treated water. Generally, the water treatment units combine primary, secondary, and tertiary treatment processes. This ensures that water treatment is efficient against all the types of pollutants.
The main primary, secondary and tertiary treatment for heavy metal contaminated water is in the tables below. I-O-B indicates Inorganic-Organic-Biological pollutants.
|Primary Treatment||Coagulation and Flocculation||Chemical Precipitation||Microfiltration|
|Types of pollutant||I-O-B||I-O||I-O-B|
|Secondary Treatment||Aerobic Process||Anaerobic Process|
|Types of pollutant||O||O|
|Tertiary Treatment||Oxidation||Distillation||Ion Exchange||Adsorption||Membrane Process|
|Types of pollutant||I-O||I-O-B||I-O||I-O-B||I-O-B|
Industrial water contaminated by heavy metals with considerable levels of contamination needs per-treatment before entering the tertiary treatment process. The most common pre-treatment for heavy metal removal is the chemical precipitation process.
Technologies for Primary Water Treatment
Primary treatment is the first step for the water purification process. After this we apply other treatments. The efficiency here is generally much lower than tertiary treatment. We can remove massive amounts of pollutants using these cost-effective and simpler technologies.
Among membrane filtration technologies, microfiltration has the largest pore size range (100-1000 nm). Microfiltration removes heavy metal ions. It also filters TDS, microorganisms, algae, bacteria. Water treated by microfiltration is used for tertiary heavy metal purification processes.
· Chemical Precipitation
It is the most widely used process for heavy metal purification. It is simple and inexpensive compared to other technologies. In this process, pH adjustment of heavy metals and reaction with chemical reagents result in the formation of insoluble particles. These are then removed by simple sedimentation. Major disadvantage is large sludge generation and secondary pollution.
· Coagulation and Flocculation
This method destabilizes the particles suspended in the polluted streams by the addition of coagulants or flocculants. It results in the sedimentation of particles. We use Ferric and aluminium salts as flocculating agents. This technique improves sludge settling and biological pollutant removal. The major chemical consumption costs and sludge disposal are their drawbacks.
Technologies for Secondary Water Treatment
Secondary water treatments depend upon naturally occurring microorganisms. These organisms convert pollutants into simpler substances. This treatment is divided into two broad categories:
· Anaerobic Treatment
Anaerobic treatment is an energy-efficient process. Here microorganisms transform pollutants in the wastewater into biogas. It takes place in the absence of oxygen. This process produces Methane-rich biogas which is used as a renewable energy source.
· Aerobic Treatment
Microorganisms convert pollutants into carbon dioxide in the presence of oxygen. Aerobic microorganisms need oxygen, so air must be continuously circulated through the tanks. Forced air from an air blower or compressor is mixed with the wastewater, where the aerobic bacteria feed on the waste present in the water source.
We use secondary treatments in removing organic pollutants. Microorganisms, as heavy metal bio-adsorbents, offer a new alternative for the removal of toxic metals in industrial wastewater. Recent research studies show that microbes may also remove the heavy metals. Aerobic up-flow systems show efficient removal of zinc (Zn), copper (Cu), manganese (Mn), and iron (Fe) with immobilized yeast Issatchenkia orientalis (Io) and Candida tropicalis (Ct) on zeolites. Cost effective and efficient industrial scale water treatment by the microbes is still in its early stages. However one can expect it to grow in future.
Technologies for Tertiary Water Treatment
Tertiary water treatment is the final step in wastewater treatment. After this we can reuse or discharge it in the environment.
This process requires minimal equipment. Chemical oxidation involves the introduction of an oxidizing agent into the wastewater. This causes electrons to move from the oxidant to the pollutants, which undergo structural modification. Oxidants commonly used in water treatment applications include:
· Chlorine dioxide
Most typical methods for tertiary water treatment is based on crystallization technology. In thermal technologies, such as distillation, sufficient energy is provided to bring the polluted water to its boiling temperature and then to vaporize. This transforms some of the water into steam. The final step is to condense the process steam as pure water.
· Ion Exchange
Ion exchange is a well-established method for hardness removal in drinking water. It is also being studied for the removal of heavy metal ions. This technology relies on exchange surfaces, as in adsorption. Hence it is important to have nano-porous resins with high specific surface area. Most common ion exchange resins available in the market are:
· sodium silicates zeolites
· polystyrene sulfonic acid
· methacrylic resins
A distinct advantage of ion exchange resins is the reversibility of the reactions. Ion exchange resins can also be used to recover valuable heavy metals present in the wastewater. Despite these advantages, they also have some important limitations. Some of these are: 1. Compulsory pre-treatment 2. Their performance is highly specific and dependent on the ions. This makes them unsuitable for simultaneous ion removal from polluted water streams.
Adsorption is the increase in substance concentration at a surface. Here we remove pollutants by promoting their adsorption on the adsorbent surface. Some advantages of adsorption are – 1. It is effective and economical.2. It has a high removal ability. 3. It is flexible in design and easy to operate. We can categorize adsorbents primarily used in wastewater treatment as
1. Carbon based nano-sorbents
2. Metal based nano-sorbents
3. Zeolite based nano-sorbents.
While discussing about this with Vaishali Sharma who is currently pursuing her masters degree in environmental science and engineering, she shared some really great information. Significant time and resources have been committed in heavy metal pollution remediation, particularly for industrial wastewater which is severely contaminated with toxic substances. When it comes to wastewater treatment, adsorption is one of the most efficient, easy, and cost-effective techniques. Because of its versatility in design and operation, high removal efficiency, and the ability to recycle and reuse most of the adsorbents through a proper desorption process, adsorption is becoming widely adopted.
· Membrane Process
Membrane processing includes several advanced and diverse technologies. It removes various types of pollutants with high efficiency. When used for heavy metal removal, membrane processing can provide a reliable solution. It generally does not need chemical additives or thermal inputs. It is simple in fabrication, operation, scale-up and control. Membrane technologies suffer from selectivity and permeability trade-off. They also have high energy consumption due to the pressure-driven inherent process. Membrane based technologies for water purification can further be classified into:
3. Reverse osmosis
4. Nano-hybrid membranes
Emerging Technologies for Heavy Metal Removal
Emerging technologies are still in their infancy. They still promise to mitigate the global water purification issue. Some of the emerging technologies are in the list below:
· Graphene Oxide Microbots
· Layered Double Hydroxides
· Metal-organic Frameworks
· Emerging Membrane Technologies
· Electro Coagulation
· Capacitive Deionization
Sustainability of Technologies
Efficiency is crucial in heavy metal water treatment technology. This becomes an important criterion when sustainability aspects are considered. For example, reverse osmosis is a reliable and widespread technology. It has high rejection efficiencies, but it can be extremely energy intensive. It also requires large investments and has very low portability. Thus, we cannot use it in most rural or remote areas where water scarcity is critical.
Similarly, some advanced materials such as latest generation adsorbers, ion exchange resins or heterogeneous photocatalysts, may not be compatible with largely accessible drinking water needs for cost aspects.
A sustainable technology for water purification should have the following characteristics:
1) maintain the cost of processed water as low as possible
2) requires little to no investment
3) be operative at low energy
4) allow recovering most of the treated water and
5) keep a low environmental fingerprint
In this blog, we have covered and evaluated some technologies in heavy metal ions waste water treatment that we commonly use. Clearly we are yet to find a universal solution for this problem. The right technology to use varies from case to case basis. It is important to note that there is no technique that can perform at a nominal, perfect 100% efficiency under real conditions.
Nonetheless, technology is progressing at such a speed that today the theoretical 100% efficiency limit is rapidly being approached. While improvements have been extremely rapid on the technical side, the cost, and sustainability aspects of each technology have progressed at a much slower rate. With increasing water pollution problems, it is necessary for industries to implement these techniques. If you liked reading this blog, please leave a comment and subscribe. Let us know what you want to read about in the next blog.