How to Treat Wastewater Containing Heavy Metal?
Wastewater containing heavy metals is a major environmental concern, as it can contaminate water sources and soil if not properly treated. Heavy metals refer to metals and metalloids like mercury, cadmium, lead, chromium, copper, zinc, nickel, etc. These metals can be toxic to humans and aquatic life even at low concentrations. Industries like mining, metal plating, tanneries, batteries, pesticides, etc. generate wastewater containing high amounts of heavy metals. Therefore, effective treatment methods are required before discharging this wastewater into the environment.
Sources of Heavy Metals in Wastewater
Heavy metals in wastewater mainly originate from industrial processes and products. Major sources include:
1· Mining and ore processing - Produces wastewater containing arsenic, cadmium, lead, mercury, zinc.
2· Metal plating - Generates wastewater with cadmium, chromium, copper, nickel, lead, tin, zinc.
3· Tanneries - Wastewater contains chromium.
4· Batteries industry - Produces cadmium, lead, lithium, manganese, nickel, silver, zinc in discharge.
5· Pesticides - May contain arsenic, lead, mercury.
6· Electronics industry - Discharges copper, cadmium, lead, nickel, zinc.
7· Textile industry - Effluents contain cadmium, chromium, copper, lead, nickel.
8· Petrochemicals - Releases chromium, lead, mercury, selenium compounds in wastewater.
Domestic sewage may also carry heavy metals from household chemicals and corrosion of plumbing systems. Proper identification of heavy metal sources is important for designing effective treatment strategies.
Characteristics of Wastewater Containing Heavy Metals
Heavy metal containing wastewater exhibits certain characteristics that influence the choice of treatment methods:
1· It has high dissolved metal concentrations, low biodegradability and variable pH.
2· Metals may be present in particulate, colloidal or dissolved forms. Dissolved ions like Cd2+, Cr6+, Ni2+ are more difficult to remove.
3· Wastewater may contain a mix of heavy metals with different properties. For example, chromium compounds cause high toxicity while nickel and zinc are relatively less toxic.
4· Other components like organic matter, salts, solids, oil/grease may also be present which can interfere with metal removal processes.
5· Composition and metal concentrations fluctuate based on the source and process generating the wastewater.
6· High total dissolved solids and hardness due to minerals affect the efficiency of certain treatments.
7· Flow rates and volumes of discharged wastewater widely vary for different industries.
Understanding these characteristics is essential for designing suitable pre-treatment steps and choosing optimal heavy metal removal techniques.
Treatment Methods
Numerous treatment methods have been developed for heavy metal removal from wastewater. They can be broadly classified into conventional, biological, chemical precipitation and adsorption methods.
Conventional Methods
1· Coagulation and flocculation involves adding coagulants like alum, ferric salts to destabilize suspended particles and emulsified oil, allowing aggregation into flocs. Flocculating agents like polyelectrolytes are used to bridge flocs into larger masses that can be separated easily. This allows removal of insoluble metal compounds along with other suspended solids. It works better when heavy metals are in particulate form rather than dissolved. Up to 90-95% removal of metals like chromium, copper, zinc, lead is possible if optimal coagulant type and dose is used. The drawback is large volume of metal-containing sludge generated.
2· Sedimentation refers to the settling of suspended particles and flocs under gravity. It allows separation of coagulated heavy metals as sludge. 50-90% removal of chromium, iron, nickel and lead can be achieved. Performance depends on particle sizes and settling characteristics. It needs long retention time for good separation.
3· Filtration is used as pre-treatment to remove solids that can interfere in subsequent steps. Sand filters, microfilters and ultrafilters have been used. Microfiltration gave 85% lead removal and nanofiltration removed 95% copper and cadmium. Membrane fouling due to particulates is a challenge.
4· Ion exchange resins exchange heavy metal ions in wastewater with non-toxic ions like H+ and Na+ from the resin. It is effective for removing dissolved ionic forms of heavy metals. Up to 99% removal of copper, cadmium, zinc, chromium can be achieved. High capital and operational costs, especially for regenerating resins, is the main disadvantage.
Biological Methods
1· Biosorption refers to certain dead microbial biomass that can adsorb heavy metal ions. Algae, bacteria, fungi and industrial wastes like sawdust have been used. Uptake depends on factors like pH, temperature, initial metal ion concentration and contact time. It is fast metal binding but has low selectivity. Uptake levels ranging from 45 to 98% have been reported for different metals. It is easy to operate but requires optimization of conditions for high performance.
2· Bioremediation involves the use of living microbes like bacteria, yeasts and algae to remove heavy metals through mechanisms like bioaccumulation, biosorption, redox reactions, etc. It requires understanding microbial responses and adaptation to high metal concentrations. High treatment costs and technical challenges exist for full-scale application.
Chemical Precipitation
1· Chemical precipitation involves converting metals to insoluble precipitates through addition of precipitants like lime, caustic soda, sulfides, etc. Over 90% removal can be achieved for many metals if optimal pH and precipitant dose is used. High sludge production containing concentrated metals is the main disadvantage.
Adsorption Methods
1· Activated carbon adsorption uses powdered or granular activated carbon which has high adsorption capacity for heavy metal ions. Up to 98% removal of lead, chromium, copper, cadmium, nickel is possible. Effectiveness depends on factors like pH, temperature, initial concentration and contact time. Cost can be high for continuous operation, regeneration is necessary after saturation.
2· Adsorption on agricultural wastes involves use of materials like rice husk, coconut shell, bark, straw which are low cost adsorbents. It works through mechanisms like chelation, ion exchange and physical adsorption. It provides good removal for metals like cadmium, lead, chromium, nickel, zinc. It is easy to operate but kinetics are slow and adsorption capacity is lower than activated carbon.
Conclusion
Treatment of heavy metal laden wastewater is essential before discharge into natural water bodies due to their toxic effects. A combination of different techniques is usually required for effective removal of multiple metals present. Preliminary treatment using coagulation, flocculation and filtration helps remove some suspended solid metals and prepares the wastewater for subsequent steps. Conventional methods provide a polishing treatment with ion exchange resins producing very high removal efficiencies. Chemical precipitation through addition of precipitants is one of the most commonly used techniques in industries for metal removal and recovery in sludge form. Biosorption serves as a simple, low cost method but has limitations in application on an industrial scale. Adsorption techniques are advantageous and activated carbon is widely used due to its high capacity and ability to treat dissolved forms of metals also. Research is ongoing to develop novel biosorbents and adsorbents to make metal removal more economical and eco-friendly. Overall, a tailored, sequential treatment plan using a combination of suitable techniques is necessary for complete treatment of heavy metal containing wastewater.
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