Pollutants from HPIs:

Thermal Power Plants (TPP):

• • The operational process involves the combustion of fossil fuels, including coal, oil, and natural gas, to generate heat. This heat is then utilized to convert water into high-pressure steam, which subsequently drives a steam turbine. The turbine is connected to a generator that produces electricity.
  • In India, the majority of TPPs primarily rely on coal as their fuel source, which contains various harmful elements. The energy efficiency of these plants is notably low, ranging from 20% to 45%. Furthermore, many TPPs lack the implementation of pollution control measures, such as flue gas desulphurization (FGD) and electrostatic precipitation.

Pollution:

• • Fly ash, which includes electrostatic precipitator ash, dry fly ash, pond ash, and mound ash, is a byproduct generated from the combustion of coal. This material is released into the atmosphere and deposited in ash ponds (a mixture of fly ash and water).
  • The failure of these ash ponds can lead to the contamination of surrounding agricultural lands, residential areas, surface water bodies, and groundwater with hazardous heavy metals and other harmful substances.
  • Heavy metals commonly found in fly ash include mercury, cadmium, arsenic, lithium, zinc, iron, copper, nickel, boron, magnesium, lead, and aluminum, which are frequently detected in both the air and water sources near thermal power plants (TPPs).
  • Additional toxic elements present in fly ash comprise fluoride and sulfur.
  • Gaseous emissions from TPPs include Carbon Dioxide (CO2), Sulfur Dioxide (SO2), Nitrogen Oxides (NOx), Particulate Matter (PM), Methane (CH4), Carbon Monoxide (CO—resulting from incomplete combustion), and Volatile Organic Compounds (VOCs).
  • Water pollution is exacerbated by heavy metal contamination stemming from acid mine drainage (AMD) associated with both open-pit and underground coal mining, as well as effluents from TPPs, which include cooling tower blowdown, ash handling wastewater, and discharges from wet flue gas desulfurization (FGD) systems.

Iron and Steel Industry:

Processes involved:

• • Unwanted impurities are eliminated through the smelting of iron ore in a blast furnace.
  • The primary impurities include sulfur, which forms iron sulfide and significantly weakens steel; lead, which enhances the machinability of steel in small amounts; and oxygen, as oxides compromise the strength of iron and steel.
  • In the blast furnace, a continuous supply of fuel (coke, which contains significantly fewer impurities than coal), iron ore, and flux (limestone, which helps remove sulfur and other impurities by converting them into slag) is maintained.
  • The resulting byproducts include liquid slag, liquid iron (known as pig iron, an intermediate product of iron ore smelting that contains oxides), and various gases.
  • The oxygen present in the iron oxides is reduced through a series of chemical reactions that generate carbon monoxide (CO) and carbon dioxide (CO2).
  • The transformation from pig iron to steel involves several stages: liquid pig iron is first converted into cast iron (cooled liquid iron that is brittle and contains more than 2% carbon), and then into wrought iron (which is weak and consists of liquid iron and slag).
  • Steel is produced from pig iron with a carbon content of up to 2.1%, although it is susceptible to corrosion. Finally, stainless steel is created by adding 10.5% chromium along with elements such as nickel, manganese, and molybdenum, rendering it resistant to corrosion.

Byproduct – Slag:

• • Slag is characterized by a high concentration of impurities, including calcium sulfide (CaS) and various oxides such as silica, alumina, magnesia, and calcium oxide (CaO), which are introduced through the iron ore or coke. A minimal fraction of slag is disposed of in landfills.
  • Cement produced from blast furnace slag exhibits reduced permeability and enhanced durability compared to traditional Portland cement. It serves as an aggregate in applications such as concrete, cement clinker, asphalt concrete, and road bases.
  • In terms of soil enhancement, the dissolution of slag creates alkalinity, which can facilitate the precipitation of metals, sulfates, and excess nutrients (including nitrogen, phosphorus, and potassium) during wastewater treatment processes.
  • As a soil conditioner, ferrous slags have been utilized to adjust soil pH levels and act as fertilizers, providing essential calcium and magnesium.

Air Pollution:

• • The industrial sector relies heavily on coal combustion, particularly in thermal power generation and coke production for blast furnaces.
  • This process contributes to air pollution, releasing particulate matter (PM2.5 and PM10), carbon dioxide, sulfur oxides (with sulfur being released as SO2 during the blast furnace operation), nitrogen oxides, carbon monoxide, hydrogen sulfide, and non-methane volatile organic compounds (NMVOCs), among other pollutants.
  • Additionally, coke ovens release naphthalene, a substance known for its high toxicity and potential carcinogenic effects.

Magnetite Contamination: 

• • Magnetite contamination denotes the occurrence of the magnetic mineral Magnetite (Fe3O4) in the environment, primarily due to anthropogenic activities including mining, steel manufacturing, and various industrial processes.
  • The presence of magnetic particles can disrupt the migratory behaviors of avian species and hinder the functionality of electronic devices, including compasses and navigation systems.

Water Pollution:

• • The contamination of water by heavy metals is primarily a result of acid mine drainage (AMD) from slag heaps.
  • The wastewater generated during the coking process, which involves heating coal in an oxygen-deprived environment (resulting in CO emissions) to eliminate volatile organic compounds (VOCs) and produce high-carbon coke, is extremely hazardous. This wastewater contains numerous carcinogenic organic substances, along with cyanide, sulfides, ammonia, and other toxic elements.
  • The leaching of slags, which are rich in oxides, can lead to the formation of highly alkaline groundwater. This occurs as the oxides in the slags react with water, resulting in an increased concentration of hydroxide ions (OH-) in the groundwater.

Cement Industry: 

• • The production process includes the following steps: extracting limestone from quarries through blasting, followed by crushing the limestone (75%) and clay (25%).
  • This mixture is then subjected to calcination, where it is heated in a kiln—an oven-like structure—at elevated temperatures to produce calcium silicate clinker, a combination of limestone and heat-altered minerals.
  • The clinker is subsequently ground with 3-5% gypsum, which helps regulate the cement’s setting time, resulting in Portland Cement.
  • Limestone, the predominant form of calcium carbonate, serves as the essential binder in cement production. It is heated in kilns to approximately 1,400 °C, utilizing coal as the fuel source.
  • During this process, carbon contained in the limestone reacts with oxygen, resulting in the release of carbon dioxide (CO2) as a byproduct; the production of one ton of cement generates at least half a ton of CO2. To mitigate CO2 emissions, a portion of the limestone can be replaced with materials such as blast furnace slag or fly ash.
  • The manufacturing of cement necessitates water for various purposes, including cooling heavy machinery and exhaust gases, operating emission control systems like wet scrubbers, and preparing slurry in kilns.
  • The emissions and effluents produced are similar to those from thermal power plants, as coal remains the primary fuel used in the kilns.

Copper Smelting Industry: 

Processes Involved: 

• • Extraction of raw chalcopyrite ore (CuFeS2 — copper iron sulfide; the most prevalent copper ore mineral; contains less than 1% Cu).
  • Enhancement of chalcopyrite ore concentration (34.5% Cu, 30.5% Fe, and 35.0% S) through processes such as crushing, grinding, and flotation purification (utilizing oils due to the affinity of certain metals for them) at the mining site.
  • Roasting of the concentrated ore is conducted in copper smelters, typically located near ports, to eliminate impurities such as sulfur, antimony, arsenic, and lead. This process removes 20-50% of sulfur in the form of highly concentrated sulfur dioxide (SO2), which is then converted into concentrated sulfuric acid for use in fertilizers, pharmaceuticals, paper bleaching, petroleum refining, and other industries.
  • The smelting of the roasted ore concentrate results in the production of matte (65% Cu), a molten amalgamation of copper sulfide (Cu2S), iron sulfide (FeS), and slag composed of iron oxide and heavy metals.
  • The conversion of matte in a converter furnace produces high-purity blister copper (~99% Cu).
  • Blister copper undergoes further refinement in an anode furnace, where it is cast into anodes to facilitate the removal of oxygen, achieving a purity of 99.5% Cu.
  • Electrolytic refining involves the deposition of copper from the anode plates onto stainless steel cathode plates, resulting in copper with a purity of 99.99%.
  • The slime produced during electrolytic refining contains valuable metals such as gold, silver, selenium, and tellurium, which are recovered in a dedicated slime treatment facility.

Copper slag:

       It serves primarily as an abrasive material for surface blast-cleaning, which is employed to refine the surfaces of metals, stones, concrete, and similar materials. Additionally, it finds applications in road construction and as a fundamental component in the manufacturing of cement, mortar, and concrete, where it acts as a raw material for clinker and both coarse and fine aggregates.

Pollution:

• • The leaching of contaminants from copper ore concentrate and slag includes substances such as radon (which is released from the natural radioactive decay of uranium and copper ores), as well as iron, arsenic, antimony, mercury, lead, cadmium, selenium, magnesium, aluminum, cobalt, tin, nickel, manganese, nitrates, and fluorides, among others.
  • Given that the majority of copper ores are based on sulfur, the smelting process generates sulfur dioxide. When sulfur dioxide levels become excessively high, industries often convert it into sulfuric acid, which acts as both an irritant and a contaminant of water sources.

Thoothukudi Sterlite Copper Plant Controversy: 

• • The Sterlite Copper Smelter operated by Vedanta in Thoothukudi, Tamil Nadu, along with Hindalco’s copper smelter in Gujarat, accounted for over 80% of India’s copper production.
  • In 2018, significant protests arose against the Thoothukudi facility due to proposals to double its production capacity from 400,000 to 800,000 tonnes annually, leading to its eventual closure.
  • The smelter was situated merely 14 kilometers from the Gulf of Mannar Marine National Park, despite regulations from the CSIR National Environmental Engineering Research Institute (NEERI) stipulating that hazardous industries like copper smelting should be located more than 25 kilometers away from ecologically sensitive areas.
  • The facility was found to be emitting harmful sulfur dioxide fumes into the air and discharging toxic effluents containing heavy metals into adjacent water bodies.
  • The Thoothukudi facility accounted for 36 percent of the national demand for refined copper.
  • It comprised a sulphuric acid production unit, a phosphoric acid production unit, and a 160 MW coal-fired power plant that supplied energy to the copper smelting operations.
  • The shutdown of this plant has had a significant impact on numerous livelihoods.

Zinc/Lead Smelting Sector:

       Zinc serves as a protective agent against corrosion in steel through the process of galvanization, while lead is primarily utilized in battery production. These two metals frequently coexist in nature as sulfide minerals. 

1. 1. Raw Materials: The primary ores used are zinc sulfide (ZnS) and lead sulfide (PbS, commonly known as galena).
   2. Beneficiation: The concentration of zinc and lead from the extracted ore occurs near the mining site, involving processes such as crushing, grinding, and flotation, which utilizes oils due to lead’s affinity for them.
   3. Roasting: The concentrated ore is subjected to high temperatures in a blast furnace, transforming it into calcine (an impure form of zinc oxide) while releasing most sulfur as sulfur dioxide (SO2). Crude molten lead, referred to as lead bullion, is similarly obtained using limestone flux and coke.
   4. Leaching: The calcine undergoes dissolution in sulfuric acid.
   5. Purification: A pure zinc sulfate solution is generated by isolating other metals, including lead and gold.
   6. Electrowinning (a specialized electrolysis technique): Zinc from the zinc sulfate solution is deposited onto aluminum cathodes, and lead is extracted from the lead bullion in a comparable manner.

By products: Gold, silver, and copper are recovered as slimes. 

      Granulated lead-zinc slag possesses appropriate particle sizes, making it suitable for use as a sand substitute in mortar and concrete applications.

Pollution: 

• • Major air contaminants include particulate matter (such as lead, zinc, arsenic, antimony, cadmium, copper, mercury, and metallic sulfates) and sulfur dioxide (SO2).
  • Wastewater sources consist of spent electrolytic baths, recovery of slimes, spent acids from hydrometallurgical processes, cooling water, and air scrubbers, among others.
  • Leaching of heavy metals from discarded slag involves elements such as cadmium, copper, lead, iron, bismuth, antimony, arsenic, and others.

Aluminium Smelting Sector: 

• • The Bayer Process is utilized to extract alumina (aluminium oxide) from bauxite ore in proximity to the mining site. This process involves the elimination of the insoluble components of bauxite by treating the ore with highly heated caustic soda (sodium hydroxide).
  • The Hall-Héroult Process generates pure aluminium through the electrolysis of alumina within an aluminium smelting facility. Given the substantial energy requirements, these smelters are typically located near power generation stations.

Pollution:

      The majority of emissions are associated with thermal power generation and the electrolysis process, including nitrogen oxides (NOx), sulfur dioxide (SO2), ammonia (NH3), and polycyclic aromatic hydrocarbons (PAHs), which are produced as a result of incomplete combustion during electrolysis.

Red Mud: 

• • Red mud refers to the highly alkaline and toxic byproduct generated from the Bayer process of alumina extraction. This residue primarily consists of iron oxides along with a range of other oxides and heavy metals.
  • Traditionally, red mud was disposed of exclusively in landfills. Recently, however, it has found applications in road construction and as a source of iron for the production of iron-rich cements and cost-effective concrete. Additionally, it is utilized in agricultural practices to enhance phosphorus cycling, mitigate soil acidity, and promote carbon sequestration.

Petroleum Refining and Petrochemicals: 

       The petroleum sector is categorized into three main segments: upstream, midstream, and downstream. The upstream segment focuses on the exploration and extraction of crude oil, the midstream encompasses the storage and transportation of crude, while the downstream is concerned with the refining process. 

Petroleum Refining: 

• • Fractional distillation: This process involves the separation of various fractions (hydrocarbon compounds) of crude oil according to their differing boiling points.
  • Conversion processes: This entails the breakdown of long-chain molecules into smaller, more valuable ones through the application of heat.
  • Treating: This step involves the removal of impurities, including sulfur, nitrogen, and heavy metals.

Pollution: 

• • Air contaminants consist of particulate matter (PM), carbon dioxide (CO2), nitrogen oxides (NOx), carbon monoxide (CO), hydrogen sulfide (H2S), sulfur dioxide (SO2), natural gas (methane), lead, and volatile organic compounds (VOCs), including carcinogenic benzene pollutants, among others.
  • Refineries employ deep injection methods to dispose of wastewater and oil byproducts into underground wells and coastal waters. Unfortunately, a portion of these waste materials infiltrates aquifers and groundwater sources.

Petrochemicals:

     The petrochemical sector involves the production of synthetic fibers, various polymers (such as PVC, polystyrene, and performance polymers), and intermediates like styrene. It also includes the creation of synthetic rubber (elastomers), synthetic detergent intermediates, and performance plastics, all utilizing hydrocarbon feedstocks (including naphtha, ethylene, propylene, and butadiene) primarily obtained from the processing of crude oil and natural gas.

Pollution: 

     Organic pollutants mainly comprise intricate polycyclic aromatic hydrocarbons (PAHs). The wastewater discharges contain hazardous substances such as phenols, cyanide, and formaldehyde.

Fertilizer Sector: 

• • Air Contaminants: The industry emits various pollutants, including particulate matter, ammonia (NH3), nitrogen oxides, sulfur, and carbon dioxide. Prilling towers, responsible for producing urea prills, are a major contributor to the release of urea dust particulates.
  • Water Contamination: The wastewater produced contains ammoniacal nitrogen, phosphates, and heavy metals such as vanadium and arsenic—utilized in the carbon dioxide removal process within ammonia production facilities—as well as chromium, which serves as a corrosion inhibitor in cooling towers, along with fluorides in varying concentrations.

Distilleries: 

• • The distillation sector utilizes various agricultural products, including sugarcane molasses, grains, fruits, sugar beets, and others, to produce alcohol through fermentation and distillation processes. Alcohol serves as a crucial component in several industries, such as chemicals, pharmaceuticals, cosmetics, beverages, food, and perfumery.
  • Distilleries rank among the industries that contribute significantly to water pollution, as the fermentation of ethanol generates substantial liquid waste characterized by elevated levels of organic matter (high BOD), nitrogen compounds (leading to eutrophication), low pH, increased temperature, high turbidity, and elevated salinity.

Paper and Pulp Industry: 

• • Pulp is a fibrous substance created by extracting cellulose fibers from sources such as wood, bagasse, fiber crops, and recycled paper.
  • Pulp mills are characterized by their high energy consumption and significant use of clean water and various chemicals, including sulfite salts, caustic soda, sodium sulfide, hydrogen peroxide, and sulphonic acid.
  • These substances are essential for the pulping process, which involves the removal of lignin, as well as for bleaching the pulp to produce paper of differing qualities and finishes.
  • The wastewater generated from these processes contains a complex mixture of organic pollutants (notably high in biochemical oxygen demand) and inorganic substances, including sodium hydroxide, sodium carbonate, sodium sulfide, chlorine dioxide, calcium oxide, and hydrochloric acid.
  • The main gaseous emissions from the industry include hydrogen sulfide, sodium sulfide, methyl mercaptan, and sulfur.

Caustic Soda:

• • Caustic soda (NaOH) is synthesized through the electrolysis of brine, which is a solution of common salt in water, utilizing either the mercury cell or membrane cell methods as part of the chloralkali process. This chemical is extensively utilized across various industries, including pulp and paper, detergents, packaging, agriculture, environmental protection, water treatment, and textiles.
  • The chlor-alkali sector produces caustic soda, soda ash, chlorine, and hydrogen, which can serve as fuel or be transformed into hydrochloric acid (HCl). These substances are essential in the production of paper, soaps, detergents, chemicals, water treatment agents, textiles, polyvinyl chloride (PVC), glass, and more.
  • Due to the environmental hazards linked to the mercury process and the dangers associated with the handling of chlorine, its vapors, and hydrogen, this industry is categorized among the 17 industries recognized as polluting.
  • The process generates wastewater from the drying of chlorine with sulfuric acid (H2SO4), and discarded cell components, such as membranes, anodes, and cathodes, can leach heavy metals into the environment.

Tannery:

Beamhouse processes:

• • The journey begins with hides sourced from slaughterhouses, which undergo soaking to eliminate dirt.
  • This is followed by liming, a process that chemically removes hair through the dissolution of hair using an alkaline solution of sulphide and lime.
  • Next, deliming occurs, where the alkaline hides are neutralized using acid ammonium salts.
  • This is succeeded by bating or puering, aimed at degrading proteins, and finally, pickling, which enhances the acidity of the hide, facilitating the penetration of chromium tannins.

Chrome tanning:

• • This method relies on the cross-linking of chromium ions with free carboxyl groups present in collagen, resulting in hides that are resistant to bacterial growth and temperature fluctuations.
  • A significant environmental concern arises from the discharge of solid waste and wastewater containing chromium.
  • The beamhouse operations, which occur prior to tanning, produce wastewater characterized by high organic content, indicated by elevated biochemical oxygen demand (BOD).
  • Air emissions are mainly associated with energy consumption, as well as the use of organic solvents and dyes, including hydrogen sulfide (H2S) and ammonia (NH3).

Sugar Industry: 

      The production process involves the harvesting of sugar cane, a tropical grass, or sugar beet, a temperate root crop, followed by juice extraction through crushing, filtration, crystallization, and drying of the resulting crystals to produce sugar. 

By products: 

• • Press mud is the waste generated during the filtration of cane juice. Its applications include use as fertilizer, soil amendment to enhance physical properties such as water retention, bio-sorbent for effectively adsorbing metal ions and contaminants, and as animal feed.
  • Molasses is a thick, dark syrup that results from the refining of sugar cane juice into sugar. It is utilized for sweetening and flavoring foods, in the production of brown sugar for baking, in ethanol manufacturing, and as animal feed and soil amendment.
  • Bagasse is the dry, fibrous material left after the crushing of sugar cane. It serves as fuel in sugar production facilities and as a raw material in the paper, pulp, and packaging industries.

Air Pollution: 

• • The incineration of sugar cane leaves, residue, and bagasse, which is used as fuel in sugar production, releases significant amounts of fly ash, sulfur dioxide, carbon monoxide, nitrogen oxides, nitrates, carbon compounds, and sulfates into the atmosphere.
  • The processes of sulphitation, which involves the introduction of sulfur dioxide into liquids, and carbonation, which saturates a liquid with carbon dioxide gas, are employed to purify cane juice. This involves the use of sulfur dioxide, carbon dioxide, lime (calcium hydroxide), and calcium carbonate, which aids in the precipitation of impurities, resulting in the production of SO2 and CO2.

GHG Emissions:

• • A significant portion of total greenhouse gas (GHG) emissions is attributed to the burning of agricultural residues, the application of synthetic fertilizers, and the combustion of fossil fuels.
  • The practice of burning sugarcane residues contributes to emissions of not only carbon dioxide (CO2), nitrous oxide (N2O), and methane but also other GHG precursors, such as carbon monoxide (CO) and non-methane volatile organic compounds (NMVOCs).

Water Consumption: 

     The cultivation of one kilogram of sugarcane necessitates approximately 1,500 to 2,000 liters of water. Following the harvest, the process of crushing one tonne of sugarcane requires an additional 1,500 to 2,000 liters of water, resulting in the generation of around 1,000 liters of wastewater. 

Water Contamination: 

• • The sugar industry is the third largest producer of wastewater, following the pulp and paper and chemical industries.
  • This wastewater is characterized by a high organic pollution load, particularly in terms of suspended solids, organic matter, press mud, and bagasse, which significantly elevates the Biological Oxygen Demand (BOD)—a measure of the amount of organic matter in water that is decomposed by bacteria and other microorganisms, consuming substantial amounts of dissolved oxygen.
  • The presence of coliform bacteria, which serve as indicator species for pollution levels, suggests a high concentration of pathogens and total dissolved solids (TDS)—inorganic salts such as calcium, magnesium, sodium, and nitrates—in the effluents produced by the sugar industry.

Highly Polluting Industries (HPIs) significantly contribute to economic growth but cause severe environmental degradation. They release hazardous waste, toxic effluents, and pollutants that harm ecosystems and public health. Key polluting sectors include chemical manufacturing, tanneries, textiles, and thermal power plants. Adoption of cleaner technologies and strict adherence to CPCB regulations is crucial. Sustainable practices like waste minimization and recycling must be prioritized. A collaborative approach is essential to balance industrial progress with environmental protection.