Solid Pollutants and Solid Pollution
Solid Pollutants are more popularly known as solid wastes. The types and quantity of solid pollutants generated in iron and steel industry differ from plant to plant. Thus, a steel plant based on direct reduced iron – electric furnace (electric arc furnace or induction furnace) route generates different types and quantities of solid wastes than the integrated steel plant based on the blast furnace – basic oxygen furnace route.
The solid wastes generated in iron and steel industry can be broadly categorized as (i) process solid wastes, and (ii) non-process solid wastes. Examples of process solid wastes are slag, dust, sludge, scrap, refractories, scale, muck, and debris etc. Examples of non-process waste materials are rubber, packing materials, electric wires, plastics and glass, and office and canteen wastes, etc. Solid waste materials are usually generated in mixed condition which means that during generation, one solid waste material gets contaminated with other waste material. These waste materials are to be segregated for their recycling and reuse.
There are two types of waste streams which generates solid wastes. One stream is related to the production process, which also includes the waste from laboratories and sludge from effluent treatment plants. The second solid waste stream is due to the maintenance of offices and canteens. The type and quantity of solid wastes’ generation depends on production process and the production technology employed. The quantity of solid waste generation is mainly dependent on two factors namely (i) the size of production unit, and (ii) efficiency of the technology.
The solid wastes generated of iron and steel industry are of two types, i.e., ferruginous wastes and non-ferruginous wastes (Fig 1). The major ferruginous wastes are iron bearing wastes and consist of iron bearing dust and fines, sludge, ferrous scrap, scale, and slag generated during welding or lancing. The major non-ferruginous wastes are lime and dolomite dust and fines, coal and coke dust, sludge (lime sludge, tar sludge, biological treatment sludge, oil sludge, and sewage sludge etc.), non-ferrous scrap, refractory wastes, ironmaking and steelmaking slag, ash (fly ash and bottom ash), and muck, and debris etc.
Fig 1 Major solid wastes of iron and steel industry
Solid wastes can be termed as hazardous wastes or non-hazardous wastes. A solid waste is hazardous because of its inherent characteristics. Some of the characteristics for the identification of hazardous solid wastes are ignitability, corrosivity, reactivity, and toxicity. Most of the solid wastes from iron and steel industry are normally non-hazardous wastes.
Hazardous solid wastes are to be segregated at source in a proper manner and are to be treated and disposed carefully in accordance with the national and international guidelines. Hazardous waste is classified on the presence of hazardous substances. The list of hazardous substances can vary a bit from one country to another. The minimum allowable quantity of hazardous substance can also vary. Hazardous substances are normally (i) explosive materials, (ii) oxidizing materials which exhibit highly exothermic reactions when are in contact with other substances, (iii) flammable materials, (iv) non-corrosive substances, which, through immediate, prolonged or repeated contact with the skin or mucous membrane, can cause inflammation, (v) harmful substances, which, if inhaled or ingested or if they penetrate the skin, and can involve limited health risks, (vi) toxic substances, (vii) carcinogenic substances, (viii) corrosive substances which may destroy living tissue on contact, (ix) infectious substances, (x) teratogenic substances, (xi ) mutagenic substances, and (xii) eco-toxic substances. The thresholds or minimum allowable percentages of these substances are important. Some substances can have zero thresholds.
In iron and steel industry, three pronged approach is normally adopted for the solid wastes. The first approach is (i) to reduce the generation of wastes by using high quality raw materials, (ii) by close control of process parameters, and (iii) by improving the efficiency of the process. The second approach is to process the generated solid wastes so that it can be recycled or reused. The third approach is to segregate the hazardous wastes from non-hazardous wastes and take all precautions in their disposal so that they have either no effect or minimum effect on the environment. High generation of solid wastes and low recycling rate increase the production cost. Higher is the recycling of the solid wastes, lower is the consumption of raw materials.
Disposal of solid wastes and their dumping are very big environmental issues and hence, there are concentrated efforts in the iron and steel industry for recycling and reuse of the solid wastes. Recycling is a process using materials (waste) into new products in order to (i) prevent waste of potentially useful materials, (ii) reduce the consumption of fresh raw materials, (iii) reduce energy usage, (iv) reduce air pollution (from incineration) and water pollution (from landfilling) by reducing the need for ‘conventional’ waste disposal, and (v) lower greenhouse gas emissions as compared to the emissions during the production without recycling. Recycling is a key component of modern waste reduction efforts and is the third component of the 3 Rs (Reduce, Reuse, and Recycle) waste hierarchy. The major advantages of recycling and reuse of the solid wastes are (i) economic advantage, (ii) saving of raw materials, (iii) conservation of resources, (iv) cleaner working environment, and (v) conservation of energy.
Unlike gaseous and liquid pollutants, solid wastes need a large land area for their dumping. Dumping of solid wastes in open space and excavated land not only creates environmental pollution in the form of dusts and leachate but also creates huge financial liability due to scarcity of land. Also, for the disposal of solid wastes, transportation facilities are required. Some of the solid wastes create difficult disposal problems because of their physical size and chemical characteristics. Some solid wastes can be even contaminated with hazardous materials.
The main sources of the generation of solid wastes are metallurgical processes, pollution control equipments and devices used for cleaning of gases, and treatment plants for contaminated water, biological water, and sewage water. The physical and chemical characteristics of the solid wastes depend on the characteristics of the materials used in the process, the process parameters, and treatment of the wastes post its generation before its disposal. These physical and chemical characteristics are important parameters since this influence the disposal of the solid wastes. The post treatment of wastes after its generation is important aspect of the solid wastes since it has a major influence on the physical characteristics of the waste on which it’s recycling and reuse potential depends.
The quantity of the solid waste materials which gets generated during a metallurgical process is dependent on different factors such as (i) the characteristics and quality of the raw materials used in the process, (ii) technology of production, (iii) process parameters, (iv) type of pollution control equipment used, and (v) maintenance and upkeep of the equipment etc. Also, it may not be feasible to collect the complete quantity of the generated solid wastes. Some quantity of these wastes gets lost in the environment or escapes into another type of wastes and contaminates them.
A large tonnage of solid wastes is getting recycled and reused either in the steel plant itself or by other industries. It is possible that the solid wastes generated in one of the metallurgical process can be recycled in another metallurgical process where it replaces equivalent quantity of basic raw material. The recycling and reuse of solid waste depends upon physical and chemical characteristics of the wastes. When the solid wastes are recycled and re-utilized then the term solid waste is replaced with the term ‘by-product’. However, most of the solid wastes for their utilization need to be processed. The recycling and reuse of solid wastes provide economic advantage over the alternative of dumping of solid wastes.
Some of the solid wastes get recycled immediately back in the process, while in most of the cases the solid wastes are collected from the process, transported, and stored for recycling in the same process or in another process in a controlled manner. In some processes, the solid wastes generated are in mixed form. In such cases the solid wastes are processed for segregation of recyclable solid wastes and non-reusable solid wastes (muck or debris). There are several stages for making solid wastes recyclable. These are (i) source segregation, (ii) collection, (iii) transportation, (iv) treatment, (v) disposal, and (vi) recycling and resource generation.
The main features of most of the solid wastes generated in steel industry are that they are considered as unwanted product and they have low intrinsic value with respect to their weight. Due to this feature, the transportation of the solid wastes to somewhat long distances constitutes a substantial cost which is quite high compared to their value. Hence, it is desirable that steel plant solid wastes are used within the steel plant or in industries situated nearby area.
Disposal of non-usable solid wastes can be onsite or offsite. The use of onsite or offsite landfills depends on land availability. Disposal practices of solid wastes which cannot be recycled or reused include stockpiling or landfilling. The major impacts on the environment from these methods of disposal result from wind transportation, surface run-off, and subsurface migration of leachate.
Wind transportation of dusts from storage piles and landfills can be minimized by proper attention to configuration (exposed surface area), moisture content, and spraying of various chemicals. Surface run-off can be controlled by method of operation. However, the main environment concern of solid wastes disposal is migration of sub-surface leachate.
Solid pollutants which are not recycled or reused are dumped in a land area usually as a land refill. The leachate produced at the land fill area affects the quality of ground water and hence needs a control. Leachate is formed by water infiltrating the dumped solid wastes. The leachate migrates through the soils under the solid wastes and is attenuated by ionic change, filtration, adsorption, complexing, precipitation, and bio-gradation. If the voids of the soil are filled with water, the leachate moves to the ground water where there is little dilution.
The dumping areas for solid wastes normally do not have facilities for the collection of leachate. Hence, the ground water under solid waste dumping sites gets contaminated and becomes unsuitable for drinking. The contamination can be due to dissolved solids, oil, pH, ammonia, chromium, manganese, phenols, cadmium, and other components. These are substantial differences in the landfill requirements for hazardous and non-hazardous wastes. It is essential that the hazardous wastes are segregated from solid wastes for their separate disposal.
The major impact of the solid waste disposal practices is the potential damage to the ground water. Endangerment to the ground water is defined as the introduction of any substance into the ground water in such a concentration that additional treatment becomes necessary for the user of the ground water, or the water becomes unfit for human consumption.
As stated earlier, the leachate is produced at the dumping site of the solid wastes. As the leachate moves through the sub-surface soils, several mechanisms can impact the nature, and hence, impacts the environment. One of these is ion exchange adsorption by clay and organic soils which can adsorb and retain metallic ions. Another mechanism is metal fixation in which metal ions binds irreversibly to the soil, or substitute with other ions of similar radii in the mineral structure. Metal cations can also react with phosphate, carbonate, or sulphide to yield a precipitate of low solubility. Heavy metals in their metallic state are generally insoluble, but the heavy metal salts can be quite soluble. Ammonia which is present in leachate is oxidized to nitrate under aerobic conditions by certain bacteria and can be nitrate by the time it reaches ground water.