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Basically, Industrial waste consists of industrial wastewater. Our environment is being polluted every day. And industrial pollution is one of main reasons for it. Factories are dumping their waste anywhere they can and it is polluting our environment. Hence, we need to control and treat the industrial water. Industrial waste water treatment for inorganic compounds can be as simple as settling or filtration and as complex as multistage chemical precipitation or ion exchange process. Typical parameters requiring treatment in industrial waste water include suspended solids, dissolved metals, nitrates, ammonia, arsenic, and sulphate. The industrial waste water constitutes a large amount of inorganic matter and these inorganic wastes can be treated by many ways namely:
Physical processes include clarification, filtration, and membrane technologies. Except for the most rigorous membrane process (reverse osmosis), physical processes will generally not remove dissolved contaminants. Clarification uses a combination of coagulation, and settling to remove suspended particles and typically involves sludge recycle. Filtration methods include bag filters, sand filters, and multimedia filters. Multimedia filters, which typically utilize anthracite coal, sand, and garnet, are probably the most common filters now in use. These filters are pressure vessels that use downflow operation to remove suspended contaminants and a periodic up flow backwash to transfer these contaminants to a waste stream. The most common membrane technologies are microfiltration, ultrafiltration, and reverse osmosis (RO). These are listed in the order of decreasing pore size, increasing removal efficiency, and increasing pressure requirements. The primary disadvantage of RO is a high-volume waste stream which often limits its applicability.
Biological treatment processes include attached growth, suspended growth, and membrane bioreactors. Attached growth processes are most common, but membrane bioreactors are a growing application. Biological treatment can be used to remove ammonia, nitrate, selenium, sulphate and dissolved metals. In an attached growth system, bacteria are attached to the media surface. Media can range from plastic to activated carbon to rock, with media diameters ranging from microns to centimetres. The attached bacteria (a Biofilm) provide a very robust process in which it is very resilient to changes in flow, pH, and contaminant concentration. Attached growth systems are the best choice for treating high or variable concentrations.
Suspended growth systems are commonly used for municipal wastewater treatment but can also be used for industrial waste water. Activated sludge is an example of suspended growth biological treatment. Suspended growth is often used for removal of nutrients (Nitrogen and Phosphorus). When properly designed, these systems can be used for both nitrification (Ammonia removal) and denitrification (Nitrate removal). Nitrification is an aerobic process, while denitrification is anaerobic process. Suspended growth is best used for relatively low contaminant concentrations. In a suspended growth system such as activated sludge processes (also aerated lagoons and aerobic digesters), waste water surrounds the free-floating micro-organisms, gathering into biological flocs. The settled flocs containing bacteria can be recycled for further treatment.
Suspended growth systems typically operate poorly when encountering high variable waste streams. Suspended growth systems also require more energy, more equipment maintenance, and are more complex to operate because they involve more equipment than attached growth systems. However, attached growth systems typically require more land, may have odour issues associated with media clogging, and may be unable to treat high wastewater flows. Consequently, urban wastewater facilities often opt for suspended growth processes, while attached growth processes are common in small to medium size operations.
Chemical treatment processes include hydroxide precipitation, sulphide precipitation, oxidation-reduction, ion exchange methods, and natural zeolites. Hydroxide precipitation typically uses lime to increase the pH. Hydrated lime or pebble lime (Sodium hydroxide), soda ash (Sodium carbonate), or magnesium hydroxide. For ease of addition and to avoid mix up of chemical solutions, liquid caustic soda or lime slurry is sometimes purchased. The pH target for hydroxide precipitation depends upon the contaminants of concern. After precipitation and subsequent clarification or filtration, acid is often added to meet discharge requirements for pH. Coprecipitation, a process in which dissolved contaminants are pulled out of solution along with precipitation of high concentrations of contaminants such as iron, manganese, and sulphate, can also help to meet discharge limits.
Oxidation reduction processes are used to transform contaminants into less soluble or more easily removed forms. For arsenic removal, oxidising agents like chlorine/sodium hypochlorite, hydrogen peroxide, ozone, or permanganate are commonly added. Conversely, reducing agents such as sodium bisulphate or metabisulphite may be added to remove contaminants such as chromium and selenium. Oxidation and reduction are typically rapid reactions but since they require chemical addition, it will increase the total dissolved solids in treated water.
Sulphide precipitation, which can achieve lower levels than hydroxide precipitation, is typically used as a polishing step to meet low metals concentrations. Sodium sulphide or Sodium hydrogen Sulphide (NaHS) is typically used. This process requires only small quantities of reagent and a short retention time. The process is typically done at neutral to high pH to avoid generating dangerous hydrogen sulphide gas.
Specific ion exchange resins from several manufacturers are available to remove dissolved metals, arsenic, and nitrate. In this process, sodium or chloride ions are exchanged for the target contaminants. Resin is relatively expensive but has a long life and can be chemically regenerated. The waste stream from ion exchange is typically much less than that generated by reverse osmosis (RO).
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