O Sarafadeen Amuda A Olanrewaju Alade Yung Tse Hung and Lawrence K Wang


22.1 Introduction 913

22.1.1 Industrial Wastewater 913

22.1.2 Process Wastes 914

22.1.3 General Wastewater Treatments 914

22.1.4 Common Wastewater Treatment Processes 915

22.1.5 Wastewater Characterization 917

22.1.6 Effluent Disposal 918

22.2 Inorganic Chemical Industries 918

22.2.1 Classification of Inorganic Chemical Industries 918

22.3 Industrial Processes, Wastewater Generation, and

Treatment in Inorganic Chemicals Production 920

22.3.1 Aluminum Fluoride 920

22.3.2 Chlor-Alkali 921

22.3.3 Chrome Pigments 926

22.3.4 Copper Sulfate 932

22.3.5 Hydrofluoric Acid 932

22.3.6 Hydrogen Cyanide 934

22.3.7 Nickel Sulfate 938

22.3.8 Sodium Bisulfite 938

22.3.9 Sodium Dichromate 941

22.3.10 Sodium Hydrosulfite 944

22.3.11 Titanium Dioxide 945

22.4 Summary 950

References 953

22.1 INTRODUCTION 22.1.1 Industrial Wastewater

All the water that is sent to the drain or is pooled together after being subjected to one process or the other in an industry is known as wastewater. It is usually classified into municipal wastewater and industrial wastewater. Industrial wastewaters are primarily from both small- and large-scale industries, which predominantly include manufacturing industries. The type of industry greatly influences the characteristics of industrial wastewater both in type and in varying amount; depending on contaminants concentrations, industrial wastewater can be classified further into strong,

medium, or weak.1 The discharge of industrial wastewater into the environment has manifold adverse effects on humans and aquatic species.

22.1.2 Process Wastes

All manufacturing industries produce peculiar wastes from their production processes. The bulk of the wastes is conveyed to the wash waters that end up as wastewaters. The washing operations include washing of raw materials, the intermediate and final products, the plant (before and after production batch), and unsolicited rain storm that washes the exposed plant parts. I norganic Process Wastes

Inorganic chemical industries, metallurgical industries, and petroleum industries are major processing industries that generate volumes of inorganic process wastes. The inorganic process wastes are characteristically toxic and may be acidic or basic in nature, but do not pose the biological problems often associated with wastewaters. It, however, poses a high problem of disposal when it combines with organic wastes because of the likely reactions that may lead to the formation of more complex hazardous compounds. Organic Process Wastes

The organic chemical industry, the food processing industry, the pulp and paper industry, the textile industry, and the petroleum industry are important industries that produce organic process wastes. Unlike inorganic process wastes, they contain dissolved and insoluble matter in the main wastewater stream; thus, they are more difficult to handle for disposal. They have its characteristic biological problems and spontaneous interaction with the surrounding environment, particularly, under high solar radiation.

22.1.3 General Wastewater Treatments

Treatment of wastewater generated in most industries is often achieved in many steps depending on the volume, shape, and nature of constituents of the wastewater. A typical wastewater treatment plant combines water treatment unit operations and processes to achieve different levels of treatment. They include the following. Preliminary Treatment

The objective of this treatment level is to reduce or eliminate nonfavorable wastewater characteristics that are likely to adversely affect the operation and efficiency of the processes and equipment of the wastewater plant. These processes include screening, comminution, flotation, flow equalization, septage handling, and odor control methods.2 The targeted wastewater characteristics include large solids, rags, abrasive grit, odor, and organic loadings.2 Primary Treatment

This treatment process employs the use of physical operations such as screening and sedimentation to partially remove suspended solids and organic matter from the wastewater. This process provides the secondary treatment with wastewater that is partially free of solids, in order to facilitate further treatment. The effluent from the primary treatment contains primarily organic matter and is characterized by a relatively high biochemical oxygen demand (BOD).2 Secondary Treatment

The secondary treatment is linked to the primary treatment in a typical wastewater treatment plant. It is designed to remove soluble and colloidal organics as well as suspended solids that are not trapped in the primary treatment. The treatment processes employed in this section of the wastewater treatment plant include activated sludge, lagoon systems, and sedimentation.2 The sludge generated as waste in this system is digested by microorganisms, mainly bacteria and protozoa.3 In general, the system involves biological processes. Advanced Treatment

In this treatment process, unit operations such as chemical coagulation, flocculation, and sedimentation followed by filtration, activated carbon, ion exchange, and reverse osmosis are employed to remove significant amounts of nitrogen, phosphorus, heavy metals, organic matters, bacteria, and viruses present in wastewater.2 It is always the last process step in the wastewater treatment plant that finally renders the treated wastewater reusable and disposable into the environment without any adverse effect (Figure 22.1).

22.1.4 Common Wastewater Treatment Processes

To meet the specified standard,4 wastewaters are often subjected to a series of treatment processes before they are discharged into the environment, particularly, water bodies. The treatment processes include physical, chemical, and biological processes that may be applied singly or collectively. The collective application of the processes can be employed in a variety of systems classified as primary, secondary, and tertiary wastewater treatment, to achieve different levels of contaminants removal.2 Physical Treatment Processes

These are age-long processes that involve the application of physical forces to remove contaminants in wastewater. It includes the following. Screening

The process involves the use of a screen to remove gross pollutants in the form of particles from a wastewater stream in order to prevent damages to downstream equipment and plant operation units. Common screening devices consist of parallel bars, rods, grating, wire mesh, and perforated plates with either circular or rectangular opening.4 The screens are further categorized according to their size of openings; these include coarse (>6 mm), fine (1.5-6 mm), very fine (0.2-1.5 mm), and micro-screens (0.001-0.3 mm).5

Tertiary treatment

_^ Secondary treatment

Primary treatment

Preliminary treatment

_^ Secondary treatment

Primary treatment

Preliminary treatment

Sludge processing facility

FIGURE 22.1 Flow diagram showing treatment levels in a wastewater treatment plant.

Sludge processing facility

FIGURE 22.1 Flow diagram showing treatment levels in a wastewater treatment plant.

221.41.2 Sedimentation

This is another widely used process in the physical treatment process of wastewater; it simply involves gravitational settling of heavy particles that are suspended in a mixture; it has wide application within the treatment plant, particularly, in the primary settling basin, where grits, particulate matter, and biological and chemical flocs are removed, in the activated sludge settling basin. Three main designs of the sedimentation or settling tank are horizontal flow, solid contacts, and inclined surface.3 Communition

The communitors are generally placed between the grit chamber and the primary settling tank in the wastewater treatment plant; they are used to pulverize large floating materials in the wastewater. This process helps reduce odors and flies. A communitor may have rotating or oscillating cutters, a barminutor (a special type of communitor), and a bar screen.5 Flotation

This is a unit operation process where air bubbles, as gas, are used to remove solid or liquid particles from the liquid wastewater. The air bubbles are often trapped in the morphology of the suspended particles and as a result of buoyant forces, the particles move up and float on the surface where they are skimmed out. The common flotation methods include dissolved air, air flotation, vacuum flotation, and chemical additives.3

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