Grégorio Crini
email: gregorio.crini@univ-fcomte.fr

Eric Lichtfouse
email: eric.lichtfouse@inra.fr

Wastewater Treatment: An Overview

Abstract During the last 30 years, environmental issues, especially concerning the chemical and biological contamination of water, have become a major concern for both society and public authorities, but more importantly, for the whole industrial world. Any activities whether domestic or agricultural but also industrial produce wastewaters or effluents containing undesirable contaminants which can also be toxic. In this context, a constant effort must be made to protect water resources. In general, conventional wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove insoluble particles and soluble contaminants from effluents. This chapter briefly discusses the different types of effluents, gives a general scheme of wastewater treatment, and describes the advantages and disadvantages of technologies available.

Abbreviations

Introduction

Actually, water pollution by chemicals has become a major source of concern and a priority for both society and public authorities, but more importantly, for the whole industrial world [START_REF] Sonune | Developments in wastewater treatment methods[END_REF][START_REF] Crini | Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment[END_REF][START_REF] Cox | Industrial liquid effluents[END_REF][START_REF] Sharma | Green chemistry for dyes removal from wastewater[END_REF][START_REF] Rathoure | Toxicity and waste management using bioremediation[END_REF]. What is water pollution? Water pollution can be defined in many ways. Pollution of water occurs when one or more substances that will modify the water in negative fashion are discharged in it. These substances can cause problems for people, animals and their habitats and also for the environment. There are various classifications of water pollution (Morin-Crini and Crini 2017). The two chief sources can be seen as point and non-point. The first refers to the pollutants that belong to a single source such as emissions from industries into the water, and the second on the other hand means pollutants emitted from multiple sources. The causes of water pollution are multiple: industrial wastes, mining activities, sewage and waste water, pesticides and chemical fertilizers, energy use, radioactive waste, urban development, etc. The very fact that water is used means that it will become polluted: any activities whether domestic or agricultural but also industrial produce effluent containing undesirable pollutants which can also be toxic. In this context, a constant effort must be made to protect water resources [START_REF] Khalaf | Green polymers and environmental pollution control[END_REF][START_REF] Rathoure | Toxicity and waste management using bioremediation[END_REF]Morin-Crini and Crini 2017).

The legislation covering liquid industrial effluent is becoming stricter, especially in the more developed countries, and imposes the treatment of any wastewater before it is released into the environment. Since the end of the 1970s, in Europe, the directives are increasingly severe and zero rejection is being sought by 2020. Currently, the European policy on water results from the Water Framework Directive (WFD) of 2000 which establishes guidelines for the protection of surface water, underground water, and coastal water in Europe (Morin-Crini and Crini 2017).

The WFD also classified chemicals into two main lists of priority substances. The first, the "Black List", involves dangerous priority substances (DPS) considered to be persistent, highly toxic or to lead to bioaccumulation. The second list, the "Grey List", gathers priority substances (PS) presenting a significant risk for the environment. The selection of these substances can either be based on individual substances of families of substances (e.g. metals, chlorobenzenes, alkylphenols, etc.) or on the basis of the industrial sector (e.g. agro-food industry, chemicals industry, metalfinishing sector, etc.). Currently, Europe is now asking industrials to innovate to reduce and/or eliminate the release of DPS and PS chemicals in their wastewaters.

Moreover, recycling wastewater is starting to receive active attention from industry in the context of sustainable development (e.g. protection of the environment, developing concepts of "green chemistry", use of renewable resources), improved water management (recycling of waste water) and also health concerns [START_REF] Kentish | Innovations in separations technology for the recycling and re-use of liquid waste streams[END_REF][START_REF] Cox | Industrial liquid effluents[END_REF][START_REF] Sharma | Advances in water treatment and pollution prevention[END_REF][START_REF] Khalaf | Green polymers and environmental pollution control[END_REF][START_REF] Rathoure | Toxicity and waste management using bioremediation[END_REF]Morin-Crini and Crini 2017). Thus, for the industrial world, the treatment of effluents has become a priority.

During the past three decades, several physical, chemical and biological technologies have been reported such as flotation, precipitation, oxidation, solvent extraction, evaporation, carbon adsorption, ion-exchange, membrane filtration, electrochemistry, biodegradation, and phytoremediation [START_REF] Berefield | Process chemistry for water and wastewater treatment[END_REF]Liu and Liptak 2000;Henze 2001;[START_REF] Harvey | Phytoremediation of polyaromatic hydrocarbons, anilines and phenols[END_REF][START_REF] Chen | Electrochemical technologies in wastewater treatment[END_REF][START_REF] Forgacs | Removal of synthetic dyes from wastewaters: a review[END_REF][START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF][START_REF] Crini | Traitement et épuration des eaux industrielles polluées[END_REF][START_REF] Cox | Industrial liquid effluents[END_REF][START_REF] Hai | Hybrid treatment systems for dye wastewater[END_REF][START_REF] Barakat | New trends in removing heavy metals from industrial wastewater[END_REF][START_REF] Rathoure | Toxicity and waste management using bioremediation[END_REF]Morin-Crini and Crini 2017). Which is the best method? There is no direct answer to this question because each treatment has its own advantages and constraints not only in terms of cost but also in terms of efficiency, feasibility, and environmental impact. In general, elimination of pollutants is done by both physical, chemical and biological means. At the present time, there is no single method capable of adequate treatment, mainly due to the complex nature of industrial effluents. In practice, a combination of different methods is often used to achieve the desired water quality in the most economical way.

After a brief discussion on the main contaminants/pollutants and the different types of effluents, this chapter proposes a general scheme of wastewater treatment and presents the advantages and disadvantages of different individual techniques used.

Water Pollution

Contamination and Contaminants

Contamination/Pollution arises from all sectors of human activity (i.e. domestic, industrial and agricultural), and is not only due to natural (petroleum, minerals, etc.) and anthropogenic causes (e.g. sewage treatment sludge or persistent organic pollutants produced by the incineration of waste) but also, and especially, to synthetic substances produced by chemical industries (e.g. dyes, fertilizers, pesticides, and so on). The terms contamination/pollution and contaminant/pollutant are often used in relation to subjects like environment, food and medicine [START_REF] Amiard | Les risques chimiques environnementaux[END_REF][START_REF] Rathoure | Toxicity and waste management using bioremediation[END_REF]. Both contaminant and pollutant refer to undesirable or unwanted substances. Pollutant refers to a harmful substance but contaminant is not necessarily harmful since contamination refers simply to the presence of a chemical substance where it should not be. This means that all pollutants are contaminants, but not all contaminants are pollutants. In this chapter, both these terms were used.

A chemical pollutant is a substance toxic for flora and fauna, and for humanity, and present at concentrations such that, in nature, it has repercussions on the environment and on health in general. Pollutants can be categorized according to the sources they are derived from, such as water pollutants, soil pollutants, air pollutants or noise pollutants [START_REF] Crini | Traitement et épuration des eaux industrielles polluées[END_REF]. Examples of pollutants known to the public and found in waters are numerous and various. The list includes nitrates, phosphates, detergents, pesticides and other crop sprays, chlorinated solvents but also metals (e.g. lead, mercury, chromium, cadmium, arsenic), dyes, organics (benzene, bisphenol A. . .), mineral derivatives (especially arsenic and cyanides) and microorganisms (e.g. bacteria, virus). Others are less well known but are considered to be high on the list of dangerous substances: volatile organic compounds (VOC), polycyclic aromatic hydrocarbons (PAH), polychlorobiphenyls (PCB), bromine-containing flame-retardants, phthalates, and many more (Liu and Liptak 2000; [START_REF] Sonune | Developments in wastewater treatment methods[END_REF][START_REF] Sharma | Advances in water treatment and pollution prevention[END_REF].

One way of measuring the quality of water is to take samples of this water and measure the concentrations of different substances that it contains, using analytical techniques such as, for instance, inductively coupled plasma (ICP) for metals, and/or determine chemical indicators or global parameters (Morin-Crini and Crini 2017). Typically, water quality is determined by comparing the physical and chemical characteristics of a sample with water quality guidelines or standards based on scientifically assessed acceptable levels of toxicity to either humans or aquatic organisms. Biological indicators using living organism such as fish can be also used. From the wastewater treatment point of view, it is also important to list the exact chemical composition of the effluents to be treated (Liu and Liptak 2000;[START_REF] Lacorte | Organic compounds in paper-mill process waters and effluents[END_REF][START_REF] Pokhrel | Treatment of pulp and paper mill wastewatera review[END_REF][START_REF] Sharma | Green chemistry for dyes removal from wastewater[END_REF][START_REF] Druart | Chemical and ecotoxicological monitoring of discharge water from a metal-finishing factory[END_REF]. Indeed, before any actions can be taken to reduce and/or eliminate any chemicals, it is necessary to identify all the dissolved substances in the effluents qualitatively and quantitatively using. However, a real effluent can be also a non-uniform mixture, colored and/or smelly, contain suspended solids (SS), immiscible liquids (e.g. oils, fats, hydrocarbons), soluble and/or biodegradable molecules, substances that can give waters redox potential, acidity, or pathogenicity problems [START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF]Crini and Badot 2010;[START_REF] Sharma | Advances in water treatment and pollution prevention[END_REF][START_REF] Sharma | Green chemistry for dyes removal from wastewater[END_REF]Morin-Crini and Crini 2017). In this case, wastewater quality can then be defined by physical, chemical and biological characteristics or general parameters [START_REF] Cooper | Removing colour from dye house waste watersa critical review of technology available[END_REF]Liu and Liptak 2000;[START_REF] Crini | Traitement et épuration des eaux industrielles polluées[END_REF]. Physical parameters include color, temperature, solids, turbidity, odor, oil and grease. Solids can be further classified into suspended and dissolved substances as well as organic and inorganic fractions. Chemical parameters associated with the organic content of industrial wastewater include the chemical oxygen demand (COD), biochemical oxygen demand (BOD), total organic carbon (TOC), and total oxygen demand (TOD). Inorganic chemical parameters include salinity, pH (acidity, alkalinity), metals, chlorides, sulfates, nitrogen, phosphorus, etc. Bacteriological parameters include coliforms, fecal coliforms, specific pathogens, and viruses. Recent books can be consulted on these topics [START_REF] Sharma | Advances in water treatment and pollution prevention[END_REF][START_REF] Sharma | Green chemistry for dyes removal from wastewater[END_REF][START_REF] Khalaf | Green polymers and environmental pollution control[END_REF][START_REF] Rathoure | Toxicity and waste management using bioremediation[END_REF]Morin-Crini and Crini 2017).

Recently, [START_REF] Druart | Chemical and ecotoxicological monitoring of discharge water from a metal-finishing factory[END_REF] investigated the chemical composition of discharge waters from a metal-finishing industry sampled over a three-month period. All these samples respected the regulatory standards. Twenty-on water parameters and 164 substances were monitored, among them organic and metallic compounds. The results indicated, that, on average, 52 substances were found with a high variability, both qualitative and quantitative. Inorganics such as calcium, sodium and chloride were present at concentrations close to g/L and metals higher than mg/L. Organics were detected at trace levels (ng/L of μg/L).

Different Types of Effluents

There are various sources of water contamination (e.g. households, industry, mines, infiltration) but one of the greatest remains its large scale use by industry [START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF][START_REF] Hai | Hybrid treatment systems for dye wastewater[END_REF]. Four categories of water are generally distinguished: (1) rainwater (runoff from impermeable surfaces), (2) domestic wastewater, (3) agricultural water and (4) industrial wastewaters [START_REF] Crini | Traitement et épuration des eaux industrielles polluées[END_REF]. The last group can be subdivided into cooling water, washing effluent (of variable composition), and manufacturing or process water (biodegradable and/or potentially toxic). In general, process waters (i.e. wastewaters or effluents) pose the greatest problems. Wastewaters differ significantly from drinking water sources (usually rivers, lakes, or reservoirs) in one important way: the contaminant levels in most drinking water sources are quite low as compared with contaminant levels in wastewaters derived from industrial-type activities [START_REF] Cooney | Adsorption design for wastewater treatment[END_REF]). However, their toxicity depends, of course, on their composition, which in turn depends on their industrial origin.

It can be noted that some effluents such as from surface treatment or cokeproduction plants are serious polluters whereas the effluent from other sectors such as the agro-food industry (including dairies, sugar mills and fruit and vegetable processing units) may be heavily loaded but the substances it contains are easily biodegradable and even recyclable. Pollution issues have a strong impact on the population. Colored effluent, for instance from pulp and paper mills or from textile mills, has a strong visual impact due to its color and is perceived by the public as an indication of the presence of dangerous pollutionhowever toxic the coloring actually is [START_REF] Lacorte | Organic compounds in paper-mill process waters and effluents[END_REF][START_REF] Pokhrel | Treatment of pulp and paper mill wastewatera review[END_REF][START_REF] Forgacs | Removal of synthetic dyes from wastewaters: a review[END_REF][START_REF] Rana | Toxic effects of pulp and paper-mill effluents on male reproductive organs and some systemic parameters in rats[END_REF][START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF][START_REF] Crini | Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment[END_REF][START_REF] Hai | Hybrid treatment systems for dye wastewater[END_REF][START_REF] Wojnárovits | Irradiation treatment of azo dye containing wastewater: an overview[END_REF]. Colored effluent can lead to nature protection associations or other stakeholders in the water bodies suing the parties responsible. In addition, it is known that paper-mill wastewater contains nutrient elements that can lead to eutrophication and thus to a heavy organic load for the aquatic environment due to the proliferation of algae at the expense of other aquatic species [START_REF] Lacorte | Organic compounds in paper-mill process waters and effluents[END_REF][START_REF] Rana | Toxic effects of pulp and paper-mill effluents on male reproductive organs and some systemic parameters in rats[END_REF]). Effluent with high levels of heavy metals from surface treatment industries is also a serious source of toxicity for aquatic ecosystems, again creating worries for the population [START_REF] Rana | Toxic effects of pulp and paper-mill effluents on male reproductive organs and some systemic parameters in rats[END_REF][START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF]Morin-Crini and Crini 2017).

The industrial sectors of agro-food, textiles, pulp industry and surface-treatment industries are today considered to be the four largest consumers of water and the most polluting, in spite of the considerable effort made to clean up the processes over the last 30 years. These activities are all energy-and water-consuming as well as highly chemically polluting. The problems encountered during wastewater treatment are generally very complex as the effluent contains pollutants of various types depending on its origin. So, there are different types of effluent to treat, each with its own characteristics requiring specific treatment processes.

Wastewater Treatment

General Scheme of Wastewater Treatment

When water is polluted and decontamination becomes necessary, the best purification approach should be chosen to reach the decontamination objectives (as established by legislation). A purification process generally consists of five successive steps as described in Fig. 1.1: (1) preliminary treatment or pre-treatment (physical and mechanical); (2) primary treatment (physicochemical and chemical); (3) secondary treatment or purification (chemical & biological); (4) tertiary or final treatment (physical and chemical); and (5) treatment of the sludge formed (supervised tipping, recycling or incineration). In general, the first two steps are gathered under the notion of pre-treatment or preliminary step, depending on the situation [START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF]Crini andBadot 2007, 2010).

Pre-treatment consists of eliminating the (floating) solid particles and all suspended substances from the effluent. This pre-treatment stage, which can be carried out using mechanical or physical means is indispensable, before envisaging secondary treatment because particulate pollution (e.g. SS, colloids, fats, etc.) will hinder later treatment, make it less efficient or damage the decontamination equipment. Primary chemical treatment such as oxidation for cyanide destruction and Cr(VI) reduction, pH adjustment, pre-reduction of a high organic load may also be required. For instance effluent from paper mills contains abundant SS such as fibres, fillers and other solids [START_REF] Pokhrel | Treatment of pulp and paper mill wastewatera review[END_REF][START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF][START_REF] Sharma | Green chemistry for dyes removal from wastewater[END_REF]). Effluents from textile mills have a very variable pH although it is often alkaline, containing a high organic Fig. 1.1 Overview of the main processes for the decontamination of contaminated industrial wastewaters load. It is therefore indispensable to pre-treat these effluents before considering secondary treatment. However, these treatments alone are, in many cases, incapable of meeting the legislation requirements.

Before its discharge into the environment or its reuse, the pre-treated effluent must undergo secondary purification treatment using the most appropriate of the biological, physical or chemical techniques available to remove the chemical pollution. In certain cases, a final or tertiary treatment (step 4 in Fig. 1.1) can also be required to remove the remaining pollutants or the molecules produced during the secondary purification (e.g. the removal of salts produced by the mineralization of organic matter). However, the use of tertiary treatment in Europe is limited, though it may be necessary in the future if new restrictions are applied. The main tertiary treatments employed to date at a few industrial sites are adsorption using activated carbons (AC), ion-exchange, membrane filtration (ultrafiltration, reverse osmosis), advanced oxidation, and constructed wetlands (CW). In Europe, most of the CW are applied for domestic sewage and municipal wastewater treatment. However, the diversity of CW configurations makes them versatile for implementation to treat industrial effluents (e.g. tannery wastewater, pulp and paper post-treated effluents).

Technologies Available for Contaminant Removal

In general, conventional wastewater treatment consists of a combination of physical, chemical, and biological processes and operations to remove solids including colloids, organic matter, nutrients, soluble contaminants (metals, organics. . .) etc. from effluents. A multitude of techniques classified in conventional methods, established recovery processes and emerging removal methods can be used (Fig. 1.2). Table 1.1 lists the advantages and disadvantages of different individual techniques [START_REF] Berefield | Process chemistry for water and wastewater treatment[END_REF]Henze 2001;[START_REF] Sonune | Developments in wastewater treatment methods[END_REF][START_REF] Chen | Electrochemical technologies in wastewater treatment[END_REF]; Pokhrel and [START_REF] Pokhrel | Treatment of pulp and paper mill wastewatera review[END_REF]Parsons 2004;[START_REF] Forgacs | Removal of synthetic dyes from wastewaters: a review[END_REF][START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF][START_REF] Chuah | Rice husk as a potentially low-cost biosorbent for heavy metal and dye removal: an overview[END_REF][START_REF] Crini | Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment[END_REF][START_REF] Crini | Non-conventional low-cost adsorbents for dye removal[END_REF][START_REF] Bratby | Coagulation and flocculation in water and wastewater treatment[END_REF]Crini andBadot 2007, 2010;[START_REF] Cox | Industrial liquid effluents[END_REF][START_REF] Mohan | Arsenic removal from waste/wastewater using adsorbentsa critical review[END_REF][START_REF] Hai | Hybrid treatment systems for dye wastewater[END_REF][START_REF] Wojnárovits | Irradiation treatment of azo dye containing wastewater: an overview[END_REF][START_REF] Barakat | New trends in removing heavy metals from industrial wastewater[END_REF][START_REF] Sharma | Advances in water treatment and pollution prevention[END_REF][START_REF] Rathoure | Toxicity and waste management using bioremediation[END_REF]Morin-Crini and Crini 2017).

Selection of the method to be used will thus depend on the wastewater characteristics [START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF][START_REF] Crini | Recent developments in polysaccharide-based materials used as adsorbents in wastewater treatment[END_REF][START_REF] Crini | Traitement et épuration des eaux industrielles polluées[END_REF][START_REF] Cox | Industrial liquid effluents[END_REF]. Each treatment has its own constraints not only in terms of cost, but also in terms of feasibility, efficiency, practicability, reliability, environmental impact, sludge production, operation difficulty, pre-treatment requirements and the formation of potentially toxic byproducts. However, among the various treatment processes currently cited for wastewater treatment, only a few are commonly employed by the industrial sector for technological and economic reasons. In general, removal of pollutants from effluents is done by physicochemical and/or biological means, with research concentrating on cheaper effective combinations of systems or new alternatives.

Conclusion

The development of cheaper, effective and novel methods of decontamination is currently an active field of research, as shown by the numerous publications appearing each year. Preserving the environment, and in particular the problem of water pollution, has become a major preoccupation for everyonethe public, industry, scientists and researchers as well as decision-makers on a national, European, or international level. The public demand for pollutant-free waste discharge to receiving waters has made decontamination of industrial wastewaters a top priority. However, this is a difficult and challenging task [START_REF] Sonune | Developments in wastewater treatment methods[END_REF][START_REF] Anjaneyulu | Decolourization of industrial effluentsavailable methods and emerging technologiesa review[END_REF][START_REF] Barakat | New trends in removing heavy metals from industrial wastewater[END_REF][START_REF] Sharma | Advances in water treatment and pollution prevention[END_REF]. It is also difficult to define a universal method that could be used for the elimination of all pollutants from wastewaters. This chapter described the advantages and disadvantages of technologies available. A multitude of techniques classified in conventional methods, established recovery processes and emerging removal methods can be used. However, among the numerous and various treatment processes currently cited for wastewater treatment, only a few are commonly used by the industrial sector for economic and technological reasons. Adsorption onto activated carbons is nevertheless often cited as the procedure of choice to remove many different types of pollutants because it gives the best results in terms of efficiency and technical feasibility at the industrial scale.

1. 1

 1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2 1.2 Water Pollution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.1 Contamination and Contaminants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 1.2.2 Different Types of Effluents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 1.3 Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.1 General Scheme of Wastewater Treatment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7 1.3.2 Technologies Available for Contaminant Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8 1.4 Conclusion . .. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20

Fig. 1 . 2

 12 Fig. 1.2 Classification of technologies available for pollutant removal and examples of techniques

Table 1 .

 1 1 Advantages and disadvantages of the main conventional methods used for the treatment of polluted industrial wastewater

		Main
	Process	characteristic(s)	Advantages	Disadvantages
	Chemical	Uptake of the	Technological simple	Chemical consumption
	precipitation	pollutants and	(simple equipment)	(lime, oxidants, H 2 S. . .)
		separation of the	Integrated physicochemi-	Physicochemical moni-
		products formed	cal process	toring of the effluent (pH)
			Both economically advan-	Ineffective in removal of
			tageous and efficient	the metal ions at low
				concentration
			Adapted to high pollutant	Requires an oxidation
			loads	step if the metals are
				complexed
			Very efficient for metals	High sludge production,
			and fluoride elimination	handling and disposal
			Not metal selective	problems (management,
			Significant reduction of	treatment, cost)
			the COD
	Coagulation/	Uptake of the	Process simplicity	Requires adjunction of
	flocculation	pollutants and		non-reusable chemicals
		separation of the		(coagulants, flocculants,
		products formed		aid-chemicals)
			Integrated physicochemi-	Physicochemical moni-
			cal process	toring of the effluent (pH)
			A large range of chemicals	Increased sludge volume
			is available commercially	generation (management,
				treatment, cost)
			Inexpensive capital cost	Low removal of arsenic
			Very efficient for SS and
			colloidal particles
			Good sludge settling and
			dewatering characteristics
			Significant reduction of
			the COD and BOD
			Interesting reduction of
			TOX and AOX (pulp and
			paper industry)
			Bacterial inactivation
			capability
			Rapid and efficient for
			insoluble contaminants
			(pigments. . .) removal
	Flotation	Separation	Integrated physicochemi-	High initial capital cost
		process	cal process
	Froth flotation		Different types of collec-	Energy costs
			tors (non-ionic or ionic)
			Efficient for small particles	Maintenance and opera-
			removal and can remove	tion costs no negligible
				(continued)

Table 1

 1

	.1 (continued)
		Main
	Process	characteristic(s)	Advantages	Disadvantages
			Insoluble organic matter
			is converted to simpler
			soluble compounds with-
			out emissions of danger-
			ous substances (WAO)